Spark plasma sintering of (U,Ce)O2 as a MOx nuclear fuel surrogate

Highlights: • Conventional and spark plasma sintering (SPS) used to manufacture UO₂ and (U, Ce)O₂ pellets. • Conventional Ce-MOx had bi-modal grain size with larger grains in Ce rich regions. • Enhanced grain growth due to redox reaction between Ce and U cations. • Ce rich islands within fuel specification for thermal reactor grade MOx for SPS pellets. • Ce content of Ce rich islands in SPS was much higher than conventional samples. Mixed uranium plutonium oxides (MOx) are currently used as nuclear fuel in thermal fission reactors and are the leading candidate for use in solid fuelled fast reactors given a wealth of irradiation data from research and prototype systems. Spark plasma sintering (SPS) has been shown to reduce the sintering temperature and time required to manufacture UO₂ fuel pellets and may benefit MOx production through the more efficient retention of volatile minor actinides (MAs) such as AmO₂ in the manufacture of these fuels. In this study we have examined SPS as an alternative field assisted sintering route to UO₂ and MOx fuel manufacture on surrogate (U_0.93,Ce_0.07)O₂ prepared by a micronized master blend (MIMAS) type process. SPS has been shown to produce pellets of high density, > 95% TD at sintering temperatures of 1573 K and hold time of 5 min, comparable to equivalent pellets produced through conventional sintering at 1973 K for 5 h. The formation of Ce-rich regions has been measured showing the size distribution of Ce-rich particles in SPS Ce-MOx is similar to conventionally sintered Ce-MOx and are appropriate for MOx fuel specifications. However, a significant difference is observed in the Ce content of the Ce-rich particles which remain close to their primary blend composition, as well as having a high degree of heterogeneity within the Ce-rich regions. The effect of the shortened sintering temperatures and times on other microstructural properties are also revealed.