The Effect of Nearfield Constraints on the Corrosion Behavior of High Burnup Spent Fuel
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The Effect of Nearfield Constraints on the Corrosion Behavior of High Burnup Spent Fuel Andreas Loida, Manfred Kelm, Bernhard Kienzler, Horst Geckeis, Andreas Bauer Forschungszentrum Karlsruhe, Institut für Nukleare Entsorgung, P.O.Box 3640, D-76021 Karlsruhe, Germany ABSTRACT The long-term immobilization for individual radioelements released from the waste form “spent fuel” in solid phases upon groundwater contact depends strongly on the (geo)chemical constraints prevailing in the repository. Related experimental studies comprise effects induced by the presence of Fe based container material, and near field materials other than Fe for a rock salt environment. The effect of the presence of an argillaceous host rock containing organic matter and pyrite on fuel alteration was studied in addition. The results have shown that oxidative radiolysis products were found to be consumed at a significant extent by the metallic Fe and by the argillaceous host rock. Under these conditions a decrease at a factor of ca.100 for both the matrix dissolution rates and the solution concentrations of U and Pu was found. There is mutual support between the matrix dissolution rates, the solution concentrations and the amounts of oxygen encountered during the experiments under various conditions controlled by the presence of near field materials under study. INTRODUCTION The long-term immobilization for individual radioelements released from the waste form “spent fuel” in solid phases upon groundwater contact depends strongly on the (geo)chemical constraints prevailing in the repository. These will be controlled predominantly by the interactions between the spent fuel and possibly intruding aqueous solutions (groundwater, brine), the near field (NF) components as container and backfill materials and the repository rock. The dissolution behavior of the fuel itself and the associated release/retention of radioelements in a rock salt environment is presently being studied by means of extensive laboratory test programs under various conditions. These studies comprise effects induced by near field compounds, e.g. by the presence of Fe based container material, by the buildup of hydrogen overpressure due to the container corrosion, by the presence of nearfield materials others than metallic Fe. Hydroxylapatite and magnetite are materials currently under investigation in saline environment. Dependent on the near field materials present in the saline system the pH values were found to be varying from the neutral (pH 6-7, presence of magnetite or apatite) to the more alkaline (pH 9-10, presence of Fe) range. Results are reported in [1,2]. However, if non-saline conditions prevail, the constraints of the near field will be dominated by different processes, possibly controlling the alteration behavior of the fuel significantly. In this context, a strong interest is focused upon repositories in argillaceous host rock formations, because they are considered for the final disposal of high level waste in the repository concepts of several countries. In s
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