Aerosol Formation During Water Ingress Into the Core of a Pebble Bed High-Temperature Reactor

In order to give an estimation of the safety of nuclear power plants, it is necessary to characterize the aerosol that develops in the event of a serious accident. Aerosol formation in a graphite-moderated high-temperature pebble bed reactor is possible when water or steam intrudes into the helium primary circuit. Because of the great temperature gradient between water droplets and the pebble surface, the evaporation of water is determined by the thin steam layer between the droplet and hot surface. The thin steam layer reduces the heat flux from the pebble to the water droplet. This effect is the well-known Leidenfrost phenomenon. The steam discharges from the steam layer at a high velocity, and is able to dislodge particles from the pebbles. The chemical reaction of the hot pebble graphite with steam results in a gasification by producing a gas mixture of H 2 O, He, CO, C0 2 , and CH 4 . In order to investigate the process of aerosol formation a test device was set up containing 1500 spherical graphite elements with a diameter of 10 mm, which can be heated up to a temperature of 1500°K. The mass concentration of the thus-produced graphite particles in the aerosol increases rapidly, when the corrosion progress has reached a specific value. The size distribution of the equivalent aerodynamic diameter is shifted to larger diameters with rising corrosion time.