Analysis of two-Phase Flow Phenomena with Conductivity Probes in Integral Reactor Safety Experiments

The thermalhydraulic behaviour of a PWR during beyond-design-basis accident scenarios is important for the verification and optimisation of accident management measures and code development. Analyses require integral experiments, such as PKL III at Siemens / KWU, which simulates a 1300 MWe KWU-design PWR with scaling factor of 1:145.
The paper presents PKL III results obtained by needle shaped conductivity probes developed in the Forschungszentrum Rossendorf [1]. The probes provided local void fractions and particle frequences within an averaging period of 2s. This time resolution is sufficient to characterize slow transients. A special flow pattern map developed for the probes enabled the calculation of the length of an average bubble. The measured information was used to determine the changing flow regimes.
At first, a secondary side feed-and-bleed procedure was investigated. Two probes were installed in the feed water line close to the steam generators. A depressurization of the secondary side caused flashing of the feed water and discharge of two-phase flow. A comparison of the measured void fraction with the density reading of a radiation densitometer installed next to the probes shows good agreement. The experimentally found flow regimes are also consistent with expected trends from flow regime maps developed by other researchers (e.g. [2]). Both show slug flow at the beginning of the secondary-side depressurization.
In the second experiment, a primary-side feed-and-bleed following steam generator tube rupture, four probes were installed at one location in the vertical part of the pressurizer surge-line, but at different radial positions. Another set of probes was installed in the horizontal pressurizer bleed-line. Two of them were used for time-of-flight measurements providing velocity values directly. Flow regimes such as stratified flow in the horizontal tube or annular flow in the vertical tube were found during the depressurization.