X. Tang, “Barium Fluoride (BaF2) Scintillation Detectors for Fast Timing Measurements in Pebble-Bed Reactor Design”, M.S. Thesis, Nuclear Engineering, Texas A&M University, College Station, TX (2021).
In this work, a barium fluoride (BaF₂) scintillation detector is investigated to determine if it can be used for fast measurement of deep burn fuel in advanced reactor designs. Pebble bed high-temperature gas-cooled reactors (HTGRs) have a multi-pass system of spherical fuel element circulation. Typically, the spherical fuel elements can be restricted from 55-hours to 100-hours of cooling time before recirculating or being used for another cycle through the system. In recent developments, a high purity germanium (HPGe) detector was utilized to detect gamma-rays where the pebble recirculates through a measuring site, and each pebble is measured for 10 seconds. However, a HPGe detector has a slow response time and therefore suffers from large dead time losses when exposed to the very high gamma-fields of a pebble (each pebble is ~ 1 kCi in radioactivity). A fast and efficient measurement system on the order of seconds is required for the pebble-bed’s detection to remedy the problem. In this work, two objectives were investigated, and their results analyzed, i) BaF₂ energy response and resolution characterization for gamma-spectroscopy of a burned pebble by broadening energy peaks based on appropriate resolution ii) time response and resolution of the BaF₂ detector. To explore the energy resolution characterization for the gamma-spectroscopy, an optimal high voltage (HV) was first determined at 2.1 kV. Data was collected with CAEN’s DT5730 digitizer using their CoMPASS software, the sampling rate of the digitizer being 250 MHz. The best energy resolution of BaF₂ was explored and found to be 16.62 1.71%. The analog to digital converter (ADC) had 16,384 energy channels corresponding to the counts for the digitizer used in this research. All the energy bins were broadened to the Gaussian pulses and eventually summed up. A MATLAB script was created to perform the convolution numerically. The spectrum from Xenon-100 pebble, provided by X-Energy was used. Secondly, to analyze the time resolution of the barium fluoride for fast measurement, a Python function was developed to process the collected data, fitted 200 pulses to the exponential curve, and the mean decay time was calculated as 0.99 and 648 ns for fast and slow components, respectively. Theoretically, the fast and slow time responses of the BaF₂ detector are 0.8 ns and 630 ns. These theoretical values were compared with the experimental data, and the error difference between them was calculated. In the future, the impact of this fast detection system on material balance and accounting will be explored by understanding how this system affects the inventory of ²³⁹Pu and ²³⁵U at a pebble bed reactor site.