Neutron Resonance Transmission Analysis of Nuclear Material Using a Portable D-T Neutron Generator

Abstract

The accurate identification and quantification of nuclear material is important for nuclear security, nonproliferation, and safeguards missions. Existing techniques have limitations in terms of their isotopic specificity, isotopic range, and their capacity to analyze thick or shielded targets. Neutron Resonance Transmission Analysis (NRTA) is an active neutron interrogation technique that addresses these challenges by offering isotope-specificity, applicability to a wide range of mid- and high-Z isotopes, and the ability to assay targets shielded by high-Z material. However, the dependence of NRTA on large beamline facilities to generate high neutron flux and achieve excellent system resolution has prevented the technique from being used for on-site applications. To overcome this limitation, this thesis presents a portable NRTA system design that utilizes a commercial-off-the-shelf D-T neutron generator. The portable system, comprising a neutron source and neutron detector assembly, each weighing less than 40 kg, enables easy transport and setup for on-site applications. The system is capable of achieving sufficient flux and resolution to identify neutron resonances up to approximately 60 eV in under an hour, making it suitable for analyzing high-Z targets with thicknesses of several centimeters and with up to several millimeters of low-Z shielding. The feasibility of the portable NRTA system for a number of nonproliferation and safeguards applications was evaluated by assaying various uranium and plutonium targets. The system successfully assayed both depleted and high enriched uranium and reactor-grade plutonium targets, accurately determining the enrichment and fissile mass content as validated by chemical analysis. The system was also able to assay combinations of thorium and uranium metal of varying enrichment, as well as small quantities of U-233 oxide under high passive gamma backgrounds to accuracies under a gram. These results highlight the potential viability of using a portable NRTA system to support nuclear nonproliferation missions that require the non-destructive assay of a diverse range of nuclear materials, including advanced reactor safeguards for the thorium fuel cycle and the confirmation of special nuclear material for arms control treaty verification.