Congruent and Incongruent Radionuclide Release During Matrix Dissolution of Partly Oxidized High Burnup Spent Fuel
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Congruent and Incongruent Radionuclide Release During Matrix Dissolution of Partly Oxidized High Burnup Spent Fuel A. Loida1, B. Grambow2 and H. Geckeis1 1 Forschungszentrum Karlsruhe - Technik und Umwelt, Institut für Nukleare Entsorgungstechnik, P.O.Box 3640, D-76021 Karlsruhe, 2 Ecole de Mines de Nantes, F- 44307 Nantes ABSTRACT With respect to the assessment of the long-term behavior of the waste form spent fuel it is of high importance to study in particular the UO2 matrix dissolution behavior and the associated release/retention of radionuclides in contact with groundwater. During long term fuel storage, fuel oxidation may not be avoided. Main issue of this work is to identify the impact on the corrosion of partly oxidized fuel of environmental conditions such as (1) the nature of solution contacting the matrix, the (2) presence/absence of CO2, (3) fixed pH values within a range between pH 7- pH 11, and (4) the presence/absence of corroding container material (Fe-powder). Dissolution tests with powdered oxidized spent fuel in various granite waters, and NaCl-brine resulted in matrix dissolution rates in the same order of magnitude for all investigated media (ca.5x10-4/d). The presence of CO2 and fixed pH values (pH 5 – 11) was without a distinct effect. The independence of the dissolution rate of the oxidized fuel matrix upon the nature of solution, pCO2, fixed pH values (5-11) can probably be explained by a masking effect of radiolysis. In presence of Fe powder the matrix dissolution rate was found to be slowed down by a factor of ca. 20, associated with strong retention effects of radionuclides. INTRODUCTION The UO2 matrix of spent nuclear fuel contains > 99.99 % of the actinide and REE inventory, > 99.9 % of the Sr inventory and > 90 % of the Cs, I inventory of the irradiated fuel. With respect to the assessment of the long-term behavior of the waste form spent fuel it is of high importance to understand in particular the UO2 matrix dissolution behavior and the associated release/retention of radionuclides in contact with groundwater. In the frame of this work we studied experimentally the impact on fuel matrix dissolution of geochemical conditions possibly encountered in close vicinity to the fuel under anoxic conditions such as (1) the nature of solution contacting the matrix (salt brine, granite water, granite bentonite water), the (2) presence/absence of CO2, (3) pH values within a range of pH 7- pH 11, and (4) the presence/absence of corroding container material (Fe and Fe corrosion products). Fuel powder was used for these tests which was partly oxidized during up to 3 or 4 years, respectively of storage in
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