Delayed Neutron & Gamma Measurements of Special Nuclear Materials, their Monte Carlo Simulations, and Applications

by Madison Andrews

Institution: Royal Military College of Canada
Department: Chemistry and Chemical Engineering/Chimie et génie chimique
Degree: PhD
Year: 2015
Keywords: delayed neutron; delayed gamma; nuclear forensics; special nuclear material; helium-3 detectors; high purity germanium detector
Record ID: 2058560
Full text PDF: http://hdl.handle.net/11264/501


Special nuclear materials (SNMs) are those which contain 233U, 235U and 239Pu. They are safeguarded by the International Atomic Energy Agency (IAEA) in addition to individual governmental organisations. Nuclear forensic analysis (NFA) is concerned with the characterisation and attribution of illicit nuclear materials, especially uranium and plutonium. Physical, chemical, and isotopic characteristics are determined with instrumentation that is ideally rapid, non-destructive, and sensitive. Delayed neutron and gamma emissions from fissioned isotopes have signature yields, energies, and temporal behaviour, their measurement allows the rapid characterisation of SNM content in a variety of matrices. Previous work developed a delayed neutron counting (DNC) prototype at the Royal Military College of Canada (RMCC). This system sent samples containing 235U content to a SLOWPOKE-2 reactor where they underwent fission. Samples were then sent via pneumatic transfer to the counting arrangement, containing six helium-3 detectors, which recorded the delayed neutron emissions. This undertaking began with the attribution of the time-dependent neutron background present in the SLOWPOKE-2 site used for DNC irradiations; the contribution of this uranium contamination was lowered from a mass equivalent of 120 to 50 ng per vial. Subsequent delayed neutron measurements of samples containing mixtures of 233U and 235U were used to determine their relative ratios (in %) with an average absolute error of ± 4 %. This thesis also included the development of a Delayed Neutron and Gamma Counting (DNGC) system intended to contribute to nuclear forensics instrumentation available to the Canadian Department of National Defence. The delayed gammas emitted from SNM were used in complement with delayed neutron measurements in an example which detected, identified, and quantified 233U content with an average relative error and accuracy of -2.2 and 1.5 %, respectively. Throughout system development detailed measurements of both delayed neutron and delayed gammas from microgram quantities of 233U, 235U, and 239Pu were performed. These were used for comparison to the simulations of delayed particle emissions and detection from the Monte Carlo code MCNP6, in collaboration with Los Alamos National Laboratory. One of the options available in MCNP6 for delayed neutron emissions was found to be discrepant with measurements ≥ 100 s after fission. These DN comparisons were released as a MCNP6 test suite to all users; the most recent release of MCNP6.1.1β resolves many of the observed discrepancies found by this work. MCNP6 simulations were also used to predict expected delayed gamma signatures useful for NFA, and compared to DNGC system measurements in a final study. Les matériaux nucléaires spéciaux (MNS) sont ceux qui contiennent les isotopes 233U, 235U et 239Pu. Ils sont réglementés par l’Agence Internationale de l’Énergie Atomique (AIEA) en plus des organisations gouvernementales individuelles. L’analyse nucléaire légale (ANL) se préoccupe de la…