Production and Fate of Gases in the Geological Repository

  pdf NTB 24-23 Production and Fate of Gases in the Geological Repository(29.61 MB)

In a deep geological repository for radioactive waste, corrosion of various metals and alloys will lead to the formation of hydrogen. In addition, organic materials will slowly degrade and generate carbon dioxide, methane and other gaseous species. Depending on local conditions, gaseous species can be consumed by chemical reactions and by microbial activity. If the resulting rate of gas generation exceeds the rate of migration of gas dissolved in the porewater of the engineered barriers or the host rock, the solubility limit will be reached, and a separate gas phase will form. The accumulation and release of gases can influence the long-term evolution of the repository.

This report addresses the so-called gas synthesis that encompasses an assessment of the hydraulic evolution associated with the production and fate of gases in the combined repository at the Haberstal site. The main objectives are to:

• present a comprehensive synthesis of processes and phenomena related to the production and fate of gases in the repository;
• assess the hydraulic evolution under consideration of the transport of gases in the repository structures;
• demonstrate the functionality of the current repository design with respect to gas and deterministically assess associated uncertainties that affect the hydraulic evolution; and
• provide underlying argumentation for the performance assessment of the repository and the subsequent derivation of safety scenarios and analyses of radiological consequences.

The assessment of gas generation and fate in the repository builds on the previous gas synthesis report from Stage 2 of the three-stage Sectoral Plan for Deep Geological Repositories. Compared to Stage 2, the current report makes advancements in particular to:

• assess the gas-related evolution of the combined repository based on the current disposal concept and phenomenological evolution;
• implement the site-specific repository configuration in accordance with the current Swiss disposal concept;
• update the waste inventory and material inventory in support structures from the provisional design of a combined repository;
• update and comprehensively survey new data for corrosion rates of metals and degradation of organics;
• update gas generation modelling and calculate new scenarios;
• implement integrated model-based analyses of gas transport for the total system (the entire repository) as well as repository components and simulate new scenarios.

The modelling workflow from gas generation to gas transport in the repository allows for a comprehensive assessment of the fate of gas and its impact on the hydraulic evolution. The quantitative analysis is achieved through model metrics that are key inputs for the performance assessment. The analysis is undertaken by means of deterministic simulation variants, in this report termed “assessment cases”, which encompass uncertainties, evaluations of the current design and scenarios. The integration of assessments at repository and component scales allows for more detailed insights into the thermo-hydraulic effects in the near-field and in the repository sealing system, the implementation of a firm abstraction methodology for the model-based assessments, and the development of argumentation for the performance assessment.

This report provides an enhanced understanding of the repository system in terms of hydraulic evolution. The synthesis of results demonstrates sensitivities to the deviations considered in the assessment cases, and key processes underpinning the phenomenological evolution of the repository. The main outcome herein is a consistent hydraulic evolution of the repository that adheres to the design principles and associated transport regimes for all considered cases. The key message is that gas pressures in the combined repository are moderate even for pessimistic cases of gas generation and accumulation in the repository.