Oxygen-to-metal ratio control during fabrication of mixed oxide fast breeder reactor fuel pellets

Oxygen-to-metal ratio (O/M) of mixed oxide fuel pellets can be controlled during fabrication by proper selection of binder (type and content) and sintering conditions. Sintering condition adjustments involved the passing of Ar--8% H2 sintering gas across a cryostat ice bath controlled to temperatures ranging from -5 to -600C to control as-sintered pellet O/M ratio. As-sintered fuel pellet O/M decreased with increasing Sterotex binder and PuO2 concentrations, increasing sintering temperature, and decreasing sintering gas dew point. Approximate relationships between Sterotex binder level and O/M were established for PuO2--UO2 and PuO2--ThO2 fuels. O/M was relatively insensitive to Carbowax binder concentration. Several methods of increasing O/M using post-sintering pellet heat treatments were demonstrated, with the most reliable being a two-step process of first raising the O/M to 2.00 (stoichiometric) at 6500C in Ar--8% H2 bubbled through H2O, followed by hydrogen reduction to specification O/M in oxygen-gettered Ar-8% H2 at temperatures ranging from 1200 to 16900C