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

Abstract

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 "internal 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 < 10µm and uniformly distributed "closed" spherical pores in the diameter range of 2-5µm. 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 (45-55µm) 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.