Analysis of thermal stratification in the primary circuit of a VVER-440 reactor with the CFX code

The thermal stratification can lead an important role in the aging of the NPP piping because of the stresses caused by the temperature differences and the cyclic temperature changes. These stresses can limit the lifetime of the piping, or lead to penetrating cracks. For the stress analyses, the determination of the thermal hydraulic parameters of the stratified flow is necessary, which can be simulated by computational fluid dynamics (CFD) codes. The results of the simulation show the time development and the breaking up of the stratification and the temperature distribution of the stratified flow. The main difficulty of these CFD simulations is the uncertainty of the boundary conditions because of the unknown flow circumstances. In this paper, some results of CFX simulations are presented concerning the pressurizer surge line, and the injection pipe of the HPIS for VVER-440 type reactors.