Passive and active detection of LEU beneath hydrogenated absorbers: simulation and validation

The Nuclear Regulatory Authority has developed a method for the detection of UF6 cylinders based on non-destructive active and passive neutron measurements. Those measurements were planned to detect the diversion of nuclear material in case of failure of safeguards surveillance. This paper shows the development of the practice and its simulation. The instrumentation set-up used by both methods was described. The simulation was made considering first the neutron source produced by 19F(α,n)24Na reaction and the 238U spontaneous fission neutrons. The AmBe source contribution was added to the neutron transport. Also it was necessary to characterize the slab detector as a matrix structure. This work was made in complexity increasing steps in order to test the different variables involved. This process began with the SLAB response modeling to an AmBe source and the attenuation of the polyethylene thickness (from 0 to 8 inches). This simulation was confirmed with measurements. After that the modeling of the whole system (AmBe source, UF6 cylinder and SLAB DETECTOR) started. Several corrections have been made related to the physical distribution of the inside cylinder material. The experience modeling with natural and 20 % enrichment was compared with the measured values. The aim was to validate the simulation to extend the results to other enrichment and uranium composition