Numerical modelling of thermal fatigue at pipe mixing points in nuclear power plants

In nuclear power plants, thermal fatigue can occur at pipe mixing points where hot and cold water combine, leading to failure at these critical locations. This study investigates the effect of temperature fluctuations on fatigue failure at critical locations and welded joints, which has received limited attention in previous research. A numerical approach was used, starting with highly-resolved unsteady conjugate heat transfer simulations to assess heat flux at the pipe wall. This was followed by structural analysis using the finite element method, and finally, a fatigue assessment to predict failure locations and estimate component lifespan. Temperature differences of 80 °C and 160 °C were investigated at the different weld locations. Results showed that at a temperature difference of 160 °C between the main pipe and the branch pipe, a full penetration butt weld would fail after 1462 to 19,119 h. A shorter failure time was observed at the stress concentration area upstream of the T-junction under the same conditions. These findings were applied to the well-documented 1998 failure at the Civaux1 plant in France, to help understand the potential causes of that failure.