Experiments on the transport, deposition and resuspension of nuclear aerosols

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 pebble bed High Temperature Reactors (HTRs). Nuclear aerosols, in particular graphite dust in size of d = [0.1; 50] μm, are deposited during operation in the primary circuit (Moormann (2008)). It is of general interest how much of these aerosols escape from the primary circuit into the containment during a depressurization scenario. The knowledge about the amount of resuspended dust allows the detailed estimate of the dose escaping the primary circuit
Flow conditions in the primary circuit range from laminar flows in the recuperator till turbulent high Reynolds number flows in the pipes and ducts. Considering the particle size distribution published by Moormann (2008), particle Stokes numbers will range from very small Stk << 1 till moderately high Stk > 1. In order to investigate the fluid mechanic behavior between the flow and the aerosol within the set of characteristic numbers, we designed a small scale gas aerosol test facility (Figure 1).

We found a turbulent square duct flow most suitable because all the flow features such as streamwise and spanwise velocity gradients, as well as vortical structures are apparent. Furthermore, there will be a wide range of experimental and numerical data for comparison.
The test facility is a small scale wind tunnel in total length of 6 m with a 10 x 10 cm² square duct section. A 500 W radial fan at the outlet of the channel accelerates the flow field from 0 up to 7.5 m/s which is equivalent to a Reynolds number of about Re = 50 k.
The inlet is equipped with a HEPA filter to clean the incoming air. A nozzle contracts the flow into a square duct which is divided into a flow formation zone (15 x d) and a test section (5 x d). In the beginning of the flow formation zone the dust feeder or the aerosol generator injects the aerosol. The length of 15 x d for the flow formation zone ensures that the test section is streamed by a well developed turbulent channel flow with an evenly distributed aerosol. Both, test section and flow formation zone, are made of transparent acrylic glass to allow optical flow field measurements, such as PIV, high speed camera imaging for the analysis of the flow field and microscopic imaging techniques for the surface particle detection.
A diffusor stage decelerates the flow before it enters the electrostatic filter for air cleaning purposes. Finally, the 500 W radial fan produces the pressure drop for the desired flow speed.
Preliminary measurements of total pressure drop in the square duct section and time averaged mean center velocity profiles for different Reynolds number will be presented on the poster.