Heat transfer performance for high Prandtl number and high temperature molten salt flow in a circular pipe and in sphere-packed pipes are evaluated with modified Tohoku-NIFS Thermofluid Loop (TNT loop) using high-temperature molten salt HTS (KNO3 : NaNO2 : NaNO3 = 53 : 40 : 7), as a stimulant of Flibe (LiF : BeF2 = 66 : 34). The modified TNT loop has much longer entrance region to develop a thermal boundary layer, which enable us to obtain more precise heat transfer data.

In the modified TNT loop experiments, the heat transfer characteristics in a circular pipe flow have good agreements with the representative correlations. It is obvious that the analogy for heat and momentum transfer is also valid for high-temperature and high-Prandtl-number molten salt flow. It is also confirmed that the heat transfer performance of sphere-packed pipes increases up to about 4 times higher than that of circular pipe, in case of relatively low flow rate. This can be effective in the Flibe blanket system from the viewpoints of moderating MHD effect and electrolysis.