Technical Report NTB 16-03

In a deep geological repository for radioactive waste, in absence of oxygen and in presence of water, corrosion of various metals and alloys will lead to the formation of hydrogen. If present, organic materials may 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 dissolved gas molecules in the pores of the engineered barriers or the host rock, the solubility limit of the gas will eventually be exceeded and the formation of a discrete gas phase will occur. Gases could continue to accumulate until the pressure becomes sufficient to be released in gaseous form.

This report deals with the evolution of gas-related processes that can influence the long-term behaviour and safety of low- and intermediate-level waste (L/ILW) and high-level waste (HLW) repositories in Opalinus Clay. The main aim is to present a synthesis of processes and pheno­mena related to repository-produced gases and to assess their influence on repository perfor­mance. A current overview of gas sources, reactions and interactions, generation, consumption, and transport is provided. Furthermore, current scientific understanding is used to define safety function indicators and criteria, which are employed to evaluate the potential influence of repository-generated gas on safety-relevant properties of engineered and natural barriers.

The assessment of gas generation, consumption and transport is addressed separately for the HLW and the L/ILW deep geological repositories. The employed methodology, which is common for both repository types, consists of the description and quantification of the potential gas sources, which include the waste, barrier components such as disposal canisters and other gas-generating repository components, and of the processes and reactions leading to the genera­tion or consumption of gas. The evolution of the gas generation rate and of the cumulative gas volume during the time frame for safety assessment is then calculated. The outcome is used as input to the modelling of gas transport and the resulting build-up of gas pressure and gas-induced water flow, which are used as safety function indicators. Uncertainties and options are com­bined into specific assessment cases, which are used to specify the expected ranges of safety-relevant parameters, to highlight the potential effects of mitigating measures aiming to decrease the amount of generated gas, and to identify areas where further research could provide the most benefit.

The main outcome of the performance assessment is that for pessimistic base cases, and even for the upper bounding cases, gas production in the HLW and L/ILW repositories does not com­promise the post-closure safety functions of the host rock and the engineered barriers. In all the cases explored there is a safety margin with regards to the defined assessment criteria. More­over, there are several design options available that can further mitigate the consequences of gas production in the repositories if necessary. In both repository types, gas production is dominated by hydrogen resulting mainly from the corrosion of carbon steel. As a result, alternative emplace­ment and building technologies, alternative disposal canister materials, or treatment of metallic waste by melting could reduce the amount of repository-generated gas during the period for safety assessment. The build-up of gas overpressure and resulting porewater displace­ment could also be mitigated by thriving gas-consuming microorganisms or by tailoring the engineered gas transport system.