The Digital Cherenkov Viewing Device (DCVD) is one of the instruments available to IAEA inspectors to verify spent nuclear fuel in wet storage. The DCVD can be used for partial defect verification, verifying that 50% or more of a fuel assembly has not been diverted. The partial defect verification relies on a comparison between measured and predicted intensities, based on operator fuel declarations. Recently, IAEA inspectors have encountered spent fuels with short cooling times where there were systematic differences between predictions and measurements. Through the Swedish support program, this deviation was investigated, by studying various modelling assumptions that could cause the discrepancy.
The predominant cause of the discrepancy was beta-decay electrons, passing through the fuel cladding and entering the water with sufficient energy to directly produce Cherenkov light. Analysis of measurement data for a set of fuels where the discrepancy was found to be pronounced revealed that for modern fuel designs with thin claddings the beta contribution is enhanced, and for short-cooled fuels additional beta-decaying isotopes are abundant and must be considered. Furthermore, the data showed that for nuclear fuels that had not reached the discharge burnup, the fuel irradiation history may cause a relative enhancement of the abundance of beta-decaying isotopes relative to other isotopes causing Cherenkov light. Other studied modelling assumptions, such as void, burnable absorbers and using binned gamma spectra, showed that they only introduced a modest bias, and proper default values and data handling can mitigate it.
A method to predict the direct beta contribution to the Cherenkov light intensity was developed, which can ensure that the observed biases will be eliminated from future verification campaigns. It is advised that this enhanced prediction method be included in the DCVD software, and made available to inspectors.