Leslie Dewan was only in sixth grade, building a mini water wheel in science class, when the last new U.S. nuclear reactor began producing power.

Now, at age 30, she’s the millennial face of next-generation nuclear. A slight look-alike to actress Amy Adams, this co-creator of a new molten salt reactor defies the stereotype of the middle-aged male nuclear scientist.

Much about the tech-savvy push to reboot nuclear power bucks tradition. Many U.S.-based startups with advanced reactor designs are backed by venture capitalists, not the U.S. government. Their impetus differs, too. Unlike American scientists of the 1950s and 19560s, who were locked in an atom-splitting Cold War race with the Soviet Union, they aim to combat climate change.

“I’m an environmentalist. I’m doing this, because I think nuclear power is the best way of producing large amounts of carbon-free electricity,” says Dewan, an MIT-trained nuclear engineer and National Geographic emerging explorer. She says the world needs nuclear—along with solar, wind, hydro, and geothermal—to cut heat-trapping emissions.

Not all environmentalists agree. In fact, nuclear power divides their ranks. Its radioactive waste and accidents—including the United States’ Three Mile Island in 1979, Ukraine’s Chernobyl in 1986 and Japan’s Fukushima Daiichi in 2011—have prompted many to argue nuclear is just too dangerous. Japan and Germany are paring back.

Yet many climate scientists say nuclear has to be part of the solution. Citing the dangers of global warming, they say there’s no realistic path forward without a “substantial role” for nuclear. Their public appeal calls for “a fresh approach to nuclear power in the 21st century.”

The next-gen designs are not your grandfather’s reactor. They promise to be safer, cheaper, and more efficient. They’re drawing venture capital from tech titans, including Microsoft co-founder Bill Gates, Amazon CEO Jeff Bezos, and former SpaceX employees.

Nearly 50 startups working in the U.S. and Canada alone have raised more than $1.3 billion in private investment, according to a “Back to the Future” report in December by Third Way, a centrist think tank. The funding continues to increase.

Some startups are small modular reactors that could be portable and prefabricated. Some boldly use nuclear fusion, long considered the holy grail for pollution-free energy. Others try innovative fuels and alternative coolants.

Here's how the new molten salt reactor would work.

How Feasible Is Molten Salt?

Dewan went with molten salt. She and fellow MIT alum Mark Massie, who previously worked at the Gates-backed nuclear startup TerraPower, have built the Waste-Annihilating Molten Salt Reactor. Their design, a modern-day variant of a tested but unused reactor developed half a century ago in a national laboratory, can run on nuclear waste.

They estimate that if all 270,000 metric tons of existing high-level nuclear waste went into their reactors, they could produce enough electricity to power the world for 72 years, even assuming projected increases in global energy demand.

How can this be? Conventional reactors, fueled by pellets of solid uranium oxide, use only 3 percent or 4 percent of uranium’s energy so the waste remains radioactive for hundreds of thousands of years. But because Dewan’s reactor uses uranium in a liquid rather than solid form, it extracts 96 percent of the energy.

View Images Leslie Dewan, a nuclear engineer and co-founder of Transatomic Power, has built a new molten salt nuclear reactor that reuses nuclear waste and automatically shuts down if there's a power outage. Photograph by Lynn Johnson, National Geographic

The liquid fuel has another advantage. If a molten salt reactor loses electric power (as happened at the Fukushima plant following the tsunami), the fuel automatically drains into an auxiliary tank and freezes solid in a few hours. Hence, no meltdown.

She and Massie launched their own company, Transatomic Power, to commercialize their molten salt technology. They’ve attracted $6 million in venture capital, including $2 million from Founders Fund—early investors in SpaceX who have this motto: “We wanted flying cars, instead we got 140 characters.”

Yet building and maintaining nuclear power plants, even smaller ones, is costly. Low natural gas prices have made some of today’s aging U.S. plants uneconomical, prompting the closure of at least four in the last two years. (Read more about these closures.)

“These are complex systems,” says Matthew McKinzie, director of the nuclear program at the Natural Resources Defense Council, an environmental group that prefers solar, wind and energy efficiency—rather than nuclear—as climate solutions.

“They look great on paper but could be difficult to realize in practice,” he says of the advanced reactors. “A lot of projects in the past have led to disappointment.” He says reactors that don’t use the light-water design common in today’s nuclear power plants will need prototypes for testing and their private funds aren’t enough to cover the cost.

In addition, “molten salt is corrosive and messy to work with,” says McKinzie, who holds a doctorate in experimental nuclear physics.

Next-gen nuclear is “absolutely doable,” says Russ Bell, senior director of new plant licensing for the Nuclear Energy Institute, an industry group. He says many new reactor designs are safe and “extremely innovative,” but since they need to be prototyped, it will take 20 to 25 years to bring them to market.

“That sounds like a long time, but in our business, things take a while,” Bell says. For example, it could take a decade or more for the five reactors currently under construction in the U.S.—the first ones since Watts Bar Unit 1 went online in Tennessee in 1996—to go from licensing to commercial operation.

Will China Take the Lead?

China might be faster. Some U.S.-based companies are looking to China, which is rapidly expanding its use of nuclear power as a way to curb the air pollution that’s choking its cities. China currently ranks fifth in nuclear capacity, but it plans to more than double that amount soon. By 2020, it could rank third—after the U.S., still the world’s leader, and France.

Bill Gates has visited China several times to seek its cooperation in developing a next-gen reactor. He chairs TerraPower, which has designed a traveling wave reactor that runs on depleted uranium and produces very little nuclear waste.

Dewan says her company wants to build its molten salt reactor first in the U.S., if at all possible. She says the U.S. Nuclear Regulatory Commission, which is looking at advanced reactors, remains the world’s gold standard for licensing.

“It’s American technology. I personally want the United States to get it first,” she says. Her company, Transatomic, plans five more years of experimental and design work before aiming to build a 20-megawatt prototype.

“We’ve been talking with the national labs about it,” she says, noting the Department of Energy has a new loan guarantee program for advanced nuclear reactors. “There’s really good buy-in from DOE for developing a wide range of technologies.”

Even if all goes well, Dewan says, it will take at least a decade to develop a commercial molten salt reactor. She’s optimistic it will happen and welcomes the work of other nuclear startups.

“It’s so cool how much new development is occurring,” says Dewan, the grown-up version of the sixth grader who managed to produce light by connecting a water wheel to a generator. “It makes me excited for the industry.”

The story is part of a special series that explores energy issues. For more, visit The Great Energy Challenge.