There are plenty of environmentalists who are fine with nuclear. The bigger issue is nuclear is substantially more expensive than renewables like solar and wind, and takes far longer to build. Plants that began construction now would most likely not be operational until 2030. That timeline makes hitting the 1.5 ‘C goals extremely difficult. A report just came out from Jesse Jenkins (Princeton prof who has no issues with nuclear) that showed nuclear is unlikely to play a major role in the US energy mix up until the year 2050.

In fact, I think most environmentalists are in favor of keeping currently existing nuclear plants that are slated to be mothballed operational.