Neutron Signatures for Nondestructive Assay in Thorium Fuel Cycles

The development of advanced reactors based on thorium fuel cycles is accelerating world-wide, with several reactors expected to come on-line by 2030. It is not yet established what the best practices for international safeguards will be for these reactors and fuel cycle facilities, and there are clear deficiencies in traditional nondestructive assay techniques when applied to 233U, which is the main fissile isotope utilized in thorium fuel cycle reactors. Nondestructive assay techniques for this isotope should be capable of withstanding the substantial gamma-ray background from 232U contamination and discriminating between 233U from 235U. To this end, we have performed several experiments at the National Criticality Experiments Research Center (NCERC) and Oak Ridge National Laboratory (ORNL) to demonstrate a set of advanced neutron signatures of 233U oxides and mixtures of 233U and 235U, using both traditional 3He-based neutron counters and a novel 4He-based scintillation detector. At NCERC, we demonstrated the passive fast neutron spectral signature of 233U3O8, the pulsed differential die-away fast neutron time profile of 233U in conjunction with a pulsed D-T neutron generator, and the isotope-specific delayed neutron time profile of 233U in conjunction with the same neutron generator. To further develop delayed neutron-based discrimination between 233U and 235U, we performed a series of experiments at ORNL using sets of small 233U and HEU plates, where each ratio of 233U to 235U yielded a distinct delayed neutron time profile shape, and pure sets of each isotope were well-separated. Since these techniques have been demonstrated on real 233U objects with significant gamma ray background from 232U contamination, it may be feasible to apply them for nondestructive assay in thorium fuel cycles.