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/