Coupled T-H-M issues relating to radioactive waste repository design and performance

In this paper. coupled thermo-hydro-mechanical (THM) issues relating to nuc lear waste repository design and performance are reviewed. Concise statemen ts. that were developed from DECOVALEX discussions, on the current state-of -knowledge are presented. Section 1 describes the THM background and the in terface with performance assessment (PA). The role of THM issues in the ove rall repository design context is amplified in Section 2, which includes a review of the processes in terms of repository excavation. operation and po st-closure stages. It is important to understand the overall context, the d etailed THM issues, the associated modelling and how these issues will be r esolved in the wider framework. Also, because uncoupled and coupled numeric al codes have been used fur this subject, there is discussion in Section 3 on the nature of the codes and how the content of the codes can be audited. To what extent does a particular code capture the essence of the problem i n hand? Consideration is also given to the associated question of code sele ction and the future of numerical codes. The state-of-knowledge statements are presented in Section 4 under 11 headings which follow the repository de sign sequence. The overview conclusion is that "A predictive THM capability is required to support repository design because precedent practice inform ation is insufficient. Many aspects of THM processes and modelling are now well understood and there is a variety of numerical codes available to prov ide solutions for different host rock and repository conditions. However, m odelling all the THM mechanisms in space and time is extremely complex and simplifications will have to be made - if only because it is not possible t o obtain all the necessary detailed supporting information. Therefor, an im portant step is to clarify the THM modelling requirement within the PA cont ext. This will help to indicate the complexity of THM modelling required an d hence the models. mechanisms, type of computing, supporting data, laborat ory and in situ testing, etc, required. An associated transparent and open audit trail should be developed." We also include comments from reviewers a nd highlight four outstanding issues which are currently being studied in t he DECOVALEX III programme. (C) 2001 Elsevier Science Ltd. All rights reser ved.