After years of bad news, 2016 looked like it might have been a turnaround for the U.S. nuclear power industry.

The year saw the opening of Tennessee's Watts Bar Unit 2, the first new nuclear reactor to go online in 20 years. State governments in New York and Illinois stepped in keep five struggling plants running in those states.

But 2017 started off with the announcement that the two-reactor Indian Point plant outside New York City would be shutting down by 2021. And work on the only reactors now under construction has been clouded by multi-billion-dollar losses by the Japanese industrial conglomerate Toshiba, whose subsidiary Westinghouse is building four reactors in Georgia and South Carolina.

The 99 reactors still operating provide about 20 percent of U.S. electric power. But while environmentalists have fought the industry for decades, its biggest opponent has turned out to be natural gas — far cheaper than nuclear, while producing half the planet-warming carbon emissions of coal when burned. And in regions like the Midwest and Northeast, where electricity is sold on deregulated markets, nuclear plants are facing financial meltdowns.

The result has been what Edward Kee, CEO of the Nuclear Economics Consulting Group, calls a "market failure" — one that may require government intervention to fix.

"Do we want nuclear in this country or not?" Kee said. "If we do, we need to fix that problem. We either need to have some additional compensation for these other attributes like zero emissions, or have government ownership or something like that. We can't expect the market to deliver nuclear power that ends up losing money."

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State governments have stepped in to keep the lights on at three plants in upstate New York and two in Illinois by offering subsidies for producing electricity without carbon emissions. Both programs were sold as part of efforts to address climate change and boost renewable sources like wind and solar power, which brought grudging support from environmental groups.

Both plans are being challenged in court — but they could become models for similar programs in other states where atomic economics are challenging.

Still, five reactors have gone offline in the past five years, and six more are expected to be shut down in the next decade. The Nuclear Energy Institute, the industry's leading trade association, says a total of 15 reactors are in danger of being shut down for economic reasons.

"We're working feverishly to prevent that," NEI spokesman John Keeley said.

But Keeley said that if government subsidizes carbon-free power sources like wind and solar, then nuclear should be supported, too. When nuclear plants have closed recently, the lost generating capacity usually gets made up by gas — which, while cleaner than coal, still results in added carbon dioxide.

"These are truly irreplaceable assets," Keeley said. "If they shut down, there is no restarting them. The utilities that own and operate them turn the licenses back over to the NRC [Nuclear Regulatory Commission]. They disperse a workforce of hundreds … you have very significant impacts both economically and environmentally."

Advocates like NEI — as well as some prominent environmentalists, like climate-science pioneer James Hansen — say nukes are needed to provide carbon-free electricity with greater reliability than renewables. In Illinois, the state will pay Chicago-based nuclear operator Exelon more than $2 billion in "zero-emissions credits" to keep the Clinton and Quad Cities nuclear plants in business for another decade. In New York, the expected subsidies could top $8 billion for Exelon and its rival Entergy.

The subsidies are based on how much carbon dioxide and other gases the plants are estimated to save, using a formula the U.S. government produced to calculate the expected social cost of climate change. And lawmakers in at least four more states — Ohio, Connecticut, Pennsylvania and New Jersey — are eying similar plans, said Doug Vine, senior energy fellow at the Center for Climate and Energy Solutions.

"The approach is certainly interesting, and I think the jury's still out on whether that will hold up," Vine said.

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The costs of operating a nuclear plant safely and securely price them out of deregulated markets, even though their fuel costs are lower than gas or coal producers. Meanwhile, the increasing use of wind power has driven electric prices negative at some points on those markets, making it harder for nukes to compete.

"Nuclear in many instances does need these states to step in and provide them some kind of financial remuneration for their environmental benefits," Vine said. The catch is "trying to come up with a way that uniformly values the desirable attributes we want, or taxes the aspects we don't want — like carbon."

In the South, where state regulators still oversee most utilities and what customers pay, nuclear plants don't face the same challenges. But the first companies to build nuclear power plants in decades have found themselves bogged down in delays that have driven costs up sharply and pushed back the expected start dates at Georgia's Plant Vogtle and the V.C. Sumner plant in South Carolina by three to four years.

Those woes also led to Toshiba's Feb. 14 announcement of $6.3 billion in losses from its U.S. nuclear projects, along with the ouster of its CEO and a decision to give up on building any new nuclear plants in America. That's going to raise "a big question mark" for other projects that power companies have been considering in the past decade, said Sara Barczak, director of the High Risk Energy Choices program at the Southern Alliance for Clean Energy.

"I don't know what would make a utility want to invest in nuclear at this point," said Barczak, whose organization is an outspoken critic of Plant Vogtle in particular. "I don't think shareholders would think that was the best idea."

Two reactors are under construction by a Georgia Power-led consortium at Plant Vogtle, near Augusta. Combined with the two existing units, Vogtle would become the largest U.S. nuclear plant when completed. But its start date has been pushed back from mid-2016 to late 2019, and the projected total costs have ballooned from $14 billion to about $20 billion, Barczak said.

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Kee said the reactors under construction at both Vogtle and Sumner are the first U.S. products of a new Westinghouse design that was supposed to be not only safer, but simpler and cheaper to build than past units. But Westinghouse had "severe" quality-control problems at a plant in Louisiana that was supposed to supply components for a series of nuclear projects.

"The concept was good, but implementing it in a world where we haven't built one in 30 years, and we're only building four of them, didn't work out so well," Kee said.

South Carolina's SCANA Energy, which has commissioned the two reactors at South Carolina's V.C. Sumner plant, says Toshiba and Westinghouse have committed to finishing that project. And Southern Company CEO Tom Fanning told analysts this week that his firm has the capability to step in and finish work at Plant Vogtle, where Southern subsidiary Georgia Power is the lead owner, if the builders can't. But Southern's chief financial officer, Arthur Beattie, said the company expects Toshiba will complete the job.

"We believe it's in their best interests from an economic perspective to complete these projects," Beattie said.