Numerical Investigation of Thermal Stratification in the Primary Circuit of VVER-440 Type Reactors

Thermal stratification in the primary loops and in the connected pipes can limit the lifetime of the piping, or lead to penetrating cracks due to the stresses caused by the temperature differences and the cyclic temperature changes. Therefore it is essential to determine the thermal hydraulic parameters of the stratified flow. The determination of the affected pipes can be based on the international operational experience and on engineering consideration. The most affected pipes in PWRs are the pressurizer surge line, the injection pipe of the emergency core cooling systems and the feedwater injection pipe of the steam generators. CFD codes can provide an appropriate tool for the examination of the development and the breaking up of the stratification and the determination of the temperature distribution. However, the challenge of the uncertainty of the boundary conditions has to be faced because of the unknown flow circumstances. According to an extensive evaluation, performed in 1998 by the VEIKI, in the VVER-440/213 units of Paks NPP the most affected pipe is the pressurizer surge line [1]. To find out the possible thermal stratification in the surge line, a temperature monitoring system was installed on the YP20 leg of the surge line of the Unit 1 of the Paks NPP in 2000. The measurements showed that during the heat-up period there is a thermal stratification almost all time in the surge line [2]. The maximum temperature differences reach 140 K (140 °C). The surge line has been modeled with the CFD code CFX-5.7. The performed transient simulations confirmed the existence of a thermal stratification in the surge line, but showed permanent recirculation of colder coolant in the lower layer, caused by the asymmetric arrangement of the surge line legs and the asymmetric connection of the two legs to the main loop. In this paper, the surge line model and the results of the transient simulations are presented. The CFD model of the injection pipe of the high pressure Emergency Core Cooling System and the performed simulations for the analysis of occurrence of thermal stratification are presented as well.