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)