The unattended fuel flow monitor (UFFM) and unattended spent fuel monitor (UFSM) monitor the movement of fresh fuel assemblies to the reactor (for those reactors utilizing MOX - mixed oxide fuel that contains plutonium - such as breeder reactors and LWRs on a MOX cycle), spent fuel assemblies from the reactor to the fuel storage pond, and spent fuel assemblies out of the storage pond.

A typical transfer sequence involving several monitor units could include a fresh fuel assembly being brought to the reactor core and a spent fuel assembly being retrieved and brought to the storage pond. The combination of neutron and γ ray signatures at the successive units characterizes the transferred material as either fresh fuel, spent fuel or other material (e.g. neutron irradiated blanket material at a breeder reactor facility). Surveillance cameras normally complement a UFFM over the fuel transfer route.

(Source: IAEA Safeguards Techniques and Equipment)