Stress analysis of PWR pressurizer surge-line during single phase and two-phase thermal stratification

Abstract

Non-uniform temperature distribution at horizontal pipes cross section leads to an asymmetric thermal stress. This condition would be considered as an ageing mechanism due to fatigue effect, known as thermal stratification. To evaluate this ageing mechanism, the stress analysis should be done in horizontal pipelines like surge-line connected the pressurizer to the hot leg of primary loop of NPPs by consideration of operational or transient conditions. During these conditions, the status of surge-line would be single phase or two-phase stratified flow. To estimate the equivalent stress in horizontal section of surge-line, the developed CFD model for both single and two-phase flow is coupled with mechanical structural analysis. In two phase-condition, the heat transfer coefficient is evaluated by employment of turbulent kinetic energy source function. The results show that the maximum stress is almost same for single and two-phase stratification, but the propagation of cold water layer intensifies the asymmetric stress in two-phase condition.