AN EXPERIMENTAL STUDY ON THE SCRUBBING CHARACTERISTICS OF GRAPHITE DUST AEROSOL IN HTGRS

The potential release of graphite dust of HTGR contributes the most to the source term and has a great impact on the safety of reactors. In water ingress accidents, the graphite dust may be released to the containment during the overpressure discharge, which not only contributes to the source term but also makes trouble for post-accident measures. An important improvement for reducing the radioactive material emissions in the light water reactor (LWR) is the aerosol pool scrubbing. The effectiveness of pool scrubbing with sphere particles has been extensively investigated. whereas, there is little research on the passive removal of dust and the source term for HTGR. This paper explores the possibility of pool scrubbing retaining the graphite particle and fission products in the overpressure discharge. The scrubber system is established and the pool scrubbing experiments with flake graphite particles and spherical silica particles are conducted. The effects of various parameters on graphite scrubbing efficiency are investigated, including water submergence depth, gas flow rate, particle size, and particle mass concentration. The decontamination factor (DF) is a key parameter to describe the scrubbing efficiency. It is shown that the DFs increase significantly with the increase of submergence depth and gas flow rate both for two kinds of particles. Dominated by the inertial deposition mechanism, the decontamination efficiency of graphite is significantly lower than silica particles for particles larger than 1 μm. As for the particles smaller than 1 μm, the difference in decontamination efficiency between graphite and silica is small attributed to the Brownian diffusion mechanism. In general, the total DF of silica is significantly higher than that of graphite particles. To improve the retention efficiency of particles, a metal packing breaking element is placed in the vessel. It benefits the breakup of large globules and the contacting of gas/liquid. In case, the total DF of graphite aerosol increased by an average factor of 2.8 times, meanwhile the pressure drop increased only by 12.4%. It is revealed that almost all particles achieve a DF of 20, corresponding to a retention efficiency exceeding 95%, at the water depth of 110cm. These results are beneficial for the design of scrubber structures that can accommodate low-pressure discharge airflow, which can achieve a high radioactive retention efficiency and maintain a relatively low pressure drop.