A NUMERICAL STUDY ON THE BEHAVIORS OF GRAPHITE AEROSOL IN THE CONTAINMENT DURING A WATER INGRESS ACCIDENT OF HIGH TEMPERATURE GAS-COOLED REACTORS

The graphite dust is a special problem for pebble-bed high temperature gas-cooled reactors (HTGR), due to the close coupling with the fission products. A large fraction of graphite particles deposit on the tube bundles of steam generator (SG) due to the particle-vortex interaction and thermophoresis effect. Once a water-ingress accident occurs, the graphite dust may resuspend due to the rapid change of flow field. A certain fraction of resuspended dust, together with some fission products, could be discharged to the containment if the overpressure protection of the primary circuit is triggered. The distributions of deposited and airborne particles in the containment are crucial for the safety analysis and source term evaluation. In this work, we use a Eulerian-Lagrangian method to numerically study the transport and deposition behaviors of graphite aerosol in the full-scale containment during a water-ingress accident. Particularly, the effects of highly irregular shape of graphite particles on the particle-wall and particle-fluid interactions are incorporated. The statistical results that as the tracking time increases, the deposition rate first drops rapidly and then exponentially decreases. The deposition fraction is 80.38% for the floors, 9.83% for the shell of the containment, and 3.55% for other surfaces. Only 6.24% of particles are suspended in the air. As the particle size increases, the deposition fraction increases first and then decreases, reaching a maximum value at 3 μm. We also discuss the effect of the down direction of overpressure relief outlet on particle deposition. The result shows a fraction of 11.68% of airborne particles, which is about 2 times than that of the up direction. Our simulation not only demonstrates that the containment has a good capability to retain the graphite dust in a water-ingress accident scenario, but also provides a support for the engineering design of the overpressure relief system of HTGRs.