Exploratory what-if analysis of some debated canister failure modes in the review of a licence application for the construction and operation of a spent nuclear fuel repository in Sweden

Abstract

Regulatory review of the licence application for construction and operation of a spent fuel repository at the Forsmark site in Sweden involves detailed assessment of both expected and hypothetical failure modes of the copper canister. The copper canister, which is supported by the bentonite buffer and the surrounding crystalline rock in the KBS-3 concept, is expected to provide complete containment of radioactive elements for very long timescales. Detailed assessment shows that there is a small probability on such timescales of canister failure due to corrosion following loss of buffer as well as mechanical failure due to large earthquakes. During the regulatory review process, it was proposed that canisters might also fail due to: (i) corrosion in anoxic oxygen gas free water, (ii) pitting corrosion, (iii) stress corrosion cracking, (iv) creep brittle failure, (v) hydrogen embrittlement. We here provisionally accept a number of alternative assumptions related to these processes as a basis for what-if analysis of their implications. The focus is not to determine the merit or to estimate probability of these cases, but rather to explore their potential significance in the context of the available knowledge about the repository environment. Simplified estimates are made of the consequences in terms of number and timing of canister failures as well as radiological impact. It is judged that poor creep ductility of copper would have larger potential consequences compared to localised corrosion phenomena. Potential corrosion failures are expected to be associated with the small fraction of deposition holes that are most extensively exposed to corrodants.