Point Kinetics-based Methods for Uranium Assay in Tagged Neutron Interrogation

Enrichment assay of large (>1 kg) uranium objects is a necessary capability for nuclear safeguards and nuclear security applications. While passive gamma ray measurements are traditionally used, these methods may be unreliable for large inhomogeneous objects. Tagged neutron interrogation (TNI) of uranium using a time-tagged external D-T neutron source allows for transmission imaging as well as fission site imaging of neutron single and double coincidences. This work develops four point kinetics-based methods of estimating enrichment from a theoretical TNI measurement. These methods expand upon traditional point kinetics models in an effort to describe the fission chain propagation of TNI measurements of bare uranium metal while retaining enough simplicity to invert them and assay enrichment. The methods focus on each of the enrichment-sensitive parameters within the point kinetics model|source-object coupling, chain-starting multiplicity, and neutron multiplication. The accuracy and sensitivities of each method are evaluated using a combination of Monte Carlo simulation data and data from TNI measurements. A study on how the coincidence response and neutron multiplication are affected by physical characteristics of large uranium objects informs what specific information needs to be known a priori or from neutron imaging to successfully assay enrichment using point kinetics-based methods.