Modelling of the spent fuel dissolution rate evolution for repository conditions. Matrix Alteration Model results and se
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Modelling of the spent fuel dissolution rate evolution for repository conditions. Matrix Alteration Model results and sensitivity analysis. Quiñones, J.1, Iglesias, E.1, Martínez-Esparza, A.2, Merino, J.3, Cera, E.3, Bruno, J.3, de Pablo, J.4, Casas, I.4, Giménez, J.4, Clarens, F.4, Rovira, M.4 1 Ciemat. Avda. Complutense22, 28040-Madrid. SPAIN 2 ENRESA, C/ Emilio Vargas 7, 28043-Madrid. SPAIN 3 ENVIROS, Passeig de Rubi, 29-31, 08197 Valldoreix, Barcelona. SPAIN 4 Dept. of Chemical Engineering. ETSEIB-UPC, Diagonal 647 H-4, 08028 Barcelona. SPAIN ABSTRACT This paper focuses on how to extrapolate current knowledge of spent fuel matrix alteration processes from laboratory to repository conditions, i.e., the influence of changes in both the environmental conditions and the range of time scale considered. Therefore, a spent fuel matrix alteration model allowing the alteration rate evolution to be predicted as a function of both the host rock considered and evaluation time scale of interest is described. At present, the model assumes that alteration of the spent fuel will start when the groundwater reaches the solid surface and that only the radiolytic species of the groundwater (oxidants generated by α-radiation of spent fuel) will produce the surface oxidation process and subsequent matrix dissolution; O2, H2O2 and OH· are the species that react with UO2(s) for oxidation of the pellet surface. The dissolution process of the surface sites that are oxidized is modelled in two steps: first, a surface co-ordination of the oxidized layer with aqueous ligands and, second, detachment (dissolution) of the product species. Taking this mechanism into account, the model gives the evolution of the spent fuel matrix alteration rate over periods as long as 1,000,000 years. In this work the matrix alteration rate results obtained for two repository environments, granitic and argillaceous, will be presented. Furthermore, a sensitivity analysis study has been performed on the influence of the following variables: type of spent fuel considered, α-dose rate evolution, α-range in groundwater, carbonate and iron concentration in groundwater, H2 partial pressure, container time failure and specific surface area of the pellet. INTRODUCTION Assessing the performance of spent fuel in a potential future geological disposal system requires an understanding of the important time-dependent phenomena influencing its behaviour over a time-scale of >106 years. A goal as demanding as this requires the development and qualification of models predicting the long-term release rate of radionuclides. Such models are usually referred to as ”Source term models”. In this paper the results and discussion of the extrapolation of the MAM from 103 - 106 years and 104 - 106 years (granitic and argillaceous host rock, respectively) are presented. This extrapolation was performed considering that failure of the waste container allows the water to come into contact with the fuel at 103 (granite repository) and 104 years (clay repository), therefore, this time was use
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