The HotBENT Experiment: objectives, design, emplacement and early transient evolution

Abstract

The High Temperature Effects on Bentonite Buffers (HotBENT) Experiment, conducted at the Grimsel Test Site, aims to obtain monitoring and sampling data from an in situ large-scale and long-term test of an engineered barrier system (EBS) consisting primarily of a bentonite buffer exposed to high temperatures (up to 200°C at the heater surface). The in situ data complemented by laboratory testing and modelling will enhance existing databases and strengthen the understanding of buffer performance at such high temperatures. A robust assessment of the effect of such higher thermal loading on the most relevant safety functions and the resulting requirements on buffer performance would allow further optimization in the design, realization and operation of geological repositories and the management of interim storage facilities. This optimization would lead to reduced repository footprints (reduced number and/or spacing of disposal canisters and buffer dimensions) or new strategies for canister loading and, hence, requirements for cooling periods. HotBENT will contribute to assessing the effects of this higher thermal loading under realistic gradients and conditions, which cannot be obtained in the laboratory. It will allow an assessment of mineralogical, physical and chemical processes, such as smectite–illite transformation or cementation processes, which could impact the swelling, thermal and hydraulic properties and, hence, the key safety-relevant properties of the bentonite buffer. HotBENT was constructed in the period 2019–21 and has been operating at target temperatures since May 2022. The objectives, design and implementation of HotBENT, as well as the early transient phase of heating and hydration, are summarized in this paper. Thematic collection: This article is part of the Sustainable geological disposal and containment of radioactive waste collection available at: https://www.lyellcollection.org/topic/collections/radioactive