Year
2012
Abstract
When safeguarding UF6 cylinders, the determination of the 235U enrichment and the 235U mass of the solidified UF6 is paramount. Current enrichment meter technology relates the ratios of various characteristic gamma-ray peak areas to the level of 235U enrichment. However, due to strong selfshielding effects, such instruments can only assess the outer-most layer of UF6. Thus, little information can be gained on the composition of the inner layers of UF6 or on the bulk 235U mass. Therefore, there are concerns that more highly enriched UF6 could be concealed inside a shell of low enriched UF6. The spontaneous fission of 238U and especially the (alpha, neutron) reaction of 234U alpha particles on 19F provide a significant neutron emission spectrum. Neutrons, compared to gamma rays, are significantly less affected by self-shielding in the UF6. Consequently, there has been increased interest in developing neutron-based enrichment measurement devices. Organic liquid scintillators possess properties favorable towards fast timing and large detector volume applications. The fast timing arises from the liquid’s very short, ns-scale fluorescence decay time. Using a relatively low cost liquid phase material also grants flexibility in designing the detector’s shape and volume. Also, pulse shape discrimination of gamma rays and neutrons is possible. Therefore the feasibility of using organic liquid scintillator detectors is being investigated using MCNPX and MCNPX-PoliMi simulations. Specifically, the possibilities of coupling measured liquid scintillator UF6 neutron energy spectra or measured neutron coincidences to algorithms to estimate the UF6’s 235U enrichment are being explored. This paper includes results from the extensive simulation study on neutron coincidences, which are found to correlate with uranium enrichment and appear to provide counting rates that will allow cylinder assays to be performed in minutes.