Fuel assemblies and reactor cores

Abstract

To provide a fuel assembly capable of increasing combustion efficiency of minor actinide and suppressing an increase in void reactivity.SOLUTION: A fuel assembly 1 has fuel rods 2 and 3 arranged in a wrapper tube 4. The fuel rod 2 has a metal fuel of U-Pu-Zr alloy therein. The fuel rod 3 has a metal fuel of U-Pu-MA-Zr alloy therein. An external diameter of the fuel rod 3 is smaller than an external diameter of the fuel rod 2. A plurality of fuel rods 2 are arranged in a regular triangle lattice in the wrapper tube 4, and the fuel rods 3 are arranged between three adjacent fuel rods 2. The external diameter of the fuel rod 3 including Pu and MA in the fuel assembly 1 is smaller than an external diameter of the fuel rod 2 that includes Pu but not MA, and thus neutrons generated in the fuel rods 2 reach the center of the fuel rods 3, so that combustion efficiency of MA in the fuel rods 3 is improved. A coolant passage cross-sectional area ratio in a cross-section of the fuel assembly 1 is small, so that it is possible to suppress an increase in void reactivity due to MA contained in the fuel assembly 1.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a fuel assembly and a core of a nuclear reactor, and more particularly to a fuel assembly containing minor actinides (MA) and a core of a nuclear reactor in which this fuel assembly is loaded. [Background technology]

[0002] Generally, a fast breeder reactor has a core placed in a reactor vessel filled with liquid sodium as a coolant. The fuel assemblies loaded into the core include multiple fuel rods containing plutonium-enriched depleted uranium (U-238), a wrapper tube surrounding the bundled multiple fuel rods, an entrance nozzle supporting the lower ends of the fuel rods and neutron shields located below the fuel rods, and a coolant outlet located above the fuel rods.

[0003] The core of a fast breeder reactor has, for example, a core fuel region having an inner core region and an outer core region surrounding the inner core region, a blanket fuel region surrounding the core fuel region, and a shield region surrounding the blanket region. In a standard homogeneous core, the plutonium enrichment of the fuel assemblies loaded in the outer core region is higher than that of the fuel assemblies loaded in the inner core region. As a result, the power distribution in the radial direction of the core is flattened.

[0004] The nuclear fuel material contained in each fuel rod of a fuel assembly can be in the form of metallic fuel, nitride fuel, or oxide fuel, of which oxide fuel is the most widely used.

[0005] Mixed oxide fuel, a mixture of Pu and depleted uranium oxides, i.e., MOX fuel pellets, are loaded to a height of approximately 80 to 100 cm in the axial center of the fuel rod. Furthermore, within the fuel rod, axial blanket regions filled with multiple uranium dioxide pellets made from depleted uranium are located above and below the MOX fuel loading region. The inner core fuel assembly loaded in the inner core region and the outer core fuel assembly loaded in the outer core region each contain multiple fuel rods filled with multiple MOX fuel pellets, as described above. The Pu enrichment of the outer core fuel assembly is higher than that of the inner core fuel assembly.

[0006] The blanket fuel region surrounding the core fuel region is loaded with blanket fuel assemblies, each of which has multiple fuel rods filled with multiple uranium dioxide pellets made from depleted uranium. Neutrons generated by nuclear fission reactions within the fuel assemblies loaded in the core fuel region leak from the core fuel region and are absorbed by U-238 within each fuel rod of the blanket fuel assemblies loaded in the blanket fuel region. As a result, the fissionable nuclide Pu-239 is newly produced within each fuel rod of the blanket fuel assembly.

[0007] Control rods are also used when starting up and shutting down a fast breeder reactor and adjusting the reactor power output. Control rods have multiple neutron absorbing rods made of boron carbide (B4C) pellets sealed in stainless steel cladding tubes, and these neutron absorbing rods are housed in a hexagonal tube, just like the inner and outer core fuel assemblies. The control rods are configured in two independent systems: the main reactor shutdown system and the backup reactor shutdown system, and an emergency shutdown of the fast breeder reactor is possible using only one of the main reactor shutdown system or the backup reactor shutdown system.

[0008] Reprocessing spent nuclear fuel from spent fuel assemblies in nuclear reactors recovers plutonium (Pu) and uranium (U), which can be reused as nuclear fuel. High-level radioactive waste (HLRW) generated during plutonium and uranium recovery contains highly radioactive fission products (FPs) and long-lived minor actinides (MAs). Japan's disposal policy for HLRW involves disposing of vitrified HLRW in deep geological formations. Since vitrified HLRW is stably buried at depths of more than 300 meters underground, public safety is ensured over the long term. However, due primarily to the presence of MAs, it takes approximately 10,000 years for their toxicity to decay to the level of natural uranium. Research into separation and transmutation of spent nuclear fuel has been conducted around the world to reduce the toxicity of spent nuclear fuel by recovering these MAs through advanced reprocessing and incorporating them into nuclear fuel for combustion in nuclear reactors, thereby shortening the time required for HLRW to decay to the level of natural uranium to several hundred years.

[0009] Japanese Patent No. 2668646 describes a method for burning MA in a fast reactor. The core of the fast reactor described in Japanese Patent No. 2668646 is loaded with a dispersed array of conventional fuel assemblies, each containing multiple first fuel rods filled with nuclear fuel material containing plutonium but not MA, and target fuel assemblies, each containing second fuel rods with a smaller outer diameter than the first fuel rods and filled with nuclear fuel material containing plutonium and MA. All of the smaller-diameter second fuel rods in the target fuel assembly are filled with plutonium and MA.

[0010] In the fuel assembly (loaded in the core of a fast reactor) shown in Figure 1 of JP 2016-8890 A, a cylindrical partition wall with a regular hexagonal cross section is located in the center of the cross section of the fuel assembly, and the cross section is divided by the partition wall into a central region and an annular region surrounding the central region. The upper and lower ends of the partition wall are open. The first fuel rods located in the central region are filled with mixed oxide fuel containing depleted uranium and Pu oxides as nuclear fuel material in a sealed cladding tube. The second fuel rods located in the annular region are filled with depleted uranium and MA as nuclear fuel material in a sealed cladding tube. The first and second fuel rods have the same outer diameter. In the fuel assembly (loaded in the core of a fast reactor) shown in Figure 8, similar to that shown in Figure 1, the first fuel rods filled with depleted uranium and Pu are located in the central region, and the second fuel rods filled with depleted uranium and MA are located in the annular region. In the fuel assembly shown in FIG. 8, the outer diameter of the second fuel rod is larger than that of the first fuel rod.

[0011] JP 2016-8890 A (Patent Publication No. 2016-8890) also discloses the structure of a fuel assembly loaded into the core of a fast reactor. This fuel assembly has an entrance nozzle connected to the lower end of a bell pipe, and the above-mentioned bulkhead is disposed within the bell pipe, with its lower end attached to a support member attached to the entrance nozzle. The lower ends of the first fuel rods arranged in the central region and the second fuel rods arranged in the annular region are supported by the support member.

[0012] On the other hand, K. FUJIMURA, et al., "Actinide-Burning Ultralong-Life FBR Concepts," Proceedings of International Conference on Fast Reactors and Related Fuel Cycles, VOLUME IV, pp. 5.16-1 to 10, October 28 to November 1, 1991, discloses that MA is contained in each fuel material in all fuel rods in a core fuel assembly in which multiple fuel rods having nuclear fuel material containing Pu are arranged. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Patent No. 2668646 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-8890 [Non-patent literature]

[0014] [Non-Patent Document 1] K. FUJIMURA, et al., “Actinide-Burning Ultralong-Life FBR Concepts,” Proceedings of International Conference on Fast Reactors and Related Fuel Cycles, VOLUME IV, P5.16-1~10, October 28-November 1, 1991. Summary of the Invention [Problem to be solved by the invention]

[0015] In Patent Publication No. 2668646, the outer diameter of the second fuel rod of the target fuel assembly is made smaller than that of the first fuel rod of a normal fuel assembly because the melting point of MA is lower than that of Pu, and therefore the fuel temperature at the center of the second fuel rod is lowered to avoid melting of the nuclear fuel material containing MA in the second fuel rod.

[0016] The core of the fast reactor described in Japanese Patent No. 2668646 is loaded with a plurality of conventional fuel assemblies, each containing a plurality of fuel rods filled with mixed oxide fuel of depleted uranium and Pu, and a plurality of target fuel assemblies, each containing a first fuel rod filled with depleted uranium and Pu arranged in a central region and a second fuel rod filled with depleted uranium and MA arranged in an annular region. In such a fast reactor core, neutrons generated by the nuclear fission of Pu in the conventional fuel assemblies are incident on the target fuel assemblies arranged adjacent to the conventional fuel assemblies, and these neutrons cause nuclear fission of MA contained in the second fuel rod in the target fuel assemblies. As a result, neutrons generated in the surrounding conventional fuel assemblies move toward the center of the target fuel assembly, but the neutron flux (neutrons / cm 2 / s) decreases toward the center of the target fuel assembly, and the fission of MA in the target fuel assembly is suppressed, resulting in a decrease in the burnup efficiency of the MA.

[0017] As described in K. FUJIMURA, et al., "Actinide-Burning Ultralong-Life FBR Concepts," Proceedings of International Conference on Fast Reactors and Related Fuel Cycles, VOLUME IV, pp. 5.16-1-10, October 28-November 1, 1991, MA contained in fuel rods placed in fuel assemblies loaded into the core of a fast reactor has a neutron absorption cross section several times larger than that of depleted uranium (mainly U-238), the main component of the nuclear fuel material in the fuel rods. This results in a higher rate of neutron absorption at the surface of the MA-containing fuel rods. This results in a decrease in neutron flux near the center of the fuel rod or at the center of the fuel assembly in which the MA-containing fuel rods are placed, resulting in a significant self-shielding effect. Furthermore, as described in the above-mentioned paper by K. FUJIMURA, et al., the addition of MA to the nuclear fuel material in the fuel assemblies loaded into the core of a fast reactor increases the void reactivity, which causes an increase in reactivity and an increase in reactor power during an Unticipated Loss of Flow (ULOF), a transient event that simulates the combination of loss of coolant flow and a scram failure.

[0018] As mentioned above, when MA is contained in the nuclear fuel material in the fuel assemblies loaded into the core of a fast reactor in order to burn MA, problems arise, such as a decrease in the burnup efficiency of MA in the fuel assemblies and an increase in void reactivity.

[0019] An object of the present invention is to provide a fuel assembly and a nuclear reactor core that can improve the burnup efficiency of minor actinides and suppress an increase in void reactivity. [Means for solving the problem]

[0020] A feature of the present invention that achieves the above-mentioned object is that a plurality of first fuel rods having a first nuclear fuel material that does not contain minor actinides are The cylindrical body is disposed inside thea plurality of second fuel rods having a second nuclear fuel material containing a minor actinide; is disposed inside the cylindrical body, The outer diameter of the second fuel rod is , the above It is smaller than the outer diameter of the first fuel rod. a second fuel rod region in which a plurality of the second fuel rods are arranged is disposed between first fuel rod regions in which a plurality of the first fuel rods are arranged within the cylindrical body, the first fuel rods are arranged in an equilateral triangular lattice within the first fuel rod region, and the second fuel rods are arranged in an equilateral triangular lattice within the second fuel rod region; The reason is that there are [Effects of the Invention]

[0023] According to the present invention, the burnup efficiency of minor actinides can be improved and an increase in void reactivity can be suppressed. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a cross-sectional view of a fuel assembly according to a first embodiment of the present invention, which is used in a fast reactor. FIG. [Figure 2] FIG. 2 is a longitudinal sectional view of the fuel assembly of the first embodiment shown in FIG. 1. [Figure 3] 1 is a longitudinal sectional view of a core of a fast reactor in which the fuel assemblies of Example 1 are loaded. FIG. [Figure 4] 4 is a cross-sectional view of the core of the fast reactor shown in FIG. 3 taken along line IV-IV. [Figure 5] 5 is an enlarged view of the arrangement of a plurality of fuel assemblies in the first embodiment loaded in the core fuel region shown in FIG. 4. FIG. [Figure 6] 5 is a cross-sectional view of an inner blanket fuel assembly loaded into an inner blanket region in the core of the fast reactor shown in each of FIGS. 3 and 4. FIG. [Figure 7] FIG. 10 is a cross-sectional view of a fuel assembly according to a second embodiment of the present invention, which is used in a fast reactor. [Figure 8] FIG. 8 is a longitudinal sectional view of the fuel assembly of the second embodiment shown in FIG. 7. [Figure 9] FIG. 10 is a longitudinal sectional view of a core of a fast reactor in which the fuel assemblies of Example 2 are loaded. [Figure 10] 9. FIG. 9 is a cross-sectional view of the core of the fast reactor taken along line IX-IX of FIG. [Figure 11] FIG. 10 is a cross-sectional view of a fuel assembly according to a third embodiment of the present invention, which is used in a fast reactor. [Figure 12] FIG. 10 is a cross-sectional view of a fuel assembly according to a fourth embodiment of the present invention, which is used in a fast reactor. [Figure 13] FIG. 10 is a cross-sectional view of a fuel assembly according to a fifth embodiment of the present invention, which is used in a fast reactor. [Figure 14] FIG. 10 is a cross-sectional view of a fuel assembly according to a sixth embodiment of the present invention, which is used in a fast reactor. [Figure 15] FIG. 10 is a cross-sectional view of a fuel assembly according to a seventh embodiment of the present invention, which is used in a boiling water reactor. DETAILED DESCRIPTION OF THE INVENTION

[0025] An embodiment of the present invention will now be described. [Example]

[0026] A fuel assembly according to a first embodiment of the present invention, which is used in a fast reactor, will be described with reference to Figures 1 and 2. Furthermore, the core of the fast reactor according to the first embodiment will be described with reference to Figures 3 to 6. The fast reactor to which this embodiment is applied is a sodium-cooled fast reactor (for example, a fast breeder reactor) which has a hard neutron spectrum and high transmutation and burnup performance of minor actinides (MA).

[0027] A fuel assembly 1 of this embodiment shown in FIGS. 1 and 2 is loaded into the core of a fast reactor. In this fuel assembly 1, a plurality of fuel rods 2 and a plurality of fuel rods 3 are arranged in a stainless steel bell pipe (cylindrical body) 4, which is a cylinder with a regular hexagonal cross section. As shown in FIG. 2 of JP 2016-8890 A, the fuel assembly 1 has a plurality of fuel rods, a bell pipe, and an entrance nozzle. Unlike JP 2016-8890 A, the fuel assembly 1 of this embodiment does not have a partition wall. The lower end of the bell pipe 4 is attached to the upper end of the entrance nozzle (not shown). In the fuel assembly 1, a plurality of fuel rods 2 and a plurality of fuel rods 3 are arranged in the bell pipe 4.

[0028] The upper end of the cladding tube 6 of the fuel rod 2 is sealed by an upper end plug 10, and the lower end of the cladding tube 6 is sealed by a lower end plug 11. Within the thus sealed cladding tube 6, there is U-Pu-Zr alloy metallic fuel, which is nuclear fuel material. Above this metallic fuel there is U-Zr alloy metallic fuel, which is blanket fuel. Below the U-Pu-Zr alloy metallic fuel there is U-Zr alloy metallic fuel. Within the cladding tube 6 of the fuel rod 2, from the lower end plug 11 upward, there are a lower blanket region 9 where U-Zr alloy metallic fuel is present, a core fuel region 7 where U-Pu-Zr alloy metallic fuel is present, and an upper blanket region 8 where U-Zr alloy metallic fuel is present. The fuel rod 2 does not contain MA. The Pu enrichment of the U-Pu-Zr alloy in the fuel rods 2 of the fuel assembly 1 with a burnup of 0 GWd / t is within the range of 13.0 wt% to 25.0 wt% (13.0 wt% or more and 25.0 wt% or less), for example, 23.5 wt%.

[0029] The upper end of the cladding tube 6A of the fuel rod 3 is sealed by an upper end plug 10A, and the lower end of the cladding tube 6A is sealed by a lower end plug 11A. Within the sealed cladding tube 6A, there is U-Pu-MA-Zr alloy metallic fuel, which is nuclear fuel material. Above this metallic fuel, there is U-Zr alloy metallic fuel, which is blanket fuel. Below the U-Pu-MA-Zr alloy metallic fuel, there is U-Zr alloy metallic fuel. Within the cladding tube 6A of the fuel rod 3, from the lower end plug 11 upward, there are a lower blanket region 9A containing U-Zr alloy metallic fuel, a core fuel region 14 containing U-Pu-MA-Zr alloy metallic fuel, and an upper blanket region 8A containing U-Zr alloy metallic fuel. The Pu enrichment of the U-Pu-MA-Zr alloy in the fuel rods 3 of the fuel assembly 1 with a burnup of 0 GWd / t is also in the range of 13.0 wt% to 25.0 wt% (13.0 wt% to 25.0 wt%), e.g., 23.5 wt%. The MA enrichment of the U-Pu-MA-Zr alloy is in the range of 3.7 wt% to 12.5 wt% (3.7 wt% to 12.5 wt%), e.g., 5.0 wt%.

[0030] Here, MA specifically refers to isotopes of Np, Am, Cm, etc., and the main nuclides are Np-237, Am-241, Am-242m, Cm-243, Cm-244, and Cm-245.

[0031] The metallic fuels of U-Pu-Zr alloy, U-Pu-MA-Zr alloy and U-Zr alloy have a solid cylindrical shape.

[0032] The outer diameter of the fuel rods 2 is 7.36 mm. In the cross section of the fuel assembly 1, the fuel rods 2 are arranged in an equilateral triangular lattice pattern within the tube 4 (see Figure 1). The outer diameter of the fuel rods 3 is 2.21 mm. In the cross section of the fuel assembly 1, one fuel rod 3 is arranged between every three adjacent fuel rods 2 and is adjacent to all of those fuel rods 2. Therefore, six fuel rods 3 are arranged around one fuel rod 2, surrounding that fuel rod 2 and adjacent to that one fuel rod 2. In one fuel assembly 1, there are 271 fuel rods 2 and 3, and 378 fuel rods 3. The total number of fuel rods 2 and 3 is 649. The lower ends of the fuel rods 2 and 3 are supported by support members (not shown) provided at the upper end of the entrance nozzle. This support member is provided with a large number of holes (through holes) for guiding liquid sodium, which is the coolant, from inside the entrance nozzle to inside the wrapper tube 4.

[0033] Wire spacers (not shown) are wound around the outer surface of the fuel rods 3. The wire spacers wound around the outer surface of the fuel rods 3 are also in contact with the outer surfaces of the fuel rods 2 adjacent to the fuel rods 3. The distance between the adjacent fuel rods 3 and 2 is maintained by the wire spacers. Coolant passages 5 (see Figures 1 and 2) are formed between the adjacent fuel rods 3 and 2, and between the adjacent fuel rods themselves. The wire spacers may be wound around the outer surface of the fuel rods 2 instead of the outer surface of the fuel rods 3. In this way, the wire spacers are wound around either the outer surface of the fuel rods 2 or 3.

[0034] [Table 1]

[0035] Table 1 compares the specifications of fuel assembly 1 of this embodiment and a conventional fuel assembly. When fuel assembly 1 and a conventional fuel assembly are loaded in separate cores, the pitch of fuel assembly 1 in the core and the pitch of the conventional fuel assembly in the core are the same, 161.42 mm. The outer diameter and number of fuel rods 2 in fuel assembly 1 of this embodiment and the outer diameter and number of fuel rods 3 are as described above. The conventional fuel assembly does not have fuel rods 3, but has fuel rods 2 with an outer diameter larger than that of fuel rods 3. The outer diameter of fuel rods 2 in the conventional fuel assembly is 7.44 mm, and the number of fuel rods 2 is 271, the same as that of fuel assembly 1.

[0036] The cross-sectional area ratio of the nuclear fuel material in the cladding tube is 37.3% in the conventional fuel assembly, but 41.5% in the fuel assembly 1, which is larger than that of the conventional fuel assembly (see Table 1). Here, the cross-sectional area ratio of the nuclear fuel material in the cladding tube will be explained. When the fuel assembly 1 and the conventional fuel assembly are loaded in the core, a coolant region exists between adjacent fuel assemblies 1 and between adjacent conventional fuel assemblies. The case of the fuel assembly 1 will be specifically explained based on FIG. 5. FIG. 5 is an enlarged partial cross-section of the core fuel region 16 shown in FIG. 4 in the core 15 of the fast reactor. In the core fuel region 16, multiple loaded fuel assemblies 1 are arranged adjacent to each other, and a coolant region 32 is formed between adjacent fuel assemblies 1. The coolant region 32 contains a coolant, such as liquid sodium, supplied to the fuel assembly 1. The width of the coolant region 32 between the fuel assemblies 1 is defined as H (see FIG. 5). A regular hexagon 33 shown by a dashed line is formed by adding 1 / 2 (H / 2) of the width H of the coolant region 32 to each of the outer surfaces of the wrapper tube 4 arranged in a regular hexagon. The cross-sectional area ratio of the nuclear fuel material in the cladding tube described above is the ratio of the cross-sectional area of ​​the nuclear fuel material in the cladding tube to the area occupied by the regular hexagon 33.

[0037] If the cross-sectional area ratio of the nuclear fuel material in a conventional fuel assembly is "1," then the cross-sectional area ratio of the nuclear fuel material in fuel assembly 1 is "1.1." In other words, the cross-sectional area ratio of the nuclear fuel material in fuel assembly 1 is 10% more than that of a conventional fuel assembly. This means that the weight of the nuclear fuel material in fuel assembly 1 is 10% more than that of a conventional fuel assembly.

[0038] Furthermore, the cross-sectional area ratio of the coolant passage in the cross section of the fuel assembly inside the inner surface of the funnel 4 of the fuel assembly is 36.6% in the conventional fuel assembly, but is 33.1% in the fuel assembly 1, which is smaller than that of the conventional fuel assembly. The cross-sectional area ratio of the coolant passage in the cross section of the fuel assembly inside the inner surface of the funnel 4 described above is the ratio of the cross-sectional area of ​​the coolant passage in the cross section of the fuel assembly inside the inner surface of the funnel 4 to the area occupied by the regular hexagon 33. If the cross-sectional area ratio of the coolant passage in the conventional fuel assembly is "1," the cross-sectional area ratio of the coolant passage in the fuel assembly 1 is "0.90." In other words, the cross-sectional area ratio of the coolant passage in the fuel assembly 1 is 10% smaller than that of the conventional fuel assembly, and the amount of coolant in the fuel assembly 1 is less than that of the conventional fuel assembly.

[0039] In order to absorb swelling of the metallic fuel due to neutron irradiation, the cross-sectional area of ​​the metallic fuel, which is a U-Pu-Zr alloy, in the fuel rod 2 is 75% of the cross-sectional area inside the cladding tube 6. Since the theoretical density of the metallic fuel is 100% TD, the smear density is 75%. In the fuel rod 2 in the fuel assembly 1, bonded sodium 12 is filled in the gap formed between the inner surface of the cladding tube 6 and the outer surface of the metallic fuel (U-Pu-Zr alloy and U-Zr alloy) present inside the cladding tube 6. The liquid level of this bonded sodium 12 is located above the top end of the upper blanket region 8. By filling the bonded sodium 12, the gap conductance between the metallic fuel, which is the U-Pu-Zr alloy and U-Zr alloy, and the cladding tube 6 is kept small. That is, the thermal conduction between the U-Pu-Zr alloy and the cladding tube 6 and between the U-Zr alloy and the cladding tube 6 increases, and the cooling of the U-Pu-Zr alloy and the U-Zr alloy is promoted. Within the cladding tube 6, above the liquid surface of the bonded sodium 12, a gas plenum 13 is formed to collect fission product gases.

[0040] In the fuel rod 3, bonded sodium 12A is also filled in the gap formed between the inner surface of the cladding tube 6A and the outer surface of the metallic fuel (U-Pu-Zr alloy and U-Zr alloy) present in the cladding tube 6A. The liquid level of this bonded sodium 12A is formed in the cladding tube 6A at a position above the upper end of the upper blanket region 8A. A gas plenum 13 is formed in the cladding tube 6A above the liquid level of the bonded sodium 12A. The difference is that the fuel element is a U-Pu-MA-Zr alloy. The smear density of the fuel is 75%.

[0041] 4 includes a core fuel region 16 loaded with fuel assemblies 1, a radial blanket region 18 surrounding the core fuel region 16 and loaded with blanket fuel assemblies 25, and a shield region 19 in which a plurality of shield assemblies 26 are arranged. The blanket fuel rods (not shown) included in the blanket fuel assemblies 25 arranged in the radial blanket region 18 contain a U-Zr alloy, which is a metallic fuel.

[0042] The core fuel region 16 has an annular inner blanket region 21 loaded with a plurality of inner blanket fuel assemblies 22. The core 15 is a radially heterogeneous core having the inner blanket region 21 in which the inner blanket fuel assemblies 22 are arranged in an annular shape.

[0043] The detailed structure of the internal blanket fuel assembly 22 will be described with reference to Figure 6. The internal blanket fuel assembly 22 has multiple blanket fuel rods 23, each of which has metallic fuel 24 made of U-Zr alloy arranged in a cladding tube. The U in the U-Zr alloy represents depleted uranium. The multiple blanket fuel rods 23 are arranged in a wrapper tube 4 of the internal blanket fuel assembly 22. The lower ends of the multiple blanket fuel rods 23, like the lower ends of the fuel rods 2 and 3 in the fuel assembly 1, are supported by support members (not shown) provided at the upper end of an entrance nozzle (not shown) connected to the lower end of the wrapper tube 4. Coolant passages 5 are formed between the blanket fuel rods 23.

[0044] In this embodiment, each of the fuel rods 2 and 3 in the fuel assembly 1 uses metal fuel as the nuclear fuel material, but instead of metal fuel, oxide fuel, for example, mixed oxide fuel, which is used in each of the inner core fuel assembly 1A and the outer core fuel assembly 1B in embodiment 2, may be used.

[0045] 3, the core 15 has an upper blanket region 27 formed above the core fuel region 16 and a lower blanket region 28 formed below the core fuel region 16, both of which are arranged inside the radial blanket region 18. The upper blanket region 8 of each fuel rod 2 in the fuel assembly 1 and the upper blanket region 8A of each fuel rod 3 form the upper blanket region 27. The lower blanket region 9 of each fuel rod 2 in the fuel assembly 1 and the lower blanket region 9A of each fuel rod 3 form the lower blanket region 28. A plurality of control rod assemblies 34 that control the reactor power are arranged in the core fuel region 16.

[0046] Before the start of operation of a fast reactor in a certain operation cycle, a plurality of fuel assemblies 1 with a burnup of 0 GWd / t, a plurality of fuel assemblies that have already been operated in one operation cycle, and a plurality of fuel assemblies that have already been operated in two operation cycles are loaded into a core 15 in a reactor vessel (not shown) of the fast reactor. In the fast reactor, refueling work is performed during a period when the operation of the fast reactor is stopped after the operation of the fast reactor in the previous operation cycle has ended and before the start of the above operation. In this refueling work, a plurality of spent fuel assemblies are removed from the core of the fast reactor, and new fuel assemblies 1, i.e., a plurality of fuel assemblies 1 with a burnup of 0 GWd / t, are loaded into the core 15. After the necessary work such as refueling work is completed, the operation of the fast reactor is started as described above.

[0047] The fast reactor has an electrical output of 750,000 kWe, a continuous operating period of 19 months, three refueling batches, and an average discharge burnup of the fuel assemblies 1 loaded in the core fuel region 16 of approximately 100 GWd / t. The core 15 shown in Figure 3 is disposed within the reactor vessel of the fast reactor. Liquid sodium, the coolant, exists within the reactor vessel. The liquid sodium flows into the entrance nozzle through multiple holes at the bottom of the entrance nozzle and rises within the bellows 4. Within the bellows, the liquid sodium rises within the coolant passage 5, contacts the outer surfaces of the cladding tubes of the fuel rods 2 and 3, and removes heat generated within the fuel rods 2 and 3. The liquid sodium, heated by this heat removal, flows out of the fuel assembly 1 from the top of the bellows 4.

[0048] According to this embodiment, the outer diameter of the fuel rods 3 containing Pu and MA arranged in the fuel assembly 1 is smaller than the outer diameter of the fuel rods 2 containing Pu but not MA, so that neutrons generated in the fuel rods 2 reach the centers of the fuel rods 3, making it easier for the MA in the fuel rods 3 to undergo nuclear fission. This improves the burnup efficiency of the MA in the fuel rods 3.

[0049] In this embodiment, one fuel rod 3 containing MA and having a small outer diameter is arranged between adjacent fuel rods 2 in the cross section of the fuel assembly 1, specifically, between every three adjacent fuel rods 2, so that the cross-sectional area ratio of the coolant passage in the cross section of the fuel assembly 1 is 10% smaller than that in a conventional fuel assembly. Therefore, an increase in void reactivity in the fuel assembly 1 due to the presence of MA can be suppressed.

[0050] Furthermore, the cross-sectional area ratio of the nuclear fuel material within the cladding tube of the fuel assembly 1, in which fuel rods 2 that do not contain MA and fuel rods 3 that are smaller in diameter than fuel rods 2 and contain MA are arranged, is 10% higher than that of a conventional fuel assembly. As a result, the weight of the nuclear fuel material in fuel assembly 1 is 10% greater than that of a conventional fuel assembly, and therefore the continuous operating period of a fast reactor with fuel assembly 1 loaded in the core can be extended by 10% for the same average discharge burnup, improving the availability of the fast reactor. [Example]

[0051] A fuel assembly according to a second embodiment of the present invention, which is used in a fast reactor, will be described with reference to Figures 7 and 8. Furthermore, the core of the fast reactor according to the second embodiment will be described with reference to Figures 9 and 10. In the fuel assembly, mixed oxide fuel is filled in the cladding tube of each fuel rod, instead of metallic fuel rods.

[0052] The inner core fuel assembly (first fuel assembly) 1A loaded in the inner core fuel zone 16A of the fast reactor core 15A and the outer core fuel assembly (second fuel assembly) 1B loaded in the outer core fuel zone 16B will be described with reference to Figures 7 and 8. The inner core fuel assembly 1A has a plurality of fuel rods 2A and a plurality of fuel rods 3A arranged in a stainless steel tube 4, which is a cylindrical tube with a regular hexagonal cross section. The outer diameter of fuel rod 2A is 7.36 mm, and the outer diameter of fuel rod 3A is 2.21 mm, which is smaller than the outer diameter of fuel rod 2A.

[0053] The upper end of the cladding tube 6 of the fuel rod 2A is sealed by an upper end plug 10B, and the lower end of the cladding tube 6 is sealed by a lower end plug 11B. Within the sealed cladding tube 6 are multiple pellets of U- and Pu-mixed oxide fuel (including UO2 and PuO2), which is the nuclear fuel material. Above these pellets are multiple pellets of depleted uranium oxide (UO2) blanket fuel. Below the multiple pellets of mixed oxide fuel, multiple pellets of depleted uranium oxide (UO2) blanket fuel are also present. Within the cladding tube 6 of the fuel rod 2A, from the lower end plug 11B upward, there are a lower blanket region 9B containing multiple pellets of UO2, a core fuel region 7A containing multiple pellets of U- and Pu-mixed oxide fuel, and an upper blanket region 8B containing multiple pellets of UO2. The fuel rod 2A does not contain MA. The Pu enrichment of the pellets of the mixed oxide fuel in the fuel rods 2A of the inner core fuel assembly 1A with a burnup of 0 GWd / t is within the range of 13.0 wt% to 26.7 wt% (13.0 wt% or more and 26.7 wt% or less), for example, 23.5 wt%.

[0054] The fuel rod 3A has a cladding tube 6A whose upper end is sealed by an upper end plug 10C and whose lower end is sealed by a lower end plug 11A. The cladding tube 6A thus sealed contains a nuclear fuel material, a mixed oxide fuel (UO X ,PuO X and MAO XAbove this mixed oxide fuel are multiple pellets of depleted uranium oxide (UO2) as blanket fuel, and below this mixed oxide fuel are multiple pellets of depleted uranium oxide (UO2) as blanket fuel. Within the cladding tube 6A of the fuel rod 3A, from the lower end plug 11C upward, there are a lower blanket region 9C containing multiple pellets of depleted uranium oxide (UO2), a core fuel region 14A containing multiple pellets of mixed oxide fuel of U, Pu, and MA, and an upper blanket region 8A containing multiple pellets of depleted uranium oxide (UO2). The Pu enrichment of the mixed oxide fuel of U, Pu, and MA in the fuel rod 3A of the inner core fuel assembly 1A with a burnup of 0 GWd / t is also within the range of 13.0 wt% to 26.7 wt% (13.0 wt% to 26.7 wt%), e.g., 23.5 wt%. The MA enrichment of the mixed oxide fuel of U, Pu and MA is in the range of 3.7 wt% to 12.5 wt% (not less than 3.7 wt% and not more than 12.5 wt%), for example, 5.0 wt%.

[0055] A holding member 32A is disposed below the lower blanket region 9B within the cladding tube 6 of the fuel rod 2A and attached to the cladding tube 6. The holding member 32A holds the lower ends of the pellets located at the lowest position within the lower blanket region 9B. A gas plenum 13A is formed within the cladding tube 6 below the holding member 32A. A through hole is formed in the holding member 32A, and this hole connects the outer surfaces of the pellets above the holding member 32A with a gap formed on the inner surface of the cladding tube 6, and the gas plenum 13A. A holding member 32B is disposed below the lower blanket region 9C within the cladding tube 6A of the fuel rod 3A and attached to the cladding tube 6A. The holding member 32B holds the lower ends of the pellets located at the lowest position within the lower blanket region 9C. A gas plenum 13B is formed within the cladding tube 6A below the holding member 32B. A through hole is also formed in the holding member 32B, and this hole connects the gap formed between the outer surface of the plurality of pellets above the holding member 32B and the inner surface of the cladding tube 6A to the gas plenum 13B.

[0056] In the cross section of the inner core fuel assembly 1A, one fuel rod 3A is arranged between every three adjacent fuel rods 2A and is adjacent to all of those fuel rods 2A. Therefore, six fuel rods 3A are arranged around one fuel rod 2A, surrounding it and adjacent to that fuel rod 2A. In one inner core fuel assembly 1A, there are 271 fuel rods 2A and 378 fuel rods 3A. The total number of fuel rods 2A and 3A is 649. The lower ends of the fuel rods 2A and 3A are supported by support members (not shown) attached to the upper end of the entrance nozzle. This support member has a number of holes (through-holes) that guide the liquid sodium coolant from the entrance nozzle into the trumpet tube 4.

[0057] A wire spacer (not shown) is wrapped around the outer surface of either fuel rod 2A or 3A. The wrapped wire spacer maintains the distance between adjacent fuel rods 3A and 2A and forms a coolant passage 5 between the adjacent fuel rods 3A and 2A.

[0058] 7 and 8, the outer core fuel assembly 1B has a plurality of fuel rods 2B and a plurality of fuel rods 3B arranged in a stainless steel tube 4, which is a cylinder with a regular hexagonal cross section. The outer diameter of fuel rod 2B is 7.36 mm, and the outer diameter of fuel rod 3B is 2.21 mm, which is smaller than the outer diameter of fuel rod 2B.

[0059] Like the fuel rod 2A, the cladding tube 6 of fuel rod 2B, whose upper end is sealed with an upper end plug 10B and whose lower end is sealed with a lower end plug 11B, contains multiple pellets of U- and Pu-mixed oxide fuel (including UO2 and PuO2), which is nuclear fuel material. Above and below the mixed oxide fuel are multiple pellets of depleted uranium oxide (UO2), which serves as blanket fuel. From the lower end plug 11B upward, the cladding tube 6 of fuel rod 2B contains a lower blanket region 9B containing multiple pellets of depleted uranium oxide (UO2), a core fuel region 7B containing multiple pellets of U- and Pu-mixed oxide fuel, and an upper blanket region 8B containing multiple pellets of depleted uranium oxide (UO2). Fuel rod 2B does not contain MA. The Pu enrichment of the pellets of the mixed oxide fuel in the fuel rods 2B of the outer core fuel assembly 1B with a burnup of 0 GWd / t is within the range of 15.6 wt% to 32.0 wt% (15.6 wt% or more and 32.0 wt% or less), for example, 28.2 wt%.

[0060] The upper end of the fuel rod 3B is sealed with an upper end plug 10B, and the lower end is sealed with a lower end plug 11B. Inside the cladding tube 6, similar to the fuel rod 3A described above, there is contained a nuclear fuel material, a mixed oxide fuel (UO X ,PuO X and MAO XThere are multiple pellets of depleted uranium oxide (UO2) in the mixed oxide fuel, and multiple pellets of depleted uranium oxide (UO2) are present above and below the mixed oxide fuel. Within the cladding tube 6A of the fuel rod 3B, from the lower end plug 11C upward, there are a lower blanket region 9C containing multiple pellets of depleted uranium oxide (UO2), a core fuel region 14B containing multiple pellets of mixed oxide fuel of U, Pu, and MA, and an upper blanket region 8C containing multiple pellets of depleted uranium oxide (UO2). The Pu enrichment of the mixed oxide fuel of U, Pu, and MA in the fuel rod 3B of the outer core fuel assembly 1B with a burnup of 0 GWd / t is also within the range of 15.6 wt% to 32.0 wt% (15.6 wt% to 32.0 wt%), e.g., 28.2 wt%. The MA enrichment of the mixed oxide fuel of U, Pu and MA is in the range of 3.7 wt% to 12.5 wt% (not less than 3.7 wt% and not more than 12.5 wt%), for example, 5.0 wt%.

[0061] In the outer core fuel assembly 1B, a holding member 32A that holds fuel pellets is also disposed within the cladding tube 6 of the fuel rod 2B below the lower blanket region 9B and attached to the cladding tube 6, similar to the fuel rod 2A. Holes, which are through-holes formed in the holding member 32A, connect the outer surfaces of the plurality of pellets above the holding member 32A to a gap formed on the inner surface of the cladding tube 6 and a gas plenum 13A. In the outer core fuel assembly 1B, a holding member 32B that holds fuel pellets is also disposed within the cladding tube 6A of the fuel rod 3B below the lower blanket region 9C and attached to the cladding tube 6A, similar to the fuel rod 3A. Holes, which are through-holes formed in the holding member 32B, connect the outer surfaces of the plurality of pellets above the holding member 32B to a gap formed on the inner surface of the cladding tube 6A and a gas plenum 13B.

[0062] The cladding tubes of the fuel rods 2A and 3A and the cladding tubes of the fuel rods 2B and 3B are not filled with bond sodium.

[0063] Each of the inner core fuel assembly 1A and the outer core fuel assembly 1B in this embodiment uses an oxide fuel, for example, a mixed oxide fuel, as a nuclear fuel material, but instead of this mixed oxide fuel, a metal fuel used in the fuel assembly 1 in Example 1 may be used.

[0064] 9 and 10, the core 15A has a core fuel region arranged inside the radial blanket region 18, which includes an inner core fuel region 16A and an outer core fuel region 16B surrounding the inner core fuel region 16A. An upper blanket region 27 located inside the radial blanket region 18 is formed above the inner core fuel region 16A and the outer core fuel region 16B. Furthermore, a lower blanket region 28 located inside the radial blanket region 18 is formed below the inner core fuel region 16A and the outer core fuel region 16B. A plurality of inner core fuel assemblies 1A are arranged in the inner core fuel region 16A of the core 15A, and a plurality of outer core fuel assemblies 1B are arranged in the outer core fuel region 16B of the core 15A. The average enrichment of plutonium in the outer core fuel zone 16B, in which the outer core fuel assemblies 1B are arranged, is higher than that in the inner core fuel zone 16A, in which the inner core fuel assemblies 1A are arranged. A plurality of control rod assemblies 34 are arranged in each of the inner core fuel zone 16A and the outer core fuel zone 16B.

[0065] The core fuel region 7A of each of the fuel rods 2A in the inner core fuel assembly 1A and the core fuel region 14A of each of the fuel rods 3A form an inner core fuel region 16A. The upper blanket region 8B of each of the fuel rods 2A and the upper blanket region 8B of each of the fuel rods 3A form a portion of an upper blanket region 27. The lower blanket region 9B of each of the fuel rods 2A and the lower blanket region 9B of each of the fuel rods 3A form a portion of a lower blanket region 28.

[0066] The core fuel region 7B of each of the fuel rods 2B in the outer core fuel assembly 1B and the core fuel region 14B of each of the fuel rods 3B form an outer core fuel region 16B. The upper blanket region 8C of each of the fuel rods 2B and the upper blanket region 8C of each of the fuel rods 3B form the remainder of the upper blanket region 27. The lower blanket region 9B of each of the fuel rods 2B and the lower blanket region 9C of each of the fuel rods 3B form the remainder of the lower blanket region 28.

[0067] 10 has an inner core fuel region 16A and an outer core fuel region 16B, and the plutonium enrichment in each outer core fuel assembly 1B loaded in the outer core fuel region 16B is higher than the plutonium enrichment in each inner core fuel assembly 1A loaded in the inner core fuel region 16A, making it possible to flatten the radial power distribution of the core 15A. The core 15A is a homogeneous two-region core.

[0068] The fast reactor in which the core 15A loaded with the fuel assemblies in this embodiment, i.e., the inner core fuel assemblies 1A and the outer core fuel assemblies 1B, is disposed in a reactor vessel has an electric output of 750,000 kWe, a continuous operation period of 18 months, and a refueling batch number of 3. The average discharge burnup of each of the inner core fuel assemblies 1A and the outer core fuel assemblies 1B loaded in the core 15A is approximately 100 GWd / t. Liquid sodium, which is the coolant in the reactor vessel, rises in a coolant passage 5 formed in each of the wrapper tubes 4 of the inner core fuel assemblies 1A and the outer core fuel assemblies 1B.

[0069] This embodiment can obtain the effects obtained in Embodiment 1. Furthermore, in this embodiment, the gas plenums formed in each of the fuel rods 2A, 3A, 2B, and 3B are located below the region filled with nuclear fuel material (located below the holding member 35A or 35B), so the axial length of the gas plenums can be made shorter than in the fuel assembly 1 of Embodiment 1. Therefore, the axial length of the core 15A can be made shorter than the axial length of the core 15 (Embodiment 1).

[0070] Furthermore, since the nuclear fuel material used in the fuel assembly of this embodiment is oxide fuel, wet reprocessing can be applied. Therefore, in this embodiment, when spent fuel is reprocessed to recover MA, the proportion of impurities FP such as rare earth elements (RE) accompanying MA can be reduced compared to the case of embodiment 1, in which each fuel material is metallic fuel and dry reprocessing is applied. This improves the economic efficiency of the core and reduces the radiation exposure from new fuel assemblies. [Example]

[0071] A fuel assembly according to a third embodiment of the present invention, which is used in a fast reactor, will be described with reference to FIG.

[0072] The fuel assembly 1C of this embodiment has six corners in its cross section. In the fuel assembly 1C, six regions (as shown in FIG. 11, the six regions are separated by dashed lines connecting the center of the cross section and each of the six corners) are formed around the center of the cross section of the fuel assembly 1C (the central axis of the fuel assembly 1C) (see FIG. 11). These six regions are first fuel rod regions 29 and second fuel rod regions 30 arranged alternately around the center of the cross section. In the three first fuel rod regions 29, a plurality of first fuel rods corresponding to the fuel rods 2 used in the fuel assembly 1 of Example 1 are arranged in an equilateral triangular lattice. The structure of the first fuel rods is the same as that of the fuel rods 2 in Example 1. Within the first fuel rod, similarly to fuel rod 2, from the bottom to the top, there are formed a lower blanket region 9 where U (depleted uranium)-Zr alloy metal fuel exists, a core fuel region 7 where U (depleted uranium)-Pu-Zr alloy metal fuel exists, and an upper blanket region 8 where U (depleted uranium)-Zr alloy metal fuel exists.

[0073] In the three second fuel rod regions 30, a plurality of second fuel rods corresponding to the fuel rods 3 used in the fuel assembly 1 of Example 1 are arranged in an equilateral triangular lattice. The structure of the second fuel rods is the same as the structure of the fuel rods 3 in Example 1. Within the second fuel rods, similar to the fuel rods 3, from the bottom to the top, a lower blanket region 9A in which U (depleted uranium)-Zr alloy metallic fuel exists, a core fuel region 14 in which U (depleted uranium)-Pu-MA-Zr alloy metallic fuel exists, and an upper blanket region 8A in which U (depleted uranium)-Zr alloy metallic fuel exists are respectively formed.

[0074] The outer diameter of the first fuel rods is 7.36 mm, the same as that of fuel rods 2. The outer diameter of the second fuel rods is smaller than that of the first fuel rods but larger than that of fuel rods 3, for example, 3.5 mm. The pitch of the second fuel rods arranged in second fuel rod region 30 is narrower than the pitch of the first fuel rods arranged in first fuel rod region 29.

[0075] A coolant passage is formed between the first fuel rods arranged in the first fuel rod region 29 by a wire spacer wrapped around the outer surface of the first fuel rod. A coolant passage is formed between the second fuel rods arranged in the second fuel rod region 30 by a wire spacer wrapped around the outer surface of the second fuel rod. For the first and second fuel rods that are adjacent near the boundary between the first and second fuel rod regions 29 and 30, a coolant passage can be formed between the adjacent first and second fuel rods by wrapping a wire spacer around the outer surface of either the adjacent first or second fuel rod.

[0076] According to this embodiment, the outer diameter of the second fuel rods containing Pu and MA arranged in the fuel assembly 1C is smaller than the outer diameter of the first fuel rods containing Pu but not MA, so that neutrons generated in the first fuel rods reach the centers of the second fuel rods, making the MA in the second fuel rods more susceptible to nuclear fission, thereby improving the burnup efficiency of the MA in the second fuel rods.

[0077] In particular, in this embodiment, the second fuel rod region 30 in which a plurality of second fuel rods are arranged is disposed between the first fuel rod regions 29 in which a plurality of first fuel rods are arranged adjacent to each other around the center of the cross section of the fuel assembly 1C, and therefore neutrons generated in each first fuel rod in each first fuel rod region 29 are irradiated to each second fuel rod in the second fuel rod region 30 present between those first fuel rod regions 29. The MA in each second fuel rod present in the second fuel rod region 30 becomes more susceptible to nuclear fission, and the burnup efficiency of the MA in the fuel assembly 1C is improved.

[0078] In the fuel assembly 1 of Example 1, the fuel rods 3 with small outer diameters are arranged between three adjacent fuel rods 2, so the outer diameter of the fuel rods 3 is restricted by the fuel rods 2 and cannot be made very large. In contrast, in the fuel assembly 1C of this example, the second fuel rod region 30 is arranged between two first fuel rod regions 29, so the outer diameter of the second fuel rods arranged in the second fuel rod region 30 can be made larger than the outer diameter of the fuel rods 3 arranged in the fuel assembly 1 of Example 1, as long as it is smaller than the outer diameter of the second fuel rods, without being restricted by the first fuel rods arranged in the first fuel rod region 29.

[0079] The arrangement of the plurality of second fuel rods in the second fuel rod region 30 is denser than the arrangement of the plurality of first fuel rods in the first fuel rod region 29, and in the cross section of the fuel assembly 1C, the area of ​​the coolant passages formed between the second fuel rods in the second fuel rod region 30 is smaller than the area of ​​the coolant passages formed between the first fuel rods in the first fuel rod region 29. As a result, the cross-sectional area ratio of the coolant passages in the cross section of the fuel assembly 1C is 10% smaller than that in a conventional fuel assembly, and an increase in void reactivity in the fuel assembly 1C due to the inclusion of MA can be suppressed.

[0080] Furthermore, the cross-sectional area ratio of nuclear fuel material within the cladding tube of fuel assembly 1C, which has second fuel rods that are smaller in diameter than the first fuel rods and contain MA in addition to the first fuel rods that do not contain MA, is higher than that of conventional fuel assemblies. As a result, the weight of nuclear fuel material in fuel assembly 1C is greater than that of conventional fuel assemblies, and therefore, for the same average discharge burnup, the continuous operating period of a fast reactor with fuel assembly 1C loaded in the core can be extended, improving the availability of the fast reactor. [Example]

[0081] A fuel assembly according to a fourth embodiment of the present invention, which is used in a fast reactor, will be described with reference to FIG.

[0082] In the fuel assembly 1D of this embodiment, 12 regions are formed around the center of the cross section of the fuel assembly 1D (the central axis of the fuel assembly 1D), which is more than the number of regions in the previously described fuel assembly 1C. Similar to the fuel assembly 1C of Example 3, these regions are delimited by dashed lines connecting the center of the cross section to the corners of the cross section and dashed lines connecting the midpoint of the side connecting adjacent corners (one side of the trumpet tube 4 of the fuel assembly 1D) to the center of the cross section ( FIG. 12 ). These 12 regions are first fuel rod regions 29A and second fuel rod regions 30A alternately arranged around the center of the cross section. Half of the 12 regions are the first fuel rod regions 29A, and the other half are the second fuel rod regions 30A.

[0083] In the six first fuel rod regions 29A, a plurality of first fuel rods corresponding to the fuel rods 2 used in the fuel assembly 1 of Example 1 are arranged in an equilateral triangular lattice pattern. The structure of the first fuel rods is the same as that of the fuel rods 2 in Example 1, and within the first fuel rods, similar to the fuel rods 2, there are metallic fuels of U (depleted uranium)-Zr alloy and metallic fuels of U (depleted uranium)-Pu-Zr alloy. In the six second fuel rod regions 30A, a plurality of second fuel rods corresponding to the fuel rods 3 used in the fuel assembly 1 of Example 1 are arranged in an equilateral triangular lattice pattern. The structure of the second fuel rods is the same as that of the fuel rods 3 in Example 3, and within the second fuel rods, there are metallic fuels of U (depleted uranium)-Zr alloy and metallic fuels of U (depleted uranium)-Pu-MA-Zr alloy, similar to the fuel rods 3.

[0084] The outer diameter of the first fuel rods is 7.36 mm, the same as that of fuel rods 2. The outer diameter of the second fuel rods is smaller than that of the first fuel rods but larger than that of fuel rods 3, for example, 3.5 mm. The pitch of the second fuel rods arranged in second fuel rod region 30A is narrower than the pitch of the first fuel rods arranged in first fuel rod region 29A.

[0085] The gaps between the fuel rods arranged in the first fuel rod region 29A and the second fuel rod region 30A are formed by wire spacers wound around the outer surfaces of the fuel rods, as in the third embodiment.

[0086] This embodiment can achieve the effects of Embodiment 3. The area of ​​the second fuel rod region 30A in this embodiment in the cross section of the fuel assembly 1D is half the area of ​​the second fuel rod region 30 in the cross section of the fuel assembly 1C in Embodiment 3. Therefore, in this embodiment, the second fuel rod region 30A in which a plurality of second fuel rods are arranged is disposed between two first fuel rod regions 29A in which a plurality of first fuel rods are arranged adjacent to each other around the center of the cross section of the fuel assembly 1D. Therefore, neutrons generated in each first fuel rod in each first fuel rod region 29A are irradiated to each second fuel rod in the second fuel rod region 30A present between the first fuel rod regions 29A. The MA in each second fuel rod present in the second fuel rod region 30A is more likely to undergo nuclear fission, further improving the burnup efficiency of the MA in the fuel assembly 1D. [Example]

[0087] A fuel assembly according to a fifth embodiment of the present invention, which is used in a fast reactor, will be described with reference to FIG.

[0088] In the fuel assembly 1E of this embodiment, eight regions are formed around the center of the cross section of the fuel assembly 1E (the central axis of the fuel assembly 1E), which is more than the number of regions in the aforementioned fuel assembly 1C. Similar to the fuel assembly 1D of Example 3, these regions are regions delimited by dashed lines connecting the center of the cross section to a corner of the cross section or the midpoint of a side connecting adjacent corners. These eight regions are first fuel rod regions 29B and second fuel rod regions 30B alternately arranged around the center of the cross section. Half of the eight regions are first fuel rod regions 29B, and the other half are second fuel rod regions 30B.

[0089] The area of ​​the second fuel rod region 30B in the cross section of the fuel assembly 1E is the same as the area of ​​the second fuel rod region 30A in the cross section of the fuel assembly 1D. The area of ​​the first fuel rod region 29B in the cross section of the fuel assembly 1E is larger than the area of ​​the first fuel rod region 29A in the cross section of the fuel assembly 1D, and is twice the area of ​​the second fuel rod region 30B.

[0090] In the four first fuel rod regions 29B, a plurality of first fuel rods corresponding to the fuel rods 2 used in the fuel assembly 1 of Example 1 are arranged in an equilateral triangular lattice pattern. The structure of the first fuel rods is the same as that of the fuel rods 2 in Example 1, and within the first fuel rods, similar to the fuel rods 2, there are metallic fuels of U (depleted uranium)-Zr alloy and metallic fuels of U (depleted uranium)-Pu-Zr alloy. In the four second fuel rod regions 30B, a plurality of second fuel rods corresponding to the fuel rods 3 used in the fuel assembly 1 of Example 1 are arranged in an equilateral triangular lattice pattern. The structure of the second fuel rods is the same as that of the fuel rods 3 in Example 3, and within the second fuel rods, there are metallic fuels of U (depleted uranium)-Zr alloy and metallic fuels of U (depleted uranium)-Pu-MA-Zr alloy, similar to the fuel rods 3.

[0091] The outer diameter of the first fuel rods is 7.36 mm, the same as that of fuel rods 2. The outer diameter of the second fuel rods is smaller than that of the first fuel rods but larger than that of fuel rods 3, for example, 3.5 mm. The pitch of the second fuel rods arranged in second fuel rod region 30B is narrower than the pitch of the first fuel rods arranged in first fuel rod region 29B.

[0092] The coolant passages between the fuel rods arranged in the first fuel rod region 29B and the second fuel rod region 30B are formed by wire spacers wrapped around the outer surfaces of the fuel rods, as in the third embodiment.

[0093] This embodiment can achieve the effects of Embodiment 3. In this embodiment, the area of ​​the second fuel rod region 30B in the cross section of the fuel assembly 1E is half the area of ​​the first fuel rod region 29B. In this embodiment, the second fuel rod region 30B, which has a smaller area and in which a plurality of second fuel rods are arranged, is disposed between the first fuel rod regions 29B, which have a larger area and in which a plurality of first fuel rods are arranged adjacent to each other around the center of the cross section of the fuel assembly 1E. Therefore, the amount of neutrons generated in each first fuel rod region 29B is much greater than the amount of neutrons generated in each first fuel rod region 29A of the fuel assembly 1D in Embodiment 4. As a result, the number of neutrons entering the second fuel rod region 30B from the first fuel rod region 29B increases, and neutrons reach the center of the cross section of all the second fuel rods in the second fuel rod region 30B. This makes it easier for MA to undergo nuclear fission in the second fuel rods, further improving the burnup efficiency of MA in the fuel assembly 1E. [Example]

[0094] A fuel assembly according to a sixth embodiment of the present invention, which is used in a fast reactor, will be described with reference to FIG.

[0095] In each of the fuel assemblies of Examples 3 to 5, a first fuel rod region in which a plurality of first fuel rods are arranged and a second fuel rod region in which a plurality of second fuel rods are arranged are alternately arranged around the center of the cross section of the fuel assembly. However, in this example, the first fuel rod region and the second fuel rod region are not alternately arranged around the center of the cross section of the fuel assembly, but the annular first fuel rod region and the second fuel rod region are alternately arranged from the center of the cross section of the fuel assembly toward the outer surface of the wrapper tube 4.

[0096] In the fuel assembly 1F of this embodiment, an annular first fuel rod region 29C in which a plurality of first fuel rods are arranged and an annular second fuel rod region 30C in which a plurality of second fuel rods are arranged are arranged to surround the center of the cross section. Each of the annular first fuel rod region 29C and the annular second fuel rod region 30C has a regular hexagonal shape, similar to the cross section of the trumpet tube 4. The first fuel rod region 29C and the second fuel rod region 30C are alternately arranged from the center of the cross section of the fuel assembly 1F toward the outer surface of the trumpet tube 4.

[0097] In the three first fuel rod regions 29C, a plurality of first fuel rods corresponding to the fuel rods 2 used in the fuel assembly 1 of Example 1 are arranged in an equilateral triangular lattice pattern. The structure of the first fuel rods is the same as that of the fuel rods 2 in Example 1, and within the first fuel rods, like the fuel rods 2, there are metallic fuels of U (depleted uranium)-Zr alloy and metallic fuels of U (depleted uranium)-Pu-Zr alloy. In the two second fuel rod regions 30C, a plurality of second fuel rods corresponding to the fuel rods 3 used in the fuel assembly 1 of Example 1 are arranged in an equilateral triangular lattice pattern. The structure of the second fuel rods is the same as that of the fuel rods 3 in Example 3, and within the second fuel rods, there are metallic fuels of U (depleted uranium)-Zr alloy and metallic fuels of U (depleted uranium)-Pu-MA-Zr alloy, like the fuel rods 3.

[0098] The outer diameter of the first fuel rods is 7.36 mm, the same as that of fuel rods 2. The outer diameter of the second fuel rods is smaller than that of the first fuel rods but larger than that of fuel rods 3, for example, 3.5 mm. The pitch of the second fuel rods arranged in second fuel rod region 30B is narrower than the pitch of the first fuel rods arranged in first fuel rod region 29B.

[0099] The coolant passages between the fuel rods arranged in the first fuel rod region 29C and the second fuel rod region 30C are formed by wire spacers wrapped around the outer surfaces of the fuel rods, as in the third embodiment.

[0100] This embodiment can achieve the effects of Embodiment 3. In this embodiment, the width of each of the annular first fuel rod region 29C and the annular second fuel rod region 30C in the direction from the center of the cross section of the fuel assembly 1F toward the outer surface of the trumpet tube 4 is uniform around the center of the cross section of the fuel assembly 1F. In particular, the annular second fuel rod region 30C is disposed between the two first fuel rod regions 29C, and the width of the second fuel rod region 30C in the direction from the center of the cross section of the fuel assembly 1F toward the outer surface of the trumpet tube 4 is uniform around the center of the cross section of the fuel assembly 1F. Therefore, neutrons generated in the first fuel rod region 29C reach the centers of all the second fuel rods disposed in the second fuel rod region 30C. This makes it easier for the MA in all the second fuel rods present in the second fuel rod region 30C to undergo nuclear fission, further improving the burnup efficiency of the MA in the fuel assembly 1F.

[0101] In each of the above-mentioned fuel assemblies 1C, 1D, 1E, and 1F, the first fuel rod and the second fuel rod use metal fuel as the nuclear fuel material, but instead of metal fuel, oxide fuel, for example, mixed oxide fuel, may be used. [Example]

[0102] A fuel assembly according to a seventh embodiment of the present invention, which is used in a boiling water reactor, will be described with reference to FIG.

[0103] In the fuel assembly 1G of this embodiment shown in Fig. 15, in the cross section of the fuel assembly 1G, a plurality of fuel rods (for example, 60 rods) are arranged in 8 rows and 8 columns in a channel box (cylindrical body) 4A. The plurality of fuel rods include a plurality of fuel rods 2C containing pellet-shaped mixed oxide fuel ((U, Pu)O2) of depleted uranium and Pu as nuclear fuel material, and a plurality of fuel rods 2D containing pellet-shaped mixed oxide fuel ((U, Pu, MA)O2) of depleted uranium, Pu, and MA as nuclear fuel material. X) as nuclear fuel material. In the cross section of the fuel assembly 1G, a water rod 31 is arranged in the center of the channel box 4A. This water rod 31 occupies an area in which four fuel rods 2C can be arranged. The outer diameter of the fuel rod 3C is smaller than the outer diameter of the fuel rods 2C.

[0104] The lower ends of fuel rods 2C and 3C and water rod 31 are supported by a lower tie plate (not shown). The upper ends of fuel rods 2C and 3C and water rod 31 are supported by an upper tie plate (not shown). The channel box 4A, which is a rectangular tube with a square cross section, has its upper end attached to the upper tie plate and extends toward the lower tie plate. Each of fuel rods 2C and 3C is bundled by a plurality of fuel spacers arranged axially and placed inside the channel box 4A. Inside the channel box 4A, coolant passages 5 are formed between each fuel rod.

[0105] In a boiling water reactor, light water as a coolant is supplied into the fuel assemblies loaded in the reactor core. From the viewpoint of compatibility between metallic fuel and light water, the fuel assembly 1G of this embodiment does not use metallic fuel as the nuclear fuel material.

[0106] In a boiling water reactor, the neutron spectrum is softer than that of the fast reactors of Examples 1 and 2, so the neutron absorption cross section of the MA contained in the fuel rods 3C becomes larger, and the self-shielding effect of the MA becomes higher. Therefore, the fuel rods 3C containing MA are not arranged in the outermost region of the fuel rod array in the fuel assembly 1G, which is adjacent to the water gap formed between the fuel assemblies loaded in the core, and are not arranged in a position adjacent to the water rods 31. Therefore, the multiple fuel rods 3C are arranged in the second row from the inner surface of the channel box 4A in the fuel rod array.

[0107] The Pu enrichment of the U and Pu mixed oxide fuel in the fuel rod 2C of the fuel assembly 1G with a burnup of 0 GWd / t is within the range of 5.0 wt% to 18.0 wt% (5.0 wt% or more and 18.0 wt% or less), for example, 10.0 wt%.

[0108] The Pu enrichment of the U, Pu, and MA mixed oxide fuel in the fuel rod 3C of the fuel assembly 1G with a burnup of 0 GWd / t is also in the range of 5.0 wt% to 18.0 wt% (5.0 wt% to 18.0 wt%), for example, 10.0 wt%. The MA enrichment of the U, Pu, and MA mixed oxide fuel is in the range of 3.7 wt% to 12.5 wt% (3.7 wt% to 12.5 wt%), for example, 5.0 wt%.

[0109] Although not shown, a plurality of fuel assemblies 1G of this embodiment are loaded into the core of a boiling water reactor.

[0110] According to this embodiment, the outer diameter of the fuel rods 3C containing Pu and MA arranged in the fuel assembly 1G is smaller than the outer diameter of the fuel rods 2C containing Pu but not MA, so that neutrons generated in the fuel rods 2C reach the center of the fuel rods 3C, making the MA in the fuel rods 3C more likely to undergo nuclear fission. This improves the burnup efficiency of the MA in the fuel rods 3C, and therefore the burnup efficiency of the MA in the fuel assembly 1G.

[0111] In this embodiment, the outer diameter of the fuel rods 3C containing MA in the cross section of the fuel assembly 1G is smaller than that of the fuel rods 2C not containing MA, and the proportion of the surrounding coolant is large, so the water-to-fuel volume ratio of the fuel rod cells is large, which suppresses the hardening of the neutron spectrum due to the presence of MA, thereby suppressing the increase in void reactivity in the fuel assembly 1G due to the inclusion of MA.

[0112] According to this embodiment, the outer diameter of the fuel rods 3C containing Pu and MA arranged in the fuel assembly 1G is smaller than the outer diameter of the fuel rods 2C containing Pu but not MA, so that neutrons generated in the fuel rods 2C reach the center of the fuel rods 3C, making the MA in the fuel rods 3C more likely to undergo nuclear fission. This improves the burnup efficiency of the MA in the fuel rods 3C, and therefore the burnup efficiency of the MA in the fuel assembly 1G.

[0113] The fuel assembly 1 or the inner core fuel assembly 1A and the outer core fuel assembly 1B may be loaded into the core of a fast reactor that uses any of lead (Pb) and lead-bismuth (Pb-Bi), helium (He) and carbon dioxide (CO2), or other gas, or molten salt, as a coolant instead of liquid sodium, rather than the fast reactors of Examples 1 and 2 that use liquid sodium as a coolant. Any of the fuel assemblies 1C, 1D, 1E, and 1F may be loaded into the core of the fast reactors of Examples 1 and 2, or further into the core of a fast reactor that uses any of lead (Pb) and lead-bismuth (Pb-Bi), helium (He) and carbon dioxide (CO2), or other gas, or molten salt, as a coolant.

[0114] The fuel assembly 1G of the seventh embodiment to be loaded into the core of a boiling water reactor may be loaded into the core of a pressurized water reactor or the core of a heavy water reactor that uses heavy water as a coolant and moderator. [Explanation of symbols]

[0115] 1, 1C, 1D, 1E, 1F, 1G... fuel assembly, 1A... inner core fuel assembly, 1B... outer core fuel assembly, 2, 2A, 2B, 2C... fuel rods (large diameter), 3, 3A, 3B, 3C... fuel rods (small diameter), 4... trumpet tube, 4A... channel box, 7, 7A, 14, 14A... core fuel area, 8, 8A, 8B, 8C... upper blanket area, 9, 9A, 9B, 9C... lower blanket area region, 15, 15A...core, 16...core fuel region, 16A...inner core fuel region, 16B...outer core fuel region, 18...radial blanket region, 21...inner blanket region, 22...inner blanket fuel assembly, 27...upper blanket region, 28...lower blanket region, 29, 29A, 29B, 29C...first fuel rod region, 30, 30A, 30B, 30C...second fuel rod region.

Claims

1. a plurality of first fuel rods having a first nuclear fuel material that is free of minor actinides are disposed within the canister; a plurality of second fuel rods having second nuclear fuel material containing the minor actinide are disposed within the tubular body; The outer diameter of the second fuel rod is smaller than the outer diameter of the first fuel rod, a second fuel rod region in which a plurality of the second fuel rods are arranged is disposed within the cylindrical body between first fuel rod regions in which a plurality of the first fuel rods are arranged; a plurality of the first fuel rods arranged in an equilateral triangular lattice pattern within the first fuel rod region, and a plurality of the second fuel rods arranged in an equilateral triangular lattice pattern within the second fuel rod region.

2. 2. The fuel assembly of claim 1, wherein the first fuel rod regions and the second fuel rod regions extending from a center of a cross section of the fuel assembly toward an inner surface of the tubular body are alternately arranged around the center of the cross section.

3. 2. The fuel assembly of claim 1, wherein the first annular fuel rod region and the second annular fuel rod region surround a cross-sectional center of the fuel assembly and are alternately arranged from the cross-sectional center toward the inner surface of the tubular body.

4. a plurality of first fuel rods having a first nuclear fuel material that is free of minor actinides are disposed within the canister; a plurality of second fuel rods having second nuclear fuel material containing the minor actinide are disposed within the tubular body; The outer diameter of the second fuel rod is smaller than the outer diameter of the first fuel rod, Within the cylindrical body having a square cross section, fuel rods including the first fuel rods and the second fuel rods are arranged in a square lattice pattern; A fuel assembly characterized in that the plurality of second fuel rods are not arranged in the outermost region of the fuel rod array, but are arranged in the second row from the inner surface of the cylindrical body in the fuel rod array.

5. 4. The fuel assembly according to claim 1, wherein each of the first nuclear fuel material and the second nuclear fuel material is either a metallic fuel or an oxide fuel.

6. 5. The fuel assembly of claim 4, wherein each of the first nuclear fuel material and the second nuclear fuel material is an oxide fuel.

7. A core of a nuclear reactor, wherein the fuel assembly according to any one of claims 1 to 3 and 5 is loaded.

8. 7. A core of a nuclear reactor, wherein the fuel assemblies according to claim 4 or 6 are loaded, and cooling water exists inside as a coolant.

Images