... all that concrete in the nuclear reactor emits a lot of CO2 when the reactor is built. If you don't use the nuclear reactor for anything but peaking, you're

... all that concrete in the nuclear reactor emits a lot of CO2 when the reactor is built. If you don't use the nuclear reactor for anything but peaking, you're not going to amortize that CO2 emission over a large number of kWH. You'd never use the nuke as a peaker because it'd cost too much money, but, it's not a good idea in this other way, too.