M.W. Swinney S.S. Chirayath, “Comparison of FBR and HFIR Monte-Carlo Simulations with Validation from Gamma Spectroscopy in Support of the NFASP Project”, 2014 ANS Winter Meeting, Annaheim, CA, 9-13 November 2014
The work presented is part of the Nuclear Forensics Analysis of Separated Plutonium for foreign fuel cycles (NFASP) project, an ongoing eort to develop a methodology for identifying intrinsic physical characteristics in weapons-grade plutonium produced by certain reactors in order to allow source attribution and improve nuclear deterrence. Previous work in the area of modeling and comparing Monte-Carlo Neutral Particle (MCNP) simulations of a Fast Breeder Reactor (FBR) and a CANadaian Dueterium Uranium (CANDU) type reactor revealed several isotopes and isotopic ratios that could be used to distinguish the source of the plutonium, presumably even after going through the PUREX process. This work involves an attempt to acquire physical representative samples that could be used to obtain measurements to compare to simulation and demonstrate the proofof- concept for the FBR case. Due to the lack of a readily available fast-spectrum reactor that the investigators could take samples from, an alternative approach had to be invented. The resulting experiment involved irradiating depleted uranium dioxide samples inside a gadolinium sheath in the High Flux Isotope Reactor (HFIR) at ORNL. To be able to accurately compare the measurement results, a MCNP model of HFIR was obtained and modified to incorporate the experimental samples, gadolinium sheath, and sample holder, and to simulate the actual irradiation of the samples as closely as possible. Once the neutron irradiated samples arrived at Texas A&M University, gamma spectroscopy measurements were completed on the samples, and several isotope radioactivities were estimated from the measurements, including Cs-134, Cs-137, Sb-125, and Ce-144. The amount of Eu- 154 in the samples made a quick, accurate measurement dicult, so results on this isotope were not reported. Due to some constraints, the MCNP model did not report Sb-125 or Ce-144 concentrations, so the experimental results compared with computational predictions in this work involved only the Cs-134 and Cs-137 concentrations.