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American Mineralogist; November 2007; v. 92; no. 11-12; p. 1946-1951; DOI: 10.2138/am.2007.2542
© 2007 Mineralogical Society of America
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Neptunium substitution in synthetic uranophane and soddyite

Amanda L. Klingensmith1 and Peter C. Burns1,2,*

1 Department of Civil Engineering and Geological Sciences, 156 Fitzpatrick Hall, Notre Dame, Indiana 46556, USA
2 Chemistry Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, Illinois 60439, USA

Correspondence: * E-mail: pburns{at}nd.edu

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.

Key Words: Soddyite • uranophane • neptunium • nuclear waste • Yucca Mountain






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