If we presume that the entire US became gradually powered by nuclear (along with solar, wind, and hydro), we probably wouldn't have ever seen an actual global decline in fossil fuel usage at any point other than perhaps coal by a little. Total fossil fuel usage today would be lower though, but this would make oil cheaper for the rest of the world which would lead to greater oil consumption in the rest of the world than what we have historically had. The US would also need to lead the rest of the world to adopt nuclear as well.