Studies of sorption of Th and Np on UO 2 and TiO 2
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Studies of sorption of Th and Np on UO2 and TiO2 Yngve Albinsson, Hans Nilsson and Anna-Maria Jakobsson Department of Nuclear Chemistry, Chalmers University of Technology, S-412 96 Gothenburg, Sweden ABSTRACT When designing an underground repository for spent nuclear fuel it is important to know if and to what extent Np and Pu are reduced. At the moment only Np has been studied, but further investigations with Pu are planned. 234 -9 Np in the pentavalent state and Th in the tetravalent state ( Th, about 10 M) have been used to study the sorption onto TiO2 (no reduction is expected) and onto UO2 with different atmospheres (air, nitrogen and nitrogen and hydrogen at 5 MPa pressure), and with different Np-12 239 -6 237 concentrations (about 10 M, Np and 10 M, Np). Th is sorbed to a high extent at pH>3 on both TiO2 and UO2. The sorption increases 3 orders of magnitude over one pH-unit when log Ka (the distribution coefficient with respect to surface area) is plotted against pH. The Np sorption on TiO2 is independent of the Np concentration and shows a slope of one when pH is plotted against log Ka. The higher sorption of Np onto UO2 compared with TiO2 indicates that Np is partially reduced even when it is in contact with air. No large differences can be observed for Np between the nitrogen and hydrogen-atmosphere, indicating that the absence of oxygen is the important factor for the reduction. The different Np concentrations had no great impact on the reduction/sorption on UO2. INTRODUCTION In an underground repository for spent nuclear fuel, several barriers for the transport of radionuclides to the environment are assumed and relied upon. One of these barriers is sorption on various surfaces in the pathways of the transporting fluid. Generally sorption is considered in the host rock and eventual clay fillings. However, sorption must also be considered on the fuel matrix itself, especially for elements with chemical properties similar to the dominating matrix element. In this study sorption of Th and Np on TiO2 and UO2 has been performed. Th and TiO2 were used for comparison since they show fairly simple sorption behaviour. Th is stable tetravalent and the dissolution of TiO2 is negligible in the studied pH-interval. The sorption has been studied in batch and on-line experiments where the pH has been varied from about 2 to 12. Np sorption has also been studied on UO2 under 5 MPa nitrogen and hydrogen atmosphere at constant pH at about 2, 4 and 7 to study if the pentavalent state is reduced by tetravalent uranium in the matrix when high pressure of H2 or N2 is used. Reduction has been indicated in previous studies of Np diffusion in bentonite clay at a normal pressure [1,2].
EXPERIMENTAL Reagents and radionuclides The chemicals used were reagent grade. All solutions were made with MilliQ-water. To a >10 M NaOH solution Ba(NO3)2 was added in order to reduce the carbonate content. The solution was filtered and diluted as needed. Experiments were performed with depleted UO2 and TiO2 as solid phases with a specific area o
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