Safeguarding Special Nuclear Material by Detecting Fast Neutrons in Liquid Scintillators.

Abstract

The number and complexity of nuclear facilities are increasing and new technologies are needed to maintain successful domestic and international safeguards efforts. Specifically, new radiation measurement systems for nuclear safeguards are needed to provide accountability of nuclear materials in facilities around the world. Previously-developed systems for the measurement of fissile mass relied on He-3 as the detection medium and neutron moderators prior to detection. This thesis explores the use of fast neutron detectors in a new safeguards instrument: the fast-neutron-multiplicity counter (FNMC). The use of fast neutron detectors such as the liquid scintillators used here provides some advantages over the previously-used detectors and analyses.

A number of experiments and simulations were performed to show the feasibility of the FNMC system. Passive neutron coincidence measurements of plutonium were performed to measure correlated neutrons from spontaneous and induced fissions. Within this study, the detection system was able to capture the time, energy, and angular distributions of neutron emission from the samples. Active-interrogation methods of uranium characterization were investigated to determine the ability of the liquid scintillators to detect induced-fission neutrons in the presence of active neutron sources. Detection timing techniques were used to identify small differences in enrichment and mass. A partial FNMC system was used to perform initial tests of the multiplicity sensitivity of the system to changes in Pu-240-effective mass. With the knowledge and tools developed in the measurements with the partial system, a full FNMC system was designed and used to quantify plutonium mass.

An optimized and efficient FNMC was shown to be able to characterize materials in fast measurement times. Few accidental counts are collected during its acquisition and each coincident detection is directly used. Therefore, the efficiency of the system can be relatively low and the system can still arrive at low statistical uncertainties in fast measurement times. The FNMC can measure gram levels of Pu-240-eff to 5% statistical uncertainty in measurement times of the order of minutes. The presented FNMC could rival the performance of traditional He-3 technology, but at a fraction of the cost.