Ultrasonic investigation system for the integrity verification of the threads used to bolt lids on casks for nuclear spent fuel

Dry storage casks filled with nuclear spent fuel assemblies are closed by bolted lids. The continuity of knowledge of the nuclear material inside the cask, once verified by Safeguards inspectors must be maintained. The work presents an ultrasonic investigation system able to recognize any attempt to tamper with the thread that could allow the seal to be removed without unscrewing it, meaning opening of the lid without leaving trace of it. For this purpose, we have defined a modification consisting of an inclusion of a thread within a larger thread (M45 instead of a M36). The proposed method is based on the analysis of the characteristic velocity and diameter estimation for distinguishing a different thread respect to the M36 in presence of large variation of the Castor V ductile material used for the storage casks.