Fabrication of high density UO2 fuel pellets involving sol-gel microsphere pelletisation and low temperature sintering

Powder-free sol-gel microsphere pelletisation and low temperature (1473 K) oxidative sintering processes were used in combination for fabrication of high density (≥ 96% TD) UO2 fuel pellets for pressurised heavy water reactors. The 'international gelation of uranium' process of BARC was modified for preparation of hydrated gel-microspheres of UO3 containing 'carbon black' pore former. The gel-microspheres were subjected to controlled air-calcination at 973 K, followed by hydrogen reduction to obtain porous, dust-free and free-flowing UO2 microspheres suitable for direct pelletisation at 225 MPa. Oxidative sintering of these pellets at 1473 K in CO2 atmosphere followed by Ar + H2 treatment led to high density (≥ 96% TD) UO2 pellets having equiaxed grains of ≤ 10 μm and uniformly distributed 'closed' spherical pores in the diameter range of 2-5 μm. Resintering of these pellets at high temperature (1973 K) for 8 hours in Ar + 8% H2 atmosphere did not show any significant change in pellet dimension or grain size. The UO2 pellets prepared by sol-gel microsphere pelletisation route had higher thermal conductivity compared to pellets of equivalent density prepared by the 'powder-pellet' route. UO2 pellets of large grain size (45-55 μm) and high density could be obtained with TiO2 dopant and high temperature sintering in Ar + H2 atmosphere. TiO2 dopant was not effective for low temperature oxidative sintering. (orig.)