Ultra fast electron beam X-ray CT for two-phase flow measurements

Nuclear reactor safety concerns for todays operated light water reactors are closely linked to thermal hydraulic phenomena in the primary and secondary coolant circuits of nuclear systems. To gain an improved understanding of phenomena associated with certain types of accidents as well as for the optimisation of fuel elements regarding the heat transfer in the reactor core simulation tools, such as system codes and more recently CFD codes are frequently applied. However, both types of tools today still exhibit some problems when treating two-phase flow scenarios. Therefore, in recent years much effort has been spent on further code development as well as on validation. This, however, was and is always accompanied by the requirement for reliable measurement data from two-phase flows under typical nuclear thermal hydraulic conditions. Experimental thermal hydraulic facilities, such as the TOPFLOW facility at FZD [1], may provide such data if they are equipped with sophisticated two-phase flow measurement sensors. One example is the wire-mesh sensor [2] that can provide phase and velocity data from steam-water two-phase flows at frame rates of up to 10 kHz and spatial resolution down to 2 mm. Since the wire mesh sensor is intrusive it has some influence on the flow. More appropriate would be tomographic imaging techniques based on either X-ray or gamma rays. However, classical CT systems, though being non-intrusive and sufficiently penetrative even for thicker metallic vessel walls, suffer from low temporal resolution, which is due to the requirement for taking different angular views of an object prior to reconstruction. So far this problem has been considered as intractable and unavoidable and tomographic techniques have almost exclusively been used to measure time averaged quantities, such as the gas hold-up.
We recently suggested [1] a computed tomography (CT) system which uses a fast moving electron beam X-ray source for the application in two-phase flow measurements. We have experimentally shown that frame rates of 10000 images per second can be reached. The static installation of the detector, however, limits the angular range of the source path which leads to so called limited-angle artefacts in the reconstructed images. Although these artefacts cannot be completely eliminated, because they are due to missing data, they can be reduced by maximizing the angular viewing range and by adequate image reconstruction algorithms [4].