Sure, you can built a thermos (vacuum around the hot storage volume, then highly reflective walls to put the radiating heat back into the storage volume). The

Sure, you can built a thermos (vacuum around the hot storage volume, then highly reflective walls to put the radiating heat back into the storage volume). The problem is that a tungsten tank filled with molten salt is heavy. If you want to store a couple of MW/h, we're quickly talking about several thousand tons. You have to suspend it somehow inside that thermos. With materials, that don't conduct heat well, but still have some strength at 2000C.