A Study on Ageing of a Canister during Long-term Storage: Evaluation of the Influence of temperature difference between Canister Sureface and Environment on corrosion

From the point of view of flexibility in the spent fuel management, it is important to provide options of dry storage technology. CRIEPI has been developing the concrete cask technology which specifically fits for the application in Japan. The most concerning issue in Japanese application is stress corrosion cracking (SCC) of a canister since the dry storage facility is usually located near the coast in Japan. For the evaluation of SCC of the canister, it is important to collect data such as the initiation of corrosion and SCC, pitting depth and crack growth rate because it is difficult to make it clear theoretically. A lot of corrosion tests were executed in the world not only for nuclear power plants but also for many fields. In the default method of corrosion test, test specimens with sea salt are put into constant temperature and humidity chamber of which condition is near the canister surface. But, taking actual conditions into account, hot canister surface is exposed different temperature and humidity conditions. To examine this influence of temperature difference between canister surface and environment, the test specimens with heater were put in the chamber. As a reference, the test specimens without heater were put in the chamber, too. The test parameters are temperature and humidity in the chamber and surface temperature of the test specimens. Basic test concept is comparison of corrosion area between two test conditions. In one case, the test condition with heater at 50 degree Celsius in the chamber which is at 30 degree, 100%RH is equivalent to that without heater in the chamber which is at 50 degree, 35%RH. The material of test specimen was stainless steel type 304L. Test duration was about 7900 hours. During the tests, the test specimens were taken out from the chamber at certain intervals and the photos were taken. The photos were binarized and the corrosion area was obtained. Compared the results of test specimens with and without heater, it was found that there is not big difference between the results of two conditions. Furthermore, after finishing the test, pitting depth was measured.