The CANDU bundle verifier (CBVB) includes a highly collimated and shielded CdTe detector. The verifier is attached to an amplifier and a portable computer. The 662 keV γ ray line from 137Cs generally dominates a spectrum for spent fuel that has cooled for longer than two years and provides a useful signature for verifying the spent fuel. For shorter cooling times, the 757/766 keV line from 95Zr/95Nb is used to verify the presence of spent fuel. The particular γ ray line to be used is selected in the SCANDU data evaluation program. A constant speed winch suspended from the spent fuel pond bridge controls the detector position. The winch speed is set for scanning either storage baskets or stacks with irradiated CANDU fuel bundles. The detector head is moved at a selected speed vertically past the face of the stacked fuel, and a scan sequence is initiated in the computer. The γ ray intensity is measured as a function of the vertical position. The high intensity peaks, indicating irradiated fuel bundles, are counted and compared with the declared information on the number of stored fuel bundles.

(Source: IAEA Safeguards Techniques and Equipment)