A nuclear terrorist attack is one of the most serious threats to the national security of the United States, and in the wake of an attack, attribution of responsibility will be of the utmost importance. Plutonium, a by-product in spent nuclear reactor fuel, can be used in a nuclear weapon when obtained from reactor fuel discharged at a low burnup (1 MWd/kg). Characteristics of plutonium reprocessed from reactor fuel depend on factors such as the reactor type (thermal or fast reactor), fuel burnup, production history and the plutonium separation process used. Detailed understanding of the plutonium isotopic composition and fission product contaminant concentrations in separated plutonium would aid nuclear forensics activities aimed at source attribution in the case of interdicted smuggled plutonium, bolstering nuclear deterrence. The study presented here shows that trace fission product to plutonium ratios are amenable for nuclear forensics attribution. Through computational reactor core physics simulations, results are obtained for weapons-grade plutonium produced in a Fast Breeder Reactor (FBR). These fission product to plutonium ratios for the FBR are further compared with results reported elsewhere for a thermal Pressurized Heavy Water Reactor. This comparison of isotopic ratios results in substantial differences between fast and thermal neutron reactor systems, leading to the determination that a suite of selected isotopic ratios can attribute separated weapons-grade plutonium to a fast or thermal neutron source reactor system.