JP6238676B2 - Fuel assemblies and cores - Google Patents

Description

  The present invention relates to a fuel assembly, and more particularly to a fuel assembly and a core suitable for application to a boiling water reactor.

  In the core of the boiling water reactor, a large number of fuel assemblies configured by housing fuel bundles in a rectangular tubular channel box are arranged. Each fuel bundle includes a number of fuel rods enclosing a plurality of fuel pellets containing uranium, an upper tie plate that supports the upper end of the fuel rod, a lower tie plate that supports the lower end of the fuel rod, and the spacing between the fuel rods. A fuel spacer for holding the fuel. In general, water that does not boil exists outside the channel box in the nuclear reactor, and water that boiled due to heat generated by the fuel rods exists in the channel box.

  In general, neutrons generated from nuclear fuel increase the probability of reaction with fissile material as it is decelerated by water, so that fuel nearer boiling water tends to promote combustion than boiling water. is there. Accordingly, since the output of the outermost fuel rod (hereinafter referred to as the outermost fuel rod) facing the inner wall of the channel box is higher, the margin for the heat removal performance of each fuel rod is greater than that of the fuel in the center of the fuel assembly. Get smaller. In particular, among the outermost fuel rods, the fuel located in the vicinity of the four corners (four corners) is most affected by non-boiling water, so that the output tends to increase.

  Thus, for example, as described in Patent Document 1, there has been proposed one in which the arrangement pitch of fuel rods in the channel box (the closest distance between the fuel rods) is different among a plurality of fuel rods. In Patent Document 1, the arrangement pitch of the outermost layer fuel rods is made larger than the arrangement pitch of the second layer fuel rods arranged adjacent to the outermost layer fuel rods, and the arrangement pitch of the second layer fuel rods is the inner width of the channel box. The fuel rods are arranged so as to be 0.9 times to 0.96 times the distance equally divided by the number of fuel rods in one row.

JP 2012-93241 A

  In order to improve the economics of the fuel assembly loaded in the core in a boiling water reactor, the power generated from the fuel rods is arranged in consideration of the distribution of boiling water and non-boiling water. The structure must be able to maintain a thermal margin.

  In Patent Document 1, although the arrangement pitch of the second layer fuel rods is set to 0.9 to 0.96 times the distance obtained by equally dividing the inner width of the channel box by the number of fuel rods in one row, the arrangement of the outermost layer fuel rods Only the fact that the pitch is larger than the arrangement pitch of the second layer fuel rods is described. In addition, the fuel rod arrangement pitch of a general boiling water reactor is from 0.96 to 0.97 times the distance obtained by equally dividing the inner width of the channel box by the number of fuel rods in a row. The fuel rod arrangement pitch is set to 0.97 times or less, and when the outermost fuel rod arrangement pitch is reduced, the flow rate of the cooling water may be reduced and the thermal margin may be reduced.

  The present invention provides a fuel assembly and a core capable of improving thermal margin and increasing nuclear fuel loading.

In order to solve the above-described problems, the present invention provides a fuel assembly in which a plurality of fuel rods containing fissile material are arranged and accommodated in a square cylindrical channel box, and faces the channel box. The diameter of the outermost fuel rods arranged in the horizontal direction is larger than the average diameter of the fuel rods in the horizontal section of the fuel assembly, and the equal arrangement pitch of the outermost fuel rods is the second layer fuel adjacent to the outermost fuel rod. greater than equal arrangement pitch of the bars, and characterized in that the switch catcher down channel box width greater than 0.97 times the distance obtained by equally dividing the array number of a row of the outermost layer fuel rods.

The present invention also provides a fuel assembly in which a plurality of fuel rods containing a fissile material are arranged and accommodated in a square cylindrical channel box in a square lattice shape, and are arranged facing the channel box. The diameter of the outermost fuel rods is larger than the average diameter of the fuel rods in the horizontal cross section of the fuel assembly, and N is the number of arrangements in the channel box of the second fuel rods adjacent to the outermost fuel rods. the sequence number of a row of the outermost fuel rods and N + 1, the same arrangement pitch of the outermost fuel rods is greater than equal arrangement pitch of the second layer fuel rods, and, aliquoted Ji catcher down channel box width N + 2 It is characterized by being greater than 0.97 times the measured distance.

  ADVANTAGE OF THE INVENTION According to this invention, the fuel assembly and core which can improve a thermal margin and can increase a nuclear fuel loading can be provided.

  Problems, configurations, and effects other than those described above will be clarified by the following description of embodiments.

1 is a horizontal sectional view of a fuel assembly according to Embodiment 1, which is an embodiment of the present invention. 1 is a schematic view of a core loaded with a fuel assembly according to Embodiment 1. FIG. 1 is a schematic view of a fuel assembly according to Embodiment 1. FIG. FIG. 6 is a horizontal sectional view of a fuel assembly according to Embodiment 2, which is another embodiment of the present invention. FIG. 6 is a horizontal sectional view of a fuel assembly according to Embodiment 3, which is another embodiment of the present invention. It is a horizontal sectional view of the fuel assembly concerning Example 4 which is other examples of the present invention.

  The inventors examined the improvement of the combustion efficiency of the fissile material in the fuel assembly. Neutrons generated from nuclear fuel materials are decelerated with water, and when they become thermal neutrons, they are absorbed again by fissile materials and cause fission. This fission efficiency is high in the region with many thermal neutrons. Since neutrons lose energy by colliding with water and become thermal neutrons, the amount of cooling water is large, and the thermal neutron flux becomes large in a region where it does not boil. In a general boiling water nuclear reactor, boiling cooling water flows in the channel box, and non-boiling cooling water flows outside the channel box and in the water rod. In particular, in the gap region between the fuel assemblies outside the channel box, there is a lot of water and a large thermal neutron flux. In other words, the fissile material is prone to fission in the region close to the gap region. Furthermore, the inventors paid attention to the characteristics of non-fissile nuclear fuel materials represented by uranium 238. This substance is a parent substance that changes to a fissile material by absorbing neutrons. Neutron absorption is easy to absorb in the energy region of thermal neutrons, just like fission, and fission materials are also easily generated in regions with large thermal neutron flux. That is, regardless of the fissionability, the nuclear fuel material may be loaded on the outermost fuel rod adjacent to the channel box. In general, since the fuel rod has a cylindrical shape, in order to increase the loading amount, it is easy to cope with fuel production by increasing the outer diameter of the circle.

  On the other hand, from the viewpoint of thermal margin, the power of the outermost fuel rod adjacent to the channel box increases due to the large thermal neutron flux. Therefore, in conventional boiling water reactors, the enrichment in the corner portion of the outermost fuel rod is important. Fuel design has been carried out with reduced emissions. If the amount of fuel loaded on the outermost fuel rod is simply increased, the thermal margin becomes more severe. Therefore, the inventors have devised a configuration that secures thermal margin. The reason for the decrease in thermal margin is the lack of cooling water. In particular, when the fuel rod diameter is increased, the cooling water flow path is reduced. Since the outermost fuel rod is adjacent to the channel box, the outer flow path is adjacent to the non-heated wall, and the inner flow path is adjacent to the second fuel rod adjacent to the outermost fuel rod. There exists only. Therefore, for the outermost fuel rods, the inner passage has a tighter thermal margin than the outer passage, and it is necessary to widen this passage.

  Therefore, the inventors made the outermost fuel rod pitch larger than the second fuel rod pitch. In a fuel assembly formed by fuel rods arranged in a square lattice arrangement, an increase in the fuel rod pitch of each layer means that the square formed by that layer becomes larger, and conversely that the fuel rod pitch becomes smaller. , Which means that the square becomes smaller. That is, the cooling water flow path becomes large between the layers having a large difference in fuel rod pitch. In addition, the fuel rod pitch of the fuel in a general boiling water reactor is 0.96 to 0.97 times the distance obtained by equally dividing the inner width of the channel box by the number of fuel rods in one row. The thermal margin can be improved by increasing the pitch.

  Accordingly, as a first embodiment of the present invention, the inventors have made the outermost fuel rod diameter larger than the average cross-sectional fuel rod diameter and the outermost fuel rod pitch larger than the second fuel rod pitch. They found that the fuel economy can be improved by increasing the outermost fuel rod pitch to be larger than 0.97 times the distance obtained by equally dividing the inner width of the channel box by the number of fuel rods in one row.

  In order to increase the loading amount of the nuclear fuel material of the outermost fuel rod, the outermost fuel rod diameter may be increased. However, as the fuel rod is increased, the gap between adjacent fuel rods is reduced and the heat removal performance is reduced. . Therefore, as a second embodiment of the present invention, the inventors reduced the number of fuel rods only in the outermost layer in order to maintain the gap between adjacent fuel rods while increasing the fuel rod diameter and increasing the amount of loaded fission material. Devised to do.

Specifically, when the number of fuel rods arranged in the reference fuel assembly is n × n rows, n−1 outermost fuel rods are arranged in a row, and (n−2) × (N-2) column. In this case, in order to maintain the thermal margin is the outermost layer of the fuel rod pitch is larger than the fuel rod pitch of the second layer. The fuel rods in the second layer and below are the same as the assembly described above. In this fuel assembly, since the number of fuel rod arrangements cannot be defined, it is not possible to define the distance obtained by equally dividing the above-mentioned channel box inner width by the number of fuel rods in a row. Therefore, by utilizing the fact that the second layer fuel rods and the second layer fuel rods of the fuel assembly are arranged in the same manner, the number of arrangement of the second layer fuel rods in a row +2 is used as a general fuel rod arrangement. This is defined as the number of fuel rods in a row. Therefore, the inventors set N + 1 as the number of arrangements of the outermost fuel rods and N + 1 as the number of arrangements of the second layer fuel rods in one row, and set the outermost fuel rod diameter to the average cross-sectional fuel rod diameter. On the other hand, the outermost fuel rod pitch is larger than the second fuel rod pitch, and the outermost fuel rod pitch is made larger than 0.97 times the distance obtained by equally dividing the inner width of the channel box by N + 2. We found that fuel economy could be improved.

  Embodiments of the present invention will be described below with reference to the drawings.

  A fuel assembly of Example 1 applied to a boiling water reactor which is a preferred embodiment of the present invention will be described in detail with reference to FIGS. 1 and 2.

  FIG. 2 is a diagram showing a schematic structure of a boiling water reactor to which a fuel assembly according to the present embodiment and a core loaded with the fuel assembly are applied. As shown in FIG. 2, the boiling water reactor (BWR) has a reactor pressure vessel (reactor vessel) 103. A reactor core shroud 102 is installed in the reactor pressure vessel 103. In the following description, the reactor pressure vessel is referred to as RPV. In the core shroud 102, a core 105 loaded with a plurality of fuel assemblies described later is disposed. The steam separator 106 and the steam dryer 107 are disposed above the core 105 in the RPV 103. An annular downcomer 104 is formed between the RPV 103 and the core shroud 102. An internal pump 115 is disposed in the downcomer 104.

  The cooling water discharged from the internal pump 115 is supplied to the core 105 from below through the lower plenum 122. The cooling water is heated when passing through the core 105 and becomes a gas-liquid two-phase flow containing water and steam. The steam separator 106 separates the gas-liquid two-phase flow into steam and water. The separated steam is further dehumidified by the steam dryer 107 and guided to the main steam pipe 108. This steam is guided to a steam turbine (not shown) to rotate the steam turbine. A generator connected to the steam turbine rotates to generate electric power. The steam discharged from the steam turbine is condensed into water by a condenser (not shown). This condensed water is supplied into the RPV 103 through the water supply pipe 109 as water supply. The water separated by the steam separator 106 and the steam dryer 107 falls and reaches the downcomer 104 as cooling water.

  FIG. 1 is a cross-sectional view of a fuel assembly 1 according to the present embodiment, and FIG. 3 is a diagram showing a configuration of the fuel assembly 1. The fuel assembly 1 includes a plurality of fuel rods 2 and 3, an upper tie plate 5, a lower tie plate 6, a plurality of fuel spacers 8, a plurality of water rods WR and a channel box 7. The fuel rods 2 and 3 are sealed by filling a plurality of fuel pellets (not shown) in a cladding tube (not shown). The lower tie plate 6 supports the lower ends of the fuel rods 2 and 3, and the upper tie plate 5 holds the upper ends of the fuel rods 2 and 3. As shown in FIG. 1, these fuel rods 2 and 3 are arranged in 10 rows and 10 columns in the cross section of the fuel assembly 1. Two water rods WR having a cross-sectional area that occupies a region where four fuel rods 3 can be arranged are arranged at the center of the cross section. The lower ends of these water rods WR are supported by the lower tie plate 6, and the upper ends thereof are held by the upper tie plate 5. The plurality of fuel spacers 8 are arranged at predetermined intervals in the axial direction of the fuel assembly 1 and have a flow path through which cooling water flows between the fuel rods 2 and 3 and between the fuel rod 3 and the water rod WR. The fuel rod 3 and the water rod WR are held so as to be formed. A square cylindrical channel box 7 having a square cross section is attached to the upper tie plate 5 and extends downward. The fuel rods 2 and 3 bundled by the fuel spacer 8 are arranged in the channel box 7.

  The inner width (L) of the channel box 7 is about 13.4 cm, and the outer diameter (R1) of the outermost fuel rod 2 disposed adjacent to the channel box 7 is about 1.08 cm. The outer diameter (R2) of the adjacent second layer fuel rods 3 is about 0.9 cm, and the outer diameter of the water rod WR is about 2.5 cm. In this embodiment, the outer diameters of the third, fourth, and fifth layer fuel rods adjacent to the second layer fuel rod 3 and disposed on the center side of the fuel assembly 1 are the second layer fuel rods. In the following description, the area occupied by the cross section of the fuel assembly 1 will be described using the second-layer fuel rod 3 for the sake of convenience.

  The water rod WR in the fuel assembly 1 is a large diameter water rod having a cross-sectional area that occupies a region where at least two second layer fuel rods 3 can be arranged. The length of the region of the fuel pellet containing fissile uranium filled in the standard fuel rod used for the outermost fuel rod 2 and the second fuel rod 3, that is, the effective fuel length is 3.7 m in this embodiment. .

As shown in FIG. 1, when the fuel assembly 1 is loaded on the core of a boiling water reactor, a control rod (CR) having a cross-shaped cross section in which one corner is inserted into the core. 9 is arranged to face 9. The channel box 7 is attached to the upper tie plate 5 by a channel fastener (not shown). The channel fastener has a gap of a necessary width between the fuel assemblies 1 so that control rods (CR) 9 can be inserted between the fuel assemblies 1 when the fuel assemblies 1 are loaded into the core. Has the function of holding. For this reason, the channel fastener is attached to the upper tie plate 5 so as to be located at a corner facing the control rod (CR) 9. In other words, the corner portion of the fuel assembly 1 facing the control rod (CR) 9 is a corner portion to which a channel fastener is attached. Each fuel pellet filled in each fuel rod such as the outermost layer fuel rod 2 and the second layer fuel rod 3 is manufactured using uranium dioxide which is a nuclear fuel material, and includes uranium-235 which is a fissile material. Yes.

  The outermost fuel rod 2 and the second fuel rod 3 have the same fuel rod pitch in each layer. The outermost fuel rod pitch P1 which is the distance between the centers of two adjacent outermost fuel rods 2 is about 1.36 cm, and the second fuel rod which is the distance between the centers of two adjacent second fuel rods 3 The pitch P2 is about 1.3 cm. The second layer fuel rod pitch P2 is smaller than the outermost layer fuel rod pitch P1. Further, the channel box inner width L is equal to 0 by a distance equally divided by the number of fuel rods in one column (in this embodiment, the number of fuel rods is 10 in 10 rows and 10 columns). Since .97 times is 1.299 cm and less than 1.3 cm, the outermost fuel rod pitch P1 is larger than this distance. Moreover, the average value of the fuel rod diameter in the cross section is about 1.0 cm, which is smaller than the outermost fuel rod diameter R1.

  The upper limit of the outermost fuel rod pitch P1 is obtained by subtracting the distance obtained by subtracting the outer diameter R1 of the outermost fuel rod 2 from the inner width L of the channel box from the number of arrangements in one row of the outermost fuel rod 2 in the channel box. The distance divided by the value. That is, in this embodiment, when the number of arrangements of the outermost fuel rods 2 in the channel box is N, (L−R1) / (N−1), and the upper limit is 1.369 cm, the outermost fuel rod pitch. P1 is 1.36 cm, which is less than the upper limit.

  According to the present embodiment, the distance between the outermost fuel rod 2 and the second fuel rod 3 can be increased, and the flow rate of the cooling water flowing between the outermost fuel rod 2 and the second fuel rod 3 is increased. As a result, the heat removal performance for the outermost fuel rod 2 with high output is improved. Further, by increasing the outer diameter R1 of the outermost fuel rod 2 to be larger than the outer diameters of the other fuel rods, the amount of nuclear fuel loaded can be increased, and the economical efficiency can be improved by reducing the frequency of fuel replacement.

  A fuel assembly of Example 2 applied to a boiling water reactor which is a preferred embodiment of the present invention will be described in detail with reference to FIG.

  FIG. 4 is a cross-sectional view of a fuel assembly according to Embodiment 2 of the present invention. The cross-shaped control rod 9 shown in FIG. 1 is omitted. The present embodiment is different from the first embodiment in that the number of arrangements in one example of the outermost fuel rod 2 in the channel box 7 is changed from 10 to 9. Thus, in this embodiment, the outermost fuel rods 2 arranged in the channel box 7 are 32, the outermost fuel rod diameter R1 is about 1.2 cm, and the outermost fuel rod pitch P1 is about 1.4 cm. .

  The second layer fuel rod pitch P2 is about 1.3 cm as in the first embodiment, the second layer fuel rod diameter R2 is also about 0.9 cm as in the first embodiment, and the outer diameter of the water rod WR is about 2.0. The inner width L of the channel box 7 is about 13.4 cm. Since the number of arrays in the channel box of the second layer fuel rods 3 is 8, 0.97 times the distance obtained by equally dividing the inner width L of the channel box by the number of arrays of the second layer fuel rods 3 +2 is 1. Since it is .299 cm and less than 1.3 cm, the outermost fuel rod pitch P1 is larger than this distance. The lower limit of the outermost fuel rod pitch P1 is determined on the basis of the case where the outermost fuel rods 2 are arranged in 10 rows and 10 columns, although the number of arrangement of the outermost fuel rods 2 in the channel box is nine. . Moreover, the average value of the fuel rod diameter in the cross section is about 1.0 cm, which is smaller than the outermost fuel rod diameter R1.

  Note that the upper limit of the outermost fuel rod pitch P1 is, for example, a distance obtained by subtracting the outer diameter R1 of the outermost fuel rod 2 from the inner width L of the channel box from the number of arrangements of the outermost fuel rods 2 in one channel box. The distance divided by the subtracted value. In other words, in this embodiment, when the number of arrangements in a row of the outermost fuel rods 2 in the channel box is N, (L−R1) / (N−1), and the upper limit is 1.525 cm, and the outermost fuel pitch P1. Is 1.4 cm, which is less than the upper limit.

  According to the present embodiment, the distance between the outermost fuel rod 2 and the second fuel rod 3 can be increased, and the flow rate of the cooling water flowing between the outermost fuel rod 2 and the second fuel rod 3 is increased. As a result, the heat removal performance for the outermost fuel rod 2 with high output is improved.

  Compared with the first embodiment, the number of arrangements of the outermost fuel rods 2 in the channel box 7 is reduced, but the outer diameter of the outermost fuel rods 2 can be increased, so that the outermost fuel rods 2 are filled. Nuclear fuel loading can be further increased. As a result, the frequency of fuel replacement is reduced, and further economic efficiency can be improved.

  A fuel assembly of Example 3 applied to a boiling water reactor which is a preferred embodiment of the present invention will be described in detail with reference to FIG.

  FIG. 5 is a cross-sectional view of the fuel assembly according to Embodiment 3 of the present invention, and is a cross-sectional view above the upper end portion of the partial-length fuel rod and within the fuel effective length range of the standard fuel rod. This embodiment is different in that the outermost fuel rods 2 arranged at the four corners (four corners) of the rectangular cylindrical channel box 7 in the first embodiment are partial-length fuel rods. Here, the partial-length fuel rod refers to a fuel rod whose effective fuel length is shorter than the effective fuel length which is the length of the region of the fuel pellet filled in the standard fuel rod. Other configurations are the same as those of the first embodiment, and the description thereof is omitted.

  Compared with the other outermost fuel rods 2, the region of the partial-length fuel rods arranged at the four corners indicated by the dotted lines in FIG. By arranging the partial length fuel rods whose length in the height direction is shorter than the other outermost fuel rods 2 at the four corners, the number of fuel rods with high output is reduced above the fuel assembly, that is, downstream of the cooling water flow. In addition, the output peaking of the cross section is reduced. This increases the thermal margin.

  In addition, the void ratio is high in the vicinity of the outermost fuel rod 2 where the output is high, and the void generated on the upstream side of the cooling water is coupled toward the downstream side, so that the gap between the outermost fuel rod 2 and the second fuel rod 3 is increased. There is a possibility of obstructing the flow of cooling water flowing through. In the present embodiment, by arranging the partial length fuel rods at the four corners, a space is formed on the downstream side of the cooling water, the voids easily flow, and the pressure loss of the fuel assembly can be reduced.

  In this embodiment, the outermost fuel rods 2 arranged at the four corners of the fuel assembly shown in FIG. 1 are the partial-length fuel rods. However, the fuel assembly shown in FIG. The outermost fuel rods 2 arranged at the four corners may be partial-length fuel rods.

  According to the present embodiment, in addition to the effects of the first embodiment and the second embodiment, it is possible to further reduce the pressure loss of the fuel assembly.

  A fuel assembly according to a third embodiment applied to a boiling water reactor which is a preferred embodiment of the present invention will be described in detail with reference to FIG.

  FIG. 6 is a cross-sectional view of a fuel assembly according to Embodiment 4 of the present invention. The fuel effective length range of the standard fuel rod is above the upper end of the partial-length fuel rod arranged as the second-layer fuel rod 3. FIG. In the third embodiment, the outermost fuel rods 2 arranged at the four corners of the fuel assembly are replaced with the partial-length fuel rods, and in this embodiment, the fuel rods are arranged at the four corners of the fuel assembly among the second fuel rods 3. The difference is that the fuel rod is a partial-length fuel rod.

  As shown by dotted lines in FIG. 6, partial-length fuel rods are arranged at the four corners of the second layer fuel rods 3 in the fuel assembly. As a result, spaces can be formed on the downstream side of the cooling water at the four corners of the second layer fuel rods 3, and the heat removal performance for the outermost layer fuel rods 2 with high output can be improved. In the present embodiment, the amount of nuclear fuel loaded in the outermost fuel rod 2 can be secured in the same manner as in the first embodiment, so that the outermost fuel rod 2 is increased while increasing the amount of nuclear fuel loaded in the outermost layer relative to the third embodiment. Among them, the flow path on the downstream side of the cooling water can be expanded, the cooling water supply amount can be increased, and the thermal margin can be improved with respect to the increase in the output of the outermost fuel rods 2 arranged at the four corners. it can.

  In this embodiment, the second-layer fuel rods 3 arranged at the four corners of the fuel assembly shown in FIG. 1 are the partial-length fuel rods. However, the fuel assembly shown in FIG. The second layer fuel assemblies 3 arranged at the four corners may be partial-length fuel rods.

  In addition, this invention is not limited to an above-described Example, Various modifications are included. For example, the above-described embodiments have been described in detail for easy understanding of the present invention, and are not necessarily limited to those having all the configurations described. Further, a part of the configuration of one embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of one embodiment. Further, it is possible to add, delete, and replace the configurations of other embodiments with respect to a part of the configurations of the embodiments.

DESCRIPTION OF SYMBOLS 1 ... Fuel assembly, 2 ... Outermost layer fuel rod, 3 ... Second layer fuel rod, 5 ... Upper tie plate, 6 ... Lower tie plate, 7 ... Channel box, 8 ... Spacer, WR ... Water rod, 102 ... Core Shroud, 103 ... reactor pressure vessel, 104 ... downcomer, 105 ... core, 106 ... steam separator, 107 ... steam dryer, 108 ... main steam pipe, 109 ... feed water pipe, 115 ... internal pump, 122 ... lower part Plenum

Claims (9)

A fuel assembly in which a plurality of fuel rods containing fissile material are arranged and accommodated in a square lattice in a rectangular channel box,
The diameter of the outermost fuel rod arranged facing the channel box is larger than the average diameter of the fuel rod in the horizontal cross section of the fuel assembly,
The same arrangement pitch of the outermost fuel rods is greater than equal arrangement pitch of the second layer fuel rods adjacent to the outermost layer fuel rods, and, aliquoted Ji catcher down channel box width array size of one row of the outermost layer fuel rods A fuel assembly characterized by being greater than 0.97 times the measured distance. The fuel assembly according to claim 1, wherein
The equal arrangement pitch of the outermost fuel rods is obtained by dividing the distance obtained by subtracting the diameter of the outermost fuel rod from the inner width of the channel box by the number of arrays obtained by subtracting 1 from the number of arrays of the outermost fuel rods. A fuel assembly characterized by being smaller. A fuel assembly in which a plurality of fuel rods containing fissile material are arranged and accommodated in a square lattice in a rectangular channel box,
The diameter of the outermost fuel rod arranged facing the channel box is larger than the average diameter of the fuel rod in the horizontal cross section of the fuel assembly,
When the arrangement number of the second layer fuel rods adjacent to the outermost fuel rods in the channel box is N, the arrangement number of the outermost fuel rods in a row is N + 1, and the outermost fuel rods are equally arranged. pitch is greater than equal arrangement pitch of the second layer fuel rods and fuel assembly, characterized in that the switch catcher down channel box width greater than 0.97 times the distance divided equally N + 2. The fuel assembly according to claim 3, wherein
The equal arrangement pitch of the outermost fuel rods is obtained by dividing the distance obtained by subtracting the diameter of the outermost fuel rod from the inner width of the channel box by the number of arrays obtained by subtracting 1 from the number of arrays of the outermost fuel rods. A fuel assembly characterized by being smaller. The fuel assembly according to any one of claims 1 to 4, wherein:
A fuel assembly, wherein the outermost fuel rods arranged at the four corners in the rectangular cylindrical channel box are partial-length fuel rods among the outermost fuel rods. The fuel assembly according to any one of claims 1 to 4, wherein:
Of the second layer fuel rods adjacent to the outermost layer fuel rods, the second layer fuel rods disposed at positions facing the outermost layer fuel rods disposed at the four corners of the rectangular tube channel box are partially extended. A fuel assembly characterized by being a fuel rod.   A core in which a plurality of fuel assemblies according to any one of claims 1 to 4 are loaded. The core according to claim 7,
Of the outermost fuel rods, the outermost fuel rods disposed at the four corners of the rectangular tube channel box are partial-length fuel rods. The core according to claim 7,
Of the second layer fuel rods adjacent to the outermost layer fuel rods, the second layer fuel rods disposed at positions facing the outermost layer fuel rods disposed at the four corners of the rectangular tube channel box are partially extended. A reactor core characterized by fuel rods.

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