Stress corrosion cracking (SCC) of welded stainless steel dry storage canisters may potentially impact systems exposed to corrosive atmospheric elements, such as those occurring near salt water bodies. Conditions important for atmospheric-related SCC include concentration and chemical species of the contaminants, temperature, and humidity. Calvert Cliffs Nuclear Power Plant conducted this historically important first inspection of spent fuel canisters to collect data for an SCC evaluation as part of the EPRI-led Extended Storage Collaboration Program (ESCP).
With no clear path for its ultimate disposition, spent fuel will remain in dry storage containers for an extended period of time, likely well beyond the current license period of 60 years. To ensure the integrity of stainless steel spent fuel containers in long-term dry storage, degradation mechanisms such as SCC must be evaluated. Several studies have shown that the materials used for dry storage canisters can in fact undergo SCC under the right conditions. By collecting data on the actual in-service conditions of stainless steel canisters—and combining that data with information on the conditions required for SCC—it will be possible to identify which, if any, dry storage canisters may be susceptible to SCC and when they may become susceptible.
To collect data relevant to determining the conditions for SCC and the corrosive environment to which stainless steel spent fuel storage canisters are exposed.
The project team selected for inspection two stainless steel canisters at Calvert Cliffs that had been in service for more than 15 years, a lead canister with the highest integrated dose and a second canister with the lowest temperature (also known as the “cold” canister). The inspection included visual examination, temperature measurements, and surface sampling. The team performed the inspection remotely, with the canisters remaining in the storage module, so tooling had to be developed for delivering the test equipment to the canister surface. For the surface sample collection, the team used both wet and dry surface methods. The wet method involved a commercially available device, but extensive testing was needed to qualify it for the specific conditions. The dry method involved a specially designed cartridge for collecting as much loose surface dust and debris as possible for later analysis. Visual inspection required insertion of a camera through the outlet vent for access to the area inside of the module for a view of nearly all of the canister surface. Surface sample and temperature measurements were performed by inserting a long tool through a narrow gap between the canister and the storage module in order to access the canister from the front opening with the door removed and temporary shielding installed.
This first-ever in-service inspection focusing on the conditions of dry storage spent fuel canisters successfully demonstrated the ability to remotely access the canister surface, obtain visual evidence of the surface condition, take temperature measurements, and collect surface samples. In terms of visual results, both canisters had substantial amounts of dust, particularly on the top surface, and a few small rust blooms were observed on the lead canister; however, the general condition of both canisters was good, with no signs of gross degradation. The measured temperatures were 124°F and the 112°F at the bottom of the lead and cold canisters, respectively. These measured temperatures were in good agreement with the predicted temperatures, but did not increase appreciably along the sides of the cold canister as predicted. The chemical analysis results confirmed very low chloride concentrations, less than 0.1 g/m2 by one or two orders of magnitude. Of particular interest was that the sample compositions resembled inland rainwater rather than seawater, indicating that the environment is more inland than marine.
Applications, Value, and Use
This inspection represents an important first step in obtaining field data, combined with lab testing and modeling, to assess SCC susceptibility in dry storage canisters under specific atmospheric conditions. Such data is an integral part of developing an overall aging management plan for the extended storage of spent fuel canisters in potentially corrosive environments.