We can move to clean afterwards but nuclear is technically "clean" energy as long as nothing bad happens. The only reason why it is not like is the risk of leakage and something like Chernobyl to happen.
You seem rather optimistic about something we shouldn't be so optimistic about. Nuclear waste isn't going to miraculously vanish. It's a concern for tens or even hundreds of thousands of years after.
With just one nuclear plant we can literally cover ALL bitcoin mining operation electricity requirements and then some (a lot more) which is why if we want free bitcoin transactions we should totally focus on nuclear plants.
I doubt it, but even if so, Bitcoin mining isn't happening in one centralized spot that you can provide energy for from one single nuclear plant. You need a lot of these nuclear plants around the world the way mining is distributed.
Things aren't as easy as they may seem when it comes to energy consumption. You can't possibly pool the entire network together with one energy source. It makes you an easy target for governments too.