The behavior of unirradiated UO 2 and uraninite under repository conditions characterized by Raman
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The behavior of unirradiated UO2 and uraninite under repository conditions characterized by Raman L. J. Bonales 1, J.M. Elorrieta 1, C. Menor-Salván 2, J. Cobos 1 1
Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Av. Complutense, 40, 28040 Madrid, Spain. 2 Centro de Astrobiología (CSIC-INTA), Ctra. Torrejón-Ajalvir, km 4, 28850 Torrejón de Ardoz, Spain. ABSTRACT Raman spectroscopy studies have been performed on one hand to identify different materials related to spent nuclear fuel (SNF), and on the other hand to study the behavior of SNF at different storage conditions. Specifically, the expected oxidation of the SNF matrix under dry storage conditions and the formation of secondary phases (SP), as a result of corrosion of SNF in a deep geological repository, have been studied. In order to perform these experiments, two protocols based on the Raman spectroscopy technique have been developed. The results show U4O9/U3O7 and U3O8 as oxidation products of UO2 powder at high temperatures in air, and the secondary phase formation (rutherfordine, UO2(CO3), soddyite, (UO2)2SiO4•2H2O, uranophane alpha Ca(UO2)2(SiO3OH)2•5H2O and kasolite, PbUO2SiO4•H2O), due to uraninite corrosion under the conditions of Sierra Albarrana (Spain). INTRODUCTION In the frame of dry interim storage of spent nuclear fuel (SNF) [1], in case of canister damage, the spent fuel matrix might be oxidized to U3O8, due to the presence of the atmospheric O2 neighboring the fuel and the high temperature of the fuel assembly (up to 400ºC) [2]. This reaction is generally described as a two-step reaction [3], UO2 → (U4O9/U3O7) → U3O8, with an increase of around 36% in volume, which might affect the integrity of the fuel pellets and therefore the release of their radionuclide content. Other possible scenario would be the final storage in a deep geological repository [4], in case groundwater reached the surface of the SNF. It has been shown that at the near field of the fuel surface (less than 50 μm) water suffers radiolysis, producing an oxidizing environment [5]. In such event, U(IV) would oxidize to U(VI) and, depending on the chemical and physical characteristics of the final storage, different secondary phases could precipitate [6]. These secondary phase precipitations could retain different radionuclides within their structures thus avoiding the migration of radionuclides to the biosphere [7]. In this work, two protocols based on Raman spectroscopy have been developed in order to study both, the oxidation of the matrix under dry storage conditions and the precipitation of the SP under wet storage conditions. The first one, named here as a laser heating method, is based on the capability of the laser excitation source of the Raman spectrometer to cause a local increase of temperature of several hundred degrees at a sample. The second method allows an easy and fast identification of each secondary phase, all of which appear as a mixture of minerals. This paper has been organized as follows. The experimental section describes
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