JP7168528B2 - fuel assembly - Google Patents

Description

The present invention relates to fuel assemblies, and more particularly to fuel assemblies suitable for use in boiling water reactors.

A core of a boiling water reactor is loaded with a plurality of fuel assemblies. The fuel assembly contains a plurality of fuel pellets containing nuclear fuel material (e.g., uranium oxide), a plurality of sealed fuel rods, an upper tie plate supporting the upper ends of the fuel rods, and supporting the lower ends of the fuel rods. It has a lower tie plate, a plurality of fuel spacers for maintaining the spacing between the fuel rods, water rods, and a rectangular tubular channel box with a square cross section. The channel box has an upper end attached to the upper tie plate and extends toward the lower tie plate and surrounds a plurality of fuel rods bounded by a plurality of axially disposed fuel spacers. A water rod is arranged in the central portion of the cross section of the fuel assembly, and a plurality of fuel rods are arranged around the water rod. The lower ends of the water rods are supported by the lower tie plate and the upper ends of the water rods are supported by the upper tie plate. Each fuel rod has a cladding tube, the lower end of which is closed with a lower end plug and the upper end of the cladding tube is closed with an upper end plug, and a plurality of fuel pellets containing nuclear fuel material are coated. It is configured by filling in a tube. Within the cladding, a gas plenum is formed above the region filled with those fuel pellets.

Mixed oxide fuel (MOX fuel) of plutonium (Pu) and uranium obtained by reprocessing uranium fuel contained in spent fuel assemblies removed from nuclear reactors is used as a nuclear fuel material. A fuel assembly containing MOX fuel, which is a nuclear fuel material, is called a MOX fuel assembly. From the viewpoint of effective use of nuclear resources, plu-thermal power generation is implemented in which MOX fuel assemblies are loaded into the core of a nuclear reactor to generate power.

The use of highly enriched MOX fuel assemblies that increase the plutonium content of the MOX fuel assemblies over conventional fuel assemblies is under consideration. At the early stage of combustion of highly enriched MOX fuel assemblies, the fuel assemblies tend to absorb thermal neutrons, so the neutron spectrum in the core becomes hard and it becomes difficult to suppress the reactivity even if control rods are inserted. . Therefore, in a nuclear reactor using highly enriched MOX fuel assemblies, the control rod worth, which indicates the reactivity suppression capability of the control rods, decreases.

An example of technology for improving the control rod value by improving such problems is described in JP-A-10-311889 and JP-A-2004-361296.

JP-A-10-311889 describes a fuel assembly for use in a boiling water nuclear power plant. This fuel assembly has a plurality of fuel rods in a square arrangement, and the upper and lower ends of the fuel rods supported by the upper tie plate and the lower tie plate (hereinafter referred to as full length fuel rods) are longer than the effective fuel length of the fuel rods. Two fuel rods having a short active fuel length (hereinafter referred to as partial length fuel rods) are arranged on each side of the outermost peripheral region of the square array of fuel rods. Only full-length fuel rods are arranged inside the outermost peripheral region. The fuel rods adjacent to the outermost part length fuel rods are full length fuel rods. Due to the structure that the outermost periphery of the fuel assembly adjacent to the control rod is a part-length fuel rod, fewer thermal neutrons are absorbed by the fuel rod at that part, so more neutrons are absorbed by the control rod. improves.

The fuel assembly described in Japanese Patent Application Laid-Open No. 2004-361296 has a fuel rod filled with highly enriched MOX fuel arranged in the center of the cross section, and the fuel rod contains depleted uranium oxide or 2 wt% or less. of plutonium oxide are placed facing the control rods. The fuel assembly has a channel box with a solid moderator on the inner or outer surface of the sidewalls of the channel box facing the control rods. The fuel assembly described in JP-A-2004-361296, like the fuel assembly described in JP-A-10-311889, increases the amount of neutrons absorbed by the control rod, and increases the control rod value. improve.

JP-A-10-311889 Japanese Patent Application Laid-Open No. 2004-361296

A fuel assembly in which a plurality of fuel rods are arranged in a square, which is described in Japanese Patent Application Laid-Open No. 10-311889, has two partial lengths at positions excluding corner portions in the outermost peripheral region in the cross section of the fuel assembly. The adjacent arrangement of the fuel rods reduces the number of fuel rods in the outermost region of the fuel assembly above the part length rods. As a result, the amount of thermal neutrons absorbed in the outermost peripheral region decreases, and the amount of thermal neutrons absorbed in the control rods arranged adjacent to the fuel assembly increases, so the value of the control rods increases.

However, in the fuel assembly described in Japanese Patent Application Laid-Open No. 10-311889, in which a plurality of part-length fuel rods are arranged in the outermost region, the fuel rods in the outermost region in the fuel assembly are located above the part-length fuel rods. Since the proportion of the light water coolant is greater in the portion , the amount of neutrons absorbed by the light water is greater than in the portion below the top of the part-length rod. As the burning of the nuclear fuel material in the fuel assembly progresses, i.e., nearing the end of the operation cycle, the amount of thermal neutrons absorbed by the nuclear fuel material decreases. The thermal neutron flux in the light water increases in the upper part, and the absorption of thermal neutrons by the light water in that part increases. As a result, the control rod worth at the end of the operating cycle deteriorates.

In addition, by arranging a plurality of part-length fuel rods in the outermost region, the amount of neutrons absorbed by light water is increased, so the swing, which indicates the amount of reactivity added to the fuel assembly at cold temperatures, is greatly reduced. When a large number of part-length fuel rods are arranged in the outermost region of the fuel assembly in order to significantly improve the control rod value, swing, that is, the amount of reactivity added to the fuel assembly at cold temperatures becomes large and negative. Become. However, a fuel assembly whose reactivity addition amount is large and negative may have a positive void coefficient, which is not preferable in terms of safety design.

In Japanese Patent Application Laid-Open No. 2004-361296, the control rod worth is improved as described above. In general, the absorption of thermal neutrons by the nuclear fuel material in the fuel rods is predominantly by fissionable nuclides such as U235. However, for medium-speed neutrons, there are also resonance absorptions by parent nuclides such as U238. Therefore, in Japanese Patent Application Laid-Open No. 2004-361296, in the cross section of the fuel assembly, the nuclear fission contained in the fuel rods arranged in the outermost peripheral region facing the inner surface of the side wall facing the control rods of the channel box Even if the ratio of nuclei is 0%, the influence of resonance absorption of neutrons such as parent nuclides on the control rod value cannot be ignored.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a fuel assembly capable of improving control rod worth throughout the operating cycle.

A feature of the present invention which accomplishes the objects set forth above is a lower fuel support member, an upper fuel support member, and a tubular channel box attached at an upper end to the upper fuel support member and extending toward the lower fuel support member. and a plurality of fuel rods having lower ends supported by the lower fuel support member and upper ends supported by the upper fuel support member and containing nuclear fuel material, the plurality of fuel rods bundled by a plurality of fuel spacers; A fuel assembly located inside the channel box,
the plurality of fuel rods includes a plurality of first fuel rods and a second fuel rod;
The active fuel length of the first fuel rod is the first active fuel length of the fuel assembly, and the active fuel length of the second fuel rod is the second active fuel length shorter than the first active fuel length,
The second fuel rod has a material-filled region filled with a material having a neutron absorption cross section smaller than that of light water. is formed over
The second fuel rods are arranged in the outermost peripheral region facing the inner surface of the side wall facing the control rods, among the plurality of side walls forming the channel box.

The second fuel rod has a material-filled region filled with a material having a neutron absorption cross section smaller than that of light water. The second fuel rods are arranged in the outermost peripheral region facing the inner surface of the side wall facing the control rods among the plurality of side walls constituting the channel box. Therefore, the amount of light water in the outermost peripheral region, which faces the inner surface of the side wall facing the control rod, is reduced, the amount of neutrons absorbed by the light water is reduced, and the control rod value is improved even at the end of the operation cycle. . Therefore, control rod worth can be improved throughout the operating cycle.

The above object includes a lower fuel support member, an upper fuel support member, a tubular channel box attached at its upper end to the upper fuel support member and extending toward the lower fuel support member, and a lower end at the lower fuel support member. a plurality of fuel rods supported by support members and having their upper ends supported by the upper fuel support member and containing nuclear fuel material; and a channel box attached to one corner of the upper fuel support member and attached to the upper fuel support member. a fuel assembly having a plurality of fuel rods bound by a plurality of fuel spacers disposed inside a channel box, the fuel assembly comprising:
the plurality of fuel rods includes a plurality of first fuel rods and a second fuel rod;
The active fuel length of the first fuel rod is the first active fuel length of the fuel assembly, and the active fuel length of the second fuel rod is the second active fuel length shorter than the first active fuel length,
The second fuel rod has a material-filled region filled with a material having a neutron absorption cross section smaller than that of light water. is formed over
By arranging the second fuel rods in the outermost peripheral region facing the inner surface of the side wall portion extending in one direction from the corner portion on the side of the channel fastener, among the plurality of side wall portions constituting the channel box. achievable.

The second fuel rod has a material-filled region filled with a material having a neutron absorption cross section smaller than that of light water. and the second fuel rods are formed in the outermost peripheral region facing the inner surface of the side wall portion extending in one direction from the corner portion on the channel fastener side among the plurality of side wall portions constituting the channel box , the amount of light water in the outermost peripheral region facing the inner surface of the side wall portion extending in one direction from the corner portion of the channel fastener side is reduced, the amount of neutrons absorbed by the light water is reduced, and the operation Control rod worth increases even at the end of the cycle. Therefore, control rod worth can be improved throughout the operating cycle.

According to the present invention, control rod worth can be improved throughout the operating cycle.

FIG. 5 is a sectional view taken along line II of FIG. 4, showing a cross section of the fuel assembly of Example 1 applied to an improved boiling water nuclear power plant, which is a preferred example of the present invention; FIG. 2 is an explanatory diagram showing the fissile plutonium enrichment distribution in the axial direction of each fuel rod included in the fuel assembly of FIG. 1; 2 is a longitudinal cross-sectional view of a reactor of an advanced boiling water nuclear plant loaded with the fuel assemblies shown in FIG. 1; FIG. FIG. 2 is a longitudinal sectional view of the fuel assembly shown in FIG. 1; FIG. 2 is a vertical cross-sectional view of the vicinity of the He filling region and gas plenum of the fuel rod H2 shown in FIG. 1; FIG. 6 is a vertical cross-sectional view of a fuel rod H4, which is another example of the fuel rod H2 shown in FIG. 5, near a gas filling region and a gas plenum; FIG. 2 is a plan view of a portion of the core loaded with the fuel assemblies shown in FIG. 1; FIG. 8 is a sectional view taken along line VI-VI of FIG. 7; 1 is a cross-sectional view of a Case 1 fuel assembly used in a boiling water reactor at a position above the top of a part length rod; FIG. FIG. 2 is a cross-sectional view of a Case 2 fuel assembly used in a boiling water reactor at a position above the top of the part length rods; FIG. 3 is a cross-sectional view of a Case 3 fuel assembly used in a boiling water reactor at a position above the top of the part length rods; FIG. 4 is a cross-sectional view of a Case 4 fuel assembly used in a boiling water nuclear reactor at a position above the top of the part length rods; FIG. 5 is a comparison of burnup-averaged control rod worths for fuel assemblies of Cases 1-4. FIG. 10 is a diagram showing the relationship between the control rod worth improvement rate (%) and the axial range when the fuel assembly horizontal cross section of case 4 is provided within a certain range from the axial upper end. Explanation showing the fissile plutonium enrichment distribution in the axial direction of each fuel rod contained in the fuel assembly of Example 2 applied to an advanced boiling water nuclear power plant, which is another preferred example of the present invention It is a diagram. Fig. 10 is a cross-sectional view of a fuel assembly of Example 3 applied to an improved boiling water nuclear power plant, which is another preferred example of the present invention; FIG. 17 is an explanatory diagram showing the fissile plutonium enrichment distribution in the axial direction of each fuel rod included in the fuel assembly of FIG. 16;

The inventors of the present invention have made various studies and found a new fuel assembly that can improve the control rod worth throughout the operation cycle. The results of this study and the outline of the newly found fuel assembly will be described below.

When increasing the enrichment of fissile plutonium in MOX fuel (mixed oxide fuel of uranium and plutonium) used in MOX fuel assemblies used in boiling water reactors (BWR), as described above, control The problem arises that the bar value is reduced.

A control rod used in a boiling water reactor has four blades containing neutron absorbing material extending in all directions from a central axis. This control rod has a cruciform cross section, and two blades extend from one corner of the fuel assembly channel box (square tube with a square cross section) in the core. facing the two side walls of the . The channel box is composed of four side walls.

In order to improve the value of the control rod, more neutrons generated in the fuel rods in the fuel assemblies and decelerated to thermal energy in the water gap regions between the fuel assemblies reach the control rods. Needs to be absorbed with a stick. Therefore, in the cross section of the fuel assembly, the outermost peripheral region of the side wall of the channel box facing the inner surface of the side wall facing the control rod is not arranged with a material that easily absorbs thermal neutrons. can be considered. Typical substances that easily absorb thermal neutrons include fissionable nuclides such as U235 filled in fuel rods. For this reason, in the cross section of the fuel assembly, among the four side wall portions of the channel box, the nuclear fuel filled in the fuel rods arranged in the outermost peripheral region facing the inner surface of the side wall portion facing the control rods. It is believed that reducing the fissile plutonium enrichment of the material will increase the control rod worth.

In addition, in fuel assemblies used in boiling water reactors, neutrons generated by fission of fissionable nuclides (e.g., U235, Pu239, etc.) contained in the nuclear fuel material in the fuel rods are mainly moderated in the water gap region. be done. This water gap region is formed between the fuel assemblies. Absorption of thermal neutrons by the nuclear fuel material in the fuel rods arranged in the outermost region of the fuel assembly is a factor that deteriorates the control rod worth. A parent nuclide such as U238 is present in the fuel rod, and this parent nuclide resonantly absorbs medium-speed neutrons. That is, of the four side walls of the channel box, the fissile plutonium of the nuclear fuel material filled in the fuel rods arranged in the outermost peripheral region facing the inner surface of each of the two side walls facing the control rods It is believed that control rod worth can be further improved by not only reducing the enrichment of , but also by eliminating nuclear fuel material.

In view of what has been said above, some of the nuclear fuel material in the fuel rods located in the outermost region facing the inner surface of each of the two sidewalls of the channel box facing the control rods is eliminated. By doing so, the amount of thermal neutrons absorbed by the outermost periphery is reduced, and the value of the control rod is considered to be improved. However, instead of removing some of the nuclear fuel material in the fuel rods arranged in that part, if light water of the same amount as the nuclear fuel material to be removed is provided in that part, the heat of the light water will be generated in that part of the outermost region. Neutron absorption occurs.

The amount of thermal neutrons absorbed by the nuclear fuel material in the fuel assembly decreases, and in particular, the thermal neutron flux in the light water increases in the above-mentioned portion of the outermost peripheral region in the fuel assembly, and the amount of thermal neutrons absorbed by the light water in that portion. increases. As a result, the control rod worth at the end of the operating cycle deteriorates.

In order to solve the problem, the inventors found that of the plurality of side walls (e.g., four side walls) that make up the channel box, two side walls facing the control rod face the inner surface of each, I came up with the idea of forming a gas-filled region filled with a gas having a neutron absorption cross section smaller than that of light water in the fuel rods arranged in the outermost region. It was thought that the control rod worth could be effectively improved by forming this gas-filled region in those fuel rods.

Based on the results of the above studies, the inventors evaluated the degree of improvement in control rod worth due to various fuel assemblies. The evaluation results will be described below.

For each of the four cases 1 to 4, the degree of improvement in control rod worth was evaluated. In Case 1, a fuel assembly used in a typical boiling water reactor, that is, a fuel assembly 10A shown in FIG. 9 is used. FIG. 9 shows a cross section at the upper portion of the fuel assembly 10A, that is, at a position above the upper ends of the below-described part length rods of the fuel assembly 10A. The channel box 16 of the fuel assembly 10A has four side walls, namely side walls 16A, 16B, 16C and 16D, with adjacent side walls connecting to each other. Each of the sidewalls 16A and 16B faces each of the two blades of the control rod 33 present within the water gap region 44. As shown in FIG. The fuel assembly 10A has a plurality of fuel rods 11 arranged in 10 rows and 10 columns inside a channel box 16 . All the fuel rods 11 arranged in ten rows and ten columns are full length fuel rods 12 whose lower ends are supported by a lower tie plate (not shown) and whose upper ends are supported by an upper tie plate (not shown). and the enrichment of fissile plutonium (Puf) contained in the filled nuclear fuel material is 5.61 wt %. The number of fuel rods 11 arranged in the channel box 16 is 78. Positions of the fuel rod array on the second row from the inner surface of the channel box 16 in FIG. Positions not marked with are part-length rods that are shorter in axial length than full-length rods, with lower ends supported by the lower tie plate and upper ends not supported by the upper tie plate. be. Two water rods WR are placed in the center of the fuel assembly cross-section for fuel economy and power flattening across the fuel assembly cross-section. In the cross section of the fuel assembly, the area occupied by the two water rods WR corresponds to the area where eight fuel rods 11 can be arranged.

Fuel assemblies 10B to 10D, which will be described later, also have part-length fuel rods arranged at the same position as the cross section of the fuel assembly 10A.

Each of the fuel assemblies 10B to 10D of Cases 2 to 4, which will be described later, has the same number of fuel rods 11 as the fuel assembly 10A of Case 1 in order to equalize the plutonium loading. 10, 11 and 12 each show a cross section at a position above the top of the part length rods in each of the fuel assemblies 10B, 10C and 10D.

In case 2, the fuel assembly 10B shown in FIG. 10 is used. Like the fuel assembly 10A, the fuel assembly 10B uses full-length fuel rods 12 as a plurality of fuel rods 11 arranged in 10 rows and 10 columns. In the fuel assembly 10A, the fuel rods 11 arranged in the outermost peripheral region 35 adjacent to the inner surface of the channel box 16 and the central region inside the outermost peripheral region 35 are respectively fuel rods 1 and nuclear fuel material. The enrichment of fissile plutonium contained in the fuel assembly 10B is the same as 5.61 wt%, whereas in the fuel assembly 10B, all the fuel rods 11 arranged in the outermost peripheral region 35 are fuel rods 3, and the outermost peripheral All of the fuel rods 11 arranged in the central region inside the region 35 are the fuel rods 2 . The enrichment of fissile plutonium contained in the nuclear fuel material filled in each fuel rod 3 is 4.30 wt%, which is lower than the enrichment of the nuclear fuel material filled in the fuel rod 1 described above. The enrichment of fissile plutonium contained in the nuclear fuel material filled in the fuel rod 2 is higher than that of the nuclear fuel material filled in the fuel rod 1 described above, and is 6.73 wt%. In the fuel assembly 10B, the fissile plutonium enrichment of the fuel rods 3 arranged in the outermost region 35 is lower than the fissile plutonium enrichment of the fuel rods 2 arranged in the central region. . However, the average enrichment of fissile plutonium in fuel assembly 10B is the same as the average enrichment of fissile plutonium in fuel assembly 10A.

In the fuel assembly 10B as well, two water rods WR having the same thickness as the water rods WR of the fuel assembly 10A are arranged in the central portion of the cross section of this fuel assembly.

In case 3, the fuel assembly 10C shown in FIG. 11 is used. The fuel assembly 10C has eight water rods WR having the same outer diameter as the fuel rods 11 instead of the two water rods WR arranged in the center of the cross section of the fuel assembly 10A of the case 1. have. In the fuel assembly 10C, eight fuel rods 1 are arranged at positions where two water rods WR are arranged in the central portion of the cross section of the fuel assembly 10A, and the eight water rods WR are arranged in a channel box 16. , four of them are arranged in each portion of the outermost peripheral region 35 facing the inner surfaces of the two side wall portions 16A and 16B facing the control rod 33. As shown in FIG. 78 fuel rods 1 and 8 water rods WR are arranged in 10 rows and 10 columns as shown in FIG. The fissile plutonium enrichment of the fuel rod 1 is 5.61 wt %, the same as that of the fuel assembly 10A. The average fissile plutonium enrichment in the fuel assembly 10C is the same as the average fissile plutonium enrichment in the fuel assembly 10A.

In case 4, the fuel assembly 10D shown in FIG. 12 is used. In the fuel assembly 10D, each portion of the outermost peripheral region 35 facing the inner surface of each of the two side wall portions 16A and 16B facing the control rods 33 among the four side wall portions of the channel box 16 in the fuel assembly 10C. , a fuel rod H1 is arranged at each position where the water rod WR is arranged. That is, in the fuel assembly 10D, fuel rods H1 are arranged in respective portions of the outermost peripheral region 35 facing the inner surfaces of the two side wall portions 16A and 16B of the channel box 16. As shown in FIG. In the fuel assembly 10D, 78 fuel rods 1 and 8 fuel rods H1 are arranged in 10 rows and 10 columns as shown in FIG. The fissile plutonium enrichment of each of the fuel rods 1 and H1 is 5.61 wt%, the same as that of the fuel assembly 10A. The average fissile plutonium enrichment in the fuel assembly 10D is also the same as the average fissile plutonium enrichment in the fuel assembly 10A. The fuel rod H1 forms a nuclear fuel material filling region filled with MOX fuel (nuclear fuel material) containing fissile plutonium inside, and a He filling region filled with helium gas, which is a gas filling region. The number of nodes in each of the fill region and He fill region is the same as the number of nodes in those regions in fuel rod H2 shown in FIG.

For each of the fuel assemblies of Cases 1 to 4 above, the control rod worth was obtained when the fuel assembly was cold, and the average value of the control rod worth that varies with burnup was obtained. FIG. 13 shows the average values of the obtained control rod worths. Each control rod value for each of the fuel bundles of cases 1-4 shown in FIG. Rod Arrangement, fuel assembly 10B above the top of the part length rods, fuel rod arrangement in the cross section shown in FIG. 10, fuel assembly 10C above the top of the part length rods. 11, and for fuel assembly 10D above the tops of the part length rods, the cross section shown in FIG. Called as sustained in direction.

Of the four side walls of the channel box 16, of the cases 1 to 4, each part of the outermost peripheral region 35 facing the inner surface of each of the two side walls facing the control rods is provided with a fuel rod. The fuel assembly 10D (Case 4) in which H1 is arranged has the largest control rod worth. The reason why the control rod worth is the largest in the fuel assembly 10D is that the fuel rods H1 arranged in the outermost peripheral region 35 as described above absorb thermal neutrons by fissile nuclides in the outermost peripheral region 35, U238 This is because the absorption of medium-velocity neutrons by parent nuclides such as , and the absorption of neutrons by light water is reduced.

Next, in order to efficiently suppress the reactivity of the core, the inventors studied where in the axial direction the cross section shown in FIG.

Generally, in a cold core, the peak of neutron flux exists in the upper part in the axial direction of the core. Therefore, by forming the fuel rod arrangement in the cross section shown in FIG. It is considered that the reactivity of

Reflecting the above considerations, the inventors arranged the fuel rod arrangement in the cross section of the fuel assembly 10D from the upper end of the effective fuel length (first effective fuel length) of the fuel assembly 10D toward the lower end. In order to ascertain to what extent the fuel effective length should be formed, the improvement rate of the control rod worth corresponding to the node is obtained downward from the upper end of the effective fuel length, and how this improvement rate changes I checked whether In addition, by using the power distribution at the cold temperature of the entire MOX equilibrium core in which all the fuel assemblies loaded in the core are MOX fuel assemblies, the control when changing the cross section at a specific node of the fuel assembly 10D is performed. The percentage improvement in bar value can be determined. When the active fuel length (first active fuel length) of the fuel assembly 10D containing plutonium is divided into 24 in the axial direction, one node is 1 node, and the active fuel length is 24 nodes. Assuming that the effective fuel length of the fuel assembly 10D is L, the length of one node is L/24.

The active fuel length of a fuel rod is the axial length of the region within the fuel rod that is filled with nuclear fuel material. The effective fuel length (first effective fuel length) of the fuel assembly is the length in the axial direction of the region in which the nuclear fuel material exists within the fuel assembly. When a fuel assembly includes a plurality of fuel rods having different effective fuel lengths, the effective fuel length of the fuel rod having the longest effective fuel length is the effective fuel length of the fuel assembly. The active fuel length L represents the active fuel length of the fuel assembly and the active fuel length of the fuel rod which is the same as the active fuel length.

When the fuel rod arrangement in the cross section shown in FIG. 12 above the upper ends of the part length rods of the fuel assembly 10D is formed over the 24 nodes of the active fuel length of the fuel assembly 10D. , when the control rod worth improvement rate is 100%, the fuel rod arrangement in the cross section shown in FIG. The change in percentage improvement in control rod worth for that node range as formed is shown in FIG.

If the fuel rod arrangement in the cross section shown in FIG. 12 at a position above the upper end of the part length fuel rods of the fuel assembly 10D is formed in the range of 18 nodes downward from the upper end of the active fuel length, That is, of the four side wall portions of the channel box 16, the fuel of the plurality of fuel rods H1 arranged in the respective portions of the outermost peripheral region 35 facing the inner surfaces of the two side wall portions facing the control rods. If the axial length of the He filling region extending downward from the upper end of the effective fuel length of the assembly 10D is set to 18 nodes (18L/24), the control rod worth improvement rate is as shown in FIG. to 100%. 18 nodes ( Lengths greater than 18L/24) only further reduce the fuel inventory of the fuel assembly 10D and do not further improve the rate of control rod worth improvement.

In the fuel assembly 10D, in the plurality of fuel rods H1 arranged in the respective portions of the outermost peripheral region 35 facing the inner surfaces of the two side walls of the channel box 16 facing the control rods, the fuel assembly 10D If the axial length of the He filling region extending downward from the upper end of the effective fuel length of is set to 2 nodes (2L/24) or more, the control rod worth improvement rate is, as shown in FIG. increase significantly. Here, the improvement rate of the control rod worth is the rate of increase of the control rod worth with respect to the control rod worth when the axial length of the He-filled region is "0". The control rod worth is 0% when the axial length of the He-filled region is "0".

For this reason, in the fuel assembly 10D including the fuel rods H1, the fuel assembly of the plurality of fuel rods H1 arranged in the respective portions of the outermost peripheral region 35 facing the inner surfaces of the side walls of the channel box 16. The length in the axial direction of the He-filled region extending downward from the upper end of the 10D effective fuel length should be within the range of 2 nodes (2L/24) or more and 18 nodes (18L/24) or less. . This significantly improves control rod worth.

In addition, "a plurality of fuel rods H1 arranged in the respective portions of the outermost peripheral region 35 facing the inner surfaces of the side walls of the channel box 16, downward from the upper end of the effective fuel length of the fuel assembly 10D. The axial length of the He-filled region extending from the bottom of the gas-filled region to the length within the range of 2 nodes (2L/24) or more and 18 nodes (18L/24) or less. The position of the boundary between H1 and the upper end of the effective fuel length (second effective fuel length) is located 2L/24 downward from the upper end of the effective fuel length (first effective fuel length) of the fuel assembly 10D; It is a position that exists within the range of positions that are positioned downward by 18L/24 from the upper end of the first effective fuel length."

Of the four side walls of the channel box 16, the fuel of the fuel assembly 10D of the fuel rods H1 arranged in the respective portions of the outermost peripheral region 35 facing the inner surfaces of the two side walls facing the control rods. If the axial length of the He-filled region extending downward from the upper end of the effective length is set to the length of 7 nodes (7L/24), as shown by the dotted line in FIG. is greater than 50%.

Preferably, the axial length of the He filling region is within the range of 7 nodes (7L/24) or more and 18 nodes (18L/24) or less, so that the control rod worth improvement rate is 50%. % to 100% or less, which further improves the control rod worth.

Further, "making the axial length of the He filling region within the range of 7 nodes (7L/24) or more and 18 nodes (18L/24) or less" means "the second fuel effective length The position of the boundary between the upper end and the lower end of the He filling region is located at a position located downward by 7L/24 from the upper end of the first effective fuel length and at a position located downward by 18L/24 from the upper end of the first effective fuel length. It is a position that exists within the range".

The He-filled region is a gas-filled region filled with a gas having a neutron absorption cross section smaller than that of light water. Any of He gas, argon gas, nitrogen gas, oxygen gas, and air is used as the gas having a smaller neutron absorption cross-section than that of light water, which is filled in the gas-filled region.

Even if a solid substance with a neutron absorption cross section smaller than that of light water is used instead of a gas with a neutron absorption cross section smaller than that of light water, the effect produced by a gas with a neutron absorption cross section smaller than that of light water, such as He gas, is obtained. be able to. Any one of graphite, beryllium, beryllium oxide, and beryllium carbide is used as the solid material having a neutron absorption cross section smaller than that of light water. A gas with a neutron absorption cross section smaller than that of light water and a solid material with a neutron absorption cross section smaller than that of light water are materials with a neutron absorption cross section smaller than that of light water. As the substance having a neutron absorption cross section smaller than that of light water, at least one of a gas having a neutron absorption cross section smaller than that of light water and a solid substance having a neutron absorption cross section smaller than that of light water may be used.

Fuel in a fuel assembly in a filling region (material filling region) of a material with a neutron absorption cross section smaller than that of light water, such as a gas with a neutron absorption cross section smaller than that of light water and a solid material with a neutron absorption cross section smaller than that of light water. The length in the axial direction extending downward from the upper end of the effective length L is within the range of 2L/24 or more and 18L/24 or less. The axial length of the material-filled region is preferably in the range of 7 L/24 to 18 L/24.

A preferred embodiment of the present invention reflecting the above study results will be described below with reference to the drawings.

A fuel assembly of Example 1 applied to an improved boiling water nuclear power plant, which is a preferred example of the present invention, will be described with reference to FIGS. 1, 2 and 4. FIG.

Before describing the fuel assembly of this embodiment, the schematic structure of a nuclear reactor of an advanced boiling water nuclear power plant (ABWR plant) to which this fuel assembly is applied will be described with reference to FIG. This nuclear reactor 20 has a reactor pressure vessel 21 in which a core 23 loaded with a plurality of fuel assemblies (not shown) is arranged. A cylindrical core shroud 22 surrounds a core 23 within a reactor pressure vessel 21 . Furthermore, an upper lattice plate 24 arranged above the core 23 is attached to the core shroud 22 , and a core support plate 25 arranged below the core 23 is attached to the core shroud 22 . A steam separator 26 located above the upper grid plate 24 in the reactor pressure vessel 21 is installed at the upper end of the core shroud 22 . Furthermore, a steam dryer 27 is arranged above the steam separator 26 and installed on the inner surface of the reactor pressure vessel 21 .

An annular downcomer 28 is formed between the outer surface of the core shroud 22 and the inner surface of the reactor pressure vessel 21 . An internal pump 30 located within the downcomer 28 extends downward through the bottom of the reactor pressure vessel 21 and is attached to the reactor pressure vessel 21 . A main steam line 31 and a feedwater line 32 are connected to the reactor pressure vessel 21 .

A lower plenum 29 is formed within the reactor pressure vessel 21 below the core 23 . A plurality of control rod guide tubes (not shown) are arranged in the lower plenum 29 . Control rods 33 (see FIG. 11) having a cruciform cross-section, which control the fission of the fuel assembly 10 (see FIG. 1), are arranged separately in their control rod guide tubes. A plurality of control rod drive mechanism housings (not shown) are attached to the bottom of reactor pressure vessel 21 and extend downwardly therefrom. Control rod drives (not shown) are disposed within respective control rod drive housings and are coupled to control rods 33 . The control rod 33 has four blades containing neutron absorbing material, and the four blades extend in all directions from the central axis of the control rod.

A plurality of fuel assemblies 10 loaded in the core 23, as shown in FIG. It has a plurality of vertically arranged fuel spacers 17 and a channel box 16 .

The plurality of fuel rods 11 includes a plurality of full-length fuel rods 12 having lower ends supported by a lower tie plate 15 and upper ends supported by an upper tie plate 14, and an axial length shorter than the full-length fuel rods 12. , a plurality of part length fuel rods 13 supported at their lower ends by a lower tie plate (lower fuel support member) 15 and unsupported at their upper ends by an upper tie plate (upper fuel support member) 14 .

The full length fuel rods 12 and the part length fuel rods 13 have a cladding tube (not shown) with a lower end plug (not shown) closing the upper end of the cladding tube to the upper end. The cladding tube is closed with a plug (not shown) and filled with a plurality of fuel pellets (not shown) made from nuclear fuel material. A gas plenum (not shown) is formed within the cladding tube above the region filled with those fuel pellets.

All the fuel rods 11 are bundled by a plurality of fuel spacers 17 arranged at predetermined intervals in the axial direction of the fuel rods 11 so as to form a space of a predetermined width serving as a cooling water flow path between them. A bundle of fuel rods 11 bundled by a plurality of fuel spacers 17 is arranged in a channel box 16 which is a rectangular tube having a square cross section. The channel box 16 is composed of four side walls 16A, 16B, 16C and 16D that are connected to each other. A channel box 16 having side walls 16A, 16B, 16C, and 16D is a rectangular tube having a square cross section. A channel box 16 is attached at its upper end to the upper tie plate 14 and extends from the upper tie plate 14 towards the lower tie plate 15 . A channel box 16 surrounds each side of the upper tie plate 14 and lower tie plate 15 . A handle 20A is provided on the upper surface of the upper tie plate 14 and extends upwardly from the upper tie plate 14 .

Attachment of the channel box 16 to the upper tie plate 14 is accomplished by channel fasteners 18 . Channel clips 43A and 43B (see FIG. 7) provided at the upper end of the channel box 16 are provided at two corners among the four corners of the upper tie plate 14, respectively. It rests on two posts that extend upwards. A channel clip 43A (Fig. 7 and 8) is removably secured to post 14A by screw 19A engaged with channel fastener 18 (see FIG. 8). Thus, channel fasteners 18 secure channel box 16 to post 14A. Another channel clip 43B is attached to the channel box 16 in the cross section of the channel box 16 at the other corner diagonally opposite the corner of the channel box 16 through which the channel clip 43A is provided. attached to the upper end of the A channel clip 43B rests on another post that extends upwardly from the upper surface of the upper tie plate 14 .

One channel fastener 18 has two leaf springs 19 . Each leaf spring 19 extends downward along the side surface of the channel box 16 from the upper end of the post 14A. Each leaf spring 19 of the channel fastener 18 extends from one corner of the channel box 16 in two orthogonal directions in the fuel assembly 10 to which the channel fastener 18 is attached. It is positioned separately opposite each outer surface.

Four fuel assemblies 10 are inserted into square rectangular holes formed in the upper grid plate 24 at their upper ends. In the four fuel assemblies 10 inserted into the rectangular holes, one leaf spring 19 of each channel fastener 18 attached to two adjacent fuel assemblies 10 comes into contact with each other (see FIG. 8). , the two fuel assemblies 10 in question are pressed against the sides of the upper grate plate 24 facing the rectangular holes formed in the upper grate plate 24 . The four fuel assemblies inserted into one rectangular hole formed in the upper lattice plate 24 are in contact with each other with the leaf springs 19 of the channel fasteners 18 attached thereto, as shown in FIG. This presses against the side of the upper grid plate 24 facing the rectangular hole. Therefore, a gap into which the control rods 33 can be inserted, that is, a water gap region 44 is formed between the four fuel assemblies 10 inserted in one rectangular hole formed in the upper grid plate 24. . When the control rods 33 are fully inserted between the fuel assemblies 10, the upper ends of the control rods 33 are positioned below the lower ends of the leaf springs 19 (see FIG. 8).

As shown in FIG. 1, the fuel assembly 10 has a plurality of fuel rods 11 arranged in a channel box 16 in a square grid of 10 rows and 10 columns in the cross section of the fuel assembly 10 . A plurality of full-length fuel rods 12 of the plurality of fuel rods 11 include 78 fuel rods 4 and 8 fuel rods (He fuel rods) H2 (see FIG. 2). There are 14 fuel rods 5 that are part length fuel rods 13 (see FIG. 2).

The effective fuel length of the fuel rods 4 is the effective fuel length L. Each fuel rod 4, as shown in FIG. 2, has a plurality of fuel pellets of MOX fuel with an enrichment of fissile plutonium (Puf) of 5.61 wt%, from the position of the lower end of the fuel effective length L, fuel It is filled up to a position 23L/24 from the lower end of the effective length L, and a plurality of fuel pellets of U ₂ O, which is depleted uranium, are placed between the position 23L/24 from the lower end of the effective fuel length L and the upper end of the effective fuel length L. is filled to The active fuel length of each fuel rod H2 is 20L/24. Each fuel rod H2 holds a plurality of fuel pellets of MOX fuel with a fissile plutonium enrichment of 5.61 wt% from the lower end of the active fuel length to the position 20L/24 from the lower end of the active fuel length. A He filling region 48 (see FIG. 5) filled with a gas having a neutron absorption cross section smaller than that of light water, for example He, is placed from a position 20L/24 from the lower end of the effective fuel length of the fuel rod 4 to the fuel assembly. It is formed over the upper end of the effective fuel length L of the body 10 . The He filling region 48 is formed in a range of 4L/24 between the position of the upper end of the effective fuel length L of the fuel rod 4, that is, the effective fuel length L of the fuel assembly 10, and the position below this position. be. The axial length of the He-filled region 48 is 4L/24. The fuel rod H2 is not filled with a nuclear fuel material such as MOX fuel in its He filling region (between the position 20L/24 from the lower end of the effective fuel length and the upper end of the effective fuel length L) 48 . The active fuel length of the fuel rod H2 is shorter than that of the fuel rod H4. The sum of the effective fuel length of the fuel rod H2 and the axial length of the He-filled region 48, which is the gas-filled region, is the same as the effective fuel length L of the fuel rod 4.

The effective fuel length L of the fuel assembly 10 is equal to the effective fuel length L of the fuel rod 4 included in the fuel assembly 10 and having the longest effective fuel length.

The fuel rod 5 has a fuel effective length of 14/24 of the fuel effective length L of the fuel rod 4, and a plurality of fuel pellets of MOX fuel having a fissile plutonium enrichment of 5.61 wt%. is filled to the fuel effective length.

In the fuel assembly 10, the average enrichment of fissile plutonium in the outermost region 35 and the average enrichment of fissile plutonium in the central region inside the outermost region 35 are the same.

Each of the fuel rods 4, 5 and H2 is sealed with helium gas, which has a high thermal conductivity, to maintain good heat conduction from the fuel pellets to the cooling water contacting the outer surface of the cladding tube of each fuel rod. Fill in the cladding tube. Filling fuel rods with helium gas, which has high thermal conductivity, has been conventionally practiced. In the fuel rod 4, a gas plenum filled with helium gas above the upper end of the effective fuel length L is formed in a cladding tube whose lower end is sealed by a lower end plug and whose upper end is sealed by an upper end plug. In the fuel rod 5, a gas plenum 49 filled with helium gas is formed above the upper end of the effective fuel length (14L/24) of the fuel rod 5 within the sealed cladding tube. In the fuel rod H2, a gas plenum 49 filled with helium gas is formed above the upper end of the He filling region 48, that is, above the upper end of the active fuel length L of the fuel rod 4. Furthermore, in each of the fuel rods 4, 5 and H2, the gap (referred to as the first gap) between the outer surface of the fuel pellet and the inner surface of the cladding within each fuel rod is also filled with helium gas.

When the control rods 33 are inserted between the plurality of fuel assemblies 10, the two blades of the control rods 33 are positioned in one of the channel boxes 16 facing the control rods 33, as shown in FIG. A corner portion, that is, one corner portion of the channel box 16 where the channel fastener 18 is arranged, faces the outer surfaces of two side wall portions 16A and 16B extending in two orthogonal directions. The channel box 16 having a square cross section has four side walls, and the two blades of the control rod 33 extend from one corner of the four side walls 16A to 16D of the channel box 16. It faces two extending side walls 16A and 16B.

Of the four side walls of the channel box 16, respective portions of the outermost peripheral region 35 facing the respective inner surfaces of the two side walls 16A and 16B, i.e. the channel box attached to the upper tie plate 14 by the channel fasteners 18. As shown in FIG. Four fuel rods H2 are arranged. In each portion of the outermost peripheral region 35 facing the inner surface of each of the side walls 16A and 16B, two fuel rods H2 are arranged adjacently in the center of this portion and from these two fuel rods H2 One fuel rod H2 is arranged with one fuel rod 4 interposed on the side of the channel fastener 18, and another fuel rod 4 is connected from the two fuel rods H2 to the opposite side of the channel fastener 18. An intervening fuel rod H2 is arranged.

Furthermore, in the cross section of the fuel assembly 10, 12 fuel rods 5 are arranged in the second row of fuel rod array from the inner surface of the channel box 16 (the fuel rod array adjacent to the innermost peripheral region 35), Two fuel rods 5 are arranged adjacent to each other in a diagonal direction of a channel box 16 in the center of the cross section of the fuel assembly 10 . In the cross section of the fuel assembly, the fuel rods 4 are arranged at positions other than the arrangement positions of the fuel rods H2 and the fuel rods 5, respectively.

In the fuel rod H2 of the fuel assembly 10, as shown in FIG. ing. The upper end of the cladding tube 45 is closed by an upper end plug 47 . Therefore, no partition member is arranged between the gas plenum 49 and the He-filled region 48 and between the He-filled region 48 and the first gap. A dashed line 50 shown in FIG. 5 indicates the upper end of the active fuel length of the fuel assembly 10 , that is, the upper end of the active fuel length of the fuel rods 4 . The He filling region 48 is located in the fuel rod H2 between the upper end of the nuclear fuel material filling region in the cladding tube 45 filled with a plurality of fuel pellets 46 and the upper end of the effective fuel length of the fuel assembly 10 indicated by the dashed line 50. A gas plenum 49 is formed within the cladding tube 45 above the upper end of the active fuel length of the fuel assembly 10 (dashed line 50).

Instead of the He-filled region 48 filled with He gas, a gas-filled region filled with any one of argon gas, nitrogen gas, oxygen gas and air may be formed in the fuel rods H2. When forming such a gas-filled region in the fuel rod H2, argon gas, nitrogen gas, oxygen gas, or air filled in the gas-filled region is injected into the gas plenum 49 filled with helium gas and the first gap. should be prevented from spreading to

A fuel assembly using any one of argon gas, nitrogen gas, oxygen gas and air as a gas having a neutron absorption cross section smaller than that of light water other than He gas will be described below.

This fuel assembly is a fuel rod in which a gas-filled region is formed instead of the He-filled region 48 in the outermost peripheral region 35 of the channel box 16 facing the inner surfaces of the side walls 16A and 16B facing the control rods 33. Place H4. In the fuel assembly 10 using the fuel rods H4, the fuel rods H4 are arranged at each position where the fuel rods H2 were arranged in the above-described fuel assembly 10 using the fuel rods H2. The fuel rod H4 will be explained using FIG.

The fuel rod H4 includes a sealed cylindrical hollow container 51 made of a zirconium alloy (for example, Zircaloy 2) containing any of argon gas, nitrogen gas, oxygen gas and air. The fuel rod H4 is placed on top of the nuclear fuel material filling region (where a plurality of fuel pellets 46 are placed) within the sealed cladding tube 45 . The upper end of cladding tube 45 is closed by upper end plug 47 . The vessel 51 is positioned between the nuclear fuel material filling region and the gas plenum 49 within the fuel rod H4. A gas-filled region 48A filled with a gas having a smaller neutron absorption cross-section than light water is formed within the container 51 and exists between the nuclear fuel material-filled region and the gas plenum 49 within the fuel rod H4. In the fuel rod H4, the sum of the axial length of the nuclear fuel material filling region filled with a plurality of fuel pellets 46 and the axial length of the container 51 in which the gas filling region 48A is formed is the fuel having the fuel rod H4. It is the same as the fuel effective length of the assembly.

The outer diameter of the container 51 in which the gas-filled region 48A is formed is smaller than the inner diameter of the cladding tube 45 of the fuel rod H4. ) is formed. Through this second gap, the gas plenum 49 communicates with the first gap formed between the outer surface of the fuel pellet 46 and the inner surface of the cladding tube 45 in the nuclear fuel material filling area. The first gap and the gas plenum 49 are also filled with helium gas in the fuel rod H4 in which the vessel 51 forming the gas filling region 48A is arranged. Since the container 51 which forms the gas filling region 48A inside is arranged in the cladding tube 45 of the fuel rod H4, the argon gas, nitrogen gas, oxygen gas or air filled in this container 51 is discharged into the first gap. And diffusion into the helium gas in the gas plenum 49 can be prevented.

A fuel assembly using any one of graphite, beryllium, beryllium oxide, and beryllium carbide as a solid material having a neutron absorption cross section smaller than that of light water will be described.

This fuel assembly is a solid material having a neutron absorption cross section smaller than that of light water in the portion of the outermost peripheral region 35 of the channel box 16 facing the inner surface of the side wall portions 16A and 16B facing the control rods 33. For example, , arranging graphite-filled solid material-filled fuel rods. Solid material filled fuel rods are fuel rods corresponding to fuel rods H2 or H4. The solid material-filled fuel rods are arranged at each position where the fuel rods H2 are arranged in the aforementioned fuel assembly 10 using the fuel rods H2.

The solid material-filled fuel rod is made of graphite, beryllium, beryllium oxide, or beryllium carbide, for example, graphite, above the upper end of the nuclear fuel material-filled region in which the plurality of fuel pellets 46 are arranged, within the cladding tube 45. Fill multiple pellets. The sum of the axial length of the graphite-filled region filled with a plurality of graphite pellets, i.e., the axial length of the solid material-filled region and the axial length of the nuclear fuel material-filled region filled with a plurality of fuel pellets 46, is the solid material-filled fuel It is the same as the active fuel length of a fuel assembly with rods.

The outer diameter of the graphite pellet is smaller than the inner diameter of the cladding tube 45 of the solid material-filled fuel rod, and a gap (called the third gap) exists between the outer surface of the graphite pellet and the inner surface of the cladding tube 45 of the solid material-filled fuel rod. ) is formed. Within the cladding tube 45 of the solid filled fuel rod, the first gap, the third gap and the gas plenum 49 are filled with He gas. The first gap and gas plenum 49 are communicated by a third gap.

The axial length of the solid substance-filled region may be shortened, and the He-filled region 48 may be formed above the solid substance-filled region by the shortened axial length of the solid substance-filled region. That is, a solid material filling region and a He filling region 48 are formed above the nuclear fuel material filling region within the cladding tube 45 of the solid material filling fuel rod. The total axial length of each of the nuclear fuel material-filled region, the solid material-filled region and the He-filled region 48 in the solid-filled fuel rod is the same as the effective fuel length of the fuel assembly having the solid-filled fuel rods. In addition, in the solid material-filled fuel rod in which the nuclear fuel material filling region, the solid material filling region and the He filling region 48 are respectively formed, instead of forming the He filling region 48, argon gas, nitrogen gas, oxygen gas and air are used. A container 51 having a gas-filled region 48A formed therein containing either may be placed above the solid substance-filled region. Axial length of each of the nuclear fuel material filling region, the solid material filling region and the container 51 in the solid material filling fuel rod arranged in the cladding tube 45 sealing the nuclear fuel material filling region, the solid material filling region and the container 51 respectively. is the same as the effective fuel length of the fuel assembly having this solid material-filled fuel rod.

The ABWR plant, which has undergone fuel replacement and maintenance inspection, is restarted for operation in the next operation cycle. The control rods 33 are withdrawn from the core 23 and the core 23 changes from the subcritical state to the critical state, and the cooling water in the core 23 (hereinafter referred to as "reactor water") flows into each fuel assembly 10 loaded in the core 23. It is heated by heat generated by nuclear fission of fissionable nuclides (for example, Pu239, etc.) contained in the nuclear fuel material. Furthermore, the reactor power is increased through the temperature increase/increase process, and the ABWR plant reaches the rated operating state (reactor water temperature is 280° C., which is the rated temperature, and the pressure inside the reactor pressure vessel 21 is the rated pressure). Rated operation of the ABWR plant continues until its operating cycle is completed.

In an ABWR plant in operation, reactor water in a reactor pressure vessel 21 is pressurized by an internal pump 30 and supplied through a lower plenum 29 into each fuel assembly 10 loaded in a core 23 . Reactor water supplied to the fuel assemblies 10 is heated by heat generated by fission of fissionable nuclides contained in the nuclear fuel material in each fuel assembly 10 . A portion of the heated reactor water becomes steam. This steam is led from the reactor core 23 to the steam separator 26 where the moisture contained in the steam is removed. The steam discharged from the steam separator 26 is led to the steam dryer 27, and moisture contained in the steam is removed by the steam dryer 27. Steam discharged from the steam dryer 27 is led to a turbine (not shown) through a main steam pipe 31 to rotate a turbine connected to a generator (not shown). Steam discharged from the turbine is condensed in a condenser (not shown) to become water, and this water is supplied as feed water to the reactor pressure vessel 21 through the feed water pipe 32 .

The fuel assemblies 10 with a burnup of 0 GWd/t loaded into the core 23 of the ABWR plant are typically loaded into the core 23 during four operating cycles. For each operation cycle, 1/4 of all the fuel assemblies 10 loaded in the core 23 complete their residence in the reactor in the 4th operation cycle of the ABWR plant whose operation has been stopped. It is taken out from the core 23 as spent fuel assemblies. The same number of fuel assemblies 10 with a burnup of 0 GWd/t as the removed spent fuel assemblies are loaded into the core 23 .

During operation of the ABWR plant, in each fuel rod 11 of the fuel assembly 10 loaded in the core 23, fission product gas generated by fission of fissionable nuclides (for example, Pu239, Pu241, etc.) contained in the fuel pellet (Xe, Kr, etc.) are released from the fuel pellets into the first gap within the cladding tube. As the burnup of the fuel assembly 10 increases, i.e., as the residence time of the fuel assembly 10 in the reactor increases, fuel pellets are released from the fuel pellets into the first gap in each of the fuel rods 14, 5 and H1. The generated fission gas accumulates within the cladding tube and increases the pressure within the cladding tube. The pressure inside the cladding tubes of the fuel rods of the fuel assemblies 10 loaded in the core 23 becomes highest when the operation of the ABWR plant in the fourth operation cycle ends. The fission gas released from the fuel pellets is directed through the first gap to the gas plenum 49 and accumulated in the gas plenum 49 within each fuel rod, the gas plenum 49 within each fuel rod being configured to reduce the pressure within the cladding tube. there is In fuel rod H2, the fission gases released from the fuel pellets are present not only in the first gap and gas plenum 49, but also in the second gap and He fill region 48 within fuel rod H2.

As described above, in the fuel rod H4 in which the container 51 filled with a gas having a neutron absorption cross-sectional area smaller than that of light water is arranged, the thickness of the container 51 is the same as that of the ABWR in the fourth operation cycle. The vessel 51 is set so that even when the plant operation is terminated, the pressure build-up in the cladding tube 45 due to the accumulation of fission gas will not cause the vessel 51 to collapse. When the container 51 with the gas-filled region 48A formed therein is arranged in the cladding tube 45 of the fuel rod H4, the fission gas released from the fuel pellets 46 cannot enter the container 51. , within the cladding tube 45 and outside its container 51 .

Each of fuel rods 4 , 5 and H 2 has a coil spring located within gas plenum 49 . This coil spring presses the upper end of the uppermost fuel pellet toward the lower end plug side of the fuel rod to prevent the fuel pellets in each fuel rod from moving and being damaged during transportation of the fuel assembly 10. ing. In the fuel rod H2, its coil spring is continuously arranged in the gas plenum 49 and the He-filled region 48 and presses the upper end of the uppermost fuel pellet toward the lower end plug side of the fuel rod. In the fuel rod H2, the He filling region 48 may be formed by filling helium gas in the sealed container 51 as in the case of the fuel rod H4 using argon gas, nitrogen gas, oxygen gas or air. good. By placing a container 51 filled with helium gas above the uppermost fuel pellets in the cladding tube 45, a He fill region 48 can be formed above the fuel pellets 46 and below the gas plenum 49. , the axial length of the coil spring located in the gas plenum 49 of the fuel rod H2 can be shortened. This coil spring presses the upper end of the uppermost fuel pellet 46 via a container 51 filled with helium gas.

During operation of the ABWR plant, some of the fast neutrons generated by the fission of fissionable nuclides contained in the fuel rods 4, 5 contained in the fuel assemblies 10 loaded in the core 23 and the nuclear fuel material in H2 are released into the water gap. The neutrons are moderated by the cooling water (light water) or the like existing in the region 44 to become thermal neutrons, which are absorbed by the control rods 33 inserted into the core 23 . Each portion of the outermost peripheral region 35 of the four side walls 16A-16D of the channel box 16 facing the inner surface of each of the side walls 16A and 16B, i.e., the channel box attached to the upper tie plate 14 by channel fasteners 18. Four fuel rods arranged in respective portions of the outermost peripheral region 35 facing each inner surface of two side wall portions 16A and 16B extending in two orthogonal directions from one corner portion of the channel fastener 18 side of 16. Since the He-filled region 48 is formed in H2, the He-filled region 48 reduces the amount of nuclear fuel material and the amount of cooling water (light water) in those portions of the outermost region 35. absorption of thermal neutrons by Therefore, the number of thermal neutrons absorbed by the control rods 33 increases. Also, by arranging the fuel rods H2 in respective portions of the outermost peripheral region 35 facing the inner surfaces of the side wall portions 16A and 16B of the channel box 16, absorption of thermal neutrons by the cooling water (light water) in those portions is reduced. Therefore, it is possible to prevent swing, that is, the amount of reactivity added to the fuel assembly at cold temperatures from becoming large negative, and the void coefficient of the fuel assembly 10 can be maintained negative. The fuel assembly 10 in which the fuel rods H2 are arranged can improve the control rod worth without making the reactivity addition amount to the fuel assembly 10 at cold temperatures significantly negative.

Further, since the fuel rods H2 are arranged in the above-described portion of the outermost peripheral region 35 in the cross section of the fuel assembly 10, the absorption of medium-velocity neutrons by parent substances such as U238 in each fuel rod is also reduced. The reduced absorption of medium-velocity neutrons by the parent material such as U238 in each fuel rod will improve control rod worth.

As described above, in the fuel assembly 10 of this embodiment, of the four side wall portions 6A to 16D of the channel box 16, each portion of the outermost peripheral region 35 facing each inner surface of the side wall portions 16A and 16B, That is, the outermost peripheral region of the channel box 16 attached to the upper tie plate 14 by the channel fastener 18 faces the inner surfaces of the side wall portions 16A and 16B extending in two orthogonal directions from one corner portion on the channel fastener 18 side. A fuel rod H2 is arranged in each portion of 35, and furthermore, in the fuel rod H2, in the axial direction, a He filling region 48 extends from the upper end of the active fuel length of the fuel assembly 10 to below this upper end. located over the upper end of the active fuel length of the fuel rod H2. Therefore, in the fuel assembly 10 of this embodiment, the cooling water (light water) in the outermost peripheral regions 35 facing the inner surfaces of the side walls 16A and 16B is removed by the He filling regions 48 of the fuel rods H2. Therefore, unlike the fuel assembly described in Japanese Patent Application Laid-Open No. 10-311889, in which part-length fuel rods are arranged in the outermost peripheral region of the cross section, the fuel assembly 10 has sidewall portions 16A and 16B thereof. The amount of cooling water (light water) in each portion of the outermost peripheral region 35 facing each inner surface of the cooling water (light water) is reduced, and the amount of neutrons absorbed by the cooling water (light water) is reduced. As the burning of the nuclear fuel material in the fuel assembly 10 progresses, that is, as the end of the operation cycle approaches, the amount of thermal neutrons absorbed by the nuclear fuel material decreases. It is an improvement over the conventional fuel assembly described in US Pat. No. 10-311889. Thus, in this embodiment, control rod worth can be improved throughout the operating cycle.

A fuel assembly having fuel rods H4, a fuel assembly having solid material-filled fuel rods forming a solid material-filled region such as a graphite-filled region, and either the He-filled region 48 or the gas-filled region 48A and its solid material filling A fuel assembly having fuel rods forming each of the regions can achieve each effect that occurs in the fuel assembly 10 having fuel rods H2.

A second embodiment of a fuel assembly applied to an improved boiling water nuclear power plant, which is another preferred embodiment of the present invention, will be described with reference to FIG.

In the fuel assembly of this embodiment, the fuel rod arrangement in the cross section is the same as the fuel rod arrangement in the cross section shown in FIG. They are arranged in 10 rows and 10 columns. The fuel assembly of this embodiment has 78 fuel rods 4, 14 partial length fuel rods 5 and 8 fuel rods H2, like the fuel assembly of the first embodiment. In the fuel assembly of this embodiment, the fuel rod H2 is different from the fuel rod H2 in the fuel assembly 10 of Embodiment 1, and each of the fuel rods 4 and 5 is the fuel rod 4 in the fuel assembly 10 of Embodiment 1. and 5, respectively. The fuel rod H2 in the fuel assembly of this embodiment and the fuel rod H2 in the fuel assembly 10 have the same fissile plutonium enrichment of 5.61 wt%, but the active fuel length and the He filling region 48 are Boundary locations are different. The position of the boundary between the effective fuel length of the fuel rod H2 of the fuel assembly 10 and the He filling region 48 is 20L/24 from the lower end of the effective fuel length L of the fuel assembly. The position of the boundary between the effective fuel length of the fuel rod H2 and the He filling region 48 is 16L/24 from the lower end of the effective fuel length L of the fuel assembly. As a result, the number of nodes of the He-filled region 48 in the fuel assembly of the present embodiment starting from the upper end of the effective fuel length of the fuel assembly is equal to the number of nodes of the He-filled region 48 of the fuel assembly 10 of the first embodiment. 4 more than The axial length of the He-filled region 48 in this embodiment is 8L/24.

This embodiment can obtain each effect produced in the first embodiment. Furthermore, in this example, the axial length of the He filling region 48 is longer than the length of the He filling region 48 of the fuel assembly 10 of Example 1, so the control rod worth in this example is larger than that in 1. In addition, in the present embodiment, since the length 48 is long, the reactivity control capability of the control rods 33 is improved in the portion where the He-filled region 48 is located in the axial direction of the core, and the neutron flux peak of the core 23 at cold temperature is suppressed. can do. Therefore, the reactivity of the core 23 is greatly suppressed, and the reactor shutdown margin is improved as compared with the first embodiment.

In the fuel assembly of this embodiment, instead of the He-filled region 48 of the fuel rod H2, a container 51 in which a gas-filled region 48A is formed, a plurality of pellets of any one of graphite, beryllium, beryllium oxide, and beryllium carbide and both the He filling region 48 and the container 51 and the solid material filling region may be arranged above the upper end of the nuclear fuel material filling region.

A fuel assembly of Example 3 applied to an improved boiling water nuclear power plant, which is another preferred example of the present invention, will be described with reference to FIGS. 16 and 17. FIG.

As shown in FIG. 16, in the fuel assembly 10E of this embodiment, a plurality of fuel rods 11 are arranged in a channel box 16 having a square lattice of 10 rows and 10 columns and a square cross section in the cross section of the fuel assembly 10E. placed inside. The plurality of fuel rods 11 includes fuel rods 6, 7 and H3, which are full length rods 12, and fuel rod 8, which is a part length rod 13. FIG. As shown in FIG. 17, the fuel assembly 10E includes 50 fuel rods 6, 28 fuel rods 7, 14 fuel rods 8, and 8 fuel rods H3.

The effective fuel lengths of the fuel rods 6 and 7 are the same as the effective fuel length of the fuel rod 4 of the fuel assembly 10 of the first embodiment, which is L. In the fuel rod 6, a plurality of fuel pellets of MOX fuel with a fissile plutonium enrichment of 6.73 wt% are filled up to a position 23L/24 from the lower end of the effective fuel length L, and depleted uranium U A plurality of fuel pellets of ₂ O are filled between the position 23L/24 from the lower end of the effective fuel length L and the upper end of the effective fuel length L. In the fuel rod 7, a plurality of fuel pellets of MOX fuel with a fissile plutonium enrichment of 4.30 wt% are filled up to a position 23L/24 from the lower end of the effective fuel length L, and depleted uranium U A plurality of fuel pellets of ₂ O are filled between the position 23L/24 from the lower end of the effective fuel length L and the upper end of the effective fuel length L.

The active fuel length of each fuel rod H3 is 16L/24. Each fuel rod H3 carries a plurality of fuel pellets of MOX fuel with a fissile plutonium enrichment of 4.30 wt% from the lower end of the active fuel length to the position 16L/24 from the lower end of the active fuel length. The He filling region 48 is moved from the lower end of the effective fuel length of the fuel rod H3, that is, the position 16L/24 from the lower end of the effective fuel length L of the fuel assembly 10E to the effective fuel length L of the fuel assembly 10E. formed over the upper end. The He filling region 48 extends from the position of the effective fuel length L of the fuel rod 6, that is, the position of the upper end of the effective fuel length L of the fuel assembly 10E to the position below the upper end of the effective fuel length L by 8L/24. formed by The axial length of the He-filled region 48 is 8L/24. The fuel rod H3 is not filled with a nuclear fuel material such as MOX fuel in its He filling region (between the position 16L/24 from the lower end of the effective fuel length L and the upper end of the effective fuel length L) 48 . The active fuel length of fuel rod H3 is shorter than that of fuel rods 6 and 7. The sum of the effective fuel length of the fuel rod H3 and the axial length of the He filling region 48, which is the gas filling region, is the same as the effective fuel length L of the fuel rods 6 and 7, that is, the effective fuel length L of the fuel assembly 10E. is.

The fuel rod 8 has a fuel effective length of 14/24 of the fuel effective length L of the fuel rod 6, and a plurality of fuel pellets of MOX fuel having a fissile plutonium enrichment of 6.73 wt%. is filled to the fuel effective length.

The fuel rods 6, 7 and 8 form a gas plenum 49 within the cladding tube above the top of the active fuel length. Fuel rod H3 forms a gas plenum 49 above He fill region 48 within the cladding tube. These gas plenums 49 are filled with helium gas. Moreover, in the fuel rods 6, 7, 8 and H3, the first gap formed between the outer surface of the fuel pellet and the inner surface of the cladding tube is also filled with helium gas. In the fuel rod H3, the gas plenum 49, the He-filled region 48 and the first gap communicate with each other.

In the cross section of the fuel assembly 10E, a plurality of fuel rods 7 are arranged in the outermost peripheral region 35 adjacent to the inner surface of the channel box 16, as shown in FIG. The plurality of fuel rods H3 are held together by channel fasteners 18 in respective portions of the outermost peripheral region 35 facing the inner surfaces of the two side walls 16A and 16B of the channel box 16 facing the control rods 33, namely the upper tie plate. In each portion of the outermost peripheral region 35 facing each inner surface of two side wall portions 16A and 16B extending in two orthogonal directions from one corner portion on the channel fastener 18 side of the channel box 16 attached to 14, As shown in FIG. 16, four are arranged.

A plurality of fuel rods 6 are arranged in a region inside the outermost peripheral region 35 (hereinafter referred to as a central region) in the cross section of the fuel assembly 10E. A plurality of fuel rods 8 are arranged in this central region. In the cross section of the fuel assembly 10E, 12 fuel rods 8 are arranged in the second fuel rod array from the inner surface of the channel box 16 (the fuel rod array adjacent to the innermost peripheral region 35), and the fuel assembly Two fuel rods 8 are arranged adjacent to each other in the direction of one diagonal line passing through one corner portion of the channel box 16 facing the control rods 33 in the center of the cross section of the body 10E. In the central region of the fuel assembly 10E, fuel rods 6 are arranged at positions other than the positions where the fuel rods H3 are arranged.

In the fuel assembly 10E of this embodiment, the average enrichment of fissile plutonium in the outermost peripheral region 35 (eg, 4.30 wt%) is the average enrichment of fissile plutonium in the central region inside the outermost peripheral region 35. degree (for example, 6.73 wt%).

In the fuel rod H3, instead of the He-filled region 48, a sealed container 51 containing any one of argon gas, nitrogen gas, oxygen gas, and air may be arranged similarly to the fuel rod H2 in the first embodiment.

In the fuel assembly 10E of this embodiment, the position of the boundary between the upper end of the effective fuel length of the fuel rod H3 and the lower end of the He filling region 48 is the position of the effective fuel length of the fuel rod H2 of the fuel assembly 10 of the first embodiment. It may be positioned at the boundary between the upper end and the lower end of the He-filled region 48, that is, at a position 20L/24 from the lower end of the effective fuel length.

In the fuel assemblies 10 and 10E, a gas-filled region filled with any one of helium gas, argon gas, nitrogen gas, oxygen gas, and air is formed inside, and fuel rods (for example, Preferably, the position of the boundary between the upper end of the effective fuel length of the fuel rod H2 or H3) and the lower end of the gas-filled region of the fuel rod H2 or H3) is 2L/24 downward from the position of the upper end of the effective fuel length L of the fuel assembly. 18L/24 (2L/24 or more and 18L/24 or less).

This embodiment can obtain each effect produced in the first and second embodiments. Furthermore, in this embodiment, by making the average enrichment of fissile plutonium in the outermost peripheral region 35 of the fuel assembly 10E lower than the average enrichment in the central region of the fuel assembly 10E, the outermost peripheral region 35 can further suppress the absorption of neutrons in Therefore, in this embodiment, the control rod worth can be further improved as compared with the first embodiment. In addition, in the outermost peripheral region 35 of the fuel assembly 10E, the output of other fuel rods adjacent to the fuel rod H3 having the gas-filled region (for example, the He-filled region 48) is rise in comparison. By reducing the fissile plutonium enrichment of the fuel rods arranged in the outermost region 35, the power peak of the fuel rods adjacent to the fuel rod H3 can be suppressed. Therefore, the fuel assembly 10E of this embodiment can improve the thermal margin more than the fuel assembly 10 of the first embodiment.

In the fuel assembly 10E of this embodiment, instead of the He-filled region 48 of the fuel rod H2, a container 51 in which a gas-filled region 48A is formed, a plurality of graphite, beryllium, beryllium oxide, and beryllium carbide. Both the pellet-filled solid material filling area and either the He filling area 48 or the container 51 and the solid material filling area may be arranged above the upper end of the nuclear fuel material filling area.

Each fuel assembly of Examples 1 to 3 described above can be used not only in an improved boiling water nuclear power plant (ABWR plant) equipped with an internal pump, but also in a boiling water nuclear power plant equipped with a recirculation system pump ( BWR plant) can also be applied.

1 to 8, 11, H1, H2, H3... Fuel rods 10, 10A, 10B, 10C, 10D, 10E... Fuel assemblies 12... Full length fuel rods 13... Part length fuel rods H1, H2, H3... He fuel rods 10, 10A, 10B, 10C, 10D, 10E ... fuel assembly 14 ... upper tie plate (upper fuel rod support member) 15 ... lower tie plate (lower fuel rod support member) 16 ... channel box , 16A, 16B, 16C, 16D... Side wall part 18... Channel fastener 19... Leaf spring 20... Reactor 21... Reactor pressure vessel 22... Core shroud 23... Core 24... Upper grid plate 25 ... core support plate, 33 ... control rods, 48 ... He filling region, 48A ... gas filling region.

Claims (11)

a lower fuel support member;
an upper fuel support member;
a tubular channel box having an upper end attached to the upper fuel support member and extending toward the lower fuel support member;
a plurality of fuel rods having lower ends supported by the lower fuel support member and upper ends supported by the upper fuel support member and containing nuclear fuel material;
A fuel assembly in which the plurality of fuel rods bundled by a plurality of fuel spacers are arranged inside the channel box,
the plurality of fuel rods includes a plurality of first fuel rods and a second fuel rod;
The active fuel length of the first fuel rod is the first active fuel length of the fuel assembly, and the active fuel length of the second fuel rod is the second active fuel length shorter than the first active fuel length. ,
The second fuel rod is internally filled with a material having a neutron absorption cross section smaller than that of light water.
a material filled region extending from the upper end of the first active fuel length to the upper end of the second active fuel length located below the upper end,
The second fuel rods are arranged in the outermost peripheral region facing the inner surface of the side wall facing the control rods, among the plurality of side walls constituting the channel box,
said substance having a neutron absorption cross section smaller than that of light water is at least one of a gas having a neutron absorption cross section smaller than that of light water and a solid substance having a neutron absorption cross section smaller than that of light water;
A fuel assembly, wherein said solid material having a neutron absorption cross section smaller than that of light water is filled in said solid material filled region. a lower fuel support member;
an upper fuel support member;
a tubular channel box having an upper end attached to the upper fuel support member and extending toward the lower fuel support member;
a plurality of fuel rods having lower ends supported by the lower fuel support member and upper ends supported by the upper fuel support member and containing nuclear fuel material;
a channel fastener attached to one corner of the upper fuel support member to attach the channel box to the upper fuel support member;
A fuel assembly in which the plurality of fuel rods bundled by a plurality of fuel spacers are arranged inside the channel box,
the plurality of fuel rods includes a plurality of first fuel rods and a second fuel rod;
The active fuel length of the first fuel rod is the first active fuel length of the fuel assembly, and the active fuel length of the second fuel rod is the second active fuel length shorter than the first active fuel length. ,
The second fuel rod has a material-filled region filled with a material having a neutron absorption cross section smaller than that of light water. It is formed over the upper end of the length,
The second fuel rods are arranged in the outermost peripheral region facing the inner surface of the side wall portion extending in one direction from the corner portion on the side of the channel fastener among the plurality of side wall portions constituting the channel box. and
said substance having a neutron absorption cross section smaller than that of light water is at least one of a gas having a neutron absorption cross section smaller than that of light water and a solid substance having a neutron absorption cross section smaller than that of light water;
A fuel assembly, wherein said solid material having a neutron absorption cross section smaller than that of light water is filled in said solid material filled region. a lower fuel support member;
an upper fuel support member;
a tubular channel box having an upper end attached to the upper fuel support member and extending toward the lower fuel support member;
a plurality of fuel rods having lower ends supported by the lower fuel support member and upper ends supported by the upper fuel support member and containing nuclear fuel material;
A fuel assembly in which the plurality of fuel rods bundled by a plurality of fuel spacers are arranged inside the channel box,
the plurality of fuel rods includes a plurality of first fuel rods and a second fuel rod;
The active fuel length of the first fuel rod is the first active fuel length of the fuel assembly, and the active fuel length of the second fuel rod is the second active fuel length shorter than the first active fuel length. ,
The second fuel rod is internally filled with a material having a neutron absorption cross section smaller than that of light water.
a material filled region extending from the upper end of the first active fuel length to the upper end of the second active fuel length located below the upper end,
The second fuel rods are arranged in the outermost peripheral region facing the inner surface of the side wall facing the control rods, among the plurality of side walls constituting the channel box,
When the length of the first effective fuel length is L, the axial length of the material-filled region extending downward from the upper end of the first effective fuel length is 2 L/24 or more and 18 L/24 or less. with a length within the range,
A fuel assembly, wherein the length in the axial direction of the material-filled region is within the range of 7 L/24 or more and 18 L/24 or less. a lower fuel support member;
an upper fuel support member;
a tubular channel box having an upper end attached to the upper fuel support member and extending toward the lower fuel support member;
a plurality of fuel rods having lower ends supported by the lower fuel support member and upper ends supported by the upper fuel support member and containing nuclear fuel material;
a channel fastener attached to one corner of the upper fuel support member to attach the channel box to the upper fuel support member;
A fuel assembly in which the plurality of fuel rods bundled by a plurality of fuel spacers are arranged inside the channel box,
the plurality of fuel rods includes a plurality of first fuel rods and a second fuel rod;
The active fuel length of the first fuel rod is the first active fuel length of the fuel assembly, and the active fuel length of the second fuel rod is the second active fuel length shorter than the first active fuel length. ,
The second fuel rod has a material-filled region filled with a material having a neutron absorption cross section smaller than that of light water. It is formed over the upper end of the length,
The second fuel rods are arranged in the outermost peripheral region facing the inner surface of the side wall portion extending in one direction from the corner portion on the side of the channel fastener among the plurality of side wall portions constituting the channel box. and
When the length of the first effective fuel length is L, the axial length of the material-filled region extending downward from the upper end of the first effective fuel length is 2 L/24 or more and 18 L/24 or less. with a length within the range,
A fuel assembly, wherein the length in the axial direction of the material-filled region is within the range of 7 L/24 or more and 18 L/24 or less. a lower fuel support member;
an upper fuel support member;
a tubular channel box having an upper end attached to the upper fuel support member and extending toward the lower fuel support member;
a plurality of fuel rods having lower ends supported by the lower fuel support member and upper ends supported by the upper fuel support member and containing nuclear fuel material;
A fuel assembly in which the plurality of fuel rods bundled by a plurality of fuel spacers are arranged inside the channel box,
the plurality of fuel rods includes a plurality of first fuel rods and a second fuel rod;
The active fuel length of the first fuel rod is the first active fuel length of the fuel assembly, and the active fuel length of the second fuel rod is the second active fuel length shorter than the first active fuel length. ,
The second fuel rod is internally filled with a material having a neutron absorption cross section smaller than that of light water.
a material filled region extending from the upper end of the first active fuel length to the upper end of the second active fuel length located below the upper end,
The second fuel rods are arranged in the outermost peripheral region facing the inner surface of the side wall facing the control rods, among the plurality of side walls constituting the channel box,
Assuming that the length of the first effective fuel length is L, the position of the boundary between the lower end of the material filling region and the upper end of the second effective fuel length is 2L/24 from the upper end of the first effective fuel length. A position that exists within a range between a position positioned below and a position positioned 18L/24 below from the upper end of the first effective fuel length,
The position of the boundary between the upper end of the second active fuel length and the lower end of the material filled region is located 7L/24 below the upper end of the first active fuel length and from the upper end of the first active fuel length. A fuel assembly having a position within a range of positions located 18L/24 below. a lower fuel support member;
an upper fuel support member;
a tubular channel box having an upper end attached to the upper fuel support member and extending toward the lower fuel support member;
a plurality of fuel rods having lower ends supported by the lower fuel support member and upper ends supported by the upper fuel support member and containing nuclear fuel material;
a channel fastener attached to one corner of the upper fuel support member to attach the channel box to the upper fuel support member;
A fuel assembly in which the plurality of fuel rods bundled by a plurality of fuel spacers are arranged inside the channel box,
the plurality of fuel rods includes a plurality of first fuel rods and a second fuel rod;
The active fuel length of the first fuel rod is the first active fuel length of the fuel assembly, and the active fuel length of the second fuel rod is the second active fuel length shorter than the first active fuel length. ,
The second fuel rod has a material-filled region filled with a material having a neutron absorption cross section smaller than that of light water. It is formed over the upper end of the length,
The second fuel rods are arranged in the outermost peripheral region facing the inner surface of the side wall portion extending in one direction from the corner portion on the side of the channel fastener among the plurality of side wall portions constituting the channel box. and
Assuming that the length of the first effective fuel length is L, the position of the boundary between the lower end of the material filling region and the upper end of the second effective fuel length is 2L/24 from the upper end of the first effective fuel length. A position that exists within a range between a position positioned below and a position positioned 18L/24 below from the upper end of the first effective fuel length,
The position of the boundary between the upper end of the second active fuel length and the lower end of the material filled region is located 7L/24 below the upper end of the first active fuel length and from the upper end of the first active fuel length. A fuel assembly having a position within a range of positions located 18L/24 below. a lower fuel support member;
an upper fuel support member;
a tubular channel box having an upper end attached to the upper fuel support member and extending toward the lower fuel support member;
a plurality of fuel rods having lower ends supported by the lower fuel support member and upper ends supported by the upper fuel support member and containing nuclear fuel material;
A fuel assembly in which the plurality of fuel rods bundled by a plurality of fuel spacers are arranged inside the channel box,
the plurality of fuel rods includes a plurality of first fuel rods and a second fuel rod;
The active fuel length of the first fuel rod is the first active fuel length of the fuel assembly, and the active fuel length of the second fuel rod is the second active fuel length shorter than the first active fuel length. ,
The second fuel rod is internally filled with a material having a neutron absorption cross section smaller than that of light water.
a material filled region extending from the upper end of the first active fuel length to the upper end of the second active fuel length located below the upper end,
The second fuel rods are arranged in the outermost peripheral region facing the inner surface of the side wall facing the control rods, among the plurality of side walls constituting the channel box,
said substance having a neutron absorption cross section smaller than that of light water is at least one of a gas having a neutron absorption cross section smaller than that of light water and a solid substance having a neutron absorption cross section smaller than that of light water;
The gas having a neutron absorption cross section smaller than that of light water is filled in the gas-filled region, which is the material-filled region;
the gas-filled region is a He-filled region filled with He gas having a neutron absorption cross section smaller than that of light water;
A fuel assembly, wherein a sealed container in which the gas-filled region is formed is arranged above an upper end of the second active fuel length within the second fuel rod. a lower fuel support member;
an upper fuel support member;
a tubular channel box having an upper end attached to the upper fuel support member and extending toward the lower fuel support member;
a plurality of fuel rods having lower ends supported by the lower fuel support member and upper ends supported by the upper fuel support member and containing nuclear fuel material;
a channel fastener attached to one corner of the upper fuel support member to attach the channel box to the upper fuel support member;
A fuel assembly in which the plurality of fuel rods bundled by a plurality of fuel spacers are arranged inside the channel box,
the plurality of fuel rods includes a plurality of first fuel rods and a second fuel rod;
The active fuel length of the first fuel rod is the first active fuel length of the fuel assembly, and the active fuel length of the second fuel rod is the second active fuel length shorter than the first active fuel length. ,
The second fuel rod has a material-filled region filled with a material having a neutron absorption cross section smaller than that of light water. It is formed over the upper end of the length,
The second fuel rods are arranged in the outermost peripheral region facing the inner surface of the side wall portion extending in one direction from the corner portion on the side of the channel fastener among the plurality of side wall portions constituting the channel box. and
said substance having a neutron absorption cross section smaller than that of light water is at least one of a gas having a neutron absorption cross section smaller than that of light water and a solid substance having a neutron absorption cross section smaller than that of light water;
The gas having a neutron absorption cross section smaller than that of light water is filled in the gas-filled region, which is the material-filled region;
the gas-filled region is a He-filled region filled with He gas having a neutron absorption cross section smaller than that of light water;
A fuel assembly, wherein a sealed container in which the gas-filled region is formed is arranged above an upper end of the second active fuel length within the second fuel rod. 3. The fuel assembly according to claim 1 , wherein said solid material filling region is a region filled with any one of graphite, beryllium, beryllium oxide and beryllium carbide. In the cross section of the fuel assembly, the average enrichment of fissile plutonium in the outermost region is lower than the average enrichment of fissile plutonium in the central region inside the outermost region. 10. The fuel assembly according to any one of Items 1 to 9 . The lower end is supported by the lower fuel support member and the upper end is not supported by the upper fuel support member, the length in the axial direction is shorter than the first fuel rod and the second fuel rod, and the nuclear fuel material is carried. a plurality of third fuel rods including
11. The fuel assembly according to any one of claims 1 to 10 , wherein said third fuel rods are arranged inside said outermost peripheral region.

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