The essential role of cement-based materials in a radioactive waste repository

Abstract

Cement-based materials are integral to radioactive waste repositories, providing versatile solutions for diverse disposal strategies. They are part of the multi-barrier system, and serve to immobilize waste materials, limit the release of radionuclides, contribute to an alkaline near-field to inhibit steel corrosion, reduce microbial activity, and slow down radionuclide transport in the repository near-field. This work delves into the adaptability of the multi-barrier systems for long-term safety, examining cases in clay and granite. Highlighting the disposal case in clay, the study emphasizes the role of cement in ensuring repository stability. The barrier system aims to minimize radionuclide release and demonstrate long-term isolation and containment of waste. The containment duration is relevant to the radionuclide’s half-life, with consideration for extended safety over extremely long periods. Cement evolves under geological conditions, undergoing a progressive process of degradation that is influenced by intricate aggregate-cement reactions and external factors, e.g., sulfates and chlorides in groundwater, the host rocks (including clays and granites), and the engineered barrier materials (including bentonite and steel), and in turn influencing mechanical stress generation and porosity. The very slow chemical alteration processes that take place at the concrete/granite interface underscore the repository stability. Corrosion of steel in the cement is expected to be slow, but its long-term structural and chemical changes remain quite unknown. Challenges remain in accurately predicting the long-term performance of the cement due to uncertainties in chemical reactivity, the impact of partial water saturation, and the kinetics of degradation processes. The manuscript advances the development of predictive modeling tools for assessing the long-term performance of cement-based barriers. The integration of experimental results with modeling efforts offers a robust framework for predicting the behavior of cementitious materials under various environmental conditions, thereby contributing to more reliable safety assessments of radioactive waste repositories. The role of cement phases in ensuring repository safety remains pivotal.