But building new plants uses a shit ton of concrete. So we’re paying the carbon cost up front, and it can take years or even decades to break even.
That's not remotely on the same scale, carbon-wise. Global output is like 4 billion tons of concrete per year, a nuclear plant uses like 12 tons per megawatt; an all-in nuclear buildout would use a tiny, tiny fraction of global concrete production and the carbon costs aren't even remotely equivalent.