If climate change is the greatest threat facing mankind, what are the odds of the big environmental groups rethinking their longstanding opposition to nuclear power?

They appear to be slim. Here's what Environmental Defense says on its website:

Serious questions of safety, security, waste and proliferation surround the issue of nuclear power. Until these questions are resolved satisfactorily, Environmental Defense cannot support an expansion of nuclear generating capacity.

And this comes from the Natural Resources Defense Council website:

New nuclear power plants are unlikely to provide a significant fraction of future U.S. needs for low-carbon energy. NRDC favors more practical, economical and environmentally sustainable approaches to reducing both U.S. and global carbon emissions, focusing on the widest possible implementation of end-use energy-efficiency improvements, and on policies to accelerate commercialization of clean, flexible, renewable energy technologies.

Supporters of nuclear energy -- including those who strongly support climate regulation to curb emissions of global warming pollutants -- say that doesn't make sense.

"They (environmentalists) love to hate the biggest thing that can move the needle with respect to climate change," says David Crane, the chief executive of NRG Energy. NRG is a member, with NRDC and EDF, of the U.S. Climate Action Partnership, an alliance of big companies and environmental groups that back a cap-and-trade program to regulate greenhouse gases.

Crane spoke last week during a lively discussion of nukes led by my colleague David Whitford at FORTUNE's Brainstorm Green conference about business and the environment. I wish we'd invited an EDF or NRDC representative onto the panel, but the focus was money, not safety, security or waste. David began the conversation by inviting everyone to "consider the evidence and think anew about something about which many of us had made up our minds."

Good idea. Many years ago, I covered protests against the Seabrook nuclear power plant in New Hampshire for a left-wing publication. My sympathies were with the protesters. Now I'm firmly undecided, and determined to learn more. Given the threat of climate change and the safety record of nuclear plants in the U.S. since Three Mile Island—especially compared the alternative of mining and burning coal—it seems like the right time to rethink nukes.

Here's what the directors of the national energy laboratories said last year in a report called A Sustainable Energy Future: The Essential Role of Nuclear Energy:

Today, nuclear energy provides 16 percent of the world's electricity and offers unique benefits. It is the only existing technology with capability for major expansion that can simultaneously provide stability for base-load electricity, security through reliable fuel supply, and environmental stewardship by avoiding emissions of greenhouse gases and other pollutants. Furthermore, it has proven reliability (greater than 90 percent capacity factor), exemplary safety, and operational economy through improved performance.

One of the signatories to the report was Steven Chu, now the energy secretary.

Here are some things I heard during the panel:

As thing stand now, we are unlikely to see the so-called nuclear renaissance that was talked about just a couple of years ago. The global economic slump is the reason why. Lenders are more risk-averse than ever, and few businesses need more capital and pose more risk than new nukes. Demand for electricity is slowing because of the recession. And natural gas prices are down, making it easier to meet new demand for electricity by building natural gas plants.

The U.S. government has set aside about $18 billion in loan guarantees for nuclear plants. That will underwrite perhaps three plants, our experts said. "I'm convinced that there will be three nuclear power plants built in the U.S. in the next 10 says," said Kevin Book, a partner at ClearView Energy Partners, a research and consulting firm.

Beyond that, it's anybody's guess. The utility industry wants to build more -- there are 24 applications for new nukes pending at the NRC, all of two to be located near to existing sites, where local support for nuclear energy is strong. No new plant has been approved since the 1980s. By contrast, there are 45 plants now under construction outside of the U.S., most in China, India and Korea, according to Book.

Like beauty, "clean" energy is in the eye of the beholder. Notice how the NRDC statement above says the group would prefer clean and renewable energy to nuclear. Well, Alan Hanson, an executive with Areva, the big French nuclear power company, says that the nuclear waste issue is closer to being solved than, say, the solar waste issue.

France, where more than 80% of the electricity comes from nuclear power, uses a safe and sophisticated system to recycle spent nuclear fuel, Hanson says. (You wouldn't expect him to say anything else, but still…) Nuclear waste can be stored on the sites of plants "for the next 500 years in we want," he said -- plenty to time to ease the transition to a renewable, low-carbon energy economy.

By contrast, he says, burning coal creates not on CO2 but mercury and other pollutants. And many solar photovoltaic panels are made of cadmium, among other things, for which there's no recycling plant. "I don't know of any part of the electricity generating world that treats its waste as well as the nuclear industry does," Hanson said.

The politics of nuclear are complicated. Chu, who's probably the smartest guy in the Obama cabinet, supports nuclear energy but Carol Browner, who's an experienced Washington power player (no pun intended) is said to be a strong opponent. Liberal Democrats on Capital Hill -- Nancy Pelosi, Henry Waxman, Barbara Boxer, Harry Reid -- also oppose nuclear power. Given a choice between nuclear and coal as a source of baseload power, they're likely to favor coal.

Crane said: "Right now the dominant wing of the Democratic Party knows they need to accommodate the coal wing of the Democratic Party in order to get energy and environmental policy passed." That leaves nuclear out of the deal-making.

President Obama hasn't said much about nuclear. It may well be that technology breakthroughs in solar, geothermal, wind or battery storage will mean that we don't need nuclear energy as a source of low-carbon power. But until those breakthroughs come along, shouldn't we keep the nuclear option open?

Nuclear cooling tower photos CC-licensed by Flickr users jster91 and PhotoDu.de.