NDT of Ductile Cast Iron from Nuclear Waste Storage Canisters

The concept for geological disposal of nuclear waste and spent nuclear fuel relies on a multi-barrier system with the copper/cast iron canister as the first barrier. The canister is designed to retain its integrity for at least 100000 years, which means that future glaciations need to be considered. A thick ice block together with hydrostatic pressure from groundwater would produce hydrostatic compressive stresses of maximum 44 MPa. A critical issue for the acceptance of the canister is to guarantee that it does not contain defects that may cause loss of integrity during design life time. Radiographic inspections of as-produced cast iron insert mock-ups of the canister were carried out to check the presence of manufacturing defects. Numerous indications were found both inside and outside the critical zone of tensile stresses. Mockups subjected to a hydrostatic compressive stress up to 130 MPa were inspected as well, and several cracks were detected in the deformed canister walls. The largest defects located in the zone of tensile stresses – slag inclusions and their agglomerates – were deemed to be critical for crack initiation. The inspection of the canisters by means of ultrasound is another useful test to assure the compliance with predefined acceptance criteria for critical defect sizes. Reliable ultrasonic inspections, however, require a good understanding of the beam’s behaviour within the inspected material. Among the physical parameters characterising the interaction of the beam with its supporting medium, ultrasonic attenuation is important because it limits the volume of the system that should be inspected, and is an input parameter for mathematical models, which play an increasingly role in nondestructive testing by allowing computer simulations. Measurements of the intrinsic longitudinal wave attenuation in as-produced cast iron were carried out in 3 different directions and first results are reported in this paper.