Np-Incorporation Into K-boltwoodite
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Np-Incorporation Into K-boltwoodite Lindsay C. Shuller1, Rodney C. Ewing1,2, and Udo Becker2 1 Materials Science and Engineering, University of Michigan 2 Geological Sciences, University of Michigan 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 quantummechanical 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. INTRODUCTION Spent nuclear fuel (SNF) is primarily composed of uranium dioxide (UO2) (95-99%), while the remaining 1-5% is composed of fission products (e.g., Cs, Sr, Tc) and transuranium elements (e.g., Pu, Np, Am). Over the very long term, the total radioactivity of SNF is dominated by the long-lived actinides [1], such as 237Np (τ1/2 = 2.1 million years). Under oxidizing conditions, the UO2 matrix of SNF alters, as the U4+ oxidizes to U6+, and the uranyl molecule, UO22+, forms complexes in solution, depending on the ground water chemistry. The oxidized uranium may precipitate as the bright yellow and orange U6+ phases that often form the corrosion rinds of altered uraninites [2,3]. The oxidation and dissolution of UO2 is accompanied by the oxidation and release of the transuranium elements and fission products. Aqueous Np5+complexes are mobile in the environment; thus, research has focused on mechanisms that may reduce the mobility of the actinide complexes. Significant Np-immobilization mechanisms include precipitation as a solid Np-oxide phase, such as NpO2 or Np2O5, incorporation of Np5+ into uranyl phases [4], and sorption and reduction of neptunyl (NpO2+)-complexes onto mineral surfaces. Several experimental studies have examined Np-incorporati
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