This concept was already actively used but mostly abandoned. A friend of mine has a (in Germany) so called "Nachtspeicher Heizung" (lit. translates to night storage heating), the English term is storage heater. https://en.wikipedia.org/wiki/Storage_heater
It is an electric heating system that has high heat capacity stones in it. If you turn it on it will use a fan to output hot air. It was invented to use the overproduction of fossil fuel plants during the night when demand was low. They have a separate meter and cheaper tariffs. With the switch to renewables they could have a second chance.
I believe the main challenge is making then somewhat smart to only store when the energy is cheap, in the past it was a very simple time based system.
I think they changed it to "post"
You have to differentiate between fusion and fission, the first one is no doubt, while looking at the time spans these projects took previously it will not save the global Energy supply in the short term. Fission is difficult to tell, since the reactors have lots of concrete to build (that creates CO2) and humanity has not found any way to get rid of the waste and contaminated building materials. It might be "greenish" but probably not sustainable (also there is a limited amount of and political problems with digging up the needed radioactive materials)