The effect of SPS processing parameters on the microstructure and thermal conductivity of ThO₂

Thorium dioxide (ThO₂), is a nuclear fuel that is expected to play a vital role in the Generation IV nuclear reactors. One of the challenges to implementation of thoria for the fuel cycle has been the difficulty in fabricating dense pellets via conventional sintering techniques. In this study, the non-conventional sintering of thoria using spark plasma sintering (SPS) was explored. A systematic investigation into the influence of processing parameters on the densification, microstructure, grain size and thermal conductivity of ThO₂ is presented. The range of sintering temperature, pressure, and hold time has been systematically varied between 1500 and 1800 °C, 50–70 MPa and 5–15 m, respectively. The results revealed that without the help of any sintering aid, pellets with a relative density of 95% theoretical density (TD) were fabricated at a sintering temperature of 1600 °C, sintering pressure of 50 MPa and a hold time of 10 min. Furthermore, the characterization of these specimens clearly indicates that by carefully selecting the processing parameters, the density, microstructure, grain size and the thermal conductivity of ThO₂ can be suitably controlled. This study shows that the use of the SPS technique can potentially solve one of the primary concerns in the front end of the thoria fuel cycle.