GB1116811A - Improvements in or relating to nuclear reactor fuel assemblies - Google Patents

Abstract

1,116,811. Fuel elements. UNITED KING- DOM ATOMIC ENERGY AUTHORITY. 22 Oct., 1965 [30 Oct., 1964], No. 44458/64. Heading G6C. A spacer grid for a cluster-type fuel element comprises a plurality of identical cells 2 of octagonal shape fabricated from strip material, alternate sides 3 of each cell 2 being secured to registering sides of adjacent cells so as to build up a complete cell structure, the outer sides of the outer cells of this structure being secured to a surrounding strip 9, and those sides 3 of each cell 2 which are not secured to registering sides of adjacent cells or to the strip 9 having inward projections 8, e.g. dimples or stampedout tongues, for contacting and locating a cylindrical fuel rod. The securing together of registering sides of adjacent cells and of sides of outer cells to the strip 9 can be effected by spot welding or by brazing. The strip 9 is provided with outward projections 10, e.g. dimples or stamped-out tongues, for locating the grid in a casing 11 forming part of the fuel element. Several spacer grids together with the four " corner " fuel rods are fabricated into a sub-assembly which is supported at the bottom of the fuel element and is able to expand and contract with temperature change, independently of the casing 11. The "corner" fuel rods are made up from sections corresponding in length to the longitudinal spacing between spacer grids. One section terminates above a spacer grid with an end cap formed with a screw - threaded spigot engaging a screwthreaded aperture in an end cap of the section below the spacer grid. The latter end cap has a recess in which is accommodated the arcuate portions of a corner cell 17 of the spacer grid, the cell being of generally cruciform section and secured by either spot welding or brazing on two adjacent sides to sides 3 of adjacent cells 2 and on the remaining two sides to the surrounding strip 9. A shim is provided between abutting faces of the end caps of the two rod sections and serves also to retain the arcuate portions of cell 17 in the recess. A locking pin is employed to prevent unscrewing of the rod sections. In an alternative construction, the grid is similar to that described above, but instead of having a square boundary has an octagonal one with alternate sides twice as long as the remaining sides. This is accomplished by leaving out the corner cells and incorporating suitably shaped strip members which are secured to two of the outer cells as well as to the octagonal surrounding strip. In both constructions, the spacer grids may be longitudinally located relative to the respective fuel element casing by securing each grid directly to the casing.