Embracing nuclear is the only way, the scientists believe, to reverse the looming threat of climate change which they blame on fossil fuels. Depending who you ask, they’re either abandoning — or leading — traditional environmentalists who for a half-century have rejected clean-burning nuclear power as too expensive or too dangerous. Opponents cite disasters at Fukushima, Chernobyl and Three Mile island…

In the documentary, which debuts on CNN Thursday at 9 p.m. ET/PT, climate change activist and author Mark Lynas says he knew publicly supporting nuclear energy would put his entire career at risk. “I’d have been much better just to keep my mouth shut,” he admits in the film. “But I couldn’t do that.”

Cavanagh said the “movie attempts to establish the proposition that mainstream environmentalists are pouring into nuclear advocacy today. They aren’t. I’ve been in the NRDC since 1979 I have a pretty good idea of where the mainstream environmental groups are and have been. I’ve seen no movement.”

Selling nuclear energy to environmentalists is a tough pitch. Hansen acknowledged that many of them won’t easily buy into it. Parts of the community operate like “a religion of sorts, which makes it very difficult,” Hansen said. “They’re not all objectively looking at the pros and cons.”