Effect of Vertical Pipe Length on the Thermal Stratification in Nuclear Power Plant Surgeline

During operating transients of the pressurizer, thermal stratification effect may occur especially in the horizontal parts of the surge line. US NRC requires consideration of thermal stratification in surge line as phenomenon that must be considered in the design basis of the surge line. Generally, the fatigue usage factor of the surge line is comparative high, due to its operating temperature and pressure transients and its thermal stratification loads. In this study we have performed some parametric fluid-structure interaction analyses with different length variables of the vertical part of the surge line to study the relationship between the magnitude of thermal stratification and the length of vertical part of the surge line. The conservativeness of the traditional finite element model for thermal stratification analysis based on the conservative assumption in the surge line was also discussed by comparison of the results of three-dimensional time transient fluid-structure interaction analysis of this study. Stresses calculated with three-dimensional time transient model were considerably reduced comparing with the traditional analysis.