Processes for the production of reactor fuel assemblies

Abstract

To provide a nuclear fuel assembly.SOLUTION: A nuclear fuel assembly for nuclear reactor cores includes at least one fuel compact including a fuel compact coating tube defining an inner volume and a plurality of fuel pins each including a pin coating tube defining an inner volume and a fissile fuel arranged in the inner volume of the pin coating tube. The plurality of fuel pins are arranged in the inner volume of the fuel compact coating tube; and the sectional shape of the fuel compact is defined by the fuel compact coating tube.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] Claim of Priority This application claims the benefit of priority of U.S. Provisional Patent Application No. 62 / 794,320, filed on January 18, 2019, the disclosure of which is incorporated herein by reference.

[0002] The presently disclosed invention generally relates to nuclear reactors, and more specifically, to fuel assemblies for use in constructing a nuclear core, and processes for producing those fuel assemblies.

Background Art

[0003] The concept of using nuclear thermal propulsion to propel a spacecraft during space travel is known. Existing propulsion concepts typically utilize a fuel assembly manufacturing process known as fuel dispersion. Fuel dispersion generates uranium-based fuel kernels, which are on the order of 200 μm to 400 μm in diameter. The kernels are often fabricated by conventional internal gelation processes or advanced sol-gel processing methods. The kernels are coated with a metal matrix material (e.g., tungsten) to encapsulate the fuel kernels. Once coated, the particles are embedded in the metal matrix material (e.g., tungsten powder) and formed into monolithic parts by metallurgical compression techniques such as hot isostatic compression molding. However, various difficulties are known to exist with fuel dispersion methods for producing fuel assemblies. For example, there are challenges in producing fuel kernels smaller than 400 μm, and similarly, there are challenges in coating the fuel kernels with refractory metals (e.g., tungsten, molybdenum, etc., but not limited to these). Inconsistent coating of fuel kernels can be a significant problem when trying to achieve a high fuel packing factor (for example, one desirable for fuel assemblies intended for use in nuclear thermal propulsion applications). Specifically, improperly coated fuel kernels can come into contact with each other and aggregate, which can lead to fuel melting and / or fuel loss. Furthermore, the unknown precise location of the coated fuel kernels within the metal matrix material can impair core neutron analysis. The need to prevent aggregation of coated fuel kernels requires the use of a sufficient amount of matrix material (tungsten powder), which can hinder the ability to achieve a sufficient packing factor for fuel assemblies used in the aforementioned nuclear thermal propulsion applications.

[0004] Therefore, the need for fuel assemblies suitable for use in nuclear thermal propulsion assemblies, and for processes to produce them, remains, at least. [Overview of the project] [Means for solving the problem]

[0005] One embodiment of the present invention provides a nuclear fuel assembly for a reactor core, the nuclear fuel assembly comprising at least one fuel cartridge, the at least one fuel cartridge having a lattice structure with an outer wall portion defining an internal volume, at least one flow channel extending through the internal volume of the lattice structure, at least one lattice site disposed within the lattice structure, and at least one fuel compact disposed within one of the corresponding lattice sites, wherein the cross-sectional shape of the at least one fuel compact is the same as the cross-sectional shape of one of the corresponding lattice sites.

[0006] Another embodiment of the present invention provides a nuclear fuel assembly for a reactor core, the nuclear fuel assembly comprising at least one fuel compact, the at least one fuel compact comprising a fuel compact cladding tube defining an internal volume, and a plurality of fuel pins, each fuel pin comprising a pin cladding tube defining an internal volume, and fissile fuel disposed within the internal volume of the pin cladding tube, the plurality of fuel pins being disposed within the internal volume of the fuel compact cladding tube, and the cross-sectional shape of the fuel compact being defined by the fuel compact cladding tube.

[0007] The accompanying drawings are incorporated herein and constitute part of this specification, illustrating one or more embodiments of the present invention, and together with this description, serve to illustrate the principles of the present invention.

[0008] Herein, the present invention will be described more fully with reference to the accompanying drawings, which illustrate some (but not all) embodiments of the invention. In fact, the invention can be embodied in many different forms and should not be construed as being limited to the embodiments described herein. Rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. [Brief explanation of the drawing]

[0009] [Figure 1] This is a perspective view of a reactor core including a fuel assembly constructed according to an embodiment of the present invention. [Figure 2] Figure 1 is a perspective view of the fuel assembly shown. [Figure 3A] Figure 2 is a perspective view of the lattice structure of the fuel assembly shown. [Figure 3B] Figure 2 is an end view of the lattice structure of the fuel assembly shown. [Figure 4] This is a partial perspective view of a fuel pin according to an embodiment of the present invention. [Figure 5] This is an end view of a fuel compact according to an embodiment of the present invention. [Figure 6A] Figure 5 is an end view of the swaging process being performed on the fuel compact shown. [Figure 6B] Figure 5 is an end view of the swaging process being performed on the fuel compact shown. [Figure 6C] Figure 5 is an end view of the swaging process being performed on the fuel compact shown. [Figure 7] This is an end view of a completed fuel compact according to an embodiment of the present invention. [Figure 8] This is a partial perspective view of the assembly of a fuel cartridge according to an embodiment of the present invention. [Figure 9] Figure 8 shows an end view of the fuel cartridge after the fuel compact has been inserted into the grid. [Figure 10A]Figure 9 is a perspective view of a fuel cartridge assembled to form a fuel assembly. [Figure 10B] Figure 9 is a perspective view of a fuel cartridge assembled to form a fuel assembly. [Modes for carrying out the invention]

[0010] The repeated use of reference letters in this specification and drawings is intended to represent the same or similar features or elements of the present invention as disclosed herein.

[0011] Herein, the present invention will be described more fully with reference to the accompanying drawings, which illustrate some (but not all) embodiments of the invention. In fact, the invention can be embodied in many different forms and should not be construed as being limited to the embodiments described herein. Rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. As used herein and in the accompanying claims, the singular forms "a," "an," and "the" refer to multiple subjects unless the context explicitly indicates otherwise.

[0012] Referring here to the figures, the reactor core 100 according to the present invention comprises a plurality of modular fuel assemblies 110, which are housed in a vessel shell 102, as best seen in Figures 1 and 2. Preferably, each fuel assembly 110 has a hexagonal cross-section and is formed by a plurality of fuel cartridges 112a, 112b, 112c, etc., as discussed in more detail below. Preferably, each fuel cartridge 112 comprises a lattice structure 114 defining a plurality of lattice sites 122 within it, and a plurality of fuel compacts 130 disposed therein. Each fuel compact 130 is housed in a correspondingly shaped lattice site 122, as best seen in Figure 8. While the fuel assemblies 110 discussed preferably have a hexagonal cross-section, it should be noted that in alternative embodiments, the fuel assemblies may have different cross-sectional shapes (e.g., triangular, square, rectangular, etc., but not limited to these).

[0013] As shown in Figures 3A and 3B, the grid structure 114 includes a plurality of sidewalls 116 defining an elongated shell, a plurality of elongated tubes 118, and a plurality of inner wall sections 120, the plurality of elongated tubes 118 being arranged within the grid structure 114 such that the longitudinal central axis of each elongated tube 118 is parallel to the longitudinal central axis of the grid structure 114, and the plurality of inner wall sections 120 defining a plurality of grid sites 122 within the grid structure. Each elongated tube 118 extends along the entire length of the grid structure 114, thereby forming a channel through which reactor coolant can flow during reactor operation. Some inner wall sections 120a of the grid structure 114 extend between pairs of elongated tubes 118, while other inner wall sections 120b extend between the corresponding elongated tubes 118 and adjacent sidewalls 116 of the grid structure 114. Therefore, each lattice site 122 does not have the same cross-sectional shape. As best seen in Figure 3B, the currently discussed embodiment of the lattice structure 114 includes 37 elongated tubes 118 and 96 complete and partial lattice sites 122. However, it should be noted that in alternative embodiments, different numbers of elongated tubes 118 and lattice sites 122 may be present. Preferably, the lattice structure 114 is formed by additive manufacturing (also known as metal 3D printing) and consists of a monolithic structure of the desired material (e.g., molybdenum, but not limited to molybdenum). Alternatively, the lattice structure 114 is formed from multiple cut sheets of the desired material (e.g., molybdenum), which are arranged in the desired lattice geometry and then resistance welded to form components.

[0014] As best seen in Figure 8, each fuel compact 130 of the fuel assembly 110 is formed such that its cross-sectional shape is the same as the cross-section of the corresponding lattice site 122. Referring further to Figures 4 and 5, each fuel compact 130 is formed by a plurality of elongated fuel pins 132, which are arranged in an elongated fuel compact covering tube 138 prior to the swaging process, as discussed in more detail below, to achieve the desired cross-sectional shape of the fuel compact. As best seen in Figure 4, each fuel pin 132 includes a pin covering tube 134, which is sealed by an upper wall 133 and a bottom wall 135, so that the pin covering tube 134 defines a sealed internal volume 139. The tube is sealed by the upper wall 133 and bottom wall 135 after the fuel pellets 136 are arranged inside the pin covering tube 134, so that the pin covering tube 134 acts as a covering for the fuel. As shown, the fuel pellet 136 is preferably used for the fuel pin. However, in alternative embodiments of the present invention, fuel kernels, fuel powder, etc., rather than pellets may be used.

[0015] Furthermore, each assembled pin-clad tube 134 and fuel pellet 136 undergoes a swaging process, during which the desired final diameter of the fuel pin 132 is achieved. The pin diameter is selected to meet the desired fuel packing factor requirements of the fuel assembly. After swaging, each fuel pin 132 is positioned within the corresponding fuel compact clad tube 138, as best shown in Figure 5. It should be noted that fuel pins 132 of different diameters are produced to achieve the desired fuel packing factor requirements, thereby reducing the assembly volume defined by the inter-grid space 141 within the fuel compact clad tube. In the preferred embodiment shown, the pin diameter is selected so that the fuel pin 132 can be positioned within the fuel compact clad tube 138 in a Warrington Seale configuration.

[0016] Referring now to FIGS. 6A through 6C, after the fuel pins 132 are arranged in a desired pattern within the fuel compact cladding tube 138, a swaging process is performed on the fuel compact 130 to achieve the desired cross-sectional shape of the compact for subsequent insertion into the corresponding lattice sites 122 (FIG. 8). In the exemplary embodiment shown, the swaging process is performed in a press 142, which deforms the fuel compact cladding tube 138 into the desired cross-sectional shape. As shown, the cross-sectional shape of the fuel compact 130 is a perfect three-lobed shape, which will enable the fuel compact 130 to be disposed in any of the lattice sites 122 that are not adjacent to the side walls 116 of the lattice structure 114.

[0017] Referring now to FIG. 8, each fuel compact 130 is press-fitted into the corresponding lattice site 122 of the lattice structure 114. Preferably, the press fit is achieved by heating the lattice structure 114 prior to inserting the fuel compact 130 into the corresponding lattice site 122. After each lattice site 122 has received the corresponding fuel compact 130 therein, the upper wall portion 133 and the bottom wall portion (not shown) are resistance welded to the upper and bottom ends of the lattice structure 114 to complete the construction of the fuel cartridge 112, as shown in FIG. 9. Both the upper and bottom wall portions include a plurality of apertures 137, each aperture 137 corresponding to the location of the lattice elongated tube 118, and allowing reactor coolant to flow therethrough.

[0018] Referring now to FIGS. 10A and 10B, each fuel assembly 110 is fabricated by stacking a desired number of fuel cartridges 112a, 112b, etc., until a fuel assembly of a desired length is achieved. The stacked fuel cartridges 112a, 112b are then integrated by methods such as, but not limited to, diffusion bonding, resistance welding, mechanical fastening, etc. In that each fuel assembly 110 is constructed from individual fuel cartridges 112 which are then fastened to one another, the material composition of the fuel within each fuel cartridge 112 can be varied from the remaining fuel cartridges 112. As such, the material gradient of each fuel assembly 110 can be varied along its axial link. For example, referring again to FIG. 2, a reduced amount of tungsten can be used in the fuel cartridge 112a forming the upper portion of the fuel assembly 110 as compared to the fuel cartridge 112c forming the bottom portion of the fuel assembly 110. It may be desirable to have an increased amount of tungsten in the bottom fuel cartridge 112c (due to the increased temperature in that portion of the core), but it may be desirable to reduce the amount of tungsten in the fuel cartridges 112a and 112b as the temperature is lower at higher locations in the core. This may be advantageous in that tungsten may be difficult to isotope enrich and is heavier than various other materials (such as molybdenum, etc.) that can be used in locations that encounter lower temperatures.

[0019] These and other modifications and variations of the present invention can be practiced by those skilled in the art without departing from the spirit and scope of the invention (which is more specifically described in the appended claims). For example, in an alternative embodiment of the invention, when constructing a fuel cartridge as described above, a fuel dispersion method may be used to fill the interior of a grid structure surrounding a flow channel, rather than arranging fuel compacts within corresponding grid sites. In addition, it should be understood that the aspects of the various embodiments can be interchanged, either whole or in part. Furthermore, those skilled in the art will recognize that the foregoing description is merely illustrative and is not intended to limit the invention as further described in such appended claims. Accordingly, the spirit and scope of the appended claims should not be limited to the exemplary description of the version contained herein.

Claims

1. A fuel compact for use with a fuel cartridge of a nuclear fuel assembly for a reactor core, wherein the fuel compact is A fuel compact covering tube that defines the internal volume, A plurality of fuel pins, each fuel pin comprising a pin-covered tube defining an internal volume, and fissile fuel disposed within the internal volume of the pin-covered tube, and Includes, The plurality of fuel pins are arranged within the internal volume of the fuel compact covering tube so as to engage with each other, the outermost fuel pins engaging with the inner surface of the fuel compact covering tube, and at least some of the fuel pins have different cross-sectional shapes from each other, but the cross-sectional shape of the fuel compact is defined by the fuel compact covering tube.

2. The fuel compact according to claim 1, wherein the fissile fuel in the plurality of fuel pins is one of a fuel pellet, a plurality of fuel kernels, and fuel powder.

3. The fuel compact is installed inside the fuel cartridge, and the fuel cartridge is A grid structure including an outer wall portion that defines the internal volume; A flow channel with a circular cross-section extending through the internal volume of the lattice structure; At least one non-circular cross-section grid site is disposed inside the grid structure. Includes, The fuel compact is located in one of the corresponding grid sites among the at least one grid site, The fuel compact according to claim 1, wherein the cross-sectional shape of the fuel compact is the same as the corresponding cross-sectional shape of the at least one grid site.

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