Controlling low temperature sintering of UO2+x

UO2 nuclear fuel pellets are typically sintered at temperatures of approximately 1700 degrees C to achieve the high densities and large grain sizes necessary for safe reactor operation. Lowering this sintering temperature is desirable in order to decrease the energy input required for fuel manufacture. Hence, the effect of temperature, time, stoichiometry and ZrO2 doping on sintering efficacy have been investigated. ZrO2 doping, coupled with hyper-stochiometry, acted as a strong sintering aid, enabling higher densities and larger grain sizes when sintering at lower temperatures and for shorter periods compared to the undoped samples. Without ZrO2 doping, at 1500 degrees C sintering was strongly sensitive to hyper-stoichiometry and only weakly sensitive to sintering duration. Addition of 0.13 mol fraction ZrO2 increased theoretical density up to 10 % and the maximum grain size from 8 mu m to 40 mu m. Addition of 0.30 mol fraction ZrO2 resulted in even greater densification, reaching 98 % of maximum theoretical density, but also formation of a secondary phase that hindered grain growth.