The corrosion behavior of one-fifth scale lid models of transport cask submerged in sea bottom

Corrosion tests have been performed for one-fifth scale lid models of typical transport cask for radioactive materials and for other crevice corrosion test specimens at a sea bottom for maximum 6 years. The effect of corrosion on the performance of the cask was evaluated based on the test results. The dissolved oxygen (DO) value measured at the test site was about 5–14 mg/l. These values were higher than DO values at the sea bottom of 200 m to 3000 m depth in the shipping route, and hence the corrosive condition was more severe than that for the actual cask. No leak was found for models with rubber gasket after immersion at the sea bottom for about 6 years. The crevice between cask body and cask lid was unchanged after immersion for about 700 days, that is, 1.4–2.8 μm. Therefore, even if the rubber gasket were deteriorated, the crevice seemed to be narrower than 10 μm, that have been regarded as the safety opening for sunken transport cask assumed in the environmental impact assessment. The maximum penetration depth at contact surfaces of lid models after immersion for 700 days was about 200 μm which was significantly lower than that of creviced specimens of type 316 stainless steel. This might be due to galvanic effect of carbon steel bolts on the corrosion of stainless steel body.