Thermal striping on the core instrumentation plate (CIP) at the bottom of the upper internal structure (UIS) of an advanced loop-type sodium-cooled fast reactor in Japan (Advanced-SFR) has been numerically investigated. At the top of the core below the CIP, the sodium at high temperature flows out from the fuel subassemblies (FSs) and the sodium at low temperature flows out from the primary control rod (PCR) and backup control rod (BCR) channels, and also the radial blanket fuel subassemblies (RBFSs) at the outer side of the core. In order to predict the thermal striping on the CIP caused by mixing fluids at different temperatures from the FSs, the PCR and the BCR channels, and the RBFSs, a numerical estimation method using a spatial connection methodology between the upper plenum analysis and the local area analysis for the target area has been developed. By using the connection methodology, the numerical simulation considering the influence of the transversal flow in the UIS and the external flow around the UIS in the upper plenum can be performed to improve the accuracy of the estimation results. In this paper, the outline of the spatial connection methodology including data transfer technique from the upper plenum analysis to the local area analysis was described. As a validation process, numerical simulation of the water experiment using the test apparatus named TAFUT which was 1/3-scaled 1/6 partial model of the upper plenum of the Advanced-SFR was performed to confirm applicability of the spatial connection methodology to a practical thermal striping problem. The numerical result of temperature distribution was compared with the measured result in TAFUT experiment. Additionally, mesh sensitivity of the local area analysis model to the numerical results was indicated by using a small and a large area models in order to suggest an appropriate local area analysis model.

Thermal striping phenomenon caused by mixing of fluids at different temperatures is one of the most important issues in design of fast breeder reactors (FBRs), because it may cause high-cycle thermal fatigue in structure. Authors have been developed a numerical simulation code MUGTHES to investigate thermal striping phenomena in FBRs and to give transient data of temperature in the fluid and the structure for an evaluation method of the high-cycle thermal fatigue problem. MUGTHES employs the boundary fitted coordinate (BFC) system and deals with three-dimensional transient thermal-hydraulic problems by using the large eddy simulation (LES) approach and artificial wall conditions derived by a wall function law. In this paper, numerical simulations of MUGTHES in T-junction piping system appear. Boundary conditions for the simulations are chosen from an existing water experiment in JAEA, named as WATLON experiment. The wall jet condition in which the branch pipe jet flows away touching main pipe wall on the branch pipe side and the impinging jet condition in which the branch pipe jet impinges on the wall surface on the opposite side of the branch pipe are selected, because significant temperature fluctuation may be induced on the wall surfaces by the branch pipe jet behavior. Numerical results by MUGTHES are validated by comparisons with measured velocity and temperature profiles. Three dimensional large-scale eddies are identified behind of the branch pipe jet in the wall jet case and in front of the branch pipe jet in the impinging jet case, respectively. Through these numerical simulations in the T-pipe, generation mechanism of temperature fluctuation in thermal mixing process is revealed in the relation with the large-scale eddy motion.