Looking at the data, it doesn't seem like the US transitioning to nuclear would have made climate change a "minor" problem: https://ourworldindata.org/grapher/annual-co-emissions-by-re...

For nearly all of the time before China outstripped the US, the EU (if you include EU-27) was a larger contributor than the US anyway. And much of the early US emissions edge came before nuclear power was invented. Global warming itself was poorly understood prior to the 1960s: https://www.scientificamerican.com/article/discovery-of-glob...

A transition to nuclear would have helped, certainly! But climate change would not be "minor."