(A) study on leaching of Cs-137 in cement matrix and long-term leach rate prediction with backfill material

Abstract

Low-level radioactive liquid wastes (LLW) are generated by the operation and maintenance activities at light water reactor (LWR). These are disposed in final disposal site after several treatment process. If underground water soaks into disposal site, the disposed drums will be corroded and the solid matrix will be contacted with water. Consequently, leaching will be initiated and radionuclides move along with the water to the environment. Therefore, the study of leaching is very important for the safety analysis of LLW disposal. In this thesis, various factors affecting the leachability of Cs-137 in cement matrix, which has been used as a major binding agent in Korea, are investigated. Specifically, the purpose of this work is to search system factors and solid factors which can reduce the leach rate. System factors include such as pressure curing, vibration curing, curing under 100% humid condition, curing with or without lid, pressure leaching and the dependency of exposed surface area. According to the experimental results, pressure curing has shown a good effect on the reduction of the leach rate. In order to analyze the experimental results, pore analysis of the cement matrices and the intrinsic diffusion coefficient measurement is carried out. Solid factors are the effect of the cement type and clay (natural zeolite catalyst) addition, ion-exchange resin (IRN-77) addition, waste solution pH, and $CO_2$ and air injection. Pottery clay and various kaoline type of clay and slag and ordinary Portland cement type I and II are used. Pottery clay has turned out to have the best capability to reduce the leach rate of Cs-137. IRN-77 has shown good retarding property on the hydration of cement pastes and the matrix of waste solution of pH 12 has shown the least leach rate. It was proved that carbon dioxide has immobilizing capability of Cs-137 in cement matrix. To analyze the experimental results, characteristics of zeolite catalysts, relationship between chemi...