Modeling of Thermo-Hydro-Mechanical Processes in a Bentonite Buffer Under High Temperature

ABSTRACT: Thermo-hydro-mechanical (THM) processes within a bentonite buffer are important for the performance of the Engineered Barrier Systems (EBS) in a deep geological repository for nuclear waste. The THM behavior of Engineered Barrier Systems (EBS) with high-level radioactive waste cannisters have been extensively studied for temperatures up to 150 °C. Recent research has been focused on investigating the behavior of the bentonite buffer at high temperatures (150 – 200 °C) which is motivated by expanding the data and knowledge base for confidence building. This study presents the ongoing HotBENT field test at the Grimsel Test Site in Switzerland. The field test consists of 4 heaters placed in series with target temperatures ranging from 175 - 200 °C in a granite tunnel. A 2-D numerical model was developed considering a cross section around Heater 1 which was maintained at 200 °C. Coupled thermal, hydrological and mechanical processes within the barrier system are modeled using the TOUGH-FLAC simulator. The model considers material zones for bentonite, concrete floor, excavation damaged zone and granitic host rock. The simulated results were compared with field test results for temperature, relative humidity and pore water pressure and the impact of thermal and hydrological parameters on the buffer performance was evaluated. The initial modeling results provide an insight into the behavior of the bentonite buffer at high temperatures. 1. INTRODUCTION Multibarrier deep geological repositories are preferred worldwide for the disposal of high-level radioactive waste. This multibarrier system will typically consist of a metal cannister containing the waste placed within a suitable host rock. The space between the cannister and the host rock is filled with bentonite. Bentonite is chosen as a backfill material for its favorable properties including low hydraulic conductivity and high swelling potential. The Engineered Barrier System (EBS) which consists of the waste cannister and the backfill material, is subjected to elevated temperatures from radioactive decay and fluid inflow from the surrounding rock. The heat transfer through the buffer also results in thermally induced excess pore water pressures and moisture flow through diffusion. As a result of these processes, the buffer material undergoes mechanical deformation through swelling and/or shrinkage. Understanding these strongly coupled thermo-hydro-mechanical (THM) processes within the bentonite is crucial when designing a safe and efficient repository.