Gamma-Blind Fast Neutron Detection for Spent Nuclear Fuel Characterization

In this thesis, the development and testing of a system for measuring the axial distribution of fast neutron emission of spent nuclear fuel rods is presented. Emphasis is placed on the novel fast neutron detector used which can reliably work in extremely high gamma fields. The detector has a sensitive volume made of a silver-activated zinc sulfide (ZnS:Ag) scintillator powder mixed with a transparent epoxy, with wavelength-shifting fibers (WLSFs) embedded in this mixture. Neutron detection happens via elastic scattering with hydrogen in the optical epoxy. The resulting recoil protons activate the ZnS:Ag and its scintillation light is partly collected by the WLSFs. Silicon photomultipliers are used to read out the bursts of scintillation light reaching the end of the fibers and a digital filter algorithm based on single photon counting is employed to recognize neutron events from clusters of captured optical photons. The thesis contains the description of the extensive characterization measurements that were performed for the detector. The main prototype had a length of 3 cm and a sensitive volume of 3 cm^3. It contained 196 WLSFs embedded at a pitch of 0.7 mm read out by four SiPMs. The testing was mainly focused on its performance in environments with a strong gamma background since this is the type of environment of the targeted spent fuel characterization. In a first step, the parameters of the filter algorithm were chosen with neutron detection efficiency and gamma rejection properties in mind. The detector achieved an intrinsic neutron detection efficiency of ~1% for neutrons emitted by a Cf-252 source. At the same time, the detector efficiently rejected gamma rays (less than 0.01 s^-1 counts) from a Co-60 source at an estimated gamma flux density of ~5e6 cm^-2 s^-1. The damage due to gamma radiation was characterized as well. After prolonged irradiation of the sensitive volume at a high gamma flux (1.8e13 cm^-2 accumulated gamma fluence from Co-60), a significant reduction of ~40% in neutron detection efficiency was observed. The overall detector characterization was extended to environments with less strong gamma backgrounds. This is intended as the groundwork for future developments applying this promising detector concept to applications in other fields than spent fuel characterization (for example fast neutron imaging or monitoring). With the relaxed requirements on gamma rejection, it was possible to tune the filter parameters and reach an intrinsic detection efficiency above 11%. Yet another set of parameters for the filter algorithm led to the detector being able to measure the arrival time of neutrons with an accuracy of ~60 ns.The thesis further reports on a spent fuel measurement campaign where the detector was used to measure the neutron emission of four known MOX and UO2 fuel samples. The fuel samples had a wide range of burn-ups and relatively long cooling times of ~25 y. The measured total neutron emission of the samples agrees well with previous measurements of the same samples that used a different technique. The measured emission of the samples relative to each other agrees within two standard deviations with the previous measurements while the absolute emission agrees within 2.5 standard deviations. For one of the samples an axial scan was performed and it was possible to reconstruct the neutron emission with a resolution of $2.5$~cm.