Sierra Club, one of the oldest, largest, and most influential grassroots environmental activist groups in the U.S., has been tentatively anti-nuke since 1974, observing now that none of its conditions of energy conservation, reactor safety, nuclear fuel disposal, weapons proliferation, or adequate regulation have ever been met, has come out unequivocably against nuclear power.
The Sierra Club remains unequivocally opposed to nuclear energy. Although nuclear plants have been in operation for less than 60 years, we now have seen three serious disasters.
Tragically, it took a horrific disaster in Japan to remind the world that none of the fundamental problems with nuclear power have ever been addressed.
Besides reactor safety, both nuclear proliferation and the required long-term storage of nuclear waste (which remains lethal for more than 100,000 years) make nuclear power a uniquely dangerous energy technology for humanity.
Plenty of links in that Sierra Club page.
Sierra Club is one of the first two of the 99 sponsors of the Forward on Climate rally, high noon Sunday 17 February 2013 on the National Mall in Washington D.C. I'll be there. How about you?