Alteration Behavior of High Burnup Spent Fuel in Salt Brine Under Hydrogen Overpressure and in Presence of Bromide
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Alteration Behavior of High Burnup Spent Fuel in Salt Brine Under Hydrogen Overpressure and in Presence of Bromide Andreas Loida, Volker Metz, and Bernhard Kienzler Institut für Nukleare Entsorgung, Forschungszentrum Karlsruhe, P.O.Box 3640, Karlsruhe, D76021, Germany ABSTRACT Recent studies have shown that in the presence of H2 overpressure, which forms due to the corrosion of the Fe based container, the dissolution rate of the spent fuel matrix is slowed down by a factor of about 10, associated with a distinct decrease of concentrations of important radionuclides. However, in a natural salt environment as well as in geological formations with chloride rich groundwater the presence of radiation chemically active impurities such as bromide must be taken in consideration. Bromide is known to react with β/γ radiolysis products, thus counteracting the protective H2 effect. In the present experiments using high burnup spent fuel, it is observed that during 212 days the matrix dissolution rate was enhanced by a factor of about10 in the presence of up to 10-3 M bromide and 3.2 bar H2 overpressure. However, concentrations of matrix bound actinides were found at the same level or below as found under identical conditions, but in the absence of bromide. In the long-term it is expected that the effect of bromide becomes less important, because the decrease of β/γ-activity results in a decrease of oxidative radicals, which react with bromide, while α-activity will dominate the radiation field. INTRODUCTION In Germany it is considered to encapsulate spent fuel rods in Fe based canisters and to dispose of in a deep geological repository. Since rock salt is still one of the favorite candidate host rocks under consideration, the access of saline solutions to the waste packages may be assumed in the case of an accident scenario. As a consequence, the container material will start to corrode at first, associated with the formation of large amounts of H2 resulting in the build up of significant H2 overpressures. Previous spent fuel corrosion experiments in 5 M NaCl brine show that an initial H2 overpressure considerably inhibits radionuclide release from the UO2 matrix [1]. Both, in experiments with H2 overpressure of 2.8 bar, formed by the corrosion of Fe powder and in Fe free experiments with externally applied 3.2 bar H2 overpressure the release rate of Sr (indicator for matrix degradation) decreased by one order of magnitude compared to spent fuel corrosion experiments without initial H2 overpressure [1]. Moreover, solution concentrations of actinides and Tc are at least two orders of magnitude lower than without initial H2 overpressure. Under these conditions it is assumed that H2 scavenge oxidative radiolysis products such as OH. radicals resulting in a decrease of both, the matrix dissolution rate and the solution concentrations of U, Np, Tc [1]. These results reflect conditions relevant for a rock salt environment in the absence of any impurities. The radiolytic reaction of H2 with oxidative radiolysis products
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