Multi-zone fuel element

Abstract

A fuel element having distinct zones or concentration gradients of fuel material along the axial, radial, or both axial and radial directions. The fuel element may be produced using an additive manufacturing process. The additive manufacturing process may facilitate the production of distinct zones or concentration gradient arrangements of the fuel element and may further allow for the incorporation of both fuel and non-fuel materials within any of the zones or gradients.

Description

[Technical Field]

[0001] This disclosure relates in general to nuclear fuel. More specifically, this disclosure relates to a fuel element having multiple zones of fuel material in at least one direction. [Background technology]

[0002] In the field of nuclear fuel, it is known that the enrichment level can be varied axially in the case of tube-packaged pellet fuel (UO2 ceramic fuel). However, radial zoning is not performed within individual fuel elements; rather, it is achieved only at the fuel assembly level by varying the enrichment level of adjacent elements.

[0003] Furthermore, known nuclear reactors use a variety of materials to control the neutron flux (and subsequent power profile) within the reactor. Examples of such materials include neutron moderators, absorbers, and fissile materials. The composition and placement of these materials affect the reactor's operation and performance. Neutron moderators, absorbers, and fissile materials utilize homogeneous elements with different compositions. This homogeneous element structure limits design flexibility and thereby impairs the ability to truly optimize reactor performance. [Overview of the project]

[0004] In one embodiment, a fuel element for use in a reactor fuel assembly includes a first zone extending along a first axis of the fuel element in a first direction and extending outward from the first axis in a second direction of the fuel element. The first zone is made of a first material. A second zone surrounds the first zone in a second direction of the fuel element and extends in the first direction. The second zone is made of a second material different from the first material. The thickness of the second zone in the second direction varies along the first direction. A third zone surrounds the second zone in a second direction of the fuel element and extends in the first direction. The third zone is made of a third material different from the first material and different from the second material. The thickness of the third zone in the second direction varies along the first direction.

[0005] In another embodiment, a fuel element for use in a reactor fuel assembly includes a first zone extending from a first axis of the fuel element in a first direction and extending along the first axis in a second direction of the fuel element. The first zone has a first material composition. A second zone extends from the first axis of the fuel element in a first direction. The second zone is adjacent to the first zone in a second direction of the fuel element. The second zone has a second material composition different from the first material composition. A third zone extends from the first axis of the fuel element in a first direction. The third zone is adjacent to the second zone in a second direction of the fuel element. The third zone has a third material composition different from the first material composition and different from the second material composition.

[0006] In yet another embodiment, a fuel element for use in a reactor fuel assembly, the fuel element having a first zone having a first material composition. The first zone includes a first central zone extending outward from a first axis in a first direction of the fuel element. The first central zone is made of a first material and has a first thickness in a first direction. A first intermediate zone surrounds the first central zone in a first direction of the fuel element. The first intermediate zone is made of a second material different from the first material and has a second thickness. A first outer zone surrounds the first intermediate zone in a first direction of the fuel element. The first outer zone is made of a third material different from the first material and different from the second material and has a third thickness. The fuel element further has a second zone having a second material composition different from the first material composition. The second zone includes a second central zone extending outward from a first axis in a first direction of the fuel element. The second central zone is made of the first material and has a fourth thickness different from the first thickness in the first direction. The second intermediate zone surrounds the second central zone in the first direction of the fuel element. The second intermediate zone is made of the second material and has a fifth thickness different from the second thickness. The second outer zone surrounds the second intermediate zone in the first direction of the fuel element. The second outer zone is made of the third material and has a sixth thickness different from the third thickness.

[0007] In another embodiment, a method for manufacturing a nuclear fuel element using additive manufacturing includes the step of forming a first zone using a first material, wherein the first zone extends along a first axis of the fuel element in a first direction and extends outward from the first axis in a second direction. The method further includes forming a second zone using a second material different from the first material, wherein the second zone surrounds the first zone in a second direction of the fuel element, extends in the first direction, and the thickness of the second zone in the second direction varies along the first direction. The method further includes forming a third zone using a third material different from the first material and different from the second material, wherein the thickness of the third zone in the second direction varies along the first direction.

[0008] In yet another embodiment, a fuel assembly for a nuclear reactor includes a plurality of fuel elements. At least one of the plurality of fuel elements includes a first zone extending from a first axis of the fuel element in a first direction of the fuel element and extending along the first axis in a second direction of the fuel element. The first zone has a first material composition. A second zone extends from the first axis of the fuel element in a first direction of the fuel element. The second zone is adjacent to the first zone in a second direction of the fuel element. The second zone has a second material composition different from the first material composition. A third zone extends from the first axis of the fuel element in a first direction of the fuel element. The third zone is adjacent to the second zone in a second direction of the fuel element. The third zone has a third material composition different from the first material composition and different from the second material composition.

[0009] For a better understanding of embodiments, other purposes, and further features of the present invention, refer to the following description used in conjunction with the accompanying drawings. Similar elements are identified by the same reference numeral. It should be understood that elements shown as a single component may be replaced by multiple components, and elements shown as multiple components may be replaced by a single component. The drawings are not to scale, and the proportions of certain elements may be exaggerated for illustrative purposes. [Brief explanation of the drawing]

[0010] [Figure 1] This is a cross-sectional view of a fuel assembly according to one embodiment of the present invention, where the cross-section is cut off by a self-dividing plane. [Figure 2] Figure 1 is a cross-sectional view of the fuel assembly, where the cross-section is cut off from the diagram in Figure 1 by a plane shifted by 1 / 8 of the torsion of the fuel elements. [Figure 3] Figure 1 is a cross-sectional view of the fuel assembly, cut along a plane parallel to the axial direction of the fuel assembly. [Figure 4] Figure 1 is a perspective view of the fuel elements of a fuel assembly. [Figure 5] Figure 3 is a cross-sectional view of the fuel element. [Figure 6] This is a cross-sectional view of the fuel element shown in Figure 3, circumscribed within a regular polygon. [Figure 7A] This is an end view of a fuel assembly according to an alternative embodiment for use in a pressurized heavy water reactor. [Figure 7B] Figure 7A is a partial side view of the fuel assembly. [Figure 8] Figures 7A and 7B show a pressurized heavy water reactor using the fuel assemblies shown. [Figure 9] Figure 3 is a cross-sectional view of the fuel element. [Figure 10] This is a cross-sectional view of a fuel assembly according to one embodiment of the present invention. [Figure 11] This is a plan view of a fuel element according to an alternative embodiment of the present invention. [Figure 12] This is a cross-sectional view of the fuel element in Figure 11, cut along line 11-11. [Figure 13] It is a cross-sectional view of a fuel element according to another alternative embodiment of the present invention. [Figure 14] It is a cross-sectional view of a fuel element according to another alternative embodiment of the present invention. [Figure 15] It is a plan view of a fuel element according to another alternative embodiment of the present invention. [Figure 16] It is a plan view of a fuel element according to another alternative embodiment of the present invention. DETAILED DESCRIPTION OF EMBODIMENTS

[0011] Configurations for various embodiments of fuel elements are disclosed. The fuel element has discrete zones or concentration gradients of fuel material in at least one direction, for example, along the axial direction, the radial direction, or both the axial and radial directions. An additive manufacturing process can be used to produce the fuel element. The additive manufacturing process facilitates production of discrete zones or concentration gradient arrangements of the fuel element, and further allows both fuel and non-fuel materials to be incorporated into any of the zones or within the gradient. Additive manufacturing is particularly well suited for production of metallic fuels, and can also be used to produce other types of fuels, including but not limited to ceramic fuels such as oxides, carbides and nitrides, and combinations of metallic and ceramic fuels known as cermets.

[0012] Nuclear fuels produced by conventional processes are limited in their ability to incorporate multiple materials, precision enrichment zoning, and functional gradients. Using additive manufacturing processes, it is possible to produce nuclear fuel elements having functional enrichment zoning and multiple materials (including different isotopes) within individual elements, which enables design of fuel systems with performance capabilities unattainable by conventional fuel manufacturing processes. Furthermore, additive manufacturing can be used to vary the enrichment within the fuel element to flatten the shape of the neutron flux in the reactor core, thereby reducing the need for neutron absorbing material.

[0013] The advantages of the fuel elements described herein include improved safety, economics, and performance of reactors for power generation, hydrogen production, isotope production, research, and space and marine applications. Furthermore, the additive manufacturing of the fuel elements described herein transforms other aspects of fuel production, including cost reduction, improved equipment design and operation, increased production and energy utilization efficiency, and improved worker safety.

[0014] The following describes in detail fuel elements having multiple lobe profiles, for which additive manufacturing processes are particularly useful. However, it is understood that additive manufacturing processes may be used to manufacture fuel elements having any desired shape. For example, a fuel element may be formed as a cylinder without lobes. Another example is that a fuel element may be formed as a plate. Furthermore, by using additive manufacturing processes, it is possible to form fuel elements into shapes that are difficult or even impossible to form by conventional manufacturing processes. Therefore, the additive manufacturing processes described herein are not limited to manufacturing fuel elements having multiple lobe profiles as described herein.

[0015] Figures 1 to 3 show a fuel assembly 10 according to one embodiment of the present invention. As shown in Figure 3, the fuel assembly 10 comprises a plurality of fuel elements 20 supported by a frame 25.

[0016] As shown in Figure 3, the frame 25 comprises a shroud 30, guide tubes 40, an upper nozzle 50, a lower nozzle 60, a lower tie plate 70, an upper tie plate 80, and / or other structures that enable the assembly 10 to function as a fuel assembly in the reactor. Without departing from the scope of the present invention, according to various embodiments, one or more of these components of the frame 25 can be omitted.

[0017] As shown in Figure 3, the shroud 25 is fitted to the upper nozzle 50 and the lower nozzle 60. The lower nozzle 60 (or other suitable structure of the assembly 10) is configured and shaped to provide a fluid communication interface between the reactor 90 in which the assembly 10 is located and the assembly 10, so as to facilitate the flow of coolant from the assembly 10 through the assembly 10 to the core. The upper nozzle 50 facilitates the direction of heated coolant from the assembly 10 to the steam generator (for PWRs), turbine (for BWRs), etc., of the power plant. The nozzles 50, 60 have shapes specifically designed to fit properly with the internal structure of the core.

[0018] As shown in Figure 3, the lower tie plate 70 and the upper tie plate 80 are preferably firmly attached to the shroud 30 or the lower nozzle 60 (and / or other suitable structural components of the assembly 10) (for example, via suitable fasteners such as welds, bolts, or screws).

[0019] The lower shaft ends of the elements 20 form pins 20a that fit into holes 70a in the lower tie plate 70, supporting the elements 20 and helping to maintain proper spacing between them. The pins 20a are fitted into the holes 70a to prevent the elements 20 from rotating around their axes or moving axially relative to the lower tie plate 70. This restriction on rotation helps ensure that all contact points between adjacent elements 20 occur at the same axial position along the elements 20 (e.g., in the self-dividing plane described below). The connection between the pins 20a and the holes 70a may be formed by welding, interference fit, non-cylindrical features that prevent rotation (e.g., keyways and splines), and / or any other suitable mechanism for restricting the axial and / or rotational motion of the elements 20 relative to the lower tie plate 70. The lower tie plate 70 includes axially extending channels (e.g., grid of openings) through which the coolant flows toward the elements 20.

[0020] The upper axial end of element 20 forms a pin 20a that fits freely into the hole 80a of the upper tie plate 80, allowing the upper pin 20a to move freely axially upward through the upper tie plate 80 while helping to maintain the spacing between elements 20. As a result, as element 20 grows axially during fission, the elongated element 20 can extend further freely within the upper tie plate 80.

[0021] As shown in Figure 4, pin 70a moves to the central part of element 20.

[0022] Figures 4 and 5 show the individual fuel elements / rods 20 of the assembly 10. As shown in Figure 5, the elongated central portion of the fuel element 20 has a four-lobed cross-section. The cross-section of the element 20 remains substantially uniform over the length of the central portion of the element 20. Each fuel element 20 has a fuel kernel 100 comprising a refractory metal and a fuel material containing fissile material.

[0023] A displacer 110 containing a refractory metal is positioned along the longitudinal axis at the center of the fuel kernel 100. The displacer 110 helps to limit the temperature at the center of such space and minimize fluctuations in the heat flux along the surface of the fuel element by displacing the fissile material that would otherwise occupy the thickest part of the fuel element 20. According to various embodiments, the displacer 110 may be eliminated entirely.

[0024] As shown in Figure 5, the fuel kernel 100 is surrounded by a refractory metal cladding 120. The cladding 120 is preferably thick, strong, and flexible enough to withstand radiation-induced expansion of the kernel 100 without failure (e.g., without exposing the kernel 100 to the environment outside the cladding 120). According to one or more embodiments, the entire cladding 120 is at least 0.3 mm, 0.4 mm, 0.5 mm, and / or 0.7 mm thick. According to one or more embodiments, the thickness of the cladding 120 is at least 0.4 mm to reduce the possibility of failure due to expansion of the cladding 120, failure due to oxidation, and / or any other failure mechanisms.

[0025] The cladding 120 can have a substantially uniform thickness in the annular direction (i.e., around the periphery of the cladding 120, as shown in the cross-sectional view of Figure 5) and over the axial / longitudinal length of the kernel 100 (as shown in Figure 4). Alternatively, according to one or more embodiments, as shown in Figure 5, the cladding 120 is thicker at the tips of the lobes 20b than at the concave intersections / areas 20c between the lobes 20b. For example, according to one or more embodiments, the cladding 120 at the tips of the lobes 20b is at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, 125%, and / or 150% thicker than the cladding 120 at the concave intersections / areas 20c. The thicker cladding 120 at the tip of the lobe 20b improves wear resistance at the tip of the lobe 20b where adjacent fuel elements 20 come into contact with each other in a self-dividing plane (described later).

[0026] The refractory metal used in the displacer 110, fuel kernel 100, and cladding 120 comprises zirconium according to one or more embodiments of the present invention. As used herein, the term zirconium means pure zirconium or zirconium combined with other alloying materials. However, other refractory metals may be used instead of zirconium without departing the scope of the present invention (e.g., niobium, molybdenum, tantalum, tungsten, rhenium, titanium, vanadium, chromium, zirconium, hafnium, ruthenium, osmium, iridium, and / or other metals). As used herein, the term "refractory metal" means any metal / alloy having a melting point above 1800 degrees Celsius (2073 K).

[0027] Furthermore, in certain embodiments, the refractory metal may be replaced with another non-fuel metal, such as aluminum. However, the use of non-refractory non-fuel metals is most suitable for reactor cores operating at lower temperatures (e.g., small cores with a height of about 1 meter and a power rating of 100 MWe or less). For use in cores with higher operating temperatures, refractory metals are preferred.

[0028] As shown in Figure 5, the central portions of the fuel kernel 100 and cladding 120 have a four-lobe profile that forms a spiral spacer rib 130. The displacer 110 may also be molded to protrude outward in the rib 130 (for example, the corners of a square displacer 110 are aligned with the rib 130). According to alternative embodiments of the present invention, the fuel element 20 may have more or fewer ribs 130 without departing from the scope of the present invention. For example, as commonly shown in Figure 5 of U.S. Patent Application Publication No. 2009 / 0252278, the fuel element may have three ribs / lobes, which are preferably equally spaced circumferentially from one another. The number of lobes / ribs 130 may depend at least in part on the shape of the fuel assembly 10. For example, a four-lobe element 20 may work well in a fuel assembly 10 with a square cross-section (for example, as used in the AP-1000). In contrast, three-lobe fuel elements can function well in hexagonal fuel assemblies (such as those used in VVERs).

[0029] Figure 9 shows various dimensions of the fuel element 20 according to one or more embodiments. Any of these dimensions, parameters, and / or ranges specified in the following table can be increased or decreased by up to 5%, 10%, 15%, 20%, 25%, 30%, 40%, 50%, or more without departing from the scope of the present invention. [Table 1]

[0030] As shown in Figure 4, the displacer 110 has a square quadrilateral cross-sectional shape, with the corners of the square quadrilateral aligned with the ribs 130. The displacer 110 forms a spiral that follows the spiral of the ribs 130, such that the corners of the displacer 110 remain aligned with the ribs 130 along the axial length of the fuel kernel 100. In alternative embodiments having more or fewer ribs 130, the displacer 110 preferably has a regular polygonal cross-sectional shape with the same number of sides as the element 20 has ribs.

[0031] As shown in Figure 6, the cross-sectional area of ​​the central portion of element 20 is preferably substantially smaller than the area of ​​square 200, where each tip of the rib 130 touches one side of the square 200. More generally, the cross-sectional area of ​​element 20 having n ribs is preferably smaller than the area of ​​a regular polygon having n sides, where each tip of the rib 130 touches one side of the polygon. According to various embodiments, the ratio of the area of ​​element 20 to the area of ​​a square (or the associated regular polygon of element 20 having more or fewer ribs 130) is less than 0.7, 0.6, 0.5, 0.4, 0.35, and 0.3. As shown in Figure 1, this area ratio approximates how much of the available space within the shroud 30 is occupied by the fuel element 20. Consequently, a lower ratio means that more space is favorably available for the coolant, which also acts as a neutron moderator, increasing the moderator-to-fuel ratio (important for neutron engineering), reducing hydraulic drag, and increasing heat transfer from element 20 to the coolant. According to various embodiments, the resulting moderator-to-fuel ratios are at least 2.0, 2.25, 2.5, 2.75, and / or 3.0 (in contrast to 1.96 when conventional cylindrical uranium oxide rods are used). Similarly, according to various embodiments, the flow area of ​​the fuel assembly 10 increases by more than 16% compared to the use of one or more conventional fuel assemblies using cylindrical uranium oxide rods. The increased flow area can reduce the pressure drop of the coolant through the assembly 10 (compared to conventional uranium oxide assemblies), which can be advantageous with respect to the pumping of the coolant through the assembly 10.

[0032] As shown in Figure 4, the element 20 is elongated in the axial direction. In the illustrated embodiment, each element 20 is a full-length element, extending from the lower tie plate 70 at or near the bottom of the assembly 10 to the upper tie plate 80 at or near the top of the assembly 10. Depending on the various embodiments and furnace designs, this can result in elements 20 that are 1 meter to over 4 meters in length (in the case of a small furnace). Thus, in a typical furnace, the element 20 may be 1 to 5 meters in length. However, the element 20 can be made longer or shorter to accommodate furnaces of any other size without departing from the scope of the present invention.

[0033] Although the illustrated element 20 is the full length in itself, the element 20 may alternatively be segmented, resulting in multiple segments coming together to form a full-length element. For example, four individual 1-meter element segments 20 can be aligned end to end to effectively create a full-length element. Additional tie plates 70, 80 can be provided at the intersections between segments to maintain the axial spacing and arrangement of the segments.

[0034] According to one or more embodiments, the fuel kernel 100 includes a combination of a refractory metal / alloy and fuel material. The refractory metal / alloy may include a zirconium alloy. The fuel material may include low-enriched uranium (e.g., U235, U233), plutonium, or thorium combined with low-enriched uranium and / or plutonium as defined below. As used herein, “low-enriched uranium” means that the entire fuel material contains less than 20% by weight of fissile material (e.g., uranium-235 or uranium-233). According to various embodiments, the uranium fuel material is enriched to 1% to 20% by weight, 5% to 20% by weight, 10% to 20% by weight, and / or 15% to 20% by weight of uranium-235. According to one or more embodiments, the fuel material contains 19.7% enriched uranium-235.

[0035] According to various embodiments, the fuel material may include a volume fraction of 3-10%, 10-40%, 15-35%, and / or 20-30% of the fuel kernel 100. According to various embodiments, the refractory metal may include a volume fraction of 60-99%, 60-97%, 70-97%, 60-90%, 65-85%, and / or 70-80% of the fuel kernel 100. According to one or more embodiments, a volume fraction within one or more of these ranges provides an alloy having beneficial properties defined by the material phase diagram of a particular alloy composition. The fuel kernel 100 may include a Zr-U alloy, which is a high-alloy fuel (i.e., a relatively high concentration of alloying components relative to the uranium component) containing either 6-phase UZr2 or a combination of 6-phase UZr2 and a-phase Zr. According to one or more embodiments, the six phases of the U-Zr binary alloy system may range from approximately 65–81 volume percent (approximately 63–80 atomic percent) of zirconium composition in the fuel kernel 100. One or more of these embodiments have been shown to result in low volume irradiation-induced expansion of the fuel element 20. According to one or more such embodiments, fission gases are encombined into the metal kernel 100 itself so that one or more embodiments of the fuel element 20 can omit the conventional gas gap from the fuel element 20. According to one or more embodiments, such expansion may be significantly less than would occur if a low alloy (a-phase only) composition were used (e.g., at least 10%, 20%, 30%, 50%, 75%, 100%, 200%, 300%, 500%, 1000%, 1200%, 1500%, or more reduction in volume percent expansion per atomic percent burnup compared to when a low alloy a-phase U-10Zr fuel is used). According to one or more embodiments of the present invention, the irradiation-induced expansion of the fuel element 20 or its kernel 100 may be less than 20, 15, 10, 5, 4, 3, and / or 2 volume percent per atomic percent burnup. According to one or more embodiments, the expansion is expected to be about 1 volume percent per atomic percent burnup.

[0036] According to one or more alternative embodiments of the present invention, the fuel kernel is replaced with a plutonium-zirconium binary alloy having the same or similar volume percentage as the U-Zr fuel kernel 100 described above, or having a different volume percentage than the U-Zr fuel kernel 100 described above. For example, since plutonium typically has a weight fraction of about 60-70% of the fissile isotope, while LEU uranium has a weight fraction of less than 20% of the fissile U-235 isotope, the plutonium fraction in kernel 100 may be substantially smaller than the corresponding uranium fraction in the corresponding uranium-based kernel 100. According to various embodiments, the volume fraction of plutonium in kernel 100 may be less than 15%, less than 10%, and / or less than 5%, and the volume fraction of the refractory metal is adjusted accordingly.

[0037] The use of the high-alloy kernel 100 according to one or more embodiments of the present invention can also result in advantageous retention of fission gases during irradiation. Oxide fuels and low-alloy metal fuels typically exhibit significant fission gas emission, which is usually contained by fuel designs that utilize a plenum within the fuel rod to contain the normally emitted fission gases. In contrast, the fuel kernel 100 according to one or more embodiments of the present invention does not emit fission gases. This is partly due to the low operating temperature of the fuel kernel 100 and the fact that fission gas atoms (specifically Xe and Kr) behave like solid fission products. According to one or more embodiments, the formation of fission gas bubbles and their migration to the outside of the fuel kernel 100 along grain boundaries does not occur. According to one or more embodiments, small (several microns in diameter) fission gas bubbles may form at sufficiently high temperatures. However, according to one or more embodiments of the present invention, these bubbles remain isolated within the fuel kernel 100 and do not form an interconnected network that facilitates fission gas emission. The metallurgical bond between the fuel kernel 100 and the cladding 120 can provide an additional barrier against the release of fission gases.

[0038] According to various embodiments, one or more fuel kernels 100 (or cladding 120 or other suitable parts of fuel element 20) of the fuel element 20 can be alloyed with a flammable poison such as gadolinium, boron, erbium, or other suitable neutron-absorbing material to form an integrated flammable poison fuel element. Different fuel elements 20 within the fuel assembly 10 may utilize different flammable poisons and / or different amounts of flammable poisons. For example, some of the fuel elements 20 in the fuel assembly 10 (e.g., less than 75%, less than 50%, less than 20%, 1-15%, 1-12%, 2-12%, etc.) may include kernels 100 having 25% by weight, 20% by weight, and / or 15% by weight or less of Gd (e.g., 1-25% by weight, 1-15% by weight, 5-15%, etc.). Other fuel elements 20 of the fuel assembly 10 (e.g., 10-95%, 10-50%, 20-50%, or more fuel elements 20 than the number of fuel elements 20 utilizing Gd) may include kernels 100 having 10 weight percent or less of Er (e.g., 0.1-10.0 weight percent, 0.1-5.0 weight percent, etc.).

[0039] According to various embodiments, the flammable toxin replaces the fuel material (rather than the refractory metal) in a fuel element 20 that does not contain the flammable toxin in its kernel 100. For example, according to one embodiment of a fuel element 20 in which the kernel 100 contains 65 volume percent zirconium and 35 volume percent uranium when the toxin is absent, the fuel element 20 contains a kernel 100 which is 16.5 volume percent Gd, 65 volume percent zirconium, and 18.5 volume percent uranium. According to one or more other embodiments, the flammable toxin replaces the refractory metal instead of the fuel material. According to one or more other embodiments, the flammable toxin in the fuel kernel 100 replaces the refractory metal and the fuel material proportionally. Therefore, according to various embodiments of these embodiments, the flammable toxin in the fuel kernel 100 may be placed in phase 6 of UZr2 or phase 6 of Zr such that the presence of the flammable toxin does not alter the phase of the UZr2 alloy or Zr alloy in which the flammable toxin is located.

[0040] A fuel element 20 having a kernel 100 containing a flammable toxin can constitute a portion (e.g., 0-100%, 1-99%, 1-50%, etc.) of the fuel elements 20 in one or more fuel assemblies 10 used in the reactor core. For example, a fuel element 20 containing a flammable toxin can be strategically positioned within the fuel assembly grid of an assembly 10 that also includes fuel elements 20 that do not contain a flammable toxin to provide power distribution control and reduce soluble boron concentration early in the operating cycle. Similarly, a selective fuel assembly 10 containing a fuel element 20 containing a flammable toxin can be strategically positioned within the core relative to an assembly 10 that does not contain a fuel element 20 containing a flammable toxin to provide power distribution control and reduce soluble boron concentration early in the operating cycle. The use of such an integrated flammable absorbent can facilitate the design of extended operating cycles.

[0041] Alternatively and / or additionally, separate non-fuel-supported combustible toxic materials may be included in the fuel assembly 10 (e.g., adjacent to the fuel elements 20, inserted into the guide tubes of a fuel assembly 10 that does not accept control rods, instead of one or more fuel elements 20, etc.). In one or more embodiments, such non-fuel combustible toxic rods can be designed as spider assemblies (referred to as combustible toxic rod assemblies (BPRAs)) similar to those used in Babcock and Wilcox or Westinghouse-designed reactors. These may then be inserted into control rod guide tubes and anchored to select fuel assemblies 10 that do not have a control bank for the initial operating cycle for reactivity control. If combustible toxic clusters are used, the combustible toxic clusters may be removed when the fuel assembly is repositioned for the next fuel cycle. According to an alternative embodiment in which a separate non-fuel-supported combustible toxic rod is placed in place of one or more fuel elements 20, the non-fuel-supported combustible toxic rod remains in the fuel assembly 10 and is discharged along with the other fuel elements 20 when the fuel assembly 10 reaches the end of its usable life.

[0042] The fuel element 20 is manufactured by powder metallurgy co-extrusion. Typically, powdered refractory metal and powdered metal fuel material (and, if included in the kernel 100, powdered flammable toxic substances) for the fuel kernel 100 are mixed, a blank for the displacer 110 is placed in the powder mixture, and the combination of powder and displacer 110 is then pressurized and sintered to form a fuel core stock / billet (for example, in a mold heated to varying degrees over various time periods to sinter the mixture). The blank for the displacer 110 may have the same or similar cross-sectional shape as the displacer 110 that is ultimately formed. Alternatively, the blank for the displacer 110 may have a shape designed to deform into the intended cross-sectional shape of the displacer 110 during extrusion. The fuel core stock (containing the materials for the displacer 110 and the sintered fuel kernel 100) is inserted into a tube of a hollow cladding 120 having a sealed tube base and an opening at the other end. Next, the opening at the other end is sealed with an end plug made of the same material as the cladding to form a billet. The billet may be cylindrical in shape, or it may have a shape closer to the final cross-sectional shape of the element 20, for example, as shown in Figures 5 and 9. The billet is then co-extruded by a die set under temperature and pressure to create an element 20, which includes the kernel 100, cladding 110, and displacer 120, to be finally formed. According to various embodiments utilizing a non-cylindrical displacer 110, the billet may be appropriately oriented with respect to the extrusion press die so that the corners of the displacer 110 align with the lobes 20b of the fuel element 20. The extrusion process can be carried out by either direct extrusion (i.e., moving the billet through a fixed die) or indirect extrusion (i.e., moving the die toward a fixed billet). This process metallurgically bonds the cladding 120 to the fuel kernel 100 and reduces the risk of the cladding 120 delaminating from the fuel kernel 100. The tubes and end plugs of the cladding 120 are metallurgically bonded to each other to seal the fuel kernel 100 within the cladding 120.Due to the high melting points of the refractory metals used in fuel element 10, powder metallurgy tends to be the preferred method for manufacturing components from these metals.

[0043] According to one or more alternative embodiments, the fuel core stock of the fuel element 20 may be manufactured by casting instead of sintering. Powdered or monolithic refractory metal and powdered or monolithic fuel material (and, if contained in the kernel 100, powdered flammable toxic material) can be mixed, melted, and poured into a mold. The mold can create a displacer subrank-shaped void in the cast kernel 100 so that a blank for the displacer 110 can be inserted after the kernel 100 has been cast, just as a cladding 120 is added to form an extruded billet. The remaining steps for manufacturing the fuel element 20 may remain the same or similar as in the embodiments above that utilize sintering instead of casting. Subsequent extrusion results in metallurgical bonding between the displacer 110 and the kernel 100, and between the kernel 100 and the cladding 120.

[0044] According to one or more alternative embodiments, the fuel element 20 is manufactured using powdered ceramic fuel material instead of powdered metallic fuel material. The remaining manufacturing steps may be the same as those described above with respect to embodiments using powdered metallic fuel material. In various embodiments of metallic fuels and ceramic fuels, the manufacturing process can result in a fuel kernel 100 comprising fuel material disposed within a base material of a metallic non-fuel material. In one or more embodiments of metallic fuels, the resulting fuel kernel 100 comprises a metallic fuel alloy kernel comprising an alloy of metallic fuel material and a base material of a metallic non-fuel material (e.g., a uranium-zirconium alloy). In one or more embodiments of ceramic fuels, the kernel 100 comprises ceramic fuel material disposed within a base material of a metallic non-fuel material (e.g., scattered throughout). According to various embodiments, the ceramic fuel material used in the manufacturing process may include powdered uranium oxide or plutonium oxide, powdered uranium nitride or plutonium nitride, powdered uranium carbide or plutonium carbide, powdered uranium hydride or plutonium hydride, or combinations thereof. In contrast to conventional UO2 fuel elements in which UO2 pellets are arranged inside a tube, the manufacturing process according to one or more embodiments of the present invention involves placing the ceramic fuel within a solid matrix of a non-fuel material (e.g., a zirconium matrix).

[0045] As shown in Figure 4, the axial winding pitch of the spiral ribs 130 is selected depending on the condition that the axes of adjacent fuel elements 10 are spaced at intervals equal to the width across the corners of the fuel element cross-sections, and may be 5% to 20% of the length of the fuel element 20. According to one embodiment, the pitch (i.e., the axial length over which the lobes / ribs rotate completely) is approximately 21.5 cm, while the total effective length of the element 20 is approximately 420 cm. As shown in Figure 3, the vertical stability of the fuel element 10 is provided by the lower tie plate 70 at the bottom, by the upper tie plate 80 at the top, and by the shroud 30 relative to the height of the core. As shown in Figure 1, the fuel element 10 has a circumferential orientation such that the lobed profiles of any two adjacent fuel elements 10 have a common plane of symmetry passing through the axes of the two adjacent fuel elements 10 in at least one cross-section of the fuel element bundle.

[0046] As shown in Figure 1, the helical twist of the fuel elements 20, combined with their orientation, ensures the existence of one or more self-dividing planes. As shown in Figure 1, in such self-dividing planes, the ribs of adjacent elements 20 are in contact with each other, ensuring adequate spacing between such elements 20. Thus, the center-to-center spacing of the elements 20 is approximately equal to the width from corner to corner of each element 20 (12.6 mm for the elements shown in Figure 5). Depending on the number of lobes 20b within each fuel element 20 and the relative geometric arrangement of the fuel elements 20, only all adjacent fuel elements 20, or only some of the adjacent fuel elements 20, are in contact with each other. For example, in the illustrated embodiment with four lobes, each fuel element 20 is in contact with all four adjacent fuel elements 20 in each self-dividing plane. However, in the embodiment of three-lobe fuel elements where the fuel elements are arranged in a hexagonal pattern, each fuel element is in contact with only three of the six adjacent fuel elements in a given self-dividing plane. The three-lobe fuel element contacts the other three adjacent fuel elements in the next axially spaced self-dividing plane (i.e., offset by 1 / 6 of a rotation from the previous self-dividing plane).

[0047] In an n-lobed element 20 where n fuel elements are adjacent to a particular fuel element 20, a self-dividing plane exists every 1 / n helical rotation (for example, every 1 / 4 helical rotation for a 4-lobed element 20 where four other fuel elements 20 are arranged in a square pattern so as to be adjacent to the fuel element 20, and every 1 / 3 helical rotation for a 3-lobed element where three fuel elements are adjacent to the fuel element (i.e., every 120 degrees around the fuel element)). The pitch of the helix may be modified to create more or fewer self-dividing planes over the axial length of the fuel element 20. According to one embodiment, each 4-lobed fuel element 20 includes multiple twists such that multiple self-dividing planes exist over the axial length of the bundle of fuel elements 20.

[0048] In the illustrated embodiment, all of the elements 20 are twisted in the same direction. However, according to an alternative embodiment, adjacent elements 20 may be twisted in opposite directions without departing from the scope of the present invention.

[0049] The formula for the number of self-dividing planes along the length of the fuel rod is as follows: N=n*L / h, where: L -- Length of the fuel rod n -- Number of lobes (ribs) and number of fuel elements adjacent to a fuel element. h -- Pitch of the spiral twist This formula is slightly different if the number of lobes and the number of fuel elements adjacent to a fuel element are not the same.

[0050] As a result of such self-separation, the fuel assembly 10 can omit the spacer grid that may have been necessary to ensure proper element spacing along the length of the assembly 10. By eliminating the spacer grid, the coolant can flow more freely through the assembly 10, thereby advantageously increasing heat transfer from the elements 20 to the coolant. However, according to alternative embodiments of the present invention, the assembly 10 may include a spacer grid without departing from the scope of the invention.

[0051] As shown in Figure 3, the shroud 30 extends axially along the entire length of the fuel element 20, forming a tubular shell surrounding the element 20. However, according to an alternative embodiment of the present invention, the shroud 30 may include axially spaced bands, each surrounding the fuel element 20. One or more such bands may be axially aligned with a self-dividing plane. Axially extending corner supports may extend between such axially spaced bands to support the bands, maintain the alignment of the bands, and reinforce the assembly. Alternatively and / or additionally, holes may be cut into the otherwise tubular / polygonal shroud 30 where the shroud 30 is not needed or desired for support. The use of a full shroud 30 facilitates better control of the separate coolant flow through each individual fuel assembly 10. Conversely, the use of a shroud with bands or holes facilitates better coolant mixing between adjacent fuel assemblies 10 and can favorably reduce the coolant temperature gradient between adjacent fuel assemblies 10.

[0052] As shown in Figure 1, the periphery of the cross-section of the shroud 30 has a shape that accommodates the reactor in which the fuel assembly 10 is used. In reactors such as the AP-1000 that utilize square fuel assemblies, the shroud has a square cross-section. However, the shroud 30 can alternatively take any suitable shape depending on the reactor in which it is used (for example, a hexagonal shape for use in a VVER reactor, as shown in Figure 1 of U.S. Patent Application Publication No. 2009 / 0252278).

[0053] The guide tube 40 provides for the insertion of control absorbent elements based on boron carbide (B4C), silver-indium-cadmium (Ag,In,Cd), dysprosium titanate (Dy2O3·TiO2), or other suitable alloys or materials (not shown) used for reactivity control, and flammable absorbent elements based on boron carbide, gadolinium oxide (Gd2O3), or other suitable materials (not shown), and is positioned within the upper nozzle 50 with the capability of elastic axial displacement. The guide tube 40 may include a zirconium alloy. For example, the arrangement of the guide tube 40 shown in Figure 1 is the arrangement used in the AP-1000 furnace (e.g., 24 guide tubes arranged in two annular rows at the positions shown in the 17×17 grid).

[0054] The shape, size, and characteristics of the frame 25 depend on the specific core in which the fuel assembly 10 is used. Therefore, those skilled in the art will understand how to fabricate a frame of the appropriate shape and size for the fuel assembly 10. For example, the frame 25 may have a shape and configuration that is suitable for a conventional nuclear power plant core, instead of a conventional uranium oxide or mixed oxide fuel assembly for the plant's core. The nuclear power plant may include core designs that were actually used before 2010 (e.g., 2, 3, or 4-loop PWRs, BWR-4s). Alternatively, the nuclear power plant may be an entirely new design specifically tailored for use with the fuel assembly 10.

[0055] As described above, the illustrated fuel assembly 10 is designed for use in AP-1000 or EPR reactors. The assembly contains a 17x17 array of fuel elements 20, 24, of which a total of 265 fuel elements 20 in the EPR, or 264 fuel elements 20 in the AP-1000, are replaced by the guide tubes 40 described above (in the AP-1000, in addition to 24 fuel elements being replaced by guide tubes, the central fuel element is also replaced by an instrumentation tube).

[0056] Element 20 preferably provides 100% of the total fissile material of the fuel assembly 10. Alternatively, a portion of the fissile material of the assembly 10 may be provided by fuel elements other than element 20 (e.g., lobeless fuel elements, uranium oxide elements, elements having different fuel ratios and / or enrichments than element 20). According to various such alternative embodiments, fuel element 20 provides at least 50 volume%, 60 volume%, 70 volume%, 75 volume%, 80 volume%, 85 volume%, 90 volume%, and / or 95 volume% of the total fissile material of the fuel assembly 10.

[0057] The use of the metallic fuel element 20 according to one or more embodiments of the present invention facilitates various advantages over conventionally used uranium oxide or mixed oxide fuels in light water reactors (LWRs) (including boiling water reactors and pressurized water reactors), such as the Westinghouse AP-1000, the AREVA EPR reactor, or the GE ABWR. For example, according to one or more embodiments, the power rating of an LWR operating with standard uranium oxide or mixed oxide fuel can be increased by up to approximately 30% by using the all-metallic fuel element 20 and / or fuel assembly 10 instead of the standard uranium oxide fuel and fuel assembly currently used in existing types of LWRs or proposed new types of LWRs.

[0058] One of the major constraints on increasing the power rating of LWRs operating with standard uranium oxide fuel was the small surface area of ​​the cylindrical fuel elements utilized by such fuel. Cylindrical fuel elements have the lowest surface area-to-volume ratio for any type of fuel element cross-sectional profile. Another major constraint for standard uranium oxide fuel was the relatively low burnup that such fuel elements could achieve while still meeting acceptable fuel performance criteria. As a result, these factors associated with standard uranium oxide or mixed oxide fuels significantly limit the extent to which existing reactor power ratings can be increased.

[0059] One or more embodiments of the all-metal fuel element 20 overcome the above limitations. For example, by eliminating the spacer grid as described above, hydraulic resistance can be reduced, and therefore the coolant flow and heat flux from the element 20 to the primary coolant can be increased. A helical twist of the fuel element 20 can increase the mixing and turbulence of the coolant, which can also increase the heat flux from the element 20 to the coolant.

[0060] Preliminary neutron and thermohydraulic analyses according to one or more embodiments of the present invention are shown below. The thermal power rating of LWR reactors can be increased by more than 30.7% (for example, the thermal power rating of EPR reactors can be increased from 4.59 GWth to 6.0 GWth). If the uranium volume fraction in the uranium-zirconium mixture is 25% and the uranium-235 enrichment is 19.7%, an EPR core with a configuration of 20 four-lobe metal fuel elements can operate for approximately 500-520 rated power operating days (EFPD) at an increased thermal power rating of 6.0 GWth when 72 fuel assemblies per batch are replaced (once every 18 months), or for 540-560 EFPDs when 80 fuel assemblies per batch are replaced (once every 18 months). • By increasing the surface area of ​​multi-lobe fuel elements, it has been shown that the mean surface heat flux of multi-lobe fuel elements is 4–5% lower than that of cylindrical uranium oxide fuel elements operating at a thermal power rating of 4.59 GWth, even at the increased power rating of 6.0 GWth. This allows for a greater safety margin against critical heat flux (e.g., increased deviation from the maximum fraction limiting the nuclear boiling margin in PWRs or the critical power ratio in BWRs). Furthermore, this allows for the possibility of using 12 fuel elements per assembly, including flammable toxins. Flammable toxins can be used to remove excess reactivity at the start of the cycle or to increase the Doppler effect during core heating. Therefore, the fuel assembly 10 can provide greater thermal output at lower fuel operating temperatures than conventional uranium oxide or mixed oxide fuel assemblies.

[0061] To utilize the increased power output of assembly 10, a conventional power plant can be upgraded (e.g., with larger and / or additional coolant pumps, steam generators, heat exchangers, pressurizers, and turbines). In fact, according to one or more embodiments, this upgrade can supply 30-40% more electricity than the existing reactor. Such a possibility avoids the need to build a complete second reactor. The refitting costs can be quickly recouped by the increased electrical output. Alternatively, a new power plant can be constructed to include appropriate features for handling and utilizing the higher thermal output of assembly 10.

[0062] Furthermore, one or more embodiments of the present invention can enable an LWR to operate at the same power rating as a standard uranium oxide or mixed oxide fuel using an existing reactor system without major reactor modifications. For example, according to one embodiment, • EPR would have the same output as when conventional uranium oxide fuel is used: 4.59 GWt; If the uranium volume fraction in the uranium-zirconium mixture is 25% and the uranium-235 enrichment is approximately 15%, then an EPR core with a configuration of 20 four-lobe metal fuel elements can operate for approximately 500-520 EFPDs when 72 fuel assemblies are replaced per batch, or for 540-560 EFPDs when 80 fuel assemblies are replaced per batch. The mean surface heat flux of element 20 is reduced by approximately 30% compared to the mean surface heat flux of a cylindrical rod with conventional uranium oxide fuel (e.g., 39.94 vs. 57.34 W / cm²). Since the coolant temperature rise through assembly 10 (e.g., the difference between inlet and outlet temperatures) and the coolant flow rate through assembly 10 remain almost the same as with a conventional fuel assembly, the reduction in mean surface heat flux results in a corresponding reduction in fuel rod surface temperature, which contributes to an increased safety margin for critical heat flux (e.g., an increased deviation from the maximum fraction limiting the nuclear boiling margin in a PWR or the critical power ratio in a BWR).

[0063] Additionally and / or alternatively, fuel assemblies 10 according to one or more embodiments of the present invention can be progressively deployed / loaded into the reactor core in place of conventional fuel assemblies. During the transition period, fuel assemblies 10 having equivalent fissile / neutron / thermal output to conventional fuel assemblies can gradually replace such conventional fuel assemblies over a series of fuel changes without altering the operating parameters of the power plant. Thus, fuel assemblies 10 can be retrofitted to existing cores that may be critical during the transition period (i.e., starting with a partial core having fuel assemblies 10 and gradually transitioning to a full core with fuel assemblies 10).

[0064] Furthermore, the fissile content of assembly 10 can be adjusted to suit a specific transition desired by the plant operator. For example, the fissile content can be appropriately increased to increase the reactor's thermal output by 0% to 30% or more compared to the use of the conventional fuel assembly that assembly 10 replaces. Thus, the power plant operator can select a specific power increase based on the capabilities of the power plant at various points in time during the upgrade, or based on the existing plant infrastructure.

[0065] One or more embodiments of the fuel assembly 10 and fuel element 20 can be used in fast reactors (as opposed to light water reactors) without departing from the scope of the present invention. In fast reactors, the non-fuel metal of the fuel kernel 100 is preferably a refractory metal, such as a molybdenum alloy (e.g., pure molybdenum or a combination of molybdenum and another metal), and the cladding 120 is preferably stainless steel (including any alloy variant thereof) or other material suitable for use with a coolant in such a reactor (e.g., sodium). Such fuel element 20 may be manufactured by the co-extrusion process described above, or by any other suitable method (e.g., vacuum melting).

[0066] As shown in Figures 7A, 7B, and 8, a fuel assembly 510 according to one or more embodiments of the present invention can be used in a pressurized heavy water reactor 500 such as a CANDU reactor (see Figure 8).

[0067] As shown in Figures 7A and 7B, the fuel assembly 510 comprises a plurality of fuel elements 20 mounted on a frame 520. The frame 520 comprises two end plates 520a, 520b that are mounted on the opposing axial ends of the fuel elements 20 (for example, by welding, interference fit, or one of the various mounting methods described above for attaching the elements 20 to the lower tie plate 70). The elements 20 used in the fuel assembly 510 are typically much shorter than the elements 20 used in the assembly 10. According to various embodiments and the furnace 500, the elements 20 and the assembly 510 used in the furnace 500 may be about 18 inches long.

[0068] The elements 20 may be positioned relative to each other within the assembly 510 such that the self-dividing plane maintains the spacing between the elements 20, as described above with respect to the assembly 10. Alternatively, the elements 20 of the assembly 510 may be spaced apart from each other such that adjacent elements 20 never touch each other, and instead, the frame 520 may be used entirely to maintain the spacing between the elements 20. Furthermore, spacers may be attached to the elements 20 or their ribs at various positions along the axial length of the elements 20 to contact adjacent elements 20 and help maintain the element spacing 20 (similar to how spacers are used on conventional fuel rods in conventional fuel assemblies for pressurized heavy water reactors to help maintain the rod spacing).

[0069] As shown in Figure 8, fuel assemblies 510 are supplied to the calandria tubes 500a of the reactor 500 (sometimes referred to in the art as the calandria 500). The reactor 500 uses heavy water 500b as moderator and primary coolant. The primary coolant 500b circulates horizontally through the tubes 500a and then to a heat exchanger, where the heat is transferred to a secondary coolant loop, typically used to generate electricity via a turbine. Typically, a fuel assembly loading mechanism (not shown) is used to load fuel assemblies 510 onto one side of the calandria tubes 500a and push out spent assemblies 510 from the opposite side of the tubes 500a while the reactor 500 is operating.

[0070] The fuel assembly 510 can be designed as a direct replacement for conventional fuel assemblies (also known in the art as fuel bundles) for existing conventional pressurized heavy water reactors (e.g., CANDU reactors). In such embodiments, the assembly 510 is supplied to the reactor 500 in place of the conventional assemblies / bundles. Such a fuel assembly 510 can be designed to have similar neutron / thermal properties to the conventional assemblies being replaced. Alternatively, the fuel assembly 510 may be designed to provide an improvement in thermal output. In such an improved embodiment, a new or upgraded reactor 500 can be designed to accommodate the higher thermal output.

[0071] According to various embodiments of the present invention, the fuel assembly 10 is designed to replace a conventional fuel assembly in a conventional reactor. For example, the fuel assembly 10 shown in Figure 1 is specifically designed to replace a conventional fuel assembly that utilizes a 17 × 17 array of UO2 fuel rods. If the guide tubes 40 of the assembly 10 are left in exactly the same position as when used in a conventional fuel assembly, and all fuel elements 20 are the same size, the pitch between fuel elements / rods remains unchanged between a conventional UO2 fuel assembly and one or more embodiments of the fuel assembly 10 (e.g., 12.6 mm pitch). In other words, the longitudinal axis of the fuel element 20 can be positioned in the same position as the longitudinal axis of a conventional UO2 fuel rod is in an equivalent conventional fuel assembly. According to various embodiments, the fuel element 20 can have a larger circumscribing diameter than an equivalent UO2 fuel rod (e.g., 12.6 mm compared to the outer diameter of a typical UO2 fuel rod of 9.5 mm). As a result, in the self-aligning plane shown in Figure 1, the cross-sectional length and width of the space occupied by the fuel elements 20 may be slightly larger than that occupied by conventional UO2 fuel rods in a conventional fuel assembly (for example, 214.2 mm in the case of fuel assembly 10 (i.e., 17 fuel elements per fuel element, with a circumscribing diameter of 20 × 12.6 mm) in contrast to 211.1 mm in the case of a conventional UO2 fuel assembly containing 9.5 mm UO2 fuel rods in a 17 × 17 array separated from each other at a 12.6 mm pitch). In a conventional UO2 fuel assembly, a spacer grid surrounds the fuel rods, increasing the overall cross-sectional envelope of the conventional fuel assembly to 214 mm × 214 mm. In fuel assembly 10, the shroud 30 similarly increases the cross-sectional envelope of fuel assembly 10. The shroud 30 may have any suitable thickness (e.g., 0.5 mm or 1.0 mm). In embodiments utilizing a shroud 30 with a thickness of 1.0 mm, the entire cross-sectional envelope of one embodiment of the fuel assembly 10 may be 216.2 mm × 216.2 mm (for example, 214 mm occupied by 17 fuel elements 20 with a diameter of 12.6 mm plus twice the thickness of the shroud 30, which is 1.0 mm).As a result, according to one or more embodiments of the present invention, the fuel assembly 10 may be slightly larger than a typical UO2 fuel assembly (214 mm × 214 mm) (e.g., 216.2 mm × 216.2 mm). The larger size may impair the ability of the assembly 10 to fit properly into the fuel assembly locations of one or more conventional reactors designed for use with conventional UO2 fuel assemblies. To address this size variation, according to one or more embodiments of the present invention, new reactors can be designed and constructed to accommodate larger fuel assemblies 10.

[0072] According to an alternative embodiment of the present invention, the circumscribing diameters of all fuel elements 20 can be slightly reduced to reduce the overall cross-sectional size of the fuel assembly 10. For example, the circumscribing diameter of each fuel element 20 can be reduced by 0.13 mm to 12.47 mm, so that the overall cross-sectional space occupied by the fuel assembly 10 remains comparable to that of a conventional 214 mm × 214 mm fuel assembly (for example, 17 fuel elements 20 with a diameter of 12.47 mm plus two shrouds with a thickness of 1.0 mm, totaling approximately 214 mm). Such a reduction in the size of the 17 × 17 array slightly alters the position of the guide tubes 40 in the fuel assembly 10 relative to the position of the guide tubes in a conventional fuel assembly. To accommodate this slight positional change of the tubes 40, the positions of the corresponding control rod array and control rod drive mechanism in the reactor can similarly be shifted to accommodate the relocated guide tubes 40. Alternatively, if the conventional in-reactor control rods are provided with sufficient clearance and tolerance, the conventionally positioned control rods can be properly fitted to the slightly shifted tubes 40 of the fuel assembly 10.

[0073] Alternatively, the diameter of the surrounding fuel elements 20 can be slightly reduced so that the entire assembly 10 fits into a conventional reactor designed for conventional fuel assemblies. For example, the circumscribing diameter of the outer row of fuel elements 20 can be reduced by 1.1 mm so that the total size of the fuel assembly is 214 mm × 214 mm (for example, 15 fuel elements 20 of 12.6 mm each, plus 2 fuel elements 20 of 11.5 mm each, plus 2 shrouds 30 with a thickness of 1.0 mm each). Alternatively, the circumscribing diameters of the two outer rows of fuel elements 20 can each be reduced by 0.55 mm so that the total size of the fuel assembly remains 214 mm × 214 mm (for example, 13 fuel elements 20 of 12.6 mm each, plus 4 fuel assemblies of 12.05 mm each, plus 2 shrouds 30 with a thickness of 1.0 mm each). In each embodiment, the pitch and position of the central 13×13 array of the fuel element 20 and guide tube 40 remain unchanged so that the guide tube 40 aligns with the control rod array and control rod drive mechanism of a conventional reactor.

[0074] Figure 10 shows a fuel assembly 610 according to an alternative embodiment of the present invention. According to various embodiments, the fuel assembly 610 is designed to replace a conventional UO2 fuel assembly in a conventional reactor while maintaining the reactor control rod arrangement designed for use with various conventional UO2 fuel assemblies. The fuel assembly 610 is generally similar to the fuel assembly 10 described above and shown in Figure 1, but includes several differences that help the assembly 610 to fit well into one or more existing reactor types (e.g., reactors using Westinghouse fuel assembly designs that utilize 17 × 17 arrays of UO2 rods) without changing the control rod positions or control rod drive mechanisms.

[0075] As shown in Figure 10, the fuel assembly contains a 17×17 array of space. The central 15×15 array is occupied by 200 fuel elements 20 and 25 guide tubes 40, as described above with respect to a similar fuel assembly 10 shown in Figure 1. Depending on the specific reactor design, if the reactor design does not utilize the central tubes 40 (i.e., 201 fuel elements 20 and 24 guide tubes 40), the central guide tubes 40 may be replaced by additional fuel elements 20. The location of the guide tubes 40 corresponds to the location of the guide tubes used in reactors designed to use conventional UO2 fuel assemblies.

[0076] The peripheral positions of the 17 × 17 array / pattern of the fuel assembly 610 (i.e., positions positioned laterally outward from the fuel elements 20) are occupied by 64 UO2 fuel elements / rods 650. As is known in the art, the fuel rods 650 may contain standard UO2 pelletized fuel placed in hollow rods. The UO2 pelletized fuel may be enriched with U-235 by only less than 20%, less than 15%, less than 10%, and / or less than 5%. The rods 650 may have a diameter slightly smaller than the circumscribing diameter of the fuel elements 20 (e.g., 9.50 mm), which slightly reduces the overall cross-sectional dimensions of the fuel assembly 610, resulting in the assembly 610 fitting better into the space allocated to conventional UO2 fuel assemblies.

[0077] In the illustrated embodiment, the fuel rod / element 650 contains UO2 pelletized fuel. However, the fuel rod / element 650 can alternatively utilize any other suitable combination of one or more fissile materials and / or nuclear fuel parent materials (e.g., thorium, plutonium, uranium-235, uranium-233, or any combination thereof). Such a fuel rod / element 650 may contain metallic and / or oxide fuels.

[0078] According to one or more alternative embodiments, the fuel rods 650 may occupy fewer positions than all 64 peripheral positions. For example, the fuel rods 650 may occupy the top row and left column of the peripheral, while the bottom row and right column of the peripheral may be occupied by the fuel elements 20. Alternatively, the fuel rods 650 may occupy any two other sides of the peripheral of the fuel assembly. The shroud 630 may be modified to surround additional fuel elements 20 located in the peripheral of the fuel assembly. The fuel assemblies modified in this way may be arranged adjacent to each other such that the rows / columns of peripheral fuel elements 650 in one assembly are always adjacent to the rows / columns of fuel elements 20 in the adjacent fuel assembly. As a result, additional space for the fuel assemblies is provided by the fact that the interface between adjacent assemblies is slightly shifted toward the assembly containing the fuel elements 650 toward the peripheral interface. Such modifications can provide the use of a greater number of higher thermal output fuel elements 20 than those provided by the fuel assembly 610.

[0079] The shroud 630 surrounds the array of fuel elements 20, separating the elements 20 from the elements 650. Due to the nozzles 50, 60, the shroud 630, the coolant flow paths formed between them, the relative pressure drops through the elements 20 and 650, and / or the increased pressure drops through the spacer grid 660 (described later) surrounding the elements 650, the coolant flow rate inside the shroud 630 and through the higher thermal output fuel elements 20 may be higher than the flow rate outside the shroud 630 and through the relatively lower thermal output fuel rods 650. The passages and / or orifices within them can be designed to optimize the relative coolant flow rates through the elements 20, 650 based on their respective thermal output and designed operating temperatures.

[0080] According to various embodiments, the moderator:fuel ratio of the fuel elements 20 of the fuel assembly 610 is less than or equal to 2.7, 2.6, 2.5, 2.4, 2.3, 2.2, 2.1, 2.0, 1.9, and / or 1.8. In the illustrated embodiment, the moderator:fuel ratio is equal to the ratio of (1) the total area within the shroud 630 available for coolant / moderator (approximately, by subtracting the total cross-sectional area occupied by the fuel elements 20 from the total cross-sectional area within the shroud 630 (assuming the guide tubes 40 are filled with coolant)) and (2) the total cross-sectional area of ​​the kernel 100 of the fuel elements 20 within the shroud 630.

[0081] According to an alternative embodiment of the present invention, the shroud 630 may be replaced by one or more annular bands, or the shroud 630 may be provided with holes as described above. The use of bands or holes in the shroud 630 facilitates the mutual mixing of the coolant between the fuel element 20 and the fuel element 650.

[0082] As shown in Figure 10, the fuel element 650 is positioned within an annular spacer grid 660 that is roughly equivalent to the outer portion of the spacer grid used in conventional UO2 fuel assemblies. The spacer grid 660 can be firmly connected to the shroud 630 (e.g., via welding, bolts, screws or other fasteners). The spacer grid 660 is preferably sized to provide the same pitch between the fuel elements 650 and the fuel elements 20 as that provided between the central fuel elements 20 (e.g., a 12.6 mm pitch between the axes of all fuel elements 20, 650). To provide such spacing, the fuel elements 650 may be positioned closer to the outside of the spacer grid 660 than to the inside of the shroud 630 and spacer grid 660. The fuel assemblies 610 and spacer grid 660 are also preferably sized and positioned to provide the same pitch (e.g., a 12.6 mm pitch) between the fuel elements 650 of adjacent fuel assemblies. However, the spacing between any of the fuel elements 20, 650 may vary with respect to the spacing between the other fuel elements 20, 650 without departing from the scope of the present invention.

[0083] According to various embodiments, the fuel element 20 provides at least 60%, 65%, 70%, 75%, and / or 80% of the total volume of all fissile material-containing fuel elements 20, 650 of the fuel assembly 610. For example, according to one or more embodiments, the fuel assembly 610 includes 201 fuel elements 20, each having a cross-sectional area of ​​approximately 70 mm², and 64 fuel elements 650, each having a diameter of 9.5 mm, the fuel elements 20 provide approximately 75.6% of the total volume of all fuel elements 20, 650 (201 fuel elements 20 × 70 mm² equals 14070 mm², and 64 fuel elements 650 × r × (9.5 / 2)² = 4534 mm², and the area of ​​the fuel elements 20, 650 is essentially proportional to the volume of the fuel elements (14070 mm² / (14070 mm² + 4534 mm²) = 75.6%).

[0084] The height of fuel assembly 610 matches the height of an equivalent conventional fuel assembly that assembly 610 can replace (for example, the standard fuel assembly height for Westinghouse or AREVA reactor designs).

[0085] The illustrated fuel assembly 610 can be used in 17x17 PWRs such as Westinghouse's 4-loop design, AP1000, or AREVA EPR. However, the design of the fuel assembly 610 may also be modified to accommodate various other reactor designs (e.g., reactor designs utilizing a hexagonal fuel assembly where the hexagonal outer perimeter is occupied by UO2 rods, while the inner position is occupied by fuel element 20, or boiling water reactors, or small modular reactors). While specific dimensions are described in relation to a particular embodiment, various alternatively sized fuel elements 20, 650 and fuel assembly 10 can be used in relation to various reactors or reactor types without departing from the scope of the invention.

[0086] Depending on the specific reactor design, additional rod positions in the fuel assembly may be replaced by UO2 rods. For example, fuel assembly 610 may contain UO2 rods only in the outermost row, but assembly 610 may alternatively include UO2 rods in the outer two rows without departing from the scope of the present invention.

[0087] According to various embodiments, a portion of the fuel assembly 610 supporting the fuel element 650 is inseparable from a portion of the fuel assembly 610 supporting the fuel element 20. According to various embodiments, the fuel element 20 is not separable as a unit from the fuel element 650 of the fuel assembly 610 (even if individual fuel elements 20, 650 can be removed from the assembly 610 based, for example, on failure of individual fuel elements). Similarly, there is no locking mechanism that selectively locks the portion of the fuel element 650 of the fuel assembly to the portion of the fuel element 20 of the fuel assembly 610. According to various embodiments, the fuel elements 20 and 650 of the fuel assembly 610 have the same design life cycle so that the entire fuel assembly 610 is used in the reactor and then removed as a single spent unit.

[0088] According to various embodiments, an increase in the thermal output of the fuel element 20 within the fuel assembly 610 can provide an improvement in output compared to all conventional UO2 fuel rod assemblies that the assembly 610 replaces. According to various embodiments, the improvement in output is at least 5%, 10%, and / or 15%. According to various embodiments, the improvement may be 1–30%, 5–25%, and / or 10–20%. According to various embodiments, the fuel assembly 610 provides at least an 18-month fuel cycle, but can also facilitate a transition to a 24-month+ or 36-month+ fuel cycle. According to one embodiment of the fuel assembly 610 using a fuel element 20 having the exemplary parameters described above with respect to the element 20 shown in Figure 10, the assembly 17 provides a 17% improvement over conventional UO2 fuel assemblies under the operating parameters specified in the following table. [Table 2] [Table 3]

[0089] Fuel assemblies 10, 510, and 610 are preferably thermodynamically designed and physically formed for use in onshore nuclear reactors 90, 500 (e.g., onshore LWRS (including BWRs and PWRs), onshore fast reactors, and onshore heavy water reactors) designed to generate heat for use in electrical and / or non-electrical purposes (e.g., desalination, chemical processing, steam generation, etc.). Such onshore nuclear reactors 90 include, among others, VVER, AP-1000, EPR, APR-1400, ABWR, BWR-6, CANDU, BN-600, BN-800, Toshiba 4S, and Monju. However, according to alternative embodiments of the present invention, fuel assemblies 10, 510, and 610 may be designed for use in offshore reactors (e.g., ships or subsea power plants, floating power plants designed to generate power for coastal use (e.g., electricity)) or other reactor applications, and may be used in them.

[0090] As described above, fuel elements can be manufactured by conventional methods, including co-extrusion or vacuum melting processes. Fuel elements may also be manufactured using additive manufacturing processes, which have been found to be particularly well-suited for the manufacture of metallic fuels. However, it is understood that additive manufacturing is not limited to metallic fuels and can be used to manufacture other types of fuels, including but not limited to ceramic fuels such as oxides, carbides, and nitrides, and combinations of metal and ceramic fuels known as cermets. Furthermore, in addition to the reactors described above, fuels manufactured using additive manufacturing techniques can be used in other reactors, including modular reactors, micro reactors, pebble-bed reactors, fast reactors, research reactors, and reactors used for space and marine applications.

[0091] Additive manufacturing, also known as 3D printing, is a process in which objects are created by depositing continuous layers of material. The deposition of material layers may be controlled by a computer that reads a computer-aided design file. Categories of additive manufacturing processes include, but are not limited to, directed energy deposition, vat photopolymerization, material injection, binder injection, powder bed fusion bonding, material extrusion, wire feeding, and sheet lamination. Electron beam additive manufacturing and plasma arc deposition additive manufacturing are particularly well suited to this application because these processes may meet the safety and quality requirements associated with nuclear fuel production.

[0092] Certain additive manufacturing processes are particularly well-suited for producing fuel elements of specific materials. For example, powder bed melting additive manufacturing can facilitate the processing of ceramic fuels, where the material is sintered in place. As another example, wire-fed additive manufacturing can facilitate the processing of metallic fuels. It should be understood that the examples given above are merely illustrative and do not imply that specific additive manufacturing processes are limited to only certain types of nuclear materials.

[0093] Fuel produced using additive manufacturing can take many forms, including cylindrical, multi-lobed, plate, and spherical shapes. Additive manufacturing makes it possible to produce fuel elements with shape factors that are extremely difficult or even impossible to produce using more conventional manufacturing techniques. Additive manufacturing allows for precise control over fission, reactivity, and the location and concentration of material for cladding, enabling more efficient, cost-effective, and safer operation across a variety of reactor applications.

[0094] Figures 11 and 12 show a fuel element 800 that can be manufactured using an additive manufacturing process. In the illustrated embodiment, the fuel element 800 has a substantially cylindrical shape. In alternative embodiments, the fuel element 800 may have any desired shape, including the lobed profile disclosed herein.

[0095] According to one embodiment, the fuel element 800 is composed of a combination of a refractory metal / alloy and a fuel material. The refractory metal / alloy may include a zirconium alloy. The fuel material may include low-enriched uranium (e.g., U235, U233), plutonium, or thorium combined with low-enriched uranium and / or plutonium. According to various embodiments, the uranium fuel material is enriched to 1% to 20% by weight, 5% to 20% by weight, 10% to 20% by weight, and / or 15% to 20% by weight of uranium-235. According to one or more embodiments, the fuel material contains 19.7% enriched uranium-235.

[0096] As shown in Figure 11, the fuel element 800 extends along the longitudinal central axis 802 between a first end 804 and a second end 806. A first axial zone 808 is provided at the first end 804 of the fuel element 800. A second axial zone 810 is provided at the second end 806 of the fuel element 800. A third axial zone 812 is provided between the first axial zone 808 and the second axial zone 810. As used herein, “axial” refers to a direction extending parallel to the longitudinal central axis 802. The lengths of each of the axial zones 808, 810, and 812 are substantially equal to each other. According to one embodiment, each of the axial zones 808, 810, and 812 has a metallic material of a different composition. As used herein, metallic materials of different compositions may mean that the materials are of different elements (e.g., uranium versus plutonium) or that the materials have different isotopic compositions (e.g., U-50Zr enriched to 5% versus U-50Zr enriched to 15%). In alternative embodiments, the axial zones may have different lengths. In yet another alternative embodiment, one or more axial zones may be made of the same metallic material. In yet another alternative embodiment, one or more axial zones may have different enrichment levels. In yet yet another alternative embodiment, the fuel element may include more or fewer axial zones.

[0097] As shown in Figure 12, the fuel element 800 has a substantially circular cross-section and includes a first radial zone 814, a second radial zone 816, and a third radial zone 818. The first radial zone 814 extends radially outward from the central axis 802 and has a first thickness. The second radial zone 816 is located radially outward from the first radial zone 814 and has a second thickness. The third radial zone 818 is located radially outward from the second radial zone 816 and has a third thickness. As used herein, “radial” refers to the direction extending away from the longitudinal central axis 802 to the edge of the fuel element 800, and “thickness” refers to the dimension of a radial zone that begins at the central axis or the inner boundary of the radial zone and ends at the outer boundary of that radial zone. According to one embodiment, each of the radial zones 814, 816, and 818 is made of a different metallic material. Here again, as used herein, metallic materials of different compositions may mean that the materials are of different elements or have different isotopic compositions. In alternative embodiments, one or more radial zones may be made of the same metallic material. In yet another alternative embodiment, one or more radial zones may have metallic materials with different enrichment levels. In yet another alternative embodiment, the fuel element may include more or fewer radial zones.

[0098] The thickness of each radial zone may vary along the length of the fuel element. For example, referring to Figure 13, the fuel element 800 may be configured such that the first radial zone 814 has a constant first thickness t1, the second radial zone 816 has a second thickness t2 that decreases as it moves along the central axis 802 from the first end 804 to the second end 806, and a third thickness t3 that increases as it moves along the central axis 802 from the first end 804 to the second end 806. The third thickness t3 increases in proportion to the decrease in the second thickness t2 so that the appearance of the fuel element 800 becomes substantially cylindrical.

[0099] With the arrangement shown in Figure 13, control over the neutron flux shape in the reactor can be improved throughout the entire service life of the fuel element during start-up and irradiation. Therefore, this arrangement enables optimization of the core with respect to the efficient use of fissile materials, increases safety margin limits, and reduces neutron leakage.

[0100] In an alternative embodiment, the thickness of the first radial zone may vary along the length of the fuel element. In other alternative embodiments, the thickness of any one or more of the radial zones may vary along the length of the fuel element according to a waveform (such as a sine wave, a square wave, a triangular wave, a sawtooth wave, etc.) or any other desired arrangement. In still other alternative embodiments, the thickness of any one or more of the radial zones may be varied such that the fuel element has a conical outer profile or any other desired shape. The zones may be configured and arranged to optimize the fuel element design to meet the requirements of a specific power plant.

[0101] Each of the axial zones may have radial zones with unique attributes separate from adjacent axial zones. For example, referring to Figure 14, the first axial zone 808 has a first thickness t 1a , a second thickness t 2a , and a third thickness t 3a respectively, and may comprise a first radial zone 814a, a second radial zone 816a, and a third radial zone 818a having the above thicknesses; the second axial zone 810 has a first thickness t 1b , a second thickness t 2b , and a third thickness t 3b respectively, and may comprise a first radial zone 814b, a second radial zone 816b, and a third radial zone 818b having the above thicknesses; the third axial zone 812 has a first thickness t 1c , a second thickness t 2c , and a third thickness t 3c respectively, and may comprise a first radial zone 814c, a second radial zone 816c, and a third radial zone 818c having the above thicknesses. The thickness t of the first radial zone 814a of the first axial zone 808 1aThe thickness t of the first radial zone 814b of the second axial zone 810 is 1b Smaller than the thickness t of the first radial zone 814c of the third axial zone 812. 1c Larger than the thickness t of the second radial zone 816a of the first axial zone 808. 2a The thickness t of the second radial zone 816b of the second axial zone 810 2b Larger than the thickness t of the second radial zone 816c of the third axial zone 812. 2c Smaller than the thickness t of the third radial zone 818a of the first axial zone 808. 3a The thickness t of the third radial zone 818b of the second axial zone 810 3b Smaller than the thickness t of the third radial zone 818c of the third axial zone 812. 3c It is larger than that.

[0102] The first radial zones 814a, 814b, and 814c of each of the axial zones 808, 810, and 812 are made from the first metallic material. The second radial zones 816a, 816b, and 816c of each of the axial zones 808, 810, and 812 are made from the second metallic material. The third radial zones 818a, 818b, and 818c of each of the axial zones 808, 810, and 812 are made from the third metallic material. The first, second, and third metallic materials are all different from each other.

[0103] The configuration shown in Figure 14 allows for improved control over the neutron flux shape within the reactor throughout the entire lifespan of the fuel elements, both during startup and irradiation. Therefore, this configuration enables core optimization for the efficient use of fissile material, improving the safety margin limits and reducing neutron leakage.

[0104] In alternative embodiments, the thickness of the radial zone of any axial zone may be greater than, less than, or equal to the thickness of the radial zone of any other axial zone. In other alternative embodiments, the first, second, and / or third metallic materials may be the same as one another. The zones can be configured and arranged to optimize the fuel element design to meet the requirements of a particular power plant.

[0105] One or more of the radial zones may have a thickness that varies along the circumferential direction of the fuel element. For example, referring to Figure 15, starting from point A and moving clockwise circumferentially around the fuel element 800 (indicated by arrow CW), the first radial zone 814 has a thickness t1 that increases to point B and then decreases again until it reaches point A. The second radial zone 816, starting from point A and moving clockwise circumferentially CW, has a thickness t2 that decreases to point B and then increases again until it reaches point A. The third radial zone 818 has a thickness t3 that remains constant along the circumferential direction of the fuel element 800. In alternative embodiments, any one of the radial zones may have any desired thickness or thickness variation along the circumferential direction of the fuel element. The zones can be configured and arranged to optimize the fuel element design to meet the requirements of a particular power plant.

[0106] The configuration shown in Figure 15 allows for improved control over the neutron flux shape within the reactor throughout the entire lifespan of the fuel elements, both during startup and irradiation. Therefore, this configuration enables core optimization for the efficient use of fissile material, improving the safety margin limits and reducing neutron leakage.

[0107] The fuel elements may be arranged such that there are concentration gradients in the axial, radial, or both axial and radial directions, rather than having separate zones. For example, referring to Figure 16, the fuel element 800 can be arranged such that its center point C is composed of U-50Zr enriched to 15%, and its peripheral portion P is composed of U-50Zr enriched to 5%. As shown in Figure 16, the darker areas represent portions of the fuel element 800 composed of U-50Zr with relatively high enrichment rates, and the lighter areas represent portions of the fuel element 800 composed of U-50Zr with relatively low enrichment rates. Thus, Figure 16 shows the overall decrease in enrichment as moving radially outward from the center point C toward the peripheral portion P. In alternative embodiments, fuel materials other than U-50Zr may be used. In other alternative embodiments, the fuel material may be enriched to any desired level and may vary according to any desired gradient. For example, the central point of the fuel element may consist of U-50Zr enriched to 5%, while the peripheral portion of the fuel element may consist of U-50Zr enriched to 15%. The zones can be configured and arranged to optimize the fuel element design to meet the requirements of a particular power plant.

[0108] The configuration shown in Figure 16 allows for improved control over the neutron flux shape within the reactor throughout the entire lifespan of the fuel elements, both during startup and irradiation. Therefore, this configuration enables core optimization for the efficient use of fissile material, improving the safety margin limits and reducing neutron leakage.

[0109] The aforementioned configurations are made possible, or at least facilitated, by the use of additive manufacturing processes. In addition to producing fuel materials using additive manufacturing processes, cladding for fuel elements can also be produced using additive manufacturing processes. Furthermore, the final form of the fuel element may be produced using additive manufacturing processes, or the fuel element may undergo further processing after the additive manufacturing process is complete. For example, complex billet shapes can be printed using additive manufacturing, and then the billet shapes can be stretched, rolled, or extruded, for example, using separately manufactured cladding. Additive manufacturing processes may also be used to form parts of fuel elements beyond metallic materials as described above. For example, cladding with any desired shape factor can be formed using additive manufacturing processes.

[0110] Fuel elements produced using additive manufacturing processes can improve control over the neutron flux shape within the reactor throughout the fuel element's lifecycle, both during startup and irradiation. Therefore, the use of additive manufacturing allows for core optimization in terms of efficient use of fissile material, improving safety margin limits and reducing neutron leakage. Furthermore, fuel elements produced using additive manufacturing processes can be designed using inherent flux shaping, thus reducing the need for external flux control systems such as separate flammable absorbers and expensive chemical control systems. The reduction in external flux control systems enables the development of advanced reactors that are smaller in size, have lower and more stable power generation costs, are less complex, and have improved operability and economics.

[0111] Although different embodiments and variations are shown and described in Figures 1 to 16, it should be understood that the disclosed features are not exclusive to each of the described embodiments. Instead, various features can be combined with the fuel element as needed. For example, the displacer or cladding configuration shown in Figure 5 can be used in conjunction with the configurations shown in Figures 11 and 12.

[0112] The illustrated embodiments described above are provided to illustrate the structural and functional principles of the present invention and are not intended to limit them. Conversely, the principles of the present invention are intended to encompass any and all modifications, alterations, and / or substitutions within the spirit and scope of the following claims.

Claims

1. A fuel element for use in a reactor fuel assembly, A first zone extending along the first axis of the fuel element in a first direction and extending outward from the first axis in a second direction of the fuel element, the first zone being made of a first material, A second zone that surrounds the first zone in the second direction of the fuel element and extends in the first direction, and is composed of a second material different from the first material, The thickness of the second zone in the second direction varies along the first direction, A third zone surrounding the second zone in the second direction of the fuel element and extending in the first direction, the third zone being composed of a third material different from the first material and different from the second material, The thickness of the third zone in the second direction varies along the first direction, and the third zone Equipped with, A fuel element in which each of the first material, the second material, and the third material is selected from the group consisting of metallic fuels, ceramic fuels, and cermets.

2. A fuel element for use in a nuclear reactor fuel assembly, A first zone extending from the first axis of the fuel element in a first direction of the fuel element and extending along the first axis in a second direction of the fuel element, having a first material composition, A second zone extending from the first axis of the fuel element in the first direction of the fuel element, adjacent to the first zone in the second direction of the fuel element, and having a second material composition different from the first material composition, A third zone extending from the first axis of the fuel element in the first direction of the fuel element, adjacent to the second zone in the second direction of the fuel element, and having a third material composition different from the first material composition and different from the second material composition. Equipped with, A fuel element in which the first zone, the second zone, and the third zone are each composed of nuclear fuel material.

3. The fuel element according to claim 2, wherein each of the first material composition, the second material composition, and the third material composition is a material composition selected from the group consisting of metals, ceramics, and cermets.

4. A fuel element for use in a fuel assembly of a nuclear reactor, A first zone having a first material combination, A first central zone extending outward from a first axis in a first direction of the fuel element, the first central zone being made of a first material and having a first thickness in the first direction, A first intermediate zone surrounding the first central zone in the first direction of the fuel element, the first intermediate zone being made of a second material different from the first material and having a second thickness, and A first outer zone surrounding the first intermediate zone in the first direction of the fuel element, the first outer zone being composed of a third material different from the first material and different from the second material, and having a third thickness, The first zone includes, A second zone having a second material combination different from the first material combination, A second central zone extending outward from the first axis in the first direction of the fuel element, the second central zone being made of the first material and having a fourth thickness different from the first thickness in the first direction, A second intermediate zone surrounding the second central zone in the first direction of the fuel element, the second intermediate zone being made of the second material and having a fifth thickness different from the second thickness, and A second outer zone surrounding the second intermediate zone in the first direction of the fuel element, the second outer zone being made of the third material and having a sixth thickness different from the third thickness, The second zone, which includes, A fuel element equipped with the above.

5. The fuel element according to claim 4, wherein each of the first material, the second material, and the third material is a material selected from the group consisting of metals, ceramics, and cermets.

6. A method for manufacturing a fuel element using additive manufacturing, A step of forming a first zone using a first material, wherein the first zone extends in a first direction of the fuel element along a first axis of the fuel element and extends outward from the first axis in a second direction, A step of forming a second zone using a second material different from the first material, wherein the second zone surrounds the first zone in the second direction of the fuel element, extends in the first direction, and the thickness of the second zone in the second direction varies along the first direction. A step of forming a third zone using a third material different from the first material and different from the second material, wherein the thickness of the third zone in the second direction varies along the first direction. Methods that include...

7. The method according to claim 6, wherein each of the first material, the second material, and the third material is a material selected from the group consisting of metals, ceramics, and cermets.

8. A fuel assembly for a nuclear reactor, wherein the fuel assembly is A plurality of fuel elements, wherein at least one of the plurality of fuel elements is A first zone extending along the first axis of the fuel element in a first direction and extending outward from the first axis in a second direction of the fuel element, the first zone being made of a first material, A second zone surrounding the first zone in the second direction of the fuel element and extending in the first direction, the second zone being made of a second material different from the first material, and the thickness of the second zone in the second direction varying along the first direction, A third zone surrounding the second zone in the second direction of the fuel element and extending in the first direction, which is composed of a third material different from the first material and different from the second material, and the thickness of the third zone in the second direction varies along the first direction, Multiple fuel elements A fuel assembly equipped with the following features.

9. The fuel assembly according to claim 8, wherein each of the first material, the second material, and the third material is a material selected from the group consisting of metals, ceramics, and cermets.

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