Boston ? Thanks to global warming, nuclear energy is hot again. Its promise of abundant, carbon emissions-free power is being pushed by the president and newly considered by environmentalists. But any expansion won’t come cheap or easy.

The enormous obstacles facing nuclear power are the same as they were in 1996, when the nation’s last new nuclear plant opened near the Watts Bar reservoir in Tennessee after 22 years of construction and $7 billion in costs.

Waste disposal, safe operation and security remain major concerns, but economics may be the biggest deterrent. Huge capital costs combine into an enormous price tag for would-be investors.

There is also fervent anti-nuke opposition waiting to be restoked. Jim Riccio of Greenpeace said nuclear advocates are exploiting global warming fears to try to revive an industry that’s too risky to fool with.

“You have better ways to boil water,” Riccio said.

But environmentalists aren’t in lockstep on the issue. Bill Chameides, chief scientist for Environmental Defense, said anything that helps alleviate global warming must be an energy option.

“I think it’s somewhat disingenuous that folks who agree that global warming is such a serious issue could sort of dismiss it out of hand,” he said. “It’s got to be at least considered.”

The U.S. has 104 commercial reactors that supply about 20 percent of the country’s power. The Department of Energy projects a 45 percent growth in electricity demand by 2030, meaning 35 to 50 new nuclear plants will be needed by then just to maintain nuclear’s share of the energy market, said Scott Peterson of the Nuclear Energy Institute, the industry’s chief lobbyist.

That growing demand, not global warming, “has been the single biggest factor in companies looking at building large nuclear plants again,” Peterson said.

The Nuclear Regulatory Commission has been notified that several companies will pursue licenses for up to 33 new reactors, with the first one online in seven years at the earliest.

Earlier this year, projects at existing plants in Illinois and Mississippi received permits for their proposed sites, but it’s no guarantee they’ll be the first projects completed.

Many of the new plants are proposed in areas that already have existing plants where there is more acceptance of nuclear energy. President Bush visited one of those spots recently when he promoted nuclear energy at the Browns Ferry’s Unit 1 reactor in Alabama.

But any major expansion will require selling nuclear in new places, where local opposition may be intense and winning approval may be costly.

“This isn’t just a bunch of environmentalists who think this is a bad idea,” Riccio said. “It’s most people who aren’t being paid to think otherwise.”

Nuclear power is produced when neutrons split the nucleus of uranium atoms, releasing heat which is used to boil water and produce the steam that drives a plant’s turbines. The process is emissions-free and the radioactive waste is contained inside the plant.

The waste is currently stored at individual plants, awaiting permanent transfer to the national Yucca Mountain Repository in Nevada. But Yucca Mountain has faced stiff opposition and won’t open until the early 2020s at the earliest.