Believing we're in a climate crisis and also being anti-nuclear are mutually exclusive positions in my mind, and opposition to nuclear from environmentalist orgs should be viewed as a massive historical mistake as it set us back decades in moving the needle on carbon emissions. The engineering side of running reactors safely is a solved problem, the US navy has > 7500 reactor-years with a perfect safety record.
Building lots of new nuclear instead of doing the cheaper option of tons of batteries and renewables, only makes sense in a few geographic locations. Not all, or even most!
Even keeping old reactors running gets super expensive as they get past their designed lifetimes, and very often doesn't make sense.
The engineering is indeed already done for electricity, and storage and renewables are cheap and getting cheaper. Nuclear is at best staying the same high cost, and getting more expensive is these large construction projects rise due to Baumol's cost disease.
Opposing more nuclear in the US in the 1980s wasn't fully irrational, the US managerial class have way overbuilt nuclear and we didn't need all the electricity. Then we didn't have much growth in
The far bigger fight for climate these days isn't electricity: it's car-centric living, it's the anti-EV and anti-battery advocates, and to some degree it's retrofitting the wide variety of highly-cost-sensitive industries, such as steel or fertilizer or concrete, to use carbon neutral methods. Or maybe sustainable aviation fuel.
Nuclear had it's chance to be a big contributor to climate action back in the mid 2000s and 2010s, it failed that challenge in Georgia at Vogtle, in South Carolnia at Summer, in the UK at Hinkley Point C, in France in Flamanville, and in Finland an Olkiluoto. Every one of those failures is a very good reason for a climate activist to oppose nuclear.