High-Cycle Thermal Fatigue in Mixing Zones: Investigations on Heat Transfer Coefficient and Temperature Fields in PWR Mixing Configurations

Piping systems of nuclear power plants include connections of branches conveying fluids at different temperatures. Thermal-hydraulic fluctuations arising from the turbulent mixing of the flows can affect the inner wall of the pipes and lead to fatigue damage. In order to assess the high-cycle thermal fatigue damages risks of the NPP mixing zones, knowledge of the temperature fluctuations and heat transfer from fluid to structure is necessary. In order to have a better knowledge of the thermal loadings in the several kinds of mixing zones of a NPP, a multi-annual R&D program has been initiated by EDF, AREVA and CEA. The experimental program uses two kinds of representative small-scale mock-ups which are called “Skin of Fluid” and “Stainless Steel FATHERINO”, for different mixing configurations. “Skin of Fluid” mock-ups are very thin ones, made of brass material, allowing visualization with an infrared camera of the fluid temperature field in the mixing zone, with minimized thermal attenuation. As a consequence, “Skin of Fluid” mock-ups allow detecting areas of interest in the mixing zone. “Stainless Steel FATHERINO” mock-ups are then used to evaluate quantitatively the thermal fluctuations and the heat transfer coefficient in the mixing zone. “Stainless Steel FATHERINO” mock-ups are made of 304L stainless steel and are 9.5mm thick. The mock-ups are instrumented with specific sensors named “Coefh”. These sensors allow measuring simultaneously the fluid and structure temperature time-histories and thus determining the heat transfer coefficient. This paper describes the experimental program and the approach used to evaluate the thermal load in the mixing zones. As an example, the application of this approach to an equal T-junction is presented hereafter.