Homogeneity of (U, M)O2 (M = Th, Np) prepared by supercritical hydrothermal synthesis

The management of neptunium, an artificial element produced in nuclear reactors that is unsuitable for recycled fuel, is crucial for the development of long-term toxicity reduction. In this study, we focused on the direct synthesis of (U,M)O2 solid solution (M = Th, Np) by extending our recent progress in hydrothermal synthesis with additives. The homogeneity of the (U,M)O2 (M = Th, Np) systems prepared by supercritical hydrothermal reactions was investigated through crystallographic analysis based on Vegard’s law, and the 23Na nuclear magnetc resonance (NMR) measurement of (U,Np,Na)O2 solid solutions. Our experimental and analytical results revealed that (i) an optimal additive is ammonium carbonate and starting uranium valence is IV in the case of (U,Th)O2+x, and (ii) an optimal additive is ethanol and starting uranium valence is VI in the case of (U,Np)O2+x, for producing the homogeneous solid solutions by hydrothermal synthesis.