JP4180612B2 - Fuel assembly - Google Patents

Description

  The present invention relates to a fuel assembly used in a boiling water reactor, and more particularly, to a fuel assembly provided with a fuel spacer for maintaining an interval between fuel bundles.

  A conventional fuel assembly for a boiling water reactor is disclosed in Patent Document 1, for example. In this fuel assembly, a plurality of fuel rods filled with a large number of fuel pellets are arranged in a 9 × 9 square lattice, and a neutron moderator is provided in the substantially central region of the square lattice. Two water rods through which water flows are arranged, and the upper and lower ends of these fuel bundles are supported by the upper tie plate and the lower tie plate, respectively, and the whole is covered with a rectangular tube-shaped channel box. ing. At this time, the fuel rod and the water rod constituting the fuel bundle are held at regular intervals by a plurality of fuel spacers arranged in the axial direction. This fuel spacer is provided at each lattice position where fuel rods are arranged, and a large number of cylindrical members into which corresponding fuel rods are inserted one by one are bundled and joined, and the outer periphery thereof is formed by a square belt-like member. It has an enclosed structure. Further, a loop spring for pressing and holding the fuel rod is provided at a joining position between a pair of adjacent cylindrical members. The loop-shaped spring functions only after the fuel rods are inserted into both of the adjacent cylindrical members, so that a pressing force is generated. Furthermore, a water rod holding member having a substantially Ω-shaped cross section for holding the water rod is joined to two of the cylindrical members adjacent to the water rod.

  Here, as described in Patent Document 2, for example, the band-shaped member is generally configured by joining substantially L-shaped band plates in combination from four side surfaces. At that time, each joining portion is welded to the outer peripheral surface of the cylindrical member so as to contact the closest cylindrical member in the outermost peripheral region of the 9 × 9 square lattice array among the many cylindrical members. This is because of the following reasons. In other words, if the joining position is not in contact with the cylindrical member, it simply joins between the end portions of the strip, so that when the part is welded, melting or vice versa is caused. This is because it becomes easy and stable welding becomes difficult, which is not preferable from the viewpoint of productivity.

  By the way, in general, in a fuel assembly for a boiling water reactor, during operation, slightly unsaturated cooling water flows from the lower tie plate and is heated by the fuel rods as it flows from the lower portion to the upper portion between the fuel rods. Boils and flows out of the upper tie plate as a two-phase flow. As a result, the void fraction of the coolant is 0% at the bottom of the fuel assembly, but reaches about 70% at the top, and the hydrogen to uranium ratio (H / U ratio), which is a factor that determines the nuclear characteristics of the fuel assembly. Is greatly different in the axial position. Here, since water is used as a moderator in boiling water reactors, this water plays an important role in promoting the fission reaction. Therefore, the fission reaction is suppressed in a small area. Therefore, the H / U ratio described above is a major factor that determines the nuclear characteristics of the fuel assembly. Accordingly, it is very important to make the H / U ratio uniform in the radial direction and the axial direction from the viewpoint of improving the fuel assembly characteristics. Conventionally, the H / U ratio has been improved by various methods. ing. Although the arrangement of the water rod described above is one of the measures, as another measure, the length of some of the fuel rods is shorter than the other fuel rods (hereinafter referred to as short fuel rods as appropriate). The H / U ratio is made uniform by utilizing a saturated water region formed at the upper part of the short fuel rod.

  In designing a fuel assembly including this short fuel rod, for example, as disclosed in Patent Document 3, the short fuel rods are arranged in a lattice arrangement from the viewpoint of improving reactivity controllability by reducing the void coefficient. It may be arranged at the outermost periphery and at a position adjacent to the water rod.

JP-A-2-163695 JP-A-3-269294 Japanese Patent Laid-Open No. 5-232273

  By the way, as described above, since the fuel spacer is provided at a plurality of positions in the axial direction, the fuel spacer applied to the fuel assembly having the short fuel rod as described above is located above the upper end of the short fuel rod. In the case of, the fuel rod does not exist at the lattice position of the short fuel rod. In response to this, in the fuel spacer positioned above the upper end of the short fuel rod, a structure for reducing the pressure loss by deleting the cylindrical member at the lattice position has already been proposed. Here, in the fuel assembly in which the short fuel rods are arranged at the outermost periphery of the grid-like arrangement and the adjacent positions of the water rods as described above, the cylindrical member at the grid position of the fuel spacer located above the short fuel rods is arranged. The following issues exist when simply trying to delete.

  That is, in the normal fuel spacer, as described above, the cylindrical member that is in contact with the belt-like member surrounding the outer periphery of the spacer is continuously in contact, thereby maintaining the structural strength of the entire spacer. For example, when an external force acts on the fuel spacer through the channel box during an earthquake or when handling the fuel assembly, this load is first transmitted to the protruding member provided on the cylindrical member. After that, it is transmitted to the cylindrical member in the outermost peripheral area of the grid-like array arranged and joined inside through the belt-like member, and further sequentially transmitted to the cylindrical member on the inner peripheral side of the grid-like array. In such a normal fuel spacer that does not delete the cylindrical member, the band-shaped member and the cylindrical member are continuously arranged in the load transmission path, and these joined bodies are integrated to exert the strength securing effect. The structural strength of the entire fuel spacer is sufficiently secured.

  On the other hand, when the cylindrical member corresponding to the shortest fuel rod at the outermost periphery is deleted, the arrangement of the cylindrical members in the outermost peripheral region of the lattice arrangement becomes discontinuous at the deletion position, so a large number of cylindrical members The effect of securing the strength of the joined body is not so much exhibited, and the structural strength of the entire fuel spacer is lowered. In particular, in the vicinity of the deletion position, the load that should be handled by the portion of the strip member that is in direct contact with the deleted cylindrical member is the contact between the two cylindrical members in the outermost peripheral region adjacent to the deletion position and the strip member. Part (hereinafter referred to as the adjacent contact part as appropriate). Therefore, in these two adjacent contact portions, the load burden is particularly increased as compared with the contact portions between the cylindrical member and the belt-like member at other positions. Therefore, if the substantially L-shaped strips described above are combined and welded together and welded to the adjacent contact portion, it becomes difficult to ensure sufficient mechanical soundness at that position, and consequently the fuel spacer. It may be difficult to ensure sufficient structural soundness, and the shape maintaining function of the fuel assembly may deteriorate.

  The object of the present invention is to reduce the pressure loss by arranging the short fuel rods at the outermost periphery of the grid-like arrangement and the position adjacent to the water rod and by deleting the cylindrical member of the spacer at the grid position of the short fuel rods. An object of the present invention is to provide a structure that can sufficiently ensure the structural integrity of a fuel spacer in a fuel assembly.

(1) In order to achieve the above object, the present invention provides a plurality of fuel rods including a plurality of short fuel rods arranged in a square lattice and having a fuel effective length shorter than other fuel rods, and the fuel rods At least one water rod arranged in a region where one or more of the fuel rods can be arranged, and a plurality of fuel spacers that maintain a mutual radial distance between the plurality of fuel rods and the water rod and are provided at a plurality of axial positions Each of the plurality of fuel spacers includes a plurality of cylindrical members that are coupled to each other and into which the fuel rods are respectively inserted, and a plurality of divided pieces that are welded and joined. and a belt-shaped member surrounding the outermost, so that the welded portion between the front Symbol plurality of divided pieces, in contact with the tubular member in the outermost peripheral region of the square lattice array, an outer peripheral surface of the said tubular member fuel being welded to In the coalescence, the plurality of short fuel rods include at least one outermost short fuel rod disposed in the outermost peripheral region, and are above the upper end of the outermost short fuel rod among the plurality of fuel spacers. At least one although there is omitted the tubular member of the first grating position corresponding to the outermost periphery length fuel rods, the welds of the front Symbol strips together, a plurality of gratings in the outermost region The cylindrical member at the third lattice position so as to be in contact with the cylindrical member at the third lattice position other than the first lattice position and the second lattice position adjacent to both sides of the first lattice position among the positions. It is also welded to the outer peripheral surface .

  According to the present invention, when the short fuel rods are arranged at the outermost periphery of the lattice arrangement and the position adjacent to the water rod, the cylindrical member of the spacer at the lattice position of the short fuel rods is deleted to reduce the pressure loss. In addition, the structural integrity of the fuel spacer can be sufficiently secured.

  Embodiments of the present invention will be described below with reference to the drawings. A first embodiment of the present invention will be described with reference to FIGS.

  FIG. 2 is a longitudinal sectional view showing the structure of the fuel assembly according to this embodiment, and FIG. 3 is a transverse sectional view taken along the line AA in FIG. 2 and 3, the fuel assembly 1 is arranged in a square grid of 9 rows and 9 columns (hereinafter appropriately shown as 9 × 9) and filled with fuel pellets (not shown) 74. Two fuel rods 2, two water rods 3 arranged in an area where seven fuel rods 2 can be arranged, and a fuel spacer 4 that maintains the radial distance between the fuel rods 2 and the water rods 3. The upper tie plate 5 and the lower tie plate 6 hold the upper and lower ends of the fuel bundle composed of the fuel rod 2 and the water rod 3, respectively, and the channel box 8 that covers the outer periphery of the above structure.

  The fuel rod 2 is composed of a fuel rod 2a whose effective length is a normal length and a short fuel rod 2b whose effective length is shorter than the fuel rod 2a. The short fuel rod 2b is adjacent to the four short fuel rods 2b1 arranged at the lattice position in the outermost peripheral region of the square lattice array and the water rod 3 in the 3 × 3 lattice region in the center of the lattice array. And two short fuel rods 2b2 arranged at the lattice positions.

  The fuel spacers 4 are provided at a plurality of axial positions as shown in FIG. At this time, as shown in FIG. 3, 74 fuel rods 2 include short fuel rods 2b having a short effective length, which are located above the fuel assembly 1 and higher than the upper end position of the short fuel rods 2b. In the upper fuel spacer, there is no fuel rod at the lattice position of these short fuel rods 2b. Accordingly, in accordance with this, the structure of the fuel spacer 4A below the upper end position of the short fuel rod 2b and the fuel spacer 4B above are slightly different. A top view and a side view showing the structure of the fuel spacers 4A and 4B are shown in FIGS.

  4 and 1, the fuel spacers 4A and 4B are arranged in a 9 × 9 square lattice pattern corresponding to the arrangement of the fuel rods 2 and welded to each other, and the corresponding fuel rods 2 are respectively inserted. A large number (74 in the spacer 4A, 68 in the spacer 4B) of the cells (cylindrical members) 9 and a pair of adjacent cells 9 and 9 are pressed to press the fuel rods 2 in the cells 9. A loop rod 10, a square band member (band) 11 that surrounds the outermost periphery of the combined body of these cells 9, and a water rod that is welded and joined to the cell 9 in the innermost peripheral region of a square lattice array. A water rod holding member 12 having a substantially Ω-shaped cross section and a substantially 1/4 cylindrical water rod holding member 13 for holding 3 in the radial direction and the axial direction, and the water rod 3 provided to the water rod holding member 13 Pressure to do And a obtain water rod retaining spring 14.

  The cell 9 has a substantially cylindrical shape, and includes two protrusions 9a and 9a for maintaining the distance between the fuel rods 2 and a loop spring 10 so as to generate an appropriate pressing force. A spring support portion (not shown) for supporting is provided (including cells 9A and 9B described later). The structures of the loop spring 10 and the spring support are not particularly shown in detail, but are known, for example, the structures similar to those disclosed in Patent Document 1 and Japanese Patent Laid-Open No. 6-273560. ing. The band 11 is formed by combining four substantially L-shaped divided pieces (band plates) 11A from the side surfaces. It is comprised by joining. At that time, each welded portion 20 is also welded to the outer peripheral surface of the cell 9 so as to be in contact with the closest cell 9 in the outermost peripheral region of the 9 × 9 square lattice array among the many cells 9. (Details will be described later). Further, the band 11 is bent so as to protrude between two cells 9 adjacent to each other in the outermost peripheral region of the square lattice-like arrangement, and a plurality of flow cells 15 for inducing a coolant flow, and a square shape on each side. Two bathtubs 16 that are provided in two and protrude toward the channel box 8 and come into contact with the inner surface thereof are formed.

  Here, the feature of this embodiment is the difference between the fuel spacer 4A and the fuel spacer 4B.

  That is, the fuel spacer 4A in FIG. 4 has a basically similar structure to the fuel spacer disclosed in Patent Document 1, but the fuel spacer 4B in FIG. 1 is different from the fuel spacer 4B shown in FIG. The lattice positions (first lattice positions) 18a, 18b, 18c, 18d corresponding to the two short fuel rods 2b1 and the cells 9 corresponding to the lattice positions 18e, 18f corresponding to the two short fuel rods 2b2 are deleted. Accordingly, the arrangement of the loop-like springs 10 in the entire spacer is entirely changed from the arrangement shown in FIG. Therefore, two cells 9A and 9B in the same row adjacent column and the same row adjacent row of the lattice positions 18e and 18f adjacent to the water rod 3 are provided with the known spring support portions on the lattice position 18e and 18f side portions. Here, loop springs 10A and 10B for pressing the fuel rods 2 in the cells 9A and 9B are provided. The two loop springs 10A and 10B are supported by a substantially cylindrical spring pressing member 19 provided at the lattice positions 18e and 18f so as to generate a pressing force to the fuel rod 2 in the cells 9A and 9B. (Details will be described later).

  Further, in the fuel spacer 4A in FIG. 4, the welded portion 20 between the divided pieces 11A of the band 11 described above is located at the lattice point at the midpoint of each side of the quadrangle formed by the outermost peripheral region of the 9 × 9 array (the lattice in FIG. It is welded to the outer peripheral surface of the cell 9 so as to contact the cell 9 at the same position as the positions 18a to 18d. On the other hand, in the fuel spacer 4B of FIG. 1, the welded portion 20 has the first lattice positions 18a, 18b, 18c, 18d and the second lattice positions 18g, 18h, 18i, 18j, 18k, which are adjacent to both sides thereof. The third lattice positions other than 18l, 18m, and 18n (lattice positions adjacent to the second lattice position in this embodiment) 18o, 18p, 18q, and 18r are welded to the outer peripheral surface of the cell 9 so as to be in contact with the cell 9 Has been. However, the fuel spacer 4A in FIG. 4 and the fuel spacer 4B in FIG. 1 are different in the welding joint position of the band 11 in this way, but the other dimensions and structure are completely the same. The shape is almost the same.

  Next, the operation of the present embodiment configured as described above will be described.

  (1) Ensuring structural soundness by band welding position The cells 9 at the first lattice positions 18a, 18b, 18c and 18d corresponding to the outermost short fuel rod 2b1 are deleted for the purpose of reducing the pressure loss as in the fuel spacer 4B. In the structure, the adjacent contact portion 21 (see FIG. 1) that contacts the two cells 9 and 9 at the second lattice positions 18g to 18n adjacent to both sides of the first lattice positions 18a to 18d of the band 11. A load burden increases especially compared with the contact part of another position. Therefore, when the divided pieces 11A and 11A of the band 11 are welded and joined to the outer peripheral surface of the cell 9 at the second grid position 18g to n where the load burden is increased, the vicinity of the second grid position 18g to n. It may be difficult to ensure sufficient mechanical soundness. In the present embodiment, when the divided pieces 11A of the band 11 are welded together, the third lattice positions 18o, 18p, 18q, 18r, etc. other than the second lattice positions 18g to 18n, where the load burden has increased, are increased. It is bonded and fixed to the outer peripheral surface of the cell 9 in Accordingly, it is possible to prevent a decrease in mechanical soundness in the vicinity of the second lattice positions 18g to n as described above, and to sufficiently secure the structural soundness of the fuel spacer 4B itself.

  (2) When the fuel spacers 4A and 4B having two types of structures are mixed due to the existence of the short fuel rod 2b as in the fuel assembly 1 of different types of spacers depending on the band welding position, these two types of fuels In the spacers 4A and 4B, if the belt-like member 10 has the same shape as described above, it is difficult to visually distinguish the difference in type from the appearance after it is once incorporated into the fuel assembly 1. . In the present embodiment, the welding joint position of the split piece 11A of the band 11 is made different for each type of fuel spacer 4. That is, in the fuel spacer 4A, the position of the welded portion 20 between the divided pieces 11A is set to a grid position at the midpoint of each side of the quadrangle formed by the outermost peripheral area of the 9 × 9 array, whereas in the fuel spacer 4B, the third grid position 18o, 18p, 18q, 18r. In this way, by using the position of the welded portion 20 of the band split piece 11A as an identifier of the difference in structure, the difference in the type of the fuel spacer 4 can be easily seen from the appearance even after being assembled in the fuel assembly 1. Can be identified.

  Needless to say, it is useful for identifying the type of the fuel spacer 4 alone. That is, for example, in a state in which the fuel spacer 4 is stacked and stored in a storage container or the like, only the spacer side surface, that is, the band 11 portion can be confirmed. Therefore, the difference can be easily confirmed visually by the difference in the welded portion 20. It becomes.

In the first embodiment described above, the case where the fuel rods 2 are arranged in a square grid of 9 rows and 9 columns (hereinafter appropriately shown as 9 × 9) has been described as an example. For example, the present invention can be applied to an arrangement of 8 rows, 8 columns, and 10 rows and 10 columns, and the same effect can be obtained in these cases.

FIG. 4 is a top view and a side view showing the structure of a fuel spacer positioned above the upper end of a short fuel rod of a fuel assembly according to an embodiment of the present invention. It is a longitudinal cross-sectional view showing the structure of the fuel assembly provided with the spacer of FIG. It is a cross-sectional view by the AA cross section in FIG. FIG. 3 is a top view and a side view showing the structure of a fuel spacer positioned below the upper end of a short fuel rod of the fuel assembly of FIG. 2.

Explanation of symbols

1 Fuel assembly 2 Fuel rod 2b1 Short fuel rod (outermost short fuel rod)
2b2 short fuel rod 3 water rod 4 fuel spacer 4B fuel spacer (spacer above the upper end of the outermost short fuel rod)
9 cell (tubular member)
11 Band (band-shaped member)
11A Divided pieces 18a to d First lattice position 18g to n Second lattice position 18o to third lattice position 20 Welded portion

Claims (1)

A plurality of fuel rods including a plurality of short fuel rods arranged in a square lattice and having a fuel effective length shorter than that of the other fuel rods, and at least one fuel rod arranged in a region where one or more fuel rods can be arranged Water rods, and a plurality of fuel spacers and a plurality of fuel spacers provided at a plurality of locations in the axial direction while maintaining a mutual radial distance between the plurality of fuel rods and the water rods. comprising a plurality of tubular members coupled to the fuel rods are inserted, respectively, and a belt-shaped member in which a plurality of split pieces are made up by welding surrounds the outermost periphery of the plurality of tubular members, prior Symbol plurality of split In the fuel assembly in which the welds between the pieces are welded and joined to the outer peripheral surface of the cylindrical member so as to be in contact with the cylindrical member in the outermost peripheral region of the square lattice array,
The plurality of short fuel rods includes at least one outermost short fuel rod disposed in the outermost peripheral region,
At least one of the plurality of fuel spacers above the upper end of the outermost short fuel rod is omitted from the cylindrical member at the first lattice position corresponding to the outermost short fuel rod ,
Welds between front Symbol plurality of divided pieces, the third grid position other than the second lattice position adjacent to both sides of the first grating position and the first lattice positions of the plurality of lattice positions in the outermost region A fuel assembly, wherein the fuel assembly is also welded to the outer peripheral surface of the cylindrical member at the third lattice position so as to contact the cylindrical member.

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