3-D Transient CFD Analysis for the Structural Integrity Assessment of a PWR Pressurizer Surge Line Subjected to Thermally Stratified Flow

The temperature difference in the fluid region due to the thermal stratification produces undesirable excessive thermal stress at the pipe in axial, circumferential, and radial directions, leading to thermal fatigue damage to the piping systems. Several nuclear power plants have so far experienced such serious mechanical damages due to excessive thermal stress as pressurizer surge line movements and its support failures, and cracks in feedwater nozzle, high pressure safety injection lines, and residual heat removal lines. An essential prerequisite for assessing the structural integrity of a piping system subjected to internally thermal stratification is to determine the transient temperature distributions in the pipe wall with accuracy. Several investigators have made efforts to determine the temperature distributions in the pipe wall by means of laboratory testing of particular geometry, field measurement of temperature, or fully theoretical predictions, or numerical calculations. However, they have addressed only 2-dimensional cases or 3-dimensional simplified cases where the existence of pipe wall thickness is neglected or the piping is assumed to be straight. However, most of piping systems at nuclear power plant are designed to have some bent parts and thick wall taking into account the wall thinning effect during the long time operation. When the conducting solid material is included in the solution domain of fluid flow and heat transfer, the problem is usually called the conjugate heat transfer problem. Recently 2-D and 3-D conjugate heat transfer calculations for a cross sectional area and full flow and heat transfer domain of a simplified straight pipe model of PWR pressurizer, respectively, have been performed by Jo et al.. In this study, to overcome the limitation in mesh generation for such a complex geometry as 3-dimensionally bent piping, 3-dimensional transient CFD calculations involving the conjugate heat transfer analysis are performed to obtain the transient temperature distributions in the wall of an actual PWR pressurizer surge line subjected to stratified internal flows either during out-surge or in-surge operation using a commercial CFD code called CFX-5.10. In addition, some key findings derived from the discussion on the CFD calculation results which should be followed in the process of thermal analysis are provided