JPH02234094A - Grid spacer for fast breeder reactor fuel assembly - Google Patents

Abstract

PURPOSE:To reduce the interaction of a fuel rod and a grid spacer by providing plural pieces of projections of different height on a grid element, and constituting the grid spacer of the grid element in which these are combined and placed. CONSTITUTION:In a hexagonal grid element 1, projections 2, 2 and 3 are placed at every other surface, and height of the projection 3 is lower than the projection 2. Accordingly, the diameter of a virtual inscribed circle 9 for enveloping the apex of each projection is larger than the diameter of a supported fuel rod 5. In such a way, even if allowances are made for a manufacturing tolerance and an increase of the diameter of the rod caused by swelling, a gap is secured between the procections 2 - 3 and the rod 5, and no strong interaction occurs. A grid spacer 4 is constituted by combining a prescribed number of elements 1, and positions of the projections 3 of each element 1 are all in the same direction. As for the spacer 4, plural pieces are placed in the axial direction of a fuel rod assembly. The position in which the projection 3 exists is placed in a state that it is rotated by 120 deg. each at every spacer 4, therefore, when some rod 5 is looked at from the upper part, it becomes equal to the case when it is all supported by the projection 2.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a grid spacer for a fuel assembly used in a fast reactor, and in particular to a grid spacer for a fast reactor fuel assembly that reduces interaction with fuel rods. Regarding spacers.

[Prior Art] A fuel assembly for a nuclear reactor has a large number of fuel rods arranged in a channel box, and these fuel rods must be maintained at correct spacing within the channel box.

For this purpose, each fuel rod is fixed at its lower end to a support structure provided in the channel box. Further, depending on the case, the upper ends of the fuel rods may be simultaneously fixed to a support structure provided on the upper end side of the fuel rods in the channel box. Additionally, support grids (grids) are placed at equal intervals in the axial direction within the channel box in the middle of each fuel rod.
Supported by Subesa. The grid spacer maintains a predetermined spacing between each fuel rod;
This prevents undesirable problems caused by variations in the spacing between fuel rods during operation of a nuclear reactor.

Conventionally, this type of grid smoother has a plurality of grid plates arranged in a grid pattern, each intersection point is welded and fixed, and a protrusion is provided inside the mesh formed by each grid plate intersecting. It is designed to support each fuel rod.

Japanese Utility Model Publication No. 48-27360 discloses that while a certain degree of restraint is applied to the fuel rods in a light water reactor fuel assembly, excessive stress that may affect the integrity of the fuel rods due to the interaction between the grid spacer and the fuel rods is disclosed. A structure is shown that resiliently holds the fuel rods from occurring. However, such methods use relatively large diameter fuel rods, the operating temperature is about 300°C, which is lower than that of fast reactor fuel rods, and the fast neutron irradiation dose is gentler than that of fast reactor fuel rods. However, when applied to fuel assemblies for fast reactors that house many small-diameter fuel rods and have severe operating conditions such as operating temperature and fast neutron irradiation, the following reasons apply: It is not necessarily suitable for practical use.

In other words, because the diameter of fast reactor fuel rods is small as mentioned above, it is difficult to adopt a fine structure for the grid and smoother, and the function of the elastic material is maintained throughout the entire period by irradiation with high-temperature and high-speed neutrons. It is also difficult to do so.

For this reason, the grid spacer of a fuel assembly for a fast reactor has a structure in which relatively simple and strong grid elements are combined, as described in Japanese Patent Publication No. 52-24199. In order to suppress movement due to fuel rod flow and reduce interaction with the grid, the fuel rods must be strongly constrained to some extent by the grid spacer, so that the gap between the two is approximately zero. This is desirable.

[Problems to be solved by the invention] In fast reactors, when the stay period of the fuel rods in the reactor increases and the amount of fast neutron irradiation increases, the volume of stainless steel that is the cladding material expands due to swelling, which causes the fuel rods to increase in volume. Strong interactions may occur if the diameter increases and the gap between them is small. If the fuel rods and the grid spacer are fixed in the axial direction due to such interaction, strong axial stress will be generated in each fuel rod due to the difference in thermal expansion caused by the temperature difference between the fuel rods, which may affect the integrity of the fuel rods. Undesirable.

An object of the present invention is to provide a structure that suppresses imaging due to coolant flow and at the same time does not significantly change the strength of interaction even when the diameter of the fuel rod increases due to swelling.

[Means for solving the UR problem] The above purpose is to reduce the height of the plurality of protrusions provided on the grid element by 2 in the grid spacer for fast reactor fuel assemblies installed in multiple stages in the axial direction of the fuel rods. Divide into more than types,
This is achieved by configuring a grid spacer with grid elements arranged in combination and changing the protrusion position for each position in the axial direction.

Three or more protrusions are required on the grid element to define the position of the fuel rods, and the height of the first protrusion is such that by using protrusions of this height, the fuel rods and the tops of the protrusions are enveloped. The gap with the first virtual inscribed circle is set to be smaller than a threshold value for occurrence of fretting. On the other hand, the height of the second protrusion is set such that when combined with the first protrusion, a second virtual inscribed circle that envelops them becomes larger than the first virtual inscribed circle. . The position of the first protrusion is set so that the relative position (direction) with respect to the fuel rod changes for each stage of the grid base.

[Function] Regarding a particular grid spacer, the fuel rods are in contact with the first protrusion, but are not in contact with the second protrusion, resulting in unstable support in this cross section. However, since the position of the first protrusion with respect to a fuel rod is shifted for each stage, the first protrusion is placed in the direction of unstable support in the grid spacers above and below this grid spacer, and the instability is resolved. Therefore, the fuel rods can obtain a supporting function substantially equivalent to that of a grid spacer composed only of the first protrusions.

[Example] Embodiments of the present invention will be described with reference to the drawings.

FIGS. 1(a) and 1(b) are a cross-sectional view and a partially enlarged view of an embodiment of the present invention, and FIG. 1(C) is an assembled view of the grid/substrate. Fig. 2 illustrates the configuration of a fuel assembly for a fast reactor using the grid spacer of the present invention, and Fig. 2 (a) is a partial longitudinal section of the fuel assembly to which the grid spacer of the present invention is applied. The top view, (b) to (d) are views showing different arrangement relationships of the first protrusion and the second protrusion in the fuel assembly of the present invention, and (e) is a view showing the state in which the fuel rod is inserted. FIG. 3 is a plan view of the grid elements seen from above.

As shown in FIG. 1(a), a hexagonal grid element 1 has protrusions 2, 2.3 on three sides arranged on alternate sides, and if all three protrusions are at the same height, the first protrusion 2. In this case, the diameter of the first virtual inscribed circle 8 encompassing the apex of the protrusion is the same as the diameter of the fuel rod 5 to be supported. Since one of these three protrusions is the second protrusion 3 with a low height, the diameter of the virtual inscribed circle 9 that envelops the apex of the actual protrusion is compared to the diameter of the supporting fuel rod 5. It's getting bigger. As shown in Figure (b), even if the diameter of the fuel rod is increased due to manufacturing tolerances or swelling, a gap is maintained between the protrusion and the fuel rod, and no strong interaction occurs.

The grid spacer 4 is made by combining a predetermined number of grid elements 1 in a honeycomb shape, as shown in FIG. It is assembled in. For convenience of explanation below, the corner where the second protrusion 3 is located will be designated as A.

As shown in Figure 2(a), the grid spacer 4
are arranged at equal intervals in the axial direction of the fuel assembly. The corner A is rotated by 120 degrees for each stage as shown in FIGS. 2(a) to 2(d). Therefore, when a certain fuel rod 5 is viewed from above, it is shown in Fig. 2 (e
), it is equivalent to the case where all the parts are supported by the first protrusion 2.

[Effects of the Invention] As explained above, according to the grid spacer of the present invention, when each fuel rod is viewed as a whole bundle, it is equivalent to being supported only by the first protrusion, so that the fuel rods are restrained. In terms of function, it is equivalent to a grid element composed only of the first protrusion with a large height, and when viewed at each stage, the diameter of the second virtual inscribed circle is large, so it is difficult to resist the increase in the diameter of the fuel rod due to swelling. Therefore, sufficient margin can be secured.

[Brief explanation of the drawing]

1(a) and 1(b) are a cross-sectional view and a partially enlarged view of an embodiment of the present invention, FIG. 1(C) is an assembled view of a grid spacer constituted by the grid element of FIG. 1, FIG. 2(a) is a partial longitudinal cross-sectional view of a fuel assembly to which the grid/substrate of the present invention is applied, and FIG. 2(b) to (d) are the first and second protrusions in the fuel assembly of the present invention. Figures illustrating different arrangement relationships, respectively. Figure (e) is a plan view of the bundle viewed from above with fuel rods inserted. DESCRIPTION OF SYMBOLS 1... Grid element 2... First protrusion 3... Second protrusion 4... Grid spacer 5... Fuel rod 6... Trumpet tube 7... Fuel rod lower support structure 8...・First virtual inscribed circle 9...Second virtual inscribed circle (8
733) Agent Patent attorney Yoshiaki Inomata (and 1 other person) (B (C) Figure 1 (a)

Claims (1)

[Claims] A grid spacer for a fast reactor fuel assembly installed in multiple stages in the axial direction of a fuel rod, the grid for a fast reactor fuel assembly comprising grid elements provided with a plurality of types of protrusions of different heights. Spacer.