Nondestructive Spent Fuel Assay Using Nuclear Resonance Fluorescence

Quantifying the isotopic composition of spent fuel is an important challenge and essential for many nuclear safeguards applications, such as independent verification of the Pu content declared by a regulated facility, shipper/receiver measurements, and quantifying isotopic input masses at a reprocessing facility. As part of the Next Generation Safeguards Initiative, NA-241 has recently funded a multilab/university collaboration to investigate a variety of nondestructive methods for determining the elemental Pu mass in spent fuel assemblies. Nuclear resonance fluorescence (NRF) is one of the methods being investigated. First modeling studies have been performed to investigate the feasibility of assaying a single fuel pin using a bremsstrahlung photon source. MCNPX modeling results indicate that NRF signals are significantly more intense than the background due to scattered interrogation photons even for isotopes with concentrations below 1percent. However, the studies revealed that the dominant contribution to the background is elastic scattering, which is currently not simulated by MCNPX. Critical to this effort, we have added existing NRF data to the MCNPX photonuclear data files and are now able to incorporate NRF physics into MCNPX simulations. Addition of the non-resonant elastic scattering data to MCNPX is in progress. Assaying fuel assemblies with NRF poses additional challenges: photon penetration through the assembly is small and the spent fuel radioactive decay and neutron activity lead to significantly higher backgrounds. First modeling studies to evaluate the efficacy of NRF for assaying assemblies have been initiated using the spent fuel assembly library created at the Los Alamos National Laboratory (LANL)