Synthesis of the Performance Assessment for a Deep Geological Repository for Radioactive Waste
Nagra (2024): Synthesis of the Performance Assessment for a Deep Geological Repository for Radioactive Waste. Nagra Technical Report NTB 24-22 Rev. 1.
This synthesis report provides a general overview of the performance assessment workflow pursued in the context of the safety case for a deep geological repository for both high-level waste (HLW) and low- and intermediate-level waste (L/ILW) at the Haberstal site in the Nördlich Lägern siting region.
The Swiss safety concept for deep geological disposal for radioactive waste relies on a staggered system of geological and engineered barriers, representing the so-called “pillars of safety” in the general safety architecture. Founded on a well-established, transparent disposal programme and on a sound scientific and technical basis, each of the seven pillars of safety performs several safety functions. These functions are overarching functional requirements on the multi-barrier system that, taken together, ensure post-closure safety.
Performance assessment represents the first stage in the general safety assessment workflow towards a robust safety case. Following international standards of transparent and traceable assessment workflows, the performance assessment addresses the post-closure safety of the repository at a level of simplification, which bridges the gap between the real repository system and its highly abstracted representation in the analysis of radiological consequences. The performance assessment workflow encompasses four steps:
- Assessment of barrier performance for each element of the multi-barrier system at the component level (“performance assessment by barrier”);
- Assessment at the level of the entire system (“total system performance”), to account for any possible thermo-hydro-mechanical interactions between the individual barrier components;
- Uncertainty quantification with special focus on parametric and conceptual uncertainties, which are associated with the simplified representation of the realistic repository system in an abstracted performance / safety assessment framework (including uncertainty propagation associated with the abstraction process);
- Performance screening, encompassing a probabilistic assessment of the expected repository performance and of deviations thereof to screen the possible paths of repository performance and to identify, bundle and formulate the safety-relevant paths in a format that can be subjected to the next step in the safety assessment workflow.
The auditability of the assessment workflow is ensured with a traceable method of proof. For this, claims are formulated, assigning one or more intended safety functions to each component of the multi-barrier system and to the repository system as a whole. The claims are rationalised by a suite of arguments. A claim can be said to be robust if it is well substantiated using sound arguments and providing convincing evidence. The robustness of a claim is strengthened by seeking multiple lines of arguments that can be assigned broadly to the following categories: (i) empirical knowledge such as experience from other radioactive waste management programmes, (ii) dedicated scientific and technical data-bases that have been elaborated specifically in the context of the repository project, and (iii) a safety-oriented repository design.
Each argument must be supported by convincing evidence along multiple lines of knowledge gain (empirical evidence, experimental evidence from targeted site investigation programmes, model-supported evidence derived from sensitivity and robustness analyses). A balanced evaluation of arguments and evidence provides important insights into the limitations of repository performance when the multi-barrier system is subjected to internal or external perturbations.
Traceable handling of uncertainty is an indispensable element in the assessment workflow. Performance assessment must provide evidence that the modelling tools and approaches applied for quantitative assessment of barrier performance at component and total system level adequately represent the safety-relevant processes and phenomena. Following well-established safety assessment principles, uncertainty is categorised as scenario uncertainty, conceptual uncertainty and parameter uncertainty. The effect, when the uncertainties of the input parameters of a physical system give rise to uncertainties in the system response, is termed propagation of uncertainty. Tracking of uncertainty propagation along the model abstraction chain is a complex process, called uncertainty quantification. It aims to bracket the confidence bounds on the expected system response of the simplified performance assessment model.
It is the role of performance assessment to screen the possible paths of repository performance and to identify, bundle and formulate the safety-relevant deviations from the expected repository performance in a format that can be subjected to radiological consequence analysis. From the performance assessment perspective, the possible paths of repository evolution are classified in terms of expected performance of the multi-barrier system and deviations thereof. Deviations from the expected performance are analysed at the component level and at the level of the entire repository system with emphasis on the likelihood of occurrence and on their relevance for safety. Those PA scenarios which are deemed relevant for long-term safety are forwarded to the next safety assessment process, called safety scenario development. Safety scenario development maps the relevant paths of barrier performance to the corresponding branches of the safety scenario event tree, representing the input to the analysis of radiological consequences.
The performance assessments presented in this report reaffirm that the repository design, with its multiple layers of defense, offers significant safety margins and is capable of meeting both national and international safety standards. The systematic book-keeping of claims, arguments and evidence at the component level confirmed the performance of the barrier components with respect to their assigned safety functions. The arguments compiled are comprising empirical knowledge, project specific databases from site investigations, dedicated experiments, and a safety-oriented repository layout, based on model-supported designs. The consistency of different lines of arguments and wide-ranging evidence from independent data sources provides confidence in the general safety concept.
The performance assessment at the system level concentrated predominantly on phenomena and processes along the backfilled and sealed underground structures. In particular it was shown that that the HLW and L/ILW sections of the repository are hydraulically and thermally decoupled from each other. The thermal- and gas-induced perturbations in the HLW repository section occur at different times, which means that detrimental superposition of the thermal- and gas-related impacts on the safety functions of the engineered and geological barriers can be excluded. Neither the thermal- nor the gas-related disturbances lead to an increased pore-water flow along the sealed and backfilled repository structures, which could otherwise give rise to increased radionuclide release. Likewise, the impact of thermal- and gas-induced perturbations on the mechanical and chemical integrity of the engineered and geological barriers of the HLW repository is modest. The layout of the repository provides large gas storage volumes, which are crucial for reducing the gas pressure build-up in the L/ILW repository section. The resulting moderate gas overpressures in the repository structures of up to 2 – 3 MPa are not expected to impair the mechanical integrity of the engineered and geological barriers. Volatile radionuclides (mainly 14C) are safely contained until they finally disappear due to radioactive decay. At the same time, the unsaturated conditions in the L/ILW emplacement caverns, with moderate gas overpressures, ensure that the release of dissolved radionuclides from the waste packages into the host rock is strongly retarded for very long times, because the mortar backfill around the waste containers represents a capillary barrier for dissolved radionuclides.
A wide range of potentially relevant deviations from expected barrier performance were specified as part of the performance screening process. The assessment of 23 PA scenarios provided clear evidence of the robustness of the repository system. The redundancy and diversity of the individual barrier components were shown to be an important feature of the multi-barrier system. None of scenarios assessed indicated a significant reduction of safety margins of the repository at the system scale. Finally, only a handful of PA scenarios were identified as cases worthy of further evaluation in the radiological consequence analysis. The motivation for complementary in-depth analyses is the fact that these scenarios deviate significantly from the expected repository evolution, which formed the basis for the provisional repository design, although there was no evidence of a relevant loss of barrier performance.
It can be concluded that the repository for HLW and L/ILW at the Haberstal site is safe over the entire assessment period, with significant safety margins under both expected and adverse conditions. The concept relying on a multi-barrier system, combined with a robust, evidence-based assessment of its performance, ensures that the repository will meet its safety objectives even considering internal or external perturbations that are even of hypothetical nature. By meticulously addressing uncertainties and thoroughly evaluating each element of the system, the performance assessment builds a strong case for the repository’s long-term safety. The transparent and traceable workflow further strengthens the conclusions, ensuring that the safety case is both credible and verifiable and providing a solid foundation for moving forward with the next stages of the safety case and overall repository development.