Study on Electrolysis-based Tritium in Water Real-Time Continuous Monitoring System

Abstract

Monitoring of tritium in water is important in the aspect of radiological protection. The tritium only emits beta ray with extremely low energy, it is categorized by hard-to-detect radionuclide. Because of its low energy, self-absorption effect is very high and detection efficiency of direct measurement of tritium is very low. Therefore, the tritium-in-water monitoring system was developed including gasification using electrolysis to reduce the self-absorption effect and specialized design of detection chamber to increase the detection efficiency. The concentration of the tritiated gas generated by electrolysis is determined by fractionation factor. The fractionation factor was estimated, and its temperature dependence was quantified. The lowest fractionation factor was obtained as 3.24 ± 0.14 at temperature of 90 ℃. A novel design of detection chamber which was specialized to detect the low energy beta ray was proposed. In the detection chamber, plastic scintillators were placed with narrow gap to reduce worthless space and increase the detection efficiency. The performance of the detection including detection efficiency and MDA for the developed chamber was evaluated using MCNP6 simulation. The highest detection efficiency was close to 100% and detection efficiency of 53.7% was shown in 32-channel chamber. The lowest MDA was shown at the chamber when 0.5 mm of plastic scintillator was placed with 48-channel and its value was 29.9 kBq/m3 with the counting time of 30 s which was lower than any other comparing studies. Because of the practical processing limitations of the acrylic which was selected as a supporting structure of plastic scintillators, a 12-channel detection chamber using 1 mm plastic scintillator was manufactured. Signal processing parameters were optimized, the maximum detection efficiency was estimated by 43.0 ± 4.32%. In the integrated system, detection efficiency was estimated by 18.8±2.4 %, and MDA for the liquid tritiated water was estimated by 5 kBq/L with measurement time of 600 min.