Technical Report NTB 02-11

Canister Options for the Disposal of Spent Fuel

Canister design concepts for the disposal of spent fuel in repositories in both crystalline and Opalinus Clay host rocks are proposed, based on a review of the functional and performance requirements for such canisters, proposed design criteria and an assessment of repository conditions and their impact on long-term performance of possible canister materials. Two proposed canister lifetime targets of 1000 and 100 000 years are considered, based on experience from a variety of performance assessment studies in a number of countries, including Switzerland. The two canister options proposed and evaluated in detail that could meet the lifetime requirements are a thick-walled (~15 cm) carbon steel canister and a composite canister with a copper external shell and a cast iron insert to provide structural integrity (the proposed SKB/Posiva canister).

The cast steel canister is at a conceptual design stage, thus from the manufacturing perspective, only the basic feasibility of fabricating a canister shell has been considered. For an evaluation of the long-term integrity, the structural behaviour of the shell under isotropic loading conditions in the repository has been considered, along with a detailed assessment of the impact of various corrosion mechanisms on canister lifetime. The corrosion evaluation indicates that the short (some decades) aerobic phase of the repository would lead to very limited general and pitting corrosion (approximately 1 cm). Subsequent anaerobic corrosion is expected to occur at a rate of 1 μm a-1. Evaluation of other mechanisms such as microbial corrosion, stress-corrosion cracking and hydrogen damage indicates that they are not expected to lead to canister breaching, thus a lifetime for a steel canister is expected to be at least 10 000 years, well in excess of the target lifetime of 1000 years. The structural analysis indicates that, for the expected total depth of corrosion of 2 cm, the canister has sufficient strength that structural loads would not lead to breaching within 10 000 years.

The corrosion assessment of the copper canister for crystalline and Opalinus Clay repository conditions suggests a lifetime of at least 100 000 years, in line with Swedish and Finnish assessments.