Assessment of Disposal of Mixed Radioactive Waste Containing Heavy Metals in Low-and Intermediate- Level Waste Disposal Facilities

Abstract

The disposal of radioactive waste is strictly regulated through comprehensive safety assessments that determine acceptable disposal concentrations and volumes based on potential risks to human health and the environment. However, in radioactive waste repositories, there exists a category known as mixed waste, which poses both radiological and chemical hazards. Regulatory frameworks addressing the chemical toxicity of such mixed waste remain unclear in most countries. In South Korea, the chemical hazard of radioactive waste is evaluated based on the Waste Control Act, which governs the management of designated(hazardous) industrial waste. However, because the disposal environment of radioactive waste differs significantly from that of conventional hazardous waste, this approach may lead to misjudgment of chemical risks. Moreover, the absence of substance-specific safety assessments may result in a lack of clear criteria regarding allowable disposal volumes for hazardous constituents in mixed waste. In this study, a comprehensive investigation was conducted on the safety assessment and characterization procedures required for the disposal of mixed waste generated from the nuclear industry. For safety assessment, long-term migration behavior of hazardous substances, specifically lead(Pb), cadmium(Cd), and antimony(Sb), within near-surface radioactive waste disposal facilities in Korea, focusing on their potential to influence both environmental safety and radionuclide mobility. Two representative facility types were modeled: the silo-type (Phase I) and the vault-type (Phase II) repositories. Using site-specific data and regulatory reports, the conceptual models were constructed with conservative assumptions on waste inventory, solubility limits, and degradation rates. The inventory mass of heavy metals was assumed as 50 tons for Pb and Cd, and 27 tons for Sb. The release of heavy metals was simulated for 100,000 years using GoldSim, and near-field transport under geochemical conditions was assessed using PHREEQC. In addition, to evaluate the potential degradation of engineered barrier performance caused by hazardous substances, competitive sorption experiments between heavy metals and radionuclides were conducted using clay minerals collected from the disposal site. Modeling results indicated that, for all heavy metals considered, the predicted concentrations remained below regulatory thresholds, suggesting that disposal of the full initial inventory would be feasible. While competitive sorption between heavy metals and radionuclides onto clay minerals was observed, it is expected to be negligible under the concentration ranges anticipated in actual disposal environments. Therefore, the disposal of heavy metals is expected not to compromise the performance of engineered barriers. In addition, this study examined the characterization procedures required for the acceptance of hazardous substances into radioactive waste disposal facilities. Leaching experiments were conducted under representative disposal conditions including high pH, groundwater ions, and radiation to identify key factors influencing the mobility of hazardous substances within the disposal environment. Furthermore, regression analysis was performed to investigate the relationships among different leaching test methods, such as Toxicity Characteristic Leaching Procedure(TCLP), Korean Elution test (KE), and ANS 16.1, which are applied to designated and radioactive waste. Based on these findings, specific criteria for the leaching behavior characterization of hazardous substances were proposed. This study provides a realistic evaluation of the potential for hazardous substances to be disposed of at the Gyeongju disposal facility. The findings are expected to contribute significantly to the development of regulations for the safe disposal of mixed waste, including waste that has been improperly stored for long periods or that is expected to be generated during nuclear power plant decommissioning. Furthermore, the safety assessment was tailored to reflect the specific environmental conditions of domestic disposal facilities, and key input parameters were experimentally derived. Therefore, the methodology presented in this study is expected to serve as a valuable framework for assessing the disposability of mixed waste.