Abstract

Alteration of spent nuclear fuel in a geological repository under oxidizing conditions may result in uranyl compounds and incorporation of Np-237 into uranyl alteration phases could impact repository performance. Powders of synthetic soddyite, (UO2)2(SiO4)(H2O)2, and uranophane, Ca[(UO2)(SiO3 OH)]2(H2O)5, were synthesized under mild hydrothermal conditions in the presence of Np5+. Synthesis experiments were conducted at various temperatures and pH of the initial solutions. Powders of soddyite exhibit increasing Np concentration with the synthesis temperature at a pH of 4, consistent with substitution of Np5+ for U6+ in the structure. In contrast, the general decrease of the Np concentration in powders of uranophane with increasing synthesis temperature is inconsistent with incorporation of Np5+ into the structure of uranophane. These results further support the possibility that uranyl phases that form in a geological repository may impact Np mobility, but also demonstrate that additional studies of the impact of the crystal structure on incorporation are needed.

You do not currently have access to this article.