At the Meuse/Haute Marne Underground Research Laboratory, Andra is studying the possibility of disposal for intermediate to high level activity – long life nuclear waste in a 500m deep clay formation. In particular, a specific technical and scientific program is carried out for several years to test the feasibility of realization and the behavior of disposal cells for high level – long life waste packages. Within the framework of this program, and as part of the European project LUCOEX (Large Underground Concept Experiment), a new phase has started at the end of 2012 with the realization of a full scale disposal cell demonstrator. Heat generated by waste packages is simulated by electrical heaters over a length of 15 m, with the aim to reach 90?C at the cell interface in 2 years. The thermo-mechanical behavior of the cell as well as the THM impact on the surrounding rock is monitored through a very complete instrumentation.

1 Introduction
1.1 General context

Andra, the French national radioactive waste management agency, is in charge of the study on the possibility of disposal for intermediate to high level activity – long life (IL/HL – LL) nuclear waste in deep geological repositories. In this aim, the Meuse/Haute Marne Underground Research Laboratory (URL) has been excavated since 2000 in a 500m deep claystone layer (Callovo-Oxfordian, COX) to characterize to confining properties of the clay-stone and demonstrate the feasibility of construction and operation of a geological disposal (Delay et al. 2007).

1.2 Callovo-Oxfordian claystones

The main level of the URL is excavated at a depth of 490m in the middle of a 135m thick argillaceous rock layer, overlain and underlain by poorly permeable carbonate formations. Argillaceous rock contains a mixture of clay minerals (40 to 45% on average) and clay-size fraction of other compositions. The clay minerals offer groundwater tightness and radionuclides retention. Silica and carbonate-rich sedimentary components contribute to high strength of the rock and stability of the underground construction.

Table 1 gives Callovo-Oxfordian claystones mechanical characteristics. Sedimentation has led to a slightly anisotropic behavior of claystones. In-situ measurements indicate a strong coupling between mechanical and hydraulic processes (Armand et al. 2011).

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