Thermal stratification and striping in a model pressurizer surge line

During the start-up of nuclear plant, hot water from the pressurizer slowly flows through the surge line to the primary system. Under some conditions, the hotter water will ride on top of the cooler water from the primary system. The resulting stratified flow has an oscillating interface. The temperature differences between the top and bottom of the surge line can cause significant deformation due to differential thermal expansion. The oscillating temperatures at the interface (thermal striping) cause thermal stresses and high-cycle fatigue. A scale model of the pressurizer surge line in a typical Babcock ampersand Wilcox pressurized water reactor was constructed to study thermally stratified flow and striping. The model closely matched elevation changes and horizontal lengths, but the inside diameter of the pipe was only 3 in. compared to 8.75 in. in the plant. The horizontal sections contained clear acrylic pipe. The hot and cold water were dyed different colors so that the interface would be clearly visible. The tests were performed at atmospheric pressure with hot water at ∼145 degree F and cold water at ∼70 degree F. Salt was added to the cold water so that density differences up to 10 lbm/ft3 could be obtained. The density and viscosity of the salt water were measured before starting the test. Results are described in this paper