Gamma Scanning at High Count Rates of Irradiated BWR Fuel for Safeguard Purposes

Gamma-ray scanning utilizing the \"'Cs radiation as a function of burnup of spent nuclear fuel has since long been used for safeguard purposes. In Sweden work has been concentrated on BWR fuel with cooling times 1-10 years. It is of interest to establish the accuracy of the method since this determines the detection limits of e. g. missing fuel pins. Also, the practical use of other radiation such as that of ' nEu and 134es should be investigated for independent verification of cooling time and other parameters. For this a Ge detector system allowing multi spectrum scaling at count rates of 100 000 events per second stored in the computer has been developed. Measurements of fuel assemblies at two facilities in Sweden are reported. Normalization between the facilities has been made using a transportable reference source. The gamma intensities have been measured with an accuracy of better than 1% and ratios K=(Gamma intensity ^^Cs)/(declared burn up) have been calculated. The standard deviation of K with no correction except for declared cooling time is about 5%. With a simple correction for power history the standard deviation of K can be reduced to less than 3 %.