Processes Controlling Radionuclide Release from Spent Fuel
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PROCESSES CONTROLLING RADIONUCLIDE RELEASE FROM SPENT FUEL
A. Loida, B. Grambow; H. Geckeis, P. Dressier, Kernforschungszentrum Karlsruhe, Institut fur Nukleare Entsorgungstechnik, Postfach 3640, 76021 Karlsruhe, FRG ABSTRACT
Dissolution of spent fuel has been studied in saline, anaerobe, carbonate free solutions. Processes controlling spent fuel dissolution and associated radionuclide release are radiolytically controlled oxidative dissolution, sorption on container, solubility and coprecipitation. Upper limits for oxidative dissolution rates are given by the production rates of 'oxidative radiolysis products. This limitation leads to a strong decrease in surface area normalized reaction rates with increasing surface to volume ratio (SAT) and imposes geometric constraints on prediction of spent fuel behavior in a repository. Solution concentrations of Am during spent fuel corrosion were about 5 orders of magnitude lower than the solubility of Am(OH) 3(s) and are likely controlled by coprecipitation. Pu concentrations may be controlled by Pu(VI) or Pu(IV) (hydr)oxides. INTRODUCTION
Extensive experimental investigations have shown that spent nuclear fuel is a very stable material, suitable for direct disposal as a waste form in a geological repository. Processes controlling potential mobilization of radionuclides must be investigated for the assessment of the long-term performance of spent fuel. The radionuclide behavior during aquatic interaction of spent fuel (corrosion etc.) is of primary concern. Important reversible and irreversible reactions which mutually control radionuclide release are the corrosion of the fuel matrix or of segregated phases, grain boundary dissolution, sorption on reaction vessels or corrosion products and formation of new phases by (co)precipitation. The complicated interrelationship of the various reactions often hampers unambiguous interpretation of
experimental results. Our work aims at characterization and qualification of radionuclide release from spent fuel with respect to disposal in salt formations. Significant results of our research program are already reported [1,2]: Due to generation of oxygen by radiolysis, even under anaerobe conditions there are sufficient reactants for fuel matrix dissolution. For experiments performed at room temperature, the extent of fuel matrix dissolution can be inferred from Sr release data. Potential contribution of Sr-release from gap or grain boundary inventories was found to be less than 0.03 %. It has been shown that an increase in the ratio of the sample surface area to the solution volume (SAT) leads to a pronounced decrease in surface area normalized spent fuel dissolution rates. Intrinsic fuel dissolution rates, based on geometric surface areas, were found to be about 5-7 mg/(m 2d) at room temperature. However, less than a monoatomic layer of the surface of spent fuel is dissolved in a year, if powdered samples were used. For interpretation of this effect new data on the relation of radiolysis and reaction rates are presented in this com
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