The HE-E experiment: comparative buffer analysis after 12 years of operation

The HE-E is an in-situ heater experiment at the Mont Terri rock laboratory (MTRL), designed to study the early, non-isothermal resaturation and thermo-hydro-mechanical (THM) behaviour of engineered barrier systems for heat-generating radioactive waste. It was initially part of the EU-PEBS project (Schäfers et al. 2014). Its main goals have been to provide data for the calibration and validation of THM models and to upscale thermal conductivity of partially saturated bentonite buffers from laboratory to field scale (Gaus et al. 2014). The HE-E experiment differs from other previous heating experiments (e.g., FEBEX, Kober et al. (2021), or Prototype, Svemar et al. (2016)) in three significantly different boundary conditions: 1) instead of compacted bentonite blocks as a buffer, 90% of the backfill volume consists of loose material, either granular buffer (bentonite) material (GBM) with a mean porosity of 44% or sand/bentonite (S/B, ratio: 65/35, porosity: 47%); the average GBM emplacement density was 1.45 versus 1.7 g/cm3 for compacted blocks, with possible density heterogeneities due to backfilling with a screw-feeder-machine (density of S/B: 1.5 g/cm3); 2) the target temperatures at the surface of the heater elements were ~140 °C (instead of ~100 °C); 3) the host rock is low-permeability Opalinus Clay (instead of crystalline rock).