Task 4 was organized because it was established, from accumulated knowledge and experience from the previous DECOVALEX I and DECOVALEX II projects, that the following important issues needed to be addressed:
Thus, Task 4 was established to derive conclusions and recommendations on practices in addressing THM issues in Performance and Safety Assessment applications, based on the findings of the DECOVALEX III project. More specifically the task objectives were:
In order to address the issues as addressed above, the Task 4 teams had:
The answers to the questionnaire received during the project period were compiled with the following main conclusions: 1) most organisations already applied standardised procedures for identifying processes and couplings to be considered in assessments; 2) a motivated statement of confidence was indeed a crucial part of a safety assessment report, and here the formal approaches were valuable; 3) judging from the answers, the impression was that most T-H-M issues were already identified, and it was rather the means of analysing the couplings that needed to be discussed.
Several examples of identified problems where T-H-M couplings were shown to be important or were judged to be potentially important to be considered directly in a safety assessment context are given such as: the migration of vapour, water and heat in partially saturated systems; the mechanical stability of the underground excavations before and during construction; the potential for and the effect of rock creep; mechanical effects such as rock fall or fracture shear displacements resulting from earthquakes; the understanding and modelling the formation and resulting hydraulic properties of a disturbed zone (EDZ) around tunnels; stress and stress change impact on fracture hydraulics; the consequences of a glacial ice cover; full thermo-hydro-mechanical couplings when analysing the resaturation of the buffer; and, finally, the heat pulse driven rock fracturing and permeability changes due to theromechanical deformation of fractures, where a full thermo-hydro-mechanical analysis is potentially needed.
In conclusion, T-H-M issues should be considered in repository R&D, both as regards modelling, field experiments and repository design and Safety Assessment. Although PA/SA are built around simplifying abstractions/assumptions, T-H-M modelling coupled with appropriate testing is still needed to understand how the hydrological system works, in order to rationalize the abstractions.
A number of PA/SA experts were invited to give presentations on PA/SA practices of different organizations from different countries, related coupled processes and their treatments in PA/SA practice. They were P. Zuidema, NAGRA, Switzerland: Safety case and THM coupling; J. Alonso, ENRESA, Spain: THM issues in the ENRESA 2000 project; M. Yui, JNC, Japan: Safety Assessment, THM(C) and Monitoring – Japanese experiences; and T. Vieno, VTT, Finland: THM Aspects in the POSIVA Safety Case.
The set of issues evolved during the course of Task 4 discussions were summarized as follows:
The main general conclusions of Task 4 were that:
Besides the one report for Task 4 for the DECOVALEX III project, Task 4 also produced a report for the BENCHPAR project supported by EC but worked with the same three BMTs of Task 2 of the DECOVALEX III project. The TSK 4 team also published a paper in the special issue of the International Journal of Rock Mechanics and Mining Sciences, Volume 42, Number 5–6: