In a PWR plant, on top of the pressurizer, there’s spray line that cools the reactor coolant. The amount of minimum flow rate is considered to be insufficient to fulfill the pipe completely during commercial operation of the plant, so it is concerned that there is a surface of water in the pipe and it may periodically sway due to condensation of the steam from the pressurizer during commercial operation. In order to identify the flow conditions in the spray line piping and assess its impact on thermal stress, flow visualization experiment were conducted. In the experiment, air is used in substitute of steam to simulate the gas phase of the pressurizer. With a full scale mock-up made of acrylic, flow under room temperature and atmospheric pressure condition was visualized and the locations where periodical swaying of water surface may be observed were identified. The period of swaying and its extent is roughly estimated based on the experiment. Based on the visualization experiment, 3 patterns of swaying of water surface were assumed and the amplitude of thermal stress fluctuation when the layer swayed instantaneously was calculated. Thus, with the assumed 3 patterns of swaying based on the visualization experiment, it was confirmed that the amplitude of thermal stress would not exceed the endurance limit defined in the Japanese design and construction code.

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