Design and simulation study of an online activity monitoring device for radioactive noble gases in Thorium-Molten-Salt-Reactor

[Background] In normal operation, Thorium molten salt reactor can produce radioactive noble gases such as Argon, Krypton and Xenon. They are expelled into the environment via the exhaust outlet after a desorption treatment. The accurate identification and measurement of the activity levels of different inert gases in exhaust emissions provide critical insights for assessing the operational status of nuclear reactors and associated off-gas treatment systems. [Purpose] This study aims to develop a coincidence monitoring device to identify nuclides and measure the radioactivity of noble gases in online model of Thorium molten salt reactor. [Methods] The monitoring device consists of a sampling chamber, an inner fifty mm diameter PIPS and an ORTEC P-type coaxial high-purity germanium detector. Using a 300 mL sampling chamber as the baseline, MCNP simulations were conducted to evaluate the detection efficiencies for four characteristic radionuclides, as well as to investigate the influence of the distance between the HPGe detector and the chamber on gamma-ray detection efficiency, thus determining the optimal chamber dimensions. The Geant4 code was employed to analyze the gamma spectra of characteristic nuclides, as well as the gamma spectra following beta-gamma coincidence. Furthermore, the minimum detectable concentrations for four characteristic nuclides were calculated based on this model. [Results] The simulation results show that the sampling chamber is optimized with dimensions of 5.3 cm in height and 4.25 cm in radius, while maintaining a 5 cm separation distance between the HPGe and the chamber. The monitoring device demonstrates levels of minimum detectable concentration ranging from 102 to 103 mBq·/m3 for four characteristic radionuclides. [Conclusions] According to the simulation results, a coincidence monitoring device is designed, and its detection and recognition capability for different noble gases is studied. Based on simulation outcomes, a coincidence monitoring device was developed, with its capability to detect and identify various noble gases systematically evaluated. Benchmarking these performance metrics against empirical calibration data constitutes a critical validation step for assessing the system's operational reliability.