HLW-Glass Dissolution and Co-Precipitation Studies
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HLW-Glass Dissolution and Co-Precipitation Studies Berthold Luckscheiter and Maria Nesovic Forschungszentrum Karlsruhe GmbH, Institut fuer Nukleare Entsorgung (INE), P.O. Box 3640, D-76021 Karlsruhe, Germany
ABSTRACT To determine the maximum attainable solution concentrations of U, Th and fission products during long-term glass corrosion, co-precipitation studies were performed. The HLW-glass GPWAK 1 was dissolved in highly acid and basic media (100 g glass/L) at 80°C and by preparing acid solutions containing the various elements in soluble form. After dissolution the pH of the solution was slowly lowered/increased and the high concentrated solutions become super-saturated and strong precipitation takes place. The found pH-dependent concentrations of the various elements reflect their different solubility, lowest concentration for Th, Zr and Fe and highest for B, alkalis and alkaline earths. To find out the solid phases controlling the solution concentration, the run of the concentrations of some elements is compared with the solubility data of their pure solid phases (e.g. hydroxides) from literature. It was found that the concentrations of Nd, Th and U in dependence on pH agree quite well with solubility data of AmOHCO3, ThO2/Th(OH)4 and Schoepite UO2·(OH)2) ·H2O. Therefore, it can be assumed that the maximum attainable concentrations of many elements are controlled by such pure solid phases.
INTRODUCTION AND OBJECTIVES Co-precipitation studies were performed with the objective to determine the maximal attainable solution concentrations of fission products, U and Th during long-term HLW glass corrosion and to find out the solid phases controlling the solution concentrations of various elements. Even after long corrosion time at high S/V ratios, we do not know if the found concentrations of the various elements in the leachate are maximal attainable concentrations, controlled by the solubility of stable secondary phases, or increase the concentrations further if the corrosion proceeds over much longer times. In glass corrosion experiments we always approximate from the under-saturated side to the maximum concentration. Instead of that, we can also approximate from the super-saturated side, e.g., by dissolution of the glass in highly acid solutions (see figure. 1). After dissolution the pH in the solution is increased and the high concentrated solution becomes super-saturated, precipitation takes place and the concentrations of less soluble elements, e.g. Lanthanides (Ln), Actinides (An), decrease strongly with rising pH. If glasses are dissolved in aqueous solutions in dependence on pH (between pH 2 and 10), the release or concentrations of the soluble elements (alkalis, B, alkaline earths) show a typical Uform, a strong increase at low as well as at high pH. The release of less soluble elements (e.g. Zr, Al, Ln, An) is high only at low pH. However, complexation reactions with anionic components in the solution can also increase the solubility at high pH by forming Al(OH)4- and anionic Ln and An (hydroxo-) carbo
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