Ultrasonic method to keep the Continuity of Knowledge of spent nuclear fuel from the Encapsulation Plant to the Geological Repository

Nowadays several countries are planning to store nuclear spent fuel in long-term geological repositories. The fuel will be preserved for thousands of years inserted in copper canisters with iron inserts. In Sweden, after the encapsulation of the fuel, canisters will be welded and transported to the geological repository for the deposition in tunnels. During transport, the Continuity of Knowledge (CoK) of spent fuel must be kept. An option could be the identification of canisters by a unique tagging system. Traditional tagging methods do not fit all the requirements foreseen in this application and then alternative ideas should be developed. The method proposed by the Joint Research Centre, in collaboration with the University of Florence (Italy), could guarantee not only a unique label for each canister but also security against falsification attempts. The idea is to combine two fingerprints: the first related to a configuration of chamfers arranged around the lid circumference and the second due the variation of the internal gap between lid and tube after the Friction Stir Welding process. The geometry and position of chamfers are designed to maximize the ultrasonic echo without affecting too much the canister structure. The ultrasonic amplitude response (identification fingerprint) acquired by a rotating transducer is strictly related to chamfers disposition and then it is unique for each container. However, in order to verify the originality of canisters, the internal gap between lid and tube is investigated by ultrasound. The ultrasonic response due to the inspection of the welding area is a “natural” fingerprint (authentication fingerprint) for canister because it is connected to the welding process and material properties. The identification and authentication fingerprints can be combined by angular matching in order to increase the robustness of the method. The description of the method and the development and testing of the ultrasonic reading system are reported in the paper.