Estimation of Ra Concentration in High-level Radioactive Waste Disposal System
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1124-Q07-18
Estimation of Ra Concentration in High-level Radioactive Waste Disposal System Yasushi Yoshida1 and Hideki Yoshikawa2 1 Inspection Development Corporation, 4-33, Muramatsu, Tokai-mura, Naka-gun, Ibaraki 319-1112, Japan. 2 Japan Atomic Energy Agency, 4-33, Muramatsu, Tokai-mura, Naka-gun, Ibaraki 319-1194, Japan ABSTRACT Concentrations of Ra in groundwater are thought to be affected by substitution reactions with alkaline earth elements contained in a mineral, because the concentration of Ra is less than the solubility of known Ra containing phases. The substitution reaction was simulated using a partition coefficient. Calcite ubiquitously exists in geological system and easy to access to react with Ra. Therefore, calcite is thought to be an important mineral for the substitution reaction. However, previous identification of reactive layers of calcite or reversibility of the substitution reaction has not been confirmed. A re-distribution experiment was therefore undertaken and it was found that an estimated 21±13 layers near the surface were reactive and within them substitution equilibrium was achieved. Using these results, a model to estimate Ra concentration was established and adopted to analyze Ra migration. The effects of substitution of Ra and Ca were reasonably simulated. INTRODUCTION Performance assessment of a geological disposal system typically includes an analysis of migration of radionuclides. Concentrations of radionuclides in groundwater are estimated based on solubility of oxides and carbonate containing phases [1-3]. Concentrations of Ra, which is one of the more important radionuclides, are thought to be limited by substitution reaction for alkaline earth elements in minerals because its concentration is less than the solubility of RaCO3(cr) or RaSO4(cr) [1]. This effect has been considered with solid solution theory by Berner [2] and with coprecipitation by Azuma et al. [3]. However, applicability of the solid solution equation of Berner [2] was not confirmed experimentally and the oversimplified assumption in Azuma et al. [3] revealed a discrepancy against experimental results. In the present study, a partition coefficient (D) [4-6] is applied to describe a substitution reaction. It expresses distribution of tracer (Tr) and carrier (Cr) element for coprecipitation and is given by: (Tr / Cr )solid (1) D= (Tr / Cr )solution where (Tr/Cr)solid and (Tr/Cr)solution are mole ratios of Tr/Cr in the solid and solution, respectively. D is derived from the results of a coprecipitation experiment and is thought to be a reasonable empirical parameter to describe distribution of trace element in coprecipitation reactions. Since alkaline earth elements and counter ligands are dominated by Ca and carbonate, respectively, in the geological environment, calcite (CaCO3) is a commonly occurring mineral. D of Ra in calcite was determined by Yoshida et al. [7] and can be converted to be a distribution coefficient (Kcp) with molar quantities of Ca in the solid phase and solution, given by: V V K cp = Ra =
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