Presumably just insulated storage of hot stuff to act as a buffer between the heat production and use for electricity generation. Very similar to what CSP plants do.

The molten sodium is a storage of energy in the form of heat. Keep it well contained and insulated, and you draw off only the heat that is needed to convert to kinetic energy, which then generates electric current through induction.

Like a monster flywheel, there are all kinds of things that might go wrong, but they're all far less scary than Chernobyl style meltdowns.

The Natrium data sheet[1] says that it is "Molten salt energy storage", so based on the diagram[2] it appears that salt (not sodium) is being pumped to to the reactor and stored in large tanks, similar to thermal mass solar installations. I assume the sodium is internal to the reactor only, but can't say for sure.

[1]:https://www.terrapower.com/wp-content/uploads/2023/03/TP_202... [2]:https://www.terrapower.com/our-work/natriumpower/