Ultrasonic Investigation of the Welding Area of Copper Canisters for Spent Nuclear Fuel

Geological repositories will be built in several countries for the long term storage of spent nuclear fuel. Sweden and Finland, for example, foresee the encapsulation of the spent fuel assemblies in copper canisters with iron inserts to be deposited in tunnels excavated about 500 metres underground in the bedrock. During the transport of canisters and the storage in the final repository, the Continuity of Knowledge (CoK) of spent fuel must be kept. Safeguards requirements suggest the implementation of a unique identification tag on copper canisters, strongly reliable and secure against falsification attempts. This paper presents a solution for the canisters’ authentication issue, using an innovative ultrasonic investigation of the internal gap between copper lid and canister tube. It provides a fingerprint strictly related to material features after the friction stir welding process. This fingerprint can be combined with the ultrasonic response of a unique code made by chamfers machined on the copper lid inner surface. The angular matching between the two fingerprints (internal gap response and chamfers’ code reading) realizes a third signature more robust and reliable. A potential practical implementation of this solution is described experimentally in this paper.