WASHINGTON — Nuclear energy, once vilified by environmentalists and facing a dim future, has become a pivotal bargaining chip as Senate Democrats hunt for Republican votes to pass climate legislation.

The nuclear power industry's long-standing campaign to rebrand itself as green is gaining footing as part of the effort to curtail greenhouse gases.

Nuclear power still faces daunting challenges, including the fate of highly radioactive reactor waste. Reactors remain a tempting target for terrorists, requiring ever vigilant security measures.

But 104 power reactors in 31 states provide one-fifth of the electricity in U.S. They also are producing 70 percent of essentially carbon-free power and are devoid of greenhouse gas emissions.

It's something the nuclear industry has hammered away at in advertising and in lobbying on Capitol Hill for nearly a decade. Only recently, however, has the message begun to resonate among both industry supporters and skeptics.

"If you want to address climate change and produce electricity, nuclear has got to be a significant part of the equation," Marvin Fertel, president of the Nuclear Energy Institute, the industry trade group, said in an interview.

Not unexpected from a top industry lobbyist. But the same is heard from Republicans and Democrats in Congress, from a growing number of environmentalists and from the White House, where nuclear power otherwise has received tepid support.

The Senate this week will kick off three committee hearings on legislation to cap greenhouse gases from power plants and large industrial facilities. The goal is to cut them about 80 percent by 2050.

The House has already passed a bill. Its chances in the Senate could hinge in part on whether it meets demands by a few Republican senators, including Lindsey Graham of South Carolina and John McCain of Arizona, that the legislation provide help to build new reactors.

"Nuclear power is pivotal to both a low carbon economy and to generate a bipartisan coalition to pass a carbon cap," says Jason Grumet, executive director of the National Commission on Energy Policy, a bipartisan group of experts created seven years ago to advise government officials on energy matters.

He says all economic models on climate legislation "assume significant increases in nuclear power" — an expansion binge unseen since the 1970s, before the Three Mile Island nuclear accident brought new reactor orders to a halt.

A study by the industry-supported Electric Power Research Institute says 45 new reactors are needed by 2030. The Energy Information Administration puts the number at 70. An analysis by the Environmental Protection Agency assumes 180 new reactors by 2050 for an 80 percent decline in greenhouse gas emissions.

The Nuclear Regulatory Commission has applications for 30 new reactors. Only a few probably would be built over the next decade, the earliest in 2016 — and then only with the government guaranteeing the private financing.

Democratic sponsors of the climate bill are far short of the 60 votes needed to overcome Republican efforts to prevent the legislation from coming to a final vote. They hope compromises could bring along uncommitted centrist Democrats and some Republicans. Along with talk of opening more offshore waters to oil drilling, support for nuclear energy is seen as the carrot that might attract Republicans.

The prospects of such a compromise appeared to brighten recently when Democratic Sen. John Kerry of Massachusetts, the climate bill's principal sponsor, and Republican Graham collaborated on a new bid to build consensus.

"Nuclear power needs to be a core component of electricity generation if we are to meet our emission reduction targets," they wrote. They called for ending "cumbersome regulations that have stalled" new reactors, measures to help utilities secure financing and expanded research to resolve the waste problem.

They outlined a framework that other Republicans might follow. Republican senators such as McCain, Lisa Murkowski of Alaska, Lamar Alexander of Tennessee as well as independent Sen. Joe Lieberman of Connecticut, have shown an interest in climate legislation — if nuclear energy plays a greater part.

To many environmentalists, it remains a choice of dealing with one overriding environmental problem, while accepting another, to some degree.

"You can't dismiss nuclear power's potential as a climate solution," says Susan Vancko of the Union of Concerned Scientists. Yet, she says, with reactors costing upward to $10 billion apiece, "this is one of the most expensive options out there" to cut greenhouse gases.

Vancko cautions against providing "almost unlimited loan guarantees" for reactors that could go bust.

A group of 14 environmental and anti-nuclear groups expressed concern in a recent letter to senators that easing licensing requirements and rushing to build new plants "would fatally undermine public confidence in the safety of U.S. reactors."

Atop the nuclear industry's wish list — 26 items covering two single-line typewritten pages — is an expansion of loan guarantees for new reactors. But it also mentions eliminating some speed bumps in the road to reactor licensing, new efforts to deal with reactor waste and an array of other items.

Some are in the Senate bill; others are likely to be added.

The goals of those calling for aggressive action on climate change have become intertwined with those pushing for more nuclear energy.

"I don't think it gets you there alone," industry official Fertel says about nuclear's role in combating global warming. "But you can't get there without it."

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