Guidance document on THM coupled processes in performance assessment (Benchpar)

The objective of this guidance document is to provide advice on how to incorporate thermo-hydro-mechanical (THM) coupled processes into performance assessments (PAs) and design studies for radioactive waste disposal in geological formations to be experienced in a European context. The document has been generated by the EU research project Benchpar: benchmark tests and guidance on coupled processes for performance assessment of nuclear waste repositories. The document starts in Section 1 with an explanation of why numerical analyses incorporating THM mechanisms are required for radioactive waste studies, and provides background material on the subject. Then, the THM processes and their interactions are explained in Section 2. Three case examples of THM numerical analysis are presented in Section 3 to illustrate the type of work that can be conducted to study the near-field, upscaling, and the far-field. The importance and priority of the THM couplings are summarised in Section 4. Recommended soft and hard auditing procedures are presented in Section 5. ln this guidance document, we emphasise especially that the most important step in numerical modelling is not executing the calculations per se, but the earlier conceptualisation of the problem regarding the dominant processes, the material properties and parameters, the engineering perturbations, and their mathematical presentations. The associated modelling component of addressing the uncertainties and estimating their influence on the results is similarly important. ln other words, the specific models and codes should be studied first to evaluate the harmony between the nature of the problem and the nature of the codes. The practical use of particular numerical techniques will then be based on a sound strategic foundation. An associated listing of bullet point recommendations and issues for future directions for this THM subject are a is given in Section 6.