Unsteady void measurements within debris beds using high speed X-ray tomography

- A high speed X-ray tomography facility has been built for the investigation on two-phase flow.
- The two-phase flow through beds of packed plastic spheres has been investigated in the facility.
- 3D-reconstructions from the measurements show the fluxes in the two-phase flow.
- The gas fraction has been calculated from the reconstruction and used for validation of the modeling.
- A new bed with closest regular spheres arrangement has been manufactured by 3D-plotter and used in the measurement.

Two-phase flow and boiling within debris beds representing a destroyed reactor core after a severe accident with core fragmentation can be simulated by using the porous media approach. In this approach, a local pressure drop and the heat transfer between the solid debris particles and the two-phase flow is modelled with the help flow-pattern maps, in which the boundaries between bubbly, slug, and annular flow are assumed. In order to support further understanding of these flows we have developed a very fast X-ray measurement device to visualize the 3D-void distribution within particle beds or porous media, which are otherwise un-accessible internally. The experimental setup uses a scanned electron beam directed in circles on a tungsten target to generate the X-rays. The particle bed, which has a diameter of 70 mm, is located between this target and a field of 256 X-ray detectors, which are arranged on a circle concentric to the target. The void distribution is reconstructed numerically from the attenuation of signals, which penetrates the particle bed and the two-phase flow inside. A 3D frame rate of up to 1000 Hz can be reached. The spatial resolution is such that bubbles with a diameter > 1.7 mm can be detected. We have investigated two-phase flows air/water through beds of packed plastic spheres (diameter between 3 and 15 mm) as well as through plastic beds, which were manufactured using a '3D-plotter'. Flow patterns can be individually determined on the basis of empirical criteria. It is confirmed, that the transition between slug and annular flow depends on the air mass flux, but the particle diameter must be considered as an additional parameter.