GB2166900A - Fuel assembly spacer grid - Google Patents

Abstract

A grid for a nuclear reactor sub-assembly comprises a plurality of cells (12) each of which has two inwardly directed projections (13; 15; 16) such that each fuel pin is engaged by the two projections of each grid but at positions displaced by 120 DEG from one grid to the next. Such an arrangement facilitates dismantling of the irradiated sub-assembly prior to reprocessing of the fuel. <IMAGE>

Description

SPECIFICATION Grids for reactor sub-assemblies This invention relates to grids for sub-assemblies of nuclear reactors.

In a reactor sub-assembly a plurality of fuel pins are held by axially spaced grids supported in an outer housing. Each grid is of cellular structure, each cell receiving one pin.

An end is arranged to restrain the pins against axial movement. Each cell has three equiangularly spaced inwardly directed pips engaging and locating the pin.

In order to disassemble or breakdown the assembly for reprocessing of the fuel in the pins, the pins may have to be withdrawn from the sub-assembly one by one, which is laboriuos, rather than two or more (or all) at the same time because the pins, which have become swollen during use, are held fairly tightly by the pips; and it is difficult to withdraw the more swollen centre portions of the pins through the relatively unswollen end grid or grids.

According to the invention a grid for a reactor sub-assembly comprise a plurality of cells each for receiving a fuel pin therethrough, each cell having two angularly spaced, inwardly directed projections.

The projections in each cell in a particular grid may be in the same relative position, as viewed axially. The projections are preferably spaced by 1200.

The invention also includes a sub-assembly comprising a plurality of pins held in a plurality of grids as above, the grids being axially spaced, the projections in successive grids along the pins being displaced by 1200 in the same sense.

The invention may be performed in various ways and one specific embodiment with possible modifications will now be described by way of exampie with reference to the accompanying drawings in which Figs. 1 to 3 show respectively axial views of part of successive grids along a sub-assembly.

The grid 10 comprises lengths of metal bent to shape and welded at 11 to form an array or cell structure, each cell 12 having six sides.

In the present case each cell 12 has two inward projections or pips angularly spaced at 1200. The pips in each cell in a particular grid are in the same relative location, for example as shown in full line at 13.

In the next grid along the pin 26 or assembly, the pips are displaced by 1200 eg. as shown at 15 Fig. 2.

In the next grid along, the pips are displaced by a further 1200 eg. as shown at 16 Fig. 3.

In the fourth grid along the pin the pips are again displaced 1200 ie. the pips are again as in Fig. 1; and so on along the pin. It will be seen that the pin is gripped by two pips in each grid, and that, as viewed axially, the pips lie in three planes at 1200.

It is possible that one or more extra grids may be required.

The objective of the design is to relieve the interference load in each cell so that pins may be more easily withdrawn. The pins now only have to be flexed to be withdrawn and the withdrawal loads should be much smaller than with three pips in each cell. If the withdrawal loads are sufficiently reduced then possibly the pins could all be pulled out together using the pin end-holding grid afer making a single laser cut around the wrapper or housing.

If the load is still too high then it can be further relieved by making laser cuts around the wrapper between grids in order to release the lateral loads on the pins. The smaller sections of wrapper plus grid can then be combed off the pins.

The pins can be released from the end-holding grid by laser cutting through a long end cap above the grid leaving the pins intact for transport to the reprocessing plant.

1. A grid for a nuclear reactor sub-assembly comprising a plurality of pin-receiving cells, each having two angularly spaced, inwardly directed projections.

2. A grid as claimed in Claim 1 in which the projections in each cell are all in the same relative positions, as viewed axially.

3. A grid as claimed in Claim 1 or 2 in which the projections in each cell are spaced by 1200.

4. A nuclear reactor sub-assembly comprising a wrapper in which a plurality of grids each as claimed in Claim 1, 2 or 3 are mounted at axially spaced positions, the projections in successive grids being displaced by 1200 in the same sense.

5. A grid substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Grids for reactor sub-assemblies This invention relates to grids for sub-assemblies of nuclear reactors. In a reactor sub-assembly a plurality of fuel pins are held by axially spaced grids supported in an outer housing. Each grid is of cellular structure, each cell receiving one pin. An end is arranged to restrain the pins against axial movement. Each cell has three equiangularly spaced inwardly directed pips engaging and locating the pin. In order to disassemble or breakdown the assembly for reprocessing of the fuel in the pins, the pins may have to be withdrawn from the sub-assembly one by one, which is laboriuos, rather than two or more (or all) at the same time because the pins, which have become swollen during use, are held fairly tightly by the pips; and it is difficult to withdraw the more swollen centre portions of the pins through the relatively unswollen end grid or grids. According to the invention a grid for a reactor sub-assembly comprise a plurality of cells each for receiving a fuel pin therethrough, each cell having two angularly spaced, inwardly directed projections. The projections in each cell in a particular grid may be in the same relative position, as viewed axially. The projections are preferably spaced by 1200. The invention also includes a sub-assembly comprising a plurality of pins held in a plurality of grids as above, the grids being axially spaced, the projections in successive grids along the pins being displaced by 1200 in the same sense. The invention may be performed in various ways and one specific embodiment with possible modifications will now be described by way of exampie with reference to the accompanying drawings in which Figs. 1 to 3 show respectively axial views of part of successive grids along a sub-assembly. The grid 10 comprises lengths of metal bent to shape and welded at 11 to form an array or cell structure, each cell 12 having six sides. In the present case each cell 12 has two inward projections or pips angularly spaced at 1200. The pips in each cell in a particular grid are in the same relative location, for example as shown in full line at 13. In the next grid along the pin 26 or assembly, the pips are displaced by 1200 eg. as shown at 15 Fig. 2. In the next grid along, the pips are displaced by a further 1200 eg. as shown at 16 Fig. 3. In the fourth grid along the pin the pips are again displaced 1200 ie. the pips are again as in Fig. 1; and so on along the pin. It will be seen that the pin is gripped by two pips in each grid, and that, as viewed axially, the pips lie in three planes at 1200. It is possible that one or more extra grids may be required. The objective of the design is to relieve the interference load in each cell so that pins may be more easily withdrawn. The pins now only have to be flexed to be withdrawn and the withdrawal loads should be much smaller than with three pips in each cell. If the withdrawal loads are sufficiently reduced then possibly the pins could all be pulled out together using the pin end-holding grid afer making a single laser cut around the wrapper or housing. If the load is still too high then it can be further relieved by making laser cuts around the wrapper between grids in order to release the lateral loads on the pins. The smaller sections of wrapper plus grid can then be combed off the pins. The pins can be released from the end-holding grid by laser cutting through a long end cap above the grid leaving the pins intact for transport to the reprocessing plant. CLAIMS

1. A grid for a nuclear reactor sub-assembly comprising a plurality of pin-receiving cells, each having two angularly spaced, inwardly directed projections.

2. A grid as claimed in Claim 1 in which the projections in each cell are all in the same relative positions, as viewed axially.

3. A grid as claimed in Claim 1 or 2 in which the projections in each cell are spaced by 1200.

4. A nuclear reactor sub-assembly comprising a wrapper in which a plurality of grids each as claimed in Claim 1, 2 or 3 are mounted at axially spaced positions, the projections in successive grids being displaced by 1200 in the same sense.

5. A grid substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.