Nuclear reactor fuel element sub-assemblies

Reference is made to fuel element assemblies for fast reactors. Normally these consist of a number of upstanding parallel pins between which liquid metal coolant flows. To facilitate insertion and withdrawal of fuel the pins are grouped in side-by-side sub-assemblies each comprising a bundle of pins enclosed by a tubular wrapper. The pins are end supported on a lower grid within the wrapper and are free to move longitudinally within a clearance limit afforded by an upper grid within the wrapper. The pins must be free to move longitudinally under irradiation growth and linear thermal expansion, but for stability of the reactor core it is desirable that the pins be held captive. Captivating the pins has been found difficult because of their close spacing and the need to maintain adequate coolant flow passages. In the arrangement described the pins have peripherally enlarged heads at one end and the heads are held captive between a fixed end abutment grid and a fixed cellular grid, with provision of coolant flow passages. The pins may have their enlarged heads at the ends which are intended to be uppermost in a reactor and be suspended from the cellular grid. Alternatively the pins may have their enlarged heads at the ends intended to be lowermost, in which case the pins are supported by the end abutment grid. (U.K.)