The main purpose of Subproject 2 is to further investigate the Key Safety Aspects identified by the Safety Advisory Group of the RAPHAEL project and to contribute to their resolution. Work package 23 targets to fill the knowledge gaps in dust production, deposition and remobilisation. Experimental and (advanced) analytical methods have been adopted to investigate all aspects of dust behaviour.

Graphite dust in the reactor core, and its contamination with fission and activation products, has been identified as a key part of the potential source term in case of pressure boundary failure and important for potential radioactive release in the environment. Although this is mostly relevant for pebble bed cores, block type cores are also prone to safety aspects of graphite dust and potential releases.

In a task performed by HZDR a small-scale test facility was designed to generate a particle-laden turbulent flow in a pebble bed. This closed loop facility comprises steel pipes, the pebble bed, a particle filter and a power unit. The flow field was dynamically downscaled from the HTR core flow to a model pebble bed by the Reynolds similarity.The aerosol particles were radioactively labelled with Fluorine 18 and the spatiotemporal distribution of the particles in the pebble bed was recorded by means of Positron Emission Tomography (PET).

Two sets of experiments were carried out and provide a data basis for Computational Fluid Dynamics (CFD) code development to compute the particle deposition and resuspension within an HTR type pebble bed.

The first set of experiments was conducted with liquid monodisperse aerosol particles and a bed of polypropylene (PP) spheres to study systematically the deposition processes and the influence of particle size and fluid velocity. PP spheres had been chosen to reduce the mass attenuation of the gamma radiation. The results of the particle concentration measurements give an indication about the overall particle deposition behaviour with respect to particle size and fluid velocity. The 3D PET-CT overlay gives spatiotemporal insight into the formation of particle deposits in the pebble bed.

In the second set of experiments, graphite spheres and radioactively labelled graphite dust were used. The dust was deposited at certain flow velocity and resuspended at a higher velocity. As a result full 3D- and time-resolved insight into the particle deposition and resuspension during flow transients was provided. The illustration below shows the time series of reconstructed PET-CT overlay of the graphite particle deposits in the pebble bed during different flow scenarios.

3D PET-CT overlay of graphite particle deposition (t = [12,38,110] min) and resuspension (t = 300 min) in a pebble bed, direction of flow aligned with x-axis, gravity against y-axis

Further information about these studies can be found in the following articles:

Barth, T., Ludwig, M., Kulenkampff, J., Gründig, M., Franke, K., Lippmann-Pipke, J., & Hampel, U. (2013). Positron emission tomography in pebble beds, part 1: liquid particle deposition. Nuclear Engineering and Design, accepted for publication.

Barth, T., Kulenkampff, J, Ludwig, M., Bras, S., Gründig, M., Franke, K. Lippmann-Pipke, J. and Hampel, U. (2013). Study of particle deposition and resuspension in pebble beds using positron emission tomography. In The 15th International Topical Meeting on Nuclear Reactor Thermalhydraulics, Pisa, Italy.