Illitization in bentonite and its effect on stress revealed by coupled THMC models for a nuclear waste repository in clay

Disposal of high-level radioactive waste in geologic repositories involves a multi-barrier system that comprises of an engineered barrier system (EBS) and the host rock (or natural barrier system, NBS). One of the common components of the EBS is a bentonite buffer material which has several beneficial features such as a low permeability as well as a high swelling and retardation capacity. Long-term chemical alteration, particularly illitization (transformation of smectite to illite), may lead to undesirable changes of these features. The occurrence of illitization, albeit extensively evidenced in geological formations, has not been conclusively confirmed for bentonite under the expected repository temperature conditions, and its impacts on other hydrological and mechanical properties have not been widely studied. This presentation starts with a review of illitization in bentonite as being studied by laboratory experiments under various conditions. Then we present a coupled thermal, hydrological, mechanical and chemical (THMC) model of a generic nuclear waste repository in a clay formation with a bentonite-based buffer, focusing on illitization and its impact of swelling stress. A large number of simulations were conducted to study various aspects associated with illitization: key chemical factors affecting illitization in different bentonites, illitization in clay rocks, change in swelling stress in different bentonites, and the effect of temperature on illitization. Because the mechanical-chemical coupling method in the THMC simulator is critical for exploratory simulations, we also explored different coupling schemes. In general, our simulations show the occurrence of illitization in the bentonite and the enhancement of illitization as a result of high temperature exposure; the degree of illitization is affected by many chemical factors and consequently varies a great deal. Illitization leads to a decrease in swelling stress in bentonite, and the degree of reduction depends on the degree of illitization, the initial conditions, and the type of bentonite.