Fuel element failure detection is important for FBR plants not only to achieve a high availability for a reactor safety operation and to secure operational reliability, but also to conduct special test attended with a fuel failure. Thus, the experimental fast reactor Joyo uses HP-Ge detector, called OnLine Gamma-ray Monitor (OLGM), to monitor Xe and Kr, which are rare gases as fission products, in its cover gas.
The OLGM is composed of a charcoal bed, gamma-ray shielding, a HP-Ge detector, valves, and piping that connects the charcoal bed to the cover gas inlet and outlet piping and a fresh argon gas supply line. As cover gas flows through the charcoal bed at room temperature, Xe and krypton isotopes are selectively adsorbed in the charcoal bed at higher efficiencies than those of 41Ar and 23Ne, which obstruct a precise gamma-ray analysis of FP nuclides, because of the difference of the adsorption coefficients between noble gases. Then, the charcoal bed is flushed with a small amount of fresh argon to release the undesirable radioactive Ar and Ne isotopes because of due to the same principle. Joyo OLGM is equipped with two charcoal beds to enable measurement in a short interval of time, and with a HP-Ge detector cooled by liquid nitrogen. In the measured gamma-ray spectrum, it can be seen that the distinct photo-electron peaks of the activated nuclides 125Xe, 127Xe, 129mXe, 131mXe, 133Xe and 135Xe are distinguished with high energy resolution.
However, the HP-Ge detector requires the time and man power to feed liquid nitrogen regularly. In this study, we examined the application of a thallium bromide (TlBr) detector to OLGM as a room temperature semiconductor detector that has the potential to achieve high gamma-ray energy resolution. The room where the OLGM is installed will be approximately 40 °C at the rated output of the reactor. And Joyo's rated operating days are 60 days. There are two major challenges here. First, the energy resolution needs to be as high as possible to identify the isotopes of Xe. The thallium bromide detector operates at room temperature and slightly higher temperatures, but the energy resolution decreases as the temperature rises. Therefore, it was decided to cool the detector with a Peltier element. The second is the stability of energy resolution and detection efficiency during continuous operation for 60 days. In this study, as the beginning of the study of the applicability of the thallium bromide detector to OLGM, we conducted experiments using a prototype detector on the temperature dependence of energy resolution and the stability of resolution and detection efficiency during long-term operation. It was confirmed that the detector can be stably cooled with a Peltier element in an environment of 50 °C using a constant temperature bath. In addition, the energy resolution was confirmed at a detector temperature of about 10 °C., and the required level for improving the energy resolution of the detector itself was determined. In addition, the time dependence of energy resolution and detection efficiency was confirmed by continuous operation for 60 days, and it was confirmed that TlBr detector could be applied to OLGM.