MIDDLETOWN, Pa. >> The concrete cooling towers that rise from a sliver of land south of Pennsylvania’s capital became symbols nearly a half-century ago of the risks of nuclear energy.

Now, a plan to restart one of the two reactors at Three Mile Island is at the leading edge of efforts to greatly expand the country’s reliance on atomic fission to meet the growing power demands of homes, businesses and data centers.

Refurbishing this aging plant — a fenced-off maze of pipes, valves, pumps and turbines in the middle of the Susquehanna River — depends on the financial backing of Microsoft. The technology giant has agreed to buy all of the electricity the plant generates, most likely starting in 2028, for 20 years.

Three Mile Island’s proposed revival reflects how vastly the perspectives on nuclear power in the United States have shifted since a cooling failure led to the partial meltdown of one of the island’s reactors in 1979.

Once the target of fierce opposition, nuclear power plants are now coveted for generating large amounts of electricity around the clock without releasing the emissions that contribute to climate change.

What’s less clear is whether expanding U.S. nuclear capacity will pencil out economically.

Resuscitating the Three Mile Island reactor that didn’t suffer a partial meltdown amounts to an early test of whether an industry known for blowing timelines and budgets can deliver on its promises.

Joseph Dominguez, the CEO of Constellation Energy, which bought the reactor in 1999, is confident that he is laying the groundwork not just to bring Three Mile Island back to life but also to add further to the company’s nuclear power fleet, the largest in the country.

“Twelve months from now, Constellation will have started on the path towards building new reactors,” Dominguez said in a recent interview.

No reactor set to close permanently has been brought back online in the United States, and only three new ones have been completed in the past quarter-century. The last two, built in Georgia, arrived seven years late and cost around $35 billion, more than twice as much as planned. Others have fared even worse: Utilities spent roughly $9 billion on two nuclear reactors in South Carolina before abandoning the project in 2017.

If Constellation and others succeed in scaling up nuclear power — currently around 19% of the U.S. electricity mix — the result would largely be due to the financial backing of some of the world’s most valuable companies. These plants would also be eligible for federal tax credits.

Microsoft is effectively paying Constellation to bring Three Mile Island back online, while Google and Amazon recently struck deals with startups developing smaller reactors that they hope will power up in the 2030s.

All three tech giants pledged to decarbonize their operations, but their data centers are consuming huge amounts of energy, making those goals more difficult to meet.

If this proves to be another false start for the nuclear industry, it will mean all the money in the world wasn’t enough to overcome the technological and regulatory hurdles to expanding the use of fission, in which atoms are split to create heat and make steam that is used to generate electricity.

“It’s an industry with a really substantial burden of proof based on all of the disappointments — all of the very expensive disappointments,” said Peter A. Bradford, who served on the Nuclear Regulatory Commission during the partial meltdown at Three Mile Island in March 1979.

It was then that America began to fall out of love with nuclear power.

Ken Bryan was on the island, working the night shift, when the telephone rang. It was just after 4 a.m., and alarms were sounding in Unit 2, the newer of the island’s two reactors. The warnings came so quickly — hundreds of them — that the control-room printer couldn’t keep up.

It later became clear that a valve designed to regulate pressure in the reactor had become stuck in an open position, allowing cooling water to escape as steam. The core overheated, and the plant released radioactive material.

“The reactor’s supposed to get hot, but not that hot,” Bryan, 80, said in an interview.

Some six miles away, Patricia Longenecker was asleep on the farm she shared with her husband and two young children. Hours later, she heard the news come over the radio, and soon it was just about all her neighbors could talk about.

“There isn’t a person in this room this evening who hasn’t asked the question, ‘How much radiation has my family received?’” she would testify that spring, during a state hearing at her children’s elementary school. “How is it going to affect our health?”

Studies ultimately found that the radiation had a “negligible” effect on people’s physical health and the environment, according to the Nuclear Regulatory Commission.

Public backlash was swift, however — and grew after the 1986 accident at the Chernobyl nuclear plant in Ukraine. Facing steep construction costs and heightened public opposition, dozens of planned reactor projects were canceled.

Three Mile Island’s undamaged reactor, down for refueling during the accident, remained offline for six years. It resumed operations in 1985 over the objections of residents and activists, providing power for nearly 34 years before shutting down in 2019 as utilities opted for cheaper electricity from natural gas plants.

By early 2023, Constellation was exploring whether to fuel it up again as long-flat U.S. electricity demand was poised to rise.

In addition, the dangers posed by climate change have become more visible in the wildfires scorching the West and the hurricanes buffeting the Southeast.

Nuclear power had also grown more popular, with 56% of U.S. adults favoring new plants, up from 43% in 2016, according to Pew Research Center surveys.

“The trade-off is: Are you concerned about the dangers of nuclear power, or are you more concerned about the dangers of climate change?” said Patrick McDonnell, who previously led Pennsylvania’s Department of Environmental Protection. “Climate change is the biggest issue we face.”

Goldman Sachs projects that U.S. power demand will grow more than 2% a year on average through the end of the decade, with data centers consuming around 8% of the country’s electricity by 2030, up from roughly 3% now.

The day Constellation announced in September that it would spend around $1.6 billion to restart Three Mile Island — a plant big enough to power more than 700,000 homes and employ upward of 700 people — the company’s stock rose 22%.