Several small bore pipes are branched off from main coolant loop in a nuclear power plant for drain or letdown system in CVCS. These pipes are often bended and connected to a horizontal pipe that leads to the closed valve. The main flow usually initiates a cavity flow in the branch pipe and hot water penetrates into it. In some case, large temperature fluctuations with a rather long period were detected at the elbow when the cavity flow reaches down to the elbow. This phenomenon is considered to be the cause of thermal fatigue crack. The purpose of this study is to clarify the mechanism of the large temperature fluctuations at the elbow of the branch pipe and evaluate thermal stress that lead to the fatigue crack. A series of mock-up experiments were conducted with vertical straight pipe (the straight-pipe) and a vertical pipe with an elbow connected to a horizontal pipe (the bent-pipe) made of acrylic resin or stainless steel. Vortex form and flow pattern were visualized using a tracer method for various main flow velocities. The liquid temperature was measured by thermo-couples and thermally stratified layer was visualized with liquid crystal. The inner wall temperature distribution around the elbow was measured by thermo-couples. The temperature distribution/fluctuation was then converted to the input of FEA code and time-dependent stress fluctuations were simulated to evaluate the thermal stress fluctuation.

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