Determining 235U enrichment in bulk uranium items using dual-energy interrogation with delayed neutron measurement

Abstract

Bulk uranium items are often measured using active neutron interrogation systems to take advantage of the relatively high penetrability of neutrons, providing the ability to measure uranium mass in large, dense configurations. Because the emitted neutrons from 235U and 238U in such configurations are, for all practical purposes, indistinguishable, established active measurement techniques require many representative calibration standards and/or well-known isotopic information to interpret the assay results (i.e., extract an isotopic mass from the effective mass). The primary objective of this work is to investigate the ability of a dual-energy neutron interrogation technique to estimate uranium enrichment for bulk materials, which has the potential to eliminate the need for a (traditionally separate) gamma isotopic measurement. Dual neutron interrogation energies were achieved by adding a deuterium–tritium (D–T) neutron generator into the measurement chamber of the Oak Ridge National Laboratory 252Cf Shuffler. This new technique exploits the change in fission rates as a function of interrogating neutron energy to independently determine the 235U and 238U content in the measurement item. Results from traditional 252Cf measurements and the new D–T measurements were then used to develop a relationship between uranium enrichment and the ratio of the two delayed neutron count rates. Technical feasibility is successfully demonstrated using the modified 252Cf Shuffler as a test bed, although several areas where refinements are both possible and needed before DEANI can be deployed as an analytical tool to meet specific measurement goals are identified.