Effect of gas species on gas-liquid two-phase flow during pool scrubbing. Preliminary consideration using nitrogen and helium

Pool scrubbing is one of the fission product (FP) removal mechanisms in nuclear severe accident (SA) countermeasures. Conventionally, pool scrubbing models have been developed using experimental results with air, nitrogen, or steam as the FP carrier gas. However, recent studies suggest there being a considerable quantity of hydrogen produced during an accident. In order to achieve a realistic assessment of SA, there is a need to evaluate the applicability of such models to cases where mixed species of gases exist. In pool scrubbing, FP removal is largely associated with flow regimes of the gas-liquid two-phase flow. This study aims to gain a deeper understanding of the effect of gas compositions on flow characteristics by performing a series of two-phase experiments, regarding single bubble and churn-turbulent regimes. Using helium and nitrogen (and their mixtures of several volumetric ratios), experimental data on two phase behaviors were obtained using cameras and wire-mesh sensors. From which, it was found that single bubbles showed little or no behavioral differences, while in a churn-turbulent flow condition, hold-up, void fraction, and bubble size distribution differed with gas compositions. More specifically, two-phase flow with a larger fraction of helium was found to contain a sizable fraction of relatively large bubbles, resulting in lower hold-up and void fractions compared to those with nitrogen. This report also discusses future research aspects in order to acquire a complete physical understanding behind the effect of gas species on gas-liquid two-phase flow. (author)