Patent Number: 041815718
Section: summary

BACKGROUND OF THE INVENTION This invention relates generally to nuclear reactors but more particularly to bracing grids for nuclear reactor fuel sub-assemblies. A nuclear reactor may have a fuel assembly comprising a plurality of fuel sub-assemblies each sub-assembly comprising a bundle of spaced fuel pins enclosed within a tubular wrapper. Where the fuel pins are closely spaced and slender as in a sub-assembly for a liquid metal cooled fast breeder nuclear reactor they may be spaced apart by a longitudinal series of bracing grids disposed at intervals along the sub-assembly to ensure that the coolant subchannels bounded by the fuel pins are maintained continuous. A typical bracing grid comprises a plurality of strips of material formed to produce a complex of hexagonally shaped cells in honeycomb pattern. Each sub-assembly may contain 250 to 350 fuel pins and the bracing grids are required to be manufactured to very close tolerances to ensure that the pins can be inserted in the longitudinal series of grids during assembly. A typical nuclear reactor of this kind with honeycomb bracing grids is disclosed in United States Pat. No. 4,036,690. Adverse tolerances in the construction of the grid can result in severe damage to the pins on insertion during assembly and on withdrawal during disassembly when the irradiated pins are fragile and it is an object of the invention to provide for a nuclear reactor a fuel sub-assembly having fuel pin bracing grids possessing a degree of compliancy whereby the fuel pins are enabled to be inserted and withdrawn collectively of the grids without causing serious damage. SUMMARY OF THE INVENTION According to the invention in a fuel pin bracing grid for a nuclear fuel sub-assembly and comprising a plurality of strips formed to produce a complex of hexagonally shaped unit cells in honeycomb pattern, each cell is formed from a discrete strip of material, each of three alternate sides of the cell having a window or opening formed therein and the remaining sides being continuous, each continuous side having an embossment to provide a guide pad for a fuel pin. The windows reduce the stiffness of the regions intermediate the ends of the cells thereby endowing the grid with sufficient compliancy for, according to one aspect, the pins to deflect resiliently the cells of successive grids on entry with low insertion loads and without causing serious damage by scoring of the pins, and, according to a second aspect, to enable the pins to be withdrawn by low withdrawal loads thereby minimising damage to brittle irradiated pins. The unit cell construction provides a strong grid with compliancy, it facilitates manufacture by jigs which maintain accurate the geometrical relationship of the cells and simplifies manufacture because the cells can be joined together by edge welds applied collectively from each face of the grid. In a preferred construction of bracing grid according to the invention the continuous sides of each unit cell have a pair of second embossments, the second embossments of each pair being disposed one on each side of the first embossments to form a linear group extending parallel to the longitudinal axis of the cells, the second embossments being of lesser radial height than the first embossments. The preferred construction of bracing grid has two levels of compliancy, firstly the compliancy of the intermediate regions of the cells as afforded by the windows and secondly the lesser compliancy afforded by the stiffer end regions of the cells wherein all the sides of the end regions are uninterrupted. The second embossments can provide additional supports for the fuel pins to resist bowing during irradiation and thereby facilitate withdrawal of the bundle of fuel pins from the wrapper and grid. The invention will reside in a nuclear fuel sub-assembly comprising a central fuel section extended at opposite ends by a locating section and a neutron shielding section and wherein the central fuel section comprises a multiplicity of elongate fuel pins spaced apart within a tubular wrapper by a series of bracing grids, each grid comprising a plurality of strips formed to produce a complex of hexagonally shaped unit cells in honeycomb pattern, each cell being formed from a discrete strip of material, each of three alternate sides of the cell having a window or opening formed therein and the remaining sides being continuous, each continuous side having an embossment to provide a guide pad for a fuel pin. A fuel sub-assembly embodying the invention has the advantage that it can be readily dismantled by cutting the wrapper to separate the end sections from the fuel section and withdrawing the fuel pins collectively from the wrapper and grid combination thereby greatly facilitating reprocessing. The invention will also reside in a liquid metal cooled fast breeder nuclear reactor of the kind wherein a nuclear fuel assembly is submerged in a pool of coolant, the fuel assembly comprising a plurality of fuel sub-assemblies upstanding in side-by-side array, each sub-assembly comprising a multiplicity of elongate fuel pins spaced apart within a tubular wrapper by a series of bracing grids each grid comprising a plurality of strips formed to produce a complex of hexagonally shaped unit cells in honeycomb pattern, each cell being formed from a discrete strip of material, each of three alternate sides of the cell having a window or opening formed therein and the remaining sides being continuous, each continuous side having a first embossment to provide a guide pad for a fuel pin and a pair of second embossments disposed one on each side of the first embossment to form a linear group extending parallel to the longitudinal axis of the cells, the second embossments being of lesser radial height than the first embossments. A liquid metal cooled fast breeder nuclear reactor of the pool kind has a fuel assembly comprising a multiplicity of slender fuel pins which are susceptible to severe bowing under irradiation; the second embossments of the bracing grids provide additional lateral support for the pins to restrain that bowing whenever the supports provided by the first embossments are inadequate because of the compliant nature of the grid.