[en] The numerical simulation code 'MUGTHES' has been developed by the authors to investigate thermal striping phenomena caused by turbulence mixing of fluids at different temperatures and to utilize with an evaluation method for high-cycle thermal fatigue. MUGTHES employs the large eddy simulation (LES) approach to predict unsteady thermal mixing phenomena and the boundary fitted coordinate (BFC) system to fit complex boundary shapes in a reactor. In this paper, thermal striping phenomena in a T-junction piping system (T-pipe) are numerically simulated as a code validation study and the simulation results are used to investigate temperature fluctuation generation mechanism in the T-pipe. The boundary conditions for the simulation are chosen from the WATLON experiment which was a water experiment conducted at JAEA using the T-pipe. In the simulation, the standard Smagorinsky model is employed as a turbulence eddy viscosity model with the model coefficient of 0.14 (=Cs). Numerical results of MUGTHES are compared with experimental velocity and temperature profiles. Applicability of MUGTHES to the thermal striping phenomena is confirmed through the numerical simulation in the T-pipe and the characteristic large-scale vortex structure which dominates thermal mixing and may cause high-cycle thermal fatigue is revealed. (author)