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.

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