Later this year, a nuclear power reactor will open in the US for the first time in two decades. But this reactor, called Watts Bar Unit 2—one of two near Spring City, Tennessee—isn't quite new. Most of it was built in the 1970s and '80s alongside Unit 1, which came online in 1996 and has performed flawlessly. The two reactors are essentially identical in terms of safety, technology, and output. But there's been one major advance in the 20 years separating their openings: widespread acceptance of fossil fuels' role in climate change, and the urgent need to wean the economy from it.

In the years that Watts Bar 2 lay fallow, policymakers and climate strategists have struggled to figure out what the future of renewable energy will look like. They have three options: find a way of cleaning up coal, build batteries capable of storing energy from capricious renewables, or go nuclear. Each has benefits and drawbacks. But nuclear is a strong contender because it is the only technology that actually exists. The Watts Bar reactors will provide power to 1.5 million households, and their only greenhouse gas emissions will come from the cars employees use to commute.

That's a good deal, but still. Show a crowd a pair of cooling towers, and at least some of them will see an atomic apocalypse featuring three-eyed fish, leafless forests, and hospital-gowned Soviet defectors with skin like glistening mayonnaise. Nuclear power may be clean, but people still question whether it is, or ever will be, safe enough.

Those fears may be moot. Safety concerns didn't delay construction on Watts Bar Unit 2 for so many years. Economics did. For all that fear, nuclear power still has the safest track record of any power source.

The Danger

Nuclear energy sources are dangerous because they emit radiation—particles and energy shed from unstable molecules trying to calm down. "Those radioactive missiles can hit the human body and damage cells or DNA," says David Lochbaum, director of the Union of Concerned Scientist's nuclear safety project. Enough radiation will give you cancer, or possibly even pass genetic mutations on to your kids. Too much can kill you outright.

But plants like Watts Bar don't release much radiation into the environment. Inside, radioactive material heats water, which turns into steam, which spins the enormous turbines that generate electricity. Plants regularly release some of that water and steam at rates prescribed by the US Nuclear Regulatory Commission, and if you live downriver or downwind of one, the radiation within will raise your chances of developing a tumor by just one tenth of one percent. You're far more likely to grow a tumor because you sneak a cigarette now and again.

But you aren't afraid of routine releases. You're terrified of another Three Mile Island, Fukushima, or Chernobyl.

These disasters were the result of a meltdown, which occurs when something impedes a reactor's ability to cool the fuel. The US, where nearly 20 percent of electricity comes from 99 nuclear plants, uses uranium. Older reactors—which is every reactor in the US, including Watts Bar Unit 2—use electric pumps to move water through the system. The Fukushima disaster showed what happens it you have pumps but no power to use them. Newer generations rely on gravity instead, draining cooling water from elevated storage tanks to send it through the reactor core.

Those updates mean serious nuclear accidents are becoming ever more rare. Since Three Mile Island in 1979, the Nuclear Regulatory Commission found that the rate of shut-down-the-reactor-level problems has dropped from 2.5 per plant per year to around 0.1 (One such happened on March 29 in Washington). Even Three Mile Island wasn't the disaster it could have been, because of that plant's layers of redundant protection.

In terms of full blown nuclear disaster, there is really only one data point: Chernobyl. Which was horrifying. But in terms of real risk? The World Health Organization estimates the disaster will claim 4,000 lives, a figure that includes everything from direct victims to people born with genetic mutations well after the meltdown in 1986. By comparison, particulate matter from coal power plants kills about 7,500 people in the US every year. Radiation is the shark attack of environmental danger: An awful way to go, but far less likely than, say, a car wreck.

Spent fuel—about a third of the uranium in a reactor's core is replaced every two years—is a bigger concern, because the US nuclear industry doesn't have anywhere to dispose of it. Used rods sit in cooling tanks for five years, until they're cold enough to encase in dry casks. But that fuel isn't harmful unless you fall into the water (hello super powers! Actually, probably just radiation poisoning). Or the plumbing fails. Spent rods stashed away in dry casks are even less worrisome, because the containers would need to be breached enough to let air get in and cause a combustion.

The only people with a truly viable argument against nuclear energy are the people who mine the fuel. "Uranium miners seem to be the ones who have the body count you can point to," says Lochbaum. Between 1950 and 2000, the US government estimates the rate of lung cancer in uranium miners was six times higher than in the general population.

The Cost

So nuclear energy, not very dangerous. Three cheers for concrete, plumbing, and preventative maintenance! Now pipe down, and listen to the irony: The nuclear industry is safe because every plant consumes billions of dollars in permitting, inspections, materials, and specialized construction decades before producing its first jolt of current. And those costs are exactly what keep this safe, sustainable energy source from really happening.

Watts Bar Units 1 and 2 were supposed to open simultaneously. But in the years since construction began in 1973, energy demand in the region had dropped. Both reactors simply cost too much to finish, so they were mothballed in 1988. Energy demand perked up enough to justify finishing Unit 1 in 1996. The only reason its owner, the Tennessee Valley Authority, voted in 2007 to resume construction is because the agency's pencil pushers were able to convince its board and shareholders that the regional economy would grow enough in the coming years to create sufficient demand.

That is probably the biggest risk with nuclear: It takes so long to see a return on investment, if one comes at all. Imagine you start building a nuclear power plant today. If, at some point in the next two decades, some hardworking genius builds a battery capable of storing wind or solar energy, scrubs the carbon out of coal emissions, or plugs the methane leaking from natural gas, the odds of there being a market for your expensive atomic energy by the time you finish construction is pretty slim.

"What we have seen from the last seven years is a number of old plants being shut down well before they are required simply because they are not able to compete on the electrical market," says M.V. Ramana, a physicist at the Nuclear Futures Laboratory at Princeton University.

The only reason that nuclear energy powers 80 percent (and falling) of France, and powered 30 percent of pre-Fukushima Japan, is because those countries don't have the wealth of natural resources the US has. And that's not just coal and natural gas (though it is mostly coal and natural gas). US solar, wind, geothermal, and hydroelectric utilities are growing fast, and getting cheaper. Currently, renewables generate over 13 percent of US energy.

Even people within the nuclear industry think it is an impractical choice. "You can make a pretty strong argument that it’s really foolish to burn a resource that's as special as nuclear energy making something as inexpensive and ubiquitous as electricity," says Arthur Ruggles, a professor of nuclear engineering at the University of Tennessee. By becoming more efficient and scaling up renewables, society could save the uranium for cool stuff like powering interplanetary spaceships.

And space ships might be necessary sooner than you'd expect, if society can't find a climate change solution it can agree on.