Incorporation of REE into Secondary Phase Studtite.
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Incorporation of REE into Secondary Phase Studtite. C. Palomo, N. Rodríguez, E. Iglesias, J. Nieto, J. Cobos and J. Quiñones Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Av. Complutense 40 28040 Madrid, Spain [email protected], [email protected] ABSTRACT The formation of uranyl peroxide phases was identified as a corrosion product of spent fuel by Hanson et al [1]. The subsequent analysis of this phase showed that metastudtite retained 241Am, 237 Np and 239Pu [2]. In this study, the retention of radionuclide Pu4+ and An3+, released from the spent fuel matrix into studtite structure, has been evaluated by the precipitation of studtite from uranyl dissolution with variable concentrations of REE (Th, Nd, Sm and Eu). Three different precipitation conditions parameters were studied: media of synthesis, time of synthesis and REE concentration. Synthesized phases were characterized by XRD and the cell parameter was calculated. The REE incorporation was determined by ICP-MS analysis. The results showed that studtite could incorporate 63% of Th in solution during its precipitation. Changes in the “a” cell parameter were identified. The results suggest that studtite coprecipitated with REE could play a role as a limiting for the REE mobility. INTRODUCTION The spent fuel (SF) is composed 95% of UO2 and 5% of actinides (e.g., 239Pu, 237Np, 241Am) and fission product (e.g., 137Cs, 90Sr, 99Tc). The UO2 is extremely insoluble under reducing conditions [3]. Alpha radiation field associated to SF is able to decompose water in radiolytic products. Those species generate oxidizing conditions in the near field of SF. UO2 is not stable under this locally oxidizing environment [4] and peroxides coming from the irradiated water oxidize the U(IV) to the more soluble U(VI) oxidation state. Then, hexavalent uranium U(VI) could react with ligands in groundwater and precipitates as secondary phase[5, 1]. The precipitated secondary phases could be capable to retain several randionuclides as Burns et al in 1997 [6] theorized. Figure 1. Structure of studtite.[9] In this way, Cs, Ba, Np and Pu were identified by radiochemical analysis of secondary phases [7, 2]. Studtite was characterized as corrosion product of SF by Hanson et al. Its structure was described by Walenta 1976 [8] and refined by Burns and Hughes (Figure 1) [9]. Possible incorporation of Np in the transformation of metaschoepite to studtite has been studied by Douglas et al [10]. EXPERIMENTAL Materials - Synthesis of studtite Solid-phase synthesis was carried out using the following reagents: uranyl nitrate hexahydrated 99%, Ln and Th dissolution of 1000 μg/l in nitric acid (1-5%wt) and H2O2 (30%).
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Two sets of precipitation experiments were performed in order to test geological repository conditions: some of them in demineralized water and other in Grimsel Groundwater, . The uranyl nitrate dissolution (0.1M) was prepared by mixing uranyl nitrate hexahydrated 99% Merk with ultrapure water or Grimsel groundwater from Switzerland in
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