The FDET measuring system includes the detector head, a several metre long extension pipe, a miniature gamma ray and neutron detector (MiniGRAND) electronics unit and a portable computer. Separate detector heads are used to measure boiling water reactor (BWR) and pressurized water reactor (PWR) type fuels. The detector head incorporates γ ray insensitive neutron detectors (four gas filled fission chamber proportional counters) and γ ray detectors suitable for measuring extremely high γ ray intensities (two gas filled ionization chambers). The neutron and γ ray signatures measured by the detectors are used to verify the highly radioactive spent fuel assemblies stored under water in spent fuel ponds. The FDET is usually installed on the guard rail of the spent fuel pond bridge or near the pond edge. To perform a measurement, the irradiated fuel assembly is lifted by the operator's crane and moved into position between the tines of the fork detector. Interactive software guides the user through the measurement procedure and simultaneously collects neutron and γ ray data. The software can also support unattended measurements.

The ratio of the neutron to γ ray data, when combined with other, complementary information, is used to characterize a particular type of fuel assembly, giving information related to its neutron exposure in the reactor, its initial fissile fuel content and its irradiation history (e.g. the number of cycles for which the assembly was in the reactor). Passive γ emission tomography, currently being tested with the help of several MSSPs, is projected to be able to detect defects at the pin level.

(Source: IAEA Safeguards Techniques and Equipment)