Simulation and Evaluation of Thermal Stratification in a Sloped Surge Nozzle Correlated With Plant Measurements

Abstract

Thermal stratification is a common phenomenon in the surge lines of Pressurized Water Reactors (PWR). The stratification temperature difference (ΔT) and cyclic action severities are most prevalent during the heatup and cooldown operations of a PWR, when the system ΔT between the pressurizer and the Reactor Coolant System (RCS) hot leg is the greatest and system inventory fluctuations are highest. This paper describes the computer simulation of thermal stratification loading in a surge line nozzle connected to the RCS hot leg to correlate to unusual behavior of plant sensor data in the hot leg and the subsequent development of a monitoring model to account for thermal stratification effects in the transient and fatigue evaluation performed in the online monitoring system. What makes this particular investigation unique is the geometry of the nozzle of interest. In many PWRs, the surge line and the surge line hot leg nozzle are horizontal at the hot leg connection. This particular nozzle is oriented at an upward angle before the attached surge line piping bends into a horizontal configuration. This orientation required a more detailed treatment of the stratification effects than has been typically developed for horizontal nozzles, with respect to both the orientation and the potentially detrimental effects of increased cyclic behavior indicated by nearby temperature sensors. This investigation combined Computational Fluid Dynamics (CFD) modeling of the system to correlate the plant data with a detailed stress model that will enable the fatigue usage factor calculation in the plant’s online transient and fatigue monitoring system.