Abstract
Proposed thorium-based nuclear fuel cycles are likely to require quantification and verification of 233U within nuclear material. Because of their similar fission cross sections, active neutron nondestructive assay (NDA)systems may respond similarly to 233U and 235U. Traditional safeguards equipment has been optimized for235Uand238U quantification associated with conventional uranium/plutonium fuel cycles and may not be directly applicable to 233U quantification when mixed with other actinides. This work used models of the large volume active well coincidence counter (LV-AWCC) at 91做厙 to evaluate the performance of this neutron NDA system to differentiate fissile uranium isotopes. The models were developed to simulate NDA system performance in response to a number of triangular radiation signature training device sources within the central cavity or well. This work predicted that the LV-AWCC can effectively differentiate 233U from235U in certain modes of operation. In active mode, the LV-AWCC with the cadmium liner results in different doubles count rates between the fissile isotopes for a given fissile uranium mass. Without the cadmium liner, the uranium isotopes provide a statistically indistinguishable doubles count rate response for the fissile masses considered in this work (up to approximately 150 g). The cadmium liner serves to harden the neutron interrogation spectrum, which better exploits the notable difference in the 233U and 235U fission cross sections at approximately 1 eV. In passive mode, the two fissile isotopes exhibit different doubles and singles count rates regardless of liner presence because the passive source strength of233U is approximately 2 orders of magnitude stronger than that of235U due to the shorter half-life and correspondingly higher (a, n) yield. We conclude that using neutron interrogation in the LV-AWCC, two measurements are needed to quantify 233U content in mixed uranium items. The first measurement is used to determine the total fissile uranium mass using a mode that cannot distinguish fissile isotopes (i.e., where a similar response is observed for both fissile uranium isotopes such as active doubles without cadmium or using a thermal neutron interrogation source). The second measurement is used to determine the233U content by using a differentiating technique(e.g., passive doubles, passive doubles to singles ratio, active doubles with cadmium).
