The majority of dry cask storage systems (DCSSs) in the United States confine used nuclear fuel within austenitic stainless steel canisters, and there is concern that some of these may become susceptible to chloride-induced stress corrosion cracking (CISCC) over the extended timeframe of their storage at independent spent fuel storage installations (ISFSIs). This report is part of an industry effort to evaluate the susceptibility to CISCC of stainless steel canisters used in DCSSs.
Due to the delayed opening of a final geological repository for used fuel generated by U.S. nuclear power plants, many plants have constructed an ISFSI, using DCSSs to relieve crowding in the spent fuel pool. To address concerns that corrosion of the stainless steel canisters may occur at some sites over extended periods of operation (on the order of 120 years or longer), the Electric Power Research Institute (EPRI) is developing an aging management plan with susceptibility criteria for stainless steel canisters.
The objective of this technical update is to calculate the time frames associated with the progression of CISCC in austenitic stainless steel canisters in DCSSs, starting with the assumption that it has initiated. This report also evaluates the tolerance of the canister to a through-wall crack. Modeling and calculations are performed that: (1) determine the growth rate of CISCC cracking (starting after the assumed point of crack initiation), (2) evaluate the effect of different ambient environments on crack growth rates, (3) calculate the mechanical flaw tolerance of the canister, and (4) determine the time taken for air to displace the inert internal atmosphere of the canister.
The flaw growth assessment first describes a crack growth rate model based on the available experimental data for atmospheric CISCC. Environmental data are then used to calculate the propagation rates, assuming prior crack initiation, that result from the duration of aqueous conditions and surface temperature calculated for various climates. This model is applied to calculate the time to propagate through the shell thickness for various locations on a hypothetical canister, starting immediately after CISCC initiation. In the flaw tolerance assessment, critical crack sizes are calculated for design basis pressure and handling loads, and representative time scales for air ingress are calculated.
Due to the relationship between surface temperature, humidity, and deliquescence, crack growth rates are dependent on the local atmospheric conditions, and calculated through-wall crack propagation times vary greatly among locations with different ambient conditions. The time required to develop conditions conducive to SCC and potentially lead to crack initiation is not evaluated in this report.
CISCC initiation is dependent on site-specific environmental conditions and the decay heat loading of each canister, and initiation would likely require a substantial period of time. The canister designs are very flaw tolerant; large flaws are required before a critical flaw size is approached. Once a flaw grows through-wall, the depressurization time is calculated to be relatively short, while the time required for air to displace the inert backfilled atmosphere is months or years.
Application, Value, and Use
These analyses are presented in advance of the Industry Susceptibility Assessment Criteria report, which will consider how the canister’s flaw tolerance affects schedules for evaluation and how environmental and DCSS parameters affect the full cycle of CISCC degradation, that is, establishment of susceptible conditions, followed by CISCC propagation,. The model and calculation framework developed in this evaluation may be further extended to perform risk-based evaluations of aging management strategies.