NEW YORK (FORTUNE) - While oil company execs defend themselves before Congress and environmentalists and power companies struggle over windpower, John Rowe of Exelon is staking out a megashare in what could be America's great atomic revival.

Since Rowe became the sole CEO of Exelon (Research) in 2002, the nuclear power operator has lit up a lot more than the Chicago skyline. Exelon is the industry's most profitable company, earning $923 million on $15.4 billion in sales in 2005; its stock, recently at $53 a share, has consistently outshone the S&P utility index.

Exelon CEO John Rowe, standing in front of the company's Three Mile Island nuclear energy plant.

More from Fortune Small banks, big problems Let us praise the venture capitalists The yin and yang of cybersecurity FORTUNE 500 Current Issue Subscribe to Fortune

(This is an excerpt from a story that ran in the May 15 issue of FORTUNE. Click here or go to fortune.com to find out how a brainy Midwestern lawyer ended up as the king of America's nukes.)

Already America's largest nuclear power operator, Exelon will become the second-largest in the world once it completes its acquisition of New Jersey's PSEG. That $13.7 billion deal, announced over a year ago, was severely scrutinized by regulators because of the size of the business it will create, but is expected to close this summer.

The new behemoth will have an estimated $27 billion in annual revenue and $3.2 billion in net earnings, employ 28,000 people, and serve nine million businesses and households in Illinois, New Jersey and Pennsylvania.

Exelon's itch to supersize was prompted by a desire for PSEG's nuclear plants. Rowe is betting that as states begin to open their electricity grids to competition, nukes will gain share by producing energy more cheaply than rival sources of power.

"I go to a nuclear power plant and love it," he says happily.

It's not surprising that a utilities boss loves nukes; what's amazing is that so many people agree. Almost 30 years after the Three Mile Island disaster, nukes have regained luster in the public eye. Today 56 percent of Americans say they favor nuclear energy, vs. 38 percent opposed, according to Gallup. (A surprising 42 percent even say they're willing to have a nuke near their home.)

Influential environmentalists like James Lovelock and Stewart Brand favor using nukes to replace coal to help slow global warming. Exelon and other utilities are in discussions with the Nuclear Regulatory Commission to build at least ten reactors.

Perhaps the biggest boost to nuclear's prospects is the Bush administration's support. Last year's federal energy bill offers billions of dollars in subsidies, tax breaks, and incentives for new plants to help reduce long-term dependency on oil. The White House also launched the Nuclear Power 2010 Initiative, a $1.1 billion effort by government and industry to start building new plants by then.

As the sole utility exec on the privately financed National Commission on Energy Policy, Rowe helped mastermind the energy bill, insiders say; that and nuclear's resurgence make him one of the most influential CEOs you've probably never heard of.

When Rowe took over Unicom (one of the two companies that merged to form Exelon) in 1998, the company's nuclear fleet was one of the industry's worst. Its plants were running at only 47 percent capacity - in fact, they were up only about half the time. Chronic safety problems had landed many on the NRC's watch list. The Zion plant on Lake Michigan had been shut down, and two more in Illinois were headed in that direction. Within the nuclear division, everyone was pointing fingers.

Rowe's first move was to replace the nuclear division's management team. The key survivor was its chief, Oliver Kingsley, a widely respected nuclear turnaround expert who had just joined Exelon.

Rowe named him COO, and Kingsley helped recruit a new generation of executives who were approachable team players. They raised safety standards, upgraded equipment, and replaced the plants' crazy quilt of management systems with a single system that all had to follow. It set a new industry standard by sending plant operators for retraining and simulations every six weeks.

Exelon's production costs are more than 50 percent lower than they were in 1998, and the nuclear plants are running at 93 percent capacity, the best performance in the industry. Analysts like Steven Fleishman of Merrill Lynch expect Exelon's stock to rise as the company applies its know-how to PSEG's plants.

(This is an excerpt from a story that ran in the May 15 issue of FORTUNE. Click here or go to fortune.com for more about Exelon's recovery.)

Research associate Patricia A. Neering contributed to this article.