Probabilistic Canister Breaching Model (PCBM) for Carbon Steel Canisters

ABSTRACT The current Swiss concept for the permanent disposal of vitrified high‐level waste and spent fuel involves the emplacement of carbon steel disposal canisters in a deep geological repository in Opalinus Clay. Breaching of the canister is most likely to occur by mechanical overload following the loss of wall thickness and/or of the degradation of material properties due to corrosion. A probabilistic canister breaching model (PCBM) has been developed to predict the distribution of breaching times, taking into account the effects of various sources of material and environmental variability and mechanistic uncertainty. The mechanical degradation modes included in the model are ductile and brittle fracture or local plastic collapse associated with the presence of defects or gross plastic collapse as a result of excessive loss of wall thickness. Breaching of the canister is predicted to occur as a result of global plastic collapse once the wall thickness has been reduced sufficiently by a period of anaerobic uniform corrosion. Flaws of a plausible size do not lead to canister breaching. The minimum and median lifetimes for spent fuel canisters are predicted to be 71 000 and 221 000 years, respectively. Canisters for reprocessed high‐level waste have longer lifetimes (minimum and median lifetimes of 133 000 and 334 000 years, respectively) because of the smaller diameter:wall thickness ratio compared with that of canisters for spent fuel. The most important factor determining the canister lifetime is the rate of anaerobic corrosion.