Large eddy simulation on the flow and thermal fluctuation of non-isothermal water mixing in reactor upper plenum

In nuclear reactors, temperature fluctuations resulting from the mixing of non-isothermal fluids in the upper plenum may induce fatigue damage to adjacent structures, a phenomenon known as thermal striping. To accurately capture temperature fluctuations in the coolant and surrounding components, we employed Large Eddy Simulation (LES) and developed a simplified hexagonal-jet model for simulating these fluctuations within the actual reactor core structure. The results were compared with those from the existing coaxial-jet model. The results show that the temperature fluctuation of the fluid under the influence of the hexagonal-jet progressively increases in the axial direction, while it significantly attenuates at the lower surface of the plate. This temperature fluctuation arises from the mixing of the jet with the crossflow. In contrast, the temperature fluctuation induced by the coaxial-jet results from the intrusion of hot fluid into the cold fluid and their subsequent mixing. The intensity of temperature fluctuations for the two models varies differently in the radial and axial directions. Furthermore, the temperature signal of the fluid influenced by the coaxial-jet is more concentrated, with a larger amplitude compared to that of the hexagonal-jet. Consequently, the results obtained by the coaxial-jet model can be regarded as a conservative estimation.