"There was a concept a long time ago that you would do a different type of reactor called a "fast reactor," that would make a bunch of another element called plutonium, and then you would pull that out, and then you would burn that. That's called "breeding" in a fast reactor. That is bad because plutonium is nuclear weapons material. It's messy. The processing you have to get through is not only environmentally difficultly, it's extremely expensive."