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. This presentation gives on overview of the complexthermo-hydro-mechanical-chemical (THMC) processes occurring during thedisposal of heat-producing high-level radioactive waste in geologicrepositories. A specific focus is on the role of compacted bentonite, which iscommonly used as an engineered backfill material for emplacement tunnelsbecause of its low permeability, high swelling pressure, and radionuclideretention capacity. Laboratory and field tests integrated with THMC modelinghave provided an effective way to deepen our understanding oftemperature-related perturbations in the engineered barrier system; however,most of this work has been conducted for maximum temperatures around100 ∘C. In contrast, some international disposal programs haverecently started investigations to understand whether local temperatures inthe bentonite of up to 200 ∘C could be tolerated with nosignificant changes to safety relevant properties. Raising the maximumtemperature is attractive for economical and safety reasons but faces thechallenge of exposing the bentonite to significant temperatureincreases. Strong thermal gradients may induce complex moisture transportprocesses while geochemical processes, such as cementation and perhaps alsoillitization effects may occur, all of which could strongly affect thebentonite and near-field rock properties. Here, we present initial investigations of repository behavior exposed tostrongly elevated temperatures. We will start discussing our current knowledgebase for temperature effects in repositories exposed to a maximum temperatureof 100 ∘C, based on data and related modeling analysis from alarge heater experiment conducted for over 18 years in the Grimsel Test Sitein Switzerland. We then show results from coupled THMC simulations of anuclear waste repository in a clay formation exposed to a maximum temperatureof 200 ∘C. We also explore preliminary data from a bench-scalelaboratory mock-up experiment, which was designed to represent the strong THMCgradients occurring in a “hot” repository, and we finally touch on afull-scale field heater test to be conducted soon in the Grimsel Test Siteunderground research laboratory in Switzerland (referred to as HotBENT).

Can we safely go to 200 °C? An integrated approach to assessing impacts to the engineered barrier system in a high-temperature repository