Measurements of local two-phase flow parameters in fuel bundle under BWR operating conditions

The Westinghouse FRIGG facility, in Vasteras/Sweden, is dedicated to the measurement of critical power, stability and pressure drop in fuel rod bundles under BWR operating conditions (steady-state and transient). The facility is particularly relevant to test modern BWR fuel designs which typically have complex features, such as part-length rods and mixing vanes that make the flow heterogeneous and challenging to accurately simulate (e.g. using sub-channel analysis codes or CFD tools). In order to support the validation of advanced thermal-hydraulics codes for detailed BWR fuel assembly simulation, new local instrumentation techniques have been tested at the FRIGG facility for the measurement of two-phase dynamic pressure (Pitot tubes) and high time resolution phase detection (optical sensor). The optical sensors were custom-made by RBI Instrumentation for the FRIGG facility and optimized for annular two-phase flow (drop/steam) under BWR operating conditions. This new instrumentation was successfully tested and allows the first-time measurement, under BWR operating conditions, of relevant two-phase flow parameters such as the local void fraction in the steam core, the local drop/steam velocity, the volumetric interfacial area, the drop collision frequency and the assessment of drop size distribution during BWR steady-state and transient operations. (C) 2017 Elsevier B.V. All rights reserved.