Fuel assemblies and fast reactor cores

Abstract

To provide a fuel assembly that can easily adjust the average MA enrichment in an internal blanket region.SOLUTION: An inner reactor core fuel assembly 7 to be loaded in an inner reactor core region 2 of a reactor core of a fast reactor has a plurality of fuel rods 10 and a plurality of fuel rods 19. The fuel rod 10 has: a lower part reactor core fuel region 12; an internal blanket region 11; and an upper part reactor core fuel region 13. In the lower part reactor core fuel region 12 and the upper part reactor core fuel region 13, a U-Pu-Zr metal fuel is arranged, and in the internal blanket region 11, a U-Zr metal fuel is arranged. The fuel rod 19 has: the lower part reactor core fuel region 12; an internal blanket region 20; and the upper part reactor core fuel region 13. In the lower part reactor core fuel region 12 and the upper part reactor core fuel region 13 of the fuel rod 19, the U-Pu-Zr metal fuel is arranged, and in the internal blanket region 20, an MA-Zr metal fuel is arranged. By adjusting the number of respective fuel rods 10 and 19, the MA enrichment in the internal blanket region 9 of the fuel assembly 7 can be easily adjusted.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a fuel assembly and a core of a fast reactor, and more particularly to a fuel assembly using metallic fuel and a core of a fast reactor in which the fuel assembly is loaded, for avoiding core damage in the event of an accident due to erroneous control rod withdrawal, thereby improving safety. [Background technology]

[0002] Generally, in a fast breeder reactor, the core is arranged in a reactor vessel, and the reactor vessel is filled with liquid sodium as a coolant. The fuel assembly loaded in the core has a number of fuel rods containing plutonium-enriched depleted uranium (U-238), a trumpet tube surrounding the bundled fuel rods, the lower ends of the fuel rods, an entrance nozzle supporting a neutron shield located below the fuel rods, and a coolant outlet located above the fuel rods.

[0003] The core of a fast breeder reactor has 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. This results in a flattened power distribution in the radial direction of the core.

[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 made of a mixture of Pu and depleted uranium oxides, i.e., MOX fuel pellets, are loaded in the fuel rod to a height of about 80 to 100 cm in the axial center. In addition, axial blanket regions loaded with multiple uranium dioxide pellets made of depleted uranium are disposed above and below the MOX fuel loading region in the fuel rod. The inner core fuel assembly loaded in the inner core region and the outer core fuel assembly loaded in the outer core region each have multiple fuel rods loaded 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 a number of fuel rods filled with uranium dioxide pellets made from depleted uranium. Neutrons generated by nuclear fission reactions in the fuel assemblies loaded in the core fuel region, and those that leak from the core fuel region, are absorbed by U-238 in each fuel rod of the blanket fuel assemblies loaded in the blanket fuel region. As a result, Pu-239, a fissile nuclide, is newly produced in each fuel rod of the blanket fuel assembly.

[0007] In addition, control rods are used when starting up and shutting down a fast breeder reactor and adjusting the reactor power output. The 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 trumpet tube with a regular hexagonal cross section, just like the inner and outer core fuel assemblies. The control rods are made up of 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] Generally, the burnup reaction rate of a fast reactor is about 3%Δk / kk'. If an accident (UTOP: Unprotected Transient Over Power) occurs due to the combination of an erroneous control rod withdrawal and a failed scram, the power density in the vicinity of the control rod may change, causing the linear power to exceed the design tolerance. If this increase in linear power during UTOP can be avoided, the thermal margin can be increased, and the safety of the reactor core can be improved. In order to avoid an increase in linear power during UTOP, it is effective to reduce the burnup reactivity and the control reactivity required for each control rod for burnup compensation.

[0009] JP 5-52981 A describes a fuel assembly for use in a fast reactor, which has a number of fuel rods filled with mixed oxide fuel containing TRUs (Np, Pu, Am, Cm, etc.).

[0010] JP 2018-71997 A describes a fast reactor core having an inner core region and an outer core region surrounding the inner core region. In this core, an inner blanket region is disposed in the inner core region. In the inner blanket region, oxide fuel including depleted uranium fuel oxide including minor actinides (MA) is present. Since oxide fuel including depleted uranium fuel oxide including MA such as Np, Am, and Cm is present in the inner blanket region, it is possible to reduce the burnup reactivity while suppressing the void reactivity, thereby reducing the reactivity applied to the core during UTOP and improving safety. In JP 2018-71997 A, the MA content in the inner blanket region is within a range of 35 wt% to 45 wt%. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Publication No. 5-52981 [Patent Document 2] JP 2018-71997 A Summary of the Invention [Problem to be solved by the invention]

[0012] In the inner blanket region of the core of the fast reactor described in JP 2018-71997 A, there is oxide fuel containing depleted uranium fuel oxide containing minor actinides. The minor actinides recovered from spent fuel by reprocessing the spent fuel are used. It is desirable that the minor actinide enrichment in the inner blanket region can be easily adjusted. In particular, when metal fuel is used as the nuclear fuel material, americium and curium among the minor actinides cannot form a homogeneous alloy with uranium. When such metal fuel is used, it is desirable that the average minor actinide enrichment in the inner blanket region can be easily adjusted.

[0013] It is an object of the present invention to provide a fuel assembly and a fast reactor core in which the average minor actinide enrichment in the inner blanket region can be easily adjusted. [Means for solving the problem]

[0014] The fuel assembly of the present invention that achieves the above-mentioned object is characterized in that a nuclear fuel material region in which nuclear fuel material is present is formed, and a first lower core fuel region, a second inner blanket region, and a first upper core fuel region are formed in this order from the lower end to the upper end of the nuclear fuel material region, a plurality of first fuel rods in which the nuclear fuel material resides, and a plurality of second fuel rods in which the nuclear fuel material resides; Within the first fuel rod, a second lower core fuel region is formed at a position corresponding to the first lower core fuel region, a second inner blanket region is formed at a position corresponding to the first inner blanket region, and a second upper core fuel region is formed at a position corresponding to the first upper core fuel region; a third lower core fuel region is formed within the second fuel rod at a position corresponding to the first lower core fuel region, a third inner blanket region is formed within the second fuel rod at a position corresponding to the first inner blanket region, and a third upper core fuel region is formed within the second fuel rod at a position corresponding to the first upper core fuel region; The nuclear fuel material present in the second inner blanket region of the first fuel rod contains uranium but not minor actinides, and the nuclear fuel material present in the third inner blanket region of the second fuel rod contains minor actinides but not uranium.

[0015] A fuel assembly having the above characteristics allows the average enrichment of minor actinides in the first inner blanket region of the fuel assembly to be easily adjusted by adjusting the number of first fuel rods and the number of second fuel rods in the fuel assembly.

[0016] It is preferable that the average enrichment of minor actinides in the first inner blanket region of a fuel assembly with a burnup of 0 GWdt be within the range of 3.7 wt% to 12.5 wt%.

[0017] By keeping the average enrichment of minor actinides in the first inner blanket region within the range of 3.7 wt% to 12.5 wt%, even in the unlikely event of an accident involving the combination of erroneous control rod removal and a failed scram, the increase in linear power in the fuel assemblies near that control rod will be minimal, the integrity of the fuel rods within the fuel assembly will be maintained, and the safety of the core will be improved.

[0018] A feature of a fast reactor core that achieves the above object is a fast reactor core having an inner core region in which a plurality of first fuel assemblies are loaded and an outer core region surrounding the inner core region and in which a plurality of second fuel assemblies are loaded, forming, from a lower end to an upper end of the inner core region, a fourth lower core fuel region, a fourth inner blanket region, and a fourth upper core fuel region, in that order, within the inner core region; the first fuel assembly is a fuel assembly having the aforementioned characteristics; A fourth lower core fuel region is formed at a position corresponding to the second lower core fuel region, a fourth inner blanket region is formed at a position corresponding to the second inner blanket region, and a fourth upper core fuel region is formed at a position corresponding to the second upper core fuel region. Effect of the Invention

[0019] According to the present invention, the average minor actinide enrichment in the inner blanket region of a fuel assembly with burnup of 0 GWdt can be easily adjusted. [Brief description of the drawings]

[0020] [Figure 1] 1 is a longitudinal sectional view of a core of a fast reactor according to a first embodiment of the present invention, which is a preferred embodiment of the present invention. [Diagram 2] 2 is a cross-sectional view taken along line II-II in FIG. 1 of an inner core fuel assembly with a burnup of 0 GWdt loaded in an inner core region of the core shown in FIG. 1. [Diagram 3] 3 is a cross-sectional view taken along line III-III in FIG. 1 of an inner core fuel assembly with a burnup of 0 GWdt loaded in an inner core region of the core shown in FIG. 1. [Figure 4] FIG. 2 is a longitudinal cross-sectional view of the inner core fuel assembly with burnup of 0 GWdt shown in FIG. 1. [Diagram 5] FIG. 2 is a longitudinal cross-sectional view of the outer core fuel assembly with burnup of 0 GWdt shown in FIG. 1. [Figure 6] FIG. 2 is a half cross-sectional view of the core of the fast reactor shown in FIG. 1. [Figure 7] FIG. 1 is a characteristic diagram showing the dependence of burnup reactivity on the average value of MA enrichment in the inner blanket region of the inner core region of a fast reactor core. [Figure 8] FIG. 11 is a vertical sectional view of an inner core fuel assembly with a burnup of 0 GWdt loaded into an inner core region of a fast reactor core according to embodiment 2 which is another preferred embodiment of the present invention. [Figure 9] FIG. 11 is a half vertical sectional view of a core of a fast reactor according to a third embodiment which is another preferred embodiment of the present invention. [Figure 10]FIG. 10 is a longitudinal cross-sectional view of an inner core fuel assembly with a burnup of 0 GWdt loaded in the inner core region of the core shown in FIG. [Figure 11] FIG. 10 is a longitudinal cross-sectional view of an outer core fuel assembly with a burnup of 0 GWdt loaded in the outer core region of the core shown in FIG. [Figure 12] FIG. 11 is a vertical sectional view of an inner core fuel assembly with a burnup of 0 GWdt loaded in an inner core region of a fast reactor core according to embodiment 4 which is another preferred embodiment of the present invention. [Figure 13] FIG. 11 is a longitudinal sectional view of an outer core fuel assembly with a burnup of 0 GWdt loaded in the outer core region of a fast reactor core of Example 4. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] An embodiment of the present invention will now be described. EXAMPLES

[0022] A core of a fast reactor according to a first embodiment, which is a preferred embodiment of the present invention, will be described with reference to FIGS.

[0023] As shown in FIG. 1 and FIG. 6, the core 1 of the fast reactor in this embodiment has an inner core region 2 arranged in the center of the core 1, an outer core region 3 surrounding the inner core region 2, a radial blanket region 25 surrounding the outer core region 3, and a reflector region 26 surrounding the radial blanket region 25. A plurality of inner core fuel assemblies 7 are loaded in the inner core region 2, and a plurality of outer core fuel assemblies 21 are loaded in the outer core region 3. A plurality of blanket fuel assemblies 27 are loaded in the radial blanket region 25, and a plurality of neutron reflectors 28 are loaded in the reflector region 26. The core 1 is an axially heterogeneous core in which an inner blanket region 6 is formed in the inner core region 2. In addition to the inner blanket region 6, the inner core region 2 has an upper core fuel region 4 located above the inner blanket region 6 and a lower core fuel region 5 located below the inner blanket region 6.

[0024] A plurality of control rod assemblies 29 are arranged in the inner core region 2 and the outer core region 3, and are inserted between the inner core fuel assemblies 7 and between the outer core fuel assemblies 21. The plurality of control rod assemblies 29 include two independent systems of control rods: main reactor shutdown system control rods (adjustment rods) and backup reactor shutdown system control rods (safety rods). The main reactor shutdown system control rods are used to adjust the change in reactivity and power distribution accompanying the combustion of nuclear fuel material. The backup reactor shutdown system control rods are installed as a backup in case the main reactor shutdown system control rods fail. A fast reactor can be shut down in an emergency by either the main reactor shutdown system control rods or the backup reactor shutdown system control rods.

[0025] The inner core fuel assembly 7 has a plurality of fuel rods 10 and a plurality of fuel rods 19 (see FIG. 4). Each of the fuel rods 10 and 19 is wrapped with a wire spacer (not shown) on the outer surface and arranged in a stainless steel wrapper tube 30, which is a cylindrical structure with a regular hexagonal cross section. The wire spacer forms a coolant passage between adjacent fuel rods through which liquid sodium, the coolant, rises. Each of the fuel rods 10 and 19 is arranged in a regular triangular lattice in the wrapper tube 30 (see FIGS. 2 and 3), and the lower end of each fuel rod is supported by an entrance nozzle (not shown) attached to the lower end of the wrapper tube 30. The wrapped wire spacer forms a gap of a predetermined width between adjacent fuel rods. This gap becomes a coolant passage through which liquid metal, the coolant, flows.

[0026] Each of the fuel rods 10 and 19 has a stainless steel cladding tube 14, the lower end of which is closed by a lower end plug 17 and the upper end of which is closed by an upper end plug 18, and is sealed. Metallic fuel material is disposed within the cladding tube 14. The fuel rod 10 uses U-Pu-Zr metallic fuel and U-Zr metallic fuel as metallic fuel material. Within the cladding tube 14 of the fuel rod 10, a lower core fuel region 12, an inner blanket region 11, and an upper core fuel region 13 are disposed from the lower end plug 17 of the cladding tube 14 upward. U-Pu-Zr metallic fuel is disposed in each of the lower core fuel region 12 and the upper core fuel region 13. U-Zr metallic fuel that does not contain minor actinides (MA) is disposed in the inner blanket region 11. The respective positions of the lower core fuel region 12 , the inner blanket region 11 and the upper core fuel region 13 within the fuel rod 10 correspond to the respective positions of the lower core fuel region 5 , the inner blanket region 6 and the upper core fuel region 4 in the inner core region 2 .

[0027] The fuel rod 19 is arranged in a cladding tube 14 sealed by a lower end plug 17 and an upper end plug 18, with a lower core fuel region 12, an inner blanket region 20, and an upper core fuel region 13 arranged from the lower end plug 17 upward. In the lower core fuel region 12 and the upper core fuel region 13, U-Pu-Zr metal fuel is arranged, as in the fuel rod 10. In the inner blanket region 20, MA-Zr metal fuel is arranged. The positions of the lower core fuel region 12, the inner blanket region 20, and the upper core fuel region 13 in the fuel rod 19 correspond to the positions of the lower core fuel region 5, the inner blanket region 6, and the upper core fuel region 4 in the inner core region 2, respectively.

[0028] The aforementioned U-Pu-Zr metallic fuel, U-Zr metallic fuel and MA-Zr metallic fuel have a solid cylindrical shape.

[0029] As shown in FIG. 3, in a cross-section of the inner core fuel assembly 7, the multiple fuel rods 10 and the multiple fuel rods 19 are intermixed with each other, specifically, arranged so that the fuel rods 10 are present between the fuel rods 19.

[0030] The inner core fuel assembly 7 has a lower core fuel region 8B, an inner blanket region 9, and an upper core fuel region 8A corresponding to the lower core fuel region 5, the inner blanket region 6, and the upper core fuel region 4 of the inner core region 2 (see FIG. 1). In the inner core fuel assembly 7, the lower core fuel region 8B corresponds to the lower core fuel region 12 of each of the fuel rods 10 and 19, the inner blanket region 9 corresponds to the inner blanket region 11 of the fuel rods 10 and the inner blanket region 20 of the fuel rods 19, and the upper core fuel region 8A corresponds to the upper core fuel region 13 of each of the fuel rods 10 and 19.

[0031] In the inner blanket region 9 of the inner core fuel assembly 7, the fuel rods 10 and 19 are arranged so that the inner blanket region 11 containing U-Zr metal fuel of the fuel rod 10 and the inner blanket region 20 containing MA-Zr metal fuel of the fuel rod 19 are mixed (see FIG. 3). In the upper core fuel region 8A of the inner core fuel assembly 7, the upper core fuel region 13 containing U-Pu-Zr metal fuel of each of the fuel rods 10 and 19 is arranged (see FIG. 2). In the lower core fuel region 8B of the inner core fuel assembly 7, the lower core fuel region 12 containing U-Pu-Zr metal fuel of each of the fuel rods 10 and 19 is arranged.

[0032] The height from the lower end to the upper end of the core 1 is, for example, 100 cm. An example of the dimensions of the lower core fuel region 5, the internal blanket region 6, and the upper core fuel region 4 in the inner core region 2 will be described. The lower end of the lower core fuel region 5 coincides with the lower end of the core 1, and the length of the lower core fuel region 5 in the axial direction of the core 1 is 40 cm. The internal blanket region 6 is located between a position 40 cm from the lower end of the core 1 and a position 60 cm from the lower end of the core 1, and the length of the internal blanket region 6 in the axial direction of the core 1 is 20 cm. The upper core fuel region 4 is located between a position 60 cm from the lower end of the core 1 and the upper end of the core 1, and the length of the upper core fuel region 4 in the axial direction of the core 1 is 40 cm. The center position of the internal blanket region 6 in the axial direction coincides with, for example, the center position of the core 1 in the axial direction.

[0033] In each of the fuel rods 10 and 19 in the inner core fuel assembly 7, the axial length of the nuclear fuel material filled region in which the metallic fuel is arranged, i.e., the axial length from the lower end of the active fuel length to the upper end of the active fuel length, is 100 cm, which is the same as the axial length of the core 1. In each of the fuel rods 10 and 19, the axial length of the lower core fuel region 12 is 40 cm, which is the same as that of the lower core fuel region 5. The axial length of each of the inner blanket regions 11 and 20 is 20 cm, which is the same as that of the inner blanket region 6. The axial length of the upper core fuel region 13 is 40 cm, which is the same as that of the upper core fuel region 4.

[0034] Bonded sodium 15, which is liquid metallic sodium, is filled in the cladding tube 14 of the fuel rods 10 and 19 in the inner core fuel assembly 7. In the fuel rod 10, the bonded sodium 15 is filled in the gaps formed between the U-Pu-Zr metal fuel and the U-Zr metal fuel, respectively, and the inner surface of the cladding tube 14. In the fuel rod 19, the bonded sodium 15 is filled in the gaps formed between the U-Pu-Zr metal fuel and the MA-Zr metal fuel, respectively, and the inner surface of the cladding tube 14. In each of the fuel rods 10 and 19, a gas plenum 16 is formed in the cladding tube 14 above the region where the bonded sodium 15 is filled.

[0035] The outer core fuel assembly 21 loaded in the outer core region 3 has a plurality of fuel rods 22 arranged in a trumpet tube 30. The lower end of each fuel rod 22 is supported by an entrance nozzle (not shown) provided at the lower end of the trumpet tube 30. The cladding tube 14 of the fuel rod 22 is also sealed by closing its lower end with a lower end plug 17 and closing its upper end with an upper end plug. The core fuel region 23 in the cladding tube 14 of the fuel rod 22 is filled with U-Pu-Zr metal fuel, which is a metal fuel material. The cladding tube 14 of the fuel rod 22 in the outer core fuel assembly 21 is also filled with bond sodium 15, which is liquid metal sodium. In the fuel rod 22, the bond sodium 15 is also filled in the gap formed between the U-Pu-Zr metal fuel and the inner surface of the cladding tube 14. In the fuel rod 22, a gas plenum 16 is also formed in the cladding tube 14 above the region filled with the bond sodium 15. The plutonium enrichment (=Pu / (Pu+U)) of the core fuel region 23 in the cladding tube 14 of the fuel rods 22 in the outer core fuel assembly 21 with a burnup of 0 GWd / t is within the range of 13 wt% to 25 wt%, for example, 25 wt%.

[0036] The height from the lower end of the active fuel length to the upper end of the active fuel length of the fuel rods 22 in the outer core fuel assemblies 21 is 100 cm, the same as the height of the core 1.

[0037] The axial length of the region filled with metallic fuel material, i.e., the active fuel length, in each of the fuel rods 10, 19 and 22 is the same.

[0038] As described above, the lower core fuel region 5, the inner blanket region 6, and the upper core fuel region 4 in the inner core region 2 are formed by the fuel rods 10 and the fuel rods 19 included in the inner core fuel assemblies 7 loaded in the inner core region 2. The plutonium enrichment of the lower core fuel region 12 and the upper core fuel region 13 of the fuel rods 10 and 19 in the inner core fuel assemblies 7 with a burnup of 0 GWd / t is also within a range of 13 wt% to 25 wt%, for example, 25 wt%. Furthermore, U-Zr metal fuel not containing MA is arranged in the inner blanket region 11 of the fuel rods 10 in the inner core fuel assemblies 7 with a burnup of 0 GWd / t, and MA-Zr metal fuel containing MA is arranged in the inner blanket region 20 of the fuel rods 19 in the fuel assemblies 7. The average MA enrichment in the inner blanket region 9 of such an inner core fuel assembly 7 is within a range of 3.7 wt% to 12.5 wt%, which will be described later, and is, for example, 8.0 wt%.

[0039] The electric output of the fast reactor having the core 1 is, for example, 750,000 kWe, the continuous operation period is 23 months, and the average discharged core fuel burnup of the fuel assemblies loaded in the core 1 is about 100 GWd / t. The fuel assemblies loaded in the core 1 are discharged from the core 1 in, for example, three batches. That is, for each of the fuel assemblies 7 and 21 loaded in the core 1, 1 / 3 of the total fuel assemblies loaded in the core 1 (1 / 3 of the fuel assemblies 7 and 21 that have experienced three cycles of operation) are discharged from the core 1 during the outage period after the fast reactor operation in one operation cycle is completed. Then, instead, each of the fuel assemblies 7 and 21 with 0 GWd / t is loaded into the core 1.

[0040] During operation of a fast reactor, gaseous fission products (FPs) produced by fission of the fissile material (e.g., Pu-239) contained in the metallic fuel in fuel rods 10 and 19 are stored in gas plenums 16 present within each fuel rod. The formation of gas plenums 16 within each fuel rod inhibits the buildup of pressure within the fuel rod due to the production of gaseous fission products.

[0041] In the inner core fuel assembly 7 with a burnup of 0 GWd / t, as described above, the fuel rods 10 having the inner blanket region 11 containing U-Zr metal fuel not containing MA, and the fuel rods 19 having the inner blanket region 20 containing MA-Zr metal fuel not containing U are mixed. The reason for mixing the fuel rods 10 and the fuel rods 19 in the inner core fuel assembly 7 in this way is that when metal fuel is used, americium (Am) and curium (Cm) among the minor actinides cannot form a homogeneous alloy with uranium (U). By mixing the fuel rods 10 having the inner blanket region 11 containing U-Zr metal fuel not containing MA, and the fuel rods 19 having the inner blanket region 20 containing MA-Zr metal fuel in the inner core fuel assembly 7, the inner blanket region 9 containing the uranium of the metal fuel and the minor actinides of the metal fuel can be easily formed in the inner core fuel assembly 7. By adjusting the number of fuel rods 10 and the number of fuel rods 19, the enrichment of minor actinides in the inner blanket region 9 of the inner core fuel assembly 7 having a burnup of 0 GWd / t can be easily adjusted. Therefore, the enrichment of minor actinides in the inner blanket region 6 in the inner core region 2 can also be easily adjusted.

[0042] The inventors have studied the change in burnup reactivity of the core 1 depending on the average enrichment (=MA / (U+MA)) of minor actinides in the internal blanket region 9 of the inner core fuel assembly 7 with a burnup of 0 GWd / t. FIG. 7 shows the change in burnup reactivity of the core 1 depending on the average enrichment of minor actinides in the internal blanket region 9 obtained from this study. In FIG. 7, the horizontal axis shows the average enrichment (wt%) of minor actinides in the internal blanket region 9 of the inner core fuel assembly 7 with a burnup of 0 GWd / t, and the vertical axis shows the burnup reactivity ($) in the inner core fuel assembly 7. Here, the minor actinides are Np, Am, and Cm contained in the spent nuclear fuel in the spent fuel assemblies of a light water reactor with a discharge burnup of 60 GWd / t.

[0043] 7, the burnup reactivity decreases as the average enrichment of minor actinides in the inner blanket region 9 increases, becomes nearly zero when the average enrichment of minor actinides approaches 8.0 wt%, and becomes a negative value as the average enrichment of minor actinides increases. The reason why the burnup reactivity becomes a negative value in this way is that the reactivity is greater at the end of the equilibrium cycle than at the beginning of the equilibrium cycle.

[0044] As shown in Fig. 7, by setting the average enrichment of minor actinides in the internal blanket region 9 of the inner core fuel assembly 7 with a burnup of 0 GWd / t within the range of 3.7wt% to 12.5wt% (3.7wt% to 12.5wt%), the absolute value of the burnup reactivity is 1$ or less. Furthermore, by setting the average enrichment of minor actinides in the internal blanket region 9 within the range of 3.7wt% to 12.5wt% (3.7wt% to 12.5wt%), even in the case of an accident in which erroneous removal of a control rod and a scram failure are combined (UTOP: Unprotected Transient Over Power), the increase in linear power in the fuel assemblies near the control rod is small, the integrity of the fuel rods is maintained, and the safety of the core is improved.

[0045] Metallic fuel containing uranium and metallic fuel containing MA are not homogeneously mixed, so metallic fuel containing uranium and MA cannot be produced. In this embodiment, the inner core fuel assembly 7 includes a plurality of fuel rods 10 having an inner blanket region 11 in which metallic fuel containing uranium but not minor actinides is present, and a plurality of fuel rods 19 having an inner blanket region 20 in which metallic fuel containing minor actinides but not uranium is present. By adjusting the number of fuel rods 10 and 19, the enrichment degree of minor actinides in the inner blanket region 9 of the inner core fuel assembly 7 can be easily adjusted even in the case of the inner core fuel assembly 7 using metallic fuel. In addition, since a plurality of inner core fuel assemblies 7 are loaded in the inner core region 2 of the core 1, the enrichment degree of minor actinides in the inner blanket region 6 of the core 1 can also be easily adjusted.

[0046] The metal fuel used in the fuel assembly of this embodiment has a higher density than the oxide fuel used in the fuel assembly described in JP 2018-71997 A, and does not contain oxygen, which has a neutron scattering effect, so that the neutron spectrum is harder and the internal conversion ratio is higher. The burnup reactivity of the metal fuel is lower than that of the oxide fuel. As a result, the average MA enrichment of the inner blanket region 9, which reduces the burnup reactivity of the inner core fuel assembly 7 having a burnup of 0 GWdt in this embodiment, can be made lower than the average MA enrichment of the inner blanket region of the fuel assembly described in JP 2018-71997 A. In the inner core fuel assembly 7 of this embodiment, even if the average MA enrichment of the inner blanket region 9 is reduced, a predetermined burnup reactivity can be obtained.

[0047] As the enrichment of MA increases, the decay heat of MA increases, and the difficulty of fuel production (the problem of heat removal during fuel production) increases. According to this embodiment, the average MA enrichment of the inner blanket region 9 of the inner core fuel assembly 7 with a burnup of 0 GWdt can be made lower than the average MA enrichment of the inner blanket region of the fuel assembly described in JP2018-71997A, and therefore the manufacturability of the fuel used in this embodiment is improved. EXAMPLES

[0048] A core of a fast reactor according to embodiment 2, which is another preferred embodiment of the present invention, will be described with reference to FIG.

[0049] In the fast reactor core of this embodiment, an inner core fuel assembly 7A shown in Fig. 8 is used instead of the inner core fuel assembly 7 used in embodiment 1. The configuration of the fast reactor core of this embodiment other than the inner core fuel assembly 7A is the same as the configuration of the fast reactor core 1 of embodiment 1. The inner core fuel assembly 7A is loaded into the inner core region 2 of the core 1 in this embodiment.

[0050] The inner core fuel assembly 7A has a plurality of fuel rods 10A and a plurality of fuel rods 19A. The configuration of the inner core fuel assembly 7A other than the fuel rods 10A and 19A is the same as that of the inner core fuel assembly 7 used in Example 1. The lower and upper ends of the cladding tubes 14 of the fuel rods 10A and 19A are also sealed by lower end plugs 17 and upper end plugs 18.

[0051] The fuel rod 10A uses U-Pu-Zr metal fuel and U-Zr metal fuel as the metal fuel material. The fuel rod 10A has a lower core fuel region 12 and an upper core fuel region 13 formed in a cladding tube 14 in the same manner as the fuel rod 10, and an inner blanket region 11A is formed between the lower core fuel region 12 and the upper core fuel region 13. U-Pu-Zr metal fuel is arranged in the lower core fuel region 12 and the upper core fuel region 13, and U-Pu-Zr metal fuel is arranged in the inner blanket region 11A.

[0052] The fuel rod 19A uses U-Pu-Zr metal fuel and U-Zr metal fuel as the metal fuel material. The fuel rod 19A has a lower core fuel region 12 and an upper core fuel region 13 formed in a cladding tube 14 in the same manner as the fuel rod 10, and an inner blanket region 20A is formed between the lower core fuel region 12 and the upper core fuel region 13. U-Pu-Zr metal fuel is arranged in the lower core fuel region 12 and the upper core fuel region 13, and MA-Pu-Zr metal fuel is arranged in the inner blanket region 20A.

[0053] The above-mentioned U-Pu-Zr metallic fuel and MA-Pu-Zr metallic fuel in this embodiment also have a solid cylindrical shape.

[0054] The plutonium enrichment in the inner blanket regions 11A and 20A of each fuel rod of the inner core fuel assembly 7A with a burnup of 0 GWdt may be in the range of more than 0 wt% and not more than 13 wt%, for example, 10 wt%. By setting the plutonium enrichment within this range, the time variation and spatial distribution of the power distribution in the core of the fast reactor of this embodiment can be made as flat as possible, and the target core reactivity can be achieved.

[0055] In this embodiment as well, the average enrichment of minor actinides in the inner blanket region 9 of the inner core fuel assemblies 7A, which have a burnup of 0 GWdt, is within the range of 3.7 wt% to 12.5 wt% (not less than 3.7 wt% and not more than 12.5 wt%).

[0056] The configuration of the inner blanket region of each of the fuel rods 10A and 19A used in the inner core fuel assembly 7A in this embodiment is shown in Table 1. Here, the number of fuel rods 10A in the inner core fuel assembly 7A is N52, and the number of fuel rods 19A is N54.

[0057] [Table 1]

[0058] In addition, "3.7≦N54×100 / (N54+N52)≦12.5" in Table 1 is the condition in which the average MA enrichment in the inner blanket region 9 of the inner core fuel assembly 7A is in the range of 3.7 to 12.5 wt%.

[0059] In this embodiment, the effects obtained in the first embodiment can be obtained. EXAMPLES

[0060] A core of a fast reactor according to a third embodiment, which is another preferred embodiment of the present invention, will be described with reference to FIGS.

[0061] As shown in FIG. 9, the core 1A of the fast reactor in this embodiment has an inner core region 2 arranged in the center of the core 1A, an outer core region 3 surrounding the inner core region 2, and a radial blanket region 33 surrounding the outer core region 3. The lower ends of the inner core region 2 and the outer core region 3 are located at the same position in the axial direction of the core 1A. Although not shown in FIG. 9, the reflector region 26 described in the first embodiment surrounds the radial blanket region 33. The height of the outer core region 3 is higher than the height of the inner core region 2, and the upper end of the outer core region 3 is located above the upper end of the inner core region 2 in the axial direction of the core 1A. The lower ends of the outer core region 3 and the inner core region 2 are located at the same position in the axial direction of the core 1A.

[0062] In the inner core region 2, a lower core fuel region 5, an inner blanket region 6, and an upper core fuel region 4 are formed from bottom to top. Furthermore, the core 1A has a lower axial blanket region 32 formed below the lower core fuel region 5 and the outer core region 3, and a sodium plenum region 31 formed above the upper core fuel region 4 and the outer core region 3. The lower ends of the lower core fuel region 5 and the outer core region 3 are located at the upper end of the gas plenum region 32.

[0063] A plurality of inner core fuel assemblies 7B are loaded in the inner core region 2, and a plurality of outer core fuel assemblies 21B are loaded in the outer core region 3. As shown in FIG. 10, the inner core fuel assembly 7B has a plurality of fuel rods 10B and a plurality of fuel rods 19B arranged in a stainless steel bell tube 30. Metal fuel, which is a nuclear fuel material, is arranged in a cladding tube 14 of each of the fuel rods 10B and 19B, the lower and upper ends of which are sealed. The metal fuel is held by a holding member 35 provided in the cladding tube 14, and is arranged above the holding member 35. A gas plenum 16A is formed in the cladding tube 14 below the holding member 35.

[0064] In the fuel rod 10B, in the nuclear fuel material filled area in which the metal fuel is arranged above the holding member 35 in the cladding tube 14, the lower core fuel area 12A, the inner blanket area 11B, and the upper core fuel area 13A are formed from the holding member 35 upward. U-Pu-Zr metal fuel is arranged in each of the lower core fuel area 12A and the upper core fuel area 13A. The inner blanket area 11B has U-Zr metal fuel. The U-Pu-Zr metal fuel arranged in each of the lower core fuel area 12A and the upper core fuel area 13A is cylindrical in shape with holes 34. The U-Zr metal fuel arranged in the inner blanket area 11B is also cylindrical in shape with holes 34. The holding member 35 provided in the fuel rod 10B also has a hole formed through the center. This hole communicates the holes 34 formed in the metal fuel with the gas plenum 16A in the fuel rod 10B.

[0065] In the fuel rod 19B, in the nuclear fuel material filled region in which the metal fuel is disposed above the holding member 35 in the cladding tube 14, the lower core fuel region 12A, the inner blanket region 20B, and the upper core fuel region 13A are formed from the holding member 35 upward. U-Pu-Zr metal fuel is disposed in each of the lower core fuel region 12A and the upper core fuel region 13A. MA-Zr metal fuel is disposed in the inner blanket region 20B. The U-Pu-Zr metal fuel disposed in each of the lower core fuel region 12A and the upper core fuel region 13A has a cylindrical shape having a void 34. The MA-Zr metal fuel disposed in the inner blanket region 20B is also cylindrical having a void 34. The holding member 35 provided in the fuel rod 19B also has a hole portion penetrating through the center. This hole portion communicates the void portion 34 formed in the metal fuel with the gas plenum 16A in the fuel rod 19B.

[0066] The U-Pu-Zr metal fuel, the U-Zr metal fuel and the MA-Zr metal fuel in this embodiment are hollow metal fuels.

[0067] The plutonium enrichment of each of the lower core fuel zone 12A and the upper core fuel zone 13A of the fuel rods 10B and 19B in the inner core fuel assembly 7B with a burnup of 0 GWd / t is the same as the plutonium enrichment of each of the lower core fuel zone 12 and the upper core fuel zone 13 of the fuel rods 10 and 19 in Example 1, for example, 25 wt%. The average minor actinide enrichment of the inner blanket zone 9 in the inner core fuel assembly 7B with a burnup of 0 GWd / t, which is formed by the inner blanket zone 11B of the fuel rods 10B and the inner blanket zone 20B of the fuel rods 19B, is within the range of 3.7 wt% to 12.5 wt% (3.7 wt% or more and 12.5 wt% or less), for example, 8.0 wt%.

[0068] The outer core fuel assembly 21B has a plurality of fuel rods 22A, each of which has a wire spacer (not shown) wound around its outer surface, arranged in a wrapper tube 30. The fuel rods 22A are filled with U-Pu-Zr metal fuel, which is a metal fuel material, in a core fuel region 23A in a cladding tube 14, both ends of which are sealed. The U-Pu-Zr metal fuel has a cylindrical shape with holes 34. The metal fuel is held by a holding member 35 provided in the cladding tube 14 of the fuel rods 22A, and is disposed above the holding member 35. A gas plenum 16A is formed in the cladding tube 14 below the holding member 35. The holes formed in the holding member 35 communicate the gas plenum 16A with the holes 34 in the U-Pu-Zr metal fuel in the fuel rods 22A.

[0069] A sodium plenum region 36 is formed in the wrapper tube 30 of the inner core fuel assembly 7B above the upper ends of the fuel rods 10B and 19B. A sodium plenum region 37 is formed in the wrapper tube 30 of the outer core fuel assembly 21B above the upper end of the fuel rod 22A. The sodium plenum region 31 of the core 1A is formed by the sodium plenum regions 36 and 37.

[0070] The lower end of the inner core region 2 is located at the same position as the lower end of the outer core region 3 in the axial direction of the core 1A (see FIG. 9). Therefore, the lower end of the active fuel length of the fuel rods 10B and 19B in the inner core fuel assembly 7B loaded in the core 1A (see FIG. 10) and the lower end of the active fuel length of the fuel rod 22A in the outer core fuel assembly 21B (see FIG. 11) are located at the same position in the axial direction of the core 1A. The active fuel length of the fuel rod 22A is longer than the active fuel length of the fuel rods 10B and 19B, and the upper end of the active fuel length of the fuel rod 22A is located above the upper end of the active fuel length of the fuel rods 10B and 19B. Therefore, the length of the sodium plenum region 37 in the outer core fuel assembly 21B in the axial direction of the core 1A is shorter than that of the sodium plenum region 36 in the inner core fuel assembly 7B.

[0071] An example of the dimensions of the lower core fuel region 5, the internal blanket region 6, and the upper core fuel region 4 in the inner core region 2 of the core 1A of this embodiment will be described. The lower end of the lower core fuel region 5 is the lower end of the inner core region 2 and coincides with the upper end of the gas plenum region 32. The lengths of the lower core fuel region 5, the internal blanket region 6, and the upper core fuel region 4 in the inner core region 2 of the core 1A in the axial direction of the core 1A are the same as those of the lower core fuel region 5, the internal blanket region 6, and the upper core fuel region 4 in the inner core region 2 of the core 1 of the first embodiment. The length of the lower core fuel region 5 in the axial direction of the core 1A is 40 cm. The internal blanket region 6 is located between a position 40 cm from the lower end of the inner core region 2 and a position 60 cm from the lower end of the inner core region 2, and the length of the internal blanket region 6 in the axial direction of the core 1A is 20 cm. The upper core fuel region 4 is located between a position 60 cm from the lower end of the inner core region 2 and the upper end of the inner core region 2, and the length of the upper core fuel region 4 in the axial direction of the core 1A is 40 cm. The center position of the inner blanket region 6 in the axial direction coincides with, for example, the center position of the inner core region 2 in the axial direction.

[0072] The inventors have studied the position of the inner blanket region 6 in the inner core region 2 of the core 1A, particularly its position in the axial direction of the inner core region 2. As a result of this study, the inventors have newly discovered the following.

[0073] In the study, the inventors investigated the change in the burnup reactivity and the void reactivity of the core 1A by shifting the center position of the inner blanket region 6 in the axial direction up and down from the center position of the inner core region 2 in the axial direction, i.e., the reference position. The burnup reactivity did not change even when the center position of the inner blanket region 6 in the axial direction was shifted up and down by 5 cm from the reference position. However, the void reactivity became more negative by shifting the center position of the inner blanket region 6 in the axial direction downward by 5 cm. For this reason, it is desirable to position the center position of the inner blanket region 6 in the axial direction in a range between the reference position and a position shifted 5 cm downward from the reference position. The further the center position of the inner blanket region 6 in the axial direction is positioned downward from the reference position within that range, the more negative the void reactivity value becomes. When the center position of the inner blanket region 6 is positioned at a position shifted downward by 5 cm from the reference position, the absolute value of the negative value of the void reactivity becomes the largest.

[0074] In this manner, by shifting the center position of the inner blanket region 6 downward, the reactor power in the upper core fuel region 4 of the inner core region 2 increases, and the number of neutrons leaking from the upper core fuel region 4 to the sodium plenum region 31 located above the upper core fuel region 4 increases, making the void reactivity of core 1A more negative.

[0075] This embodiment can obtain the effects obtained in the first embodiment. Furthermore, in this embodiment, since the length of the inner core region 2 in the axial direction of the core 1A is shorter than that of the outer core region 3, the void reactivity of the core 1A can be reduced. The metal fuel arranged in the fuel rods 10B and 19B in the inner core fuel assembly 7B and the fuel rods 22A in the outer core fuel assembly 21B loaded in the core 1A of this embodiment is cylindrical and forms holes 34, so that the smear density of the metal fuel is 75%, and the swelling of the metal fuel caused by the burning of the fissile material contained in the metal fuel can be absorbed. Therefore, in the fuel rods 10B and 19B in the inner core fuel assembly 7B and the fuel rods 22A in the outer core fuel assembly 21B, it is not necessary to fill the bond sodium 15 inside, unlike the fuel rods 10 and 19 in the inner core fuel assembly 7 and the fuel rods 22 in the outer core fuel assembly 21 used in the first embodiment.

[0076] In this embodiment, when the inventory of nuclear fuel materials in the inner core region 2 is the same as that in the outer core region 3, the axial length of core 1A in the inner core region 2, where the contribution of void reactivity is large, is made shorter than that length in the outer core region 3, thereby making it possible to reduce the void reactivity of core 1A compared to when the axial length of core 1A in the inner core region 2 is the same as that length in the outer core region 3. EXAMPLES

[0077] A core of a fast reactor according to a fourth embodiment, which is another preferred embodiment of the present invention, will be described with reference to FIGS.

[0078] The core of the fast reactor of this embodiment has a configuration in which the inner core fuel assembly 7 loaded in the inner core region 2 in the core 1 of embodiment 1 is replaced with the inner core fuel assembly 7C shown in Fig. 12, and further the outer core fuel assembly 21 loaded in the outer core region 3 is replaced with the outer core fuel assembly 21C shown in Fig. 13. Each of the inner core fuel assembly 7C and the outer core fuel assembly 21C uses oxide fuel instead of metal fuel as the nuclear fuel material. The other configuration of the core of the fast reactor of this embodiment is the same as the other configuration of the core 1 of embodiment 1.

[0079] In the inner core fuel assembly 7C having a burnup of 0 GWdt loaded in the inner core region 2 of the core in this embodiment, the lower core fuel region 12B and the upper core fuel region 13B in the sealed cladding tubes 14 of the fuel rods 10C and 19C are filled with a plurality of fuel pellets made of mixed oxide fuel (MOX fuel) of depleted uranium oxide and plutonium oxide. In the sealed cladding tube 14 of the fuel rod 10C, the inner blanket region 11C is filled with a plurality of fuel pellets made of depleted uranium oxide fuel not containing minor actinides. In the sealed cladding tube 14 of the fuel rod 19C, the inner blanket region 20C is filled with a plurality of fuel pellets made of oxide fuel containing minor actinides and depleted uranium.

[0080] In the present embodiment, a plurality of fuel pellets made of mixed oxide fuel (MOX fuel) of depleted uranium oxide and plutonium oxide are filled in the core fuel region 23A in the sealed cladding tube 14 of the plurality of fuel rods 22A in the outer core fuel assembly 21C with a burnup of 0 GWdt, which is loaded in the outer core region 3 of the core. The plutonium enrichment in the core fuel region 23A is also the same as the plutonium enrichment in the core fuel region 23 of the fuel rods 22 in the first embodiment.

[0081] Each of the fuel pellet packings in fuel rods 10C, 19C and 22A is a solid fuel pellet.

[0082] In the inner core fuel assembly 7C in this embodiment, fuel rods 10C having an inner blanket region 11C containing oxide fuel of depleted uranium not containing minor actinides, and fuel rods 19C having an inner blanket region 20C containing oxide fuel of minor actinides and depleted uranium are mixed. Therefore, according to this embodiment, even in the inner core fuel assembly 7C using oxide fuel, the enrichment degree of minor actinides in the inner blanket region 9 of the inner core fuel assembly 7C having a burnup of 0 GWdt can be easily adjusted by adjusting the number of fuel rods 10C and 19C. In addition, since a plurality of inner core fuel assemblies 7C are loaded in the inner core region 2 of the core in this embodiment, the enrichment degree of minor actinides in the inner blanket region 6 of the core can be easily adjusted. EXAMPLES

[0083] A core of a fast reactor according to a fifth embodiment, which is another preferred embodiment of the present invention, will be described with reference to FIGS.

[0084] The fast reactor core of this embodiment has a configuration in which the fuel rods 10A and 19A containing metallic fuel of the inner core fuel assembly 7A loaded in the inner core region 2 in the fast reactor core of Example 2 are replaced with fuel rods 10C and 19C containing oxide fuel of the inner core fuel assembly 7C used in Example 4, and further the fuel rods 22 containing metallic fuel of the outer core fuel assembly 21 loaded in the outer core region 3 are replaced with fuel rods 22A containing oxide fuel of the outer core fuel assembly 21C used in Example 4.

[0085] The configuration of the outer core fuel assembly 21C loaded in the core of the fast reactor of this embodiment is the same as the configuration of the outer core fuel assembly 21C used in the fourth embodiment. In the inner core fuel assembly 7C loaded in the core, the inner blanket region 11C of the fuel rod 10C and the inner blanket region 20C of the fuel rod 19C each contain plutonium, similar to the inner core fuel assembly 7A described above. That is, the inner blanket region 11C of the fuel rod 10C does not contain minor actinides, and is filled with a plurality of fuel pellets made of oxide fuel containing depleted uranium and plutonium. The inner blanket region 20C of the fuel rod 19C is filled with a plurality of fuel pellets made of oxide fuel containing depleted uranium, plutonium, and minor actinides.

[0086] According to this embodiment, the above-mentioned core structure makes it possible to obtain the effects obtained in the second embodiment. [Explanation of symbols]

[0087] 1,1A...core, 2...inner core region, 3...outer core region, 4,8A,13,13A...upper core fuel region, 5,8B,12,12A...lower core fuel region, 6,9,11,11A,11B,20,20A,20B...inner blanket region, 7,7A,7B...inner core fuel assemblies, 10,10A,10B,19,19A,19B,22,22A...fuel rods, 16,16A...gas plenum, 21,21B...outer core fuel assemblies, 23,23A...core fuel region, 31,36,37...sodium plenum region, 32...gas plenum region.

Claims

1. A fuel assembly in which a nuclear fuel material region in which nuclear fuel material is present is formed, and a first lower core fuel region, a first inner blanket region, and a first upper core fuel region are formed in this order from a lower end to an upper end of the nuclear fuel material region, a plurality of first fuel rods in which the nuclear fuel material resides, and a plurality of second fuel rods in which the nuclear fuel material resides; Within the first fuel rod, a second lower core fuel region is formed at a position corresponding to the first lower core fuel region, a second inner blanket region is formed at a position corresponding to the first inner blanket region, and a second upper core fuel region is formed at a position corresponding to the first upper core fuel region; Within the second fuel rod, a third lower core fuel region is formed at a position corresponding to the first lower core fuel region, a third inner blanket region is formed at a position corresponding to the first inner blanket region, and a third upper core fuel region is formed at a position corresponding to the first upper core fuel region; The nuclear fuel material present in the second inner blanket region of the first fuel rod contains uranium but not minor actinides, and the nuclear fuel material present in the third inner blanket region of the second fuel rod contains minor actinides but not uranium.

2. 2. The fuel assembly of claim 1, wherein the nuclear fuel material present in the third inner blanket region of the second fuel rod includes plutonium in addition to minor actinides.

3. 3. The fuel assembly according to claim 1, wherein the nuclear fuel material having a solid shape is disposed within a sealed cladding tube of each of the first fuel rod and the second fuel rod, and bonded sodium is filled between the nuclear fuel material and the cladding tube.

4. 4. The fuel assembly according to claim 1, wherein the nuclear fuel material is a metallic fuel.

5. 5. The fuel assembly of claim 4, wherein said nuclear fuel material is disposed within a sealed cladding of each of said first and second fuel rods in a hollow configuration.

6. 6. The fuel assembly according to claim 4 or 5, wherein the average enrichment of minor actinides in the first inner blanket region is within the range of 3.7 wt% to 12.5 wt%.

7. 7. The fuel assembly according to claim 4, wherein the plutonium enrichment of the nuclear fuel material present in the third inner blanket region is in the range of greater than 0 wt. % to 13 wt. %.

8. 4. The fuel assembly according to claim 1, wherein the nuclear fuel material is an oxide fuel.

9. 9. The fuel assembly according to claim 1, wherein the first fuel rods and the second fuel rods are arranged in a mixed manner in a cross section of the fuel assembly.

10. A core of a fast reactor having an inner core region loaded with a plurality of first fuel assemblies and an outer core region surrounding the inner core region and loaded with a plurality of second fuel assemblies, a fourth lower core fuel region, a fourth inner blanket region, and a fourth upper core fuel region are formed in the inner core region, in that order, from a lower end to an upper end of the inner core region; The first fuel assembly is a fuel assembly according to any one of claims 1 to 9, A core of a fast reactor, characterized in that the fourth lower core fuel region is formed at a position corresponding to the second lower core fuel region, the fourth inner blanket region is formed at a position corresponding to the second inner blanket region, and the fourth upper core fuel region is formed at a position corresponding to the second upper core fuel region.

11. The fast reactor core according to claim 10, wherein a position of an upper end of the inner core region is lower than a position of an upper end of the outer core region.

12. 12. A core of a fast reactor according to claim 10 or 11, wherein a center position in the axial direction of the fourth inner blanket region is located within a range between a center position in the axial direction of the inner core region and a position shifted 5 cm downward from the center position in the axial direction of the inner core region.

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