Influence of the Specific Surface Area on Spent Nuclear Fuel Dissolution Rates
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1124-Q02-02
Influence of the Specific Surface Area on Spent Nuclear Fuel Dissolution Rates J. Quiñones*, E. Iglesias, N, Rodriguez, J. Nieto. CIEMAT. High Level Waste Unit. Avda. Complutense, 22. 28040. Madrid.
ABSTRACT This paper focuses on the influence of the evolution of the specific surface area during the alteration process of the spent nuclear fuel. We describe results of a spent fuel Matrix Alteration Model (MAM) that allows the alteration rate evolution to be predicted as a function of the host rock considered and evaluation time scale of interest. The changes produced by the value of the UO2 specific surface area on the MAM model (presented in a previous MRS conference) are here analyzed. The matrix alteration rates obtained with the MAM model (for granitic environment) are presented and compared to those performed for Spent Fuel Stability project (SFS). Furthermore, a sensitivity analysis study has been performed on the influence of the following variables: influence of the initial power size distribution and the initial oxidation state. A strong dependence of the alteration rates of the spent fuel on the specific surface area is found. These results are presented for a specific scenario, but they could be extrapolated to different environments depending on the input file. INTRODUCTION The Matrix Alteration Model (MAM) developed by CIEMAT, UPC (Universidad Politécnica de Cataluña), ENVIROS, and financially supported by ENRESA (Empresa Nacional de Residuos) [1, 2] works in any scenario, depending on the environment of the final option for high level radioactive waste disposal (Deep Geological Repository: DGR). MAM predicts the matrix altered rate of the spent fuel pellet with a defined geometry under different environmental conditions. Model description and the considered hypothesis are detailed in [1-3]. There are still some processes not considered by the model [2], such as influence of precipitated secondary phases over the pellet; improvement is being developed by the MICADO coordinated action. Obtained values so far using MAM have been used in performance assessment studies for ENRESA [1] and the SFS project [3], in lab experiments for validation [4] and for long term extrapolation [2, 3]. The possibility of working with different host rocks (granite, clay and salt) and the capability for long term repository extrapolation gives the MAM a great versatility. METHODS At present, the model assumes that alteration of the spent fuel will start when the groundwater reaches and contact the solid surface and that only the radiolytic species of the groundwater (oxidants generated by α-radiation of spent fuel) 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 alteration process of the U surface sites is modeled 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
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