Evaluation, Refurbishment, and Characterization of Fast Neutron Scintillators for the TREAT Fuel Motion Monitoring System

This paper details with work being performed to evaluate, refurbish, and characterize proton-recoil scintillator (PRS) assemblies being used in a 360-channel detector array for the measurement of fast neutrons in the Fuel Motion Monitoring System (FMMS) at the Transient Reactor Test Facility (TREAT). The TREAT FMMS measures the real-time movement and displacement of experimental fuel during transient experiments, correlating fast-neutron signals to the mass of fuel in each of 360 pixels at the center of the reactor's core. The detectors in the FMMS have stood dormant for over 20 years and significant degradation must be addressed before the system can be brought back to operational status. The PRS refurbishment process began by decoupling the scintillators from their matched photomultiplier tubes (PMTs) and treating the PRSs with ethanol. This process cleans the surfaces, removing paint and legacy labels/markings. During this procedure many physical variations were observed. After cleaning, the PRSs were measured using a digital caliper and the dimensions documented. Following this, a well-controlled fiber-coupled 280-nm light-emitting diode light source was used to stimulate the PRSs to produce an emission spectrum inside a light integration sphere. This emission spectrum was measured using an ultra-violet light enhanced optical spectrometer. Spectral measurements were recorded from 185-nm to 580-nm in 0.4-nm steps. The PRSs were then lightly sanded on the nonPMT mating surfaces and painted with commercially available reflective paint. A final emission spectrum was recorded for each PRS after painting.