By summer’s end, authorities expect the new reactor at this complex along the Chickamauga Reservoir, a dammed section of the Tennessee River extending northward from Chattanooga, to steadily generate enough electricity to power 650,000 homes. Although the opening of a new nuclear facility used to draw protesters and angry rhetoric, the Tennessee Valley Authority’s Watts Bar reactor has been mostly welcomed by local residents — and even some advocates concerned about climate change.

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“It’s a big step forward for clean energy, and we really have to be pushing that as hard as we can for the sake of the climate – all sources of clean energy, which includes nuclear,” said MIT atmospheric scientist Kerry Emanuel.

He and a group of influential climate scientists, led by former NASA researcher James Hansen, have recently made a strong push for nuclear, arguing that the energy source “will make the difference between the world missing crucial climate targets or achieving them.”

But while nuclear reactors account for the lion’s share of the carbon-free electricity generated in the United States, the industry faces this new set of circumstances in a state of near-crisis. A combination of very cheap natural gas and deregulated energy markets in some states has led to a growing number of plant closures in recent years.

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Even as Watts Bar engineers and planners busily tested their new reactor, Exelon, the nation’s biggest utility for nuclear, with 23 reactors, announced that it would be closing two plants in Illinois, citing financial losses and the state’s failure to pass energy legislation that would help support nuclear plants.

“We are supposed to be adding zero-carbon sources, not subtracting, or simply replacing, to just kind of tread water,” said U.S. Energy Secretary Ernest Moniz recently — before the Exelon news drove the point further home.

The turn for the industry could represent bad news for U.S. carbon emissions: As more plants shut down, and with wind and solar not yet able to offset the electricity-generating capacity of nuclear, emissions could actually increase in certain regions.

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Yet even if the country decided tomorrow to recommit to nuclear power plants in the name of climate change, it would still take many years to build more of them. They also would be difficult to finance in many electricity markets. Watts Bar 2, the plant’s second reactor, is nothing if not a symbol of the travails involved in getting massive nuclear plants running — it was originally permitted in the 1970s, but construction halted in 1985.

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That matters because the extent to which adding nuclear energy helps battle climate change depends not only on the nature of the electricity generation itself but also on the time frame. To not miss international targets, which seek to keep global warming below 2 degrees or even 1.5 degrees Celsius above late-19th-century levels, emissions cuts have to happen fast. But as Watts Bar itself demonstrates, new nuclear can take a long time to build.

“Nuclear cannot provide a short-term solution to climate change because it takes so long to bring new plants online,” said Allison Macfarlane, a professor at George Washington University and a former chair of the Nuclear Regulatory Commission.

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Here in rural Rhea County — home to just more than 32,000 people, and where the Tennessee Valley Authority also controls the river itself with the Watts Bar dam — adding new nuclear seems a fairly uncontentious affair. In the cavernous room holding the Watts Bar plant’s twin turbines, which are driven by steam generated in the two nuclear reactors, an almost equally vast American flag hangs above a wall of windows.

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The siting here of the country’s first new reactor in decades is no doubt in part because of the unique nature of the TVA, a New Deal-era government-controlled corporation with a vast base of municipal utilities and other large customers that buy its power.

“At a time when other regions of the country are relying on less-reliable sources of energy, our region is fortunate that TVA opened the last nuclear reactor of the 20th century and is opening the first reactor of the 21st century,” said Sen. Lamar Alexander (R-Tenn.), a major supporter of nuclear energy.

While hardly the country’s biggest nuclear installment in terms of sheer capacity to generate electricity, Watts Bar is no less overpowering. As you approach the site along two-lane highways, the plant’s two cooling towers — each more than a football field and a half in height — surge out of the landscape.

On a recent Tuesday, only one tower — Watts Bar 1, the one licensed in 1996 — could be seen wafting water vapor into the atmosphere. But that should change when Watts Bar 2 starts full operations.

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Even then, the plant will represent a rather halting step into new nuclear in the United States. Original planning for this $ 4.7 billion unit occurred many decades ago, and rather than presenting any major technological departure, it uses a design quite consistent with the current U.S. fleet of 99 reactors. “Novel’s not typically the kind of word that you look for in nuclear,” said Joe Grimes, the TVA’s chief nuclear officer.

In contrast, two new nuclear units being built by the Southern Company, in Georgia, and two more under construction in South Carolina, will employ a new, more-streamlined technology called an “advanced pressurized water reactor,” which is thought to be an even greater safety improvement over older technologies.

Watts Bar has made safety strides in another way — it was the first U.S. plant to fully comply with the Nuclear Regulatory Commissions’s new orders after the 2011 Fukushima Daiichi nuclear disaster in Japan. The regulations require more backup generation and other safety features on site in case of an unexpected catastrophe.

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Yet that has not stopped critics from faulting aspects of the plant’s layout, including the use of an “ice condenser” design around the reactor — amounting to buckets upon buckets of ice hanging in a chamber, hopefully to provide an added layer of defense in case of a major reactor problem.

In the environmental community, though, reaction to the new reactor appears fairly muted. “From a safety standpoint, which is our focus, the plant seems in good condition,” said David Lochbaum, head of the Nuclear Safety Project at the Union of Concerned Scientists. Lochbaum also praised the TVA’s openness in its engagement with the public over the new reactor, a process in which he participated.

“Watts Bar 2 is going to be one of the last nuclear power plants built in the United States,” added Jonathan Levenshus, a representative of the Sierra Club’s Beyond Coal campaign in Tennessee. “Right now, wind power and solar energy are so much cheaper than new nuclear plants.” But Levenshus did not critique Watts Bar 2 in particular on safety or other grounds; he was simply pessimistic about the future of nuclear power in general.

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When Watts Bar 2 comes fully online, Grimes said, the TVA will move from a portfolio that is 34 percent powered by nuclear energy to one close to 40 percent — a change sure to drive down greenhouse-gas emissions substantially.

TVA spokesman Scott Brooks said it is not clear precisely how much the new reactor will help Tennessee comply with the Obama administration’s Clean Power Plan (assuming that plan survives court challenges). But it’s notable that Tennessee is missing from the long list of states suing to oppose the plan.

Scientists and other advocates worried about climate change have been sounding the alarm about nuclear energy’s decline.

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“The tragedy is that we have subtracted more clean energy in the last two years by premature nuclear-plant closing, than we have added from all of the solar that has gone online,” said Emanuel of MIT.

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While some environmentalists welcome nuclear, though, an increasingly influential environmental left is insisting on a completely fossil- and nuclear-free future that is instead based on wind, solar, water and batteries. Democratic presidential contender Sen. Bernie Sanders’s climate-change plan, for instance, calls for a “completely nuclear-free clean energy system for electricity.”

It all makes for some pretty odd politics. Moniz recently noted, for instance, that after the closure of the Vermont Yankee nuclear plant in 2014 and more burning of natural gas, the New England region saw a rise in carbon-dioxide-gas emissions. “That’s the kind of confluence of data that we would like to avoid more of,” Moniz said. And now, nuclear’s defenders are predicting the same outcome when Exelon closes the two Illinois plants.

The globe presents, in many, ways a similar story. The “nuclear renaissance” is well underway in China, where more than 20 nuclear plants are under construction, according to the World Nuclear Association. Overall, more than 60 plants are under construction globally.

But this is offset by the reality that other regions, such as Germany, will see more plant closures in the future. Germany has already scrapped 9 of its 17 reactors in the wake of the Fukushima catastrophe and plans to close eight more by 2022. Japan’s fleet of nearly 50 nuclear plants, too, remains almost entirely offline in the wake of the Fukushima disaster, with only two plants having restarted.

Nuclear’s defenders, such as the TVA’s Grimes and Moniz, assert that despite the clear benefit of nuclear energy to the climate system, the energy source is not being valued properly in energy markets. One reason is because there is no price on carbon — a policy that would make natural gas and coal more expensive to burn.

“I believe that short-term decision-making and short-term pricing decisions around nuclear are very unfortunate in a long term for clean energy in this country,” Grimes said.

“The nuclear renaissance never happened, and it doesn’t look like it’s going to happen unless something changes,” Macfarlane added. “Were there a price on carbon, things may change significantly. But there isn’t a price on carbon, and it doesn’t look like we’re going to get one.”

Defenders further point out that nuclear generation — while hardly without risk — compares favorably with coal and gas not only from a climate perspective but also when it comes to air pollution, and thus public health. Hansen, for instance, has published data suggesting that in a world without nuclear power, further reliance on coal and natural gas for electricity would have resulted in more than a million added deaths because of air pollution.

Nonetheless, there’s little doubt that lingering fears of radioactive contamination, in the end, drive continuing resistance to nuclear — even if in the Tennessee Valley, people may not be so concerned.

“What we’re up against is the fact that our brains are wired to perceive risks in a way that suited us as very primitive people and is completely irrational today,” said MIT’s Emanuel. “The same person who is scared to fly in an airplane will drive across the country in a Volkswagen.”

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