Fuel bundles having twisted ribbon fuel rods for nuclear thermal propulsion applications, structures for manufacture, and methods of manufacture - Patents.com

Abstract

The fuel bundle has a plurality of twisted ribbon fuel rods in a hexagonal or circular packing arrangement in a core region encased in a multi-layer casing. The arrangement of the twisted ribbon fuel rods is facilitated by a rod seating fixture with a seating surface having a plurality of protrusions that form receiving spaces for the ends of the twisted ribbon fuel rods. The manufacture of the fuel bundle incorporates fiber manufacturing techniques and, optionally, penetration of gaps in the core region with an infiltrant. The twisted ribbon fuel rod manufacturing system has subsystems for imparting twist periodicity to the extruded ribbons, inspecting the twisted extruded ribbons, and cutting the twisted extruded ribbons to length. The inspection provides feedback for sorting the twisted ribbon fuel rods and for adjusting the operation of the subsystems. The fuel bundle (and optional fuel bundle support) may be incorporated into a fuel assembly of a nuclear propulsion nuclear fission reactor structure, for example, of a nuclear thermal propulsion engine.

Description

[Technical field]

[0001] Related Application Data This application is based on and claims priority to U.S. Provisional Application No. 63 / 317,477, filed March 7, 2022, the entire contents of which are incorporated herein by reference, pursuant to 35 U.S.C. § 119.

[0002] Technical Field and Industrial Applicability The present disclosure relates generally to nuclear fission reactors, particularly for propulsion, and structures related to nuclear fission reactors. Such nuclear propulsion nuclear fission reactors may be used in a variety of applications suitable for gas-fired reactor designs, such as space, lunar, and terrestrial environments. In particular, the present disclosure relates to twisted ribbon fuel rodlets formed from compositions including fissionable fuel components and assembled into fuel bundles that are incorporated into fuel assemblies for thermal propulsion reactors, structures for manufacturing the twisted ribbon fuel rods and for assembling the fuel bundles, and methods for fabricating such twisted ribbon fuel rods and fuel bundles. [Background technology]

[0003] In the following description, reference is made to certain structures and / or methods. However, the following references should not be construed as an admission that these structures and / or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and / or methods do not qualify as prior art against the present invention.

[0004] Development and testing of twisted ribbon fuel took place in the USSR from about 1975 to about 1990 and is reported in Burns et al., Nuclear Thermal Propulsion Reactor Materials, edited by P. Tsvetkov, Nuclear Materials, London: IntechOpen, 2020. (U,Zr)C fuel was used in the low temperature section of the Soviet reactor design (i.e., propellant exit gas temperature ≤ 2500K) and (U,Zr,Nb)C was used in the high temperature section of the reactor core. The fuel ribbons were extruded, twisted on a longitudinal axis, sintered, and assembled into tubes. The ribbon dimensions and twist rate were (1.5mm x 2.8mm, S = 30mm).

[0005] The simple tube restraint system used in this previous study allowed several failure modes. As reported in Lanin, "Nuclear Rocket Engine Reactor", Springer Series in Materials Science, Volume 170, Wang et al. (eds.), Springer-Verlag Berlin Heidelberg (2013), high temperature multi-cycle operation damaged the twisted ribbon fuel. For example, a twisting failure mode was observed, caused by the plasticity of the fuel elements at high temperature and the increased axial forces with increasing reactor differential pressure. Also, high temperature hydrogen ablation of the insulation and casing materials was observed. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] U.S. Patent Application Serial No. 16 / 999,244 [Non-patent literature]

[0007] [Non-Patent Document 1] Burns et al., “Nuclear Thermal Propulsion Reactor Materials,” edited by P. Tsvetkov, Nuclear Materials, London: IntechOpen, 2020 [Non-Patent Document 2] Lanin, "Nuclear Rocket Engine Reactor", Springer Series in Materials Science, Volume 170, Wang et al. (eds.), Springer-Verlag Berlin Heidelberg (2013) Summary of the Invention [Problem to be solved by the invention]

[0008] There is a need for improvements in twisted ribbon fuel rods, structures for manufacturing twisted ribbon fuel rods and assembling fuel bundles, and methods for fabricating such twisted ribbon fuel rods and fuel bundles. In particular, there is a need for improvements related to radial restraint of twisted ribbon fuel rods in fuel bundles, and the structures and materials of the fuel bundles and their casings. Thus, the method of radial restraint for twisted ribbon fuel rods in fuel bundles uses geometrically specific end fasteners, supports, and fiber architectures, as well as manufacturing methods to address failure modes, improve the internal distribution and restraint of the twisted ribbon fuel rods, improve distribution forces, and strengthen the twisted ribbon fuel rods (and the materials of the fuel bundle casing) against spallation by keeping the entire fuel bundle in radial compression.

[0009] Additionally, improvements related to the manufacture of twisted ribbon fuel rods are disclosed that provide manufacturing methods including preparation of materials for the twisted ribbon fuel rods, extrusion of the ribbon fuel rods and twisting the ribbon fuel rods to form twisted ribbon fuel rods with uniform properties, and improved manufacturability including defect detection in the manufactured twisted ribbon fuel rods. The improved twisted ribbon fuel rods contribute to improved overall fuel bundle performance and reduced failures.

[0010] Improved force distribution is also disclosed in the fuel assembly, particularly in hot ductile regions of the fuel assembly, such as the outlet fuel bundle region. The geometry of the fuel bundle support, such as a tapered peripheral surface, transfers the axial load of the twisted ribbon fuel rods caused by the reactor pressure differential from the fuel bundle and its components to the outer surface of the fuel assembly. [Means for solving the problem]

[0011] One embodiment of a fuel bundle includes a multi-layer casing having an interior volume defining a nuclear reactor core, and a plurality of twisted ribbon fuel rods arranged within the nuclear reactor core, the plurality of twisted ribbon fuel rods having a composition including a fissile fuel component, the cross sections of the plurality of twisted ribbon fuel rods arranged in a hexagonal packing arrangement or a circular packing arrangement in a cross section perpendicular to a longitudinal axis of the fuel bundle.

[0012] An embodiment of a rod seating fixture for arranging a plurality of twisted ribbon fuel rods to form a nuclear reactor core of a fuel bundle comprises a seating surface including a plurality of protrusions. The plurality of protrusions are distributed on the seating surface and have a height from a base surface of the seating surface. The protrusions include a plurality of sides, a first portion of the plurality of sides including an angled region joining a second portion of the plurality of sides at an angle other than 90 degrees. Sides of the plurality of adjacent protrusions define a receiving space configured to seat an end of a twisted ribbon fuel rod. The receiving space is configured to seat an end of a twisted ribbon fuel rod with either line contact between a side of the plurality of adjacent protrusions defining the receiving space and a corner of the end of the twisted ribbon fuel rod, or with area contact between at least one angled region of the plurality of adjacent protrusions defining the receiving space and a long side of the end of the twisted ribbon fuel rod.

[0013] An embodiment of a method for manufacturing a fuel bundle includes forming a core region of the fuel bundle, the forming of the core region including seating a first end of each of a plurality of twisted ribbon fuel rods (having a composition including a fissionable fuel component) into a respective receiving space of a rod seating fixture, the rod seating fixture including a seating surface having a plurality of protrusions, the plurality of protrusions being distributed on the seating surface and having a height from a base surface of the seating surface, and a side of the plurality of adjacent protrusions defining a respective receiving portion; attaching an end cap to a second end of each of the plurality of twisted ribbon fuel rods to form a pre-bundle; optionally introducing an infiltrant into the pre-bundle to fill interstices of the assembled twisted ribbon fuel rods; placing the pre-bundle into a multi-layer casing including an inner layer, an inner intermediate layer, an outer intermediate layer, and an outer layer; removing the rod seating fixture and the end cap; and optionally removing the infiltrant.

[0014] An embodiment of a twisted fuel rod fabrication system includes a ribbon initial cooling zone, a ribbon tensioning subsystem, a ribbon twisting subsystem, a ribbon cutting subsystem, a ribbon inspection zone, a length sensor subsystem, and a rod sorting and recovery subsystem. The twisted fuel rod fabrication system is enclosed within an inert atmosphere chamber. The ribbon tensioning subsystem includes a tensioning roller configured to contact a surface of the extruded ribbon and is mounted to the ribbon twisting subsystem for rotation about an axis of the extruded ribbon's processing path. The ribbon inspection zone includes one or more non-destructive inspection devices, and the rod sorting and recovery subsystem includes structure and components for sorting the rods based on inspection performed by the one or more non-destructive inspection devices.

[0015] One embodiment of a method for manufacturing twisted ribbon fuel rods includes mixing a carbon source with a plurality of oxide powders to form a mixture, at least one oxide powder having a composition including an oxide of a fissionable fuel component, forming the mixture into an intermediate powder, subjecting the intermediate powder to carbothermal reduction, mechanically processing the carbothermally reduced intermediate powder to form a feedstock of solid solution carbide powder including the fissionable fuel component, and forming the feedstock into a billet, where forming the feedstock includes the steps of heated thermoplastic mixing, extruding the billet to form an extruded ribbon, and processing the extruded ribbon to form the twisted ribbon fuel rod. The carbon source may be a phenolic resin or carbon black, and the feedstock may be (U a Zr b Nb c ) C d wherein 0.05 <a<0.4、0<b<0.95、0<c<0.4、および0.7<d<1である。

[0016] Processing the extruded ribbon to form twisted ribbon fuel rods includes contacting a surface of the extruded ribbon at a nip of tensioning rollers of a ribbon tensioning subsystem, where one or both tensioning rollers are spring loaded in a direction parallel to the processing path of the extruded ribbon, rotating the ribbon tensioning subsystem about an axis of the processing path of the extruded ribbon while maintaining tension on the extruded ribbon to impart a twist to the extruded ribbon, performing image analysis on the twisted extruded ribbon to identify defects and sorting the twisted ribbon fuel rods based on the results of the image analysis. Processing the extruded ribbon to form twisted ribbon fuel rods is performed under an inert atmosphere.

[0017] One embodiment of a method for controlling twist rate during manufacturing of twisted ribbon fuel rods includes receiving an image of the twisted ribbon, converting the received image into a binary ribbon profile by comparing each pixel in the received image to a predefined threshold, generating a theoretical ribbon profile according to a ribbon model including a target twist rate, generating a plot by convolving the ribbon profile with the theoretical ribbon profile for a range of twist rates, and determining a calculated twist rate for the twisted ribbon as the x-coordinate of the point in the plot having a maximum value.

[0018] An embodiment of a fuel assembly includes a fuel assembly external structure, a plurality of fuel bundles including a multi-layer casing having an interior volume defining a nuclear reactor core, a plurality of twisted ribbon fuel rods arranged within the nuclear reactor core, the plurality of twisted ribbon fuel rods having a composition including a fissionable fuel component, and a fuel bundle support structure including a body having an outer peripheral side and a plurality of openings in the body. The fuel assembly external structure has an inner surface including a plurality of facets and a tapered section. The inner surface of the tapered section is oriented radially inwardly with respect to a longitudinal axis of the fuel assembly external structure in a direction from an inlet end of the fuel bundle support structure to an outlet end of the fuel bundle support structure, and an outer surface of the multi-layer casing of each of the plurality of fuel bundles conformally mates with the plurality of facets of the inner surface of the fuel assembly external structure. The outer peripheral side of the fuel bundle support structure is oriented radially inwardly with respect to an axis of the fuel bundle support structure that is normal to the top surface in an axial direction from the top surface of the fuel bundle support structure to the bottom surface of the fuel bundle support structure, and the outer peripheral side of the fuel bundle support structure conformally fits with the tapered section of the inner surface of the fuel assembly external structure.

[0019] One embodiment of the fuel bundle support comprises a body including a top surface, a bottom surface, an outer circumferential side joining the top surface and the bottom surface, and a plurality of openings in the body from the top surface to the bottom surface. In an axial direction from the top surface to the bottom surface, the outer circumferential side of the fuel bundle support structure is oriented radially inward with respect to an axis of the fuel bundle support structure that is normal to the top surface. The plurality of openings in the body of the fuel support structure are defined by a plurality of walls extending from the top surface to the bottom surface, and for each of the plurality of openings, an area of ​​the opening at the top surface is less than an area of ​​the opening at the bottom surface.

[0020] Embodiments of the disclosed fuel bundles, fuel assemblies, fuel bundle supports, and methods of construction and fabrication have application in a wide range of fields, including in various nuclear fission reactor designs, as well as aerospace and industrial applications.

[0021] The foregoing summary, as well as the following detailed description of the embodiments, can be better understood when read in conjunction with the appended drawings, in which: It is to be understood that the depicted embodiments are not limited to the precise arrangements and instrumentalities shown. [Brief description of the drawings]

[0022] [Figure 1] 1 is a schematic perspective cutaway view of an embodiment of a fuel bundle having twisted ribbon fuel rods and a multi-layer casing including compliant insulation layers and layers having multi-directional fibers; FIG. [Figure 2A] FIG. 2 is a schematic diagram of a twisted ribbon fuel rod showing some dimensional aspects. [Figure 2B] FIG. 2 is a schematic diagram of a twisted ribbon fuel rod showing some dimensional aspects. [Figure 2C] FIG. 2 is a schematic diagram of a twisted ribbon fuel rod showing some dimensional aspects. [Diagram 3] 2 is a flow chart illustrating various steps in one embodiment of a method for manufacturing a fuel bundle having twisted ribbon fuel rods and multi-layer casings. [Figure 4A] FIG. 2 is a schematic perspective view of a rod-shaped object seating fixture. [Figure 4B] FIG. 4B is an enlarged view of region P1 in FIG. 4A. [Figure 5A] FIG. 5C is a schematic diagram showing the placement and arrangement of twisted ribbon fuel rods during assembly of a fuel bundle, including details of the rod seating fixtures in an enlarged view of area P2 of FIG. 5B. [Figure 5B] FIG. 5C is a schematic diagram showing the placement and arrangement of twisted ribbon fuel rods during assembly of a fuel bundle, including details of the rod seating fixtures in an enlarged view of area P2 of FIG. 5B. [Figure 5C] FIG. 5C is a schematic diagram showing the placement and arrangement of twisted ribbon fuel rods during assembly of a fuel bundle, including details of the rod seating fixtures in an enlarged view of area P2 of FIG. 5B. [Figure 5D]FIG. 5C is a schematic diagram showing the placement and arrangement of twisted ribbon fuel rods during assembly of a fuel bundle, including details of the rod seating fixtures in an enlarged view of area P2 of FIG. 5B. [Figure 5E] FIG. 5C is a schematic diagram showing the placement and arrangement of twisted ribbon fuel rods during assembly of a fuel bundle, including details of the rod seating fixtures in an enlarged view of area P2 of FIG. 5B. [Figure 6A] 6A is a schematic diagram illustrating the arrangement of twisted ribbon fuel rods in an assembled fuel bundle in which the twisted ribbon fuel rods are arranged in a hexagonal packing arrangement (FIG. 6A). [Figure 6B] FIG. 6C is a schematic diagram illustrating the arrangement of twisted ribbon fuel rods in an assembled fuel bundle in which the twisted ribbon fuel rods are arranged in a circular packing arrangement (FIG. 6B). [Figure 7] FIG. 6B is an enlarged view of region P3 in FIG. 6A. [Figure 8] FIG. 2 is a schematic perspective view of the fuel bundle during assembly after the end caps are installed; [Figure 9] FIG. 2 is an enlarged view of the fuel bundle during assembly showing the ends of the twisted ribbon fuel rods and filler rods seated in the rod seating fixtures. [Figure 10A] FIG. 2 is a schematic diagram of a fuel bundle during assembly with a portion of the support housing removed to reveal the twisted ribbon fuel rods and filler rods. [Figure 10B] 2 is a schematic diagram of a fuel bundle during assembly with the entire support housing removed; [Figure 11A] 1A-1C illustrate a fuel bundle at various points in the process of fiber mandrel wrapping of a multi-layer casing. [Figure 11B] 1A-1C illustrate a fuel bundle at various points in the process of fiber mandrel wrapping of a multi-layer casing. [Figure 11C] 1A-1C illustrate a fuel bundle at various points in the process of fiber mandrel wrapping of a multi-layer casing. [Figure 12]FIG. 2 is a schematic perspective view of the end of an assembled fuel bundle after the end cap has been removed. [Figure 13] FIG. 2 is a schematic diagram illustrating removal of infiltrant from interstices of assembled twisted ribbon fuel rods. [Figure 14A] 1A-1C are images showing twisted ribbon fuel rod configurations at different stages during assembly of a prototype fuel bundle. [Figure 14B] 1A-1C are images showing twisted ribbon fuel rod configurations at different stages during assembly of a prototype fuel bundle. [Figure 14C] 1A-1C are images showing twisted ribbon fuel rod configurations at different stages during assembly of a prototype fuel bundle. [Figure 15A] FIG. 2 is a pictorial perspective view of a prototype fuel bundle with the various layers exposed. [Figure 15B] FIG. 2 is a pictorial perspective view of a prototype fuel bundle with the various layers exposed. [Figure 16] 1 is a flow diagram illustrating a general process for one embodiment of a method for manufacturing twisted ribbon fuel rods. [Figure 17A] 4 is a flow diagram illustrating in greater detail various steps in one embodiment of a method for manufacturing twisted ribbon fuel rods. [Figure 17B] 4 is a flow diagram illustrating in greater detail various steps in one embodiment of a method for manufacturing twisted ribbon fuel rods. [Figure 17C] 4 is a flow diagram illustrating in greater detail various steps in one embodiment of a method for manufacturing twisted ribbon fuel rods. [Figure 18A] FIG. 13 is an SEM image showing a solid solution carbide powder feedstock formed with a phenolic resin carbon source. [Figure 18B] FIG. 1 shows relevant X-ray diffraction patterns. [Figure 19A] FIG. 1 shows an SEM image showing a solid solution carbide powder feedstock formed with a carbon black carbon source. [Figure 19B]FIG. 1 shows relevant X-ray diffraction patterns. [Figure 20A] FIG. 1 illustrates one embodiment of a mixing cup for blending feedstock to form a billet for an extruder. [Figure 20B] FIG. 1 illustrates one embodiment of an arbor press for extracting a billet from a mixing cup. [Figure 21A] FIG. 1 illustrates a twisted fuel rod manufacturing system including subsystems and components. [Figure 21B] FIG. 1 illustrates a twisted fuel rod manufacturing system including subsystems and components. [Figure 22] FIG. 2 illustrates details of the ribbon tensioning subsystem. [Diagram 23] FIG. 1 is an image of a twisted fuel rod manufacturing system showing various subsystems and components enclosed within an inert atmosphere chamber. [Figure 24A] 1 is an image of the subsystems and components of a twisted fuel rod fabrication system. [Figure 24B] 1 is an image of the subsystems and components of a twisted fuel rod fabrication system. [Figure 24C] 1 is an image of the subsystems and components of a twisted fuel rod fabrication system. [Figure 24D] 1 is an image of the subsystems and components of a twisted fuel rod fabrication system. [Figure 24E] 1 is an image of the subsystems and components of a twisted fuel rod fabrication system. [Figure 24F] 1 is an image of the subsystems and components of a twisted fuel rod fabrication system. [Figure 25A] FIG. 1 illustrates a process for monitoring and correcting twist rate in a fuel rod according to some embodiments. [Figure 25B] FIG. 1 illustrates a process for monitoring and correcting twist rate in a fuel rod according to some embodiments. [Figure 25C]FIG. 1 illustrates a process for monitoring and correcting twist rate in a fuel rod according to some embodiments. [Figure 26A] FIG. 2 is a fully assembled view illustrating an embodiment of a fuel assembly including a plurality of fuel bundles having twisted ribbon fuel rods. [Figure 26B] FIG. 2 illustrates a cross-sectional view of an embodiment of a fuel assembly including a plurality of fuel bundles having twisted ribbon fuel rods. [Figure 26C] FIG. 1 is an enlarged view of area P4 illustrating an embodiment of a fuel assembly including a plurality of fuel bundles having twisted ribbon fuel rods. [Figure 27] FIG. 2 is a cross-sectional perspective view of a fuel assembly including a plurality of fuel bundles having twisted ribbon fuel rods. [Figure 28] FIG. 2 is an enlarged perspective cross-sectional view of an exit region of a fuel assembly including a plurality of fuel bundles having twisted ribbon fuel rods, showing a fuel bundle support and a portion of the exit fuel bundle; [Figure 29A] FIG. 2 is a schematic plan bottom view of an embodiment of a fuel bundle support; [Figure 29B] FIG. 2 is a schematic plan side view of an embodiment of a fuel bundle support; [Diagram 30] FIG. 29C is a perspective view of the fuel bundle support from FIGS. 29A and 29B. [Figure 31A] FIG. 31 is a cross-sectional perspective view of the fuel bundle support from FIGS. 29A-29B and 30. [Figure 31B] FIG. 31B is an enlarged view of region P5 in FIG. 31A. [Diagram 32] FIG. 4 is an end view of another embodiment of an assembled fuel bundle. [Figure 33A] FIG. 33 illustrates the assembled fuel bundle of FIG. 32 seated on a fuel bundle support. [Figure 33B] FIG. 33B is an enlarged view of region P6 in FIG. 33A. [Figure 34A] 1A-1C show various internal views of a fuel assembly and are cross-sectional views showing the contoured inner surface of the fuel assembly external structure. [Figure 34B]1A-1C show various internal views of the fuel assembly, the view being taken at section II showing the contoured inner surface of the fuel assembly external structure. [Figure 34C] FIG. 1 shows various internal views of the fuel assembly, with an enlarged view showing the contoured inner surface of the fuel assembly external structure. [Figure 35A] FIG. 1 is a view of a fuel assembly viewed along the longitudinal axis of the fuel assembly toward the exit region illustrating an embodiment of a conformal interface between an outer surface of a multi-layer casing of the fuel bundle and an inner surface of the fuel assembly external structure. [Figure 35B] 1 is a perspective view illustrating an embodiment of a conformal interface between an outer surface of a multi-layer casing of a fuel bundle and an inner surface of a fuel assembly external structure. FIG. [Diagram 36] FIG. 4 is a perspective view of an alternative embodiment of a fuel bundle support. [Figure 37A] 4 is a schematic plan bottom view of another embodiment of a fuel bundle support; FIG. [Figure 37B] 4 is a schematic plan side view of another embodiment of a fuel bundle support; [Figure 38] FIG. 1 is an enlarged cross-sectional view of the exit region of a fuel assembly seated on a lower reactor tube sheet illustrating the axial load distribution associated with the fuel bundle support. [Figure 39] FIG. 13 illustrates the variation of helix angles used to fabricate the fuel bundle external structure. [Diagram 40] 1 is a flow diagram illustrating various steps in an embodiment of a method for manufacturing a fuel assembly including a plurality of fuel bundles having twisted ribbon fuel rods and multi-layer casings. [Diagram 41] 1 is a schematic cross-sectional side view of an embodiment of a nuclear propulsion nuclear fission reactor structure within a vessel having a fuel assembly including a plurality of fuel bundles having twisted ribbon fuel rods and multi-layer casings; FIG. [Diagram 42] FIG. 1 is a schematic cross-sectional top view of an embodiment of a nuclear propulsion nuclear fission reactor structure within a vessel having a fuel assembly including a plurality of fuel bundles having twisted ribbon fuel rods and multi-layer casings; [Diagram 43] 1 is a schematic partial cross-sectional view of a nuclear thermal propulsion engine; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] In some cases, the dimensions of the respective components have been appropriately adjusted for clarity. For clarity, in some cases, only some of the named features in the figures are labeled with reference numerals.

[0024] 1 is a partial cutaway schematic perspective view of one embodiment of a fuel bundle 10. The fuel bundle 10 includes a plurality of twisted ribbon fuel rods 20 forming a core region 22 and a multi-layered casing 30. Within the core region 22, the twisted ribbon fuel rods 20 are oriented in the same general direction, with the longitudinal axis of each of the rods 20 extending substantially parallel to the direction of the longitudinal axis 12 of the fuel bundle 10 from a first end 14 of the fuel bundle 10 to a second end 16 of the fuel bundle 10, where substantially parallel includes a deviation of up to 5 degrees from parallel. The deviation from parallel may vary along the axial length (L) of the twisted ribbon fuel rods 20 due to twist periodicity accuracy over the length (L).

[0025] As shown in the schematic diagrams of the twisted ribbon fuel rod 20 in Figures 2A-2C, an embodiment of the twisted ribbon fuel rod 20 extends from a first end 21a to a second end 21b, has a length dimension (L) along a longitudinal axis A of the rod 20, and is twisted with a periodicity of S / 2. As shown in Figure 2C, the cross-sectional shape of the twisted ribbon fuel rod 20 perpendicular to the longitudinal axis is a rectangle having a short side 24 having a length (h) and a long side 26 having a length (D), where D>h. Also, as seen in Figure 26C, the envelope surface 28 of the twisted ribbon fuel rod 20 is a circle. When arranged in the core region 22, the envelope surfaces 28 of adjacent twisted ribbon fuel rods 20 are in tangential contact. The geometry of the twisted ribbon fuel rods 20 and their organization and arrangement within the core region 22 results in gaps 40 between adjacent twisted ribbon fuel rods 20 that provide a path for a coolant, such as a propellant gas in a gas-cooled nuclear thermal propulsion (NTP) reactor, to pass through the core region 22 from the first end 14 of the fuel bundle 10 to the second end 16 of the fuel bundle 10. In an exemplary embodiment, the twisted ribbon fuel rods 20 have an axial length (L)=150 mm, and in cross section, dimensions of short side length (h)=1.2 to 2.0 mm (nominal 1.5 mm), long side length (D)=2.5 to 3.2 mm (nominal 2.8 mm), and envelope diameter=3 to 5 mm (nominal 3.6 mm).

[0026] 1, the multi-layer casing 30 has multiple layers. In an exemplary embodiment, the multi-layer casing 30 has four layers: an inner layer 32, an inner middle layer 34, an outer middle layer 36, and an outer layer 38. In alternative embodiments, the multi-layer casing 30 can have more than four layers, including, for example, three middle layers.

[0027] The inner layer 32 is a flexible compressed layer that is hydrogen resistant and has low thermal conductivity. An exemplary material suitable for the inner layer 32 is a graphite compressed felt insulation layer, an example of which is commercially available as Activated Carbon Felt ACF1000 from Ceramaterials, Inc., Dingmans Ferry, Pennsylvania. Both 1 mm and 2 mm thicknesses may be utilized. In an exemplary embodiment, the graphite felt is surface converted to ZrC, which may be commercially performed, for example, by Advanced Ceramic Fibers, LLC, Idaho Falls, Idaho. The surface conversion to ZrC provides improved survival in the H2 environment at the hot end (~2700K) of the NTP reactor, resulting in improved performance of the fuel bundle. Thus, in some embodiments, the surface conversion to ZrC may be implemented in fuel bundles 10 located toward the outlet end of the fuel assembly, but may not be necessary for fuel bundles at the cooler inlet end of the fuel assembly.

[0028] The inner intermediate layer 34 is a composite reinforced compression layer. An exemplary material suitable for the inner intermediate layer 34 has a combination of unidirectional carbon fibers and weft-wise elastic fibers. The weft-wise elastic fibers provide the compressive force. Typically, these materials for the inner intermediate layer 34 take the form of a tube that slides over the underlying structure. One example of a suitable inner intermediate layer 34 is commercially available as Carbon Fiber Elastic UD 1012 from Eurocarbon BV, The Netherlands. Different nominal diameters are available and may be used based on the required dimensions. For example, nominal diameters of 40 mm (1012 / 40), 60 mm (1012 / 60), and 80 (1012 / 80) may be utilized.

[0029] The outer intermediate layer 36 is a compressed prepeg or dry wound layer that provides a first compressed structure for the multi-layer casing 30. An exemplary material suitable for the outer intermediate layer 36 is a carbon fiber prepreg fabric having a biaxial fiber orientation of +45° / -45°. One example of a suitable outer intermediate layer 36 is commercially available as HexTow AS4C from Hexcel Corporation, Stamford, Connecticut. A filament count tow of at least 6,000 may be utilized for strength purposes. Graphitized yarn may also be used in the outer intermediate layer 36.

[0030] Additional intermediate layers may be included in the multi-layer casing 30 and, when present, are positioned between the inner intermediate layer 34 and the outer intermediate layer 36 .

[0031] The outer layer 38 is also a compressed prepreg layer, providing a second compressed structure to the multi-layer casing 30. An exemplary material suitable for the outer layer 38 is a circumferentially oriented unidirectional prepreg fabric. Examples of suitable outer layers 38 are commercially available as prepreg unidirectional carbon fiber fabrics, such as FibreGlast 2114 from Fibre Glast Developments Corp, Brookville, Ohio, and spread tow fabrics, such as TeXtreme™ 5021 from Oxeon AB, Boras, Sweden. Alternatively, a carbon fiber and carbon matrix composite (C / C) may be used. For some applications, such as where the operating temperature of the core region is less than 1600K, a SiC fiber and SiC matrix composite (SiC / SiC) may be used.

[0032] In the exemplary embodiment, the material of the outer middle layer 36 is not the same as the material of the outer layer 38 .

[0033] Table 1 details an exemplary embodiment of the middle layer of the multi-layer casing 30.

[0034] [Table 1]

[0035] The basket weave layer of the inner intermediate layer 34 is intended to provide the best insulation possible with CF mandrel wrapping. The gap between the tows in this layer can be adjusted from 1 mm to 5 mm. Twisted tows and CF yarns can also be used. The additional intermediate layer is to prevent matrix material from the outer intermediate layer from seeping into the inner intermediate layer. CF tow or high density TeXtreme™ material from Oxeon AB, Boras, Sweden can be used for one or more additional intermediate layers. The outer intermediate layer 36 provides hoop strength to the multi-layer casing 30.

[0036] 3 is a flow diagram illustrating various steps in one embodiment of a method for manufacturing a fuel bundle having twisted ribbon fuel rods and a multi-layer casing. Method S100 includes assembling twisted ribbon fuel rods 20 into a core region 22, enclosing the assembled twisted ribbon fuel rods in a multi-layer casing 30, and completing the process of forming the fuel bundle.

[0037] Steps S110 and S120 assemble the twisted ribbon fuel rods 20 into the core region 22. In an exemplary embodiment, the ends of the twisted ribbon fuel rods 20 are seated in rod seating fixtures 200 (step S120). A support housing 210 may be coupled to the rod seating fixture 200 (step S110). The support housing is a temporary structure that facilitates assembly of the twisted ribbon fuel rods 20 into the rod seating fixtures 200 and is removed prior to encapsulating the assembled twisted ribbon fuel rods into the multi-layer casing. One embodiment of the rod seating fixture 200 is shown in FIG. 4A, and an embodiment of the support housing 210 coupled to the rod seating fixture 200 is shown in FIGS. 5A-5E. 5A-5E also show a fuel bundle in various stages of assembly as the twisted ribbon fuel rods 20 are seated in the rod seating fixture 200. In the illustrated embodiment, a support housing 210 is attached to the rod seating fixture 200 (e.g., at the periphery of the rod seating fixture 200) and extends axially away from the rod seating fixture 200. The twisted ribbon fuel rods 20 are then supported along their longitudinal lengths by the support housing 210. The rod seating fixture 200 and / or the support housing 210 may be made from a suitable material, such as nylon.

[0038] The rod seating fixture 200 has a seating surface 220 including a plurality of protrusions 225. FIG. 4B is an enlarged view of region P1 of FIG. 4A. The plurality of protrusions 225 are distributed on the seating surface 220 and have a height (H) from a base surface 230 of the seating surface 220. Opposite the seating surface 220 along the longitudinal axis C, the rod seating fixture 200 has a mandrel mating feature 232. The mandrel mating feature 232 may include, for example, a male or female feature (see FIG. 8) that engages with a cooperating mounting feature on a mandrel winding machine for rotation about the longitudinal axis C. The seating surface 220 and the mandrel mating feature 232 are connected by an end cap surface 235.

[0039] A plurality of twisted ribbon fuel rods 20 are seated on the seating surface 220 with the ends of each of the twisted ribbon fuel rods 20 within the receiving spaces 240 between the protrusions 225. As can be seen in FIGS. 4B and 5C, the sides of the protrusions 225 include angled regions 245. These angled regions 245 join other sides of the protrusions 225 at angles other than 90 degrees. The angled regions 245 are positioned and sized such that the dimensions between the sides of adjacent protrusions 225 that define the receiving spaces 240 fit the ends of the twisted ribbon fuel rods 20. Additionally, the angled regions allow the twisted ribbon fuel rods 20 to rotate about their respective longitudinal axes (A) without interference with the raised protrusions 225. For example, each end of the twisted ribbon fuel rod 20 within the receiving space 240 may contact the projections 225 that define the receiving space 240 in up to four locations, namely, points a, b, c, and d. Points a and c are on the angled region 245 of the projection 225, and points b and d are on other regions of the side. Thus, each side of the twisted ribbon fuel rod 20 may contact one angled region 240 of one projection 225 and one other region on a second projection. The distance (L1) between points a and b is sized to fit the short side 24 of the twisted ribbon fuel rod 20, whose length (h) is equal to the distance L1, and the distance (L2) between points b and c is sized to fit the long side 26 of the twisted ribbon fuel rod 20, whose length (D) is equal to the distance L2. The contact between the twisted ribbon fuel rod 20 and the angled region 245 of the projection 225 can be a line contact or an area contact.

[0040] 5A-5B are schematic diagrams showing the placement and arrangement of the twisted ribbon fuel rods during assembly of the fuel bundle (step S120). FIG. 5A is a perspective view, and FIG. 5B is a plan view. The ends of the twisted ribbon fuel rods 20 are seated in the receiving space 240 of the rod seating fixture 200, and the length of the twisted ribbon fuel rods 20 extends axially. The length of the twisted ribbon fuel rods 20 is supported by the support housing 210 (for the twisted ribbon fuel rods 20 along the periphery) and by tangential contact between the envelope surface 28 of the twisted ribbon fuel rods 20 and the support housing 210 (for the twisted ribbon fuel rods 20 seated inward from the periphery).

[0041] 5D is another schematic diagram showing the placement and configuration of the twisted ribbon fuel rods during assembly of the fuel bundle (step S120). With respect to FIGS. 5A-5B, in FIG. 5D, more twisted ribbon fuel rods 20 are seated within the receiving space 240 of the rod seating fixture 200.

[0042] The arrangement of the twisted ribbon fuel rods 20 in the core region 22 has translational symmetry, as illustrated by dashed arrows T1, T2, and T3 in FIG. 5D. The translational symmetry aligns the twisted ribbon fuel rods 20 in rows, columns, and diagonals. The translational symmetry improves the neutronic properties of the fuel bundle 10 and the fuel assembly formed therewith. The translational symmetry of the twisted ribbon fuel rods 20 also improves the thermal-hydraulic performance of the fuel bundle 10 by preventing flow channeling through gaps in the ribbon packing arrangement that may otherwise occur, for example, in a circular packing arrangement (compare the hexagonal packing arrangement of FIG. 6A with no voids at the perimeter with the circular packing arrangement of FIG. 6B with voids at the perimeter). Additionally, the translational symmetry ensures that each twisted ribbon fuel rod 20 experiences radial compression from the outer fiber winding architecture of the multi-layer casing 30.

[0043] 5E is another schematic diagram showing the placement and configuration of the twisted ribbon fuel rods during assembly of the fuel bundle (step S120), with the twisted ribbon fuel rods 20 seated in the receiving space 240 of the rod seating fixture 200.

[0044] 6A and 6B are schematic diagrams illustrating the arrangement of twisted ribbon fuel rods within an assembled fuel bundle 20. FIG.

[0045] In FIG. 6A, the twisted ribbon fuel rods 20 are arranged in a hexagonal packing arrangement. At the periphery of the core region 22, the core region 22 has a nominal circular geometry. However, the radial distance of the circumferential circular geometry may not coincide with an integer multiple of the twisted ribbon fuel rods 20. If not, there will be some spaces at the ends of the assembled twisted ribbon fuel rods 20 that are not large enough to accommodate the full size of the twisted ribbon fuel rods 20. This is especially true for the hexagonal packing arrangement, where such spaces occur where the hexagonal packing arrangement meets the circular geometry. However, such spaces are large enough (relative to the gaps 40 between adjacent twisted ribbon fuel rods 20 providing a path for the coolant) that if left unfilled, these spaces would adversely affect the coolant flow within the fuel bundle by providing preferential flow paths or by lowering the differential pressure drop between the first end 14 and the second end 16. To address these issues, filler rods 260 are placed within the spaces. The filler rod 260 may have a composition with or without fissile fuel components. For example, in some embodiments, the composition of the filler rod 260 is ZrUNbC, and in other embodiments, the composition of the filler rod 260 is Zrc. The filler rod 260 may have a suitable geometric shape to occupy a space. For example, the filler rod may have a cross section with a shape of a semicircle, or may have a convex outer surface joined by a chord. The filler rod 260 has a longitudinal length such that it extends from the rod seating fixture 200 the same length as the twisted ribbon fuel rods 20. One end of the filler rod 260 is seated in the rod seating fixture 200. As can be seen in FIG. 7, which is an enlarged view of region P3 in FIG. 6A, the end of the filler rod 260 has a peg end 265 extending from an end face 270 of the filler rod 260. The peg end 265 has a cross-sectional area that is less than the cross-sectional area of ​​the filler rod 260 and has a height approximately the same as the height (H) of the projection 225 on the seating surface 220 of the bar seating fixture 200 .The peg end 265 fits within the receiving space 240 (see FIG. 9 ) and is sufficiently rotatable within the receiving space 240 to allow the filler rod 260 to conform to an orientation that mates with the surface of the nearest twisted ribbon fuel rod 20 while simultaneously mating with the inner surface 270 of the support housing 210.

[0046] Also, as already shown in FIG. 5D, the hexagonal packing arrangement of the twisted ribbon fuel rods 20 in the core region 22 has translational symmetry, as illustrated by dashed arrows T1, T2, and T3. The translational symmetry aligns the twisted ribbon fuel rods 20 in rows (T2), columns (T1), and diagonals (T3). In an exemplary embodiment, the rows (T2) and columns (T1) are oriented at 90 degrees to each other, and the diagonal (T3) is oriented at 45 degrees to both the rows (T2) and columns (T1). In FIG. 6A, the translational symmetry is centered on an axial centerline 280 of the core region 22 and extends to the outermost twisted ribbon fuel rods 20 on the periphery of the core region 22. Depending on the diameter of the core region, the axial centerline 280 in the hexagonal packing arrangement can be centered on the twisted ribbon fuel rods 20. Alternatively, for a particular diameter of the core region, the size of the twisted ribbon fuel rods 20 may be varied to result in the axial centerline 280 of the hexagonal packing arrangement being at the center of the twisted ribbon fuel rods 20. Preferably, a pattern with symmetry, such as N=199, is used compared to a pattern without symmetry, such as N=210.

[0047] In FIG. 6B, the twisted ribbon fuel rods 20 are arranged in a circular packing arrangement. When the twisted ribbon fuel rods 20 in the core region 22 are arranged in a circular packing arrangement, filler rods 260 are not required. The circular packing arrangement of the twisted ribbon fuel rods 20 in the core region 22 has translational symmetry, as illustrated by dashed arrows T1, T2, and T3 in FIG. 6B. The translational symmetry aligns the twisted ribbon fuel rods 20 in rows, columns, and diagonals, but the angular separation between the row (T2) and column (T1) is greater than 90 degrees. The diagonal (T3) is oriented at the bisector of the angle between the row (T2) and column (T1). In FIG. 6B, the translational symmetry is not centered on the axial centerline 280 of the core region 22 and does not extend to the outermost twisted ribbon fuel rods 20 on the periphery of the core region 22. Rather, the translational symmetry is offset from an axial centerline 280 of the core region 22 and extends radially inward from the outer periphery of the core region 22 to the twisted ribbon fuel rods 20. Depending on the diameter of the core region, the axial centerline 280 in a circular packing arrangement may be centered about the twisted ribbon fuel rods 20. Additionally, a circular packing arrangement results in asymmetric regions (see, e.g., regions NS1 and NS2).

[0048] After assembling the twisted ribbon fuel rods 20 with their ends seated in the rod seating fixtures 200 (step S120), the end caps 300 are attached (step S130). FIG. 8 is a schematic perspective view of the fuel bundle during assembly after the end caps 300 are attached. The surfaces of the end caps 300 oriented toward the twisted ribbon fuel rods 20 in the core region 22 have surface features that interface with the twisted ribbon fuel rods 20 to secure the arrangement of the twisted ribbon fuel rods 20. For example, in some embodiments, the surfaces of the end caps 300 that interface with the twisted ribbon fuel rods 20 have an arrangement of protrusions 225 similar to the seating surfaces 220 of the rod seating fixtures 200. In other embodiments, the surfaces of the end caps 300 that interface with the twisted ribbon fuel rods 20 have a grid assembly or mesh that fits over the ends of the twisted ribbon fuel rods 20. The grid assembly or mesh may interface with all ends of the twisted ribbon fuel rods 20 or may interface with a subset of the ends of the twisted ribbon fuel rods 20. For example, the ends of the twisted ribbon fuel rods 20 in only some of the columns, rows, and diagonals and combinations thereof may interface with the grid assembly or mesh, with the tangential contact between the twisted ribbon fuel rods 20 secured with the grid assembly or mesh providing additional support holding the entire twisted ribbon fuel rod 20 arrangement in place.

[0049] The end cap 300 also connects to the support housing 210 and has a mandrel mating feature 232 that engages a cooperating mounting feature on the mandrel winding machine for rotation about the longitudinal axis D.

[0050] 9 is an enlarged view of the fuel bundle during assembly, showing the ends of the twisted ribbon fuel rods 20 and filler rods 260 seated in the rod seating fixtures 200. In FIG. 9, the twisted ribbon fuel rods 20 and filler rods 260 are shown partially transparent to facilitate viewing of the twisted ribbon fuel rods 20 and filler rods 260 within the receiving spaces 240 between the projections 225.

[0051] After the end cap 300 is installed, the twisted ribbon fuel rods 20 and filler rods 260 (if present) seated in the rod seating fixture 200 and end cap 300 are prepared for application of the multi-layer casing 30. In one embodiment, the lower support housing 210 is removed to form a pre-bundle in which the twisted ribbon fuel rods 20 and filler rods 260 (if present) are seated in the rod seating fixture 200 and end cap 300. FIG. 10A is a schematic of a fuel bundle during assembly with a portion of the support housing 210 removed to expose the twisted ribbon fuel rods 20 and filler rods 260 in the core region 22. In another embodiment, a pre-bundle is formed by attaching additional sections of the support housing 210 to completely encapsulate the assembled twisted ribbon fuel rods 20 (and any filler rods 260 that may be present). FIG. 10B is a schematic diagram of the fuel bundle during assembly, with the entire support housing 210 removed.

[0052] After the pre-bundle is formed, the method S100 continues with an optional infiltrant being introduced into the pre-bundle to occupy the interstices 40 of the assembled twisted ribbon fuel rods 20 (step S140). The infiltrant is more easily introduced where the pre-bundle includes a support housing 210 that acts as a mold to contain the infiltrant. In addition, using a support housing allows vacuum assisted techniques to be used to infiltrate the interstices 40 with the infiltrant. The infiltrant has a low enough viscosity that it flows into the interstices 40 throughout the interior volume defined by the support housing 210 and then solidifies. The solidified infiltrant provides support to the assembled twisted ribbon fuel rods 20, holding the configuration and relative spacing of the assembled twisted ribbon fuel rods 20 in place. The solidified infiltrant also protects the assembled twisted ribbon fuel rods 20 from damage during subsequent steps, such as encapsulating the assembled twisted ribbon fuel rods 20 (and any filler rods 260 that may be present) within the multi-layer casing 30 (see steps S150a and S150b). Additionally, the presence of the infiltrant prevents debris from entering the gaps 40 therebetween. In some embodiments, the infiltrant is paraffin wax, which is heated to facilitate its penetration into the assembled twisted ribbon fuel rods 20 and solidifies after cooling.

[0053] After optional infiltrant infiltration, the pre-bundle assembled twisted ribbon fuel rods 20 (and possible filler rods 260) are encapsulated within a multi-layer casing 30. Two encapsulation methods are disclosed: a manual lay-up method (step S150a) and a mandrel wrapped method (step S150b).

[0054] In the manual lay-up method (S150a), the materials forming the inner layer 32, inner intermediate layer 34, outer intermediate layer 36, and outer layer 38 are each manually applied to form the various layers of the multi-layer casing 30. In applying the inner layer 32, the material (such as graphite compressed felt insulation) is overwrapped onto the exposed core region 22 with an amount of material, for example, about 15 mm, extending axially beyond each end of the core region 22 and overlapping the rod seating fixture 200 and the end cap 300. The material of the inner layer 32 is secured in place, for example, by stitching the material with carbon fiber or ZrC thread. The material of the inner intermediate layer 34 is then applied over the inner layer. The inner intermediate layer 34 is secured in place, for example, by carbon fiber or ZrC thread. In some cases, the inner intermediate layer 34 is a braided material and is slid axially over the pre-bundle. The outer intermediate layer 36 is applied over the inner intermediate layer 34 by wrapping the pre-bundle under tension with +45 / -45 pre-preg carbon fiber cloth. Multiple layers of +45 / -45 pre-preg carbon fiber cloth may be used. The outer layer 38 is applied by overwrapping with a tensioned circumferentially oriented unidirectional pre-preg cloth. The overwrapping of the outer layer 38 is preferably done at a helix angle of 80 degrees or more (e.g., greater than 80 degrees to less than 90 degrees, greater than 85 degrees to less than 90 degrees, or greater than 86 degrees to less than 88 degrees). In some embodiments, the overwrapping of the outer layer 38 utilizes non-geodesic winding parameters, and the helix angle of the fiber winding may be varied as a function of the length below the pre-bundle to optimize strength at a particular axial location. During overwrapping with the outer layer 38, sufficient wrapping is applied so that the overall outer diameter of the wrapped pre-bundle exceeds the desired final diameter of the fuel bundle by about 2 mm so that it can be machined to the final dimensions. The wrapped pre-bundle is then cured at a temperature that cures the various layers into a multi-layer casing 30. An exemplary curing process typically involves curing at 94° C. using an increasing temperature heating schedule according to manufacturer specifications.

[0055] In the mandrel winding method (step S150b), the materials forming the inner layer 32, the inner middle layer 34, the outer middle layer 36, and the outer layer 38 are each applied using a filament winding process in which continuous fibers or rovings are applied to a pre-bundle that acts as a mandrel in the filament winding process to form the various layers of the multi-layer casing 30. The incorporation of CNC manufacturing techniques allows for maximum control over fiber placement and uniformity.

[0056] In applying the inner layer 32, a material (such as graphite compressed felt insulation) is overwrapped onto the exposed core region 22 with an amount of material, e.g., about 15 mm, extending axially beyond each end of the core region 22 and overlapping the bar seating fixtures 200 and end caps 300. The inner layer 32 material is secured to stay in place, e.g., by stitching the material with carbon fiber or ZrC threads 344. Figure 11A shows the inner layer 32 material 342 applied to a pre-bundle.

[0057] The inner intermediate layer 34 material 346 is then applied over the inner layer 32. The inner intermediate layer 34 is secured in place, for example, by carbon fiber or ZrC yarn. In some cases, the inner intermediate layer 34 is a braided material and is slid axially over the pre-bundle. Figure 11B shows an example of the inner intermediate layer 34 material being applied to the pre-bundle by slid axially over the pre-bundle.

[0058] The outer intermediate layer 36 is applied over the inner intermediate layer 34 by wrapping the pre-bundle under tension with +45 / -45 pre-preg carbon fiber cloth. Multiple layers of +45 / -45 pre-preg carbon fiber cloth may be used. The outer layer 38 is applied by overwrapping with a tensioned circumferentially oriented unidirectional pre-preg cloth. Both the outer intermediate layer 36 and the outer layer 38 are applied using a mandrel wrapping technique, with FIG. 11C showing an example of mandrel wrapping of the outer layer 38. The overwrapping of the outer layer 38 is preferably done with a helix angle (θ) of 80 degrees or more (e.g., greater than 80 degrees to less than 90 degrees, greater than 85 degrees to less than 90 degrees, or greater than 86 degrees to less than 88 degrees). In some embodiments, the overwrapping of the outer layer 38 utilizes non-geodesic winding parameters, and the helix angle of the fiber winding may be varied as a function of length down the pre-bundle to optimize strength in a particular axial configuration. Additionally, using the mandrel wrapping technique, one or both of the outer intermediate layer 36 and outer layer 38 extend axially past the interface 310 and at least partially past the end cap surface 235, and preferably past the end cap surface 235 to the mandrel mating feature 232. The mandrel rotates (R) and oscillates the pre-bundle about axis D, causing the tow wraps to move in a corresponding oscillating direction during mandrel wrapping.

[0059] A variety of mandrel wrapping methods may be used, particularly for the outer layer 38. Exemplary mandrel wrapping methods include wet wrapping, prepreg wrapping, or dry wrapping with vacuum resin infusion. It should be noted that the infiltrant protects the core region 22 from penetration by the mandrel wrapping method material into the core region 22.

[0060] In both the manual lay-up method (step S150a) and the mandrel wrapping method (step S150b), sufficient wrapping is applied during overwrapping with the outer layer 38 so that the overall outer diameter of the wrapped pre-bundle exceeds the desired final diameter of the fuel bundle by about 2 mm so that it can be machined to the final dimensions.

[0061] In both the manual lay-up method (step S150a) and the mandrel wrapping method (step S150b), the wrapped pre-bundle is then cured at a temperature that solidifies the various layers into the multi-layer casing 30. An exemplary curing process typically involves curing at 94° C. using an increasing temperature heating schedule according to manufacturer specifications.

[0062] In both the manual lay-up method (step S150a) and the mandrel wrapping method (step S150b), the additional step of applying a shrink wrap may be included after application of the outer layer 38 and prior to curing.

[0063] After forming the multi-layer casing 30 by either the manual lay-up method (step S150a) or the mandrel wrapping method (step S150b), the encapsulated pre-bundle is further processed into a final form. This includes removing the rod fixture 200 and end caps 300 and surfacing the outer surface of the multi-layer casing 30 to final dimensions (step S160). The rod seating fixture 200 and end caps 300 may be removed, for example, by cutting the ends at the interface 310 between the rod seating fixture 200 and end caps 300 and the core region 22 or a short distance axially inward from the interface 310, i.e., less than 3-5 mm. The surfacing includes one or more of machining the outer diameter of the fuel bundle 10 to final dimensions, machining the outer diameter of the fuel bundle to be concentrically rounded, or machining the outer diameter of the fuel bundle to a uniform outer diameter. An exemplary machining process includes centerless grinding.Figure 12 is a schematic perspective view of the end of an assembled fuel bundle 10 after the end cap 300 has been removed.

[0064] If present, the infiltrant is removed from the gap 40 (S170). Removal of the infiltrant may include heating or dissolving with a solvent such as acetone (depending on the infiltrant). FIG. 13 is a schematic illustrating removal of the infiltrant 320 from the gap 40 of an assembled twisted ribbon fuel rod 20. Removal of the infiltrant may occur after removal of one or both of the rod seating fixture 200 and end caps 300 and completion of surface finishing. Optionally, the infiltrant 320 may be removed after removal of one or both of the rod seating fixture 200 and end caps 300, and further machining and surface finishing may occur after infiltrant removal.

[0065] Additional steps of heat treatment, pyrolysis, and prepreg firing may also be included in method S100. For example, pyrolysis may improve bonding between the various fibers and matrix material in the fiber structure. Also, for example, repeated infiltrant infiltration and polymer infiltration and pyrolysis (PIP) cycles may add density to the multi-layer casing 30 to improve its strength. In some embodiments, vacuum infusion PIP or spray PIP on the outer diameter surface of the multi-layer casing 30 may be used to concentrate the densification increase on the outside of the multi-layer casing 30, which leaves voids in the area adjacent to the inner diameter surface of the multi-layer casing 30, advantageously improving the insulation performance of the multi-layer casing 30.

[0066] The disclosed method (S100) of manufacturing a fuel bundle having twisted ribbon fuel rods and multi-layer casing has several advantages resulting from the compression bundling structure. For example, compression bundling strengthens the fuel bundle 10 by applying a cylindrical preload on the assembled twisted ribbon fuel rods 20, thereby mitigating or avoiding cracking of individual ribbons. Also, for example, compression bundling prevents bending failure of the twisted ribbon fuel rods 20 after the brittle-ductile transition at .about.1800° C. Also, for example, compression bundling mitigates the adverse effects of cracks in the twisted ribbon fuel rods 20, should they occur, since the twisted ribbon fuel rods 20 cannot move axially and are constrained at all other rotational angles. Here, even a fractured twisted ribbon fuel rod 20 within the fuel bundle will have minimal impact on the reactor dP, preventing runaway axial dP forces on the twisted ribbon fuel rod 20, preserving reactor life and preventing fuel overheating caused by insufficient cooling in the choked flow regions of the reactor. Also, for example, compression bundling allows for a high technology readiness level manufacturing method that can organize the ribbons and avoid random ribbon arrangements that have low volumetric efficiency.

[0067] 14A-14C are images showing twisted ribbon fuel rod configurations at different stages during assembly of a prototype fuel bundle.

[0068] 15A-15B are images showing a prototype fuel bundle in two perspective views with the various layers exposed. For example, in the prototype fuel bundle 10 in FIGs. 15A-15B, visible are twisted ribbon fuel rods 20, inner layer 32, inner middle layer 34, and outer layer 38.

[0069] 16 is a flow diagram illustrating a general process for one embodiment of a method (S400) for manufacturing twisted ribbon fuel rods. The general process includes providing a feedstock of solid solution carbide powder containing fissionable fuel components (S410), processing the feedstock to form a billet (S420), and extruding the billet or billets into twisted ribbon fuel rods (S430). Both processing the feedstock to form a billet (S420) and extruding the billet or billets into twisted ribbon fuel rods (S430) are performed under inert atmosphere or vacuum conditions.

[0070] 17A-17C are flow diagrams illustrating in more detail the various steps in one embodiment of a method (S500) for manufacturing twisted ribbon fuel rods. The method (S500) begins with mixing oxide powders with a carbon source to form a mixture (S505). In an exemplary embodiment, the oxide powders include oxides of fissile fuel components such as zirconium oxide nanopowder (<50 nm), niobium oxide (<1 μm), and uranium oxide. The carbon source may be either phenolic resin or carbon black. Additional components used to form the mixture include one or more of a diluent, a dispersant, and a binder. Table 2 shows the components and their amounts for an exemplary embodiment using (i) phenolic resin as the carbon source (composition 1) and (ii) carbon black as the carbon source (composition 2).

[0071] [Table 2]

[0072] The processing steps to form the intermediate powder differ based on the carbon source: step S510 describes the steps when the carbon source is a phenolic resin, and step S520 describes the steps when the carbon source is carbon black.

[0073] When the carbon source is a phenolic resin, the mixture (e.g., having composition 1) is poured into a mold and cured to form an intermediate (S512). An exemplary curing process is heating to 120° C. for 4 hours. The intermediate is then pyrolyzed in an inert atmosphere or vacuum (S514). An exemplary pyrolysis process is heating to 800° C. in argon. After pyrolysis, an intermediate powder is formed (S516). An exemplary process for forming the intermediate powder includes mechanically breaking apart the pyrolyzed intermediate, for example, with a mortar and pestle or mill, and then sieving to obtain a powder having a diameter of less than 100 microns (μm). In an exemplary embodiment, the intermediate powder is a solid particle with a size in the range of 1-100 μm, and is composed of a matrix of solid carbon (from the pyrolyzed phenolic resin solid particles) with fine oxide nanopowders dispersed within the matrix.

[0074] If the carbon source is carbon black, the mixture (e.g., having composition 2) is dried to form an intermediate (S522). An exemplary drying process is evaporating the isopropyl alcohol with or without heating. After drying, an intermediate powder is formed (S524). An exemplary process for forming the intermediate powder includes mechanically breaking apart the pyrolyzed intermediate, for example, by a mortar and pestle or mill, and then sieving to obtain granules having a diameter of less than 2 microns, alternatively 1-2 microns. In an exemplary embodiment, the granules are agglomerates with a size in the range of 1-2 μm, and are composed of oxide and carbon nanopowder held together by surface forces.

[0075] The powder from step S510 or the granules from step S520 are formed into pellets (with or without a binder, such as a 10 wt% binder in a polyvinyl alcohol solution) and then subjected to a process of carbothermal reduction (S530). An exemplary carbothermal reduction process involves heating to 1300° C. to 1700° C. in an inert atmosphere, such as argon, or in vacuum.

[0076] The carbothermally reduced material is then mechanically processed to form a solid solution carbide powder feedstock containing fissionable fuel components (S535). The mechanical processing can be any suitable mechanical processing, such as milling. The mechanical processing can also include sieving to select particle size. The composition of the feedstock can be determined according to the formula (U a Zr b Nb c ) C d wherein 0.05 <a<0.4、0<b<0.95、0<c<0.4、および0.7<d<1である。

[0077] 18A and 18B show an SEM image (FIG. 18A) of a solid solution carbide powder feedstock formed with a phenolic resin carbon source and an associated X-ray diffraction pattern (FIG. 18B). Visible in the SEM image 600 is the feedstock powder, which has an observed size that is <500 nm (for the majority of the particles in the feedstock powder), with a few particles approaching 1 micron in size. The X-ray diffraction pattern 610 shows peaks for ZrC and UO2. The X-ray diffraction pattern 610 of the solid solution carbide powder feedstock formed with the phenolic resin carbon source has a UO2 peak because the material has not fully reacted to form the solid solution carbide and there is still UO2 present in the feedstock. During subsequent processing, such as extrusion processing and heat treatment, the remaining UO2 reacts to form the desired solid solution carbide. The treated material then exhibits an X-ray diffraction pattern consistent with a single phase material, with peaks characteristic of ZrC, but shifted to lower diffraction angles due to uranium doping and an increase in the lattice constant in the crystal structure.

[0078] 19A and 19B are SEM images (FIG. 19A) showing a solid solution carbide powder feedstock formed with a carbon black carbon source, and the associated X-ray diffraction pattern (FIG. 19B). Visible in the SEM image 650 is the feedstock powder, which has an observed size that is <500 nm (for the majority of the particles in the feedstock powder), with a few particles approaching 1 micron in size. The X-ray diffraction pattern 660 shows a peak for ZrC. Since this material is a single phase solid solution carbide with uranium uniformly doped in the ZrC phase, the doping of uranium slightly shifts the ZrC peak in the diffraction pattern to lower 2θ, but there are no additional peaks when the uranium is fully incorporated into the structure.

[0079] One difference between the solid solution carbide powder feedstock formed with a phenolic resin carbon source and the solid solution carbide powder feedstock formed with carbon black is that phenol produces a two-phase mixture (ZrC+UO2) that reacts further in later processing steps. Otherwise, the powder particle size and behavior in further processing are similar between the solid solution carbide powder feedstock formed from the two different carbon sources.

[0080] A feedstock of solid solution carbide powder containing fissile fuel components is used to form a billet for extrusion into twisted ribbon fuel rods 20. Referring to Figure 17B, forming the billet includes heated thermoplastic mixing to disperse and homogenize the feedstock in the blended components (S540) and cooling to form a billet (S550), which is then removed from the mixer apparatus (S550).

[0081] The heated thermoplastic mixing step (S540) mixes a feedstock of solid solution carbide powder containing fissionable fuel components with blended ingredients under an inert atmosphere. The blended ingredients include polymers such as poly(ethylene vinyl acetate) and high density polyethylene (HDPE) polymers, paraffin, and waxes such as carnauba. Stearic acid is also included in the blend as a dispersant for the ceramic particles. In one embodiment of the blend, the blended ingredients are 40% backbone polymer (such as poly(ethylene vinyl acetate) and HDPE) / 60% wax. Optionally, one or more of nickel nanopowder, diamond nanopowder, and carbon fiber may be added to the blend. An exemplary composition of the blend is presented in Table 3.

[0082] [Table 3]

[0083] The blend is placed in a high shear mixer under inert atmosphere or vacuum for heated thermoplastic mixing. A suitable high shear mixer is the FlackTec DAC1100-VAC, available from FlackTek, Inc., Landrum, South Carolina. In some embodiments, the polymer blend ingredients are first placed in the high shear mixer and melted under shear mixing, and then the feedstock powders and remaining blend ingredients are added to the molten polymer blend ingredients. The high shear mixer is used to provide heat to the blend (resulting in thermoplastic melting) and homogenize the composition. The temperature of the blend during the shear mixer mixing process is indirectly monitored by a thermocouple or RTD thermistor placed under the mixing cup of the shear mixer. The preferred temperature range for shear mixing is 100-125°C. Exemplary parameters for mixing are 1600 rpm for 300 second cycles. A minimum of 3 cycles (300 seconds each), alternatively 3-5 cycles are used. Speeds can be slowed down and times shortened to control the temperature of the blend to fall within the aforementioned range of 100-125° C. Exemplary total blend times are up to about 30 minutes, although other times can be used as long as there is a homogenous distribution of particles in the thermoplastic blend, agglomerates are broken up, and the temperature of the blend does not exceed 125° C., for example, to avoid damage to the steric acid dispersant.

[0084] FIG. 20A shows an embodiment of a mixing cup for blending feedstocks to form a billet for an extruder. The mixing cup 700 contains the ingredients of the blend 705 and a primary atmosphere 710 and is sealed at the mouth end by a cup insulation layer 715 and a cap 720. The primary atmosphere 710 may be an inert atmosphere and protects the fissile fuel-containing ingredients (such as ZrUC) from oxidation. Argon may be used as the primary atmosphere 710. The mixing cup 700 may be made of a high heat transfer material, such as aluminum. The mixing cup 700 is placed in a containment vessel 730 of a shear mixer, with a mixer insulation layer 735 between the mixing cup 700 and the containment vessel 730. Additionally, the containment vessel 730 is sealed at the mouth end by a cover 740. A secondary protective atmosphere 745, which may be an inert atmosphere such as argon, occupies the gap within the containment vessel 730. Suitable materials for the cup insulation 715 and mixer insulation 735 include silicone-based insulating materials.

[0085] A thermocouple or RTD thermistor 750 is embedded in the mixer insulation layer 735. The thermocouple or RTD thermistor is preferably attached to a microprocessor with wireless data transmission capability so that the temperature of the formulation 705 can be actively determined in the high shear mixing process. The program containing the mixing parameters used by the high shear mixer can be automatically or manually modified to maintain the formulation 705 within the minimum and maximum temperature limits of the components of the formulation. Particular care should be taken to ensure that components with lower melting points do not evaporate in the mixing process.

[0086] After mixing, the blended feedstock is cooled under an inert atmosphere or vacuum to form a billet (S545). Cooling can be ambient cooling or forced air cooling with an inert gas.

[0087] After partial cooling, the billet is removed from the mixer equipment (S550). Partial cooling maintains some ductility of the billet, facilitating removal by, for example, an arbor press 775. An exemplary operating temperature range for partial cooling of the billet is 70° C. to 110° C. In an exemplary embodiment, the billet is removed from the mixer equipment using an arbor press. FIG. 20B illustrates an embodiment of an arbor press 775 that extracts the billet 770 from the mixing cup 700. The arbor press 775 has a central flow passage 780 defined by an outer wall 785. A wiper system, such as a sleeve 790 having flexible wings 795, is disposed on the outer surface of the arbor press 775. The arbor press 775 is placed inside the mouth of the mixing cup 700 and is forced into the area of ​​the mixing cup 700 where the billet 770 is to be placed (F). This action forces the billet 770 into the central passage 780 of the arbor press 775 while the flexible wings 795 wipe the inner surface of the mixing cup 770. After the billet 770 is fully contained within the central passage 780, the arbor press 775 (with the billet 770 contained therein) may be removed from the mixing cup 700. The billet 770 may then be ejected from the central passage 780 and saved for use in forming twisted ribbon fuel rods 20, for example, by extrusion. Removal of the billet 770 is performed under a protective atmosphere, for example, in a glove box having an inert atmosphere such as argon. Extraction of the billet 770 by the arbor press 775 minimizes surface defects on the billet 770 that, if present, may be transferred to the extruded ribbon during the extrusion process.

[0088] Referring to FIG. 17C, the process of forming the billet 770 into twisted ribbon fuel rods 20 includes loading the billet 770 into an extruder (S555) and extruding the ribbon through a heated die (S560). As the extruded ribbon exits the heated die, an inert atmosphere flows over the extruded ribbon to initiate solidification (S565). The extruded ribbon is then captured between tension rollers (S570), which rotate about the axis of the extruded ribbon while applying tension to the extruded ribbon to impart a twist to the extruded ribbon (S575). Image analysis is performed on the twisted extruded ribbon (S580), and the twisted extruded ribbon is cut into rods in response to input from a length sensor (S585). The twisted ribbon fuel rods 20 are then sorted in response to output from the image analysis (S590).

[0089] 21A and 21B show a twisted fuel rod manufacturing system including subsystems and components. The twisted fuel rod manufacturing system 800 is downstream from the exit of a heated die 802 of an extruder 804, preferably a piston extruder. The twisted fuel rod manufacturing system 800 has various zones, subsystems, and components. In sequence along the process path (PP), the twisted fuel rod manufacturing system 800 includes a ribbon initial cooling zone 806, a ribbon tensioning subsystem 808, a ribbon twisting subsystem 810, a ribbon cutting subsystem 812, a ribbon inspection zone 814, a length sensor subsystem 816, and a rod sorting and recovery subsystem 818.

[0090] The ribbon initial cooling zone 806 is a region between the exit of the heated die 802 and the ribbon tensioning subsystem 808. External cooling, such as forced air cooling with an inert atmosphere, may be located within the ribbon initial cooling zone 806. In FIG. 21A, the external cooling is provided by a nozzle 820, which directs an inert atmosphere, such as argon, into the process path. The distance of the ribbon initial cooling zone 806 within the process path may vary, but has a size sufficient to allow the extruded ribbon 822 to begin to solidify. This distance may be influenced by any external cooling conditions, such as forced air cooling conditions. In an exemplary embodiment, the forced air cooling conditions and the size of the ribbon initial cooling zone 806 cool the extruded ribbon 822 from an initial temperature of about 110° C. (at the exit of the heated die 802) to a temperature within the range of 60° C. to 80° C. at which the extruded ribbon 822 is sufficiently pliable such that (i) the surface of the extruded ribbon 822 may be touched without leaving artifacts on the surface, and (ii) a periodic S / 2 twist may be imparted to the extruded ribbon 822.

[0091] The extruded ribbon 822 passes through the ribbon initial cooling zone 806 and between tensioning rollers 824 of the ribbon tensioning subsystem 808. FIG. 22 shows details of the ribbon tensioning subsystem. The tensioning rollers 824 contact the extruded ribbon 822 and rotate at a rotational speed (R1). Typically, the tensioning rollers 824 contact the long side 26 of the extruded ribbon 822. The material of the tensioning rollers 824, e.g., polyurethane, allows some slippage of the extruded ribbon 822 to maintain tension in the extruded ribbon 822 in the ribbon initial cooling zone 806. In some embodiments, the tensioning rollers 824 rotate at a rotational speed (R1) that matches or is 2 to 5% greater than the output speed of the extruder 804, and the slippage of the tensioning rollers 824 relative to the surface of the extruded ribbon 822 maintains tension. Additionally, one or both tensioning rollers 824 are spring loaded (M) in a direction parallel to the process path (PP) to accommodate changes in extrusion speed without breakage of the extruded ribbon 822. In FIG. 22, one of the tensioning rollers 824 is mounted on a spring arm 828. A funnel 826 helps direct the extruded ribbon 822 into the nip of the tensioning rollers 824. As can be seen in FIGS. 21A and 21B, additional cooling can be provided near the ribbon tensioning subsystem 808, for example, by a cooling fan 830.

[0092] The ribbon tensioning subsystem 808 is mounted on a stage 832 of the ribbon twisting subsystem 810. The stage 832 has an annular or similar shape with a central opening that allows the passage of the extruded ribbon 822. The stage 832 may incorporate a coaxial sliding ring system. A drive system, such as a stepper motor (SM) attached to the stage 832 by a belt 834, rotates (R2) the stage 832 about the axis 836 of the extruded ribbon 822, i.e., about the axis of the process path (PP). The rotation (R2) of the stage 832 moves the mounted ribbon tensioning subsystem 808, which maintains tension in the extruded ribbon 822 while the stage 832 rotates (R2).

[0093] The rotation (R2) of stage 832 is controllable, and during initial operation, stage 832 rotates at a faster rate to establish an initial periodicity S / 2 of extruded ribbon 822. After the periodicity S / 2 of extruded ribbon 822 is established, the rotation of stage 832 reaches approximately steady state with adjustments based on feedback from inspections performed in ribbon inspection zone 814.

[0094] The combined action of the ribbon tensioning subsystem 808 and the ribbon twisting subsystem 810 imparts a twist with periodicity S / 2 to the extruded ribbon 822. The twisting of the extruded ribbon 822 occurs in the ribbon initial cooling zone 806 between the exit of the heated die 802 and the tensioning rollers of the ribbon tensioning subsystem 808. The twisted ribbon fuel rods 20 can have different twist periodicities to accommodate reactor designs, such as pressure differentials within the fuel bundle 10 and / or fuel assembly during operation.

[0095] The ribbon cutting subsystem 812 includes a cutting device 840 that operates based on signals received from the length sensor subsystem 816. After passing through the tension rollers 824 and the opening in the stage 832, the extruded ribbon 822 passes through an operating zone of the cutting device 840. The cutting device 840 may include a cutting surface, such as a razor blade or the like, that is mounted to grips on either side of the processing path (PP) and has a reciprocating motion, e.g., pneumatically actuated, that closes to cut the extruded ribbon 822 to the length of the rod and opens after the cut to allow additional lengths of extruded ribbon 822 to pass through. This reciprocating cutting motion is repeated to cut the extruded ribbon 822 to form each twisted ribbon fuel rod 20. A funnel is positioned upstream from the operating zone in the processing path (PP) (see FIG. 24C), and the outlet of the funnel helps stabilize the extruded ribbon 822 within the operating zone for cutting.

[0096] The ribbon inspection zone 814 includes subsystems and components for inspecting the extruded ribbon 822 and identifying defects. In some embodiments, a visual inspection system (VIS) 850, such as a camera, is positioned in the ribbon inspection zone 814 to inspect the extruded ribbon 822 as it moves through the processing path (PP) and past the inspection points. One or more other non-destructive inspection techniques, such as radiography, computed tomography, and ultrasonic inspection, may also be included. The properties of the extruded ribbon 822 that are inspected include twist periodicity (S / 2), twist regularity (e.g., consistency of S / 2 over the length of the extruded ribbon), distance between peaks and / or valleys in the profile of the extruded ribbon 822, surface finish defects, and combinations thereof. Examples of surface finish defects include die shrinkage or bulging, surface breaks, surface porosity, and periodic surface defects caused by gradual damage of the die geometry over time. Optical analysis of information from the visual inspection system (VIS) (and optionally analysis of information from other non-destructive inspection techniques) may be used to control the bar sorting and recovery subsystem 818. In addition, analysis of the collected information may be analyzed, for example, by machine learning, and used to provide feedback control to other processing equipment, such as the extruder 804, ribbon tensioning subsystem 808, ribbon twisting subsystem 810, etc., which may then be adjusted to mitigate or correct identified defects. Analysis may also be used to identify maintenance requirements.

[0097] In an alternative embodiment, ribbon inspection occurs in ribbon initial cooling zone 806 and the subsystems and components of ribbon inspection zone 814 are incorporated into ribbon initial cooling zone 806. Inspection in both ribbon initial cooling zone 806 and ribbon inspection zone 814 may also be used.

[0098] A length sensor subsystem 860 is disposed within the process path (PP). In one embodiment, the length sensor subsystem 860 includes a laser line scanner and a photodiode detector 862. When the extruded ribbon 822 moving along the process path (PP) breaks the detection plane between the laser line scanner and the photodiode detector, the length sensor subsystem 860 sends a signal to the ribbon cutting subsystem 812, which then cuts the extruded ribbon 822. The distance along the process path (PP) from the location of the reciprocating cutting action in the ribbon cutting subsystem 812 to the detection plane between the laser line scanner and the photodiode detector in the length sensor subsystem 860 determines the length (L) of the twisted ribbon fuel rod 20. The position of the detection plane between the laser line scanner and the photodiode detector in the length sensor subsystem 860 can be adjusted to adjust the length (L) of the twisted ribbon fuel rod 20, for example, by moving the attachment point between the length sensor subsystem 860 and a rail 864.

[0099] After being cut to length, the twisted ribbon fuel rods 20 are sorted and collected by a rod sorting and collection subsystem 870. For example, one embodiment of the rod sorting and collection subsystem 870 includes a sorting chute 872 and a sorting tray 874. The entrance of the sorting chute 872 is positioned such that the cut twisted ribbon fuel rods 20 enter the sorting chute 872. The exit from the sorting chute 872 is positioned such that the cut twisted ribbon fuel rods 20 are directed to the sorting tray 874. The sorting chute 872 is movable (e.g., by a servo motor (SM)) to orient the outlet toward a first bin 876, e.g., a first orientation for collecting twisted ribbon fuel rods 20 that pass the inspection criteria (as determined by the analysis of the information from the ribbon inspection zone 814), or to orient the outlet toward a second bin 878, e.g., a second orientation for collecting twisted ribbon fuel rods 20 that do not pass the inspection criteria (as determined by the analysis of the information from the ribbon inspection zone 814).

[0100] In FIG. 21B , the twisted fuel rod manufacturing system 800 is supported on an adjustable platform 880 that can be used to adjust the distance between the exit of the heating die 802 and the twisted fuel rod manufacturing system 800, for example, to adjust the length of the ribbon initial cooling zone 806.

[0101] FIG. 23 is an image of a twisted fuel rod manufacturing system showing various subsystems and components. The twisted fuel rod manufacturing system 800 is enclosed within an inert atmosphere chamber 890. A portion of the heated die 802, such as the outlet, is also enclosed within the inert atmosphere chamber 890. Alternatively, the entire extruder 804 or a portion of the extruder 804 may also be enclosed within the inert atmosphere chamber 890. As an example, the inert atmosphere chamber 890 may be fabricated from polycarbonate. The inert atmosphere chamber 890 may include a sealable access port. In FIG. 23, an access port 892 allows access to the chamber that functions as the sorting tray 874.

[0102] 24A-24F are images of subsystems and components of a twisted fuel rod manufacturing system 800, including a ribbon initial cooling zone 806 and ribbon tensioning subsystem 808 (first top side view) (FIG. 24A), a ribbon tensioning subsystem 808 (second top side view) and ribbon twisting subsystem 810 (FIG. 24B), a ribbon cutting subsystem 812 in a side view (FIG. 24C) and a bottom view (FIG. 24D), a length sensor subsystem 816 (FIG. 24E), and a rod sorting and recovery subsystem 818 (FIG. 24F).

[0103] 25A-25C are diagrams illustrating a process for monitoring and correcting twist rate in a fuel rod according to some embodiments. As shown in FIG. 25B, an image 940 of an extruded ribbon is captured. For each pixel in the image, the system compares it to a binary threshold 942 and generates a binary ribbon image 944. In the illustrated embodiment, black indicates any portion of the raw image 940 that shows the ribbon, and the background is white. real If represents a surface, then the integral 946 represents an area (e.g., the area of ​​the shadow cast by the ribbon). a can be used to adjust the scaling.

[0104] The target shape of the ribbon is based on a set of ribbon parameters 948, including a target twist rate. With these parameters, the system has a ribbon model 950 that describes what the ribbon should look like. This model is projected 952 onto a plane to create a target surface shape 954. model If represents the surface of the target surface shape, then the corresponding integral 956 represents the area. b can be used for scaling.

[0105] In Figure 25C, the actual surface 962 and the model surface 964 are convolved 966 over the torsion rate using an integral 968. The upper limit of the outer integral is the torsion rate, t, and the lower limit is 1. A plot of the convolution as a function of torsion rate, t, is shown in plot 970. In this plot, a maximum occurs at the calculated torsion rate. In plot 970, the calculated torsion rate is the x-coordinate of the maximum point 972.

[0106] There are at least two intuitive reasons to understand why the maximum point identifies the calculated twist rate. First, since the system is performing a convolution over the twist rates, it transforms the measured twist surface integral results into twist rate space, so that the maximum will occur at the actual twist rate. Second, the convolution is essentially the same as the correlation between the measured and theoretical ribbons over discrete twist rates. From this, the peak will be at the twist rate where the correlation is highest, which is the twist rate of the measured ribbon.

[0107] Figure 25A is a flow chart of this overall process. Ribbon images may be captured continuously or periodically (e.g., every second, every tenth of a second, or every millisecond). The upper boxes 902-910 of the flow chart appear continuously or periodically to process the most recent ribbon image 902. As illustrated in Figure 25B, the summation generates a binary image by comparing the pixels of the most recent ribbon image to a threshold (904), and then radially sums the binary ribbon image (906).

[0108] In some embodiments, the system detrenches the twist profile (908). This involves performing a linear regression over the summed and integrated twist profile threshold and subtracting it. This removes potential effects from how the ribbon is illuminated (e.g., the threshold may vary over the length of the ribbon as a result of greater light exposure at one end of the ribbon). The result of these steps is to produce an updated twist profile 910.

[0109] The lower portion of the flow chart begins when twist control is activated (912). To generate the appropriate projection from the ribbon model 950, the system identifies the platform rotation and extruder exit distance. The system converts the target twist rate to θ radians (914) and sends a command to rotate the twist platform by θ-2π radians (916). θ in this case is the twist rate converted to radians of platform rotation, and is therefore a variable that depends on both the desired twist rate and the distance from the extruder exit to the tensioner wheel that actually grips the extrusion. One of the reasons for these two steps is to get the ribbon close to the final desired twist rate, which is achieved by the θ-2π radians of initial rotation. This puts the system close enough to the target twist rate that it is possible to accurately measure the current twist rate and begin the PID control loop. Otherwise the system would be somewhat unstable when attempting to measure what is essentially an infinite twist rate (the initial extrusion state).

[0110] In some embodiments, images of the ribbon are taken at a given frame rate of the camera and the twist rate is calculated from each new frame. Some embodiments use a simple moving average over the last X frames for the twist rate calculation. The platform moves the requested number of radians, the twist rate is continuously calculated at the frame rate of the camera, and when the rotating platform finishes its movement, the current twist rate (whether instantaneous rate or a moving average) is passed to a PID control loop to generate the next platform movement.

[0111] As illustrated in Figure 25C, the system convolves the twist profile with a theoretical twist profile (918) and then finds the maximum of the convolution (920), e.g., point 972 on the plot of Figure 25C. The x-coordinate of this point is the calculated twist rate. This calculated twist rate is then passed to a PID controller (922) and / or to a motor control module (924).

[0112] For best performance, the twist rate should be in and remain in a desired range. Thus, the process checks whether the calculated twist rate is within the desired range (926). If so, the system sends a control signal to lock the twist (928). If the calculated twist rate is not within the desired range, the flow loops back to calculate a new convolution. As noted above, the upper steps 902-910 of the flow are always in progress, so by returning to the convolution step 918, a new convolution is calculated using the new latest twist profile 910.

[0113] Locking the twist rate means reducing or completely eliminating the system's ability to control the rotating platform. This process achieves a very stable twist rate and avoids constantly changing the rotational position of the platform. Below a certain threshold it is preferable to just stop the control. The system still calculates the current twist rate as fast as images are received and there is a check to see if the calculated current twist rate is still within some tolerance of the target rate. This is useful to avoid possible "slippage" of the twist (for example when an extrusion turns half a turn in the tensioner assembly and untwists half a turn). When this occurs the twist lock is lost and the control system takes over to restore the desired twist rate.

[0114] The twist rate can be tracked using a variety of units. The system typically reports the twist rate to the user in units of mm / full twist, but internally these units are pixels / full twist. This allows the system to apply a pixel / mm scaling factor that is solely a function of the physical camera setup and image plane after the twist rate in pixel space has been calculated. An embodiment can go either way since this is simply a constant factor.

[0115] This method of measuring twist rate has several advantages. First, it is robust to the position and orientation of the ribbon within the camera's field of view. Second, it reduces the amount of ribbon that needs to be imaged for the twist rate to be accurately known. (In contrast, pure Fourier-based methods require a ribbon length at least twice the twist rate; in many cases, this is not possible due to the limited size of extrusion handling systems.) Third, it can be adapted to any ribbon aspect ratio. And, fourth, it can be utilized to detect asymmetries in ribbon twist.

[0116] 26A-26C illustrate an embodiment of a fuel assembly including a plurality of fuel bundles having twisted ribbon fuel rods. In FIG. 26A, the fuel assembly 1000 is illustrated in a fully assembled state. The fuel assembly 1000 includes a fuel assembly external structure 1002, an inlet connection assembly 1004 coupled to an inlet end of the fuel assembly external structure 1002, and an outlet connection assembly 1006 coupled to an outlet end of the fuel assembly external structure 1002. The fuel assembly external structure 1002 can include one or more sections that house and contain the fuel bundles. In an exemplary embodiment, the fuel assembly external structure 1002 includes a lower fuel assembly external structure 1010 and an upper fuel assembly external structure 1012. During manufacture, the internal components of the fuel assembly 1000, such as the fuel bundles and any supporting structures, are inserted into the lower fuel assembly external structure 1010, and then the upper fuel assembly external structure 1012 is attached to the lower fuel assembly external structure 1010, e.g., at the braze joint 1014. The braze joint 1014 is in a region of the fuel assembly external structure 1002 toward the inlet end, which is not exposed to temperatures that exceed the melting point of the braze material.

[0117] The inlet connection assembly 1004 and the outlet connection assembly 1006 may be formed integrally with the upper and lower fuel bundle external structures 1012 and 1010, respectively. For example, fiber fabrication techniques may be used to form these structures. In operation, coolant flows through the fuel assembly 1000. The coolant flows through the inlet fuel bundles and into the inlet connection assembly 1004 (F I ), exiting the outlet connection assembly 1006 (F O ) will be done.

[0118] In FIG. 26B, a fully assembled fuel assembly 1000 is illustrated in a cross-sectional view. Inside the fuel assembly external structure 1002 are multiple fuel bundles and internal components including, optionally, one or more fuel bundle supports 1020. In the illustrated embodiment, there are five fuel bundles (10a, 10b, 10c, 10d, 10e) arranged along a longitudinal axis 1022 of the fuel assembly 1000, with the inlet fuel bundle 10a located closest to the inlet end of the fuel assembly 1000 and the outlet fuel bundle 10e located closest to the outlet end of the fuel assembly 1000. In operation, the fuel bundles 10 within the fuel assembly external structure 1002 are exposed to progressively higher temperatures. For example, the inlet fuel bundle 10a is typically exposed to temperatures of 200 to 610 K, and sequentially in the coolant flow direction, the fuel bundles are exposed to the following temperatures: The temperature in the second fuel bundle 10b is 610 to 1020K, the temperature in the third fuel bundle 10c is 1020 to 1430K, the temperature in the fourth fuel bundle 10d is 1430 to 1850K, and the temperature in the outlet fuel bundle 10e is 1850 to 2250K. After the operating temperature in the fuel bundle exceeds about 1850K, the twisted ribbon fuel rods are susceptible to ductile failure. To provide mechanical support and distribute axial loads on the fuel bundle 10, one or more fuel bundle supports 1020 are disposed adjacent to the fuel bundles having a design temperature above 1850K. For example, the outlet fuel bundle support 1020 is disposed below (in the coolant flow direction) the outlet fuel bundle 10e, and the intermediate fuel bundle support 1020 is disposed between the outlet fuel bundle 10e and the preceding (in the coolant flow direction) fuel bundle 10d. 26B includes two fuel bundle supports 1020, which are attached below all of the fuel bundles 10, thereby isolating the load on each. Including a fuel bundle support 1020 below every fuel bundle 10 may be mechanically advantageous, but may have to be balanced against the core neutronic properties associated with having a fuel bundle support 1020 below every fuel bundle 10.

[0119] FIG. 26C is an enlarged view of region P4 from FIG. 26B illustrating details of a fuel bundle support 1020 arranged within the fuel bundle external structure 1002. The outer circumferential surface of the fuel bundle support 1020 includes a tapered surface 1026. For a fuel bundle support 1020 in an intermediate arrangement between fuel bundles 10, the tapered surface 1026 contacts a corresponding portion of the inner surface of the fuel bundle external structure 1002 where there is a reduction in cross-sectional diameter. For example, in FIG. 26C, the inner surface of the fuel bundle external structure 1002 changes from a diameter D1 to a diameter D2 (where D2 is less than D1) at a tapered section 1030. For the outlet fuel bundle support 1020 located below (in the direction of coolant flow) the tapered surface 1026 contacts a corresponding portion where there is a reduction in inner diameter as the coolant flow path transitions from the fuel bundle 10 to the outlet connection assembly 1006, which may be associated with an inner surface of the fuel assembly external structure 1002, the outlet connection assembly 1006, or a combination thereof. For example, in FIG. 26C , the reduction in inner diameter as the coolant flow path transitions from the fuel bundle 10 to the outlet connection assembly 1006 is at a tapered section 1032. As an alternative to or in addition to the tapered section 1032, a cleat 1036 or other structural support projecting radially inward from the inner diameter surface may provide a seating surface for the outlet fuel bundle support 1020.

[0120] If the dimension of the inner surface of the fuel assembly external structure 1002 is larger than the outer diameter of the outer surface of the multi-layer casing 30 of the fuel bundle 10, a spacer 1040 or other inner diameter feature is used to provide a physical space between the inner diameter of the fuel assembly external structure 1002 and the outer diameter of the fuel bundle 10. The spacer 1040 may be used to increase the radial thermal resistance profile of the fuel assembly 1000 in the arrangement in which the spacer 1040 is present. The spacer 1040 may be formed on the inner diameter of the fuel assembly external structure 1002 and spirally wrapped around the outer diameter of the fuel bundle 10 installed in that arrangement, or a separate piece may be wrapped around the fuel bundle 10 prior to installation to fill the gap between the outer surface of the multi-layer casing 30 of the fuel bundle 10 and the inner surface of the fuel assembly external structure 1002. Alternatively, fuel bundles 10 having different diameters may be used in areas of the fuel assembly external structure 1002 having different inner diameters.

[0121] Figure 27 is a cross-sectional perspective view of a fuel assembly including a plurality of fuel bundles having twisted ribbon fuel rods, and Figure 28 is an enlarged cross-sectional perspective view of an outlet region of a fuel assembly including a plurality of fuel bundles having twisted ribbon fuel rods, showing a fuel bundle support and a portion of the outlet fuel bundle. Figures 27 and 28 provide additional views of the internal features and structure of the fuel assembly 1000 in the outlet region.

[0122] 29A is a schematic plan bottom view of an embodiment of a second fuel bundle support, and FIG. 29B is a schematic plan side view of an embodiment of a fuel bundle support. The fuel bundle support 1020 includes a body portion 1100 having a top surface 1102, a bottom surface 1104, and an outer circumferential side surface 1106. In an exemplary embodiment, the outer circumferential side surface 1106 includes alternating flat surfaces 1108 and curved surfaces 1110. Additionally, the outer circumferential side surface 1106 is angled with respect to an axis 1112 that is normal to the surfaces 1102, 1104 and is centered in the body portion 1100 in the plan bottom view. The angled outer circumferential side surface 1106 is shown in FIG. 29B as having an angle alpha (α) that is axial from the top surface 1102 to the bottom surface 1104 and is angled radially inward with respect to the axis 1112. The angle alpha (α) may vary from 3 to 20 degrees, alternatively from 5 to 15 degrees. The angled peripheral side 1106 forms the tapered surface 1026 shown and described in connection with FIGS. 26C, 27 and 28.

[0123] The fuel bundle support 1020 also includes openings 1120 in the body portion 1100 that extend from the top surface 1102 to the bottom surface 1104. In the exemplary embodiment, the fuel bundle support 1020 has two types of openings 1120.

[0124] In a first opening type, the walls defining the individual openings are oriented with respect to the plane of the top surface 1102 at an angle beta (β) greater than 90 degrees such that the area of ​​the openings at the top surface 1102 is smaller than the area of ​​the openings at the bottom surface 1104 (see FIGS. 31A-31B). Exemplary values ​​for angle beta (β) are in the range of 93 to 105 degrees. As an example, the walls defining the individual openings of the first opening type may form a truncated pyramid or cone, with a truncated end oriented toward the top surface 1102 and a base end oriented toward the bottom surface 1104.

[0125] In FIG. 29A, the first aperture type 1120a is a truncated pyramid that forms a diamond shape at the top surface 1102 and the bottom surface 1104, although other geometric shapes such as other parallelograms may be used. As can be seen in FIG. 29A at the bottom surface 1104, the plurality of first aperture types 1120a are arranged in a grid pattern, with adjacent first aperture types 1120a separated by walls 1122. The walls 1122 meet at intersections 1124. The top surface 1102 has a similar grid pattern with walls and intersections, but has thicker walls (as can be seen in FIGS. 31A-31B). In an exemplary embodiment, the walls 1122 on the top surface 1102 have a length of 1 mm or more and 2 mm or less, such as 1.3 to 1.6 mm, and the walls 1122 on the top surface 1102 have a length of 1.7 to 2.6 mm. 2 etc., 4mm 2 The following regions for the openings of the first opening type 1120a are formed: In an exemplary embodiment, the walls 1122 on the bottom surface 1104 have a length equal to or greater than 1.5 mm and equal to or less than 2.5 mm, such as 1.9 to 2.2 mm, and the walls 1122 on the bottom surface 1104 have a length equal to or greater than 2.25 mm and equal to or less than 2.6 mm, such as 1.9 to 2.2 mm. 2 etc., 6.25mm 2 The area for the opening of the first opening type 1120a is formed as follows, where the area for the opening of the first opening type 1120a formed by the wall 1122 on the bottom surface 1104 is greater than the area for the opening of the first opening type 1120a formed by the wall 1122 on the top surface 1102.

[0126] In the second aperture type, the walls defining the individual apertures are oriented at an angle of about 90 degrees (where about means that the angle varies by ±1 degree, taking into account manufacturing tolerances) with respect to the plane of the top surface 1102 such that the area of ​​the apertures at the top surface 1102 is equal (within ±2%) to the area of ​​the apertures at the bottom surface 1104. As an example, the walls defining the individual apertures of the second aperture type 1120b may form a right angle with respect to the top surface 1102 and the bottom surface 1104. In some embodiments, the second aperture type 1120b is a right circular cylinder with a first circular end oriented towards the top surface 1102 and a second circular end oriented towards the bottom surface 1104, or the second aperture type 1120b is an elongated polygon with a first end oriented towards the top surface 1102 and a second end oriented towards the bottom surface 1104.

[0127] In FIG. 29A, the second aperture type 1120b is a right circular cylinder that forms a circle at the top surface 1102 and the bottom surface 1104, although other geometric shapes may be used as discussed above. As can be seen in the bottom surface 1104 of FIG. 29A, the plurality of second aperture types 1120b are arranged in regions 1130 that are radially inward from the periphery 1132 of the bottom surface 1104 and radially outward from the grid pattern formed by the first aperture type 1120a. These regions 1130 are arranged such that the area of ​​the openings of the first aperture type 1120 would be 1 mm2 if the first aperture type 1120a were continued radially outward as a continuation of the grid pattern. 2 10. The fuel bundle 1000 may have a cooling path in that region of the fuel assembly 1000 and the fuel bundle 10 may have a cooling path in that region of the fuel assembly 1000. In the exemplary embodiment, the second opening type 1120b is a hole having a diameter of 1 mm (±0.1 mm).

[0128] FIG 30 is a perspective view of the fuel bundle support 1020 from FIGs 29A and 29B. The perspective view is from the top and side of the fuel bundle support 1020, showing the bottom surface 1104 and the outer circumferential side surface 1106. Both the first opening type 1120a and the second opening type 1120b are shown in FIG 30.

[0129] FIG 31A is a cross-sectional perspective view of the fuel bundle support 1020 from FIGS. 29A-29B and 30, and FIG 31B is an enlarged view of region P5 of FIG 31A. The perspective view shows the top surface 1102 of the fuel bundle support 1020 having a plurality of first opening types 1120a in a grid pattern, with adjacent first opening types 1120a separated by walls 1122, and the walls 1122 of adjacent first opening types 1120a meeting at intersections 1124. As previously mentioned, the plurality of first opening types 1120a are arranged in a grid pattern with walls 1122 and intersections 1124, with the grid pattern with walls and intersections of the top surface 1102 being similar to the grid pattern with walls and intersections of the bottom surface, but the walls 1122 are thicker at the top surface 1102 compared to the bottom surface 1104. As can be seen from the close-up view of the cross section shown in FIG. 31B, the surface 1140 of the wall 1122 is oriented at an angle delta (Δ) from a first end 1142 (corresponding to the top surface 1102) to a second end 1144 (corresponding to the bottom surface 1104) with respect to the direction of the axis 1112. Exemplary values ​​for the angle delta (Δ) are in the range of 3 to 15 degrees. Typically, all walls of the first opening type 1120a, e.g., all four walls, are oriented at the angle delta. In other embodiments, at least two of the walls are oriented at the angle delta (where the remaining walls are perpendicular to the top and bottom surfaces). Orienting the surface 1140 of the wall 1122 of the first opening type 1120a to form a first opening type 1120a with an area of ​​the opening at the top surface 1102 smaller than the area of ​​the opening at the bottom surface 1104 contributes to minimizing the pressure drop across the fuel bundle support 1020 during operation of the fuel assembly.

[0130] The selection of materials for the fuel bundle support 1020 may be based, at least in part, on the location within the fuel assembly 1000 in which the fuel bundle support 1020 will be installed and the expected temperatures during operation for that location. For example, for expected operating temperatures of 2000 K or greater (i.e., hot regions of the fuel assembly), the fuel bundle support 1020 may be formed from a carbide, such as zirconium carbide (ZrC) or zirconium niobium carbide (ZrNbC). And, for example, for expected operating temperatures of less than 2000 K (i.e., cold regions of the fuel assembly), the fuel bundle support 1020 may be formed from tungsten, molybdenum, or other refractory metals or refractory metal alloys, such as Zircaloy-4 alloy. Of course, the materials used for the hot region fuel bundle supports may also be used for the cold region fuel bundle supports. In the embodiments of Figures 26B and 26C, the outlet fuel bundle support 1020 is formed from zirconium carbide and the intermediate fuel bundle supports 1020 (if present) are formed from either tungsten or zirconium carbide or niobium zirconium carbide.

[0131] When formed from tungsten, an exemplary manufacturing method includes powder metallurgy techniques to form a near net shape green body of the fuel bundle support 1020, followed by sintering. The sintered body is then machined to form the first aperture type 1120a in a grid pattern by forming the initial holes, then wire EDMing the first aperture type 1120a using a 3-axis EDM machine that allows for the formation of various angled surfaces, followed by any finish machining, forming the second aperture type 1120b with an EDM hole popper or drill, etc. Tungsten and other refractory metal fuel bundle supports 1020 may also be manufactured using additive manufacturing processes.

[0132] When formed from a carbide such as zirconium carbide, exemplary manufacturing methods include gel casting techniques, spark plasma sintering, and injection molding, followed in each case by machining. For example, a slurry of zirconium carbide can be cast to form a green body and sintered. The casting mold can be a negative mold of the final geometry (including one or both aperture types) to properly allow for shrinkage and form a near-net-shape part. The sintered body is then machined (if necessary) to form the first aperture type 1120a in a grid pattern, such as by forming the initial holes and then wire EDMing the first aperture type 1120a using a three-axis EDM machine that allows for the formation of various angled surfaces, followed by any finish machining. For gel cast or injection molded parts, these parts may only require surface grinding of the top and bottom surfaces 1104 and 1102 and the peripheral side surface 1106 on the tapered surface defined by the angle alpha (α). Regardless of how it is manufactured, the inner wall surface 1140 of the opening 1120 can be left as-manufactured and does not need to be machined, for example by EDM, or reinforced to increase production cycle speed.

[0133] FIG. 32 is an end view of another embodiment of an assembled fuel bundle. In this embodiment, the shape of the periphery of the assembled fuel bundle 10 deviates from the circular geometry shown and described in FIG. 12. Instead, the multi-layer casing 30 has an outer surface that includes flat regions 1146 and curved regions 1148 (in cross section, these regions are considered as straight segments 1150 and curved segments 1152, respectively). Due to the hexagonal pattern of the twisted ribbon fuel rods 20 in the core region 22, there are regions of the sides of the core region that are flat (e.g., F1 and F2), which are transformed into flat regions 1146 when the assembled twisted ribbon fuel rods are placed into the multi-layer casing 30. Other regions (e.g., C1) associated with the presence of the filler rods 260 result in curvature of the sides of the core region, which are transformed into curved regions 1148 when the assembled twisted ribbon fuel rods are placed into the multi-layer casing 30. The presence of curved regions 1148 contributes to the ability to tightly wind the fibers of the multi-layer casing 30 during manufacture using fiber manufacturing techniques.

[0134] FIG. 33A illustrates the assembled fuel bundle of FIG. 32 seated on a fuel bundle support, and FIG. 33B is an enlarged view of region P6 of FIG. 33A. The twisted ribbon fuel rods 20 are visible through the openings 1120 in the fuel bundle support 1020. The twisted ribbon fuel rods 20 are aligned with the grid pattern intersections 1124. In this manner, the twisted ribbon fuel rods 20 are supported by the grid pattern of the fuel bundle support 1020 when there is an axial force (in the direction of the longitudinal axis 12 of the fuel bundle 10) within the fuel bundle 10. The twisted ribbon fuel rods 20 at the periphery of the core region 22 are similarly supported by the body portion 1100 of the fuel bundle support 1020 in region 1130 when there is an axial force. The ends of the multi-layer casing 30 may also be supported in region 1130. Additionally, a majority of the cross-sectional area of ​​the gaps 40 between the twisted ribbon fuel rods 20 are aligned with the openings 1020, either the first opening type 1120a or the second opening type 1120b, which facilitates the flow of coolant through the fuel assembly 1000 during operation with minimal pressure drop.

[0135] FIG. 34A is a cross-sectional view of the fuel assembly 1000 showing the fuel bundles (outlet fuel bundle 10e and fourth fuel bundle 10d) toward the outlet end of the fuel assembly 1000 and the contoured inner surface 1160 of the fuel assembly external structure 1002. FIG. 34B corresponds to a view at section II of FIG. 34A. The contoured inner surface 1160 has facets. The facets are located on both the lower fuel assembly external structure 1010 and the upper fuel assembly external structure 1012. In the exemplary embodiment, the facets include flat sections 1162 and curved sections 1164 that extend in the longitudinal direction of the fuel assembly external structure 1002. The flat sections 1162 and the curved sections 1164 may alternate on the contoured inner surface 1160. The flat section 1162 and the curved section 1164 correspond in shape and size, respectively, to the flat region 1146 and the curved region 1148 of the embodiment of the multi-layered casing 30 shown, for example, in FIG.

[0136] Also, as can be seen in FIG. 34C , the contoured inner surface 1160 has an angled surface 1166 (angled with respect to the longitudinal axis of the fuel assembly external structure 1002) and a region (TA) having both facet formations, e.g., alternating flat sections 1162 and curved sections 1164, that correspond, for example, to the alternating flat surfaces 1108 and curved surfaces 1110 of the outer circumferential side 1106 of the fuel bundle support 1020.

[0137] When the outer surface of the multilayer casing 30 of the assembled fuel bundle 10 conformally mates, directly or indirectly, with the contoured inner surface 1160 of the fuel assembly external structure 1002, the non-circular shape of the outer surface prevents rotation of the assembled fuel bundle 10 relative to the fuel assembly external structure 1002. The shape of the fuel bundle support 1020 defined by the outer circumferential surface 1106 is similarly non-circular and functions to prevent rotation of the fuel bundle support 1020 relative to the fuel assembly external structure 1002. In addition, the non-rotation of the assembled fuel bundles 10 and fuel bundle supports 1020 relative to the fuel assembly external structure 1002 also prevents the assembled fuel bundles 10 and fuel bundle supports 1020 from rotating relative to each other, which contributes to maintaining alignment of the coolant flow paths between the assembled fuel bundles 10 within the fuel assembly 1000, as well as between the assembled fuel bundles 10 and the openings 1120 (both the first opening type 1120a and the second opening type 1120b) in the fuel bundle supports 1020.

[0138] Other locking mechanisms for preventing movement of the fuel bundle support 1020 relative to the assembled fuel bundle 10 may also be used, such as locking pins or keyways.

[0139] 35A and 35B are views of the fuel assembly taken along the longitudinal axis of the fuel assembly 1000 toward the outlet region ( FIG. 35A ) illustrating an embodiment of a form-fitting interface between the outer surface of the multi-layer casing 30 of the fuel bundle 10 and the contoured inner surface 1160 of the fuel assembly external structure 1002, as well as a perspective view ( FIG. 35B ) illustrating an embodiment of a form-fitting interface between the outer surface of the multi-layer casing 30 of the fuel bundle 10 and the contoured inner surface 1160 of the fuel assembly external structure 1002. In FIG. 35A , the outer surface of the multi-layer casing 30 of the assembled fuel bundle 10 conformally mates indirectly with the contoured inner surface 1160 of the fuel assembly external structure 1002 with a spacer 1040 or other inside diameter feature positioned between the assembled fuel bundle 10 and the fuel assembly external structure 1002. 35A, the view is along the longitudinal axis 280 toward the outlet end of the fuel assembly 1000, looking through the outlet fuel bundle 10e and the fuel bundle support 1020 and out the outlet connection assembly 1006. Thus, in the central region 1170, one is looking out the outlet connection assembly 1006 along the gap 40, while in the outer radial region 1172, one is looking out the wall of the transition section 1174 of the outlet connection assembly 1006 along the gap 40. In FIG. 35B, the outer surface of the multi-layer casing 30 of the assembled fuel bundle 10 conformally mates directly with the contoured inner surface 1160 of the fuel assembly external structure 1002.

[0140] In alternative embodiments, one or both of the curved section 1164 of the contoured inner surface 1160 of the fuel assembly external structure 1002 and the curved region 1148 of the fuel bundle support 1020 may be formed by a plurality of short or narrow planar surfaces that form a discontinuous curve or that generally approximate a curved shape. For example, a series of three, four, five, or more short or narrow planar surfaces may approximate a continuous curved curvature over a defined length. In such cases, gaps formed by minor mismatches between the inner surface 1160 of the fuel assembly external structure 1002, the outer peripheral side 1106 of the fuel bundle support 1020, and the outer surface of the multi-layered casing 30 of the fuel bundle 10 may be optionally filled, for example, by spacers 1040 or other inside diameter features, as disclosed herein.

[0141] The fuel bundle support 1020 may have alternative embodiments. FIG. 36 is a perspective view of an alternative embodiment of the fuel bundle support 1020. In the embodiment of FIG. 36, a plurality of first ribs 1180 are oriented non-parallel to a plurality of second ribs 1182. The first ribs 1180 have teeth periodically arranged along their length that interdigitate with teeth periodically arranged along their length of the second ribs 1182 such that, when assembled, the first ribs 1180 and the second ribs 1182 form a common planar surface (which in FIG. 36 corresponds to the top surface 1102). An end 1184 of the first rib 1180 and an end 1186 of the second rib 1182 are received in receiving slots 1190 in the outer peripheral side 1106 of the fuel bundle support. Some or all of the various contacts between the first rib 1180 and the second rib 1182, between the first rib 1180 and the perimeter side 1106, and between the second rib 1182 and the perimeter side 1106 may be secured together, for example, by autogenous weld joints. In this alternative embodiment, the ribs 1180, 1182 may be shaped to have walls within the openings 1192 that form a right angle with the top surface 1102 or an obtuse angle with the top surface 1102 (similar to the angle beta (β) shown in FIG. 31B).

[0142] 37A and 37B are schematic plan bottom (FIG. 37A) and side (FIG. 37B) views of another alternative embodiment of a fuel bundle support. In FIG. 37A and 37B, the fuel bundle support 1020 is similar to that shown and described in connection with FIG. 29A-29B, FIG. 30, and FIG. 31A-31B, except that in the alternative embodiment shown in FIG. 37A-37B, the outer peripheral side 1106 of the fuel bundle support 1020 forms a hexagon, and the outer peripheral side 1106 does not have a curved surface 1110 or the curved surface is limited to a portion of the outer peripheral side 1106 that corresponds to an arc having an angle gamma (γ) of ±5 degrees to either side of an apex 1196 of the hexagon. In other aspects, many of the structure, features, and functionality of the embodiment of the fuel bundle support 1020 of Figures 29A-29B, 30, and 31A-31B are also present in the alternative embodiment of the fuel bundle support 1020 of Figures 37A and 37B, which includes a first type of opening 1120a, walls 1122 meeting at an intersection 1124, angled peripheral sides having an angle alpha (α) that is between 3 and 20 degrees, walls defining individual openings oriented with respect to the plane of the top surface 1102 at an angle beta (β) that is greater than 90 degrees, such as between 93 and 105 degrees, such that the area of ​​the openings at the top surface 1102 is smaller than the area of ​​the openings at the bottom surface 1104, and the surface (within the openings) of the walls 1122 are oriented at an angle delta (Δ) that is between 3 and 15 degrees with respect to the direction of the axis 1112 from a first end (corresponding to the top surface 1102) to a second end (corresponding to the bottom surface 1104).

[0143] 38 is an enlarged cross-sectional view of the outlet region of a fuel assembly seated on a lower reactor tube sheet illustrating the axial load distribution associated with the fuel bundle support. The pressure differential associated with the coolant flowing through the fuel assembly 1000, typically an inlet pressure of about 34 bar (about 500 PSI) to about 69 bar (about 1000 PSI), creates axial loads (indicated by block arrows DP in FIG. 38) on components inside the fuel assembly 1000, including the fuel bundle 10 with the twisted ribbon fuel rods 20 and the fuel bundle support 1020. The axial loads resulting from the operational pressure differentials are typically tolerable in the brittle region of the fuel assembly 1000, i.e., the region where the operating temperature is below the brittle-ductile transition temperature of the twisted ribbon fuel rods 20, typically about 1850 K. However, in the ductile region of the fuel assembly 1000, i.e., in the region of the higher operating temperature region, such as about 1850 K or higher, the twisted ribbon fuel rods 20 may undergo ductile failure under axial loads resulting from operational pressure differentials. To reduce the axial loads resulting from the operational pressure differentials of the twisted ribbon fuel rods 20, each fuel bundle support 1020 transfers at least a portion of the axial loads applied to the fuel bundle support 1020 to the fuel bundle external structure 1002 (FIG. 28 shows the outlet region of the fuel assembly 1000 and thus illustrates the lower fuel bundle external structure 1010). 28, an axial load applied to the top surface 1102 of the fuel bundle support 1020 between the fourth fuel bundle 10d and the outlet fuel bundle 10e is transferred to the lower fuel bundle external structure 1010 through direct or indirect contact between the tapered surface 1026 of the fuel bundle support 1020 and the tapered section 1030 of the lower fuel bundle external structure 1010. Also, for example, as shown in FIG. 28, an axial load applied to the top surface 1102 of the fuel bundle support 1020 below the outlet fuel bundle 10e (in the direction of coolant flow) is transferred to the lower fuel bundle external structure 1010 through direct or indirect contact between the tapered surface 1026 of the fuel bundle support 1020 and the tapered section 1032 of the outlet connection assembly 1006.

[0144] For any fuel bundle 10 downstream in the coolant flow direction from the fuel bundle support 1020, the respective fuel bundle 10 experiences only an axial force from the differential pressure associated with the coolant flowing through that fuel bundle 10 (or fuel bundles) to the next fuel bundle support 1020 in the coolant flow direction. With reference to FIG. 38, the axial force from the differential pressure associated with the coolant flowing through the fourth fuel bundle 10d (and any upstream fuel bundles not separated from the fourth fuel bundle 10d by a fuel bundle support 1020) is transmitted radially by the fuel bundle support 1020 to the fuel assembly external structure 1002 (see section SA), and the axial force from the differential pressure associated with the coolant flowing through the outlet fuel bundle 10e is transmitted radially by the outlet fuel bundle support 1020 to the fuel assembly external structure 1002 (see section SB). The axial force at the fuel assembly external structure 1002 is carried by the lower reactor tube sheet 1200 on which the fuel bundle 1000 is seated.

[0145] The fuel assembly 1000 includes a space 1198 between the bottom surface 1104 of the intermediate fuel bundle support 1020 and a downstream (coolant flow direction) fuel bundle, such as the outlet fuel bundle 10e. The space 1198 is formed by any suitable means, such as an outer circumferential structure or a protrusion on the bottom surface 1104 of the intermediate fuel bundle support 1020. The space 1198 provides a volume for the coolant to mix and restore turbulence after being excited through the openings 1120 of the intermediate fuel bundle support 1020. Mixing within this space can improve the thermal properties of the coolant. Additionally, the space 1198 provides an expansion volume for internal components of the fuel assembly 1000, such as the twisted ribbon fuel rods 20, that may undergo thermal expansion in the longitudinal direction.

[0146] The fuel assembly external structure 1002 may be formed, for example, from SiC fibers and manufactured by fiber processing techniques, such as, for example, a mandrel wrapping method. A variety of mandrel wrapping methods may be used. Exemplary mandrel wrapping methods include wet wrapping, prepreg wrapping, or dry wrapping with vacuum resin infusion. A polymer infiltration and pyrolysis (PIP) process may also be used.

[0147] In the exemplary embodiment, the mandrel has an outer surface that is a negative geometry corresponding to the contoured inner surface 1160 of the fuel assembly external structure 1002. For example, the mandrel outer surface may include a negative geometry corresponding to alternating flat sections 1162 and curved sections 1164 extending longitudinally of the mandrel. The mandrel outer surface may also include diameter variations corresponding to different diameters of the contoured inner surface 1160, including angled surfaces corresponding to areas (TA).

[0148] The fuel assembly external structure 1002, especially if made of SiC / SiC fiber, may be reinforced to withstand additional radial loads (see above regarding axial loads transferred to the fuel assembly external structure 1002) that occur in the configurations where the fuel bundle supports 1020 are arranged. One example of reinforcement is the use of high angle circumferential wrapping in those configurations. FIG. 39 illustrates the variation of helix angles used to fabricate the fuel assembly external structure 1002. In FIG. 39, the configuration 1220 corresponds to the configuration of the fuel bundle supports 1020 of the assembled fuel assembly 1000, the configuration 1230 corresponds to the configuration of the fuel bundle 10, and the configuration 1240 is the end of the fuel assembly external structure 1002. In the configuration 1230, the helix angle (relative to the longitudinal axis 1222) is 30 to 60 degrees, alternatively 50 to 55 degrees. However, in arrangement 1220, the helix angle (relative to longitudinal axis 1222) is 70 to 85 degrees, alternatively 75 to 85 degrees. The regions between arrangements 1220 and 1230, and between 1230 and 1240, have helix angles that transition between the regions in each adjacent arrangement. A higher helix angle strengthens the fuel assembly external structure 1002, allowing it to withstand greater forces. A higher helix angle may result in additional wraps and additional diameter features on the outer diameter of the fuel assembly external structure, but such additions may be compensated for in the moderator and reactor structure.

[0149] Additionally or alternatively, the crimp reinforcement may be placed outside the fuel assembly external structure, especially where lower temperatures allow for the use of structural materials such as nickel superalloys or ductile materials. If present, the crimp reinforcement would be located relative to the fuel bundle support 1020.

[0150] Although FIG. 39 is described in the context of the fuel assembly external structure 1002, the description applies to the lower fuel assembly external structure 1010 as well as the upper fuel assembly external structure 1012.

[0151] A fuel assembly incorporating multiple fuel bundles (having twisted ribbon fuel rods and multi-layer casing, and optionally, one or more fuel bundle supports) may be manufactured by any suitable means. Figure 40 is a flow diagram illustrating various steps in an embodiment of a method S2000 for manufacturing a fuel assembly including multiple fuel bundles having twisted ribbon fuel rods and multi-layer casing, and optionally, one or more fuel bundle supports.

[0152] In step S2010, the fuel assembly external structure is prepared, for example, by attaching components for attaching an outlet flow adapter to the outlet end of the lower fuel assembly external structure. This facilitates later attachment of the outlet connection assembly 1006 to the outlet end of the fuel assembly external structure. An attachment component (such as a flange or short pipe section or sleeve), typically formed from a metal alloy, is attached to the outlet end, for example, by vacuum brazing or other process capable of forming an essentially leak-tight joint. The components inside the fuel assembly external structure, including one or more fuel bundles with twisted ribbon fuel rods and multi-layer casing 10, and (optionally) the fuel bundle support 1020, are then inserted in a suitable order to achieve the desired arrangement of each fuel assembly within the fuel assembly external structure, as well as its positioning relative to one another, i.e., stacked or unstacked longitudinally according to the expected operating temperature or neutron property conditions.

[0153] For example, a fuel bundle support is inserted S2020 into the lower fuel assembly external structure and seated on the associated support feature toward the outlet end of the fuel assembly such that the first inserted fuel bundle support becomes the outlet fuel bundle support. A fuel bundle having twisted ribbon fuel rods and multi-layer casing 10 is inserted S2030 into the lower fuel assembly external structure 1010 and seated on the top surface 1102 of the outlet fuel bundle support. Additional fuel bundle supports 1020 and fuel bundles having twisted ribbon fuel rods and multi-layer casing 10 are alternately stacked (steps S2040, S2050, and S2055) until the lower fuel assembly external structure 1010 is filled and assembly of the lower fuel assembly is complete. Additional fuel bundles and optionally inlet fuel bundle supports are stacked on the assembled lower fuel assembly (whose sizes and dimensions correspond to the internal features and volumes of the upper fuel assembly external structure 1012), and the upper fuel assembly external structure 1012 is positioned on top of the stack and coupled to the lower fuel assembly external structure 1010 S2060. The inlet coupling assembly 1004 is then attached to the inlet end of the fuel assembly external structure via a mounting component S2070.

[0154] Fuel bundles having twisted ribbon fuel rods and multi-layer casing 10 as disclosed herein, and (optionally) fuel bundle supports 1020 (and fuel assemblies 1000 formed therefrom) may be incorporated into a nuclear fission reactor structure. Typically, the fuel assemblies are positioned within a block of moderator that is used to thermalize fast neutrons. Nuclear control means, such as a rotating peripheral control drum, may be used to control the reactivity of the core. The entire core is disposed within a pressure boundary that is connected to a converging-diverging nozzle.

[0155] 41 is a schematic cross-sectional side view of an embodiment of a nuclear propulsion nuclear fission reactor structure in a vessel having a fuel assembly including a plurality of fuel bundles having twisted ribbon fuel rods and a multi-layer casing 10. The embodiment of the nuclear fission reactor structure 2100 includes a plurality of fuel assemblies 2105 (e.g., fuel assemblies 1000 formed from fuel bundles having twisted ribbon fuel rods and a multi-layer casing 10, and (optionally) one or more fuel bundle supports 1020) disposed within an active core region 2110 of the nuclear fission reactor structure 2100 (the active core region 2110 is an interior region in which moderator blocks are disposed, with fuel assembly portions within the moderator blocks). At the inlet and outlet of the fuel assemblies 2105, coupling assemblies (such as an inlet coupling assembly 2115 and an outlet coupling assembly 2120) provide fluid communication for the propellant supplied to and discharged from each of the fuel assemblies 2105. Thus, the inlet connection assembly 2115 connects to or interfaces with the inlet openings of multiple fuel assemblies 2105, and the outlet connection assembly 2120 connects to or interfaces with the outlet openings of multiple fuel assemblies 2105.

[0156] An interface structure 2125, which may or may not include a supplemental radial constraint, faces radially outward of the active core region 2130, and a reflector 2135 faces radially outward of the interface structure 2125. A first surface of the interface structure 2125 conforms to an outer surface of the active core region 2110, and a second surface of the interface structure 2125 conforms to an inner surface of the reflector 2135. The inner surface of the reflector 2135 is oriented toward the active core region 2110, and the interface structure 2125 functions to fit the geometry of the outer surface of the active core region 2110 to the geometry of the inner surface of the reflector 2135, thus enabling various arrangements for the fuel assemblies 100 within the moderator block 2155, such as a hexagonal pattern resulting in a hexagonal interface with the interface structure 2125 or a concentric ring pattern resulting in a circular interface with the interface structure 2125.

[0157] FIG. 42 is a schematic cross-sectional top view of an embodiment of a nuclear-propelled nuclear fission reactor structure 2100 in a vessel 2140. A plurality of control drums 2145, each comprising a neutron absorber body 2150, are disposed within a volume of a reflector 2135, such as an annular section of an outer portion of a cylindrically shaped control drum. The control drum 2145 itself is made of a neutron-reflecting material, as is the reflector 2135. The neutron absorber body 2150 is made of a neutron-absorbing material and is movable, such as by rotation, between a first position and a second position, the first position being radially closer to the active core region than the second position. In an exemplary embodiment, the first position is radially closest to the active core region and the second position is radially furthest from the active core region. The neutron absorber body 2150 is movable between the first position and the second position to control the reactivity of the active core region 2110. In the illustrated embodiment, the neutron absorber body 2150 is rotatable from a first radially closer position to a second position by rotation (R4) about the axis of the control drum 2145. However, other radial positions and / or movement directions may be implemented as long as the various positions to which the neutron absorber body 2150 may be moved provide control of the reactivity of the active core region 2110. In some embodiments, when the multiple neutron absorber body 2150 are each at a first radially closer position, each of the multiple neutron absorber body 2150 is radially equidistant from the axial centerline of the active core region 2110. Other control concepts may also be implemented, such as adjusting neutron leakage by opening and closing a portion of the reflector 2135.

[0158] The nuclear fission reactor structure may further include a vessel 2140. Figures 41 and 42 illustrate a schematic of one embodiment of a nuclear fission reactor structure 2100 having a vessel 2140. The nuclear fission reactor structure 2100, including an active core region 2110, an interface structure 2125, an inlet coupling assembly 2115 and an outlet coupling assembly 2120, a reflector 2135, and a plurality of control drums 2145 having neutron absorber bodies 2150, is contained within the interior volume of the vessel 2140.

[0159] 41, the motor 2160 is operably attached to rotate to the control drum 2145 by a drum shaft 2165. The motor 2160 may be housed in a pressure boundary extension of the vessel 2140 or alternatively not, in which case a seal would be required around the drum shaft 2165. A motor internal to the vessel 2140 may also be implemented.

[0160] Embodiments of the container 2140 are formed from machine forgings and typically use high strength aluminum or titanium alloys due to weight considerations. The container 2140 can be multiple components that are then assembled together, for example with fasteners. However, in other embodiments, the container 2140 can be one continuous component or an assembly welded together.

[0161] Additional disclosure relating to nuclear fission reactor structures and components thereof can be found in U.S. patent application Ser. No. 16 / 999,244, the entire contents of which are incorporated by reference.

[0162] The present disclosure is also directed to a reactor thermal propulsion engine including a nuclear fission reactor structure 2100 within a vessel 2140 within a reactor section 2170. The reactor thermal propulsion engine further includes a shield 2175, a turbomachinery 2180, and a nozzle section 2185 attached to or supported by the vessel 2125, for example, consistent with that shown in FIG.

[0163] It is contemplated that various support and auxiliary equipment may be incorporated into the disclosed nuclear fission reactor structures and nuclear thermal propulsion engines. For example, at least one of moderators (such as zirconium hydride, beryllium, beryllium oxide, and graphite), control rods for launch safety, neutron sources to assist in start-up, and scientific instruments (such as temperature sensors or radiation detectors) may be incorporated into the nuclear propulsion nuclear fission reactor structures.

[0164] The disclosed arrangements pertain to any configuration in which a heat source, including a fissile nuclear fuel composition, is incorporated into a fuel bundle. Although generally described herein in the context of a gas-cooled nuclear thermal propulsion reactor (NTP reactor), the structures and methods disclosed herein are applicable to other nuclear reactor systems.

[0165] The nuclear propulsion nuclear fission reactor configurations disclosed herein may be used in any suitable application, including, but not limited to, non-terrestrial power applications, space power, space propulsion, and naval applications including submersibles.

[0166] Although particular embodiments have been mentioned, it will be apparent that other embodiments and modifications can be devised by those skilled in the art without departing from the spirit and scope thereof, and it is intended that the appended claims be construed to include all such embodiments and equivalent variations. [Explanation of symbols]

[0167] T1 column T2 row T3 Diagonal 10 Fuel Bundles 10a, 10b, 10c, 10d, 10e fuel bundles 12 Longitudinal axis 14 First end 16 Second end 20 Twisted ribbon fuel rods 22 Core Region 24 Short Side 26 Long Side 28 Envelope 30 Multi-layer casing 32 Inner layer 34 Inner middle layer 36 Outer middle layer 38 Outer layer 40 Gap 100 fuel assembly 200 Stick-shaped object seating fixture 210 Support housing 220 Seating surface 225 Protrusion 230 Base surface 232 Mandrel Mating Feature 235 End Cap Surface 240 Reception Space 245 Angled Area 260 Filler Rod 265 Peg End 270 End face 270 Inner surface 280 Axial centerline 300 End Cap 310 Interface 342 Material 344 Carbon fiber or ZrC yarn 346 Material 600 SEM images 610 X-ray diffraction pattern 650 SEM images 660 X-ray diffraction patterns 700 Mixing Cup 705 Compounds 710 Primary atmosphere 715 Cup Insulation Layer 720 Cap 730 Containment Vessel 735 Mixer insulation layer 740 Cover 745 Secondary Protective Atmosphere 750 Thermocouple or RTD Thermistor 770 Billet 775 Arbor Press 780 Central Channel 785 Exterior Wall 790 Sleeve 795 flexible wing 800 Twisted Fuel Rod Manufacturing System 802 Piston Extruder Heating Die 804 Extrusion Machine 806 Ribbon initial cooling zone 808 Ribbon Tensioning Subsystem 810 Ribbon Twisting Subsystem 812 Ribbon Cutting Subsystem 814 Ribbon Inspection Zone 816 Length Sensor Subsystem 818 Rod-shaped object sorting and recovery subsystem 820 Nozzle 822 Extrusion Ribbon 824 Tension Roller 826 Funnel 828 Spring Arm 830 Cooling Fan 832 Stage 834 Belt 836 Axis 840 Cutting device 860 Length Sensor Subsystem 862 Photodiode Detector 864 Rail 870 Rod-shaped object sorting and recovery subsystem 872 Sorting Shoot 874 Sorting Tray 880 Adjustable Platform 890 Inert Atmosphere Chamber 892 Access Port 902~910 Upper box 910 Latest Torsion Profile 940 Extrusion Ribbon Images 942 Binary Threshold 944 binary ribbon images 946 integral 948 Ribbon parameters 950 Ribbon Model 954 Target Surface Shape 956 Corresponding integral 962 Actual Surface 964 Model Surface 968 Integral 970 Plots 972 maximum points 1000 fuel assemblies 1002 Fuel assembly external structure 1004 Inlet Connection Assembly 1006 Outlet Connection Assembly 1010 Lower fuel assembly external structure 1012 Upper fuel assembly external structure 1014 Brazed joint 1020 Fuel bundle support 1026 Tapered Surface 1030 Tapered Section 1032 Tapered Section 1036 Cleat 1100 Main unit 1102 Top surface 1104 Bottom 1106 Outer periphery 1108 plane 1110 Curved surface 1112 Axis 1120a First opening type 1120b Second opening type 1122 Wall 1124 Intersection 1130 area 1132 Periphery 1140 Surface 1144 Second End 1146 Planar area 1148 Curved Area 1150 Straight Segments 1152 Curved Segment 1160 Inner surface 1162 Plane Section 1164 Curved Section 1170 Central area 1172 Outer radial area 1174 Transition Section 1180 First Rib 1182 Second Rib 1184 End 1186 End 1190 Receiving Slot 1192 Opening 1196 Vertex 1198 Space 1200 Lower reactor tube sheet 1220 Placement 1222 Longitudinal axis 1230 Placement 1240 placement 2100 Nuclear fission reactor structure 2100 Nuclear propulsion nuclear fission reactor structure 2105 Fuel assembly 2110 Active core region 2115 Inlet Coupling Assembly 2120 Outlet Connection Assembly 2125 Interface structure 2130 Active core region 2135 Reflector 2140 Container 2145 Control drum 2150 Neutron absorber body 2155 Moderator Block 2160 Motor 2165 Drum shaft 2175 Shield 2180 Turbomachinery 2185 Nozzle Section

Claims

1. 1. A fuel bundle comprising: a multi-layer casing having an interior volume defining a reactor core; a plurality of twisted ribbon fuel rods arranged within the core, the plurality of twisted ribbon fuel rods defining a core region; Equipped with the plurality of twisted ribbon fuel rods have a composition including a fissionable fuel component; the multi-layer casing includes an inner layer, an inner middle layer, an outer middle layer, and an outer layer; the inner layer is a graphite compressed felt insulation layer; the inner intermediate layer is a composite reinforced compression layer; the outer intermediate layer is a first compressed prepreg layer; The fuel bundle, wherein the outer layer is a second compressed prepreg layer.

2. Within a cross section perpendicular to a longitudinal axis of the fuel bundle, the cross sections of the plurality of twisted ribbon fuel rods are arranged in a hexagonal packing arrangement; 10. The fuel bundle of claim 1, wherein the hexagonal packing arrangement extends to outermost twisted ribbon fuel rods at the periphery of the core region.

3. The fuel bundle of claim 2 further comprising a plurality of filler rods at a plurality of locations around the periphery of the core region.

4. Within a cross section perpendicular to a longitudinal axis of the fuel bundle, the cross sections of the plurality of twisted ribbon fuel rods are arranged in a circular packing arrangement; 10. The fuel bundle of claim 1, wherein the circular packing arrangement extends from an outer periphery of the core region to twisted ribbon fuel rods inward.

5. The fuel bundle of claim 4 , wherein the periphery of the core region has an asymmetric region.

6. 1. A rod seating fixture for arranging a plurality of twisted ribbon fuel rods to form a nuclear reactor core of a fuel bundle, said rod seating fixture comprising: a seating surface including a plurality of protrusions; the plurality of protrusions are distributed on the seating surface and have a height from a base surface of the seating surface; The protrusion has a plurality of sides, and a first portion of the plurality of sides includes an angled region that joins a second portion of the plurality of sides at an angle other than 90 degrees.

7. further comprising a mandrel mating feature; the mandrel-mating feature faces the seating surface along a longitudinal axis of the rod seating fixture; 7. The rod seating fixture of claim 6, wherein the mandrel mating feature is connected to the seating surface by an end cap surface.

8. 8. The rod seating fixture of claim 6 or 7, wherein sides of a plurality of adjacent projections define a receiving space configured to seat an end of a twisted ribbon fuel rod.

9. 9. The rod seating fixture of claim 8, wherein the receiving space is configured to seat the end of the twisted ribbon fuel rod with line contact between the side surfaces of the plurality of adjacent protrusions defining the receiving space and corners of the end of the twisted ribbon fuel rod.

10. 9. The rod seating fixture of claim 8, wherein the receiving space is configured to seat the end of the twisted ribbon fuel rod with surface contact between an angled region of at least one of the plurality of adjacent protrusions defining the receiving space and a long side of the end of the twisted ribbon fuel rod.

11. 1. A method of manufacturing a fuel bundle, comprising: forming a core region of the fuel bundle, the forming of the core region including seating first ends of each of a plurality of twisted ribbon fuel rods in respective receiving spaces of a rod seating fixture, the rod seating fixture including a seating surface having a plurality of protrusions distributed on the seating surface and having a height from a base surface of the seating surface, sides of a plurality of adjacent protrusions defining the respective receiving spaces; attaching an end cap to a second end of each of the plurality of twisted ribbon fuel rods to form a pre-bundle; Optionally, introducing an infiltrant into the pre-bundle to fill interstices between the assembled twisted ribbon fuel rods; placing the pre-bundle within a multi-layer casing including an inner layer, an inner middle layer, an outer middle layer, and an outer layer; removing the rod seating fixture and end cap; and optionally removing said infiltrant; the plurality of twisted ribbon fuel rods have a composition including a fissionable fuel component; the inner layer is a graphite compressed felt insulation layer; the inner intermediate layer is a composite reinforced compression layer; the outer intermediate layer is a first compressed prepreg layer; The method of manufacturing a fuel bundle wherein the outer layer is a second compressed prepreg layer.

12. supporting an axial length of the twisted ribbon fuel rods seated in the core region of the fuel bundle with a support housing, the support housing contacting an outer periphery of the rod seating fixture; 12. The method of manufacturing a fuel bundle as recited in claim 11, further comprising the step of removing said support housing prior to placing said pre-bundle within said multi-layer casing.

13. 13. The method of manufacturing a fuel bundle as recited in claim 12, further comprising the step of completely encapsulating the assembled twisted ribbon fuel rods, and optionally wherein introducing the infiltrant into the pre-bundle comprises vacuum assisted infiltration.

14. 14. The method of manufacturing a fuel bundle as recited in claim 11, wherein placing the pre-bundle within the multi-layer casing includes manual layup of one or more of the inner layer, the inner intermediate layer, the outer intermediate layer, and the outer layer.

15. 14. The method of manufacturing a fuel bundle as recited in claim 11, wherein placing the pre-bundle within the multi-layer casing includes mandrel wrapping one or more of the inner layer, the inner intermediate layer, the outer intermediate layer, and the outer layer.

16. Within a cross section perpendicular to a longitudinal axis of the core region, the cross sections of the plurality of twisted ribbon fuel rods are arranged in a hexagonal packing arrangement; 14. The method of manufacturing a fuel bundle as defined in any one of claims 11 to 13, wherein the hexagonal packing arrangement extends to outermost twisted ribbon fuel rods at the periphery of the core region.

17. 17. The method of manufacturing a fuel bundle as recited in claim 16, wherein placing the pre-bundle within the multi-layer casing includes manual layup of one or more of the inner layer, the inner intermediate layer, the outer intermediate layer, and the outer layer.

18. 17. The method of manufacturing a fuel bundle as recited in claim 16, wherein placing the pre-bundle within the multi-layer casing includes mandrel wrapping one or more of the inner layer, the inner intermediate layer, the outer intermediate layer, and the outer layer.

19. 17. The method of fabricating a fuel bundle as recited in claim 16, further comprising the step of positioning a plurality of filler rods in a plurality of arrangements about the periphery of the core region, wherein an end of each of the plurality of filler rods is seated within the rod seating fixture.

20. 20. The method of manufacturing a fuel bundle as recited in claim 19, wherein placing the pre-bundle within the multi-layer casing includes manual layup of one or more of the inner layer, the inner intermediate layer, the outer intermediate layer, and the outer layer.

21. 20. The method of manufacturing a fuel bundle as recited in claim 19, wherein encasing the pre-bundle within the multi-layer casing includes mandrel wrapping one or more of the inner layer, the inner intermediate layer, the outer intermediate layer, and the outer layer.

22. Within a cross section perpendicular to a longitudinal axis of the core region, cross sections of the plurality of twisted ribbon fuel rods are arranged in a circular packing arrangement; 14. The method of manufacturing a fuel bundle as defined in any one of claims 11 to 13, wherein the circular packing arrangement extends from the outer periphery of the core region to twisted ribbon fuel rods inward.

23. 23. The method of manufacturing a fuel bundle as recited in claim 22, wherein placing the pre-bundle within the multi-layer casing includes manual layup of one or more of the inner layer, the inner intermediate layer, the outer intermediate layer, and the outer layer.

24. 23. The method of manufacturing a fuel bundle as recited in claim 22, wherein encasing the pre-bundle within the multi-layer casing includes mandrel wrapping one or more of the inner layer, the inner intermediate layer, the outer intermediate layer, and the outer layer.

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