Reports published on gas production in deep geological repositories

Nagra recently published a synthesis report and a series of further reports on the production, consumption and transport of gas in deep geological repositories.

Chemical and microbial processes produce gas in a sealed repository. Nagra has been investigating the impacts of gases on long-term safety since 1997. The reports summarise this work and provide an overview of the current state of knowledge on this topic.

Gas-induced processes can have an influence on the long-term safety of a repository for low- and intermediate-level waste (L/ILW) or high-level waste (HLW) in the Opalinus Clay host rock. The synthesis report contains an overview of gas sources and gas production rates from the waste itself, the repository installations and the waste containers; this allows accumulated gas volumes to be calculated. The results of these calculations are used for modelling gas transport and the resulting gas pressure build-up. «The analyses and calculations have shown that gas production does not compromise the safety functions of the host rock and the engineered barriers in either type of repository – even under pessimistic assumptions», says Nikitas Diomidis, Section Head Materials Performance at Nagra.

The reports also present various options for reducing gas production in repositories even further if required. In both repository types, gas production is dominated by hydrogen generated mainly by the corrosion of carbon steel. Steel is currently foreseen as the material for the disposal canister for HLW. Alternative canister materials, but also alternative construction and emplacement technologies or treatment of metallic waste by melting, could reduce the gas volumes. A gas pressure build-up could also be mitigated through use of appropriately designed underground and access structures and the controlled removal of gas along the access structures.

With a view to ongoing optimisation of the repositories, Nagra is examining the topic of gas as part of its research and development work.

Picture: NWMO

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