Problems and research performed

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:

• clarifying the role of THM processes for PA;
• demonstration analysis of disposal system stability;
• scale-dependent properties relevant to repository design and performance assessment; and
• technical auditing demonstration of the overall modelling and a specific numerical codes.

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:

1. to provide concrete examples on when T-H-M couplings may need to be considered in a quantitative fashion in post-closure performance assessment of nuclear waste repositories in hard rock formations and when T-H-M couplings not need to be considered in such assessments; and
2. to provide a practical approach for the general problem of simplifications of T-H-M analyses such that they can be properly incorporated in Performance Assessment analysis, and to evaluate uncertainties introduced through such simplifications.

In order to address the issues as addressed above, the Task 4 teams had:

1. analysed two major T-H-M experiments (Task 1 and Task 2) and three different Bench Mark Tests (Task 3) set-up to explore the significance of T-H-M in some potentially important safety assessment applications;
2. compiled and evaluated the use of T-H-M modelling in safety assessments at the time (year 2000), using a questionnaire and answers from the concerned funding organizations in the DECOVALEX III project; and
3. organised a forum of interchange between PA-analysts and THM-modellers at each DECOVALEX III workshop.

Achievements and outstanding issues

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:

• Are most THM-related FEPs both identified and sufficiently understood?
• Can we formulate workable performance measures for judging the relevance of THM coupling?
• Can the identified FEPs be managed through the appropriate combination of design, process modelling and scenario analysis?
• Do we need to consider coupled HM mechanisms outside the near-field?
• Is there a need to couple THM with transport of radioactive nuclides (apart from the indirect coupling though hydrogeology)?
• What do we need to now as regards short term EBS evolution and monitoring and its relation to System Safety?
• Is the interaction with the ‘PA-people’ actually taking place within your organisation and is it used to inform, set priorities and define the needed level of accuracy? Where should (further) THM-related R&D focus?

The main general conclusions of Task 4 were that:

1. the full development of T-H-M modelling was still at an early stage during the DECOVALEX III project time, and it was not evident whether the current codes could provide the adequate information that was required for PA/SA;
2. the current situation may not directly imply that all existing coupled mechanisms must be represented numerically in PA/SA. Modelling could be conducted for specific purposes and match the required confidence level with the modelling objective;
3. coupled THM modelling must incorporate uncertainties, in the conceptual model and in data assessment;
4. the emphasis on the need for THM modelling differs among disciplines. For geological radioactive waste disposal in crystalline and other similar hard rock formations , it is essential to understand the stress-permeability couplings when interpreting stress and permeability field data, understand the coupled processes involved in the re-saturation of the near-field, understand the coupled processes involved in the development of an Excavated Disturbed Zone and understand the coupled processes involved in the impact of large-scale and significant climatic events, like glaciations and permafrost.

Publications

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:

• J.A. Hudson, O. Stephansson, J. Andersson, Guidance on numerical modelling of thermo-hydro-mechanical coupled processes for performance assessment of radioactive waste repositories, Int. J. Rock Mech. Min. Sci., 2005; 42(5–6):850–870.