An overview of inverse modeling methods applied to the thermal and diffusion experiments at the Mont Terri underground rock laboratory

Various countries are considering consolidated clay formations as suitable host rocks for the deep disposal of radioactive waste. The main purpose of experiments performed at the Mont Terri Underground Research Laboratory (URL), Switzerland, is to develop experimental tools and modeling methods to characterize the properties of the clay formations. Among all the experiments carried out, HE-C is dedicated to the characterization of the thermal behaviour of the rock, and DI is a diffusion tracer test. The HE-C experiment has consisted in measuring the time evolution of the rock temperature submitted to a heating source during 250 days in order to determine the thermal conductivity parameter of the clay. Temperature data are collected at different vertical levels. In DI experiment, HTO and stable iodine were injected in a borehole. Concentration is monitored in the injection system. At the end of the experiment, tracer concentrations were measured along several profiles extracted from an over-core. For the DI experiment, the properties investigated are accessible porosity and effective diffusion coefficient. Modeling of these two experiments involve the same partial differential equation, a parabolic-type equation. The presence of a bedding plane combined with an anisotropy of the medium requires a three-dimensional modeling. Equations are solved numerically by CAST3M (CEA code) using a Finite Element scheme for HE-C and a Mixed Hybrid Finite Element scheme for DI. (authors)