Assessing the Feasibility of Using Neutron Resonance Transmission Analysis (NRTA) for Assaying Plutonium in Spent Fuel Assemblies

Neutron resonance transmission analysis (NRTA) is an activeinterrogationnondestructive assay (NDA) technique capableof assaying spent nuclear fuel to determine plutonium content.Prior experimental work has definitively shown the techniquecapable of assaying plutonium isotope composition in spentfuel pins to a precision of approximately 3 percent, with aspatial resolution of a few millimeters. As a grand challengeto investigate NDA options for assaying spent fuel assemblies(SFAs) in the commercial fuel cycle, Idaho National Laboratoryhas explored the feasibility of using NRTA to assay plutoniumin a whole SFA. The goal is to achieve a Pu assay precision of1 percent. The NRTA technique uses low-energy neutronsfrom 0.1-40 eV, at the bottom end of the actinide-resonancerange, in a time-of-flight arrangement. Isotopic compositionis determined by relating absorption of the incident neutronsto the macroscopic cross-section of the actinides of interest inthe material, and then using this information to determine theareal density of the isotopes in the SFA. The neutrons used forNRTA are produced using a pulsed, accelerator-based neutronsource. Distinguishable resonances exist for both the plutonium(239,240,241,242Pu) and uranium (235,236,238U) isotopes of interest inspent fuel. Additionally, in this energy range resonances exist forsix important fission products (99Tc, 103Rh, 131Xe, 133Cs, 145Nd,and 152Sm), which provide additional information to supportspent fuel plutonium assay determinations. Based on extensivemodeling of the problem using Monte Carlo-based simulationcodes, our preliminary results suggest that by rotating an SFAto acquire two orthogonal views, sufficient neutron transmissioncan be achieved to assay a SFA. In this approach multiplescan information for the same pins may also be unfolded topotentially allow the determination of plutonium for sub-regionsof the assembly. For a 17x17 pressurized water reactor SFA, asimplified preliminary analysis indicates the mass of 239Pu may bedetermined with a precision on the order of 5 percent, withoutthe need for operator-supplied fuel information or operationalhistories.