Positron emission tomography in pebble beds. Part 2: Graphite particle deposition and resuspension

The release of radio-contaminated carbonaceous dust from the pebble bed of a high temperature reactor (HTR) plays a major role for the safety assessment of accidental scenarios. In the present investigation, graphite particle deposition and resuspension in a fluid dynamically scaled HTR pebble bed was studied by means of positron emission tomography (PET). The particle laden turbulent flow in a model pebble bed was generated by an air driven small-scale test facility at ambient conditions. The pebble bed geometry was recorded by a 3D gamma ray computed tomography (CT) scan. Technical graphite dust was radioactively labelled and dispersed into the turbulent flow field of the test facility. The flow scenarios were adapted to study particle deposition and resuspension in succession. Firstly, the radio-labelled graphite particles were dispersed at low fluid velocities to study the multilayer formation during a pure deposition regime. Afterwards, 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 complete insight into the particle multilayer formation and removal. Similarities to other particle deposition and resuspension studies in turbulent flows were found.