Unpredicted gas accumulation causes some negative effects into a safety system of nuclear power plant, such as, deteriorations of cooling performance and component integrity. Some locations elevated higher than their vicinities, namely, inverse U-bend, heat exchanger and various type of valves, tend to make gases remain inside. Even though the gas accumulation has to be avoided, there have been a lack of studies that attempt to identify the accumulative mechanism of gas. In this study, in order to understand what causes the accumulation of non-condensable gases, it was observed how the gases accumulated after they had been generated by pressure drop and heating. First, the sudden pressure drop produced cavitation during the flow passed through orifices which had various inner diameters of 1 through 5 mm. The cavitation triggered the phase change and the separation of dissolved gases simultaneously. Although the cavitation bubbles recovered immediately, the separated gases flowed with water along the inverse U-bend. As increasing the water velocity, the differential pressure between before and after the orifice increased and the amount of separated gas increased as well. When the amount of separated gases was above a certain value, the gases began to accumulate from the top of the inverse U-bend. When the inner diameter of orifice was larger, the differential pressure at the onset of gas accumulation was lower. Using same orifice, the faster the water velocity, the higher the accumulation rate. Second, the continuous heat of approximately 5 kW caused dissolved gas separation. The higher the temperature of water, the more the generated gas. When the velocity exceeded about 0.32 m/s, all bubbles were swept out by flow and escaped the inverse U-bend. When the velocity was less than about 0.17 m/s, whole gases that entered into the inverse U-bend accumulated. In the velocity range between 0.17 and 0.32 m/s, the faster the water velocity, the lower the onset temperature of the gas accumulation. After the onset of gas accumulation, due to the water temperature was higher when the water flowed faster, the accumulation rate became higher as increasing the velocity. In conclusion, the sizes and amount of gases entering into the inverse U-bend affected to the rate of gas accumulation. For advancing this study, we are planning quantify the injecting gas amount and investigate the correlation between gas accumulation and various variables, such as, temperature, flowrate of water and gas and so on.


Gas Accumulation at Inverse U-Bend Resulted From Sudden Pressure Drop and Temperature Increase

Division / Sponsors: 
Power Division & Nuclear Engineering Division (NED)

Power Conference
Paper Proceedings: https://asmedigitalcollection.asme.org/POWER/POWER2020/volume/83747

International Conference on Nuclear Engineering
Paper Proceedings: https://asmedigitalcollection.asme.org/ICONE/ICONE2020/volume/83761

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