By changing the composition ratio in Flibe to decrease its melting temperature, it becomes possible to design the TBM under the temperature design limits for ferritic steel. The accompanied demerit due to the increase in viscosity and degradation in heat transfer performance is overcome by introducing sphere-packed pipe as the first wall. The empirical correlation for heat transfer performance is derived for several sizes and materials of the spheres. Through the present analysis, there exist design windows for the Flibe TBM. This possibility is strongly linked to the demo reactor development since the structural material development for higher temperature condition can lead to the usage of Flibe with higher melting temperature and better heat transfer performance, which could be available under higher heat flux in the demo reactor.