Abstract. This presentation gives on overview of the complex
thermo-hydro-mechanical-chemical (THMC) processes occurring during the
disposal of heat-producing high-level radioactive waste in geologic
repositories. A specific focus is on the role of compacted bentonite, which is
commonly used as an engineered backfill material for emplacement tunnels
because of its low permeability, high swelling pressure, and radionuclide
retention capacity. Laboratory and field tests integrated with THMC modeling
have provided an effective way to deepen our understanding of
temperature-related perturbations in the engineered barrier system; however,
most of this work has been conducted for maximum temperatures around
100 ∘C. In contrast, some international disposal programs have
recently started investigations to understand whether local temperatures in
the bentonite of up to 200 ∘C could be tolerated with no
significant changes to safety relevant properties. Raising the maximum
temperature is attractive for economical and safety reasons but faces the
challenge of exposing the bentonite to significant temperature
increases. Strong thermal gradients may induce complex moisture transport
processes while geochemical processes, such as cementation and perhaps also
illitization effects may occur, all of which could strongly affect the
bentonite and near-field rock properties. Here, we present initial investigations of repository behavior exposed to
strongly elevated temperatures. We will start discussing our current knowledge
base for temperature effects in repositories exposed to a maximum temperature
of 100 ∘C, based on data and related modeling analysis from a
large heater experiment conducted for over 18 years in the Grimsel Test Site
in Switzerland. We then show results from coupled THMC simulations of a
nuclear waste repository in a clay formation exposed to a maximum temperature
of 200 ∘C. We also explore preliminary data from a bench-scale
laboratory mock-up experiment, which was designed to represent the strong THMC
gradients occurring in a “hot” repository, and we finally touch on a
full-scale field heater test to be conducted soon in the Grimsel Test Site
underground research laboratory in Switzerland (referred to as HotBENT).