RANGERS - Methodology for design and performance assessment of engineered barrier systems in a salt repository for HLW/SNF: synthesis report

Abstract

Salt formations have long been recognized as a highly favorable host rock for the final disposal of high-level radioactive waste (HLW) in deep geological repositories. Their unique properties, including exceptional impermeability, self-healing capabilities, and thermal conductivity, make them a reliable natural barrier for the deep disposal of radioactive waste. This report focuses on the development and application of a methodology for assessing the integrity and performance of the Engineered Barrier System (EBS) within salt-based repositories, a critical component of the multi-barrier system ensuring safe radioactive waste disposal. The RANGERS project, a collaboration between BGE TECHNOLOGY GmbH (BGE TEC) and Sandia National Laboratories (SNL), leverages decades of expertise from Germany and the United States to establish a unified approach for the design, evaluation, and performance assessment of EBS in salt repositories. The methodology developed under this project provides a comprehensive framework for addressing the regulatory, geotechnical, and safety requirements for HLW repositories, ensuring containment and isolation over regulatory timeframes. At the core of this methodology is the integration of Features, Events, and Processes (FEP) analysis to evaluate the loads and stresses acting on the EBS, predict its evolution, and assess its performance through rigorous numerical simulations. The approach begins with the selection of a geological site and repository design, followed by the definition of a tailored sealing concept. The repository system, comprising the geological site, infrastructure, and EBS, is subjected to FEP analysis to identify relevant processes affecting the EBS’s structural and functional integrity. This analysis informs the integrity assessment, which evaluates the EBS’s capacity to withstand thermomechanical, hydraulic, and chemical loads over regulatory timeframes. The integrity of the repository system is verified through a dual-path framework, consisting of an Integrity Demonstration under a reference scenario and an Integrity Evaluation under alternative scenarios. This comprehensive approach, as outlined in regulatory guidelines, provides a deeper understanding of repository robustness and reliability. The Integrity Demonstration ensures the sealing function by analyzing hydraulic resistance and structural stability under thermal-hydraulic-mechanical-chemical (THMC) conditions. It is inspired by engineering standards such as EUROCODE, utilizing partial safety factors to address uncertainties in loads and resistance, ensuring structural integrity of the EBS. The Integrity Evaluation, meanwhile, focuses on the hydraulic evolution of the EBS under alternative scenarios, enhancing robustness and resilience by optimizing its design. While mechanical analysis dominates integrity demonstration, the hydraulic evolution assessed in alternative scenarios directly contributes to radiological performance, aligning these evaluations within the broader performance assessment framework. Building on this approach, a robust and comprehensive modeling concept has been developed to derive systematic numerical analyses for both integrity and performance assessments. This advanced concept enables precise and rigorous safety evaluations, ensuring the long-term containment of high-level radioactive waste (HLW) and spent nuclear fuel (SNF). By leveraging recent advancements in performance assessment (PA) codes and the availability of powerful computational resources, the concept integrates a wide range of FEPs into unified numerical models. These models employ variable discretization strategies tailored to the specific focus of each investigation, allowing for optimized resolution and computational efficiency. This ap- proach ensures that all critical aspects of repository performance are captured with sufficient accuracy, enhancing confidence in the overall safety and robustness of the repository system. The present report consolidates two separate studies into a synthesis document: one focusing on the development of the methodology, and the other on its application to a generic repository system in salt formations through numerical analyses. Key findings from the RANGERS project, derived from the application of this methodology, confirm that the engineered barrier system (EBS) can withstand THMC repository conditions, maintain structural integrity, and ensure effective containment under repository conditions: • Thermal Evolution and Backfill Compaction: The crushed salt backfill, which serves as the long-term sealing component in repository drifts, achieves significant compaction within approximately 1,000 years. Elevated temperature, generated by the decay heat from HLW and SNF, facilitate rapid compaction in the early stages, ensuring effective containment within decades of emplacement. Thermal conductivity of the salt rock contributes to efficient heat dissipation, minimizing the impact on the structural stability of the repository. • Mechanical integrity: The repository system demonstrates robust mechanical stability under thermomechanical loads. Dilatancy zones, which form locally around the drift and shaft walls, are predicted to remain spatially limited and recover naturally over time due to salt’s creep and convergence properties. This recovery ensures the repository’s structural integrity over extended timescales. It has been shown that the sealing components in the EBS will be subject to compressive stresses over time as a result of the creep behaviour of the salt. • Hydraulic Sealing Performance: Hydraulic assessments reveal minimal fluid migration through the shaft seal, with calculated inflow volumes into the infrastructure remaining below significant levels for up to 50,000 years. This ensures the hydraulic sealing function of the EBS under both normal and extreme scenarios. The evolving permeability of the compacted crushed salt is shown to effectively prevent advective transport of potential contamined fluids, further contributing to the containment function. • Gas Generation and Transport: Simulations indicate low gas pressures within the repository due to the initial migration of evaporated water into the surrounding salt rock, driven by elevated temperatures. This process reduces the potential for gas-induced hydraulic loading, ensuring that the EBS’s structural integrity is maintained over time. Further analyses have shown that, at low gas pressures, the influence on the compaction of crushed salt is relatively modest, allowing the backfill to consolidate as intended. However, at elevated gas pressures, the impact becomes more pronounced, with the potential to significantly delay the compaction process by several decades, or in extreme cases, even centuries. Under conditions of extremely high gas pressure, the compaction process may not only be halted entirely but could also reverse, leading to the dilation of the backfill material. • Integrated Performance Assessment: The methodology effectively integrates the results of numerical simulations into an overarching performance assessment framework. This includes evaluating the evolution of the EBS under the reference and alternative scenarios. These assessments have shown that gases generated within the repository will be dissolved in the crystalline water of the salt. It is therefore unlikely that high gas pressure will develop in a salt repository. The developed numerical simulations can now be further elaborated for the consideration of radiological assessment. The findings emphasize the suitability of salt formations and the engineered barrier system for radioactive waste disposal. The RANGERS methodology offers a structured workflow for designing, evaluating, and optimizing EBS performance in salt repositories. By focusing on the integrity and functionality of each EBS component and its evolution, the methodology reduces uncertainties in performance assessment models and ensures compliance with stringent regulatory standards. The project RANGERS provides a solid foundation for advancing EBS integrity assessments, optimizing repository designs, and improving long-term safety for HLW disposal. The results validate the EBS’s capacity to maintain its structural and functional integrity over regulatory timeframes, supporting sustainable, long-term radioactive waste management in salt repositories. Through its rigorous and adaptable framework, the RANGERS project establishes a new standard for the safe and effective disposal of radioactive waste in salt formations.