Coprecipitation Experiments Using Simulated Spent Fuel Solutions in the Presence of Metallic Iron in Synthetic Bentoniti
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Coprecipitation Experiments Using Simulated Spent Fuel Solutions in the Presence of Metallic Iron in Synthetic Bentonitic-Granitic Water Under Oxidising Conditions Javier Quiñones1,*, Ángel González de la Huebra1, Aurora Martínez Esparza2 1 Nuclear Fission Department, Ciemat, Avda Complutense 22. 28040 – Madrid, Spain 2 ENRESA, C/ Emilio Vargas 7, 28043 – Madrid, Spain ABSTRACT This paper presents the results obtained from coprecipitation experiments of uranyl solutions in the presence of metallic iron and/or its alteration phases in synthetic bentonitic-granitic composition water. Experiments were done under oxidising conditions at room temperature. The pH range covered was 7.4 – 8.8. Changes in the uranium concentrations and the characterisation of the secondary phases formed in the experiments were done using XRD and SEM-EDS and are presented herein. Final uranium concentration values were in the range of 2·10-5 – 5·10-4 mol (kg of H2O)-1. In all cases, results from these experiments did not show evidence of a clear effect due to the presence of iron (metallic or previously corroded) on the uranium concentration. These data were similar to those obtained by coprecipitation in similar conditions but in absence of iron material. Boltwoodite was observed [K2(UO2)2(SiO3)2(OH)2·3H2O] in iron surface materials and characterized in all experiments. Based on the experimental data obtained (uranium concentration in solution and bulk solid phase characterisation) we propose the following surface-mediated reaction: 2 K+ + 2 H4SiO4 + 2 UO2+2 + 3 H2O ⇔ K2(UO2)2(SiO3)2(OH)2·3H2O + 6 H+ INTRODUCTION The influence of corroded engineering barriers on spent fuel alteration behaviour is one of the important issues to be addressed in performance assessment studies. This paper focuses on a granite repository and tries to establish the influence of iron (and its secondary phases) on U in solution and the possible formation of secondary phases under oxidising conditions. Although bedrock and the buffer material help maintain reducing conditions in the repository, radiation from spent fuel (principally alpha radiation after ~ 1000 years) can produce oxidants, which may corrode repository materials. Corrosion of the spent fuel may release, among other components, uranyl ions from the fuel. The migration of uranyl ions in soils can be affected by interactions with various soils minerals, including sorption and precipitation of uranyl solids. Several authors have demonstrated sorption and/or coprecipitation of uranium with iron minerals in the laboratory and in nature [1-3]. Liger et al. [1] concluded that the main process of immobilisation of U in natural systems is reduction of U(VI) by Fe(II). Taking into account this work, a set of coprecipitation experiments *
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were done in order to evaluate the influence of iron on the uranium concentration close to spent fuel (i.e., a reduction in the uranium concentration) as a consequence of secondary phase formation.
Table I. Mole fraction rat
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