Controlling low temperature sintering of UO2+x

UO ₂ nuclear fuel pellets are typically sintered at temperatures of approximately 1700 °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 ZrO ₂ doping on sintering efficacy have been investigated. ZrO ₂ 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 ZrO ₂ doping, at 1500 °C sintering was strongly sensitive to hyper-stoichiometry and only weakly sensitive to sintering duration. Addition of 0.13 mol fraction ZrO ₂ increased theoretical density up to 10 % and the maximum grain size from 8 µm to 40 µm. Addition of 0.30 mol fraction ZrO ₂ resulted in even greater densification, reaching 98 % of maximum theoretical density, but also formation of a secondary phase that hindered grain growth.