Boiling water reactor and fuel assembly

Abstract

To provide a fuel assembly capable of setting a void coefficient to be negative while increasing the content of plutonium, and a boiling-water reactor equipped with the same.SOLUTION: The boiling-water reactor comprises: a reactor pressure vessel 2; and a plurality of fuel assemblies 3 loaded on a reactor core 5 in the reactor pressure vessel 2. Each fuel assembly 3 is provided with a plurality of fuel rods 23, and a channel box 21 surrounding the periphery of the fuel rods 23, and cooling water flows outside and inside the channel box 21. When the reactor core 5 is operated at a rating output, the cooling water is in a non-boiling state outside the channel box 21, and a deceleration effect for neutrons emitted from the fuel rods 23 is large. In the channel box 21, the cooling water is in a boiling-state, and the deceleration effect for the neutrons emitted from the fuel rods 23 is small.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a boiling water nuclear reactor and a fuel assembly to be loaded into the boiling water nuclear reactor. [Background technology]

[0002] Plutonium produced in the reprocessing of spent fuel is mixed with uranium and loaded into a nuclear reactor as fuel (hereafter referred to as "mixed oxide fuel"). In order to utilize more plutonium, it is necessary to increase the ratio of the weight of plutonium to the total weight of uranium and plutonium in the fuel (hereafter referred to as the "plutonium content").

[0003] Increasing the plutonium content of the fuel increases the ratio of fissile plutonium in the plutonium, increasing the fission reaction and making it impossible to maintain the criticality of the reactor. Even if the plutonium content of the fuel is increased, it is possible to reduce the fission by increasing the neutron energy, and thus maintain the criticality. In the field of boiling water reactors, a boiling water reactor (hereinafter referred to as a "reduced-moderation spectrum boiling water reactor") has been proposed in which multiple fuel rods are densely arranged in a channel box of a fuel assembly and voids are generated in the channel box during operation to increase the neutron energy. Examples of reduced-moderation spectrum boiling water reactors are described in Patent Document 1 and Patent Document 2.

[0004] In conventional reduced-moderation spectrum boiling water reactors, nuclear fission can be reduced and criticality can be maintained by reducing the amount of cooling water (moderator) and increasing neutron energy. However, when the neutron energy is increased, the rate of change in reactivity (hereinafter referred to as the "void coefficient") when the bubble volume fraction of the boiling cooling water (hereinafter referred to as the "void fraction") increases becomes positive, and the safety of the reactor cannot be ensured. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 11-94972 [Patent Document 2] JP 2000-19280 A Summary of the Invention [Problem to be solved by the invention]

[0006] A configuration for making the void coefficient negative in a reduced moderation spectrum boiling water reactor is described in, for example, Patent Document 2. In the technology described in Patent Document 2, two types of fuel, short fuel assemblies whose fuel length is 50% or less of the fuel length of normal fuel assemblies and normal fuel assemblies, are loaded into the core, and the void coefficient is made negative by utilizing the fact that when the void fraction increases, the amount of neutron leakage increases at the top of the short fuel where there are no fuel rods. However, the technology in Patent Document 2 uses short fuel assemblies whose effective fuel length is half or less of the fuel length of normal fuel assemblies, so the amount of plutonium used cannot be increased.

[0007] Thus, in conventional technology, there is a problem in that it is difficult to simultaneously increase the plutonium content of the mixed oxide fuel in order to utilize more plutonium and make the void coefficient negative.

[0008] An object of the present invention is to provide a fuel assembly capable of increasing the plutonium content while making the void coefficient negative, and a boiling water reactor loaded with the fuel assembly. [Means for solving the problem]

[0009] The boiling water reactor according to the present invention comprises a reactor pressure vessel and a plurality of fuel assemblies loaded in a core inside the reactor pressure vessel. The fuel assembly comprises a plurality of fuel rods and a channel box surrounding the periphery of the fuel rods, and cooling water flows inside and outside the channel box. When the core is operated at rated power, the cooling water is in a non-boiling state outside the channel box and has a large moderation effect on neutrons emitted by the fuel rods, and the cooling water is in a boiling state inside the channel box and has a small moderation effect on neutrons emitted by the fuel rods.

[0010] The fuel assembly according to the present invention includes a plurality of fuel rods and a channel box surrounding the fuel rods, and can be loaded into a core inside a reactor pressure vessel of a boiling water reactor. When the fuel assembly is loaded into the core and the core is operated at rated power, cooling water flows inside and outside the channel box, and outside the channel box, the cooling water is in a non-boiling state, which has a large moderation effect on neutrons emitted by the fuel rods, and inside the channel box, the cooling water is in a boiling state, which has a small moderation effect on neutrons emitted by the fuel rods. Effect of the Invention

[0011] According to the present invention, it is possible to provide a fuel assembly capable of increasing the plutonium content while making the void coefficient negative, and a boiling water reactor loaded with this fuel assembly. [Brief description of the drawings]

[0012] [Figure 1] FIG. 4 is a cross-sectional view of a fuel assembly according to a first embodiment of the present invention, taken along the arrow AA in FIG. 3. [Diagram 2] 1 is a schematic diagram showing a configuration of a boiling water reactor according to a first embodiment. [Diagram 3] FIG. 2 is a longitudinal sectional view of a core, showing a fuel assembly according to a first embodiment. [Figure 4]FIG. 4 is a cross-sectional view of four fuel assemblies taken along the arrow BB in FIG. 3 . [Diagram 5] FIG. 13 is a diagram showing an example of a change in void coefficient with respect to a channel box inner / outer flow passage area ratio in a fuel assembly. [Figure 6] FIG. 11 is a cross-sectional view of a fuel assembly according to a second embodiment of the present invention. [Figure 7] FIG. 11 is a cross-sectional view of a fuel assembly according to a third embodiment of the present invention. [Figure 8] FIG. 11 is a cross-sectional view of a fuel assembly according to a fourth embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] The fuel assembly according to the present invention has a mixed oxide fuel in which plutonium and uranium are mixed. The boiling water reactor according to the present invention can be loaded with the fuel assembly.

[0014] In the fuel assembly according to the present invention, even if the fuel assembly has a high plutonium content, the cooling water can be boiled inside the channel box to reduce the water density and reduce the flow path area of ​​the cooling water, and outside the channel box, the cooling water can be non-boiled to increase the water density and increase the flow path area of ​​the cooling water (i.e., by reducing the flow path area ratio inside and outside the channel box, as described below), thereby reducing the degree of neutron moderation inside the channel box and increasing the degree of neutron moderation outside the channel box (i.e., neutrons are in a low-moderation state inside the channel box and a high-moderation state outside the channel box), thereby making it possible to make the void coefficient negative.

[0015] In conventional thermal neutron reactors, neutrons are highly moderated inside and outside the channel box to cause efficient nuclear fission, so if the plutonium content of the fuel increases, the reactor cannot maintain criticality. In conventional reduced-moderation spectrum boiling water reactors, the neutrons are moderated by reducing the amount of cooling water inside and outside the channel box, so criticality can be maintained, but the void coefficient becomes positive.

[0016] The fuel assembly according to the present invention has the above-mentioned configuration and can increase the plutonium content while making the void coefficient negative. Hereinafter, a boiling water reactor and a fuel assembly according to an embodiment of the present invention will be described with reference to the drawings. In the drawings used in this specification, the same or corresponding components are designated by the same reference numerals, and repeated description of these components may be omitted. EXAMPLES

[0017] 2 is a schematic diagram showing the configuration of a boiling water reactor according to a first embodiment of the present invention. The boiling water reactor 1 according to this embodiment includes a reactor pressure vessel 2, a fuel assembly 3 according to this embodiment, a core shroud 4, a core 5, a core support plate 6, an upper grid plate 7, a plurality of control rods 12, and a plurality of internal pumps 13. The boiling water reactor 1 is an advanced boiling water reactor (hereinafter referred to as "ABWR") in which cooling water is supplied to the core 5 by an internal pump 13 provided on a lower head 2A, which is the bottom of the reactor pressure vessel 2.

[0018] The reactor pressure vessel 2 includes a core 5 and a core shroud 4 surrounding the core 5. The core 5 can be loaded with a plurality of fuel assemblies 3 according to this embodiment. A core support plate 6 arranged at the bottom of the core 5 and an upper lattice plate 7 arranged at the top of the core 5 are attached to the inner surface of the core shroud 4. A shroud head 8 that covers the core 5 is attached to the upper end of the core shroud 4. Inside the reactor pressure vessel 2, a plurality of steam-water separators 9 are attached to the shroud head 8 and extend upward. Above the steam-water separator 9, a steam dryer 10 is attached to the inner surface of the reactor pressure vessel 2.

[0019] An annular downcomer 14 surrounding the core shroud 4 is formed between the inner surface of the core shroud 4 and the inner surface of the reactor pressure vessel 2. A plurality of internal pumps 13 are attached to the bottom head 2A of the reactor pressure vessel 2 at the position of the downcomer 14. An impeller 13A of each internal pump 13 is disposed inside the downcomer 14.

[0020] A plurality of control rod drive mechanism housings 16 are attached to the reactor pressure vessel 2, inside the internal pump 13. Each control rod drive mechanism housing 16 penetrates the bottom head 2A and extends downward beyond the bottom head 2A. A control rod drive mechanism (not shown) is installed inside each control rod drive mechanism housing 16.

[0021] A plurality of control rod guide tubes 15 are arranged below the reactor core 5 inside the reactor pressure vessel 2. Each of the control rod guide tubes 15 is installed at the upper end of a control rod drive mechanism housing 16 and extends upward. The upper ends of the control rod guide tubes 15 reach the position of the lower ends of the fuel support brackets 11 installed on the core support plate 6. The control rods 12 are arranged inside the control rod guide tubes 15 and connected to a control rod drive mechanism inside the control rod drive mechanism housing 16, and are moved up and down by the control rod drive mechanism.

[0022] A neutron instrumentation tube 17 housing a neutron detector is installed inside the reactor pressure vessel 2 .

[0023] 3 is a vertical cross-sectional view of the reactor core 5, showing a fuel assembly 3 according to this embodiment. The fuel assembly 3 includes a channel box 21, a plurality of fuel rods 23, a lower tie plate 22, an upper tie plate 24, and a plurality of fuel spacers 25, and can be disposed in the reactor core 5.

[0024] Each fuel rod 23 has its lower end supported by lower tie plate 22 and its upper end supported by upper tie plate 24 .

[0025] The fuel spacers 25 are arranged at multiple locations in the axial direction (length direction) of the fuel assembly 3, and surround and bundle the multiple fuel rods 23 so that a predetermined gap is formed between the fuel rods 23. The gaps formed between the fuel rods 23 become flow paths for cooling water.

[0026] The channel box 21 is a cylindrical body having a quadrangular (e.g., square) cross section, and surrounds the bundle of fuel rods 23 bound by fuel spacers 25. The upper end of the channel box 21 is attached to the upper tie plate 24 by a channel fastener (not shown). Cooling water flows through the outside and inside of the channel box 21. The cross section of the channel box 21 may be of any shape, and may be, for example, a square or hexagonal shape. If the cross section of the channel box 21 is square, this is preferable because the fuel assembly 3 according to this embodiment can be easily applied to the current boiling water reactor 1.

[0027] The region outside the channel box 21, that is, the region between adjacent channel boxes 21, is a region where saturated water exists and is called a gap water region 42.

[0028] Next, the fuel assembly 3 according to this embodiment will be described. The fuel assembly 3 according to this embodiment is loaded into the reactor core 5.

[0029] FIG. 1 is a diagram showing a cross section (horizontal section) of a fuel assembly 3 according to this embodiment, and is a cross section taken along the arrow AA in FIG.

[0030] The fuel assembly 3 is configured by closely arranging 165 fuel rods 23 inside a channel box 21 having a square cross section. In this embodiment, the fuel rods 23 have an outer diameter of 8.0 mm and are spaced apart from each other by 2.0 mm. The channel box 21 has an inner width of 134.1 mm and a wall thickness of 2.5 mm. Tie rods 30 made of Zircaloy-2 are arranged at the inner corners of the channel box 21.

[0031] The fuel assembly 3 includes an outermost layer fuel rod 23a as the fuel rod 23 located in the outermost layer (the outermost part in the cross section of the fuel assembly) of the fuel assembly 3 among the fuel rods 23. Since the outermost layer fuel rod 23a is located in the outermost layer of the fuel assembly 3, it is adjacent to the cooling water outside the channel box 21 with the channel box 21 in between.

[0032] As shown in Fig. 3, the upper and lower ends of the fuel rods 23 and the tie rods 30 are held by the upper tie plate 24 and the lower tie plate 22, and are held at multiple points in the axial direction (length direction) by fuel spacers 25. The fuel spacers 25 are arranged at regular intervals in the axial direction.

[0033] As described above, the region outside the channel box 21, that is, the region between adjacent channel boxes 21, is the gap water region 42 where saturated water exists (FIG. 1).

[0034] A fuel assembly lattice 43 is defined in each of the fuel assemblies 3. The fuel assembly lattice 43 is a region including the fuel assembly 3 and the gap water region 42. The boundary (outer periphery) of the fuel assembly lattice 43 is separated by a distance a from the outside of the channel box 21. The fuel assembly lattice 43 will be described below with reference to FIG. 4.

[0035] Fig. 4 is a cross-sectional view of four fuel assemblies 3, taken along the arrow BB in Fig. 3. Fig. 4 shows four adjacent control rods 12 and four fuel assemblies 3 arranged in an area surrounded by the four control rods 12. The area between the adjacent fuel assemblies 3 is a gap water region 42.

[0036] The fuel assembly lattice 43 is an area obtained by equally dividing a square area (indicated by dashed lines in FIG. 4) having vertices at the center points of four adjacent control rods 12 in a cross section of the fuel assembly 3 of the core 5 including the fuel assemblies 3 and the control rods 12 into four. In this embodiment, the fuel assembly lattice width, which is the width of the fuel assembly lattice 43, is 154.9 mm. The distance between the boundary of the fuel assembly lattice 43 and the channel box 21 (distance a shown in FIG. 1) is 7.9 mm.

[0037] The fuel assembly 3 according to the present embodiment has the above configuration and does not have a water rod that is included in the conventional fuel assembly, so the flow path area of ​​the cooling water flowing inside the fuel assembly 3 is small and the amount of water inside the fuel assembly 3 is small. Since the fuel assembly 3 does not have a water rod through which non-boiling water flows, the cooling water inside the channel box 21 can be brought into a boiling state, that is, a state in which voids are generated, in a cross section at the axial center of the nuclear fuel material filling area of ​​the fuel rods 23 during operation at the rated output of the core 5. Therefore, the boiling water reactor 1 according to the present embodiment realizes a reduced moderation spectrum boiling water reactor with high neutron energy inside the fuel assembly 3 by bringing the cooling water inside the channel box 21 into a state in which voids are generated, in a cross section at the axial center of the nuclear fuel material filling area of ​​the fuel rods 23 (i.e., at a position 1 / 2 of the active fuel length) during operation.

[0038] The fuel rod 23 uses mixed oxide fuel (MOX fuel) produced by mixing uranium oxide and plutonium oxide as the nuclear fuel material, and has a configuration in which a plurality of fuel pellets produced from this MOX fuel are filled and sealed in a cladding tube. The axial length of the region filled with the nuclear fuel material of the fuel rod 23, i.e., the effective fuel length, is about 370 cm, the same as that of fuel loaded in existing boiling water reactors. A gas plenum is formed in the cladding tube of the fuel rod 23 above the upper end of the region filled with the nuclear fuel material.

[0039] 5 is a diagram showing an example of a change in void coefficient with respect to the channel box inner / outer flow passage area ratio in the fuel assembly 3. The operation of the fuel assembly 3 according to this embodiment will be described with reference to FIG.

[0040] First, the channel box inner / outer flow passage area ratio will be described with reference to Fig. 1. In the following description, the flow passage of the cooling water outside the channel box 21 in the fuel assembly lattice 43 will be referred to as the "external flow passage of the channel box 21", and the flow passage of the cooling water inside the channel box 21 in the fuel assembly lattice 43 will be referred to as the "internal flow passage of the channel box 21". In addition, the flow passage area of ​​the external flow passage of the channel box 21 will be referred to as the "external flow passage area of ​​the channel box 21", and the flow passage area of ​​the internal flow passage of the channel box 21 will be referred to as the "internal flow passage area of ​​the channel box 21".

[0041] The channel box inner / outer flow path area ratio is the ratio of the internal flow path area of ​​the channel box 21 to the external flow path area of ​​the channel box 21 (internal flow path area of ​​the channel box 21 / external flow path area of ​​the channel box 21). The flow path area is the flow path area of ​​the cooling water in the cross section of the fuel assembly 3 (area on the surface shown in FIG. 1). The external flow path area of ​​the channel box 21 is the flow path area of ​​the cooling water in the region between the outside of the channel box 21 and the boundary of the fuel assembly lattice 43. The internal flow path area of ​​the channel box 21 is the flow path area of ​​the cooling water in the region inside the channel box 21 where the fuel rods 23 and tie rods 30 are not present.

[0042] In the boiling water reactor 1 according to this embodiment, the fuel assembly 3 does not have a water rod inside through which non-boiling water flows, and when the core 5 is operated at rated power, non-boiling cooling water (gap water) flows in the external flow path of the channel box 21, and cooling water that has become boiling due to heat generated by the fuel rods 23 flows in the internal flow path of the channel box 21. Therefore, the outside of the channel box 21 has a greater moderation effect on neutrons emitted by the fuel rods 23 than the inside of the channel box 21, and the inside of the channel box 21 has a smaller moderation effect on neutrons emitted by the fuel rods 23 than the outside of the channel box 21. That is, the fuel assembly 3 according to this embodiment puts neutrons in a low-moderation state inside the channel box 21 and puts neutrons in a high-moderation state outside the channel box 21.

[0043] Figure 5 shows an example of the change in void coefficient with respect to the channel box inner / outer flow passage area ratio for a fuel assembly 3 with an average plutonium content (ratio of plutonium mass to the sum of plutonium mass and uranium mass) of 15% by weight. As shown in Figure 5, when the channel box inner / outer flow passage area ratio is 2.1 or less, the void coefficient is negative. Note that, in order to maintain criticality in a boiling water reactor 1, it is generally considered that the upper limit of the plutonium content in terms of design is 15% by weight.

[0044] The void coefficient shown in Fig. 5 was obtained by an analysis simulating the average operating conditions of the boiling water reactor 1 as follows. That is, while the cooling water in the external flow passage of the channel box 21 is in a non-boiling state with a void fraction of 0%, the infinite multiplication factor of neutrons when the void fraction inside the channel box 21 is 40% corresponding to rated operation, and the infinite multiplication factor of neutrons when the void fraction inside the channel box 21 has risen to 70% were obtained by analysis, and the rate of change of the infinite multiplication factor of neutrons with respect to the change in the void fraction was obtained as the void coefficient.

[0045] In the example shown in Fig. 5, the channel box inner / outer flow passage area ratio at which the void coefficient becomes negative is 2.1 or less, but this value of 2.1 may vary depending on the operating conditions of the boiling water reactor 1. Therefore, in general, the channel box inner / outer flow passage area ratio can be set to a predetermined value or less at which the void coefficient becomes negative, which is determined from the relationship between the channel box inner / outer flow passage area ratio and the void coefficient. Such a value can be determined by the analysis described above.

[0046] The inventor discovered that in addition to reducing the water density by bringing the inside of the channel box 21 into a boiling state, by reducing the flow path area of ​​the cooling water inside the channel box 21 while bringing the cooling water outside the channel box 21 into a non-boiling state to increase the water density, and by increasing the flow path area of ​​the cooling water outside the channel box 21, i.e., by reducing the flow path area ratio inside and outside the channel box, the neutrons inside the channel box 21 can be put into a reduced velocity state while the neutrons outside the channel box 21 can be put into a highly moderated state, thereby making it possible to make the void coefficient negative.

[0047] As described above, in the fuel assembly 3 according to this embodiment, by setting the channel box inside / outside flow passage area ratio to a predetermined value or less (2.1 or less in the example shown in FIG. 5) at which the void coefficient is negative, when the core 5 is operated at rated power, the neutrons emitted by the outermost fuel rod 23a can be in a highly moderated state, and the neutrons emitted by the fuel rods 23 other than the outermost fuel rod 23a can be in a low-moderated state. The outermost fuel rod 23a is adjacent to the cooling water outside the channel box 21 in a non-boiling state and is influenced by this cooling water, and has a large moderation effect on the emitted neutrons, and can lower the neutron energy to an energy region where the void coefficient is negative. On the other hand, the fuel rods 23 other than the outermost fuel rod 23a of the fuel assembly 3 have a small moderation effect on the emitted neutrons due to the cooling water inside the channel box 21 in a boiling state, and can maintain the neutron energy in a high state. If the neutron energy is high, criticality can be maintained even if the plutonium content is high. Therefore, the fuel assembly 3 according to this embodiment can have a negative void coefficient as a whole, while increasing the plutonium content.

[0048] In addition, when partial-length fuel rods shorter than normal fuel rods are used as the fuel rods 23, the fuel assembly lattice 43 (cross section of the fuel assembly 3) has a region where partial-length fuel rods exist and a region where partial-length fuel rods do not exist. In the region where partial-length fuel rods exist, the internal flow passage area of ​​the channel box 21 is smaller than that of the region where partial-length fuel rods do not exist, so that the neutron energy tends to be higher and the void coefficient tends to be positive. For this reason, when the fuel rods 23 include partial-length fuel rods, it is preferable to use the internal flow passage area of ​​the channel box 21 at the cross section where the internal flow passage area of ​​the channel box 21 is the smallest for calculating the channel box internal / external flow passage area ratio. That is, when the fuel rods 23 include partial-length fuel rods, it is preferable to use the internal flow passage area of ​​the channel box 21 at the cross section where the partial-length fuel rods exist as the internal flow passage area of ​​the channel box 21 used for calculating the channel box internal / external flow passage area ratio.

[0049] In the fuel assembly 3 according to this embodiment, the flow passage area ratio inside and outside the channel box is 1.97. Therefore, as shown in Fig. 5, the fuel assembly 3 according to this embodiment can make the void coefficient negative even if the average plutonium content is 15% by weight, and the safety of the reactor can be improved. Since the boiling water reactor 1 according to this embodiment has the fuel assemblies 3 according to this embodiment loaded in the core 5, even if the plutonium content of the fuel is increased, the void coefficient can be made negative as long as the average plutonium content is 15% by weight or less, and the safety of the reactor can be improved.

[0050] According to this embodiment, in a reduced moderation spectrum boiling water reactor, it is possible to make the void coefficient negative while increasing the plutonium content of the fuel. Therefore, in this embodiment, it is possible to realize a fuel assembly and a boiling water reactor that can improve the safety of the reactor.

[0051] For example, when the core 5 is downsized without changing the channel box 21 and the fuel rods 23, if the fuel assembly lattice 43 (Figure 1) is made smaller and the distance between the boundary of the fuel assembly lattice 43 and the channel box 21 (distance a shown in Figure 1) is reduced to 5.4 mm, the flow path area ratio inside and outside the channel box becomes 2.89 and the void coefficient becomes positive (Figure 5). EXAMPLES

[0052] Fig. 6 is a diagram showing a cross section (horizontal section) of a fuel assembly 3 according to a second embodiment of the present invention, and corresponds to Fig. 1 in the first embodiment. In the following, the fuel assembly 3 according to the present embodiment will be mainly described with respect to differences from the fuel assembly 3 according to the first embodiment.

[0053] In the fuel assembly 3 according to this embodiment, the outer diameter of the fuel rods 23 is larger than that of the fuel assembly 3 according to the first embodiment. The fuel assembly 3 according to this embodiment is configured by closely arranging 140 fuel rods 23 inside a channel box 21 having a square cross section (horizontal section). In this embodiment, the fuel rods 23 have an outer diameter of 8.8 mm and a gap between them of 2.0 mm. The channel box 21 has an inner width of 134.1 mm and a wall thickness of 2.5 mm. Tie rods 30 made of Zircaloy-2 are arranged at the inner corners of the channel box 21. The width of the fuel assembly lattice 43 (fuel assembly lattice width) is 154.9 mm.

[0054] In the fuel assembly 3 according to this embodiment, the flow passage area ratio inside and outside the channel box is 1.90. Therefore, as shown in Fig. 5, in the fuel assembly 3 according to this embodiment, even if the average plutonium content is 15% by weight, the void coefficient can be made negative and smaller than the value in Example 1, thereby improving the safety of the nuclear reactor. Furthermore, in this embodiment, since the outer diameter of the fuel rods 23 is larger than that in Example 1, the weight of plutonium used in the fuel can be increased compared to Example 1, even if the plutonium content is the same as in Example 1, 15% by weight.

[0055] In the boiling water reactor 1 according to the present embodiment, the fuel assemblies 3 according to the present embodiment are loaded in the core 5, so that in a reduced moderation spectrum boiling water reactor, the void coefficient can be made negative even if the plutonium content of the fuel is increased, thereby improving the safety of the reactor. EXAMPLES

[0056] Fig. 7 is a diagram showing a cross section (horizontal section) of a fuel assembly 3 according to a third embodiment of the present invention, and corresponds to Fig. 1 in the first embodiment. In the following, the fuel assembly 3 according to the present embodiment will be mainly described with respect to differences from the fuel assembly 3 according to the first embodiment.

[0057] In the fuel assembly 3 according to this embodiment, the outer diameter of the fuel rods 23 is larger than that of the fuel assembly 3 according to the first embodiment. The fuel assembly 3 according to this embodiment is configured by closely arranging 117 fuel rods 23 inside a channel box 21 having a square cross section (horizontal section). In this embodiment, the fuel rods 23 have an outer diameter of 9.9 mm and a gap between them of 2.0 mm. The channel box 21 has an inner width of 134.1 mm and a wall thickness of 2.5 mm. Tie rods 30 made of Zircaloy-2 are arranged at the inner corners of the channel box 21. The width of the fuel assembly lattice 43 (fuel assembly lattice width) is 154.9 mm.

[0058] In the fuel assembly 3 according to this embodiment, the flow passage area ratio inside and outside the channel box is 1.81. Therefore, in the fuel assembly 3 according to this embodiment, even if the average plutonium content is 15% by weight, as shown in Fig. 5, the void coefficient can be made negative and smaller than the value in Example 1, thereby improving the safety of the nuclear reactor. Furthermore, in this embodiment, since the outer diameter of the fuel rods 23 is larger than that in Example 1, the weight of plutonium used in the fuel can be increased compared to Examples 1 and 2, even if the plutonium content is 15% by weight, which is the same as in Examples 1 and 2.

[0059] In the boiling water reactor 1 according to the present embodiment, the fuel assemblies 3 according to the present embodiment are loaded in the core 5, so that in a reduced moderation spectrum boiling water reactor, the void coefficient can be made negative even if the plutonium content of the fuel is increased, thereby improving the safety of the reactor. EXAMPLES

[0060] Fig. 8 is a diagram showing a cross section (horizontal section) of a fuel assembly 3 according to a fourth embodiment of the present invention, and corresponds to Fig. 1 in the first embodiment. In the following, the fuel assembly 3 according to the present embodiment will be mainly described with respect to differences from the fuel assembly 3 according to the first embodiment.

[0061] The fuel assembly 3 according to this embodiment has a larger number of fuel rods 23 than the fuel assembly 3 according to embodiment 1. The fuel assembly 3 according to this embodiment is configured by closely arranging 192 fuel rods 23 inside a channel box 21 having a square cross section (horizontal section). In this embodiment, the fuel rods 23 have an outer diameter of 7.3 mm and a gap between them of 2.0 mm. The channel box 21 has an inner width of 134.1 mm and a wall thickness of 2.5 mm. Tie rods 30 made of Zircaloy-2 are arranged at the inner corners of the channel box 21. The width of the fuel assembly lattice 43 (fuel assembly lattice width) is 154.9 mm.

[0062] In the fuel assembly 3 according to this embodiment, the flow passage area ratio inside and outside the channel box is 2.04. Therefore, in the fuel assembly 3 according to this embodiment, even if the average plutonium content is 15% by weight, as shown in Fig. 5, the void coefficient can be made negative, and the safety of the nuclear reactor can be improved. Furthermore, in this embodiment, since the number of fuel rods 23 is greater than that of the first embodiment, the output per fuel rod can be reduced more than that of the first embodiment, and the linear power density, which is the thermal margin, can be reduced.

[0063] In the boiling water reactor 1 according to the present embodiment, the fuel assemblies 3 according to the present embodiment are loaded in the core 5, so that in a reduced moderation spectrum boiling water reactor, the void coefficient can be made negative even if the plutonium content of the fuel is increased, thereby improving the safety of the reactor.

[0064] In the first to fourth embodiments, the boiling water reactor 1 has been described as an ABWR. The boiling water reactor according to the present invention may be a normal boiling water reactor (hereinafter referred to as "BWR") or an economical simplified boiling water reactor (hereinafter referred to as "ESBWR"). The BWR is provided with a recirculation pump and is configured to circulate the cooling water by flowing water (cooling water) as a coolant to the outside of the reactor pressure vessel and then flowing the cooling water back into the downcomer inside the reactor pressure vessel. The ESBWR is provided with a configuration in which an internal pump is not required in the ABWR. The BWR and the ESBWR can also be provided with the configuration of the core of the boiling water reactor 1 described in the first to fourth embodiments and can be loaded with the fuel assemblies 3 described in the first to fourth embodiments.

[0065] The present invention is not limited to the above-described embodiments, and various modifications are possible. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to an embodiment having all of the configurations described. It is also possible to replace a part of the configuration of one embodiment with the configuration of another embodiment. It is also possible to add the configuration of another embodiment to the configuration of one embodiment. It is also possible to delete a part of the configuration of each embodiment, or to add or replace another configuration. [Explanation of symbols]

[0066] 1...boiling water reactor, 2...reactor pressure vessel, 2A...bottom head, 3...fuel assembly, 4...core shroud, 5...core, 6...core support plate, 7...upper lattice plate, 8...shroud head, 9...steam separator, 10...steam dryer, 11...fuel support bracket, 12...control rod, 13...internal pump, 13A...impeller, 14...downcomer, 15...control rod guide tube, 16...control rod drive mechanism housing, 17...neutron instrumentation tube, 21...channel box, 22...lower tie plate, 23...fuel rod, 23a...outermost fuel rod, 24...upper tie plate, 25...fuel spacer, 30...tie rod, 42...gap water region, 43...fuel assembly lattice.

Claims

1. A reactor pressure vessel; a plurality of fuel assemblies loaded into a core inside the reactor pressure vessel; A plurality of control rods; Equipped with The fuel assembly includes a plurality of fuel rods and a channel box surrounding the fuel rods, does not include a water rod inside through which non-boiling water flows, and cooling water flows outside and inside the channel box; In a cross section of the fuel assembly, a square region having vertices at the center points of four adjacent control rods is equally divided into four regions to define a fuel assembly lattice, an area of ​​the cooling water flow passage in the cross section outside the channel box in the fuel assembly lattice is defined as an external flow passage area of ​​the channel box; an area of ​​the cooling water flow passage in the cross section of the channel box in the fuel assembly lattice is defined as an internal flow passage area of ​​the channel box; When the core is operating at rated power, Inside the fuel assembly lattice and outside the channel box, the cooling water is in a non-boiling state, and the moderating effect on the neutrons emitted by the fuel rods is large, Inside the channel box, the cooling water is in a boiling state due to heat generated by the fuel rods, and the moderating effect on the neutrons emitted by the fuel rods is small. a ratio of the internal flow path area of ​​the channel box to the external flow path area of ​​the channel box is equal to or less than a predetermined value such that a void coefficient is negative; 1. A boiling water reactor comprising:

2. Among the fuel rods, the fuel rods located in the outermost layer of the fuel assembly are outermost layer fuel rods, The outermost fuel rods are adjacent to the cooling water outside the channel box with the channel box in between, When the core is operating at rated power, The outermost fuel rods have a large moderation effect on the emitted neutrons due to the cooling water outside the channel box, which is in a non-boiling state, The fuel rods other than the outermost fuel rods of the fuel assembly have a small moderation effect on the emitted neutrons due to the cooling water inside the channel box, which is in a boiling state.

2. The boiling water reactor of claim 1.

3. The fuel assemblies have an average plutonium content of 15% by weight or less.

2. The boiling water reactor of claim 1.

4. When the reactor core is operated at rated power, the cooling water inside the channel box is in a boiling state in a cross section at the axial center of the nuclear fuel material filling region of the fuel rod.

2. The boiling water reactor of claim 1.

5. The channel box has a square cross section.

2. The boiling water reactor of claim 1.

6. Located in an area surrounded by four control rods, A plurality of fuel rods; A channel box surrounding the fuel rod; Equipped with It does not have a water rod inside through which non-boiling water flows, It can be loaded into the core inside the reactor pressure vessel of a boiling water reactor, In a cross section, a square region having vertices at the center points of four adjacent control rods is divided into four equal regions, and the fuel assembly lattice is defined as the fuel assembly lattice. When the reactor core is loaded and the reactor core is operating at rated power, Cooling water flows inside and outside the channel box, an area of ​​the cooling water flow passage in the cross section outside the channel box in the fuel assembly lattice is defined as an external flow passage area of ​​the channel box; an area of ​​the cooling water flow passage in the cross section of the channel box in the fuel assembly lattice is defined as an internal flow passage area of ​​the channel box; Inside the fuel assembly lattice and outside the channel box, the cooling water is in a non-boiling state, and the moderating effect on the neutrons emitted by the fuel rods is large, Inside the channel box, the cooling water is in a boiling state due to heat generated by the fuel rods, and the moderating effect on the neutrons emitted by the fuel rods is small. a ratio of the internal flow path area of ​​the channel box to the external flow path area of ​​the channel box is equal to or less than a predetermined value such that a void coefficient is negative; A fuel assembly comprising:

7. Among the fuel rods, the fuel rods located in the outermost layer of the fuel assembly are outermost layer fuel rods, The outermost fuel rods are adjacent to the cooling water outside the channel box with the channel box in between, When the core is operating at rated power, The outermost fuel rods have a large moderation effect on the emitted neutrons due to the cooling water outside the channel box, which is in a non-boiling state, The fuel rods other than the outermost fuel rods of the fuel assembly have a small moderation effect on the emitted neutrons due to the cooling water inside the channel box, which is in a boiling state. The fuel assembly of claim 6.

8. The average plutonium content is 15% by weight or less; The fuel assembly of claim 6.

9. When the reactor core is operated at rated power, the cooling water inside the channel box is in a boiling state in a cross section at the axial center of the nuclear fuel material filling region of the fuel rod. The fuel assembly of claim 6.

10. The channel box has a square cross section. The fuel assembly of claim 6.

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