The unwavering opposition among several major environmental organizations isn't sitting well with Hansen, and he's comparing them to, yes, the very global warming skeptics they often lampoon.
"It is analogous to climate deniers. Their minds are made up, facts don't matter much," Hansen said in an email exchange.
Hansen told National Journal he hasn't had discussions with green groups on the topic since he wrote the letter along with scientists Ken Caldeira, Tom Wigley, and Kerry Emanuel.
"I cannot change their position. That change will require pressure from environmentalists. People need to understand the situation and put pressure on the environmental organizations, for example by withholding financial support until they become (honestly) open-minded and scientific," said Hansen, who left NASA last year but remains affiliated with Columbia University's Earth Institute.
Hansen doesn't think getting environmental groups to embrace reactors is a lost cause. "I am hopeful that some of them are beginning to change, but it is too early to tell."
He also believes that there's more support among the green-minded than the formal positions of environmental groups might suggest. "Environmental groups and environmentalists are two very different entities," said Hansen, who first testified about global warming before Congress in the 1980s.
"It seems to me that there are a lot of environmentalists who are beginning to look into the facts and appreciate the potential environmental advantages of intelligent development of nuclear power," he said, calling rejection of nuclear a de facto acceptance of hydraulic fracturing for gas and continued reliance on coal.
To be sure, there's a long list of financial and political barriers to building new reactors in the United States.
Steve Kerekes, a spokesman for the Nuclear Energy Institute, the industry's main trade group, said environmental opposition isn't anywhere near the biggest hurdle to building the first new U.S. reactors in decades.
Even a major antinuclear group isn't claiming that activists are what's hindering the industry's long-hoped-for but slow-to-materialize "renaissance" of new U.S. construction. "Wall Street and Main Street have both rightly abandoned nuclear power," said Jim Riccio, a nuclear-power analyst with Greenpeace.
The U.S. natural-gas boom has driven down gas costs and helped make the fuel highly attractive to power companies, while renewables like wind and solar — though still a very small fraction of U.S. electricity — are on the march too. The Japanese nuclear disaster also put a spotlight on safety concerns.
And growth in U.S. power demand is slow, which Kerekes points to as a key reason why the number of reactors under development is at the lower end of the industry's earlier forecasts.
Just a small handful of nuclear projects are going forward. Power companies Southern and SCANA are building four new reactors in Georgia and South Carolina, while the Tennessee Valley Authority is completing construction of a reactor that it had abandoned in the late 1980s.