Reprocessing, nuclear fuel fabrication, or uranium enrichment require large facilities that contain many glove-boxes, tanks and pipes where uranium and/or plutonium can build up. During the cleaning and dismantling of these facilities, the quantity of fissile materials need to be evaluated for nuclear material accounting and control, criticality safety, radiation safety and waste management. Holdup measurements are usually performed using gamma-ray techniques, although neutron measurements with 3-He detectors are also used. During the measurement, many factors can influence the result, such as the location of the source, its distribution and isotopic composition.
Neutron time-of-flight event detection and fast coincidence counting are powerful techniques that can be used to discriminate neutron from gamma and establish the location of nuclear materials. MCNPX-PoliMi is used to model Pu-240 source in a glove-box and an array of plastic scintillators around the inspected object. A post-processing macro based on ROOT simulates the response of the acquisition system (energy threshold, energy resolution, time jitter, etc.). The neutron time-of-flight technique is performed to identify fission signatures. Afterwards, using time-of-flight distribution, the location is established and activity calculated.