I **think** that nuclear fusion could be made clean, but don't know enough about the physics of that to be certain.

One of the potential problems with these gigantic plants is that they cost billions of dollars, so if someone screws up in the design or construction there is a strong desire to cover it up and to bribe the inspector. With billions at stake, a million-dollar bribe seems quite reasonable and very difficult to turn down. Perhaps I am just being silly.