Integration of the Short-term Evolution of the Engineered Barrier System (EBS) with the Long-term Safety Perspective

Abstract

The main aim of the project PEBS (Long-term Performance of the Engineered Barrier System) is to evaluate the sealing and barrier performance of the EBS with time, through development of a comprehensive approach involving experiments, model development and consideration of the potential impacts on long-term safety functions. The experiments and models cover the full range of conditions from initial emplacement of wastes (high heat generation and EBS resaturation) through to later stage establishment of near steady-state conditions, i.e. full resaturation and thermal equilibrium with the host rock. The intention is to integrate the knowledge obtained in the project in a manner that will lead to a more convincing connection between the initial transient state of the bentonite barrier and its long-term state that provides the required isolation of the wastes. The work within the project builds on existing knowledge and experience generated during recent years and supported by ongoing national and EC research programmes. The project intends to provide a more complete description of the THM and THM-C (thermo-hydro-mechanical-chemical) evolution of the EBS system, a more quantitative basis for relating the evolutionary behaviour to the safety functions of the system and a further clarification of the significance of residual uncertainties for long-term performance assessment. It is noted that there are other phenomena relevant to the engineered barrier system that have not been dealt with in the PEBS project. Gas production and transport processes in bentonite barriers have been studied in some detail in the FORGE project (Shaw 2014). In addition, ongoing activities within the IGD-TP (Implementation of Geological Disposal Technology Platform) are examining microbial issues relevant to geological disposal. The present report focuses in particular on the importance of uncertainties arising from potential disagreement between the process models for THMC evolution and the relevant laboratory and in situ experiments performed both within and outside PEBS. The implications for extrapolation of results are treated, with particular emphasis on extrapolation of long-term performance. The report builds on the approach presented in PEBS Deliverable 1.1 (SKB et al. 2012), which described the treatment of the early evolution of the EBS in safety assessments from a number of European national programs based on their studies published in the period 2002 - 2010. Despite the differences in repository concepts, which include crystalline, and clay host rocks and copper and carbon steel canisters, the safety functions defined for the engineered clay barriers are similar. The key processes occurring in the EBS in the early evolution of the repository that may affect the long-term performance are similar in all concepts on a fundamental level. However, the significance of the processes for long-term safety as well as the treatment of the processes in the safety assessment can differ among the different concepts. In Deliverable 1.1, the safety assessment methodology was discussed with particular emphasis on safety functions, which are a tool that is used for the evaluation of the long-term performance of individual repository components. The safety function approach as applied to a bentonite barrier is reiterated here in Chapter 2 to provide a framework for the subsequent discussion, as the use of safety function indicators arises frequently throughout the report in the discussion of the significance of uncertainties for long-term performance. The main body of the present report describes the performance of the bentonite barrier in the context of four "cases" that were defined at the outset of the project in D1.1. These cases represent a method of examining the various THMC processes that occur during the evolution of the EBS, along with their impacts on long-term performance, in a manner that provides for each case a specifically different focus. The four cases are outlined in Chapter 3, including the reasons for their selection and the main work in the project that contributes to evaluating them. The four defined cases provide a good basis for discussing all aspects of evolution of the EBS, which forms the content of Chapter 4 and focuses on the best understanding of the processes and the associated uncertainties. Chapter 5 uses the safety function approach to elaborate the importance of the derived uncertainties in process understanding in terms of assessment of the long-term performance of the bentonite barrier, focusing in particular on the degree to which hydraulic and swelling properties are likely to be affected over the long term as a consequence of the early thermohydraulic evolution phase. However, because the significance of the uncertainties in terms of disposal system performance depends on other considerations as well, in particular the specific repository design concepts and the barrier functions of the host rock and canister, the consequences of the uncertainties are discussed Chapter 5 in the context of the national safety cases.