Design and commissioning of a small-scale gas/aerosol test facility for investigation of deposition and resuspension of nulcear aerosol particles

Nuclear aerosol deposition and the assessment of its resuspension during a design basis accident in the primary circuit are a key issue in the development and certification of advanced (Very) High Temperature Reactors ((V)HTRs). It is of general interest how much of these nuclear aerosol particles escape from the primary circuit into the containment during a depressurization scenario. The knowledge about the amount of resuspended material will allow detailed estimate
of dose escaping the primary circuit (Fontanet et al. (2009)).
Kissane (2009) and Moormann (2008) list different sources of radioactive graphite dust production. Regarding pebble bed HTRs, there is mechanical movement and thereby friction between the pebbles due to loading and unloading of the core. Furthermore, different kind of impurities such as oil, water and air ingresses lead to dust production due to oxidation and peeling of the pebble surfaces. Referring to VAMPYR filter experiments at AVR J¨ulich, Moormann (2008) reports about airborne concentration (C = 0..50 μg/m³), geometrical size distribution (dP = 0.1..50 μm) and chemical composition of graphite dust. Annual amount of dust production in the AVR is estimated to be around 5 kg/yr and surface loading is calculated to be approximately 10 g/m² after 16 years operation. Furthermore, Kissane (2009) expects that the partially graphitized binder is a major source of carbonaceous dust production. Scaling these dust production contributions up to a 400 MWt pebble bed core, the annual dust production is around 100 kg/yr. The graphite dust particles circulate in helium pressure boundary (HPB) which is characterized by a cooling pressure of about 11 to 55 bar and coolant temperature up to 950°C. Flow conditions range from laminar flows in the recuperator to turbulent high Reynolds number flows in pipes and ducts. Depending on the local conditions, there is deposition and esuspension of graphite dust mainly moderated by diffusion, thermophoresis, turbophoresis, particle inertia, gravitational settling and electrostatic precipitation. In order to investigate transport, deposition and resuspension phenomena, we focus on single effect studies of nuclear aerosol particles in a well developed turbulent flow field.