Development of high-temperature-steam resistant UN via the addition of UB2

A composite UN fuel containing 10wt% UB2 has been manufactured via spark plasma sintering using different milling methods prior to sintering, and the resulting pellets characterised to understand the effects of UB2 location and morphology on UN sintering behaviour and oxidation performance. Differences in microstructure and phases present were observed, with planetary ball milling leading to smaller UB2 inclusions as well as the formation of a UBN phase on sintering. Composite pellets showed an increase in the steam oxidation onset temperature when compared to UN at similar density and manufactured from the same feedstock. Of particular note was the behaviour of one sample with a comparably low density (∼92 %) which had an onset temperature of 823 K and a significantly reduced rate of reaction compared to monolithic UN at similar density. This provides the first confirmatory evidence that UB2 limits the UN-steam reaction by some other mechanism than simply promoting a high-density microstructure. This is supported by examination of post-oxidation composite material, which shows a varied and more complex morphology compared to reference UN samples, including large apparently-bound agglomerates and limited free fine particulate.