Fuel assemblies and reactor cores

Abstract

To provide a fuel assembly capable of suppressing an occurrence of dryout of a liquid film.SOLUTION: A fuel assembly 13 is formed by arranging fuel rods 1-4, 6 and 8 whose lower ends are supported by a lower tie plate and whose upper ends are supported by an upper tie plate, and fuel rods 5 and 7 (partial-length fuel rods) whose lower ends are supported by the lower tie plate and whose upper ends are not supported by the upper tie plate, in a channel box 17. The effective fuel length of the fuel rods 5 is 2 / 3 of that of the fuel rods 6, and the effective fuel length of the fuel rods 7 is 9 / 10 of that of the fuel rods 6. The fuel rods 5 are respectively arranged at four corners in a third row from the inner face of the channel box 17 in a fuel rod array. The fuel rods 7 are arranged at the third from the fuel rods 1 arranged at the respective corners in the second row in each of two orthogonal directions along the inner surface of the channel box 17, with the fuel rods 1 arranged at the respective corners in the second row from the inner surface of the channel box 17 used as base points in the fuel rod array.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a fuel assembly and a core of a nuclear reactor, and more particularly to a fuel assembly and a core of a nuclear reactor suitable for application to a boiling water reactor. [Background technology]

[0002] In order to effectively utilize uranium resources, reduce the amount of radioactive waste generated, and effectively utilize plutonium, development of reduced-moderated spectrum boiling water reactors is underway based on light water reactor technology. In a reduced-moderated spectrum boiling water reactor, a plurality of fuel assemblies are arranged in the core, each of which is made up of a plurality of fuel rods filled with nuclear fuel material arranged in a rectangular channel box with a square cross section. In this fuel assembly, the fuel rods are arranged densely, and the proportion of water, which is a moderator, in the channel box is significantly reduced. Therefore, in the fuel assemblies loaded in the core of a reduced-moderated spectrum boiling water reactor, the fast neutrons generated by the fission of the fissionable material contained in each fuel material in the fuel rods are not moderated much, and relatively high-energy neutrons are used for the fission of the fissionable material.

[0003] JP 2020-118526 A describes a fuel assembly to be loaded into the core of a reduced moderation spectrum boiling water reactor. In this fuel assembly, a plurality of fuel rods filled with MOX fuel are arranged in an equilateral triangular lattice, with a small pitch between the fuel rods and a small amount of water present inside. When a plurality of fuel rods are arranged in an equilateral triangular lattice in a channel box having a square cross section, gaps having a triangular cross section are formed near the inner surface of the channel box, and the cooling water rising in the channel box is likely to rise through the gaps. For this reason, the flow rate of the cooling water rising through the gaps having a triangular cross section increases. In the fuel assembly described in JP 2020-118526 A, water exclusion rods attached to the inner surface of the channel box are arranged in each gap having a triangular cross section. The arrangement of the water exclusion rods further reduces the amount of water in the channel box. In such fuel assemblies loaded into the core of a reduced-moderation spectrum boiling water reactor, the neutron spectrum is hardened to improve the conversion of uranium-238 to plutonium-239 and the void reactivity coefficient can be made more negative.

[0004] JP 2019-178896 A describes a fuel assembly in which a plurality of fuel rods filled with fissile plutonium are arranged in 10 rows and 10 columns in a channel box. This fuel assembly includes two types of part-length fuel rods that contain fissile plutonium and have different effective fuel lengths. One of the part-length fuel rods has a longer effective fuel length than the other. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2020-118526 A [Patent Document 2] JP 2019-178896 A Summary of the Invention [Problem to be solved by the invention]

[0006] JP 2020-118526 A describes a fuel assembly to be loaded into the core of a reduced-moderation spectrum boiling water reactor. In general, fuel assemblies loaded into the core of a reduced-moderation spectrum boiling water reactor, like this fuel assembly, have multiple fuel rods arranged in a regular triangular lattice in a channel box with a square cross section, so that a triangular gap is inevitably formed between the fuel rods arranged in the outermost peripheral region of the fuel rod array and the inner surface of the channel box. Due to the formation of this triangular gap, the cooling water rising in the fuel assembly may rise unevenly (concentrated) in the gap, so the cooling water rising in the central region of the cross section of the fuel assembly may decrease, and the heat removal performance of the multiple fuel rods arranged in the central region may decrease.

[0007] In order to solve such problems, in JP 2020-118526 A, water exclusion rods are arranged in the respective triangular gaps near the inner surface of the channel box. The arrangement of these water exclusion rods requires the preparation of a large number of water exclusion rods, the lower end of each water exclusion rod being supported by a lower tie plate, and the upper end of each water exclusion rod being supported by an upper tie plate. Furthermore, it is necessary to bundle each water exclusion rod and each fuel rod while maintaining a predetermined interval between each water exclusion rod and each fuel rod by a plurality of fuel spacers. As a result, the installation of the water exclusion rods complicates the structure of the fuel assembly.

[0008] In consideration of the above circumstances, the inventors have considered a fuel assembly in which the fuel rod arrangement in the rectangular cylindrical channel box of the fuel assembly is changed from a regular triangular lattice to a square lattice as shown in JP 2019-178896 A, as described in detail later, and the fuel rods are densely arranged to harden the neutron spectrum during operation of the reactor, and no water exclusion rods are installed. In this study, the inventors found that in the cross section of the fuel assembly, in the fuel rods arranged in the second and third layers from the inner surface of the channel box in the fuel rod arrangement, liquid film dryout may occur due to an increase in power.

[0009] For this reason, it is desirable to realize a fuel assembly that can suppress the occurrence of liquid film dryout.

[0010] A first object of the present invention is to provide a fuel assembly and a nuclear reactor core capable of suppressing the occurrence of liquid film dryout.

[0011] A second object of the present invention is to provide a fuel assembly and a nuclear reactor core capable of suppressing a decrease in the inventory of nuclear fuel materials. [Means for solving the problem]

[0012] A first aspect of the present invention for achieving the first object described above is a lower fuel support member, an upper fuel support member; a channel box which is a rectangular tube having a square cross section and whose upper end is attached to the upper fuel support member and extends toward the lower fuel support member; In the channel box, a square lattice pattern dense A number of fuel rods filled with nuclear fuel material are arranged in the 、 A fuel assembly comprising: The fuel rods are each formed of a plurality of fuel rods each having a lower end supported by the lower fuel support member and an upper end supported by the upper fuel support member. full length A fuel rod, a lower end of which is supported by the lower fuel support member and an upper end of which is not supported by the upper fuel support member, and an effective fuel length is full length Several rods shorter than the fuel rods 1st part length Contains fuel rods, the full length fuel rods and the first part length fuel rods arranged in a cross section of the fuel assembly are arranged symmetrically about a central axis of the fuel assembly; The above 1st part length The fuel rods are arranged in the second and third rows from the inner surface of the channel box in the cross section of the fuel assembly. and a position where the heat generation coefficient of the fuel rod is the largest in a cross section of the assumed fuel assembly when all the fuel rods in the fuel assembly are assumed to be full-length fuel rods, and a position where the heat generation coefficient is approximately the same as the largest heat generation coefficient. The advantage of this method is that it can be placed in each of the following locations.

[0013] 1st part lengthThe fuel rods are arranged in the second and third rows from the inner surface of the channel box in the fuel rod arrangement in the cross section of the fuel assembly, so that the occurrence of liquid film dryout in the fuel assembly can be suppressed. By suppressing the occurrence of liquid film dryout in the fuel assembly, the thermal margin of the fuel assembly can be improved, and the thermal output during operation of the nuclear power plant can be increased.

[0014] The second feature of the present invention for achieving the second object described above is that Combustion The fee is a plurality of second part-length fuel rods having an effective fuel length shorter than that of the full-length fuel rods and longer than that of the first part-length fuel rods; Contains the second part length fuel rods arranged in a cross section of the fuel assembly are arranged point symmetrically about a central axis of the fuel assembly; The second part-length fuel rods are arranged in the second and third rows from the inner surface of the channel box in the fuel rod arrangement in the cross section of the fuel assembly, and are arranged at positions where the heat generation coefficient of the fuel rods in the cross section of the assumed fuel assembly is the next largest heat generation coefficient after the heat generation coefficient that is approximately the same as the largest heat generation coefficient, and at positions where the heat generation coefficient is approximately the same as the second largest heat generation coefficient. The point is...

[0015] 2nd part length The effective fuel length of the fuel rod is 1st part length Since it is longer than the effective fuel length of the fuel rod, it is possible to suppress a reduction in the loading amount of nuclear fuel material in the fuel assembly.

[0016] Preferably, 2nd part length The fuel rods are arranged in each of the four second corners of the second row. full length The above-mentioned is arranged at the second corners of each of two directions perpendicular to each other along the inner surface of the channel box, with the fuel rod as the base point. full length In addition to being disposed at the third fuel rod, the fuel rods disposed at each of the four second corners of the second row are full length The fuel rod is used as a base point, and the fuel rod is disposed at the second corner of each of the two orthogonal directions. full length Also located on the first and second fuel rods is .

[0017] 2nd part length The fuel rods were arranged at the second corners of each of the two orthogonal directions. full length In addition to being placed on the third fuel rod, there are also two fuel rods placed at the second corners of each of the two orthogonal directions in the second row. full lengthSince the fuel rods are also arranged in the first and second positions, the degree of reduction in the amount of nuclear fuel material loaded in the fuel assembly is reduced. Effect of the Invention

[0018] According to the first aspect of the present invention, it is possible to suppress the occurrence of liquid film dryout in a fuel assembly.

[0019] According to the second aspect of the present invention, it is possible to suppress a reduction in the loading amount of nuclear fuel material in the fuel assembly. [Brief description of the drawings]

[0020] [Figure 1] FIG. 2 is a cross-sectional view (II cross-sectional view of FIG. 3 ) of a fuel assembly according to a first embodiment of the present invention, which is applied to a reduced-moderation spectrum boiling water reactor. [Diagram 2] FIG. 2 is an explanatory diagram showing the enrichment of fissile plutonium in each fuel rod contained in the fuel assembly shown in FIG. [Diagram 3] FIG. 2 is a vertical cross-sectional view of the fuel assembly shown in FIG. 1 (a cross-sectional view taken along line III-III in FIG. 1). [Figure 4] FIG. 2 is a vertical sectional view of a reduced moderation spectrum boiling water nuclear power plant in which the fuel assemblies shown in FIG. 1 are loaded. [Diagram 5] FIG. 2 is an explanatory diagram illustrating the distribution of heat coefficients of the fuel rods in a cross section of the fuel assembly shown in FIG. 1, in which all of the part-length fuel rods are replaced with full-length fuel rods. [Figure 6] FIG. 11 is a cross-sectional view of a fuel assembly according to a second embodiment of the present invention, which is applied to a reduced moderation spectrum boiling water reactor. [Figure 7] FIG. 7 is an explanatory diagram showing the enrichment of fissile plutonium in each fuel rod contained in the fuel assembly shown in FIG. 6. [Figure 8] FIG. 7 is an explanatory diagram illustrating the distribution of heat coefficients of the fuel rods in a cross section of the fuel assembly shown in FIG. 6, in which all of the part-length fuel rods are replaced with full-length fuel rods. [Figure 9]FIG. 11 is an explanatory diagram showing the relationship between the flow rate (core flow rate) of cooling water and MCPR in each of the fuel assembly of Example 2 (with part-length fuel rods) and the fuel assembly without the part-length fuel rods. [Figure 10] FIG. 11 is a cross-sectional view of a fuel assembly according to a third embodiment of the present invention, which is applied to a reduced moderation spectrum boiling water reactor. [Figure 11] FIG. 11 is a cross-sectional view of the fuel rod shown in FIG. [Figure 12] FIG. 1 is a cross-sectional view of a conventional fuel assembly. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The inventors have studied the suppression of liquid film dryout in a fuel assembly in which the fuel rods are arranged in a dense square lattice pattern in a rectangular cylindrical channel box.

[0022] Therefore, in this study, the inventors assumed a conventional fuel assembly in which a plurality of fuel rods are densely arranged in a square lattice pattern in a channel box that is a rectangular cylinder with a square cross section. This assumed conventional fuel assembly 13C is shown in FIG. 12. The fuel assembly 13C is loaded into the core of a reactor of a reduced moderation spectrum boiling water nuclear power plant. In the fuel assembly 13C, a plurality of fuel rods 11C are arranged in 13 rows and 13 columns in a channel box 17 that is a rectangular cylinder with a square cross section. These fuel rods 11C are arranged in a square lattice pattern.

[0023] As shown in Fig. 12, the four support rods 12 are arranged one at each of the four corners of the outermost region (first row from the inner surface of the channel box 17) of the fuel rod arrangement in the cross section of the fuel assembly 13C. Water gap regions 39 in which saturated water exists are formed between the fuel assemblies 13C loaded in the core. The water gap regions 39 exist facing each of the four side surfaces, i.e., the outer surfaces, of the channel box 17 of one fuel assembly 13C. A control rod 34 having a cross-shaped cross section is inserted into the water gap regions 39 formed between the fuel assemblies 13C.

[0024] In this fuel assembly 13C, a plurality of fuel rods 11C highly enriched in fissile plutonium are arranged in the center of the cross section, and a plurality of other fuel rods 11C containing burnable poisons such as gadolinia are arranged in the outermost region. In the center of the cross section, the fuel rods 11C containing highly enriched fissile plutonium are arranged densely, so there is less cooling water (light water) to act as a moderator, and the thermal neutron flux is smaller (i.e., the neutron spectrum is harder).

[0025] On the other hand, in the outer periphery of the fuel assembly 13C near the water gap region between the fuel assemblies 13C loaded in the core, the neutron spectrum becomes softer due to the influence of the cooling water in the water gap region 39. As a result, the fission reaction of the fissile plutonium becomes active in the outer periphery of the cross section of the fuel assembly 13C, and the output of each of the fuel rods 11C arranged in the second and third rows from the inner surface of the channel box 17 in the fuel rod arrangement becomes high. This state continues throughout the operation cycle of the nuclear power plant. As the output of a specific fuel rod 11C arranged in the outer periphery of the fuel assembly 13C becomes high, a phenomenon occurs in which the outer surface of the fuel rod 11C dries out (liquid film dryout), which leads to the burnout of that fuel rod 11C. This becomes a bottleneck in the thermal margin of the operation of the reactor, and limits the rated operating output of the fuel assembly 13C.

[0026] As a result of investigating measures to solve such problems, the inventors discovered that the occurrence of liquid film dryout in the fuel rods in a fuel assembly can be suppressed by arranging part-length fuel rods in the second and third rows from the inner surface of the channel box in the fuel rod arrangement in the cross section of the fuel assembly.

[0027] Additionally, it would be desirable to have a higher loading of nuclear fuel material in a fuel assembly having part-length rods.

[0028] An embodiment of the present invention that takes the above-mentioned results into consideration will be described below. The embodiment of the present invention described below can be applied to a reduced-moderation spectrum boiling water reactor, which is a boiling water reactor. The reduced-moderation spectrum boiling water reactor includes a boiling water reactor (BWR) that uses cooling water as a coolant and circulates the cooling water by causing the cooling water to flow out of the reactor pressure vessel by a recirculation pump and then flowing back into the reactor pressure vessel, an advanced boiling water reactor (ABWR) that has an internal pump and circulates the cooling water inside the reactor pressure vessel, and an economical simplified boiling water reactor (ESBWR) that does not use the internal pump of the ABWR. EXAMPLES

[0029] A fuel assembly according to a first embodiment of the present invention, which is used in a reduced moderation spectrum boiling water nuclear power plant, will be described with reference to FIGS. 1, 2, 3, and 4. FIG.

[0030] First, the structure of a reactor of a reduced-moderation spectrum boiling water nuclear power plant 20 in which the fuel assemblies of this embodiment are loaded into the reactor core will be described with reference to Fig. 4. The reactor 21 of this reduced-moderation spectrum boiling water nuclear power plant 20 is an ABWR.

[0031] The nuclear reactor 21 has a reactor pressure vessel 22, and a core 23 loaded with a plurality of fuel assemblies 13 (see Figs. 1, 2 and 3) is disposed in the reactor pressure vessel 22. In the reactor pressure vessel 22, a cylindrical core shroud 24 surrounds the core 23, and a shroud head 25 disposed above the core 23 is installed at the upper end of the core shroud 24. A plurality of steam-water separators 28 are attached to the shroud head 25 and extend upward. Furthermore, a steam dryer 29 is installed in the reactor pressure vessel 22 above the steam-water separators 28. An annular downcomer 32 is formed between the outer surface of the core shroud 24 and the inner surface of the reactor pressure vessel 22. An internal pump 26 disposed in the downcomer 32 extends downward through the bottom of the reactor pressure vessel 22 and is attached to the bottom of the reactor pressure vessel 22. The internal pump 26 has an impeller 27. A main steam line 37 and a feedwater line 38 are connected to the reactor pressure vessel 22 .

[0032] An upper grid plate 30 is disposed above the core 23 and attached to the inner surface of the core shroud 24. A core support plate 31 is disposed below the core 23 and attached to the inner surface of the core shroud 24. A plurality of fuel support brackets 33 are installed on the core support plate 31.

[0033] A lower plenum 36 is formed below the core 23 within the reactor pressure vessel 22. A plurality of control rod guide tubes (not shown) are arranged in the lower plenum 36. Each control rod 34, which has a plurality of neutron absorbing rods filled with a neutron absorbing material (e.g., boron carbide) and has a cross-shaped cross section, is separately arranged in each control rod guide tube. A plurality of control rod drive mechanisms 35 are installed at the bottom of the reactor pressure vessel 22 and extend downward from the bottom of the reactor pressure vessel 22, and each control rod drive mechanism 35 is separately connected to the control rod 34.

[0034] 3, the fuel assembly 13 loaded in the reactor core 23 includes a plurality of fuel rods 11, a lower tie plate (lower fuel support member) 14, an upper tie plate (upper fuel support member) 15, a plurality of fuel spacers 18 arranged in the axial direction, and a channel box 17. The fuel assembly 13 is a MOX fuel assembly containing mixed oxide fuel (MOX fuel) of uranium oxide and plutonium oxide.

[0035] Each fuel rod 11 has a cladding tube (not shown), the lower end of which is sealed with a lower end plug (not shown) and the upper end of which is sealed with an upper end plug (not shown), and is configured by filling the cladding tube with a plurality of fuel pellets (not shown) containing nuclear fuel material (MOX fuel). A gas plenum (not shown) is formed within the cladding tube above the nuclear fuel material filling area in which the fuel pellets are filled. The lower end of each fuel rod 11 is supported by a lower tie plate 14, and the upper end of each fuel rod 11 is supported by an upper tie plate 15. The upper tie plate 15 is provided with a handle 16.

[0036] The fuel rods 11 are bundled together by a number of fuel spacers 18 arranged in the axial direction. The bundled fuel rods 11 are arranged in a channel box 17, whose upper end is attached to the upper tie plate 15 and extends downward. The channel box 17 is a rectangular tube with a square cross section. 165 fuel rods 11 are arranged in a square lattice pattern in the channel box 17 with 13 rows and 13 columns. Cooling water passages 19 are formed between the fuel rods 11. The diameter of the fuel rods 11, i.e., the outer diameter of the cladding tube, is 8.0 mm.

[0037] The fuel assembly 13 has four support rods 12 in addition to the fuel rods 11. As shown in FIG. 1, the four support rods 12 are arranged at each of the four corners of the outermost region (first row from the inner surface of the channel box 17) of the fuel rod arrangement in the cross section of the fuel assembly 13. Each support rod 12 is made of metal, and is preferably manufactured from a metal with a small neutron absorption cross section. The support rods 12 do not contain nuclear fuel material. The lower end of the support rod 12 is screwed into the lower tie plate 14 and attached to the lower tie plate 14. The upper end of the support rod 12 is inserted into a hole formed in the upper tie plate 15 and held by the upper tie plate 15. Each support rod 12 plays a role of holding each fuel spacer 18 at a predetermined position in the axial direction.

[0038] The lower tie plates 14 of the fuel assemblies 13 loaded in the core 23 are supported by fuel support fittings 33 installed on the core support plate 31. Each fuel support fitting 33 supports the lower tie plates 14 of four fuel assemblies 13. The upper ends of the channel boxes 17 of the four fuel assemblies 13 whose lower tie plates 14 are supported by one fuel support fitting 33 are inserted into one of the squares formed in the upper grid plate 30. The upper grid plate 30 has a plurality of squares with a cross section formed therein that penetrate the upper grid plate 30. The upper ends of the four fuel assemblies 13 supported by each fuel support fitting are inserted into each of the squares, and the upper ends are supported by the upper grid plate 30.

[0039] Between the fuel assemblies 13 loaded in the core 23, a water gap region 39 in which saturated water exists is formed, similar to between the fuel assemblies 13C shown in FIG. 12. This water gap region 39 exists facing each of the four side surfaces of the channel box 17 of one fuel assembly 13. A control rod 34 connected to a control rod drive mechanism 35 is inserted between the four fuel assemblies 13 supported by the fuel support metal fitting 33 through a control rod insertion hole (not shown) having a cross-shaped cross section formed in the center of the fuel support metal fitting 33 and penetrating the fuel support metal fitting 33. The control rod drive mechanism 35 inserts the control rod 34 between the fuel assemblies 13 and withdraws the control rod 34 from between the fuel assemblies 13. The control rod 34 inserted between the fuel assemblies 13 exists in the water gap region 39.

[0040] The arrangement of the multiple fuel rods 11 in the channel box 17 in the cross section of the fuel assembly 13 will be specifically described with reference to Fig. 1. The multiple fuel rods 11 in the fuel assembly 13 include fuel rods 1, 2, 3, 4, 5, 6, 7, and 8. The number of each of the fuel rods 1 to 8 in the fuel assembly 13 is shown in Fig. 2. There are 12 fuel rods 1, 16 fuel rods 2, 8 fuel rods 3, 8 fuel rods 4, 4 fuel rods 5, 89 fuel rods 6, 8 fuel rods 7, and 20 fuel rods 8.

[0041] Among the multiple fuel rods 11, the fuel rods 1 to 4, 6, and 8, which are full-length fuel rods, have their lower ends supported by the lower tie plate 14 and their upper ends supported by the upper tie plate 15. The axial length of the nuclear fuel material filled area in each of the fuel rods 1 to 4, 6, and 8, i.e., the active fuel length, is the same. The lower ends of the fuel rods 5 and 7 are supported by the lower tie plate 14, but their upper ends are not supported by the upper tie plate 15. The active fuel length of each of the fuel rods 5 and 7 is shorter than the active fuel length of each of the fuel rods 1 to 4, 6, and 8, and each of the fuel rods 5 and 7 is a partial length fuel rod. The active fuel length of each of the fuel rods 5 and 7 is longer than 1 / 2 of the active fuel length of each of the fuel rods 1 to 4, 6, and 8, and the active fuel length of the fuel rod 7 is longer than the active fuel length of the fuel rod 5. The effective fuel length of fuel rod 5 is 2 / 3 of the effective fuel length of each of fuel rods 1 to 4, 6 and 8, and the effective fuel length of fuel rod 7 is 9 / 10 of the effective fuel length of each of fuel rods 1 to 4, 6 and 8.

[0042] When the effective fuel length of a long partial length fuel rod (e.g., fuel rod 7) is L1 and the effective fuel length of a short partial length fuel rod (e.g., fuel rod 5) is L2, the effective fuel lengths L1 and L2 can be determined so as to satisfy the following equation (1).

[0043] {(L-L1)×n1+(L-L2)×n2} / (L×n)≦0.072 …(1) Here, L is the effective fuel length of the full-length fuel rod corresponding to each of fuel rods 1 to 4, 6, and 8, n1 is the number of long partial-length fuel rods (e.g., fuel rod 7), n2 is the number of short partial-length fuel rods (e.g., fuel rod 5), and n is the total number of fuel rods contained in fuel assembly 13.

[0044] Equation (1) indicates that the effective fuel length L1 of the long partial-length fuel rods and the effective fuel length L2 of the short partial-length fuel rods are set so that the ratio of the total reduction in the effective fuel length caused by placing two types of partial-length fuel rods with different effective fuel lengths in a fuel assembly ((L-L1) x n1 + (L-L2) x n2) to the total effective fuel length (L x n) of all full-length fuel in the fuel assembly in which no partial-length fuel rods exist is 7.2% or less.

[0045] As long as formula (1) is satisfied, the effective fuel length L1 of the long partial length fuel rod may be other than 9L / 10, and the effective fuel length L2 of the short partial length fuel rod may be other than 2L / 3.

[0046] A specific method for determining the effective fuel length L2 of the short partial length fuel rod (e.g., fuel rod 5) and the effective fuel length L1 of the long partial length fuel rod (e.g., fuel rod 7) using formula (1) will be described. A core performance calculation for the fuel assembly is performed to obtain the distribution of heat generation coefficients in the cross section of the fuel assembly (e.g., see FIG. 5), and the positions of the short partial length fuel rods and the long partial length fuel rods in the cross section of the fuel assembly are determined. Furthermore, the number of short partial length fuel rods (n2) and the number of long partial length fuel rods (n1) are determined based on the determined positions of the short partial length fuel rods and the long partial length fuel rods. Then, the effective fuel length L of the full length fuel rod, the number n of the full length fuel rods when all the fuel rods in the fuel assembly are full length fuel rods, the number n1 of the long partial length fuel rods, and the number n2 of the short partial length fuel rods are substituted into formula (1), and an arbitrary value is substituted into either the effective fuel length L1 or the effective fuel length L2 (for example, the effective fuel length L2) to calculate the other effective fuel length (for example, the effective fuel length L1) to which no arbitrary value is substituted. By performing such calculations of formula (1) in several patterns, the optimal effective fuel length L1 and the effective fuel length L2 are comprehensively confirmed, and the effective fuel lengths L1 and L2 of the long partial length fuel rods and the short partial length fuel rods are determined so as to be maximized while the MCPR satisfies the reference value (design value).

[0047] Each of the fuel rods 1 to 7 is filled with MOX fuel, which is a nuclear fuel material, in a sealed cladding tube, and the nuclear fuel material does not contain burnable poisons. The enrichment of fissile Pu in the MOX fuel in each of the fuel rods 1 to 7 is described below based on Figure 2. The enrichment of fissile Pu in fuel rod 1 is 3.6 wt%, the enrichment of fissile Pu in fuel rod 2 is 5.0 wt%, the enrichment of fissile Pu in fuel rod 3 is 6.2 wt%, the enrichment of fissile Pu in fuel rod 4 is 8.8 wt%, the enrichment of fissile Pu in fuel rod 5 is 11.5 wt%, and the enrichment of fissile Pu in each of the fuel rods 6 and 7 is 12.1 wt%. The fuel rods 8 are filled with a nuclear fuel material, such as depleted uranium, and contain a burnable poison, such as gadolinium (Gd). The concentration of gadolinium in the fuel rods 8 is 9.0 wt%. The fuel rods 8 do not contain fissile Pu.

[0048] Fuel rods 6 and 7 have the highest enrichment of fissile Pu in the fuel assembly 13. Fuel rod 5 has the next highest enrichment of fissile Pu after fuel rods 6 and 7. The enrichment of fissile Pu in each of fuel rods 4, 3, 2, and 1 is smaller than that of fuel rod 5, and the enrichment of fissile Pu decreases in the order of fuel rods 4, 3, 2, and 1.

[0049] The above values ​​of the enrichment of fissile Pu and the concentration of burnable poisons are values ​​for the fuel assembly 13 with a burnup of 0 GWd / t, which includes each of the fuel rods 1 to 8. The fuel assembly 13 with a burnup of 0 GWd / t is a new fuel assembly, which has not experienced the operation of a nuclear reactor.

[0050] The positions of the fuel rods 1 to 8 in the cross section of the fuel assembly 13 will be described.

[0051] The fuel rods 1 are arranged in the outermost region of the fuel rod array in the channel box 17 and in the second row from the inner surface of the channel box 17. In the corners facing the control rods 34 in the cross section of the fuel assembly 13, the fuel rods 1 are arranged second from the support rods 12 in the outermost region in each of two orthogonal directions along the inner surface of the channel box 17, starting from the support rods 12 arranged at each of the four corners of the outermost region. Furthermore, in the corners facing the control rods 34, the fuel rods 1 are also arranged at the corners of the second row. The arrangement of the fuel rods 1 in the outermost region and the second row in the corners facing the control rods 34 is also performed in the other three corners in the cross section of the fuel assembly 13, other than the corners facing the control rods 34.

[0052] At the corners facing the control rods 34, the fuel rods 8 are arranged between the support rods 12 and the fuel rods 1 in each of the two orthogonal directions along the inner surface of the channel box 17 in the outermost peripheral region. Also, in the two orthogonal directions in the outermost peripheral region, other fuel rods 8 are arranged adjacent to the fuel rods 1 adjacent to the fuel rods 8 adjacent to the support rods 12. This arrangement of fuel rods 8 in the outermost peripheral region at the corners facing the control rods 34 is also performed at the other three corners in the cross section of the fuel assembly 13 other than the corners facing the control rods 34. The other fuel rods 8 are adjacent to the fuel rods 1 on the opposite side to the fuel rods 8 adjacent to the support rods 12 and the fuel rods 1 in the outermost peripheral region. .difference In addition, in its outermost region, the fuel rods 8 are also arranged in the center of each square piece formed by the four support rods 12. In the fuel assembly 13, the fuel rods 8 are arranged only in the outermost region.

[0053] The fuel rods 3 are arranged in the second row from the inner surface of the channel box 17 in the fuel rod arrangement in the cross section of the fuel assembly 13. In the corner portion facing the control rod 34, the fuel rods 3 are arranged adjacent to the fuel rods 1 arranged in the corner of the second row in each of two orthogonal directions along the inner surface, starting from the fuel rod 1 arranged in the corner of the second row as the base point. In the other three corner portions other than the corner portion facing the control rod 34, the fuel rods 3 are arranged in the same manner as in the corner portion facing the control rod 34.

[0054] The fuel rods 4 are arranged in the second row from the inner surface of the channel box 17 in the fuel rod arrangement in the cross section of the fuel assembly 13. In the corners facing the control rods 34, the fuel rods 4 are arranged adjacent to the above-mentioned fuel rods 3 arranged in the second row in each of the two orthogonal directions starting from the fuel rod 1 arranged at the corner of the second row. In the other three corners other than the corner facing the control rod 34, the fuel rods 4 are arranged in the same manner as in the corner facing the control rod 34.

[0055] The fuel rods 5 are arranged in the third row from the inner surface of the channel box 17 in the fuel rod arrangement in the cross section of the fuel assembly 13. In the corner portion facing the control rod 34, the fuel rods 5 are arranged at the corners of the third row. Specifically, in the other three corner portions other than the corner portion facing the control rod 34, the fuel rods 5 are arranged in the same manner as in the corner portion facing the control rod 34.

[0056] The fuel rods 7 are arranged in the second row from the inner surface of the channel box 17 in the fuel rod arrangement in the cross section of the fuel assembly 13. In the second row, the fuel rods 7 are arranged adjacent to the fuel rods 4 on the opposite side to the fuel rods 3.

[0057] The multiple fuel rods 6 are not arranged in the outermost region, but together with the multiple fuel rods 7, are arranged in a central region from the second row of the fuel rod arrangement to the central axis side of the fuel assembly 13 in the cross-section of the fuel assembly 13, which does not include the four corner portions described above in the cross-section of the fuel assembly 13.

[0058] The fuel rods 1 to 8 arranged as described above are arranged in a point symmetry around the central axis of the fuel assembly 13 in the cross section of the fuel assembly 13. In the four corners in the cross section of the fuel assembly 13, the enrichment of fissile Pu decreases from the fuel rod 5 arranged at the corner of the third row from the inner surface of the channel box 17 in the fuel rod arrangement toward the outermost peripheral region. In each of the four corners, as fuel rods 11, there are the fuel rod 5 arranged at the corner of the third row, the fuel rod 1 arranged at the corner of the second row in the second row from the inner surface of the channel box 17, the fuel rods 3 arranged adjacent to both sides of this fuel rod 1, and the fuel rods 8 arranged adjacent to both sides of the support rods 12 arranged at the corners in the outermost peripheral region, the fuel rods 1 arranged adjacent to each of these fuel rods 8, the fuel rods 8 arranged on the opposite side of each fuel rod 1 from the support rods 12, and the fuel rods 2 arranged adjacent to each of these fuel rods 8. In each corner, the enrichment of fissile Pu in the fuel rods 11 in the third row, the fuel rods 11 in the second row, and the fuel rods 11 in the outermost region decreases in the order of the third row, the second row, and the outermost region. The enrichment of fissile Pu in the fuel rods 11 arranged in these four corners is smaller than that of the fuel rods 11 in the third row, i.e., the fuel rods 6 adjacent to the fuel rods 5. In other words, the enrichment of fissile Pu in the fuel rods 11 arranged in each corner is smaller than the enrichment of fissile Pu in the fuel rods 6 and the fuel rods 7 arranged in the central region.

[0059] In the reduced moderation spectrum boiling water nuclear power plant 20, all the control rods 34 are inserted into the core 23 at the end of one operation cycle, and the operation of that operation cycle ends. Fuel exchange and maintenance and inspection of the nuclear power plant 20 are carried out while the operation of the nuclear power plant 20 is stopped. In the fuel exchange, some of the fuel assemblies 13 loaded in the core 23 are removed from the reactor 21 to a fuel storage pool (not shown) as spent fuel assemblies, and new fuel assemblies (fuel assemblies with a burnup of 0 GWd / t) stored in the fuel storage pool are loaded into the core 23 in the reactor 21. In the fuel exchange, for example, when 1 / 4 of all the fuel assemblies 13 loaded in the core 23 when the operation of the nuclear power plant 20 is stopped are removed from the core 23 as spent fuel assemblies, new fuel assemblies 13 in the same number as the number of the removed spent fuel assemblies are loaded into the core 23. As a result, before the nuclear power plant 20, in which new fuel assemblies 13 have been loaded into the core 23, begins operation in the next operating cycle, the core 23 contains fuel assemblies 13 with a burnup of 0 GWd / t that have not experienced operation in the nuclear power plant 20, fuel assemblies 13 that have experienced operation in one operating cycle, fuel assemblies 13 that have experienced operation in two operating cycles, and fuel assemblies 13 that have experienced operation in three operating cycles.

[0060] The fuel assembly 13 with a burnup of 0 GWd / t contains a burnable poison because it has fuel rods 8. The concentration of the burnable poison contained in the fuel rods 8 present in the fuel assembly 13 with a burnup of 0 GWd / t is set so that the burnable poison disappears at the end of one operation cycle by the operation of the nuclear power plant 20. Therefore, the remaining fuel assemblies 13 present in the core 23 that have experienced operation in one or more operation cycles do not contain the burnable poison at the start of operation of the nuclear power plant 20 in the above-mentioned next operation cycle.

[0061] After the fuel exchange and the maintenance and inspection of the nuclear power plant 20 are completed, the operation of the nuclear power plant 20 in the next operation cycle is started. When the internal pump 26 is driven, the cooling water present in the downcomer 32 in the reactor pressure vessel 22 is pressurized by the impeller 27 of the internal pump 26. The pressurized cooling water is supplied from the fuel support bracket 33 to the channel box 17 of the fuel assembly 13 through the lower plenum 36, and rises through the cooling water passage 19 formed between the fuel rods 11 in the channel box 17. While rising through the cooling water passage 19, the cooling water is heated by heat generated by the nuclear fission of fissile material (e.g., fissile Pu) contained in the nuclear fuel material present in the fuel rods 11. A part of the heated cooling water becomes steam, and the cooling water becomes a gas-liquid two-phase flow containing water and steam.

[0062] This two-phase gas-liquid flow is discharged from the upper end of the fuel assembly 13, i.e., from the core 23, and flows into the steam separator 28. In the steam separator 28, the two-phase gas-liquid flow is separated into water and steam. The separated water is discharged from the steam separator 28 to the downcomer 32, and flows down the downcomer 32 as cooling water, and is pressurized by the internal pump 26. The separated steam is led from the steam separator 28 to the steam dryer 29, where moisture contained in the steam is removed. The steam discharged from the steam dryer 29 after moisture has been removed is discharged from the reactor pressure vessel 22 to the main steam pipe 37, and is led to a steam turbine (not shown) through the main steam pipe 37. The steam rotates the steam turbine, which rotates a generator (not shown) connected to the steam turbine. Electricity is generated by the rotation of the generator. The steam discharged from the steam turbine is condensed in a condenser (not shown) to become water. This condensed water is supplied as feed water to the reactor pressure vessel 22 through the feed water piping 38 .

[0063] The horizontal width of the cooling water passage 19 formed between the fuel rods 11 is narrower than the sum of the width of the gap between the fuel rods 11 arranged in the outermost region of the fuel rod array and the inner surface of the channel box 17 in the cross section of the fuel assembly 13 and the width of the water gap region 39 present between the fuel assemblies 13. Therefore, in the above-mentioned central region in the cross section of the fuel assembly 13, the amount of cooling water (light water) serving as a moderator per fuel rod 11 (fuel rods 6 and 7) is much smaller than the amount of cooling water serving as a moderator per fuel rod arranged in the outermost region facing the gap between the fuel rods 11 arranged in the outermost region of the fuel rod array and the inner surface of the channel box 17 and the water gap region 39. Therefore, the neutron spectrum becomes harder toward the center of the above-mentioned central region in the cross section of the fuel assembly 13. On the other hand, the neutron spectrum becomes softer in the gap between the fuel rods 11 arranged in the outermost region of the fuel rod array and the inner surface of the channel box 17 and in the outermost region close to the water gap region 39.

[0064] In the fuel assembly 13 in this embodiment, as described above, the neutron spectrum becomes softer toward the outer periphery, so that the fuel rods 11 closer to the inner surface of the channel box 17 have higher thermal output. On the other hand, the fuel rods 8 arranged in the outermost region of the fuel assembly 13 contain burnable poisons. For this reason, the output of the fuel rods 11 in the outermost region of the fuel assembly 13 is suppressed, and the output of the fuel rods 11 arranged in the second row from the inner surface of the channel box 17 in the fuel rod arrangement is the highest in the fuel assembly 13. In addition, as the operation of the nuclear power plant 20 progresses in a certain operation cycle and the burnup of fissile Pu contained in each fuel material in the fuel rods 11 progresses, the thermal output of the fuel rods 11 arranged in the second row from the inner surface of the channel box 17 decreases, so that the thermal output of the fuel rods arranged in the third row from the inner surface of the channel box 17 is the highest in the fuel assembly 13.

[0065] When designing a nuclear reactor, it is necessary to evaluate the MCPR (Minimum Critical Power Ratio). MCPR is used as an index for evaluating the thermal margin of a BWR. Critical thermal power is the fuel assembly thermal power at which nucleate boiling transitions to film boiling. MCPR is the minimum critical power ratio among the fuel assemblies loaded in the core, among the critical power ratios (CPR) defined as the critical thermal power / fuel assembly generated thermal power.

[0066] The critical thermal output is a value that depends on the design of the fuel assembly, and the more thermally-hydraulic the fuel assembly is, the higher it is. It is necessary to determine the thermal output of the reactor during operation so that the critical power ratio falls within the regulated value. In other words, if there are fuel rods with high thermal loads in the fuel assembly during design, the critical thermal output of the fuel assembly will decrease. As a result, in order to keep the critical power ratio below the regulated value, the thermal output of the reactor during operation must be reduced. This brings about economic problems due to a decrease in the amount of power generated by the reactor, and safety problems such as the need to ensure sufficient operating margin.

[0067] The liquid film dryout that occurs in the fuel assembly will be described below. To explain this liquid film dryout, the inventors have assumed a fuel assembly having a configuration in which all of the fuel rods 5 and 7, which are part-length fuel rods in the fuel assembly 13 of this embodiment shown in Fig. 1, are replaced with full-length fuel rods. This assumed fuel assembly will be referred to as a first assumed fuel assembly below.

[0068] In the above-mentioned first assumed fuel assembly, as in the fuel assembly 13 of this embodiment, the neutron spectrum becomes harder toward the center in the cross section of the first assumed fuel assembly. Also, in the outermost peripheral region of the fuel rod array, the neutron spectrum becomes softer. Therefore, the thermal output of the fuel rods near the inner surface of the channel box 17 becomes higher. Since the fuel rods containing burnable poisons are arranged in the outermost peripheral region of the fuel rod array, the output of the fuel rods in the outermost peripheral region of the first assumed fuel assembly is suppressed, and the output of the fuel rods arranged in the second row from the inner surface of the channel box 17 in the fuel rod array becomes the highest in the first assumed fuel assembly. Also, as the operation of the nuclear power plant 20 progresses in a certain operation cycle and the burnup of the fissile Pu contained in each fuel material in the fuel rods 11 progresses, the thermal output of the fuel rods arranged in the second row from the inner surface of the channel box 17 decreases, and the thermal output of the fuel rods arranged in the third row from the inner surface of the channel box 17 becomes the highest in the first assumed fuel assembly.

[0069] In a boiling water reactor, the thermal margin of the fuel rods is maintained by removing heat from the fuel rods due to the presence of sufficient cooling water on the surface of the fuel rods. During operation of the reactor, as described above, the cooling water flows into the channel box 17 from the lower tie plate of the first hypothetical fuel assembly loaded in the reactor core, and flows out from the upper tie plate. The heat generated by the nuclear fission of the fissile Pu in the full-length fuel rods causes the cooling water to boil in the first hypothetical fuel assembly, and some of it becomes steam. As a result, a gas-liquid two-phase flow rises inside the first hypothetical fuel assembly loaded in the reactor core.

[0070] The flow pattern of the gas-liquid two-phase flow depends on the respective velocities and pressures of the cooling water and steam, but is most affected by the volumetric ratio of the cooling water and steam. On the upstream side of the first assumed fuel assembly, the volumetric ratio of steam to the cooling water is small, and the cooling water flows in most of the first assumed fuel assembly. Therefore, on the upstream side of the first assumed fuel assembly, a flow pattern of gas bubble flow is formed, in which steam bubbles flow in the cooling water. On the downstream side of the first assumed fuel assembly, the heat generated in the fuel rods sufficiently boils the cooling water rising between the fuel rods, and the volumetric ratio of steam to the cooling water becomes sufficiently large. Therefore, on the downstream side of the first assumed fuel assembly, a flow pattern of annular mist flow is formed, in which a gas-liquid two-phase flow flows between the fuel rods and a liquid film flows on the outer surface of the fuel rod.

[0071] Under conditions where the thermal output of the fuel rod is high, it is conceivable that the liquid film flowing on the outer surface of the fuel rod will all evaporate. This phenomenon in which the liquid film on the outer surface of the fuel rod all evaporates and the outer surface of the fuel rod dries is called liquid film dryout. When a liquid film exists on the outer surface of the fuel rod, the heat generated in the fuel rod can be sufficiently removed by the boiling and evaporation of the liquid film. On the other hand, when liquid film dryout occurs, the heat in the fuel rod can only be removed by convection of the steam containing the surrounding liquid droplets. Compared to the heat removal by boiling and evaporation, the heat removal by convection has a lower heat removal performance, and it is not possible to remove the heat generated in the fuel rod. As a result, the temperature of the fuel rod rises sharply, and this rapid rise leads to the burnout of the fuel rod.

[0072] In the first assumed fuel assembly, the second and third rows from the inner surface of the channel box 17 of the fuel rod arrangement eye Since the fuel rods arranged in each of the above have the highest thermal output in the first assumed fuel assembly, liquid film dryout occurs in these fuel rods, and the critical thermal output of the first assumed fuel assembly is determined. In other words, if the occurrence of liquid film dryout in the fuel rods 11 arranged in each of the above-mentioned second and third rows can be suppressed, the critical thermal output of the first assumed fuel assembly can be increased, and the economy and safety of the reactor can be improved.

[0073] As described above, liquid film dryout occurs under the condition of annular mist flow where the volume ratio of steam to cooling water is sufficiently large, and therefore it is assumed to occur near the upper end of the active fuel length of the full length fuel rods in the first assumed fuel assembly, where a gas-liquid two-phase flow flows that has sufficiently received the heat generated in the fuel rods 11. Therefore, in the first assumed fuel assembly, liquid film dryout is predicted to occur near the upper end of the active fuel length in the fuel rods arranged in the second and third rows from the inner surface of the channel box 17 in the fuel rod arrangement.

[0074] The inventors calculated the heat coefficient of each fuel rod in the cross section of the first assumed fuel assembly by core performance calculation. The calculated heat coefficient of each fuel rod in the cross section of such a fuel assembly is shown in FIG. 5. In the first assumed fuel assembly, the enrichment of fissile Pu of the full-length fuel rod corresponding to fuel rod 5 is the same as that of fuel rod 5 in fuel assembly 13, and the enrichment of fissile Pu of the full-length fuel rod corresponding to fuel rod 7 is the same as that of fuel rod 7 in fuel assembly 13. The first assumed fuel assembly that was the subject of the core performance calculation does not include a partial length fuel rod. The heat coefficient of each fuel rod in the cross section of the first assumed fuel assembly calculated by the core performance calculation is shown in FIG. 5. The heat coefficient of a fuel rod represents the relative heat value of each fuel rod in the first assumed fuel assembly when the average heat value of the fuel rods in the first assumed fuel assembly is set to 1.

[0075] In FIG. 5, "1, 2, 3, ..., 13" written horizontally are column numbers of the fuel rod arrangement in the cross section of the first assumed fuel assembly. Also, "1, 2, 3, ..., 13" written vertically are row numbers of the fuel rod arrangement in the cross section of the first assumed fuel assembly. The position of each fuel rod is expressed by (column number, row number). In FIG. 5, each position of (1, 1), (1, 13), (13, 1), and (13, 13) in the cross section of the first assumed fuel assembly indicates the positions of four corners in the outermost region of the fuel rod arrangement in the cross section. Since the support rods 12 are arranged at these corner positions, the heat generation coefficients at these positions are "0.00". Note that the corner portion of the cross section of the first assumed fuel assembly where the support rod 12 arranged at the position (1, 1) exists faces the control rod 34 inserted in the water gap region 39.

[0076] In the cross section of the first assumed fuel assembly, the fuel rods arranged in the cross section are arranged in point symmetry around the central axis of the fuel assembly (located at the position (7,7)).

[0077] According to the heat generation coefficients of the fuel rods shown in Fig. 5, the heat generation coefficient of the fuel rod 11 arranged at the position (3,3) is "1.43", which is the largest in the cross section of the first assumed fuel assembly. The heat generation coefficients of the fuel rods 11 arranged at the positions (3,11), (11,3) and (11,11) are "1.42", which is almost the same as the heat generation coefficient of the fuel rod 11 arranged at the position (3,3). The fuel rods 11 arranged at the positions (3,3), (3,11), (11,3) and (11,11) are arranged at the corners of the third row from the inner surface of the channel box 17 in the fuel rod arrangement in the cross section of the fuel assembly.

[0078] According to FIG. 5, the fuel rods 11 with the next highest heat coefficients after the fuel rods 11 arranged at the positions (3,3), (3,11), (11,3), and (11,11) are the fuel rods 11 arranged at the positions (2,5), (2,9), (5,2), (5,12), (9,2), (9,12), (12,5), and (12,9). The heat coefficients of the fuel rods 11 arranged at the positions (2,5) and (5,2) are "1.40". The heat coefficients of the fuel rods 11 arranged at the positions (2,9), (5,12), (9,2), (9,12), (12,5), and (12,9) are "1.39", which is almost the same as the heat coefficients of the fuel rods 11 arranged at the positions (2,5) and (5,2). The fuel rods 11 arranged at each of the positions (2,5), (2,9), (5,2), (5,12), (9,2), (9,12), (12,5) and (12,9) are arranged third from the fuel rods 11 arranged at each corner of the second row in each of two perpendicular directions along the inner surface of the channel box 17, starting from the fuel rods 11 arranged at each corner of the second row from the inner surface of the channel box 17 in the fuel rod arrangement in the cross-section of the fuel assembly.

[0079] Taking into consideration this distribution of heat generation coefficients, there is a possibility that liquid film dryout may occur in fuel rods 11 (full-length fuel rods) located at the above positions (3,3), (3,11), (11,3), (11,11), (2,5), (2,9), (5,2), (5,12), (9,2), (9,12), (12,5), and (12,9).

[0080] The inventors came up with the idea of ​​arranging partial length fuel rods at each of the fuel rod positions (3,3), (3,11), (11,3), (11,11), (2,5), (2,9), (5,2), (5,12), (9,2), (9,12), (12,5) and (12,9) where liquid film dryout may occur, in order to suppress the occurrence of liquid film dryout. As described above, by arranging partial length fuel rods at each of the positions (3,3), (3,11), ..., (12,5) and (12,9) where liquid film dryout may occur, it is possible to avoid the placement of nuclear fuel material near the upper end of the active fuel length of the fuel assembly 13 where liquid film dryout is expected to occur, and to suppress the occurrence of liquid film dryout. The active fuel length of the fuel assembly 13 is the same as the active fuel length of the full length fuel rods in the fuel assembly 13.

[0081] 1, in the fuel rod arrangement in the cross section of the fuel assembly 13, the fuel rods 5 are arranged at the positions (3,3), (3,11), (11,3), and (11,11). In addition, the fuel rods 7, which are longer than the fuel rods 5, are arranged at the positions (2,5), (2,9), (5,2), (5,12), (9,2), (9,12), (12,5), and (12,9). Since the heat coefficient of the position where the fuel rods 7 are arranged is smaller than the heat coefficient of the position where the fuel rods 5 are arranged, the effective fuel length of the fuel rods 7 can be made longer than that of the fuel rods 5.

[0082] In other words, liquid film dryout may occur at the upper end of the active fuel length on the outer surface of the cladding tube of the fuel rod 11 (long fuel rod) arranged at each of the positions (3,3), (3,11), (11,3), and (11,11) shown in FIG. 5. Liquid film dryout occurs at a point on the outer surface of the cladding tube. The liquid film dryout occurrence region where such liquid film dryout may occur extends downward from the upper end of the active fuel length in the axial direction of the fuel rod 11. The length of the liquid film dryout occurrence region in the axial direction from the upper end of the active fuel length downward is considered to correspond to the heat coefficient of the fuel rod 11 arranged at the above position. The larger the heat coefficient, the longer the length of the liquid film dryout occurrence region. In order to suppress the occurrence of liquid film dryout, the fuel rods 11 arranged at the positions (3,3), (3,11), (11,3), and (11,11) may be fuel rods 5 (partial length fuel rods) having an active fuel length shorter than the active fuel length of the fuel rods 6 by the length of the liquid film dryout occurrence region. Also, the active fuel length of the fuel rods 7 arranged at the positions (2,5), (2,9), (5,2), (5,12), (9,2), (9,12), (12,5), and (12,9), which are longer than the fuel rods 5, is longer than the active fuel length of the fuel rods 5 by the amount that the heat generation coefficient at the positions where the fuel rods 7 are arranged is smaller than the heat generation coefficient at the positions (3,3), (3,11), (11,3), and (11,11).

[0083] That is, taking into consideration the heat generation coefficients at each of the positions (3,3), (3,11), (11,3), and (11,11), and the heat generation coefficients at each of the positions (2,5), (2,9), (5,2), (5,12), (9,2), (9,12), (12,5), and (12,9), in order to determine the effective fuel length L2 of the partial length fuel rods located at each of the positions (3,3), (3,11), (11,3), and (11,11), and the effective fuel length L1 of the partial length fuel rods located at each of the positions (2,5), (2,9), (5,2), (5,12), (9,2), (9,12), (12,5), and (12,9), the calculation of equation (1) is performed several times as described above, and the effective fuel lengths L1 and L2 are determined so as to be maximized while the MCPR satisfies the reference value (design value).

[0084] In addition, the length of the liquid film dryout occurrence region in the axial direction on the outer surface of the cladding tube of the fuel rods 11 (full-length fuel rods) arranged at each of the positions (2,5), (2,9), (5,2), (5,12), (9,2), (9,12), (12,5), and (12,9) is considered to correspond to the heat coefficient of the fuel rods 11 arranged at those positions. Since the heat coefficient of the fuel rods 11 arranged at the positions (2,5) etc. is smaller than that of the fuel rods 11 arranged at the positions (3,3) etc., the length of the liquid film dryout occurrence region in the axial direction of the fuel rods 11 arranged at the positions (2,5) etc. is shorter than that of the fuel rods 11 arranged at the positions (3,3) etc. In order to suppress the occurrence of liquid film dryout, the fuel rods 11 arranged at the positions (2,5) etc. may be fuel rods 7 (partial-length fuel rods) having an effective fuel length shorter than the effective fuel length of the fuel rods 6 by the length of the liquid film dryout occurrence region and longer than the effective fuel length of the fuel rods 5.

[0085] According to this embodiment, the partial length fuel rods are arranged in the second and third rows from the inner surface of the channel box in the fuel rod array, and therefore it is possible to suppress the occurrence of liquid film dryout in the fuel assembly 13. By suppressing the occurrence of liquid film dryout in the fuel assembly 13, it is possible to improve the thermal margin of the fuel assembly, and it is possible to increase the thermal output during operation of the nuclear power plant 20.

[0086] In particular, fuel rods 5 which are partial length fuel rods are arranged at each corner of the third row from the inner surface of the channel box 17 in the fuel rod arrangement in the cross section of the fuel assembly 13, and fuel rods 7 which are partial length fuel rods are arranged third from fuel rods 1 arranged at each corner of the second row in each of two orthogonal directions along the inner surface of the channel box 17, starting from fuel rods 1 arranged at each corner of the second row from the inner surface of the channel box 17 in the fuel rod arrangement in the cross section of the fuel assembly 13. By arranging the fuel rods 5 and 7 in this way, the occurrence of liquid film dryout in the fuel assembly 13 can be further suppressed.

[0087] The arrangement of fuel rods 5, which have a shorter effective fuel length than fuel rods 7, at each corner of the third row from the inner surface of the channel box 17 in the fuel rod arrangement in the cross-section of the fuel assembly 13 can contribute to further suppressing the occurrence of liquid film dryout in the fuel assembly 13.

[0088] In each of two orthogonal directions along the inner surface of the channel box 17, starting from the fuel rods 11 arranged at the corners of the second row from the inner surface of the channel box 17 in the cross section of the fuel assembly 13, the active fuel length of the fuel rod 7 arranged at the third position in the second row of the fuel rod array from the fuel rods 11 arranged at the corners of the second row is longer than the active fuel length of the fuel rods 5 arranged at the corners of the third row from the inner surface of the channel box 17 in the fuel rod array, so that the reduction in the amount of nuclear fuel material loaded in the fuel assembly can be suppressed. By suppressing the reduction in the amount of nuclear fuel material loaded, that is, by increasing the amount of Pu loaded, the volume of spent nuclear fuel is ultimately reduced.

[0089] In the case where liquid film dryout does not occur in the fuel rods 5 arranged in each corner of the third row (e.g., (3,3)) from the inner surface of the channel box 17 in the fuel rod arrangement in the cross section of the fuel assembly 13, and in the fuel rods 7 arranged from the fuel rods 1 arranged in each corner of the second row from the inner surface of the channel box 17 in each of two orthogonal directions along the inner surface of the channel box 17, starting from the fuel rods 1 arranged in each corner of the second row from the inner surface of the channel box 17, it is presumed that liquid film dryout will occur in, for example, the fuel rods 3 and 4 arranged in the first (3,2) and second (4,2) fuel rods from the fuel rods 1 arranged in each corner of the second row, which has the second highest heat coefficient after the fuel rod 7, in each of two orthogonal directions along the inner surface of the channel box 17, starting from the fuel rods 1 arranged in each corner of the second row from the inner surface of the channel box 17.

[0090] That is, in each of the two orthogonal directions along the inner surface of the channel box 17, starting from the fuel rod 1 arranged at each corner of the second row from the inner surface of the channel box 17, the fuel rods 5 arranged at each corner of the third row from the inner surface of the channel box 17, and the fuel rods 1 arranged at each corner of the second row from the inner surface of the channel box 17, even if the effective fuel length of each of the fuel rods 1 arranged at the corners of the second row to the fuel rods 7 arranged at the third row is shorter than the length at which the occurrence of liquid film dryout can be avoided, liquid film dryout occurs in each of the fuel rods 3 and 4 arranged at the first and second rows from the fuel rods 1 arranged at each corner of the second row, so it does not contribute to improving the thermal margin. Furthermore, the shorter the effective fuel length of the partial length fuel rods, the more the volume of the nuclear fuel material in the fuel assembly decreases, and there is a concern that the amount of fuel loaded in the fuel assembly will decrease. For this reason, it is desirable to secure the effective fuel length of the partial length fuel rods as long as possible if liquid film dryout can be avoided in the corresponding fuel rods. In this embodiment, as the fuel rods arranged in the corners ((3, 3) etc.) of the third row from the inner surface of the channel box 17 in the cross section of the fuel assembly 13, which have the largest heat generation coefficient and the largest thermal load, partial length fuel rods with a short effective fuel length were used. In addition, in each of the two orthogonal directions along the inner surface of the channel box 17, starting from the fuel rods arranged in the corners of the second row from the inner surface of the channel box 17 in the above fuel rod arrangement, the third fuel rod from the fuel rods arranged in the corners of the second row, that is, the fuel rods arranged in the corners of the third row, have a lower heat generation coefficient and a smaller thermal load than the fuel rods arranged in the corners of the third row, and partial length fuel rods with a longer effective fuel length than the partial length fuel rods arranged in the corners of the third row were used. As a result, the effective fuel length of the partial length fuel rods can be maximized while avoiding liquid film dryout, and the reduction in the fuel loading amount of the fuel assembly due to the use of partial length fuel rods can be minimized.

[0091] In this embodiment, the support rods not containing nuclear fuel material arranged at the four corners of the outermost region of the fuel rod array reduce the power of the corners of the fuel assembly 13, and thus contribute to flattening the power distribution in the cross section of the fuel assembly 13. That is, the cross-sectional area of ​​the water gap region 39 formed between the corners of the four fuel assemblies 13 facing each other is larger than the cross-sectional area of ​​the water gap region 39 formed between the side walls of the channel boxes 17 facing each other in the adjacent fuel assemblies 13, and more thermal neutrons are generated in the water gap region 39 formed between the corners of the four fuel assemblies 13. For this reason, the power of the corners of the fuel assembly 13 in the cross section tends to increase, but the arrangement of the support rods at the four corners of the outermost region of the fuel rod array reduces the power of the corners of the fuel assembly 13, and flattens the power distribution in the cross section of the fuel assembly 13.

[0092] At the four corners in the cross section of the fuel assembly 13, the enrichment of fissile Pu decreases from the fuel rods 5 arranged at the corners in the third row from the inner surface of the channel box 17 in the fuel rod array toward the outermost region. This decrease in the enrichment of fissile Pu from the fuel rods 5 arranged at the corners in the third row from the inner surface of the channel box 17 toward the outermost region in the fuel rod array at each corner of the cross section of the fuel assembly 13 results in a further flattening of the power distribution in the cross section of the fuel assembly 13.

[0093] The placement of the burnable poison in the outermost region of the fuel rods 8 can further increase the value of the burnable poison in the fuel rods 8 . EXAMPLES

[0094] A fuel assembly according to a second embodiment of the present invention, which is used in a reduced moderation spectrum boiling water nuclear power plant, will be described with reference to FIGS. 3, 4, 6 and 7. FIG.

[0095] The reactor 21 of the reduced moderation spectrum boiling water nuclear power plant 20 in which the fuel assemblies of this embodiment are loaded into the reactor core is an ABWR.

[0096] The arrangement of the fuel rods 11 in the channel box 17 in the cross section of the fuel assembly 13A of this embodiment will be specifically described with reference to FIG. 6. The fuel rods 11 in the fuel assembly 13A include fuel rods 1, 2, 5, 6, 7, 8, 9, and 10. The fuel rods 1, 2, 5, 6, 7, and 8 arranged in the fuel assembly 13A are the same as the fuel rods 1, 2, 5, 6, 7, and 8 arranged in the fuel assembly 13 of the first embodiment. That is, the enrichment and number of fissile Pu in the fuel rods 1, 2, 5, 6, and 7 arranged in the fuel assembly 13A are the same as the enrichment and number of fissile Pu in the fuel rods 1, 2, 5, 6, and 7 arranged in the fuel assembly 13 of the first embodiment. The fuel rods 8 arranged in the fuel assembly 13A are filled with nuclear fuel material, such as depleted uranium, and contain, for example, gadolinium (Gd) as a burnable poison. The gadolinium concentration and the number of the fuel rods 8 are the same as those of the fuel rods 8 arranged in the fuel assembly 13 of Example 1. The fuel assembly 13A of this example also has the fuel rods 11 arranged in a square lattice with 13 rows and 13 columns.

[0097] The arrangement of the fuel rods 1, 2, 5, 6, 7, and 8 in the fuel assembly 13A is the same as the arrangement of the fuel rods 1, 2, 5, 6, 7, and 8 in the fuel assembly 13 in the first embodiment. The arrangement of the fuel rods 9 and 10 used in the fuel assembly 13A will be described. Each of the fuel rods 9 and 10 is arranged in the second row from the inner surface of the channel box 17 in the fuel rod arrangement in the cross section of the fuel assembly 13. In the corner portion facing the control rod 34, in the second row from the inner surface of the channel box 17, in each of the two orthogonal directions with the fuel rod 1 as the base point, the fuel rod 9 is arranged adjacent to the fuel rod 1 arranged at the corner of the second row. In the other three corner portions other than the corner portion facing the control rod 34, the fuel rods 9 are arranged in the same manner as in the corner portion facing the control rod 34. At the corners facing the control rods 34, the fuel rods 10 are arranged adjacent to the fuel rods 9 arranged in the second row in each of the two orthogonal directions in the second row. At the other three corners other than the corner facing the control rods 34, the fuel rods 4 are arranged in the same manner as at the corners facing the control rods 34.

[0098] Each of the fuel rods 9 and 10 is a partial length fuel rod. The effective fuel length of each of the fuel rods 9 and 10 is the same as that of the fuel rod 7, and is 9 / 10 of the effective fuel length of each of the fuel rods 1 to 4, 6 and 8. The enrichment of fissile Pu in the fuel rod 9 is 6.2 t%, and the enrichment of fissile Pu in the fuel rod 10 is 8.8 t%. Furthermore, the fuel assembly 13A includes eight fuel rods 9 and eight fuel rods 10.

[0099] The inventors assumed a fuel assembly having a configuration in which all of the fuel rods 5, 7, 9, and 10, which are part-length fuel rods in the fuel assembly 13A of this embodiment shown in FIG. 6, are replaced with full-length fuel rods. This assumed fuel assembly is hereinafter referred to as a second assumed fuel assembly. The inventors calculated the heat coefficient of each fuel rod in the cross section of the second assumed fuel assembly by core performance calculation. The calculated heat coefficient of each fuel rod in the cross section of such a second assumed fuel assembly is shown in FIG. 8.

[0100] In the cross section of the second assumed fuel assembly, the fuel rods arranged in the cross section are arranged in point symmetry around the central axis of the fuel assembly (located at the position (7,7)).

[0101] According to the heat coefficient of the fuel rods shown in FIG. 8, the heat coefficient of the fuel rod 11 arranged at the position (3,3) is "1.40", which is the largest in the cross section of the second assumed fuel assembly. The heat coefficient of the fuel rods 11 arranged at each of the positions (3,11), (11,3) and (11,11) is "1.38", which is almost the same as the heat coefficient of the fuel rod 11 arranged at the position (3,3). The fuel rods 11 arranged at each of the positions (3,3), (3,11), (11,3) and (11,11) are arranged at each corner of the third row from the inner surface of the channel box 17 in the fuel rod arrangement in the cross section of the fuel assembly. The fuel rods 5, which are part-length fuel rods (the fuel active length is a full-length fuel rod, for example, 2 / 3 of the fuel active length of the fuel rod 6), are arranged at each corner of the third row of the fuel assembly 13A.

[0102] According to Fig. 8, the heat coefficient of the fuel rods 11 arranged at each of the positions (2,5), (2,9), (5,2), (5,12), (9,2), (9,12), (12,5), and (12,9) is "1.38" or "1.37". The fuel rods 11 arranged at each of the positions (2,5), (2,9), (5,2), (5,12), (9,2), (9,12), (12,5), and (12,9) are arranged third from the fuel rods 11 arranged at each corner of the second row in each of two orthogonal directions along the inner surface of the channel box 17, starting from the fuel rods 11 arranged at each corner of the second row from the inner surface of the channel box 17 in the fuel rod arrangement in the cross section of the fuel assembly.

[0103] According to Fig. 8, the heat coefficient of the fuel rods 11 arranged at each of the positions (2,4), (2,10), (4,2), (4,12), (10,2), (10,12), (12,4), and (12,10) is "1.38" or "1.39". The fuel rods 11 arranged at each of the positions (2,4), (2,10), (4,2), (4,12), (10,2), (10,12), (12,4), and (12,10) are arranged second from the fuel rods 11 arranged at each corner of the second row in each of two orthogonal directions along the inner surface of the channel box 17, starting from the fuel rods 11 arranged at each corner of the second row of the above fuel rod array.

[0104] 8, the heat coefficient of the fuel rods 11 arranged at each of the positions (2,3), (2,11), (3,2), (3,12), (11,2), (11,12), (12,3), and (12,11) is "1.36" or "1.37". The fuel rods 11 arranged at each of the positions (2,3), (2,11), (3,2), (3,12), (11,2), (11,12), (12,3), and (12,11) are arranged one rod away from the fuel rods 11 arranged at each corner of the second row in each of two orthogonal directions along the inner surface of the channel box 17, starting from the fuel rods 11 arranged at each corner of the second row of the above fuel rod array.

[0105] In the cross section of the fuel assembly 13A, in two orthogonal directions along the inner surface of the channel box 17, starting from the fuel rod 1 arranged at each corner of the second row from the inner surface of the channel box 17, the fuel rod 7 arranged at the third position from the fuel rod 1 arranged at each corner of the second row, the fuel rod 10 arranged at the second position from the fuel rod 1 arranged at each corner of the second row, and the fuel rod 9 arranged at the first position from the fuel rod 1 arranged at each corner of the second row are all part-length fuel rods. The active fuel length of each of the fuel rods 7, 9, and 10 is a full-length fuel rod, for example, 9 / 10 of the active fuel length of the fuel rod 6.

[0106] The configuration of the second hypothetical fuel assembly 13A, which has fuel rods 1, 2, 6, and 8 that are full-length fuel rods and fuel rods 5, 7, 9, and 10 that are part-length fuel rods, is the same as the configuration of the first hypothetical fuel assembly 13 used in Example 1, which has fuel rods 1, 2, 3, 4, 6, and 8 that are full-length fuel rods and fuel rods 5 and 7 that are part-length fuel rods. This is because the enrichment of fuel rods 1, 2, and 6 and the number of fuel rods 1, 2, and 6 in the second hypothetical fuel assembly are the same as the enrichment of fuel rods 1, 2, and 6 and the number of fuel rods 1, 2, and 6 in the first hypothetical fuel assembly. The gadolinium concentration of fuel rod 8 and the number of fuel rods 8 in the second hypothetical fuel assembly are the same as the gadolinium concentration of fuel rod 8 and the number of fuel rods 8 in the first hypothetical fuel assembly. The enrichment of each full-length fuel rod corresponding to fuel rods 5, 7, 9 and 10 in the second hypothetical fuel assembly and the number of each of these full-length fuel rods are the same as the enrichment of each full-length fuel rod corresponding to fuel rods 5 and 7, respectively, and full-length fuel rods 3 and 4 in the first hypothetical fuel assembly, and the number of each full-length fuel rod corresponding to fuel rods 5 and 7, respectively, and full-length fuel rods 3 and 4.

[0107] Thus, although the configuration of the second assumed fuel assembly is the same as that of the first assumed fuel assembly, the values ​​of the heat generation coefficients at the positions of the fuel rods shown in Fig. 8 obtained for the second assumed fuel assembly are generally smaller than the values ​​of the heat generation coefficients at the positions of the fuel rods shown in Fig. 5 obtained for the first assumed fuel assembly. The reason why the values ​​of the heat generation coefficients at the positions of the fuel rods in the second assumed fuel assembly are smaller than the values ​​of the heat generation coefficients at the positions of the fuel rods in the first assumed fuel assembly will be explained below.

[0108] The duration of one operation cycle in the reduced-moderation spectrum boiling water nuclear power plant in which a plurality of second assumed fuel assemblies are loaded in the core is different from the duration of one operation cycle in the reduced-moderation spectrum boiling water nuclear power plant in which a plurality of first assumed fuel assemblies are loaded in the core. Specifically, the duration of one operation cycle in the reduced-moderation spectrum boiling water nuclear power plant using the second assumed fuel assemblies is longer than that in the reduced-moderation spectrum boiling water nuclear power plant using the first assumed fuel assemblies. The burnup of the second assumed fuel assemblies loaded in the core of the reduced-moderation spectrum boiling water nuclear power plant is also different from the burnup of the first assumed fuel assemblies loaded in the core of the reduced-moderation spectrum boiling water nuclear power plant. Specifically, the burnup of the second assumed fuel assemblies loaded in the core of the reduced-moderation spectrum boiling water nuclear power plant is greater than the burnup of the first assumed fuel assemblies loaded in the core of the reduced-moderation spectrum boiling water nuclear power plant.

[0109] As described above, the duration and burnup of one operation cycle when the second assumed fuel assembly is used are different from those when the first assumed fuel assembly is used, and the power of the full-length fuel rods in the second assumed fuel assembly is different from that of the full-length fuel rods in the first assumed fuel assembly. As a result, the values ​​of the heat generation coefficients at the arrangement positions of the fuel rods shown in Figure 8 for the second assumed fuel assembly are generally smaller than the values ​​of the heat generation coefficients at the arrangement positions of the fuel rods shown in Figure 5 for the first assumed fuel assembly.

[0110] In the fuel assembly 13A, the lower ends of the fuel rods 1, 2, 6, and 8 are supported by the lower tie plate 14 and the upper ends are supported by the upper tie plate 15. The active fuel lengths of the fuel rods 5, 7, 9, and 8 are the same. The lower ends of the fuel rods 5, 7, 9, and 10 are supported by the lower tie plate 14, but the upper ends are not supported by the upper tie plate 15. The active fuel lengths of the fuel rods 5, 7, 9, and 10 are shorter than the active fuel lengths of the fuel rods 1, 2, 6, and 8, and the active fuel lengths of the fuel rods 5, 7, 9, and 10 are part-length rods. The active fuel lengths of the fuel rods 5, 7, 9, and 10 are longer than half the active fuel lengths of the fuel rods 1, 2, 6, and 8, and the active fuel lengths of the fuel rods 7, 9, and 10 are longer than the active fuel length of the fuel rod 5. The active fuel length of fuel rod 5 is 2 / 3 of the active fuel length of each of fuel rods 1 to 4, 6 and 8, and the active fuel length of each of fuel rods 7, 9 and 10 is 9 / 10 of the active fuel length of each of fuel rods 1 to 4, 6 and 8.

[0111] The inventors evaluated the MCPR in the fuel assembly 13A of this embodiment using a cooling water flow evaluation method. The evaluation result of the MCPR is shown in FIG. 9. In the evaluation result shown in FIG. 9, the first case in which a fuel assembly having a part-length fuel rod is loaded in the core is shown by a solid line, and the second case in which a fuel assembly having a part-length fuel rod is loaded in the core is shown by a broken line. In the first case, the fuel assembly loaded in the core is the fuel assembly 13A of the second embodiment. In the second case, the fuel assembly loaded in the core has a configuration in which the part-length fuel rod in the fuel assembly 13A is replaced with a full-length fuel rod. The horizontal axis of FIG. 9 indicates the core flow rate (%), and the core flow rate during rated operation of a reactor with a reactor power of 100% is set to 100%. The MCPR was evaluated by changing the core flow rate in the range of 80% to 120%. The vertical axis of FIG. 9 indicates the MCPR obtained by changing the core flow rate.

[0112] The MCPR in the first case, where fuel assemblies with partial-length fuel rods are loaded into the core, is higher than the MCPR in the second case, where fuel assemblies without partial-length fuel rods are loaded into the core. The MCPR increases with fuel assemblies containing partial-length fuel rods. The MCPR in the first case is about 5% higher than that in the second case. To improve the MCPR, it is better to load fuel assemblies containing partial-length fuel rods into the core.

[0113] This embodiment can obtain the effects of the embodiment 1. Furthermore, in this embodiment, in the fuel rod arrangement in the cross section of the fuel assembly 13A, starting from the fuel rod 1 arranged at each corner of the second row from the inner surface of the channel box 17, the fuel rod 10 which is a partial length fuel rod is arranged at the first fuel rod from the fuel rod 1 arranged at each corner of the second row in each of two orthogonal directions along the inner surface of the channel box 17, and in the fuel rod arrangement, starting from the fuel rod 1 arranged at each corner of the second row from the inner surface of the channel box 17, the fuel rod 9 which is a partial length fuel rod is arranged at the first fuel rod from the fuel rod 1 arranged at each corner of the second row in each of two orthogonal directions along the inner surface of the channel box 17, starting ... the two orthogonal directions along the inner surface of the channel box 17, so that the occurrence of liquid film dryout in the fuel assembly 13A can be further suppressed than in the fuel assembly 13 of the embodiment 1.

[0114] In the fuel assembly 13A of this embodiment, in addition to the fuel rods 5 and 7 which are part-length fuel rods, the fuel rods 9 and 10 which are part-length fuel rods are arranged, so that the amount of loaded nuclear fuel material is reduced compared to the fuel assembly 13 of Example 1. However, since the effective fuel length of each of the fuel rods 9 and 10 is longer than that of the fuel rod 5, the degree of reduction in the amount of loaded nuclear fuel material in the fuel assembly 13A is small. EXAMPLES

[0115] A fuel assembly according to a third embodiment, which is another preferred embodiment of the present invention and is used in a reduced moderation spectrum boiling water nuclear power plant, will be described with reference to Figs. 3, 4, 10 and 11.

[0116] The reactor 21 of the reduced moderation spectrum boiling water nuclear power plant 20 in which the fuel assemblies of this embodiment are loaded into the reactor core is an ABWR.

[0117] In the fuel assembly 13B of this embodiment, the fuel rods 11B are also arranged in a square lattice with 13 rows and 13 columns. The arrangement of the fuel rods 11B in the fuel assembly 13B is the same as the arrangement of the fuel rods 11 in the fuel assembly 13A of the second embodiment. In the fuel assembly 13B, the fuel rods 5, 7, 9, and 10, which are part-length fuel rods used in the fuel assembly 13A of the second embodiment, are arranged in the same positions as in the fuel assembly 13A. As shown in FIG. 11, the fuel rods 11B are configured by filling a cladding tube 41 with a plurality of fuel pellets 40 made of nuclear fuel material.

[0118] In the fuel assembly 13A of Example 2, four fuel rods 5 which are partial length fuel rods with an effective fuel length of 2 / 3 and a total of 24 fuel rods 5, 7, 9 and 10 which are partial length fuel rods with an effective fuel length of 9 / 10 are used, and thus the amount of nuclear fuel material loaded is reduced by 2.3% compared to the fuel assembly 13 of Example 1. In a reduced moderation spectrum boiling water reactor, it is desirable to increase the amount of nuclear fuel material loaded in the fuel assembly.

[0119] Therefore, in this embodiment, in order to compensate for the reduction in the amount of loaded nuclear fuel material in the fuel assembly 13B due to the increase in the number of part-length fuel rods (fuel rods 5, 7, 9, and 10) in the fuel assembly 13B, the diameter of the fuel rods 11B is set to 8.1 mm, which is larger than the diameter of the fuel rods 11 in the fuel assembly 13A of embodiment 2, which is 8.0 mm. This makes it possible to compensate for the reduction in the amount of loaded fissile material in the fuel assembly due to the increase in the number of part-length fuel rods by increasing the amount of loaded nuclear fuel material due to the increase in the diameter of the fuel rods.

[0120] The increased diameter d of the fuel rods to compensate for the reduced fissile material loading in the fuel assemblies w Since depends on the effective fuel length of the part-length fuel rods arranged in the fuel assembly, it is sufficient to set it so as to satisfy the relationship of the following equation (2).

[0121] π×{(d w -2×δ w ) / 2} 2 ×Ls w -π×{(d w / o -2×δ w / o ) / 2} 2 ×Ls w / o >0 …(2) Here, Ls w is the total effective fuel length of all fuel rods arranged in a fuel assembly with partial length rods, d w is the diameter of the fuel rods arranged in a fuel bundle having part-length rods, δ w is the thickness of the cladding of the fuel rods in the fuel assembly with part-length rods, Ls w / o is the sum of the effective fuel lengths of all fuel rods in a fuel assembly that does not have a partial length rod, d w / o is the diameter of the fuel rods in the fuel assembly that does not have part-length rods, δ w / o is the cladding thickness of a fuel rod located in a fuel assembly that does not have a part-length rod.

[0122] The diameter d of the fuel rods arranged in the fuel assembly having the part-length rods is set so as to satisfy the formula (2). w is taken to be the diameter d of a fuel rod disposed in a fuel assembly that does not have a part-length rod, excluding the thickness of the cladding tube 41. w / o By setting the fuel assembly 13B having part-length fuel rods to be greater than the nuclear fuel material loading amount in a fuel assembly not having part-length fuel rods, the nuclear fuel material loading amount in the fuel assembly 13B having part-length fuel rods can be increased compared to the nuclear fuel material loading amount in a fuel assembly not having part-length fuel rods.

[0123] According to this embodiment, it is possible to obtain the respective effects obtained in the embodiment 2. Furthermore, in this embodiment, the diameter of the fuel rods 11B arranged in the fuel assembly 13B having the part-length fuel rods is set so as to satisfy the formula (2), so that the loading amount of the nuclear fuel material in the fuel assembly 13B having the part-length fuel rods can be increased compared to the loading amount of the nuclear fuel material in the fuel assembly not having the part-length fuel rods. [Explanation of symbols]

[0124] 1,2,3,4,6,8...fuel rod (full length fuel rod), 5.7.9.10...fuel rod (partial length fuel rod), 11,11B...fuel rod, 12...support rod, 13,13A,13B...fuel assembly, 14...lower tie plate (lower fuel support member), 15...upper tie plate (upper fuel support member), 17...channel box, 20...reduced moderation spectrum boiling water nuclear plant, 21...reactor, 22...reactor pressure vessel, 23...core, 34...control rod, 39...water gap region, 40...fuel pellets, 41...cladding tube.

Claims

1. a lower fuel support member; an upper fuel support member; a channel box which is a rectangular tube having a square cross section and whose upper end is attached to the upper fuel support member and extends toward the lower fuel support member; A fuel assembly comprising: a plurality of fuel rods packed with nuclear fuel material and arranged in a square lattice pattern in the channel box, the fuel rods include a plurality of full-length fuel rods having lower ends supported by the lower fuel support member and upper ends supported by the upper fuel support member, and a plurality of first part-length fuel rods having lower ends supported by the lower fuel support member and upper ends not supported by the upper fuel support member, and having an effective fuel length shorter than that of the full-length fuel rods, the full length fuel rods and the first part length fuel rods arranged in a cross section of the fuel assembly are arranged symmetrically about a central axis of the fuel assembly; A fuel assembly characterized in that the first part-length fuel rods are arranged in the second and third rows from the inner surface of the channel box in the fuel rod arrangement in the cross section of the fuel assembly, and are arranged at a position where the heat coefficient of the fuel rod is the largest and a position where the heat coefficient is approximately the same as the largest heat coefficient in the cross section of an assumed fuel assembly assuming that all of the fuel rods in the fuel assembly are full-length fuel rods.

2. The fuel rod is a plurality of second part length fuel rods having an effective fuel length shorter than that of the full length fuel rods and longer than that of the first part length fuel rods; the second part length fuel rods disposed in a cross section of the fuel assembly are arranged in point symmetry about a central axis of the fuel assembly; 2. The fuel assembly according to claim 1, wherein the second part-length fuel rods are arranged in the second and third rows from the inner surface of the channel box in the fuel rod arrangement in the cross section of the fuel assembly, and at positions where the heat generation coefficients of the fuel rods in the cross section of the assumed fuel assembly are the next largest heat generation coefficient and the next largest heat generation coefficient, respectively.

3. 3. The fuel assembly according to claim 2, wherein the first part-length fuel rods are arranged at the four first corners of the third row, and the second part-length fuel rods are arranged third from the full-length fuel rods arranged at the respective second corners in two perpendicular directions along the inner surface of the channel box, starting from the full-length fuel rods arranged at the four second corners of the second row.

4. 4. The fuel assembly according to claim 2 or 3, wherein the effective fuel length L1 of the second part-length fuel rod is L1, the number of the second part-length fuel rods in the fuel assembly is n1, the effective fuel length of the first part-length fuel rod is L2, the number of the first part-length fuel rods in the fuel assembly is n2, the effective fuel length of the full-length fuel rod is L, and the number of all the fuel rods included in the fuel assembly is n, the effective fuel length L1 of the second part-length fuel rod and the effective fuel length L2 of the first part-length fuel rod satisfy the relationship of the following formula (1). {(L-L1)×n1+(L-L2)×n2} / (L×n)≦0.072…(1)

5. 5. The fuel assembly of claim 4, wherein the first part length fuel rods have an active fuel length that is 2 / 3 of the active fuel length of the full length fuel rods, and the second part length fuel rods have an active fuel length that is 9 / 10 of the active fuel length of the full length fuel rods.

6. 4. The fuel assembly according to claim 3, wherein the second part-length fuel rods are arranged not only as third fuel rods from the full-length fuel rods arranged at the second corners in each of two orthogonal directions along an inner surface of a channel box, the full-length fuel rods being taken as a starting point for each of the four second corners of the second row, but also as first and second fuel rods from the full-length fuel rods arranged at the second corners in each of the two orthogonal directions, the full-length fuel rods being taken as a starting point for each of the four second corners of the second row.

7. 7. The fuel assembly of claim 6, wherein said first part length fuel rod is disposed adjacent a portion of said second plurality of part length fuel rods disposed in said second row.

8. the plurality of full length fuel rods includes a plurality of fuel rods containing the nuclear fuel material and no burnable poison, and a plurality of fuel rods containing the nuclear fuel material and the burnable poison; 8. The fuel assembly according to claim 1, wherein the fuel rods containing the nuclear fuel material and the burnable poison are arranged in an outermost region of a fuel rod arrangement in a cross section of the fuel assembly.

9. 8. The fuel assembly according to claim 1, wherein a support rod having a lower end supported by the lower fuel support member and an upper end supported by the upper fuel support member, holding a fuel spacer bundling the plurality of fuel rods, and containing no nuclear fuel material is disposed at each of the four corners of the outermost peripheral region of the fuel rod arrangement in a cross section of the fuel assembly.

10. 10. The fuel assembly according to claim 1 , wherein the diameter dw of the fuel rod satisfies the following formula (2): Lsw is a sum of the effective fuel lengths of all the fuel rods arranged in the fuel assembly having the partial-length fuel rods, dw is a diameter of the fuel rod arranged in the fuel assembly having the partial-length fuel rods, δw is a thickness of the cladding tube of the fuel rod arranged in the fuel assembly having the partial-length fuel rods, Lsw / o is a sum of the effective fuel lengths of all the fuel rods arranged in the fuel assembly not having the partial-length fuel rods, dw / o is a diameter of the fuel rod arranged in the fuel assembly not having the partial-length fuel rods, and δw / o is a thickness of the cladding tube of the fuel rod arranged in the fuel assembly not having the partial-length fuel rods. π×{(d ｗ -2×δ ｗ ) / 2} ２ ×Ls ｗ -π×{(d ｗ／ｏ -2×δ ｗ／ｏ ) / 2} ２ ×Ls ｗ／ｏ >0…(2)

11. 11. The fuel assembly according to claim 1, wherein the plurality of fuel rods in the fuel rod array in the channel box are arranged in 13 rows and 13 columns.

12. A nuclear reactor core, comprising a plurality of fuel assemblies according to any one of claims 1 to 11 loaded therein.

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