Study of particle deposition and resuspension in pebble beds using positron emission tomography

This work describes an experimental study about the transport behavior of carbonaceous dust in a High Temperature Reactor (HTR) pebble bed. Carbonaceous dust may be formed during reactor operation and deposits on the inner surfaces of the HTR primary circuit. In case of a loss of coolant accident (LOCA) this dust may be resuspended and released into the environment. Since the dust is a carrier of fission products this dust is a considerable source term and has to be analyzed with respect to such accidental scenarios.
The particle deposition and resuspension behavior was experimentally investigated in a pebble bed by means of Positron Emission Tomography (PET). The turbulent flow field was generated by an air-driven small scale test facility. The pebble bed was fluid mechanically downscaled by the pebble bed related Reynolds number. The pebble bed geometry was analyzed by a 3D gamma ray computed tomography scan for the precise determination of the pebble orientation and the bed porosity.
Two sets of PET experiments were conducted. First, monodisperse liquid aerosol particles were labeled with the radioisotope (18F) and dispersed into the turbulent flow field. PET was used for the temporal and spatial recording of the particle deposition process.
In the second set of PET measurements, the liquid particles were replaced by technical graphite dust matching the particle size distribution usually found in the HTR primary circuit. The graphite dust was also labeled with 18F before being dispersed into the turbulent flow field. Deposition experiments were conducted at rather low fluid velocities to purely study the particle precipitation. Subsequently, the fan power of the facility was stepwise increased and the PET scanner recorded the particle resuspension process online. The existing data sets give a unique 3D and time resolved insight into the particle transport process in such a complex geometry. The data can be further used for CFD code development to predict the particle behavior during a LOCA in an HTR.