Concrete is used as the main engineering barrier in low and intermediate level radioactive waste disposal facilities. As the time passed, the radionuclides stored in repository may contact with groundwater and leak into the ecosystem through the rock media. In this process, the radionuclides can react with calcite via sorption or coprecipitation, because calcite is the major mineral of concrete. Under the various background conditions in repository, frequent dissolution-precipitation reactions can happen. Dissolution of Sr-coprecipitated calcite may be different from that of SrCO3(s) which can mislead the safety performance of radioactive Sr and the estimate of Sr mobility based on the solubility of SrCO3(s). Strontium is not only one of the fission products but also emits beta rays with a long half-life almost 29 years. The strontium may be released or retarded by the dissolution-precipitation reactions in repository. In this study, the dissolution of Sr-coprecipitated with respect to calcite was tested in various environment conditions. The Sr-coprecipitated calcite, (Sr,Ca)CO3(s) was synthesized by coprecipitation method in alkaline condition. The 250 mL of 0.1 M of CaCl2 solution was mixed with 250 mL of 1.14 mM SrCl2·6H2O solution. Then, independently prepared 500 mL of 0.1 M Na2CO3 solution was mixed with the mixed solution of CaCl2 and SrCl2. The precipitates could be made and they were aged for 3 days at room temperature. Then, the supernatant was separated by the centrifugation and the solid at the bottom was dried in an oven at temperature 80°C. After that, the Srcoprecipitated calcite powder was washed using the DI water several times and dried again before use. Characterization of solid powder was conducted by XRD and SEM, and the ICP-MS and ICP-AES were used to analyze the concentrations of Ca and Sr. The batch dissolution experiment was conducted with a solid-to-solution ratio of 10 g/L groundwater in polyethylene tubes. The oxidative groundwater was synthesized by simulating the chemical composition of KAERI Underground Research Tunnel (KURT) DB-3 groundwater. Different temperatures and pHs were prepared and tested for the release of Sr and Ca from the coprecipitated (Sr,Ca)CO3(s) to compare the results with the release of Sr and Ca from SrCO3(s) and CaCO3(s), respectively. Such as, these results will be used to provide better understanding of Sr release and mobility in various repository environments.

• Nara Jeong(Pohang University of Science and Technology (POSTECH))
• Wooyong Um(Pohang University of Science and Technology (POSTECH)) Corresponding author