Effect of Local Void Fractions on Subcooled Boiling Oscillations in Natural Circulation Boiling Loop

Passive heat removal using natural circulation boiling loops (NCBLs) is well established from the view point of the safety of boiling water reactors (BWRs). Nevertheless, NCBLs suffer from a large number of nonlinear flow oscillations due to low driving force for circulations. The local measurements of void fraction a are essential to clearly understand the mechanism of flow oscillations. In this work, we have measured the impedance from the needle probe using LCR meter and quantified time-averaged local a from the measured impedance under oscillating flow conditions in the NCBL. Impedance probes have been successfully used in estimating void fractions and bubble frequencies. (authors)