It is common practice to examine discharged nuclear fuel assemblies to confirm that the characteristics of the spent fuel are as declared. In particular, the amount of spent fuel in an assembly or bundle and the burnup to which the fuel was subjected are of interest. One serious difficulty, however, is that the measured and expected count rates have limited accuracy, so the diversion of a single fuel rod (or perhaps a small number of rods) cannot be definitively detected. And there is the further limitation that the counting techniques are sensitive to only the fuel rods toward the outside of an assembly. In this work we report on gamma counting simulations of discharged CANDU-19 fuel bundles with a missing inner fuel rod, and we show how the missing rod can be identified. The radiological characteristics of the fuel rods in a discharged CANDU bundle were determined with a burnup and depletion code. This simulation produced a detailed , and the resulting counting signals in a gamma detector were then obtained from Monte Carlo transport simulations. The variation in the gamma signal was investigated as a detector was moved azimuthally around the bundle. Fourier analysis of this signal was able to detect a missing inner-ring fuel pin. The advantage to this approach is that it is not dependent on absolute count rates. Rather, it is based on the detection of abnormal spatial frequencies in the count rate.
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