Measurement of subchannel void fraction in 5 x 5 rod bundles using an impedance void meter

In order to improve the ability to predict nuclear reactor behaviour, reliable measurement of the void fraction in a subchannel is essential. A subchannel impedance void meter (SCIVM) is developed and applied in air-water two-phase flow in 5 x 5 rod bundles, which consists of a six-strip emitter electrode and six-strip receiver electrode to cover the whole subchannel. Because of the lack of accurate void fraction calculation in the subchannel, the developed impedance void meter is calibrated by a four-sensor conductivity probe with a quartic polynomial function. The void fractions in different subchannels are measured with a sampling frequency of 10 kHz to capture the dynamic features of the rapidly changing phase distribution in two-phase flow. The phase distributions in rod bundles are discussed based on the void fractions measured in different subchannels, and among all flow regimes, the cap turbulent flow has the largest void fraction differences between the inner, side and corner subchannel on the same cross-section due to the effect of the casing tube. Moreover, the dynamic features of the subchannel void fraction are analysed for different flow regimes, including bubbly, cap bubbly, cap turbulent, churn turbulent, and annular flow. It is demonstrated that the newly designed subchannel impedance void sensor is an effective tool to measure the void fraction in the subchannel and figure out its dynamic features. Highlights A novel SCIVM is designed to measure the void fraction in the subchannel. The measured subchannel void fraction is calibrated with a four-sensor conductivity probe and verified with empirical drift-flux correlations. The differences in void fraction are discussed for different subchannels on the same cross-section. The dynamic features of the subchannel void fraction are analysed for different flow regimes.