Application of Eddy Current Testing to SCC Detection for Canisters in Concrete Cask System

Concrete cask system is widely used in the United States, but it has not been put into practical use in Japan. To apply the concrete cask system to the storage of spent fuels in Japan, it is necessary to prevent Stress Corrosion Cracking (SCC) which may occur on the surface of canister. Occurring SCC needs three factors, i.e., residual stress, environment, and material. Removing just one of these factors can prevent occurring SCC. However, prevention effect may be degraded by aging effect, therefore we are considering about introducing periodical inspection of SCC to our concrete cask system during long storage period. Some nondestructive inspections are proposed for SCC detection of canisters. The eddy current testing (ECT), one of them, is superior in depth measurement accuracy, but the detection accuracy at the welding zone is low due to the magnetic noise. The level of magnetic noise depends on material, welding amount, welding method, and so on. We researched that magnetic noise of the test piece which has the same condition of the canister used in the United States is equal to 2 mm defect signal. The noise prevents detection of the initial stage SCC. In addition, welding induces tensile residual stress, and welding zone is sensitive for SCC occurring. From the above, it is necessary to decrease magnetic noise at the welding zone to detect the SCC of the canister. To decrease the magnetic noise at welding zone, the magnetic saturation method can be used. The magnetic saturation can improve heterogeneity of magnetic permeability by applying an external magnetic field. We adopt permanent magnets as the external magnetic field and strengthen magnetic force by considering composition of the magnet and its layout without changing material of the magnets. As the result, we succeeded to reduce the magnetic noise to the level of one-tenth at the welding zone while maintaining the sensor compact, and the detection accuracy of our new ECT system reached enough level to detect initial SCC.