The CANDU spent fuel bundle counter (VIFB) is an unattended system that monitors a strategic location in the spent fuel bundle pathway of an on-load refuelled power reactor. Collimated γ ray detectors detect the fuel bundle as it passes. The proper placement of detectors and use of the appropriate algorithm for the facility enable the device to count the bundles as they pass and record the direction in which they are moving, even when two bundles are moving together, which is normally the case.

(Source: IAEA Safeguards Techniques and Equipment)