Thermal striping phenomena caused by mixing of fluids at different temperature is one of the most important issues in design of Fast Breeder Reactors (FBRs), because it may cause high-cycle thermal fatigue in structure and affect the structural integrity. A numerical simulation code MUGTHES has been developed to investigate thermal striping phenomena and to estimate high cycle thermal fatigue in FBRs. In this study, numerical simulation for the WATLON experiment which was the water experiment of a T-junction piping system (T-pipe) conducted in JAEA was carried out to validate the MUGTHES and to investigate the relation between the mechanism of temperature fluctuation generation and the unsteady motion of large eddy structures. In the numerical simulation, the large eddy simulation (LES) approach with standard Smagorinsky model was employed as eddy viscosity model to simulate large-scale eddy motion in the T-pipe. The mesh as the same with the previous study as reference, the finer mesh and the coarser mesh arrangements were employed to estimate the Grid Convergence Index (GCI) for uncertainty quantification in the validation process. The modified method of the GCI estimation based on the least squire version could successfully quantify uncertainty. Through the numerical simulations, it was indicated that the fine mesh arrangement could improve the temperature distribution in the wake. It could be found that the thermal mixing phenomena in the T-pipe were caused by the mutual interaction of the necklace-shaped vortex around the wake from in the front of the branch jet, the horseshoe-shaped vortex and the Karman’s vortex motions in the wake.

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