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

This report describes the application of gamma-ray computed tomography (CT) and positron emission tomography (PET) for the investigation of particle transport in a model of a High Temperature Reactor (HTR) pebble bed. Micron sized liquid and graphite particles were radioactively labelled and the deposition and resuspension of particle multilayers in the pebble bed was studied. The particle laden turbulent flow in a model pebble bed was scaled by Reynolds similarity and was generated by an air driven small-scale test facility at ambient conditions. The pebble bed geometry was recorded by a CT to determine the bed porosity and pebble orientation. Two sets of experiments were performed. Firstly, radio-labelled liquid particles were dispersed into the flow to study pure particle deposition over a wide fluid velocity range. In a second set of experiments, radio-labelled graphite particles were injected into the flow and particle deposition and resuspension was studied in succession. Activity spots appeared on the front sides of the single pebbles indicating the formation of particle multilayers on the pebble surfaces. After the deposition experiment, the aerosol generator was switched off and the fan speed was stepwise increased to induce particle resuspension. A time resolved 3D PET-CT overlay provides a completely new insight into the particle multilayer formation and removal.