Non-destructive assay verification of CANDU type dry storage canisters

An NDA method for qualitative attribute (Method H) verification of CANDU spent fuel baskets in dry storage canisters has been developed, successfully tested, and is currently being used by the IAEA. The method is based on a canister's gross gamma vertical profile scan, as measured by a small size CdTe detector coupled to a portable multichannel analyser (PMCA) operated in multichannel scanning (MCS) mode. The detector is placed in one of the canister's vertical verification tubes and lowered at a constant speed. Simultaneously the PMCA is recording the canister's gross gamma axial profile which gives a characteristic and unique 'fingerprint'. This fingerprint exhibits a number of broad peaks equal to the number of CANDU spent fuel baskets in the canister. The canister's fingerprint is a specific attribute of the verified canister which can be remeasured from time to time. It has been demonstrated that spent fuel canisters containing baskets of fuel bundles with cooling time between five and ten years can be verified with short (three minute) scans. There are no expected difficulties for reverification of spent fuel of longer and shorter cooling time. To increase the confidence level of Method H reverification, gross gamma scanning is supplemented by gamma spectrometric measurements at randomly chosen points by the same CdTe detector. For spent fuel of cooling time more than five years the 137Cs 662 keV peak proved to be the only spent fuel specific attribute. It is easy to detect this gamma peak within a one minute measurement time. (author). 3 refs, 3 figs