Technical Basis for the Use of a Correlated Neutron Source in the Uranium Neutron Coincidence Collar

Active neutron coincidence systems are commonly used by international inspectorates to verify a material balance across the various stages of the nuclear fuel cycle. The Uranium Neutron Coincidence Collar (UNCL) is one such instrument; it is used to measure the linear density of ²³⁵U (g ²³⁵U/cm of active length in assembly) in fresh light water reactor fuel in nuclear fuel fabrication facilities. The UNCL and other active neutron interrogation detectors have historically relied on americium lithium (²⁴¹AmLi) sources to induce fission within the sample in question. Californium-252 is under consideration as a possible alternative to the traditional ²⁴¹AmLi source. This work relied upon a combination of experiments and Monte Carlo simulations to demonstrate the technical basis for the replacement of ²⁴¹AmLi sources with ²⁵²Cf sources by evaluating the statistical uncertainty in the measurements incurred by each source and assessing the penetrability of neutrons from each source for the UNCL.