Year
2019
Abstract
The Uranium Neutron Coincidence Collar (UNCL) has been used for over thirty years for the verification of the 235U content of LWR nuclear fuel and research is currently underway to investigate a new design for the collar. All of the prior measurement approaches for the neutron collar have deployed active neutron interrogation using an external AmLi or 252Cf neutron source to induce fission reactions in the 235U. The approach currently being simulated uses the UO2 fuel rods to self-interrogate the 235U in the fuel assembly. The interrogation source is the 238U spontaneous fission neutrons in the fuel assembly which scatter through the assembly prior to inducing a fission reaction in the 235U. As the enrichment in the fuel rods increases, the 234U content also increases, and this provides an increase to the measured neutron rates that is proportional to the 235U. Longer gate lengths can be used in the passive mode because accidental coincidence background is significantly reduced as there is no contribution to the singles from an external interrogation source. The Monte Carlo modeling has demonstrated several promising advantages to this passive technique. The calibration curves are close to linear which reduces the measurement uncertainties especially for high 235U loading. In passive mode, both the singles and doubles rates provide useful signatures to verify the 235U content in LWR fuel assemblies and these responses are less sensitive to burnable poison biases than active techniques.