Technical basis for use of a correlated neutron source in the uranium neutron coincidence collar

Abstract

Nuclear nonproliferation efforts rely on a variety of safeguards to protect sensitive materials in nuclear facilities. The enrichment of fresh light-water-reactor fuel assemblies is verified by several inspectorates using the uranium neutron coincidence collar (UNCL), which uses neutrons from an americium lithium (241AmLi) source to interrogate the assemblies from one side. Eighteen 3He tubes on the other three sides are used to count the coincidence neutrons from the induced fission reactions. Experiments have shown that 252Cf could also be used to complete these measurements, providing several benefits over the use of the standard 241AmLi source. The UNCL is one of the many instruments that will be available for training purposes in the China Center of Excellence for Nuclear Security (COE), which is located in Beijing, China. This thesis contains a detailed characterization of the response of this detector with 252Cf as compared with 241AmLi and an analysis of the technical basis for the use of 252Cf in place of 241AmLi in the Antech N2071 Neutron Coincidence Collar. This thesis (1) discusses the development a benchmarked, high-fidelity model of the UNCL using Monte Carlo N-Particle Extended (MCNPX), version 2.7.4.a; (2) fully characterizes the detection parameters, including the efficiency profile, die-away time, and deadtime parameters; and (3) demonstrates the technical basis for the replacement of 241AmLi sources with 252Cf sources by assessing the penetrability of neutrons from each source, evaluating the statistical uncertainty in the measurements incurred by each source, and investigating the possibility of a higher effective average number of neutrons produced per fission using 252Cf rather than 241AmLi. This work demonstrates the suitability of 252Cf as a substitute for 241AmLi and in fact shows approximately a 7.5% improvement in counting statistics over the traditional interrogation source at 4% enrichment.