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 released graphite dust may not only contribute to the source term but also make trouble for post-accident measures. 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. It is shown that the decontamination factor (DF) increases significantly with the increase of submergence depth and gas flow rate both for two kinds of particles. Almost all particles achieve a DF of 20, corresponding to a retention efficiency exceeding 95%, at the water depth of 110cm. Dominated by the inertial deposition mechanism, the decontamination efficiency of graphite is significantly lower than silica particles for particles larger than 1 μm. To improve the retention efficiency of particles, a metal packing breaking element is placed in the vessel. 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%. 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. (authors)