DECOVALEX-THMC Project. Task A. Influence of near-field coupled THM phenomena on the performance of a spent fuel repository. Report of Task A2

This report presents the research works performed for the Phase 2 of Task A of the international co-operative research project DECOVALEX-THMC (2003-2007), in which the development and calibration of mathematical models for Lac-du-Bonnet granite and the MX-80 bentonite, using a variety of laboratory and field experiments are the major topics of research. The objective is to assess the implications of coupled THM processes in the near field of a typical repository on its long-term performance, with special concerns on the impact of rock damage and bentonite behaviours on the performance and safety assessments of underground nuclear waste repositories. The mathematical models used by all teams are based on the framework of Biot's theory of poroelasticity. However, the models are extended beyond the classical Biot's theory, in order to include such effects as thermal effects, unsaturated behaviour of the bentonite and damage behaviour of the granite. The results show that the behaviour of both materials are very complex, and an accurate scientific understanding is not currently achieved for repository safety assessment. However, the achievement have clarified a number of scientific and technical issues related to rock damage and bentonite behaviour and helped to establish adequate confidence in identifying the key safety features in the near field of a hypothetical repository, using coupled THM scoping calculations. These key features would be the temperature evolution, the resaturation and stresses in the bentonite, the extent of the inner damage zone in the rock, and the change in hydraulic and mechanical properties in the rock