Prediction of Long-Term Geochemical Change in Bentonite Based on the Interpretative THMC Model of the FEBEX In Situ Test

Abstract

Since nuclear energy is crucial in the decarbonization of the energy supply, one hurdle to

remove is the handling of high-level radioactive waste (HLW). Disposal of HLW in a deep geological

repository has long been deemed a viable permanent option. In the design of a deep geological

repository, compacted bentonite is the most commonly proposed buffer material. Predicting the longterm

chemical evolution in bentonite, which is important for the safety assessment of a repository,

has been challenging because of the complex coupled processes. Models for large-scale tests and

predictions based on such models have been some of the best practices for such purposes. An 18-yearlong

in situ test with two dismantling events provided a unique set of chemical data that allowed

for studying chemical changes in bentonite. In this paper, we first developed coupled thermal,

hydrological, mechanical, and chemical (THMC) models to interpret the geochemical data collected

in the in situ test and then extended the THMC model to 200 years to make long-term prediction

of the geochemical evolution of bentonite. The interpretive coupled THMC model shows that the

geochemical profiles were strongly affected by THM processes such as evaporation/condensation,

porosity change caused by swelling, permeability change, and the shape of concentration profiles

for major cations were largely controlled by transport processes, but concentration levels were

regulated by chemical reactions, and the profiles of some species such as pH, bicarbonate, and sulfate

were dominated by these reactions. The long-term THMC model showed that heating prolongs

the time that bentonite becomes fully saturated in the area close to the heater/canister; however,

once the bentonite becomes fully saturated, high concentrations of ions in bentonite near the heater,

which was observed in the field test, will disappear; illitization continues for 50 years but will not

proceed further.