Development of Neutron Energy Spectrum Based Nuclear Forensics Attribution Methodology Using Trace Nuclide Ratios in Weapons-Grade Plutonium

A study is currently in progress at Texas A&M University on the computational and experimental methods for reliably predicting and measuring unique physical characteristics of separated weapons-grade plutonium produced by certain reactor types, specifically a Fast Breeder Reactor (FBR) and a Pressurized Heavy Water Reactor (PHWR). These reactor types will likely be operating in a non-safeguarded manner in some countries. Both the FBR and PHWR fuels produce weapons-grade plutonium when discharged at a low burnup of about 1 MWd/kg. We anticipated that the differences in neutron energy spectra as well as fission yield curves between the two reactor types would result in variations in isotopes of plutonium and fission products. These unique plutonium and fission product isotope concentrations will thus have the possibility of containing information capable of attributing separated weapons-grade plutonium to a fast or thermal neutron energy spectrum system as the source of the interdicted material. The computational part of the project utilizes the MCNPX- 2.7 radiation transport code to model the reactor cores, perform burnup cycles and estimate the resulting isotopics of the discharged fuel. Previous work on elemental decontamination factors using PUREX separations indicated the need for ratios comprised of isotopes of the same element, in order to be independent of reprocessing scheme and efficiency. Specific fission product and actinide same element isotope ratios were selected based upon several factors including, the amount of isotope production, detection capability, and whether ratio production or loss behavior is a function of our reactor parameters of interest. A suite of same element isotope ratios were then selected which would be useful for accurate nuclear forensics attribution of the source reactor for interdicted weapons-grade plutonium. The work presented here includes the development and validation of an inverse analysis database methodology for the attribution of a source reactor-type from measured same element isotope ratio data. The proposed methodology includes a maximum likelihood calculation from a basic library comparison to predict reactor parameters such as burnup level, cooling time, and neutron energy spectrum for various reactors.