Characterisation of sintered UO2 pellets relevant to fuel design and performance

High density sintered UO2 pellets are used in the fuel for Indian Pressurised Heavy Water Reactors (PHWR). To-date more than 8000 fuel bundles have been irradiated in RAPS-1 with a success rate of 99.89%. Currently, from fabrication considerations, efforts are directed towards characterisation and quality control of UO2 powder and pellets. While the specification requirements on (a) moisture and halogen control and (b) density and redensification phenomenon could be met by improved manufacturing processes, certain specific problems were encountered in the areas of pellet microstructure and visual quality standards. Systematic studies were undertaken which have enabled a correlation of fuel performance with grain size in the range (5 - 300 microns) at the power ratings and fuel burnups as obtaining in PHWR. It has been observed that higher grain size has certain beneficial effect on fuel performance as also speculated by other workers. UO2 pellets exhibiting the phenomenon of tear-outs, hairline cracks, micro-cracks, end-capping and surface pits were investigated and revised quality control standards have been evolved. (auth.)