The Model JCC-71 Neutron Coincidence Collar is a passive/active neutron counter for the measurement of the 235U content per unit length in fresh PWR, BWR and CANDU fuel assemblies. The JCC-71 unit can also be used to measure the plutonium content of MOX fuel. The system design is based on technology transfer from the Los Alamos National Laboratory.
The JCC-71 system is made up of four counter banks, each composed of high-density polyethylene for the moderation of the fission neutrons. Each bank contains several 3He detectors for the detection of neutrons. The counter can operate in both an active mode and a passive mode. For the passive mode, all four counter banks are used around the fuel assembly. If operated in the active mode, one bank of detectors is replaced with a polyethylene bank containing only an Americium-Lithium (AmLi) interrogating source. (The AmLi source must be ordered separately.)
In the active mode, the AmLi source is required to interrogate the fuel, and coincidence counting of the induced fission neutrons from 235U is performed. The AmLi source is contained in a tungsten source bottle and placed inside the polyethylene bank. The AmLi neutrons are thermalized in the polyethylene and induce fission in the 235U. The average energy from the induced fission is higher than the moderated AmLi neutrons and gives fast neutron multiplication which allows the measurement to penetrate into the interior of the fuel assemblies. For HEU fuel, cadmium liners can be added to improve neutron penetrability.