DECOVALEX-THMC Project. Task A. Influence of near field coupled THM phenomena on the performance of a spent fuel repository. Report of Task A1: Preliminary scoping calculations

The DECOVALEX-THMC project is an ongoing international co-operative project that was stared in 2004 to support the development of mathematical models of coupled Thermal (T), Hydrological (H), Mechanical (M) and Chemical (C) processes in geological media for siting potential nuclear fuel waste repositories. The general objective is to characterise and evaluate the coupled THMC processes in the near field and far field of a geological repository and to assess their impact on performance assessment: - during the three phases of repository development: excavation phase, operation phase and post-closure phase; - for three different rocks types: crystalline, argillaceous and tuff; - with specific focus on the issues of: Excavation Damaged Zone (EDZ), permanent property changes of rock masses, and glaciation and permafrost phenomena. The project involves a large number of research teams supported by radioactive waste management agencies or governmental regulatory bodies in Canada, China, Finland, France, Germany, Japan, Sweden and USA, who conducted advanced studies and numerical modelling of coupled THMC processes under five tasks. This report presents the definition of the first phase, Task A-1, of the Task A of the project. The task is a working example of how interaction between THMC modelling and SA analysis could be performed. Starting with the technical definition of the Task A, the report presents the results of preliminary THM calculations with a purpose of an initial appreciation of the phenomena and material properties that must be better understood in subsequent phases. Many simplifications and assumptions were introduced and the results should be considered under these assumptions. Based on the evaluation of the multiple teams' results, a few points of concern were identified that may guide the successive phases of Task A studies: 1. The predicted maximum total stress in the MX-80 bentonite could slightly exceed the 15 MPa design pressure for the container. The MX-80 bentonite exhibits very nonlinear THM behaviour and small variations in the assumed input properties could significantly influence the results. A systematic model calibration with laboratory data will have to be performed in order to predict stresses in the bentonite with more confidence. 2. The preliminary results show that a damage zone could be formed around the waste emplacement boreholes; the extent of this damage zone, as well as its hydraulic and mechanical properties, should be better predicted. 3. In addition to unsaturated properties of both the rock and buffer, the variation of rock permeability with stress or strain could have significant effects on the resaturation time of the bentonite. Permeability functions specific to the granite under consideration will have to be developed