> The eight 345 MW advanced sodium cooled reactors would provide Meta with up to 2.8 GW of carbon-free, baseload energy. Each reactor comes with the Natrium technology’s innovative built-in energy storage system providing the capacity to boost total output to 4 GW of power.

For energy storage, is it storing the hot water, or using batteries to store generated electricity?

Sodium cooled. They will store heat in a big thermos of molten salt.