The use of small neutron detector arrays for the assessment of nuclear materials comprising 235-U, 238-U and 240-Pu

O'Donnell, Helen and Joyce, Malcolm (2020) The use of small neutron detector arrays for the assessment of nuclear materials comprising 235-U, 238-U and 240-Pu. PhD thesis, Lancaster University.

Text (2020O'DonnellPhD)
main_FINAL_PASSED.pdf - Published Version
Available under License Creative Commons Attribution-NonCommercial-NoDerivs.
Download (43MB)

Abstract

Neutron metrology has important applications in areas such as nuclear safeguards, quality assurance, and nuclear power operations. To date, the most common detectors for performing neutron metrology have been 3He detectors. With 3He production in decline, it has become supply-constrained and alternatives are being sought as potential replacements. Liquid organic scintillation detectors are an option, and in particular the relatively recent development of EJ-309 liquid scintillant is a forerunner for the replacement of 3He. This thesis is a study of the appropriateness of small arrays of organic liquid scintillation detectors for the characterisation of 235U, 238U, and 240Pu using total-neutron-counting and coincidence counting during active and passive assessments. Constraints of the equipment and methods are also reported. To do this, five studies have been carried out with a number of cubic EJ-309 detectors; comprising a combination of experimental investigations and MCNP simulations. They have assessed: optimal detector geometries; the use of a neutron shielding cube for measurements of forensic-style samples; an investigation into the applicability of the detectors for total-neutron-counting during the active assay of a broad-range of 235U enrichments; the measurement of total-neutron-count of spontaneous fission neutrons from 238U and a proof-of-concept of a plutonium-contaminated material drum monitor using coincidence counting. The neutron shielding cube and broad-range of 235U experiments suggest that the EJ-309 detectors are suitable for measuring small samples of 235U actively using total-neutron-count. The investigations into measuring the ultra-low-level emission of spontaneous fission neutrons from 238U, and the assessment of 1 mg of PuO2/PuF4 in a 200 litre drum, are evidence of the system’s ability to measure particularly small neutron fluxes, in total-neutron-counting-mode and coincidence-mode. The detectors performed well in all investigations when the length of time of assay was chosen appropriately. In most tests, the EJ-309 detectors outperform comparable 3He detector-based systems, due to their ability to detect fast neutrons directly. The lack of efficiency of the detectors below 0.5 MeV does discount them for thermal neutron detection.