Frequency and Location Dependent Attenuation in Copper Tubes for Radioactive Waste Deposit

The present paper describes the progress in understanding the amplitudes and attenuation changes acting on the inspection, by the Phased Array UT, of copper canisters for the final disposal of spent nuclear fuel. Previous studies showed the existence of a low pass filtering effect on the attenuation coefficient, due to the heterogeneous grain size distribution, along the path of UT waves, thus affecting both the detectability of defects and their POD determination. Consequently, the difference between the first and second back wall echoes were not sufficient to determine the local attenuation (within the inspection range), which affects the signal response obtained by the individual defect. These deviations were taken into account by manual adaption of the modelled data, elaborating an attenuation that is the superposition of a 0 value from back-wall echo and a  taking into account the deviations between measured and modelled data values. To confirm the experimental evidence and achieve a deeper insight, experiments were carried out onto test samples cut from a full-size copper canister characterized by different attenuation properties. Data were analysed both in the time and in the spectral domains and a model explaining the observed behaviour was implemented.