JPS648799B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a fuel rod support grid for triangularly arranging nuclear reactor fuel rods, and more particularly to a fuel rod support grid for hexagonal fuel assemblies for sodium-cooled fast reactors.

The core of a nuclear reactor is usually made up of a collection of fuel assemblies. Each fuel assembly is supported and arranged by an axially spaced grid of fuel assemblies. The support grid defines the shape of the cells that receive the fuel rods and maintains the correct distance between the fuel rods.

Some nuclear reactors use hexagonal fuel assemblies with fuel rods arranged in a triangular arrangement or pitch.
Here, the triangular arrangement of fuel rods refers to an arrangement in which three imaginary lines connecting the centers of any three adjacent fuel rods in a fuel assembly form an equilateral triangle. Although support grids for triangular arrays have existed in the past, they have suffered from manufacturing and technical drawbacks, as discussed below.

A hexagonal grid for a triangular array of fuel rods consists of a number of corrugations superimposed with deeply undulating strips, which are surrounded by strips that form the grid enclosure. .

The overlapping of the corrugated strips creates a double wall section within the grid that impedes flow through the fuel assembly.

The depth of the corrugation of the stripes is not constant from the time of manufacture due to wear of the press die, for example. Experience has shown that changes in the depth of these waveforms are not a mixture of increases and decreases, but one of them is unevenly distributed, and the depth of a certain batch of stripes becomes slightly larger (or smaller) than the design value. . Currently used overlay methods accumulate errors and distort the height of the completed grid. This problem of distortion is caused by the pressing operation, but it first appears during the stacking operation, at which time the only way to adjust the dimensions is to utilize the flexibility of the dimples (spring curved portions). In practice, the cumulative error can sometimes be greater than the flexibility of the dimples, rendering the grid unusable due to the inability to insert some fuel rods.

Various manufacturing issues can cause the dimple position to vary from its correct position, and because of this, and because small changes in dimple position also change the distance between the dimple contact points within the cell, the fuel rods can become unnecessarily stiff. Or it will be loosely fixed.

Annular grids with a space in the center are currently manufactured by first stacking them, cutting them into hexagons, enclosing the outer surface to form a complete grid, and removing the center section to provide a strip on the inner surface. There is. A complete grid is required because the strips are positioned, aligned, and supported during welding with respect to previously welded strips. The center cannot simply be omitted since it is required during the manufacturing process. The materials and labor used to form the core are now wasted. Additionally, the annular grid must ultimately include at least two rows of fuel rod support holes to maintain a mechanically sound structure during the cutting operation.

It is therefore an object of the present invention to provide a fuel rod support grid for supporting fuel rods in a triangular arrangement that can be made into an annular or hexagonal shape without overlapping strips and without extra steps.

With this object in mind, the present invention provides a fuel rod support grid comprising interconnected metal strips supporting nuclear reactor fuel rods in a triangular arrangement, in which the strips are arranged as a plurality of concentric rings. and a plurality of webs are arranged to extend radially between the adjacent rings and are fixed to the adjacent rings to form a triangular array of cells for receiving reactor fuel rods. located in the fuel rod support grid.

This configuration of strips and webs can be applied to fuel assemblies with polygonal cross sections such as hexagons. The example described below is for use with hexagonal fuel assemblies.

Next, the present invention will be explained along with embodiments of the present invention shown in the accompanying drawings.

The grid shown in FIG. 1 supports 37 cylindrical fuel rods 1 arranged in a triangular array 2 to form a hexagonal assembly. The strip 3 is formed into a hexagonal ring 4 and welded at the corners 5. The radial webs 22 are provided only between adjacent rings 4 and are welded to the strips 3 at each end to form cells 9 for receiving the fuel rods 1. The spring curvature or dimple 20 is doubled and integral with the web 22. Second
The central cell 8 shown has a dimple on the ring 4 of the stripe.

3 and 4 show details of the double dimple 20 of FIG. 1, and FIGS. 5 and 6 show details of the single dimple 21 of FIG. 2 for supporting the central fuel rod. It is a figure showing details. These dimples 20, 21 are integral to the strip or web and are of the same material.

The grid of the present invention is not created using a superposition method and does not suffer from cumulative error problems. The grid is assembled within a welded fixture that receives and temporarily supports the flat strips in the appropriate shape and position. Differences in cell size are caused by welding fixtures and can be detected and corrected before grid fabrication. The cells of the grid are formed by selecting and arranging the webs 22 to provide intimate contact. When using a butt joint as shown in FIG. 7, close contact can be achieved by using webs of slightly different lengths, selected as necessary. When using staggered seam welding, as shown in FIG. 8, webs of the same size can be used since the joints can be slid to make intimate contact. The net change in cell size is not cumulative across the grid. In such a configuration, even if there is an error, it is within a range that can be absorbed by the dimples.

The flat plate-like single and double dimples 20, 21 of the present invention have line contact portions 16 (FIGS. 1 and 2), so that slight deviations in fuel rod position will not cause loosening.

The completed grid has no double walls, so there is less obstruction to flow.

Although the grid shown in FIG. 1 is useful in some applications, it may be necessary to arrange the fuel rods so closely together that there is no adequate line-of-sight clearance between the fuel rods to accommodate the hexagonal rings 4 of the flat strips 3. It cannot accept bar arrays.

The embodiment of FIG. 2 uses corrugated strips 10 to form a hexagonal ring 4. This grid has fuel rods closely spaced with no line-of-sight gaps between adjacent rings of fuel rods.

In this configuration, dimples are provided on both the web and the stripes. Figures 3-6 show details of double dimple 20 (Figure 4) and single dimple 21 (Figures 5 and 6). The central fuel rod 8 can also be supported by more than two dimples as shown in FIG.

The manufacturing method of this second embodiment can use webs 22 of varying length extending only between adjacent rings 4 or a staggered joint. The same advantages are obtained as in the first embodiment. Errors in the corrugation depth of the corrugated stripes 10 only affect the cells adjacent to a particular strip and are not cumulative across the grid and are thus within the adjustment range of the support dimples. The corrugated strips 10 can be supported with welded fixtures to provide the flexibility necessary to accommodate depth tolerances by using staggered seams or webs of different lengths.

Figure 2 shows dimple 2 not used in position 23.
1 is shown. These dimples 21 have the same standardized corrugated stripes 1 on all hexagonal rings 4.
Since 0 is used, it is in this position. These dimples may be omitted.

In accordance with the present invention, a web and strip structure is used to join the web and strip while being formed into the desired grid shape and supported within a welding machine and a welding fixture that includes a grooved plate. The strip is cut from a prefabricated standard strip having preformed dimples at appropriate spacing to the appropriate length to form the required ring. Standard strips may be of standard length or in large rolls. The cut strip is inserted into the groove of the plate of the welding fixture to form a hexagonal ring of a predetermined size. Insert the web into the groove in the plate of the welding fixture so that the web is in the proper position for welding to the ring. It is at this stage that the webs are selected to have slightly different lengths so that the webs are in tight contact with the rings and accommodate slight differences in distance between the rings.

Welding may be electron beam welding, resistance welding, laser welding or other welding methods.

FIG. 7 shows a butt joint between the web and the strip, strip 3 being resistance welded at point 26 and tangent line 27.
Welded by electron beam welding etc.

FIG. 8 shows a staggered joint (groove and tongue joint) between the web and the strip welded at the double contact line 28. According to the staggered joint, the standard web length is adjusted so as to absorb slight errors in the spacing between the rings by moving the tongue piece 30 in and out of the groove 31 while maintaining the welding double contact line 28. Allows flexibility in suitability. This feature reduces or eliminates the need to use webs of different lengths.

Although the grids of FIGS. 1 and 2 are illustrated for relatively small fuel assemblies, in practice the grids can be adapted to support larger numbers of fuel rods for larger fuel assemblies.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing an embodiment using the corrugated strip of the present invention, and FIG.
Figures 4 and 4 are detailed views of the double dimple along the lines - and - of Figure 1, respectively, and Figures 5 and 6 are detailed views of the double dimple along the lines - and - of Figure 2, respectively.
FIGS. 7 and 8 are detailed views of the single dimple along the strip and web. FIGS. 1...fuel rod, 4,10...stripe, 9,18...
...Cell, 22...Web.

Claims (1)

[Scope of Claims] 1. A fuel rod support grid comprising interconnected metal strips supporting nuclear reactor fuel rods in a triangular arrangement, the strips being arranged in a plurality of concentric rings; and a plurality of webs are provided to extend radially between the adjacent rings and are fixed to the adjacent rings to form a triangular array of cells for receiving reactor fuel rods. Fuel rod support grid. 2. A fuel rod support grid according to claim 1, wherein said ring is formed of a corrugated strip allowing the fuel rods to be arranged in a closely spaced triangular arrangement. 3. A fuel rod support grid according to claim 1 or 2, wherein the webs have different lengths. 4. The fuel rod support grid according to claim 1, 2 or 3, which has a spring curved portion having a portion in line contact with the fuel rod.