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Np-Incorporation Into K-boltwoodite

Published online by Cambridge University Press:  01 February 2011

Lindsay C. Shuller ,
Rodney C. Ewing  and
Udo Becker
Lindsay C. Shuller
Affiliation:
Materials Science and Engineering, University of Michigan
Rodney C. Ewing
Affiliation:
Materials Science and Engineering, University of Michigan Geological Sciences, University of Michigan
Udo Becker
Affiliation:
Geological Sciences, University of Michigan
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Abstract

Np-237 (τ1/2 = 2.1 million years) is a potentially important contributor to the total dose for a geologic repository under oxidizing conditions. Further, the Np5+-complexes are mobile aqueous species. Several processes may limit the transport of Np, as well as other actinides: i) the precipitation of Np-solids, ii) the incorporation of Np into secondary uranium phases, and iii) the sorption and reduction of Np-complexes on Fe-oxide surfaces. This study utilizes quantum-mechanical calculations to determine the most energetically favorable Np5+-incorporation mechanisms into uranyl phases, where Np5+-substitution for U6+ requires a charge-balancing mechanism, such as the addition of H+ into the structure. Experimental results suggest that uranyl structures with charged interlayer cations have a greater affinity for Np5+ than uranyl structures without interlayer cations. Therefore, the uranyl silicate phase boltwoodite (KUO2(SiO3OH)(H2O)1.5) is selected for this computational investigation. The charge-balancing mechanisms considered to occur with substitution include: i) addition of H+, ii) substitution of Ca2+ for K+, and iii) substitution of P5+ for Si4+. While the incorporation energy results (1-3 eV)are higher than energies expected based on current experimental studies, solid-solution calculations are used to estimate the limit of Np incorporation for the P5+ substitution mechanism (10 ppm at ̃100°C). The electronic structure of the boltwoodite structure provides insight into the electron density that may be involved in the incorporation of Np into the structure.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 1107: Scientific Basis for Nuclear Waste Management XXXI , 2008 , 455
Copyright
Copyright © Materials Research Society 2008

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