THM Modelling of the Bentonite Buffer at a Crystalline Site – Water Uptake and Homogenization

Abstract

The planned final storage for used nuclear fuel in Canada is a deep geological repository situated at a depth of more than 500 m. Currently, two types of host rocks are considered for the repository: crystalline and sedimentary rock. The modelling described here considers a generic crystalline host rock, i.e., the conditions are not site specific. The used nuclear fuel will be stored within copper-coated Used Fuel Containers (UFC), placed inside Highly Compacted Bentonite (HCB) blocks, one such unit is called a ‘Buffer Box’. The HCB buffer boxes will then be placed in a stacked configuration in the underground room and the gap remaining between the HCB blocks and rock walls will be filled with Gap Fill Material (GFM) consisting of granular bentonite. After emplacement, heat will flow from the UFC through the clay buffer and dissipate into the surrounding rock, while the bentonite buffer will take up water from the rock and undergo swelling. The finite element solver, CODE_BRIGHT, was utilized to study the Thermo-Hydro-Mechanical (THM) behavior of the bentonite-based materials in a generic crystalline host rock during the water uptake phase. The study has focused on the THM evolution and mechanical final state of the bentonite buffer. Dry density distribution and development of swelling pressures of the buffer, and displacement of the UFCs were analyzed. A sensitivity analysis of different air gaps between the HCB blocks and various initial dry densities of the bentonite buffer was carried out to examine how they could affect the evolution and final state. The numerical results show that the final dry density, stress field, and stress acting on the UFC were significantly affected by these different initial states. The numerical simulations also indicate that air gaps between the HCB blocks would be sealed by the swelling bentonite, but the final dry density in these regions would become lower than that in the surrounding volumes.