Trace Fission Product Ratios for Nuclear Forensics Attribution of Weapons-Grade Plutonium Separated from Fuel Irradiated in Thermal and Fast Reactor Types

A research project is currently in progress at Texas A&M University on computational and experimental methods for reliably predicting and measuring unique intrinsic 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 fuel burnup of about 1 MWd/kg. It was 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, minor actinides, and fission products. The computational part of the project utilizes and estimate the resulting isotopics of actinides and trace elements in the discharged fuel. The specific fission products and actinides selected for nuclear forensics analyses were chosen based upon several parameters including, the amount of isotope production, half-life, activity, probability of detection, and the isotope’s Plutonium Uranium Recovery by EXtraction (PUREX) decontamination factor. Ratios of the selected isotopes’ concentrations and activities per 1 kg of total plutonium, with a decontamination factor of 106 applied, were calculated for both reactor types. A comparison of the PHWR results and FBR results was done to identify isotopes with greater reactor dependencies. A suite of isotopic ratios were then selected which would be useful for quick and accurate nuclear forensics attribution of the source reactor for interdicted weapons-grade plutonium. An uncertainty analysis associated with the Monte Carlo methodology was completed using the computational model to predict the mean and standard deviation in the amount of production for each chosen isotope. Fuel samples for both reactors will be irradiated with replicated neutron spectra at the Oak Ridge National Lab-High Flux plutonium from fission products and uranium. The experimental data will become available in the latter half of this investigation for comparison and verification of the computational results.