Mandrel-wound eccentric monolithic fuel assembly core, fuel assembly, reactor incorporating the same, and method of manufacture.

Abstract

An insulated fuel assembly core comprises multiple fuel monoliths, an exhaust support plate, an exhaust shield assembly, and an insulating layer, wherein the multiple fuel monoliths are axially positioned along the longitudinal axis, and each of the multiple fuel monoliths has an eccentric cylindrical shape and a composition containing fissile fuel components. Channels in the exhaust support plate are oriented so that the propellant gas flow through the exhaust support plate does not collide with the exhaust shield assembly. The insulated fuel assembly core is manufactured by forming a tensioned fuel monolith stack mandrel assembly using mandrel spacers and internal tension-imparting components, and by forming an insulating layer on the outer surface of the tensioned fuel monolith stack mandrel assembly by mandrel winding. The insulated fuel assembly core can be incorporated, for example, into the fuel assembly of a nuclear propulsion fission reactor structure of a nuclear thermal propulsion engine.

Description

Technical Field

[0001] [Related Application Data] This application is based on U.S. Provisional Application No. 63 / 459,450, filed on April 14, 2023, and claims priority under 35 U.S.C. § 119 to that document, the entire content of which is incorporated herein by reference.

[0002] [Technical Field and Industrial Applicability] The present disclosure generally relates to nuclear fission reactors and structures associated with nuclear fission reactors, and more particularly to structures for propulsion. In particular, a fuel assembly core has stacked monolithic fuel body parts, an insulating layer, an exhaust support plate, and an inlet flow structure. The fuel assembly core is mandrel-wound to form an assembled insulated fuel assembly core, which is positioned within a fuel assembly outer structure to form a fuel assembly. The fuel assembly is incorporated into a nuclear thermal propulsion reactor, which can be used in various applications suitable for gas reactor designs (e.g., space environments, lunar surface environments, and terrestrial environments, etc.).

Background Art

[0003] In the following discussion, certain structures and / or methods are referenced. However, the following references should not be construed as admitting that these structures and / or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and / or methods are not entitled to be prior art against the present invention.

[0004] Twisted ribbon fuel configurations have long been used in reactor design (see Burns et al., "Nuclear Thermal Propulsion Reactor Materials," Nuclear Materials, edited by P. Tsvetkov, London: IntechOpen, 2020), with (U, Zr)C fuel used for the low-temperature portion of the reactor design (i.e., propellant exit gas temperature ≤ 2500K) and (U, Zr, Nb)C used for the high-temperature portion of the reactor core. These conventional ribbon fuel configurations were assembled into tubes with insulating layers of NbC and ZrC and an outer casing of carbide graphite material.

[0005] However, such conventional assemblies have allowed several failure modes. These include cracking of the insulating layer, layer separation, and ablation forming holes. Each of these failure modes may allow hot propellant gases (e.g., hydrogen) to penetrate the layered tubular structure, attack the layers, and ultimately cause casing failure. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] U.S. Patent Application No. 18 / 118,193 [Patent Document 2] U.S. Patent Application No. 16 / 999,244 [Non-patent literature]

[0007] [Non-Patent Document 1] Burns et al., “Nuclear Thermal Propulsion Reactor Materials”, Nuclear Materials, edited by P. Tsvetkov, London: IntechOpen, 2020 [Overview of the project] [Problems that the invention aims to solve]

[0008] To address these failure modes, and more generally, to reduce defects and improve performance, there is a need for improvements in nuclear thermal propulsion reactors. In particular, there is a need for improvements relating to the geometry of the fuel itself, as well as improvements relating to the structure, arrangement, and manufacture of the fuel assembly, including various components of the fuel assembly (e.g., exhaust support plates and exhaust shield assemblies). In addition, improvements relating to the manufacture of fuel assemblies including insulated fuel assembly cores have been disclosed, which offer improved manufacturability. [Means for solving the problem]

[0009] Embodiments of an insulated fuel assembly core include a plurality of fuel monoliths, an exhaust support plate, an exhaust shield assembly, and an insulating layer. The plurality of fuel monoliths are axially positioned along the longitudinal axis of the insulated fuel assembly core, and each of the plurality of fuel monoliths has an eccentric cylindrical shape and a composition containing fissile fuel components.

[0010] An embodiment of a method for manufacturing an insulated fuel assembly core includes the steps of forming a tensioned fuel monolith stack mandrel assembly and forming an insulating layer on the outer surface of the tensioned fuel monolith stack mandrel assembly by mandrel winding. The steps of forming a tensioned fuel monolith stack mandrel assembly are: assembling a plurality of fuel monoliths into a stack along a longitudinal axis, each of which has an eccentric cylindrical shape and includes a first end surface, a second end surface, a side surface connecting the first end surface to the second end surface, and a plurality of first channels extending axially from the first end surface to the second end surface; inserting an alignment drill rod through one of the plurality of first channels in the direction of the longitudinal axis; and attaching an exhaust support plate to the first end of the assembled stack to form a core stack, the exhaust support plate including a first end surface, a second end surface, a circumferential surface connecting the first end surface to the second end surface, and a plurality of second channels extending from the first end surface to the second end surface, and attaching the exhaust support plate is done through one of the plurality of second channels Steps include: inserting an alignment rod through one; inserting a first set of tensioning cables through first portions of multiple first channels in multiple fuel monoliths and through first portions of multiple second channels in exhaust support plates; attaching multiple second mandrel spacers to a second end of a core stack; inserting a first set of tensioning cables through multiple second mandrel spacers; and attaching multiple first mandrel spacers to a first end of a core stack, wherein the first end of the core stack includes an exhaust support plate attached to the first end of an assembled stack; and through (i) multiple first mandrel spacers, (ii) second portions of multiple second channels in exhaust support plates, (iii) second portions of multiple first channels in multiple fuel monoliths, and (iv) multiple first mandrel spacersThe procedure includes the steps of inserting a second set of tensioning cables, applying tension to the first set of tensioning cables and the second set of tensioning cables, and attaching a first threaded mandrel cap to the distal ends of a plurality of first mandrel spacers and attaching a second threaded mandrel cap to the distal ends of a plurality of second mandrel spacers.

[0011] The disclosed insulated fuel assembly core can be incorporated into a fuel assembly by positioning the insulated fuel assembly core within the fuel assembly outer structure and by attaching the inlet connection assembly to the inlet end of the fuel assembly outer structure. Furthermore, multiple fuel assemblies can be positioned within one of several fuel assembly openings within a moderator block to form a nuclear fission reactor structure.

[0012] The disclosed insulated fuel assembly cores, fuel assemblies, and embodiments of construction and fabrication methods have applications in various nuclear fission reactor designs and in a wide range of fields, including aerospace and industrial applications.

[0013] The above overview and the following detailed description of the embodiments can be better understood when read in conjunction with the accompanying drawings. It should be understood that the embodiments depicted are not limited to the precise arrangements and means shown. [Brief explanation of the drawing]

[0014] [Figure 1A] This is a perspective view of an embodiment of the fuel monolith. [Figure 1B] This is a diagram of the end surface of the fuel monolith. [Figure 1C] This is a perspective view of another embodiment of the fuel monolith. [Figure 1D] This is a diagram of the end surface of the fuel monolith. [Figure 2A]Perspective view of an embodiment of an insulated fuel assembly core. [Figure 2B] Cross-sectional view of the insulated fuel assembly core shown in FIG. 2A, taken at X1-X1. [Figure 2C] Enlarged view of area P1 in FIG. 2B, showing the end portion of the inlet of the insulated fuel assembly core in cross-section. [Figure 2D] Enlarged view of area P2 in FIG. 2B, showing a portion of the exhaust end of the insulated fuel assembly core in cross-section. [Figure 3A] Perspective view of an embodiment of a fuel assembly. [Figure 3B] Cross-sectional view of the fuel assembly shown in FIG. 3A, taken at X2-X2. [Figure 3C] Enlarged view of area P3 in FIG. 3B, showing the end portion of the inlet of the fuel assembly in cross-section. [Figure 3D] Enlarged view of area P4 in FIG. 3B, showing a portion of the exhaust end of the fuel assembly in cross-section. [Figure 4] Cross-section of a portion of the exhaust end of a fuel assembly, annotated to illustrate the flow through the exhaust support plate. [Figure 5] Perspective view of an embodiment of an exhaust support plate. [Figure 6A] End view of the exhaust support plate shown in FIG. 5. [Figure 6B] First cross-sectional view of the exhaust support plate shown in FIG. 5. [Figure 6C] Second cross-sectional view of the exhaust support plate shown in FIG. 5. [Figure 6D] End view of an alternative embodiment of the exhaust support plate. [Figure 6E] Various cross-sectional views of an alternative embodiment of the exhaust support plate. [Figure 6F] Various cross-sectional views of an alternative embodiment of the exhaust support plate. [Figure 6G]These are cross-sectional views of various alternative embodiments of the exhaust support plate. [Figure 6H] These are cross-sectional views of various alternative embodiments of the exhaust support plate. [Figure 6I] This is an end view of an alternative embodiment of the exhaust support plate. [Figure 6J] These are cross-sectional views of various alternative embodiments of the exhaust support plate. [Figure 6K] These are cross-sectional views of various alternative embodiments of the exhaust support plate. [Figure 6L] These are cross-sectional views of various alternative embodiments of the exhaust support plate. [Figure 7] This is a perspective view of an embodiment of a truncated cone-shaped section of an exhaust shield assembly. [Figure 8A] Figure 7 is an end view of a truncated cone-shaped section of the exhaust shield assembly shown. [Figure 8B] Figure 7 is a side view of the truncated cone-shaped section of the exhaust shield assembly shown. [Figure 8C] Figure 7 is a cross-sectional view of a truncated cone-shaped section of the exhaust shield assembly shown. [Figure 8D] This is a perspective view of an embodiment of a tubular section of an exhaust shield assembly. [Figure 8E] This is a longitudinal cross-sectional view of an embodiment of a tubular section of an exhaust shield assembly. [Figure 8F] This is a magnified view of area P12 in Figure 8E. [Figure 9A] This is a perspective view of a first embodiment of an integrated exhaust shield assembly. [Figure 9B] This is a longitudinal cross-sectional view of a first embodiment of an integrated exhaust shield assembly. [Figure 9C] This is a perspective view of a second embodiment of an integrated exhaust shield assembly. [Figure 10]This is a flow diagram illustrating various steps in an embodiment of a method for manufacturing an insulated fuel assembly core and fuel assembly. [Figure 11A] This is a perspective view of an embodiment of a tensioned fuel monolith stack mandrel assembly. [Figure 11B] This is a cross-sectional view of the tensioned fuel monolith stack mandrel assembly shown in Figure 11A, taken at X5-X5. [Figure 11C] Figure 11B is an enlarged view of area P5, showing a cross-sectional view of the threaded mandrel end cap and mandrel spacer at the first end of the tensioned fuel monolith stack mandrel assembly. [Figure 11D] Figure 11B is an enlarged view of area P6, showing a cross-sectional view of the inlet end of the fuel assembly core, which is insulated from the mandrel spacer in the first part of the tensioned fuel monolith stack mandrel assembly. [Figure 11E] Figure 11B is an enlarged view of area P7, showing a cross-sectional view of the first portion of the exhaust end of the insulated fuel assembly core in the second portion of the tensioned fuel monolith stack mandrel assembly. [Figure 11F] Figure 11B is an enlarged view of area P8, showing a cross-sectional view of the mandrel spacer in the third part of the tensioned fuel monolith stack mandrel assembly and the second part of the exhaust end of the insulated fuel assembly core. [Figure 11G] Figure 11B is an enlarged view of area P9, showing a cross-sectional view of the threaded mandrel end cap and mandrel spacer at the second end of the tensioned fuel monolith stack mandrel assembly. [Figure 12A] This is a perspective cross-sectional view showing the arrangement of tension components in different parts of a tensioned fuel monolith stack mandrel assembly. [Figure 12B]This is a perspective cross-sectional view showing the arrangement of tension components in different parts of a tensioned fuel monolith stack mandrel assembly. [Figure 12C] This is a perspective cross-sectional view showing the arrangement of tension components in different parts of a tensioned fuel monolith stack mandrel assembly. [Figure 13] This is a cross-sectional view of an alternative embodiment of a tensioned fuel monolith stack mandrel assembly. [Figure 14A] Figure 13 is an enlarged view of area P5', showing a cross-sectional view of the mandrel tensioning cap and mandrel spacer at the first end of an alternative embodiment of a tensioned fuel monolith stack mandrel assembly. [Figure 14B] Figure 13 is an enlarged view of area P9', showing a cross-sectional view of the mandrel tensioning cap and mandrel spacer at the second end of an alternative embodiment of a tensioned fuel monolith stack mandrel assembly. [Figure 14C] A perspective view of the first end of an alternative embodiment of a tensioned fuel monolith stack mandrel assembly at various points in time during assembly. [Figure 14D] A perspective view of the first end of an alternative embodiment of a tensioned fuel monolith stack mandrel assembly at various points in time during assembly. [Figure 14E] A perspective view of the first end of an alternative embodiment of a tensioned fuel monolith stack mandrel assembly at various points in time during assembly. [Figure 14F] A perspective view of the first end of an alternative embodiment of a tensioned fuel monolith stack mandrel assembly at various points in time during assembly. [Figure 14G] This is a perspective view of the second end of an alternative embodiment of a tensioned fuel monolith stack mandrel assembly after assembly. [Figure 14H]This is a perspective view of the second end of an alternative embodiment of a tensioned fuel monolith stack mandrel assembly after the addition of tensioning cables. [Figure 15] This is a flow diagram illustrating various steps in an embodiment of a method for preparing a tensioned fuel monolith stack mandrel assembly. [Figure 16] This is a schematic cross-sectional view of a twin-bore hydraulic tensioner system. [Figure 17] This image shows an embodiment of wrapping an insulating layer over a tensioned fuel monolith stack mandrel assembly. [Figure 18] This image shows a perspective view of an embodiment of an insulated fuel assembly core. [Figure 19] This image shows a diagram of an embodiment of an insulated fuel assembly core along its longitudinal axis. [Figure 20A] This figure is a cross-sectional side view of an embodiment of a nuclear-powered nuclear fission reactor structure inside a container, which comprises a fuel assembly as disclosed herein. [Figure 20B] This is an enlarged view of area P10 in Figure 20A, showing a cross-sectional view of the inlet end of the fuel assembly. [Figure 20C] This is an enlarged view of area P11 in Figure 20A, showing a cross-sectional view of the exhaust end of the fuel assembly. [Figure 21] This figure is a cross-sectional top view of an embodiment of a nuclear-powered nuclear fission reactor structure inside a container, which comprises a fuel assembly as disclosed herein. [Figure 22A] This is a partial cross-sectional view of an embodiment of a nuclear thermal propulsion engine incorporating a nuclear propulsion fission reactor structure as disclosed herein. [Figure 22B] Figure 22A is an exploded view of the reactor subsystem of the nuclear thermal propulsion engine. [Modes for carrying out the invention]

[0015] In some cases, the dimensions of each component have been appropriately adjusted for clarity. For ease of viewing, in some cases, only some of the features that are named in the diagram are labeled with reference numbers.

[0016] Figure 1A is a perspective view of an embodiment of the fuel monolith 10, and Figure 1B is a view of the end surface of the fuel monolith 10. The fuel monolith 10 has a body portion 20 including a first end surface 22, a second end surface 24, and a side surface 26 connecting the first end surface 22 to the second end surface 24. The fuel monolith 10 has a plurality of channels 30 that extend through the body portion 10 from an inlet opening in the first end surface 22 to an outlet opening in the second end surface 24. The plurality of channels 30 provide a passage for propellant gas (e.g., hydrogen) to flow through the fuel monolith 10 during operation. The flow of propellant gas through the entire plurality of channels 30 also cools the fuel monolith 10.

[0017] In some embodiments, the multiple channels 30 are arranged symmetrically with respect to the longitudinal axis 40 of the main body 20 of the fuel monolith 10. For example, the multiple channels 30 can be arranged in a group of concentric rings, comprising a central channel 30', an inner ring 52 of channel 30'', an intermediate ring 54 of channel 30'''', and an outer ring 56 of channel 30''''. The channels of any one ring can be distributed uniformly or non-uniformly in the circumferential direction. The concentric rings can be distributed uniformly or non-uniformly in the radial direction. In some embodiments, the axis of each of the multiple channels 30 is parallel to the longitudinal axis 40. In other embodiments, the axis of a first portion of the multiple channels 30 is parallel to the longitudinal axis 40, and the axis of a second portion of the multiple channels 30 is oblique to the longitudinal axis 40. For example, the axis of the central channel 30', and one or more of the axes of (a) the multiple channels 30'' in the inner ring 52 and (b) the multiple channels 30'''' in the intermediate ring 54 can be parallel to the longitudinal axis 40, and the axes of the multiple channels 30'''' in the outer ring 56 can be oblique to the longitudinal axis 40.

[0018] In a cross-section perpendicular to the longitudinal axis 40, the main body 20 of the fuel monolith 10 has an elliptical shape having a first diameter D1 different from a second diameter D2 (where the first diameter is perpendicular to the second diameter), resulting in the main body 20 of the fuel monolith 10 having an eccentric cylindrical shape (i.e., an eccentric right cylinder of height H1 with an elliptical end surface). As an ellipse, the first diameter D1 is the major axis of the ellipse, and the second diameter D2 is the minor axis of the ellipse. In exemplary embodiments, the difference in length between the first diameter D1 and the second diameter D2 is greater than 0.5 mm and up to 5 mm, and alternatively, the difference in length is greater than 1.0 mm or greater than 2.5 mm, and less than 2.0 mm or less than 2.5 mm or less than 4.0 mm. In yet another alternative example, the length difference is approximately 2.0 mm (e.g., 1.8 ± 0.1 mm, 1.6 ± 0.1 mm, 1.4 ± 0.1 mm, or 1.2 ± 0.1 mm), or approximately 2.0 mm to approximately 5.0 mm (e.g., 4.5 ± 0.1 mm, 4.0 ± 0.1 mm, 3.5 ± 0.1 mm, 3.0 ± 0.1 mm, or 2.5 ± 0.1 mm). In an exemplary embodiment, the eccentricity of the body portion 20 of the fuel monolith 10 is greater than zero and up to 0.6, alternatively greater than zero and up to 0.4, or greater than zero and up to 0.15, for example, within the range of 0.01, 0.02, 0.05, or 0.10 to 0.6, 0.5, 0.4, 0.3, 0.2, 0.15, 0.10, or 0.05.

[0019] The eccentricity of the main body 20 of the fuel monolith 10 facilitates the alignment of the multiple channels 30 within the fuel monolith when the fuel monoliths are stacked together to form a fuel assembly core. In some embodiments, the first diameter D1 is 27 mm and the second diameter D2 is 26 mm, resulting in a 1.0 mm locking feature. In other embodiments, the first diameter D1 is 29.9 mm and the second diameter D2 is 28.9 mm, resulting in a 1.0 mm locking feature. The locking feature functions to maintain the alignment of the channels 30 across the multiple fuel monoliths 10 without the need for other mechanical locking features (e.g., alignment tubes, trapped spheres in pockets, keyways, or other geometric locking means). The first diameter D1 and the second diameter D2 can have other sizes, and their difference D1-D2 is the size of the locking feature. As the difference in diameters increases, the strength of the locking feature increases. However, when the difference in diameter decreases to less than 1 mm, the alignment of the channels 30 between adjacent fuel monoliths 10 becomes difficult, resulting in the need for supplemental locking features (e.g., other mechanical locking features). Examples of appropriate sizes for locking features range from 1.0 mm to 4.0 mm.

[0020] In an optional embodiment, in a cross-section perpendicular to the longitudinal axis 40, the main body 20 of the fuel monolith 10 has a circular shape, and the fuel monolith 10 has a straight cylindrical shape and zero eccentricity.

[0021] In exemplary embodiments, the fuel monolith 10 has a composition containing fissile fuel components. One example of a fissile fuel component is a uranium-based material (e.g., ZrUC or UC-ZrC-NbC). Other uranium-based materials include UO2, UO2 and W cermets, UN and W cermets, and UO2 and Mo cermets. In exemplary embodiments, the composition is a solid solution containing fissile fuel components. In one example, the composition of the fuel monolith 10 contains uranium having a U-235 assay between 5 percent and 20 percent. Other compositions that can be processed by injection molding techniques to form the fuel monolith 10 are also possible.

[0022] Figure 1C is a perspective view of another embodiment of the fuel monolith 10', and Figure 1D is a view of the end surface of the fuel monolith 10'. The fuel monolith 10' and fuel monolith 10 are substantially the same, and disclosures relating to fuel monolith 10 are applicable to fuel monolith 10'. Furthermore, similar features are labeled with similar reference numbers. One difference between fuel monolith 10' and fuel monolith 10 is the number and arrangement of the channels 30, with fuel monolith 10' having more channels 30'' in the outer ring 56 than fuel monolith 10. Another difference between fuel monolith 10' and fuel monolith 10 is the angular rotation of the channels 30 in the continuous ring and the resulting radial alignment. Another difference between fuel monolith 10' and fuel monolith 10 is that instead of a single through-hole size, the coolant hole size is varied, which enhances the cooling of the fuel monolith on the outer surface 26. As will be further used herein, fuel monolith 10 and fuel monolith 10' are interchangeable, and references to fuel monolith 10 can be substituted by references to fuel monolith 10'.

[0023] Figure 2A is a perspective view of an embodiment of the insulated fuel assembly core 100, and Figure 2B is a cross-sectional view of the insulated fuel assembly core 100 shown in Figure 2A, taken along X1-X1, where X1-X1 coincides with the longitudinal axis 102 of the insulated fuel assembly core 100. The insulated fuel assembly core 100 extends from a first end 104 to a second end 106. The first end 104 is the inlet end for propellant gas during operation, and the second end 106 is the outlet end for propellant gas during operation. In the insulated fuel assembly core 100, a plurality of fuel monoliths 10 are stacked with their end faces together to form the fuel assembly core 110. The insulated fuel assembly core 100 further includes one or more outer insulating layers 120, an exhaust support plate 130, and an exhaust shield assembly 150.

[0024] Figure 2C is an enlarged view of area P1 in Figure 2B, showing a cross-section of the inlet end of the insulated fuel assembly core 100, and Figure 2D is an enlarged view of area P2 in Figure 2B, showing a cross-section of a portion of the exhaust end of the insulated fuel assembly core 100. In both Figures 2C and 2D, several fuel monoliths 10 are shown stacked, with their end faces abutting each other at their respective interfaces 112, and the channels 30 aligned along the axial length of the fuel assembly core 110. The insulator 120 is located radially outside the fuel assembly core 110 and includes an inner insulating layer 120a and an outer insulating layer 120b. At the second end 106, the exhaust support plate 130 is stacked on top of the fuel assembly core 110, with the end face of the exhaust support plate 130 in contact with the end face of the adjacent fuel monolith 10 at interface 114, and the exhaust shield assembly 150 is fitted to the exhaust support plate 130 (see area A).

[0025] The exhaust support plate 130 includes a plurality of channels 132 extending through the body of the exhaust support plate 130 from an inlet opening 134 at a first end face 136 to an outlet opening 138 at a second end face 140. The plurality of channels 132 provide a passage for propellant gas (e.g., hydrogen) to flow through the exhaust support plate 130 during operation. The inlet openings 134 of the plurality of channels 132 are aligned with the plurality of channels 30 in the fuel monolith 10 of the fuel assembly core 110 and can be arranged similarly to the arrangement of the plurality of channels 30 in the fuel monolith 10, for example, having the same symmetrical arrangement and / or being arranged in a group of concentric rings with uniform or non-uniform circumferential and / or radial distributions. The first end surface 136 of the exhaust support plate 130 is planar to facilitate contact with the end surface of the adjacent fuel monolith 10, and the second end surface 140 of the exhaust support plate 130 is concave to facilitate the direction of the propellant gas flow exiting from the outlet opening 138.

[0026] The exhaust support plate 130 is formed from a material having good corrosion resistance to the exhausted propellant gas. Resistance to both chemical corrosion and wear corrosion is preferred. Exemplary materials for the exhaust support plate 130 are tungsten, tungsten-based alloys, or W-Re alloys (e.g., having an Re content between 3 and 50 wt%), or carbide-based materials (e.g., ZrC). In some exemplary materials for the exhaust support plate 130, the material can be dispersion-strengthened with carbides and oxides (e.g., HfC, ThO2, and / or La2O3).

[0027] In an exemplary embodiment, the exhaust shield assembly 150 includes a truncated cone-shaped section 152 and a tubular section 154. In an exemplary embodiment, the truncated cone-shaped section 152 of the exhaust shield assembly 150 transitions from an elliptical shape at a first end (designed to fit the elliptical shape of the eccentric cylindrical shape of the fuel monolith 10 and the exhaust support plate 130) to a circular shape at a second end, where the truncated cone-shaped section 152 is attached to the tubular section 154.

[0028] Region A in Figure 2D shows a conical section 152 of the exhaust shield assembly 150 fitted into the exhaust support plate 130. The circumferential surface 142 of the exhaust support plate 130 is tapered, and the inner diameter surface of the conical section 152 of the exhaust shield assembly 150 is correspondingly tapered, providing a contact area between the circumferential surface 142 and the inner diameter surface of the conical section 152. In region A, the wall thickness is tapered so that the outer diameter surface of the end portion 156 of the exhaust shield assembly 150 is aligned with the circumferential surface of the fuel assembly core 110 (for example, at the same radial distance as the circumferential surface of the fuel assembly core 110). This feature results in the size of the outer diameter surface of the fuel assembly core 110 being maintained beyond the interface 114, which makes it possible to reduce potential stress points in the insulator 120 at that location.

[0029] The exhaust shield assembly 150 is formed from a material having good corrosion resistance to the exhausted propellant gas. Resistance to both chemical corrosion and wear corrosion is preferred. Exemplary materials for the exhaust shield assembly 150 are tungsten, tungsten-based alloys, or W-Re alloys (e.g., having an Re content between 3 and 50 wt%), or carbide-based materials (e.g., ZrC). In some exemplary materials for the exhaust shield assembly 150, the material can be dispersion-strengthened with carbides and oxides (e.g., HfC, ThO2, and / or La2O3).

[0030] In an alternative embodiment, the transition of the fuel monolith 10 from an eccentric cylindrical shape to a circular shape can occur within the exhaust support plate 130, where the first end face 136 of the exhaust support plate 130 has an elliptical shape corresponding to the eccentric cylindrical shape of the fuel monolith 10, and the second end face 140 of the exhaust support plate 130 is circular.

[0031] Figure 3A is a perspective view of an embodiment of the fuel assembly 200, and Figure 3B is a cross-sectional view of the fuel assembly 200 shown in Figure 3A, taken along X2-X2, where X2-X2 coincides with the longitudinal axis 202 of the fuel assembly 200. The fuel assembly 200 includes a fuel assembly core 100 insulated within a fuel assembly outer structure 210, extending from a first end 206 to a second end 208. The first end 206 is the inlet end for propellant gas during operation, and the second end 208 is the outlet end for propellant gas during operation.

[0032] Figure 3C is an enlarged view of area P3 in Figure 3B, showing a cross-section of the first end 206 of the fuel assembly 200. At the first end 206, the fuel assembly 200 includes an inlet connection assembly 220. The inlet connection assembly 220 includes an orifice plate 222, a flow stabilizer 224, an inlet housing 226, a biasing element 228, and a seal 230. The inlet connection assembly 220 is fitted to the fuel assembly outer structure 210, for example, by a crimp 232. During operation, propellant gas enters the inlet connection assembly 220 through the opening 234, passes through the flow stabilizer 224 and the orifice plate 222, and enters the insulated fuel assembly core 100. The channels 238 in the flow stabilizer 224 and the openings 240 in the orifice plate 222 are aligned with the channels 30 in the fuel monolith 10 within the insulated fuel assembly core 100. In the upper volume 236 of the inlet housing 226, the propellant gas flow is turbulent, and the channels 238 in the flow stabilizer 224 and the openings 240 in the orifice plate 222 reduce the turbulent flow of the propellant gas before it enters the insulated fuel assembly core 100. A biasing element 228 (e.g., a spring) accommodates changes in the axial dimensions of the insulated fuel assembly core 100 (e.g., from thermal expansion). The biasing element 228 is positioned in a slot within the inlet housing 226, and the body of the inlet housing 226 shields the biasing element 228 from the propellant gas flowing through the inlet connection assembly 220.

[0033] Figure 3D is an enlarged view of area P4 in Figure 3B, showing a cross-sectional view of a portion of the second end 208 of the fuel assembly 200. The structure and features of the portion of the second end 208 of the fuel assembly 200 shown in Figure 3D are the same as those shown and described in relation to the exhaust end portion of the insulated fuel assembly core 100 shown in Figure 2D, with the addition of a fuel assembly outer structure 210 outside the insulator 120.

[0034] It should be noted that the truncated cone-shaped section 152 and the tubular section 154 can be manufactured as separate pieces and joined together, for example, by welding. However, welding may leave a seam joint between the two pieces. Alternatively, the truncated cone-shaped section 152 and the tubular section 154 can be manufactured as a single unit, for example, by additive manufacturing or machining (e.g., by sinker electrical discharge machining (EDM) from a billet), so that there is no welded seam joint between the two pieces.

[0035] Figure 4 is a cross-section of a portion of the exhaust end 206 of the fuel assembly 200, annotated to illustrate the flow 300 through the channel 132 of the exhaust support plate 130. As further described herein, the axis of the channel 132 in the peripheral region of the exhaust support plate 130 is angled with respect to the longitudinal axis 202 of the fuel assembly 200, and the flow 300 through these angled channels 132 is also angled with respect to the longitudinal axis 202 of the fuel assembly 200, and is angled sufficiently to avoid the flow impacting the inner surface of the exhaust shield assembly 150, particularly in the truncated conical section 152. In exemplary embodiments, the flow 300 through the angled channels 132 is parallel to the inner surface of the truncated conical section 152. Furthermore, as further described herein, the axis of the channel 132 in the central region of the exhaust support plate 130 is parallel to the longitudinal axis 202 of the fuel assembly 200, and the flow 300 through these non-angled channels is also parallel to the longitudinal axis 202 of the fuel assembly 200.

[0036] Figure 5 is a perspective view of an embodiment of the exhaust support plate 130, and Figures 6A to 6C are an end view (Figure 6A), a first section view (Figure 6B), and a second section view (Figure 6C) of the exhaust support plate 130 shown in Figure 5, illustrating various features disclosed herein. Angled and non-angled channels are observable in Figures 6A to 6C. For example, in Figure 6A, the channel 132 in the central region 144 has an axis parallel to the longitudinal axis 202 of the fuel assembly 200, and in the plan view of the second end face 140, the inlet opening 134 and the outlet opening 138 are aligned and can be viewed straight through these parallel channels 132 when viewed along the longitudinal axis 202. In contrast, the channels 132 in the peripheral region (i.e., radially outward from the central region 144) have axes angled with respect to the longitudinal axis 202 of the fuel assembly 200, and in the plan view of the second end face 140, the inlet opening 134 and the outlet opening 138 are not aligned, and the inner surfaces of these non-parallel channels 132 can be seen when viewed in the direction along the longitudinal axis 202.

[0037] Figure 6B is a cross-sectional view of the exhaust support plate 130 shown in Figure 6A, taken at X3-X3, where X3-X3 coincides with the first diameter of the exhaust support plate 130 (for example, the minor diameter in the case where the exhaust support plate 130 has an elliptical shape to correspond to the eccentric cylindrical shape of the fuel monolith 10). The second end face 140 is concave, and its surface is above a radius R1. The cross-section in Figure 6B includes three channels 132. The axis of the central channel is positioned to coincide with the longitudinal axis 102 of the insulated fuel assembly core 100, and the channels are parallel. Two channels are located outward from the central channel and each has an axis oriented at an angle α with respect to the longitudinal axis 202.

[0038] Figure 6C is a cross-sectional view of the exhaust support plate 130 shown in Figure 6A, taken at X4-X4, where X4-X4 coincides with the second diameter of the exhaust support plate 130 (for example, the major diameter in the case where the exhaust support plate 130 has an elliptical shape to correspond to the eccentric cylindrical shape of the fuel monolith 10). The first diameter along X3-X3 is perpendicular to the second diameter along X4-X4. The cross-section in Figure 6C includes seven channels 132. The three central channels include a central channel positioned so that its axis coincides with the longitudinal axis 102 of the insulated fuel assembly core 100, and two channels outside the central channel whose axes are parallel to the longitudinal axis 102. Each of these three channels is a parallel channel. Outward from the parallel channels are non-parallel channels. On each side of the parallel channels, there are two non-parallel channels. In each of the two groupings, the first non-parallel channel has an axis oriented at an angle θ with respect to the longitudinal axis 202, and the second non-parallel channel has an axis oriented at an angle β with respect to the longitudinal axis 202. The first non-parallel channel is radially inward of the second non-parallel channel.

[0039] In exemplary embodiments, R1 is in the range of 10 mm to 50 mm (wherein the second end face 140 can optionally be planar without a radius), angle α is in the range of 0 to 20 degrees (e.g., 4 to 5 degrees, or 8 to 10 degrees), angle θ is in the range of 0 to 20 degrees (e.g., 4 to 6 degrees, or 8 to 12 degrees), angle β is in the range of 0 to 20 degrees (e.g., 6 to 10 degrees, or 10 to 15 degrees), and H2 (height of exhaust support plate 130) is in the range of 5 mm to 50 mm (e.g., 12 mm to 20 mm). In some embodiments, α < θ < β, for example, α = 4.6 degrees, θ = 5.4 degrees, and β = 8 degrees.

[0040] The central region 144 is shown in Figures 6A to 6C as constituting the central channel and the first ring of the channel, and the peripheral region is shown as constituting the second ring and the third ring of the channel. However, the boundary between the central and peripheral regions, and which channels or rings of channels are located within each region, can vary based on the design of the insulated fuel assembly core.

[0041] Figures 6D–6L include end views (Figures 6D and 6I) and various cross-sectional views (Figures 6E–6H and 6J–6L) of alternative embodiments of the exhaust support plate 130'. The alternative embodiments of the exhaust support plate 130' are substantially similar to the exhaust support plate 130 shown and described herein, and disclosures relating to the exhaust support plate 130 are applicable to the exhaust support plate 130'. Furthermore, similar features are labeled with similar reference numerals. Differences between the exhaust support plate 130 and the alternative embodiments of the exhaust support plate 130' include the number and arrangement of the channels 132' (both radial and circumferential) and the longitudinal orientation of the channels 132' (i.e., the angle of the centerline of the channel 132' with respect to the centerline of the central channel 132').

[0042] Details of the location and orientation of channel 132' are shown in the various cross-sectional views corresponding to the annotated sections, as follows: For the sections in Figures 6D to 6L, Figure 6E corresponds to section BB, Figure 6F corresponds to section CC, Figure 6G corresponds to section DD, and Figure 6H corresponds to section EE. For the section in Figure 6I, Figure 6J corresponds to section FF, Figure 6K corresponds to section GG, and Figure 6L corresponds to section HH.

[0043] Generally, the various angles shown and labeled in Figures 6D through 6L are described in Tables 1 and 2 below.

[0044] [Table 1] [Table 2] These angles orient the channels 132, 132' so that they are essentially parallel to the plane of the elliptical loft 142 (e.g., within ±5 degrees parallel, or alternatively, within ±1 degree parallel). These angles ensure that the openings of the channels 132, 132' on the second end face 140 (i.e., the surface formed by R1) do not intersect each other. The position of the channel inlet opening 134 is such that the inlet opening 134 fits snugly with the channel 30 in the fuel monolith 10.

[0045] The exhaust shield assembly 150 can be formed from separate parts that are joined together, or it can be formed as a single unit. For example, the truncated cone-shaped section 152 and the tubular section 154 can be formed separately and joined together, for example, by resistance welding. Alternatively, the single unit exhaust shield assembly 150' can have the truncated cone-shaped section 152' and the tubular section 154' formed as a single unit, which eliminates the need for a welded joint.

[0046] Figure 7 is a perspective view of an embodiment of a truncated cone-shaped section 152 of the exhaust shield assembly 150, showing various features disclosed herein, and Figures 8A–8C are an end view (Figure 8A), a side view (Figure 8B), and a cross-sectional view (Figure 8C) of the truncated cone-shaped section 152 shown in Figure 7. In the case where the first end of the truncated cone-shaped section 152 is attached to the exhaust support plate 130, the exhaust support plate 130 has a corresponding elliptical shape having a first diameter (D3) smaller than a second diameter (D4). The second end of the truncated cone-shaped section 152 has a third diameter D5, where diameter D5 is smaller than diameter D3. The exhaust shield assembly 150 has an overall length L1 and two longitudinal sections (a first longitudinal section 158 and a second longitudinal section 164), the first longitudinal section 158 having an outer surface angled with respect to the longitudinal centerline 162, a constant thickness T, and a length L2, and the second longitudinal section 164 having an outer surface parallel to the longitudinal centerline 160, a tapered thickness, and a length L3. Length L3 is smaller than length L2.

[0047] Figures 8D to 8E are perspective views (Figure 8D) and longitudinal sections (Figure 8E) of an embodiment of a tubular section 154. In this embodiment, the tubular section 154 has a constant inner and outer diameter along its entire length. Figure 8F is an enlarged view of area P12 in Figure 8E, showing a cross-section of the end surface at the inlet end of the tubular section 154. The end surface 172 joins the inner surface 174 and outer surface 176 of the tubular section 154 and includes tapered surfaces 182, 184. The tapered surfaces 182, 184 are angled relative to each other at an angle λ, and the apex of the angle forms a ridge 186 that extends along the end surface 172 of the integrated exhaust shield assembly 150''. The angle λ can be, for example, 20 to 30 degrees. The raised portion 186 and tapered surfaces 182, 184 provide the geometric shape for a resistance projection welding joint when assembling the tubular section 154 into the truncated conical section 152.

[0048] An alternative embodiment of the integrated exhaust shield assembly is a single-piece structure. Figures 9A and 9B illustrate an embodiment of the integrated exhaust shield assembly 150'. The integrated exhaust shield assembly 150' is shown in a perspective view (Figure 9A) and a longitudinal section view (Figure 9B). The integrated exhaust shield assembly 150' includes a conical section 152' and a tubular section 154', and is manufactured as a single part, for example, by additive manufacturing. The features of the conical section 152' are substantially the same as those of the exhaust shield assembly 150 shown and described herein, and disclosures relating to the exhaust shield assembly 150 are applicable to the conical section 152' of the integrated exhaust shield assembly 150'. Furthermore, similar features are labeled with similar reference numerals.

[0049] Figure 9C shows a further alternative embodiment of the integrated exhaust shield assembly 150'', in which the exhaust support plate 130'' is further incorporated as part of the integrated structure of the integrated exhaust shield assembly 150''.

[0050] Figure 10 is a flow diagram illustrating various steps in an embodiment of a method for manufacturing an insulated fuel assembly core and a fuel assembly. The method S100 for manufacturing an insulated fuel assembly core 100 includes the steps of: preparing a fuel monolith 10 in step S110; assembling the fuel monolith 10 in step S120; attaching auxiliary components (e.g., exhaust support plate 130 and exhaust shield assembly 150, etc.); preparing a tensioned fuel monolith stack mandrel assembly in step S140; applying an insulating layer 120 in step S150; and disassembling components used in the mandrel assembly (e.g., mandrel winding component and tensioning component, etc.). The insulated fuel assembly core 100 can be manufactured by positioning the insulated fuel assembly core 100 within the fuel assembly outer structure 210 in step S170. Additional finishing processes of the fuel assembly 200, such as abrasive polishing of the external surface, can be performed as needed, which can promote bonding of the insulating layer 120.

[0051] Step S110 prepares the fuel monolith 10, which may include the manufacturing and post-manufacturing of the fuel monolith 10, or may include only the post-manufacturing of a pre-manufactured fuel monolith 10. The manufacturing of the fuel monolith 10 can be carried out by various techniques. In an exemplary embodiment, the fuel monolith 10 is formed by an injection molding technique (e.g., ceramic injection molding), followed by degreasing (e.g., a heat-based or chemical-based degreasing technique) and sintering. Ceramic injection molding is very precise and repeatable, resulting in the dimensions and shape of the fuel monolith 10 being strictly controlled and substantially the same between production processes (within manufacturing tolerances) (in particular the side surfaces 26 and the eccentricity of the body portion 20 of the fuel monolith 10). The first end surface 22 and the second end surface 24 are also manufactured to be flat and parallel.

[0052] If necessary, the manufactured fuel monolith 10 can undergo post-manufacturing processing. For example, the surface of the manufactured fuel monolith 10 can be post-manufacturing processed, for example, by grinding to the final dimensions (e.g., by double-sided grinding and lapping techniques). Also, if necessary, the channels 30 of the manufactured fuel monolith 10 can be post-manufacturing processed to form the channels 30 or to finish the channels 30 to the final shape, size, and / or configuration. Other post-manufacturing processing, such as wet polishing techniques, can be used to provide a smooth surface finish or coating (e.g., Nb2C or ZrC, depending on the fuel type).

[0053] Step S120 involves assembling a plurality of fuel monoliths 10. In an exemplary embodiment, the plurality of fuel monoliths 10 are stacked with their end faces facing each other. The eccentric shape of the fuel monoliths 10 provides guides for aligning the channels 30 of the stacked fuel monoliths 10. Optionally, additional guides (e.g., drill rods 480) can be inserted into one or more channels 30 (typically into parallel channels) and used to align the stacked fuel monoliths 10. Preferably, at least two drill rods 480 and up to six or seven drill rods 480 (depending on the number of parallel channels allowed) are used as additional guides. For alignment accuracy, the clearance of the drill rods 480 relative to the internal diameter of the channel 30 is as small as possible, for example, 25 to 75 microns.

[0054] Step S130 involves attaching the exhaust support plate 130 and the exhaust shield assembly 150 to the assembled fuel monoliths 10 to form the fuel assembly core 110. The eccentric shape of the exhaust support plate 130 provides a guide for aligning the channels 132 of the exhaust support plate 130 with the channels 30 of the stacked fuel monoliths 10. Optionally, additional guides (e.g., drill rods) can be used to facilitate such alignment. The exhaust shield assembly 150 is then attached to the exhaust support plate 130. This is done, for example, by fitting the end portion 156 of the conical section 152 of the exhaust shield assembly 150 onto the circumferential surface 142 of the exhaust support plate 130.

[0055] The exhaust support plate 130, as well as the truncated conical section 152 and tubular section 154 forming the exhaust shield assembly 150, can be formed as separate pieces or as a single integrated structure. The separate pieces can be joined by welding (e.g., resistance welding) or by mechanical joints. When using resistance welding, adding rhenium to the alloy of the materials to be welded assists the resistance welding process. Manufacturing techniques for forming the exhaust support plate 130, as well as the truncated conical section 152 and tubular section 154 forming the exhaust shield assembly 150, as separate pieces include 3D wire electrical discharge machining (3D wire EDM) and EBEAM printing calibrated for tungsten production (e.g., Arcam Spectra H electron beam melting system). EBEAM printing produces a roughened surface finish on the outer surface of the final part, which may be advantageous in the subsequent application of the insulating layer 120. When manufactured as separate parts, the joining surfaces have tight tolerances to facilitate the mating of the separate pieces. For example, 3D wire EDM manufacturing can be used with true profile tolerances (e.g., 0.1 mm with respect to the internal bore and 0.03 mm with respect to the outside of the elliptical loft area). Manufacturing techniques for forming the truncated conical section 152 and tubular section 154 (or subsets of these pieces) for forming the exhaust support plate 130, as well as the exhaust shield assembly 150, as a single integrated structure include additive manufacturing techniques. The advantage of additive manufacturing these pieces is the elimination of joints. Eliminating joints in these pieces reduces the possibility of errors during manufacturing, allows pieces to be manufactured faster, and increases the strength of the structure. However, integrated manufacturing (in particular by additive manufacturing) affects, for example, the surface roughness of the flow channel 132, which affects the flow 300 through the flow channel 132.Post-manufacturing processes (such as grinding or wet polishing techniques to provide a smooth surface finish, or high-temperature coating) can be used to reduce the as-manufactured surface roughness of the flow channel 132 (similar to the post-manufacturing processes of channel 30).

[0056] Step S140 prepares the tensioned fuel monolith stack mandrel assembly 400. The structures and processes for preparing the tensioned fuel monolith stack mandrel assembly 400 are discussed further herein, in particular in relation to Figures 10A–10G, 11A–11C, and 12.

[0057] Step S150 involves applying an insulating layer 120 to the tensioned fuel monolith stack mandrel assembly 400. The content and location of the insulating layer 120 can be varied based on thermal requirements. For example, the insulating layer on the radially innermost part (as opposed to the radially outermost insulating layer), and the insulating layer on the fuel assembly 200 at the second end 208 (as opposed to the first end 206) are exposed to the highest temperatures and are most susceptible to hydrogen corrosion. Expected operating conditions include an inlet hydrogen flow temperature of approximately 300K during full power thrust, a mid-range fuel assembly temperature of approximately 1,300K during full power thrust, and an exhaust hydrogen flow temperature of approximately 2750K during full power thrust. In high ISP embodiments, the exhaust hydrogen flow temperature can be 3000K. Therefore, geometric and material selections are made to provide the best resistance at these locations to the expected operating conditions.

[0058] As can be seen in Figures 2B to 2D, the insulating layer 120 has multiple layers. In an exemplary embodiment, the insulating layer 120 has two layers (an inner insulating layer 120a and an outer insulating layer 120b). In an alternative embodiment, the insulating layer 120 can have three or more layers, including, for example, one or more intermediate layers between the inner insulating layer 120a and the outer insulating layer 120b. The intermediate layers may or may not be insulating layers (for example, structural layers that provide composite reinforcement and compressive forces).

[0059] The inner insulating layer 120a is a flexible and compressible layer that is hydrogen-resistant and has low thermal conductivity. In some embodiments, the inner insulating layer 120a is made from random-orientation matting of ZrC fibers. The random-orientation matting is roll-wrapped around the tensioned fuel monolith stack mandrel assembly 400. The random-orientation matting is tightly wrapped, but it does not need to provide structural support. The main role of the inner insulating layer 120a is to provide high-temperature resistance to high-temperature hydrogen and to provide thermal shielding to other insulating layers and the fuel assembly outer structure 210. In exemplary embodiments, the inner insulating layer 120a is 1 mm thick, while in alternative embodiments, the inner insulating layer 120a is 2 mm or thicker. The fiber density of the random-orientation matting of ZrC can be 40-60%, with 60-40% void spaces, respectively. Generally, larger void spaces are desired to reduce through-layer radial thermal conductivity and therefore to increase the performance of the inner insulating layer 120a and the overall insulating layer 120.

[0060] In alternative embodiments, the inner insulating layer 120a is made from ZrO2, SiC, carbon fibers, carbon fibers with surface conversion to ZrC, or carbon fibers completely converted to ZrC, and the inner layer is formed either by using tow strands and mandrel wrapping or by using woven material and roll wrapping. Chemical compatibility with hydrogen and high temperatures is a major factor to consider when selecting the insulating material type, format (tow-to-felt-to-woven), and application method (mandrel wrapping-to-roll wrapping).

[0061] In a further alternative embodiment, the inner insulating layer 120a can be, for example, a thin tungsten spiral-wrapped foil having a thickness of 25 to 50 microns. This tungsten layer can be used to seal the hydrogen channel, providing high resistance to the flowing hydrogen escaping from the fuel assembly core 110.

[0062] Another exemplary material suitable for the inner insulating layer 120a is to manufacture the inner insulating layer 120a separately and then apply it to the fuel assembly core 100. In this regard, carbon fibers can be basket-woven on a solid zirconia oxide mandrel, impregnated with phenols, and fired at 1300°C to form a low-temperature C / C composite. The C / C composite is removed from the mandrel and its surface is converted to ZrC. Surface conversion to ZrC provides improved survival in the hydrogen environment at the high-temperature end (approximately 2700K) of the fuel assembly 200 and also provides improved performance of the fuel assembly 200. Therefore, in some embodiments, surface conversion to ZrC can be implemented in the portion of the fuel assembly 200 toward the outlet end, but not in the lower-temperature inlet end of the fuel assembly 200. The resulting composite can then be laminated on top of the fuel assembly core 100 as the inner insulator 120a.

[0063] The inner insulating layer 120a is typically applied dry without a binder. This reduces the thermal conductivity of the inner insulating layer 120a. The dry inner insulating layer 120a is held in place by subsequent layers of insulating layer 120.

[0064] The inner insulating layer 120a can use various geometric wrapping patterns (e.g., helical wrapping patterns or basket weave helical patterns). Basket weave helical patterns are used to reduce fiber density and therefore reduce radial thermal conductivity in both the inner insulating layer 120a and the other layers of insulating layer 120. Basket weave patterns are formed by increasing the fiber band width in the CNC mandrel wrapping control software beyond the actual fiber width when applied to the mandrel. By increasing this fiber band width, it is possible to achieve an open weave with high porosity. For example, a basket weave pattern can have 2 mm thick fibers oriented at 45 degrees, with approximately 2 mm of space between each tow.

[0065] The inner insulating layer 120a can optionally be held in place by grip features, which consist of a roughened surface finish or other mechanical features (e.g., spike-like protrusions) on the outer circumferential surface of the fuel assembly core 110. Applying other layers of the insulating layer 120 (e.g., outer insulating layer 120b) compresses the inner insulating layer 120a to engage with the grip features. The grip features can be positioned in separate regions along the length of the fuel assembly core 110 to prevent movement of the inner insulating layer 120a relative to the fuel assembly core 110 during thermal cycles. The grip features can also be applied to the outer circumferential surfaces of the exhaust support plate 130 and / or exhaust shield assembly 150.

[0066] The outer insulating layer 120b is produced by mandrel wrapping using a twin-tow machine and is fabricated using carbon fibers. In an alternative embodiment, the outer insulating layer 120b is fabricated using SiC fiber or ZrO2 fiber tows. Wrapping using twin tows (No. 56) may be advantageous over wrapping with a single tow, because it has been observed that the tension of a single fiber causes deflection within the insulated fuel core assembly 100. Twin-tow wrapping orients the two tows 180 degrees apart, which prevents this deflection.

[0067] The outer insulating layer 120b can consist of multiple layers (e.g., 1 to 20 sublayers) depending on the type of fiber used for insulating material winding and the overall thickness of the insulating material. In an exemplary embodiment, the inner sublayers (e.g., sublayers 1 to 3) are applied dry without a binder to reduce thermal conductivity. The dry sublayers are held in place by subsequent sublayers of the outer insulating layer 120b. The outermost sublayers (e.g., the outermost sublayers 3 to 5) are applied wet using a phenolic resin (Bakelite Synthetics' BK 5236M phenol-impregnated resin, or similar). The low viscosity of this phenolic resin is desirable for the wet lamination stage of mandrel winding. The ideal viscosity of the phenolic resin is in the range of 300 to 700 Cps.

[0068] The insulating layer 120 (for example, the sublayers of the inner insulating layer 120a and the outer insulating layer 120b) is applied over the axial length of the tensioned fuel monolith stack mandrel assembly 400, which is greater than the length of the fuel assembly core 100. This is done so that any changes in the tow winding (for example, resulting from changes in the orientation of the winding equipment in each turnaround region) do not occur in locations corresponding to the fuel assembly core 100. For example, the turnaround regions can be located where the mandrel spacers of the tensioned fuel monolith stack mandrel assembly 400 are positioned. By causing any changes in the tow winding to occur axially away from the fuel assembly core 100, the winding of the insulating layer 120 on the fuel assembly core 100 becomes more uniform.

[0069] For the application of the insulating layer in step S150, axial tension can be applied to the tensioned fuel monolith stack mandrel assembly 400. For example, a pneumatic chuck can be incorporated into the winding system and attached to the axial end of the tensioned fuel monolith stack mandrel assembly 400, for example, via threaded mandrel end caps 440, 470, and tension can be applied to the tensioned fuel monolith stack mandrel assembly 400 from the winding equipment. Depending on the amount of tension already applied to the tensioned fuel monolith stack mandrel assembly 400 from the internal tension-applying component (as discussed herein), only a small amount of axial tension (e.g., less than 1500 Newtons (337 lbf), e.g., 222 Newtons (50 lbf), 445 Newtons (100 lbf), or 1245 Newtons (280 lbf)) is required to be applied to the tensioned fuel monolith stack mandrel assembly 400 in order to keep the tensioned fuel monolith stack mandrel assembly 400 upright during the application of the insulating layer. Excessive axial tension (e.g., more than 445 Newtons) may cause stretching of the internal tension-applying component, and thus a gap may be created between the fuel monoliths 10. Furthermore, excessive axial tension increases the overall torque applied to the tensioned fuel monolith stack mandrel assembly 400 due to bearing drag on the idle chuck to which axial tension is applied. When higher tensions are used, the idle chuck can be actively driven using a motor synchronized with the primary drive chuck.

[0070] Once the outer sublayer is impregnated with phenolic resin, the wound and impregnated tensioned fuel monolith stack mandrel assembly 400 is then cured according to a heat treatment schedule applicable to the phenolic resin. The curing process can be carried out in a controlled manner in a suitable heating system (e.g., a tubular heating system with an insulator), which allows for uniform heating of the phenolic resin while the wound and impregnated tensioned fuel monolith stack mandrel assembly 400 is slowly rotated inward using the rotation axis of the mandrel winding machine. Slowly rotating the wound and impregnated tensioned fuel monolith stack mandrel assembly 400 helps maintain straightness during the phenolic resin curing cycle.

[0071] Further details and / or alternative details relating to mandrel-wrapped insulators are contained in U.S. Patent Application No. 18 / 118,193, the entirety of which is incorporated herein by reference.

[0072] In step S160, the mandrel wrapping component and tensioning component are disassembled from the hardened tensioned fuel monolith stack mandrel assembly 400. The structures and processes for disassembling the mandrel wrapping component and tensioning component are discussed further herein, in particular in relation to Figures 10A–10G, 11A–11C, and 12.

[0073] Once disassembled, the insulated fuel assembly core 100 can be optionally post-machined and then inserted into the fuel assembly outer structure 210. Post-machining may include, for example, machining to final dimensions. An exemplary machining process includes centerless grinding.

[0074] Step S170 involves inserting the insulated fuel assembly core 100 into the fuel assembly outer structure 210, for example, by inserting the insulated fuel assembly core 100 into the fuel assembly outer structure 210.

[0075] Once inserted into the fuel assembly outer structure 210, other components can be attached as needed to form the fuel assembly 200. For example, the inlet connection assembly 220 can be attached, for example, using a crimp 232.

[0076] Figure 11A is a perspective view of an embodiment of the tensioned fuel monolith stack mandrel assembly 400, and Figure 11B is a cross-sectional view of the tensioned fuel monolith stack mandrel assembly 400 shown in Figure 11A, taken at X5-X5.

[0077] The tensioned fuel monolith stack mandrel assembly 400 extends along an axis 402 from a first end 404 to a second end 406 and includes (i) a fuel assembly core 100 (shown with an insulating layer 120), (ii) an internal tensioning component (see also Figures 11C–11G and Figures 12A–12C), (iii) at the first end 404, a plurality of mandrel spacers 420 in a first mandrel spacer section 430 and a first threaded mandrel end cap 440, and (iv) at the second end 406, a plurality of mandrel spacers 450 in a second mandrel spacer section 460 and a second threaded mandrel end cap 470. The exposed section of the first mandrel spacer section 430 has a length L4 and lies between the first end 104 of the fuel assembly core 100 and the first threaded mandrel end cap 440. The exposed section of the second mandrel spacer section 460 has a length L5 and lies between the second end 106 of the fuel assembly core 100 and the second threaded mandrel end cap 470.

[0078] Figures 11C to 11G show enlarged views of various parts of the tensioned fuel monolith stack mandrel assembly 400.

[0079] Figure 11C is an enlarged view of area P5 in Figure 11B, showing in cross-section the threaded mandrel end cap 440 and several mandrel spacers 420 at the first end 404 of the tensioned fuel monolith stack mandrel assembly 400. The mandrel spacers 420 are positioned adjacent to the first end 104 of the insulated fuel assembly core 100 as axial extensions of the insulated fuel assembly core 100. The mandrel spacers 420 are typically cylindrical or eccentric body portions and include a plurality of holes 422, which are aligned with the channels 30 in the fuel monolith 10 when positioned adjacent to the first end 104 of the insulated fuel assembly core 100. One mandrel spacer (typically the outermost mandrel spacer 420a) includes a threaded surface 424, by which the first threaded mandrel end cap 440 can be attached via a complementary threaded surface 442 of the first threaded mandrel end cap 440. The threaded surface 424 can be on a fastener attached to the mandrel spacer, or it can be directly incorporated into the surface of the mandrel spacer. Other mounting means besides threaded connections can be used (e.g., fasteners, biased connectors, or welding). The first threaded mandrel end cap 440 also includes an axial projection 444. The axial projection 444 engages with a holder assembly (not shown) for rotation of the tensioned fuel monolith stack mandrel assembly 400 during the application of the insulating layer 120.

[0080] Figure 11D is an enlarged view of area P6 in Figure 11B, showing a cross-section of the mandrel spacer 420 in the first portion of the tensioned fuel monolith stack mandrel assembly and the first end 104 of the insulated fuel assembly core 100. In Figure 11D, the interface 434 between the fuel monolith 10 at the first end 104 of the insulated fuel assembly core 100 and the mandrel spacer 420 at the end of the first mandrel spacer section 430 is shown.

[0081] Figure 11E is an enlarged view of area P7 in Figure 11B, showing a cross-section of the first portion of the second end 106 of the insulated fuel assembly core 100 in the second portion of the tensioned fuel monolith stack mandrel assembly 400. The mandrel spacer 450 is a typically cylindrical or eccentric body and has a different size from the mandrel space 420 to fit into the internal space of the tubular section 154 of the exhaust shield assembly 150. The mandrel spacer 450 includes a plurality of holes 452. The ends of the mandrel spacer 450 interface with the second end face 140 of the exhaust support plate 130, and the holes 452 in the mandrel spacer 450 are aligned with channels 132 in the exhaust support plate 130 (which themselves are aligned with channels 30 in the fuel monolith).

[0082] Figure 11F is an enlarged view of area P8 in Figure 11B, showing in cross-section the mandrel spacer 450 in the third portion of the tensioned fuel monolith stack mandrel assembly 400 and the second portion of the second end 106 of the insulated fuel assembly core 100. The mandrel spacer 450 is a continuation from the exhaust shield assembly 150 and is positioned as an axial extension of the insulated fuel assembly core 100.

[0083] Figure 11G is an enlarged view of area P9 in Figure 11B, showing a cross-section of the second threaded mandrel end cap 470 and mandrel spacer 450 at the second end 406 of a tensioned fuel monolith stack mandrel assembly 400. One mandrel spacer (typically the outermost mandrel spacer 450a) includes a threaded surface 454, by which the second threaded mandrel end cap 470 can be attached via a complementary threaded surface 472 of the second threaded mandrel end cap 470. The threaded surface 454 can be on a fastener attached to the mandrel spacer, or it can be directly incorporated into the surface of the mandrel spacer. Other mounting means besides threaded connections can be used (e.g., fasteners, biased connectors, or welding). In some embodiments, the outermost mandrel spacer 450a can be the same as the outermost mandrel spacer 420a (and therefore the hole 456; the hole 456 is not used in the outermost mandrel spacer 450a). The second threaded mandrel end cap 470 also includes an axial projection 474. The axial projection 444 engages with a holder assembly (not shown) for rotation of the tensioned fuel monolith stack mandrel assembly 400 during the application of the insulating layer 120.

[0084] Mandrel spacers 420 and 450 can be made from aluminum (e.g., 6061 aluminum) or stainless steel (e.g., 17-4PH stainless steel or 304 stainless steel), or other suitable materials.

[0085] Internal tensioning components are shown in Figure 11B, in the combination of Figures 11C to 11G, and in the combination of Figures 12A to 12C. The internal tensioning components include a first set of tensioning cables 490a and a second set of tensioning cables 490b. The first set of tensioning cables 490a is shorter than the second set of tensioning cables 490b. Embodiments of the tensioned fuel monolith stack mandrel assembly 400 use up to 16 to 20 tensioning cables 490a and up to 6 to 10 tensioning cables 490b. The number of tensioning cables can vary based on the dimensions of the component and the number of channels / holes. Additionally, the number of tensioning cables can be less than the number of channels / holes, in which case the tensioning cables are inserted into channels / holes arranged in a symmetrical pattern (when viewed along the longitudinal axis).

[0086] The first set of tensioning cables 490a extends from a first end on the end surface of the outermost mandrel spacer 420a to a second end on the second end surface 140 of the exhaust support plate 130. Thus, the first set of tensioning cables 490a extends through the first mandrel spacer 420 in the first mandrel spacer section 430, through the fuel monolith 10 of the fuel assembly core 100, and through the exhaust support plate 130. In the exhaust support plate 130, the first set of tensioning cables 490a is located within a non-parallel channel 132 around the exhaust support plate 130.

[0087] The first set of tension-applying cables 490a are tensioned and then terminated at the first and second ends, respectively, in order to maintain the tension in the first set of tension-applying cables 490a. Termination can be done by appropriate means (for example, by crimps 494 or by termination fixtures).

[0088] The second set of tensioning cables 490b extends from a first end on the end surface of the outermost mandrel spacer 420a to a second end on the end surface of the outermost mandrel spacer 450a. Thus, the second set of tensioning cables 490b extends through the first mandrel spacer 420 in the first mandrel spacer section 430, through the fuel assembly core 100 (including the fuel monolith 10, the exhaust support plate 130, and the exhaust shield assembly 150), and through the second mandrel spacer 450 (both the second mandrel spacer 450 at the second end 106 of the fuel assembly core 100 and the second mandrel spacer section 450 in the second mandrel spacer section 460). In the exhaust support plate 130, the second set of tensioning cables 490b are located radially inward from the non-parallel channel 132 in which the first set of tensioning cables 490a are located, and are situated within both the parallel channel 132 and the non-parallel channel 132.

[0089] The second set of tensioning cables 490b are tensioned and then terminated at the first and second ends, respectively, in order to maintain the tension in the second set of tensioning cables 490b. Termination can be done by appropriate means (for example, by crimps 492 or by termination fixtures).

[0090] It should be noted that not all channels 30 of the fuel monolith 10 and not all channels 132 of the exhaust support plate 130 contain internal tensioning components. Rather, some channels 30, 132 are left empty in the tensioned fuel monolith stack mandrel assembly 400.

[0091] The tension cable can be any suitable cable, such as stainless steel cable or tungsten cable made from 316L. Examples include Bare 7x49 (SS or W) and Bare 1x19 (SS) commercially available cables from Carl Stahl Sava Industries, Inc. (Riverdale, New Jersey).

[0092] Also illustrated is a drill rod 480, which is used to align the components of the fuel assembly core 100 during assembly (for example, when stacking the fuel monolith 10 and exhaust support plate 130). The drill rod 480 provides strength to the tensioned fuel monolith stack mandrel assembly 400 and helps prevent arching deformation, but the drill rod 480 does not apply large compressive forces, if any, to the tensioned fuel monolith stack mandrel assembly 400. In some embodiments, the drill rod 480, positioned within the central channel 30' of the fuel monolith 10, may include a threaded end for the application of a threaded end, which can be used to tension the drill rod 480 within the channel 30'. In addition, the threaded drill rod allows for the application of compressive forces to the tensioned fuel monolith stack mandrel assembly 400, for example, by screwing in a end nut.

[0093] Figures 12A to 12C are perspective cross-sectional views showing the arrangement of tension components in different parts of the tensioned fuel monolith stack mandrel assembly 400.

[0094] Figures 11A-11G and 12A-12C depict the tensioned fuel monolith stack mandrel assembly 400, including the insulating layer 120, after the application of the insulating layer 120 (i.e., after step S150), but before the disassembly of the mandrel winding component and tensioning component (i.e., before step S160). The tensioned fuel monolith stack mandrel assembly 400 before the application of the insulating layer 120 (i.e., after step S140 and before step S150) would be the same as those depicted in Figures 11A-11G and 12A-12C, except that it does not have the insulating layer 120.

[0095] Returning to step S160 in method S100 and Figure 10, the disassembly of the mandrel winding and tensioning components may include removing the mandrel winding components (e.g., threaded mandrel end caps 440, 470, etc.) and the internal tensioning components (e.g., mandrel spacers 420, 450 and tensioning cables 490a, 490b, etc.). Typically, not all internal tensioning components are directly accessible after the application of the insulating layer 120, and therefore, the cured tensioning fuel monolith stack mandrel assembly 400 is intended to be post-processed to allow for the removal of at least some internal components. For example, the cured insulating layer 120 is intended to be cut to form the finished ends of the insulated fuel assembly core 200. Such cuts can be located in the turnaround region of the mandrel winding process and can be positioned close enough to the first end 104 or second end 106 of the fuel assembly core 100 to provide access to the internal tensioning components at each location. For example, once accessible, the ends 492, 494 of the tensioning cables 490a, 490b at one end can be removed, for example, by cutting, and the cables can be removed from the hardened tensioning fuel monolith stack mandrel assembly 400 by pulling the other end of the tensioning cables 490a, 490b to pull the cables through the channel / hole.

[0096] Figure 13 is a cross-sectional view of an alternative embodiment of the tensioned fuel monolith stack mandrel assembly. The alternative embodiment of the tensioned fuel monolith stack mandrel assembly 400' is substantially similar to the tensioned fuel monolith stack mandrel assembly 400 shown and described herein, and the disclosures relating to the fuel monolith 10 are applicable to the fuel monolith 10'. Furthermore, similar features are labeled with similar reference numerals.

[0097] In an alternative embodiment of the tensioned fuel monolith stack mandrel assembly 400', threaded mandrel end caps 440, 470 are replaced by mandrel tensioning caps 440', 470' at the first end 404' and the second end 406', respectively. Additionally, one or more tensioning cables in the inner grouping of the channel are replaced by drill rods 480'. In an exemplary embodiment, a number of drill rods 480' (e.g., five to nine drill rods 490', alternatively seven drill rods 490') are used. The drill rods 480' are slip-fitted tightly into the internal channel 30' of the fuel element, the support plate, and the spacers of the tensioned fuel monolith stack mandrel assembly 400'. The drill rods 480' are threaded at each end, allowing threaded nuts to be used to tension the drill rods 480'. The threaded end of the drill rod 480' extends beyond the end surface of the mandrel tensioning caps 440' and 470', and a threaded nut is attached to the threaded end and tightened, allowing the fuel monolith stack mandrel assembly 400' to be placed under tension. Using the drill rod 490' instead of the tensioning cable simplifies assembly and increases rigidity to the tensioned fuel monolith stack mandrel assembly 400' by resisting twisting during winding.

[0098] Figure 14A is an enlarged view of area P5' in Figure 13, showing a cross-sectional view of the mandrel tensioning cap 440' and mandrel spacer 420' at the first end 404' of an alternative embodiment of the tensioned fuel monolith stack mandrel assembly 400'. Figure 14B is an enlarged view of area P9' in Figure 13, showing a cross-sectional view of the mandrel tensioning cap 470' and mandrel spacer 450' at the second end 406' of an alternative embodiment of the tensioned fuel monolith stack mandrel assembly 400'.

[0099] Figures 14C to 14F are perspective views of the first end 404' of an alternative embodiment of the tensioned fuel monolith stack mandrel assembly 400' at various points in time during assembly. In the first step (see Figure 14C), the drill rods 490' are inserted into the channel 30', for example, by sliding them. Typically, the drill rods 490' will be positioned symmetrically around the centerline of the fuel monolith stack mandrel assembly 400'. Threaded nuts 482 are screwed onto and tightened over the ends of each drill rod 490'. Then (see Figure 14D), the first spacers 484 are added over the ends of each drill rod 490'. The first spacer 484 has geometrically shaped countersunk recesses 486 for each opening in the first spacer 484 through which the drill rods 490' will protrude, and an additional hole 488 that aligns with a hole in the mandrel tensioning cap 440' which is covered by the first spacer 484 but is not occupied by the drill rods 490' at this point. The geometrically shaped countersunk recesses 486 for the first spacer 484 have a circular or other shape that allows a tool to be inserted and fitted with a threaded nut 482, facilitating tightening by turning the threaded nut 482 with the tool. Another set of drill rods 490' are inserted into the channel 30' through the holes in the first spacer 484, for example by sliding, and the threaded nuts 482' are screwed onto the ends of each drill rod 490' and tightened (see Figure 14E). Next (see Figure 14F), a second spacer 484' is added over the ends of each drill rod 490'. The second spacer 484' has countersunk recesses 486' for each opening in the second spacer 484' through which the drill rod 490' will protrude, providing clearance for tightening threaded nuts 482'. In addition, a central drill rod 490'' is added and tensioned by threaded nuts 482''.

[0100] Figure 14G is a perspective view of the second end of an alternative embodiment of a tensioned fuel monolith stack mandrel assembly after assembly. At the second end, the drill rods 490' extend from the channel beyond the end surface of the mandrel tensioning cap 470', and threaded nuts are mounted on each drill rod 490'. Two spacers (a first spacer 484 and a second spacer 484') are present and mounted, and threaded nuts 482' are added in the same manner as described with respect to the first end. However, at the second end, each spacer includes a geometrically shaped countersunk recess 486' (e.g., hexagonal) which fits around the outer circumference of the threaded nut 482' mounted on the end of the drill rod 490', facilitating tightening by rotating the drill rod 490' from the other end while the threaded nut 482' is supported against the recess 486'.

[0101] Subsequently, tension-applying cables 490c may be added as described herein in relation to a first embodiment of the fuel monolith stack mandrel assembly 400 and as shown in Figure 14H.

[0102] Figure 15 is a flow diagram illustrating the various steps in one embodiment of a method for preparing a tensioned fuel monolith stack mandrel assembly. The method S200 for preparing a tensioned fuel monolith stack mandrel assembly 400 includes the steps of: inserting a drill rod 480 into a fuel assembly core 100 S220; attaching a first set of tensioning cables 490a S230; attaching mandrel spacers 420, 450 at the exhaust end and inlet end of the fuel assembly core 100 S240; attaching a second set of tensioning cables 490b S250; and applying tension to the tensioning cables 490a, 490b S260. Once the assembly is tensioned, threaded mandrel end caps 440, 470 are attached to form a tensioned fuel monolith stack mandrel assembly 400.

[0103] Method S200 optionally uses an alignment jig (see step S210) to assist in setting up and aligning various components and for alignment support during the tensioning process. Setting up in the alignment jig can occur at any time before tensioning the tensioning cables (step S260). After tensioning is completed in step S250, the assembly is removed from the alignment jig (step S270). Removal from the alignment jig can occur either before or after attaching the threaded mandrel end caps 440, 470 (step S280).

[0104] If necessary, additional processing of the tensioned fuel monolith stack mandrel assembly 400 may be performed before proceeding to applying the insulating layer 120 (step S150) (e.g., dimensional verification and correction as needed, with respect to straightness, etc.).

[0105] Step S220 involves inserting one or more drill rods 480 into the fuel assembly core 100. The fuel monolith 10 is aligned in the stack using the internal diameter of the channel 30 and the drill rods inserted into the channel 30. If an alignment jig or other support is used, it can be used to cradle the fuel assembly core 100 while inserting one or more drill rods 480. The clearance of the drill rods relative to the internal diameter of the channel 30 is as small as possible, for example, a clearance of 25 to 75 microns. The drill rods provide rotational constraints, and at least two drill rods 480 are used, and alternatively, six to seven drill rods 480 are used. At least one drill rod 480 is provided in the central channel 30' and extends axially beyond each end of the fuel assembly core 100. It is also possible, optionally, for others from the multiple drill rods 480 to extend axially beyond each end of the fuel assembly core 100. These axially extending drill rods 480 provide alignment for the subsequent mounting of the mandrel spacers 420, 450.

[0106] Step S230 installs the first set of tensioning cables 490a. The first set of tensioning cables 490a are installed in a plurality of non-parallel channels 132 in the peripheral region of the exhaust support plate 130. The use of non-parallel channels 132 guides the first set of tensioning cables 490a into each channel 30'''' in the outer ring 56 of the exhaust support plate 130. Each second end of the first set of tensioning cables 490a has a termination 494, and each first end of the first set of tensioning cables 490a is inserted through the channel 30 in the fuel monolith until the respective termination 494 seats against the exhaust support plate 130. The terminations 494 may be chamfered and shortened so that they fit into the void space between the exhaust support plate 130 and the truncated conical section 152 of the exhaust shield assembly 150. In an alternative embodiment, one or more or all of the first set of tensioning cables 490a can be replaced by threaded rods, and the termination 494 can be a threaded nut.

[0107] Step S240 involves installing the exhaust end mandrel spacer 450 and the inlet end mandrel spacer 420. An axially extending drill rod 480 is inserted into the holes 422 in the mandrel spacers 420 and 450, and the mandrel spacers 420 and 450 are installed sequentially to construct the respective mandrel spacer sections 430 and 460. The number of mandrel spacers 420 and 450 can vary, as long as the lengths of the first mandrel spacer section 430 and the second mandrel spacer section 460 provide sufficient run-off space for the turnaround area of ​​the helical winding process used in forming the insulating layer 120. Sufficient run-off space results in the helical winding process being uniform over the entire length of the finished insulated fuel assembly 100.

[0108] Mandrel spacers can be installed in any sequence between the exhaust end and the inlet end, as long as the appropriate type of mandrel spacer is used at each end. The inlet end mandrel spacer 420 has the same basic geometry as the fuel monolith 10 (see Figure 11D). The exhaust end mandrel spacer 450 has a first exhaust end mandrel spacer 450 with an end surface that fits into the second end face 140 of the exhaust support plate 130. Furthermore, exhaust end mandrel spacers 450 positioned within the exhaust shield assembly 150 have a diameter that fits within the tubular section 154 of the exhaust shield assembly 150, while exhaust end mandrel spacers 450 positioned outside the exhaust shield assembly 150 have a larger diameter (for example, a diameter that matches the outer diameter of the tubular section 154 of the exhaust shield assembly 150) so that the fiber tow has a smooth surface for fiber landing during subsequent mandrel winding.

[0109] Simultaneously with or at a separate time from the installation of the mandrel spacers 420 and 450, the first set of tensioning cables 490a are inserted into the hole 422 in the inlet end mandrel spacer 420.

[0110] Step S250 installs the second set of tensioning cables 490b. The second set of tensioning cables 490b is installed from one distal end of one of the mandrel sections 430, 460, through holes 422, 452 in the respective mandrel spacers 420, 450, and through channels in the exhaust support plate 130 and fuel monolith 10 that are matched accordingly. Typically, the second set of tensioning cables 490b is installed starting from the exhaust end by inserting the first end of each tensioning cable 490b into the respective holes 452 in the exhaust end mandrel spacer 450. The holes 422, 452 in the respective mandrel spacers 420, 450 are selected so that the second set of tensioning cables 490b extends through holes 30 in the intermediate ring 54 of the fuel monolith 10. Each second end of the second set of tension-applying cables 490b has a termination 492, and each first end of the second set of tension-applying cables 490b is inserted (depending on which end insertion begins) until the respective termination 492 seats against the end faces of the respective end mandrel spacers 420, 450. In an alternative embodiment, one or more or all of the second set of tension-applying cables 490b can be replaced by threaded rods, and the termination 492 can be a threaded nut.

[0111] At the same time as or at a different time as the second set of tensioning cables 490b is installed, the second set of tensioning cables 490a is inserted into the hole 422 in the inlet end mandrel spacer 420.

[0112] The tensioning cable 490b of the second set can be the same tensioning cable 490a of the first set, or it can be a different tensioning cable from the tensioning cable 490a of the first set.

[0113] In an exemplary embodiment, the first set of tensioning cables 490a contains 18 tensioning cables 490a, and the second set of tensioning cables 490b contains 6 tensioning cables 490b. Other arrangements may also be used, such as 7 internally threaded drill rods and 4 to 8 externally tensioned cables, for example, 7 internally threaded drill rods and 4 to 8 externally tensioned cables.

[0114] Step S260 applies tension to the tensioning cables 490a and 490b. The cable tensioning is performed to balance the forces acting on the fuel monolith stack mandrel assembly 400. For example, one or two pairs of tensioning cables (i.e., two or four tensioning cables) from the same set of tensioning cables (i.e., the first set of tensioning cables 490a or the second set of tensioning cables 490b) can be simultaneously tensioned to balance any harmful moments inside the fuel monolith stack mandrel assembly 400. Otherwise, such harmful moments would tend to cause the fuel monolith stack mandrel assembly 400 to bend. When applying both forces to a pair of tensioning cables, opposing tensioning cables are used.

[0115] The tensioning sequence begins with the central grouping of the second set of tensioning cables 490b. It should be noted that the first set of tensioning cables 490a is more easily tensioned from the inlet side due to the arrangement of structures at the exhaust end (e.g., exhaust end mandrel spacer 450 and exhaust shield assembly 150). On the other hand, the second set of tensioning cables 490b can be tensioned from either end. For convenience, the assembly sequence and insertion of the tensioning cables 490a and 490b can result in both the first set of tensioning cables 490a and the second set of tensioning cables 490b being tensioned from the same end (e.g., the first end 404 corresponding to the first end 104 (or inlet end) of the insulated fuel assembly core 100).

[0116] Once each tensioning cable is tensioned to its final tension, its end is terminated. Termination can be done by appropriate means (for example, by crimp 492 or by a termination fixture).

[0117] When tension is applied to the second set of tensioning cables 490b, the tensioning sequence continues with the tensioning of the first set of tensioning cables 490a, which proceeds in a similar manner to that of the tensioning of the second set of tensioning cables 490b.

[0118] In one embodiment, a twin-bore hydraulic tensioner system can be used to apply tension to a fuel monolith stack mandrel assembly. Figure 16 is a schematic cross-sectional view of the twin-bore hydraulic tensioner system. The twin-bore hydraulic tensioner system 500 can be used to provide uniform tension to two cables simultaneously and can be used during the tensioning step S260.

[0119] In some embodiments, end plates are positioned above the end ends of the respective mandrel spacer sections 430, 460, and they distribute the force associated with applying tension to the tension cables 460a, 490b. In exemplary embodiments, the force in each tension-applying cable 460a, 490b is approximately 85 N / M to approximately 450 N / m, and alternatively approximately 225 N / m (20 lbf to 100 lbf, alternatively 50 lbf).

[0120] In optional embodiments, a stack of small Belleville washers can be used between the end plate and the end. The addition of this high-load spring makes the overall tensioned assembly more resilient to harmful deflection when the assembly process completes the installation of the mandrel spacer 420.

[0121] In other optional embodiments, a high-tensile monofilament plastic wire replaces one, more, or all of the metal tension-bearing cables. The high-tensile monofilament plastic wire maintains the tension applied to the cable better than the metal tension-bearing cables. When used, tension can be applied to the high-tensile monofilament plastic wire until the tension produces elastic or plastic deformation, after which termination can be applied.

[0122] Although the sequence of steps S220 to S250 is described above in that order, other orders of steps S220 to S250 are also possible. For example, some or all of the mandrel spacers 420, 450 can be installed before the installation of the first set of tensioning cables 490a by, for example, placing the mandrel spacers 420, 450 on the drill rod 480. Also, for example, the second set of tensioning cables 490b can be installed before the installation of all mandrel spacers 420, 450 is completed by, for example, installing the second set of tensioning cables 490b after installing the mandrel spacers 420 of the first mandrel spacer section 430.

[0123] (a) Each first end 494 of the first set of tensioning cables 490a is adjacent to the second end face 140 of the exhaust support plate 130, and each second end of the first set of tensioning cables 490a is positioned to protrude from the first set of holes 422 in the most distal mandrel spacer 420 at the other end of the assembly (e.g., the first end 404) and to be in a position where tension can be applied; and (b) Each first end 492 of the second set of tensioning cables 490b is Other modifications may be used, as long as the sequence results in an assembly ready for tensioning in step S260, with the second end of the second set of tensioning cables 490b protruding from the holes 422 of the second set in the most distal mandrel spacer 420 at the other end of the assembly (e.g., the first end 404) and positioned to be tensioned.

[0124] Step S280 involves attaching threaded mandrel end caps 440 and 470 in preparation for the application of the mandrel-wrapped insulating layer 120 (step S150). The threaded mandrel end caps 440 and 470 can also be attached earlier in method S200 when access to the end to which the threaded mandrel end caps are attached is no longer required.

[0125] Figure 17 is an image 550 showing an embodiment of the winding of an insulating layer on a tensioned fuel monolith stack mandrel assembly 400. What can be seen in image 550 is a basket weave pattern, which reduces the fiber density and therefore reduces the radial thermal conductivity in both the inner insulating layer 120a and the outer insulating layer 120b. The spacing between toes in the basket weave pattern can be adjusted from 1 mm to 5 mm. The helical angle of the fiber winding can be kept constant or can be varied as a function of the length down the pre-bundle to optimize the strength at a particular axial location. Helical angles (θ) greater than 80 degrees (e.g., greater than 80 degrees and less than 90 degrees, greater than 85 degrees and less than 90 degrees, or greater than 86 degrees and less than 88 degrees) can be used. CNC manufacturing techniques provide maximum control over fiber placement and uniformity.

[0126] Figure 18 is an image 600 showing a perspective view of an embodiment of the insulated fuel assembly core 100, and Figure 19 is an image 700 showing a diagram of an embodiment of the insulated fuel assembly core 100 along the longitudinal axis 102. Note the light that can be seen through the parallel channels 602 in the first inner ring of the fuel assembly core 110.

[0127] Figure 20A is a schematic cross-sectional side view of an embodiment of a nuclear-powered nuclear fission reactor structure in a vessel, which comprises a fuel assembly including an insulated fuel assembly core (as described herein) within a fuel assembly outer structure. Embodiments of the nuclear fission reactor structure 1100 include a plurality of fuel assemblies 1105 (e.g., a fuel assembly 200 formed from an insulated fuel assembly core 100 and a fuel assembly outer structure 210) located within the active core region 1110 of the nuclear fission reactor structure 1100 (the active core region 1110 is an internal region in which a moderator block 1155 is located, and the fuel assemblies 1105 are located within the moderator block 1155). At the inlet and outlet portions of the fuel assemblies 1105, inlet connection assemblies 1115 (e.g., including an inlet connection assembly 220) and outlet connection assemblies 1120 (e.g., including an exhaust shield assembly 150) provide fluid communication for the propellant supplied to and exhausted from each of the fuel assemblies 1105. Therefore, the inlet connection assembly 1115 is connected to or interfaces with the inlet plenum, and the outlet connection assembly 1120 is connected to or interfaces with the exit plenum.

[0128] An interface structure 1125 (which may or may not include supplemental radial constraints) lies radially outward of the active core region 1110, and a reflector 1135 lies radially outward of the interface structure 1125. The first surface of the interface structure 1125 conforms to the outer surface of the active core region 1110, and the second surface of the interface structure 1125 conforms to the inner surface of the reflector 1135. The inner surface of the reflector 1135 is oriented toward the active core region 1110, and the interface structure 1125 functions to fit the geometry of the outer surface of the active core region 1110 into the geometry of the inner surface of the reflector 1135, thus enabling various geometric shapes and arrangements for the active core region 1110 (e.g., cylindrical or polygonal).

[0129] Figure 20B is an enlarged view of area P10 in Figure 20A, showing a cross-section of the inlet end of the fuel assembly 1105, including the inlet connection assembly 1115. Note how the seal 230 contacts the inner surface of the opening 1118, providing a degree of freedom of motion for changes in axial dimensions due to thermal expansion.

[0130] Figure 20C is an enlarged view of area P11 in Figure 20A, showing a cross-section of the exhaust end of the fuel assembly 1105. Figure 20C includes the exhaust support plate 130 and the exhaust shield assembly 150.

[0131] Figure 21 is a schematic cross-sectional top view of an embodiment of the nuclear-powered nuclear fission reactor structure 1100 within the vessel 1140. Multiple control drums 1145 (each containing a neutron absorber body 2150) are positioned within the volume of the reflector 1135 (for example, within an annular section in the outer portion of the cylindrically shaped control drum). The control drums 1145 themselves, like the reflector 1135, are made from a neutron-reflecting material. The neutron absorber body 1150 is made from a neutron-absorbing material and is movable between a first position and a second position, for example, by rotation R4. In the first position, the neutron absorber body 1150 is radially closer to the active core region 1110 than when the neutron absorber body 1150 is in the second position. In an exemplary embodiment, the first position is radially closest to the active core region 1110, and the second position is radially furthest from the active core region 1110. The neutron absorber body 1150 is movable between the first and second positions to control the reactivity of the active core region 1110. In the illustrated example, the neutron absorber body 1150 is rotatable in conjunction with the movement of the control drum 1145 from the first radially closer position to the second position by rotation (R4) of the control drum 1145 around its axis. However, other radial positions and / or directions of movement can also be implemented, as long as the various positions to which the neutron absorber body 1150 can be moved provide control of the reactivity of the active core region 1110. In some embodiments, when the multiple neutron absorber body sections 1150 are each located closer together in the first radial direction, each of the multiple neutron absorber body sections 1150 is equidistant radially from the axial centerline of the active core region 1110. Other control concepts can also be implemented, such as adjusting neutron leakage by opening and closing portions of the reflector 1135.

[0132] The nuclear fission reactor structure may further include a vessel 1140. Figures 20A and 21 schematically illustrate an embodiment of the nuclear fission reactor structure 1100 with the vessel 1140. The nuclear fission reactor structure 1100 (which includes an active core region 1110, an interface structure 1125, an inlet connection assembly 1115 and an outlet connection assembly 1120, a reflector 1135, and a plurality of control drums 1145 with a neutron absorber body 1150) is housed within the internal volume of the vessel 1140.

[0133] As shown in Figure 20A, the motor 1160 is movably mounted to the control drum 1145 by the drum shaft 1165 for rotation. The motor 1160 can be housed within the pressure boundary extension of the vessel 1140, or alternatively, it can not be housed, in which case a seal is required around the drum shaft 1165. It is also possible for the motor to be mounted inside the vessel 1140.

[0134] Embodiments of the container 1140 are formed from machined forgings and generally use high-strength aluminum alloys or titanium alloys due to weight considerations. The container 1140 can be made up of multiple components, which are then assembled together, for example, by fasteners. However, in other embodiments, the container 1140 can be a single continuous component or a welded assembly.

[0135] Further disclosures relating to nuclear fission reactor structures and their components can be found in U.S. Patent Application No. 16 / 999,244, the entirety of which is incorporated by reference.

[0136] Furthermore, this disclosure is directed toward a nuclear thermal propulsion engine that includes a nuclear fission reactor structure 1100 within a vessel 1140 in a reactor section 1170. The nuclear thermal propulsion engine further includes a shielding section 1175, a turbomachinery section 1180, and a nozzle section 1185 that is attached to or supported by the vessel 1140, as is consistent with those shown in Figures 22A and 22B, for example.

[0137] Figure 22B is an exploded view of the reactor section 1170 of the nuclear thermal propulsion engine shown in Figure 22A. The components of the reactor section 1170 include an exit plenum 1190, a vessel 1140, a reflector 1135, a moderator 1155, a fuel assembly 1105, a flanged core barrel 1192, an internal shielding component 1194, an inlet plenum 1196, a reactor head 1198, and a control drum 1145.

[0138] It is intended that various support and auxiliary equipment can be incorporated into the disclosed nuclear fission reactor structure and nuclear thermal propulsion engine. For example, at least one of the following can be incorporated into the nuclear propulsion nuclear fission reactor structure: moderators (e.g., zirconium hydride, beryllium, beryllium oxide, and graphite), control rods for launch safety, neutron sources to assist in startup, and scientific instruments (e.g., temperature sensors or radiation detectors).

[0139] The disclosed configurations relate to any configuration in which a heat generation source containing a fissile nuclear fuel composition is incorporated into the fuel bundle. Although generally described herein in relation to gas-cooled nuclear thermal propulsion reactors (NTP reactors), the structures and methods disclosed herein may also be applicable to other fission reactor systems.

[0140] The nuclear-powered fission reactor structures disclosed herein, but not limited to, can be used in appropriate applications, including extraterrestrial power generation, space power, space propulsion, and naval applications (including submarines).

[0141] While specific embodiments have been referenced, it is evident that other embodiments and variations can be devised by those skilled in the art without departing from the spirit and scope thereof. The appended claims are intended to be construed to include all such embodiments and equivalent variations. [Explanation of Symbols]

[0142] 10 Fuel Monoliths 10' Fuel Monolith 20 Main body 22 First end surface 24 Second end surface 26 Side surface 30 channels 30' Central Channel 30'' channel 30''' channels 30 channels 40 Longitudinal axis 52 Inner ring 54 Intermediate ring 56 Outer ring 100 Insulated Fuel Assembly Cores 102 Longitudinal axis 104 First end 106 Second end 110 Fuel Assembly Core 112 Interfaces 114 Interfaces 120 Insulator 120a Inner insulating layer 120b outer insulation layer 130 Exhaust support plate 130' Exhaust support plate 130'' Exhaust support plate 132 channels 132' channel 134 Inlet opening 136 First end face 138 Exit opening 140 Second end face 140' Second end face 142 Circumferential surface 144 Central area 150 Exhaust Shield Assembly 150' Integrated Exhaust Shield Assembly 150'' Integrated Exhaust Shield Assembly 152 Truncated cone-shaped section 152' Truncated cone-shaped section 154 Tubular sections 154' Tubular section 154'' tubular section 156 End section 158 First longitudinal section 160 Longitudinal centerline 162 Longitudinal centerline 164 Second longitudinal section 172 End surface 174 Inner surface 176 Outer surface 182 Tapered surface 184 Tapered surface 186 Ridge 200 Fuel Assembly 202 Longitudinal axis 206 First end 208 Second end 210 Fuel Assembly Outer Structure 220 Inlet connection assembly 222 Orifice Plate 224 Flow Stabilizer 226 Entrance Housing 228 biasing elements 230 stickers 232 Crimp 234 Opening 236 Upper volume 238 channels 240 opening 300 flows 400 Tensioned Fuel Monolith Stack Mandrel Assembly 400' Tensioned Fuel Monolith Stack Mandrel Assembly 402 axis 404 First end 404' First end 406 Second end 406' Second end 420 Mandrel Spacer 420' Mandrel Spacer 420a Outermost mandrel spacer 422 Hole 424 Threaded Surface 430 First Mandrel Spacer Section 434 Interface 440 First threaded mandrel end cap 440' Mandrel Tension-Imposing Cap 442 Threaded surface 444 Axial protrusion 450 Mandrel Spacer 450' Mandrel Spacer 450a Outermost mandrel spacer 452 Hole 454 Threaded surface 455 biasing elements 456 Hole 460 Second Mandrel Spacer Section 470 Second threaded mandrel end cap 470' Mandrel Tension-Imparting Cap 472 Threaded surface 474 Axial protrusion 480 Drill Rod 482 Threaded Nut 482' Threaded Nut 482'' threaded nut 484 First Spacer 484' Second spacer 486 Countersunk hole recess 486' Countersunk hole recess 488 Hole 490' Drill Rod 490'' Center Drill Rod 490a First set of tensioning cables 490b Second set of tensioning cables 490c tension-applying cable 492 First terminal, crimp 494 First terminal, crimp 500 Twin Bore Hydraulic Tensioner System 550 images 600 images 602 channels 700 images 1100 Nuclear fission reactor structure 1105 Fuel Assembly 1110 Active Core Area 1115 Inlet connection assembly 1118 Opening 1120 Outlet connection assembly 1125 Interface Structure 1135 Reflector 1140 Container 1145 Control Drum 1150 Neutron absorber main body 1155 Moderator 1160 Motor 1165 Drum Shaft 1170 Reactor Section 1175 Shielding section 1180 Turbomachinery Section 1185 Nozzle Section 1190 Exit Plenum 1192 Flanged Core Barrel 1194 Internal shielding component 1196 Entrance Plenum 1198 Reactor Head D1 First diameter D2, the second diameter D3 First diameter D4, the second diameter D5, the third diameter H1 Height H2 Exhaust support plate 130 height L1 Overall length L2 Length L3 Length L4 Length L5 Length P1 Area P2 Area P3 Area P4 Area P5 Area P5' Area P6 Area P7 Area P8 Area P9 Area P9' Area P10 Area P11 Area P12 Area R1 radius R4 rotation T thickness α angle β angle θ angle γ1 angle γ2 angle γ3 angle γ4 angle γ5 angle γ6 angle φ1 angle φ2 angle φ3 angle φ4 angle φ5 angle φ6 angle λ angle

Claims

1. An insulated fuel assembly core, Multiple fuel monoliths, Exhaust support plate and Exhaust shield assembly and Insulating layer and, Includes, Each of the aforementioned fuel monoliths has an eccentric cylindrical shape, The plurality of fuel monoliths are positioned axially along the longitudinal axis of the insulated fuel assembly core, Each of the aforementioned fuel monoliths is an insulated fuel assembly core having a composition containing fissile fuel components.

2. The insulated fuel assembly core according to claim 1, wherein each of the plurality of fuel monoliths includes a first end surface, a second end surface, a side surface connecting the first end surface to the second end surface, and a plurality of first channels extending axially from the first end surface to the second end surface.

3. The exhaust support plate includes a first end face, a second end face, a circumferential surface connecting the first end face to the second end face, and a plurality of second channels extending from the first end face to the second end face. The insulated fuel assembly core according to claim 1 or 2, wherein the plurality of second channels include a first portion of the second channel having a channel axis parallel to the longitudinal axis of the insulated fuel assembly core, and a second portion of the second channel having a channel axis not parallel to the longitudinal axis of the insulated fuel assembly core.

4. The insulated fuel assembly core according to claim 3, wherein the second portion of the second channel is located radially outward from the first portion of the second channel.

5. The insulated fuel assembly core according to claim 4, wherein the opening of the second portion of the second channel on the first end face is located around the first end face, and the opening of the second portion of the second channel on the second end face is located around the second end face.

6. The insulated fuel assembly core according to claim 5, wherein the projection of the channel axis of the second portion of the second channel does not intersect the exhaust shield assembly.

7. The insulated fuel assembly core according to any one of claims 3 to 6, wherein the second end face is concave.

8. The insulated fuel assembly core according to claim 7, wherein the exhaust shield assembly includes a truncated conical section at a first end and a tubular section at a second end.

9. The plurality of fuel monoliths define a fuel assembly core having an inlet end and an exhaust end, The first end surface of the exhaust support plate is in contact with the exhaust end of the fuel assembly core. The insulated fuel assembly core according to claim 8, wherein the first end of the exhaust shield assembly is fitted to the exhaust support plate.

10. The insulated fuel assembly core according to claim 9, wherein the insulating layer is an outer layer extending over at least a portion of the outer circumferential surface of the fuel assembly core and at least a portion of the outer surface of the exhaust shield assembly.

11. The insulated fuel assembly core according to claim 10, wherein the outer circumferential surface of the fuel assembly core is completely covered by the insulating layer.

12. The insulated fuel assembly core according to claim 11, wherein the outer surface of the exhaust shield assembly is completely covered by the insulating layer.

13. The insulated fuel assembly core according to any one of claims 1 to 12, wherein the insulating layer includes an inner insulating layer and an outer insulating layer.

14. Fuel assembly, Fuel assembly outer structure, An insulated fuel assembly core according to any one of claims 1 to 13, the fuel assembly core being located within the fuel assembly outer structure, Fuel assembly, including.

15. The fuel assembly further includes an inlet connection assembly, The fuel assembly according to claim 14, wherein the inlet connection assembly is attached to the inlet end of the fuel assembly outer structure.

16. A nuclear fission reactor structure, A speed reducer block including multiple fuel assembly openings, A plurality of fuel assemblies according to claim 14 or 15, wherein each of the plurality of fuel assemblies is located in one of the plurality of fuel assembly openings, Includes, A nuclear fission reactor structure in which, in a cross-section of the moderator block perpendicular to the longitudinal axis of the nuclear fission reactor structure, the plurality of fuel assemblies are distributedly arranged within the moderator block.

17. A method for manufacturing an insulated fuel assembly core according to any one of claims 1 to 13, The steps include forming a tensioned fuel monolith stack mandrel assembly, The steps include forming the insulating layer by winding it around a mandrel, Methods that include...

18. The step of forming the tensioned fuel monolith stack mandrel assembly is: The steps include forming the aforementioned multiple fuel monoliths into a stack, The steps include inserting one or more alignment drill rods axially through the assembled stack, The steps include attaching the exhaust support plate to the first end of the assembled stack to form a core stack, The steps include inserting a first set of tensioning cables through the exhaust support plate and the plurality of fuel monoliths, The steps include attaching a plurality of first mandrel spacers to the first end of the core stack, The steps include inserting the first set of tensioning cables through the plurality of first mandrel spacers, The steps include attaching a plurality of second mandrel spacers to the second end of the core stack, (i) inserting a second set of tensioning cables through the plurality of second mandrel spacers, (ii) the exhaust support plate, (iii) the plurality of fuel monoliths, and (iv) the plurality of first mandrel spacers, The steps include applying tension to the tension-applying cables of the first set and the tension-applying cables of the second set, The steps include attaching a first threaded mandrel cap to the distal end of the plurality of first mandrel spacers, and attaching a second threaded mandrel cap to the distal end of the plurality of second mandrel spacers, The method according to claim 17, including the method described in claim 17.

19. Each first end of the tension-applying cable in the first set is adjacent to the exhaust support plate, and each second end of the tension-applying cable in the first set is adjacent to the distal end of the plurality of first mandrel spacers. The method according to claim 18, wherein the first end of each of the second set of tension-applying cables is adjacent to the distal end of the plurality of second mandrel spacers, and the second end of each of the second set of tension-applying cables is adjacent to the distal end of the plurality of first mandrel spacers.

20. A method for manufacturing an insulated fuel assembly core, The steps include forming a tensioned fuel monolith stack mandrel assembly, The steps include forming an insulating layer on the outer surface of the tensioned fuel monolith stack mandrel assembly by winding the mandrel around it, Methods that include...

21. The step of forming the tensioned fuel monolith stack mandrel assembly is: A step of assembling a plurality of fuel monoliths into a stack along a longitudinal axis, wherein each of the plurality of fuel monoliths has an eccentric cylindrical shape and includes a first end surface, a second end surface, a side surface connecting the first end surface to the second end surface, and a plurality of first channels extending axially from the first end surface to the second end surface, The steps include inserting the alignment drill rod through one of the plurality of first channels in the direction of the longitudinal axis, A step of attaching an exhaust support plate to the first end of the assembled stack to form a core stack, wherein the exhaust support plate includes a first end face, a second end face, a circumferential surface connecting the first end face to the second end face, and a plurality of second channels extending from the first end face to the second end face, and the step of attaching the exhaust support plate includes inserting the alignment drill rod through one of the plurality of second channels, The steps include inserting a first set of tensioning cables through first portions of the plurality of first channels in the plurality of fuel monoliths and through first portions of the plurality of second channels in the exhaust support plate, The steps include attaching a plurality of second mandrel spacers to the second end of the core stack, The steps include inserting the first set of tensioning cables through the plurality of second mandrel spacers, A step of attaching a plurality of first mandrel spacers to the first end of the core stack, wherein the first end of the core stack includes the exhaust support plate attached to the first end of the assembled stack, (i) inserting a second set of tensioning cables through the plurality of first mandrel spacers, (ii) second portions of the plurality of second channels in the exhaust support plate, (iii) second portions of the plurality of first channels in the plurality of fuel monoliths, and (iv) the plurality of first mandrel spacers, The steps include applying tension to the tension-applying cables of the first set and the tension-applying cables of the second set, The steps include attaching a first threaded mandrel cap to the distal end of the plurality of first mandrel spacers, and attaching a second threaded mandrel cap to the distal end of the plurality of second mandrel spacers, The method according to claim 20, including the method described in claim 20.

22. The method according to claim 21, wherein the channel axis of each of the first portions of the plurality of second channels in the exhaust support plate is non-parallel to the longitudinal axis of the insulated fuel assembly core.

23. The method according to claim 22, wherein the openings of the first portion of the plurality of second channels on the second end face are located around the second end face.

24. The step of forming the tensioned fuel monolith stack mandrel assembly further includes the step of attaching an exhaust support plate to the first end of the core stack, The method according to claim 23, wherein the projection of the channel axis does not intersect with the exhaust shield assembly.

25. Each first end of the tension-applying cable of the first set is adjacent to the second end face of the exhaust support plate, and each second end of the tension-applying cable of the first set is adjacent to the distal end of the plurality of first mandrel spacers. The method according to any one of claims 21 to 24, wherein the first end of each of the second set of tension-applying cables is adjacent to the distal end of the plurality of second mandrel spacers, and the second end of each of the second set of tension-applying cables is adjacent to the distal end of the plurality of first mandrel spacers.

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