Assessment of a full-scale in-situ heater experiment based on the French high-level waste disposal cell concept conducted in the Callovo-Oxfordian claystone

An in-situ heater experiment, known as ALC1605, is being carried out as a part of the research and demonstration program of the Cigéo project, the deep geological disposal facility for intermediate-level long-lived and high-level radioactive waste in France. This experiment is conducted in the Meuse/Haute-Marne underground research laboratory located at a depth of 490 m within the Callovo-Oxfordian claystone (COx) layer.

The ALC1605 experiment consists of a 28.5 m long horizontal steel-cased micro-tunnel. The heat emitted by the high-level waste (HLW) packages is mimicked by heater devices installed between 10 m and 25 m. This is the second full-scale heater prototype of the HLW disposal cell concept. Unlike the first full-scale prototype, the annular gap between the steel sleeve and the rock formation is filled with an alkaline grout material to reduce the corrosion of the sleeve.

The main objectives of this experiment are to study: (i) the thermomechanical behavior of the steel sleeve, and (ii) the thermo-hydro-mechanical (THM) behavior of the rock, with a particular attention to the effect of the backfill material by comparing the experimental results with its predecessor.

At this stage, the temperature in the sleeve has reached a steady state with a maximum value of 85 °C. The backfill material modifies the loading/ovalization process of the sleeve compared to the experiment without backfill material, resulting in vertical convergence and horizontal divergence. Furthermore, analyses of the two anisotropy planes were carried out before and after the heating to study the intrinsic anisotropy of the COx behavior. These analyses show that the pore pressure distribution due to the micro-tunnel excavation plays a non-negligible role in the thermally induced pore pressure, accentuating the anisotropic THM response of the COx.