Aerosol retention in HTGRs: A study on pool scrubbing of graphite and silica particles

Abstract

The modular high-temperature gas-cooled reactor (HTGR) is recognized for the inherent safety, i.e., eliminating the possibility of core meltdown. Therefore, the source term becomes the main concern for HTGR accidents, with special focus on the graphite aerosol. However, there is few studies on the active control of graphite aerosol for HTGR. In this study, we propose a preliminary scheme for aerosol retention through pool scrubbing in HTGR water-ingress and loss-of-coolant accidents. The scrubbing characteristics of non-spherical graphite particles are investigated for different gas flow rates and submergences, and compared to the spherical silica particles. For both silica and graphite particles, the trend of decontamination factor (DF) is similar: slowly declining and then rapidly increasing with the increasing particle size. Further analysis with Stokes number indicates that the diffusion dominates below a critical Stokes number, while the inertia impact becomes the main mechanism when above this critical Stokes number. Especially, the DF of graphite particles is significantly lower than that of silica particles in the inertia-controlled regime, due to its irregular shape, porous structure, and hydrophobic nature. A bubble-breaking element is introduced to enhance the retention efficiency of graphite particles, by promoting gas–liquid mixing and more importantly, reducing bubble size in the rising zone. The result shows an average improvement of 135% in scrubbing efficiency, accompanied by a minimal 12.4% increase in pressure drop. This study demonstrates that by introducing an optimized scrubber to the current reactor design, the source term of graphite aerosol can be largely reduced for HTGR accidents with fairly low cost.