Experimental and Computational Forensics Characterization of Weapons-Grade Plutonium Produced in a Fast Reactor Neutron Environment

A terrorist attack using an improvised nuclear device is one of the most serious dangers facing the United States. The work presented here is part of an effort to improve nuclear deterrence by developing a methodology to attribute weapons-grade plutonium to a source reactor by measuring the intrinsic physical characteristics of the interdicted plutonium. In order to demonstrate the developed methodology, plutonium samples were produced from depleted uranium dioxide (DUO₂) surrogates irradiated in a fast-neutron environment. In order to replicate the neutron flux in a fast-neutron-spectrum reactor and obtain experimental samples emulating weapons-grade plutonium produced in the blanket of a fast breeder reactor, DUO₂ samples were placed in a gadolinium sheath and irradiated in the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory. Previous computational work on this topic identified several fission products that could be used to distinguish between reactor types (fast and thermal reactors), specifically: ¹³⁷Cs, ¹³⁴Cs, ¹⁵⁴Eu, ¹²⁵Sb, ¹⁴⁴Ce, ⁸⁵Rb, ¹⁴⁷Pm, and ¹⁵⁰Sm along with the plutonium isotopes. Simulations of the fast neutron irradiation of the DUO₂ fuel surrogates in the HFIR were carried out using the Monte Carlo radiation transport code MCNPX 2.7. Comparisons of the predicted values of plutonium and fission product concentrations to destructive and nondestructive assay measurements of neutron-irradiated DUO₂ surrogates are presented here. The agreement between the predictions and gamma spectroscopic measurements in general were within 10% for ¹³⁴Cs, ¹³⁷Cs, ¹⁵⁴Eu, and ¹⁴⁴Ce. Additional experimental results (mass spectroscopy) agreed to within 5% for the following isotopes: ⁸⁵Rb, ¹⁴⁷Pm, ¹⁵⁰Sm, ¹⁵⁴Eu, ¹⁴⁸Nd, ¹⁴⁴Ce, and ²³⁹Pu. Two indicator isotopes previously suggested to differentiate between the reactor types were ruled out for use in the attribution methodology; ¹²⁵Sb was ruled out due to the difficulty in accurately predicting its concentration, and ²⁴²Pu was ruled out because of its low content in weapons-grade plutonium.