Assessment of canister degradation for the encapsulation of spent nuclear fuel: Key research issues encountered in recent regulatory reviews and government decision making in Sweden

In January 2022, the Swedish government approved the development of a final repository for spent nuclear fuel at the Forsmark site. Leading up to this decision, and after completion of the main review phase in 2018, the Swedish Radiation Safety Authority (SSM) was asked by the Government to comment on new reported experiments related to degradation mechanisms for both the canister’s copper shell and cast iron insert. In particular, decision makers were concerned to ensure that such studies did not raise new questions regarding the suitability of the canister design.

In this paper, we provide an overview and assessment of the relevant degradation mechanisms:

• Localised sulphide corrosion of copper: Although previously ruled out based on the condition that available sulphide would be consumed by general corrosion, for high sulphide loads passive films may form. Localised corrosion could then be induced by tensile stresses (SCC) or other factors (pitting corrosion).
• Anoxic corrosion of copper: Although contradictory to conventional thermodynamic understanding, the detection of hydrogen evolution in certain laboratory experiments and an alleged higher than expected corrosion during a 20 year field experiment (LOT) have been proposed as evidence for this process.
• Radiation induced effects: The initial radiation field is expected to have a limited effect on rates of corrosion and to some extent affect the properties of canister materials.
• Strain hardening of cast iron: The implications for the canister’s mechanical integrity of deformation events that do not directly lead to canister failure (e.g. large earthquakes) have been proposed as being important in safety assessment.

It has been postulated that these processes represent a significant detriment for canisters, with suggestions that containment lifetimes could be reduced by orders of magnitude. Such claims have, however, typically been made in the absence of relevant data relating to repository conditions, and it has thus not been straightforward for decision makers to assess their accuracy.

SSM´s reviews generally found the first item to be most significant since experiments indeed reveal that passivating films could form under certain conditions. Even so, differences in overall repository performance were assessed to be limited, primarily because any appreciable effect would be restricted to a small number of deposition holes in which it was theoretically possible to sustain combined relatively large mass-transfer rates and high sulphide concentrations. The second item was judged to be of negligible significance, with no plausible connection to the LOT experiments, where detected corrosion is considered to be associated with oxidising conditions. For the third and fourth items, limited effects on canister integrity analysis can be expected and SSM looks for work to be conducted to ensure safety margins are maintained with respect to mechanical performance of canisters.

Although the fruitful Swedish canister debate did not result in a need to modify regulatory conclusions regarding compliance with regulatory criteria, experiences show that new scientific information can always emerge which must be considered in a site specific context regarding e.g. detailed canister design and production as well as safety assessment.