Hold-up Measurement based on Fast Coincidence Counting and
Neutron time-of-flight

Reprocessing, nuclear fuel fabrication, or uranium enrichment require large facilities that contain many glove-boxes, tanks and pipes where nuclear materials can build up. The quantity of fissile materials needs to be evaluated for nuclear material accounting and control, criticality safety, radiation protection and waste management. The location of the source and attenuation of radiations (neutrons and gamma-rays) are two key factors for the evaluation of nuclear material quantity. Indeed, if the source’s location is unknown and/or radiations attenuations are not estimated, they become important sources of uncertainties for the evaluation of nuclear material quantity. A new system based on fast coincidences between inorganic and organic scintillators is studied to reduce uncertainties associated with the source localization and attenuations. An array of scintillators is simulated around the inspected object. MCNPX-PoliMi code is used to model a mix of plutonium and americium oxide in a large object. Then a postprocessing macro based on ROOT simulates the realistic response of the acquisition system (real and accidental coincidences, energy threshold, energy resolution, time jitter, etc.). The attenuation estimation and the discrimination between prompt fission neutrons and neutrons from (alpha,n) reactions are obtained by multiplicity measurement. Then the neutron time-offlight technique is performed to identify fission signatures and locate the source. Afterwards, plutonium mass could be evaluated.