Three-dimensional behaviour of a prototype radioactive waste repository in fractured granitic rock

Abstract

An investigation of the three-dimensional coupled thermohydromechanical behaviour of a prototype repository in fractured granitic rock is presented. The pre-placement behaviour of the repository is first considered, making use of a full three-dimensional simulation of the problem. An effective continuum approach, augmented with discrete features with a high hydraulic conductivity, is employed. The method adopted is found to be able to simulate accurately the highly anisotropic flow regime observed at the pre-placement phase. The major features of the full repository experiment under applied heating conditions were then successfully simulated. The range of buffer hydration rates, the thermal response of the repository, and the associated mechanical response were successfully simulated. Approaches to capture both the transient microstructural behaviour of the compacted bentonite (MX-80 type) and a MX-80 pellet material are incorporated. The repository behaviour was shown to be strongly influenced by complex coupled processes, including interactions between the system components. The adoption of a three-dimensional modelling approach proved to be essential to realistically simulate the behaviour of a repository incorporating anisotropic flow behaviour. Finally, potential impacts of the processes considered on performance of the barrier system and in safety assessment are considered.