Monte Carlo Simulations For Time-of-flight Epithermal Neutron Activation Analysisfor Isotopic Signatures

There is a need for a fast, non-destructive technique to monitor nuclear material throughout the fuel cycle. Analyzing unique signatures can enhance the ability to monitor and verify nuclear-material activities. Epithermal neutron activation analysis (ENAA) is a tool that can leverage unique isotopic resonances of interest. An epithermal neutron time-of-flight facility (TOF) is being developed at the Penn State Breazeale Reactor (PSBR) for use as a nuclear material verification tool. This unique TOF facility will implement a novel mechanical chopper to pulse epithermal neutrons up to 40 eV with energy resolution of approximately 2% or better. Previous work has focused on the design of the mechanical chopper that will create this unique source of nearly monochromatic epithermal neutrons. Conventional disc and Fermi choppers were evaluated for their ability to effectively pulse neutrons up to 40 eV. Mechanical limitations of these designs led to a novel redesign of a piston-based chopper. The kinematic and neutronics capabilities of this design have been investigated and will enhance the neutron source for optimized ENAA. This work focuses on MCNP6 simulations to assess the expected performance of the proposed chopper. Specifically, modeling of pulsed ENAA with the proposed chopper system is being examined for its utility as a nuclear materials verification technique. In parallel with this research, a comprehensive review of isotopic resonances with energies of interest will allow for an optimized selection of samples for interrogation. Simulations include static interrogation with a full reactor neutron spectrum source, as well as nearly monoenergetic neutron sources. These simulations will be used as a baseline for comparison with dynamic simulations. The dynamic simulations will be discretized in time to model the TOF effect of the mechanical neutron chopper in operation. This will be accomplished by coupling the kinematics of the neutron chopper with independent MCNP6 simulations to discretize each simulation in time. This work is relevant to the National Nuclear Security Administration’s (NNSA) mission of preventing nuclear weapons proliferation and reducing the threat of nuclear terrorism by complementing research on the signals and source terms for nuclear nonproliferation, and is a part of the MTV NNSA Consortium.