Computational Analyses of the Thermal Stress Induced by Coolant Mixing in a T-Joint

In this paper we report computational fluid dynamics and thermal stress finite element analyses of a T-joint component as used in a Swiss nuclear power plant under realistic loading conditions and simplified boundary conditions. In this report the focus is on the thermal stress analyses, and therefore only those fluid dynamics simulations used for thermal stress analyses are presented. The local stress amplitudes, simulated using elastic finite element simulations, are compared to a regulatory fatigue life curve to estimate the local distribution of low cycle fatigue damage and crack initiation probability in the T-joint. The case studies reported here have been selected with the purpose of analyzing the different ways in which the fluid structure interaction induces thermal stress in the mixing tee. In case I only the thermal stress induced by the mixing turbulence is simulated, i.e. with constant mass flow rate boundary conditions. In case II a peak in the flow rate in the main pipe, typical for transient (startup) plant conditions, is addressed and analyzed in detail. Only in the latter case is low cycle fatigue cracking to be expected in the form of non-propagating (i.e. not throughwall) surface cracks or crazing.