Nuclear Forensics Methodology for Source Reactor-Type Discrimination of Chemically Separated Plutonium

Abstract

The growing nuclear threat has heightened the need for developing nuclear forensics analysis techniques that contribute to nuclear material source attribution, thereby strengthening nuclear deterrence. The objective of this research was to develop a nuclear forensics methodology that is capable of source reactor-type discrimination of chemically separated weapons-usable plutonium. The developed methodology utilizes plutonium and fission product intra-element isotope ratios within the plutonium sample to predict characteristics of the irradiated material, including burnup, time since irradiation, and reactor type.

The MCNPX-2.7 and MCNP6 radiation transport codes were used to model reactor cores, perform burnup simulations, and estimate the isotopics of the discharged fuel. Ratios of intra-element isotopes (fission products and plutonium) were identified which contribute to resolving the parameters of burnup, time since irradiation, and reactor type. The simulation results were used to generate a reactor-dependent library of intra-element isotope ratio values as a function of burnup and time since irradiation. A maximum likelihood calculation was utilized to compare the simulated intra-element isotope ratio values contained in the reactor library to the same ratio values measured in the sample. The result is a likelihood value which is proportional to the probability of observing the measured intra-element isotope ratios given the reactor type and parameters.

In order to validate the nuclear forensics methodology developed, two experimental irradiation campaigns were performed, resulting in two distinct UO₂ fuel samples containing weapons-usable plutonium. The first was designed to replicate weapons-usable plutonium produced in the blanket of a fast breeder reactor, by irradiating depleted UO₂ fuel samples in a pseudo-fast neutron spectrum within the High Flux Isotope Reactor at Oak Ridge National Laboratory. The second irradiation was designed to represent weapons-usable plutonium produced in a natural uranium fueled thermal reactor, by irradiating natural UO₂ fuel samples in a thermal neutron spectrum at the University of Missouri Research Reactor. The irradiated samples were subjected to nondestructive and destructive analyses to measure the plutonium and fission product isotope ratios. The methodology performed well for both experimentally irradiated cases, identifying the source reactor model and adequately predicting the burnup and time since irradiation. The work presented here served to develop and validate a nuclear forensics source reactor-type discrimination methodology.