Experimental Testing of a Nuclear Forensics Methodology for Reactor-Type Attribution of Chemically Separated 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 fast and thermal reactor-types. These reactor types will likely be operating in a non-safeguarded manner in some countries, and have the ability to produce weapons-grade plutonium when discharged at a burnup less than 5 GWd/MTU. The authors anticipated that differences in neutron energy spectra as well as fission yield curves between the 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 weaponsgrade plutonium to a fast or thermal neutron energy spectrum system as the source of the interdicted material. The computational part of the project utilizes MCNP radiation transport codes 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. A nuclear forensics methodology has been developed which is capable of attributing weapons-grade plutonium to a source reactor-type from measurements of intra-element isotopic ratios. The methodology has the ability to predict reactor parameters such as burnup and time since irradiation, in addition to the reactor-type. In order to demonstrate the methodology, two research reactor irradiations were performed resulting in two distinct fuel samples containing bred plutonium. The irradiated samples were subject to nondestructive and destructive analyses to measure the plutonium and fission product concentrations. The work presented here includes an overview of the development and validation of the nuclear forensics attribution methodology.