Technical Basis for the 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 ( 241 AmLi) source to interrogate the assemblies from one side. Eighteen 3 He tubes on the other three sides are used to count the coincidence neutrons from the induced fission reactions. Experiments have shown that 252 Cf could also be used to complete these measurements, providing several benefits over the use of the standard 241 AmLi 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 252 Cf as compared with 241 AmLi and an analysis of the technical basis for the use of 252 Cf in place of 241 AmLi 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 241 AmLi sources with 252 Cf 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 252 Cf rather than 241 AmLi. This work demonstrates the suitability of 252 Cf as a substitute for 241 AmLi and in fact shows approximately a 7.5% improvement in counting statistics over the traditional interrogation source at 4% enrichment.

Comparison Of ²⁵²Cf Time Correlated Induced Fisssion With AmLi Induced Fission On Fresh MTR Research Reactor Fuel

“…Even though AmLi produced 1.21 times higher number of IF events in the fuel than 252 Cf,252 Cf provided higher number of doubles count rates. This proves that the boost in the doubles count rate is due to boost in combined multiplicity of IF+SF due to the TCIF effect described by the figures 2-8(a) and 2-8(b) [7][18]…”

“…Multiplicity counting is just an extension of coincidence counting to the collection of higher multiples of neutrons. Singles, doubles, and triples follow the following three equations 14, 15, and 16 given below [7][12]…”

Utilization of the Differential Die-Away Self-Interrogation Technique for Characterization and Verification of Spent Nuclear Fuel

“…For example, a 0.2 g/cm 2 change in density results in a ~8% change in the doubles rate. Previous benchmarks of active interrogation of the LANL fresh fuel rods have had disagreement between simulation and experiment as well [94], and the cause for the disagreement was ultimately undetermined. There has also been extensive work done identifying nuclear data issues leading to disagreement, particularly with nubar values [95,96]; however this area of study requires much more data and experimental work to confirm the correct values.…”

Safeguards Technology for Thorium Fuel Cycles: Research and Development Needs Assessment and Recommendations

“…Hence, regular replacement of 252 Cf sources may be required for uninterrupted operations. Additionally, should an SF source be used for the active assay of fissile material in terms of counting real coincidences from IF, it may be significant to properly distinguish the contribution of the interrogating source from the response of the interrogated item [55,56]. *The intensities of these isotopic sources scale with the decay of the driving isotope.…”

“…This assay method is referred to as the Time Correlated Induced Fission method (TCIF), which was recently developed for use of an SF source, such as 252 Cf, in place of the traditional 241 AmLi source in an active detector [63]. This method has also been successfully developed for the assay of uranium enrichment in fresh LEU fuel [55]. In this and similar cases, however, the isotope of interest is 235 U, and the low probability of fission of 238 U is considered an advantage of the TCIF method.…”