Back in 2009, Simon Irish, an investment manager in New York, found the kind of opportunity that he thought could transform the world while — in the process — transforming dollars into riches.

Irish saw that countries around the globe needed to build a boggling amount of clean-power projects to replace their fossil fuel infrastructure, while also providing enough energy for rising demand from China, India, and other rapidly growing countries. He realized that it would be very hard for renewables, which depend on the wind blowing and the sun shining, to do everything. And he knew that nuclear power, the only existing form of clean energy that could fill the gaps, was too expensive to compete with oil and gas.

But then, at a conference in 2011, he met an engineer with an innovative design for a nuclear reactor cooled by molten salt. If it worked, Irish figured, it could not only solve the problems with aging nuclear power, but also provide a realistic path to dropping fossil fuels.

“The question was, ‘Can we do better than the conventional reactors that were commercialized 60 years ago?” Irish recalled. “And the answer was, ‘Absolutely.’”

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Irish was so convinced that this new reactor was a great investment that he bet his career on it. Nearly a decade later, Irish is the CEO of New York City-based Terrestrial Energy, a company that expects to have a molten-salt reactor online before 2030.

Terrestrial is far from alone. Dozens of nuclear startups are popping up around the country, aiming to solve the well-known problems with nuclear power — radioactive waste, meltdowns, weapons proliferation, and high costs.

There are reactors that burn nuclear waste. There are reactors designed to destroy isotopes that could be made into weapons. There are small reactors that could be built inexpensively in factories. So many ideas!

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To former Secretary of Energy Ernest Moniz, an advisor to Terrestrial, it feels as if something new is underway. “I have never seen this kind of innovation in the sector,” he said. “It’s really exciting.”

Other reactors, like Terrestrial’s molten-salt-cooled design, automatically cool down if they get too hot. Water flows through conventional reactors to keep them from overheating, but if something halts this flow — like the earthquake and tsunami in Fukushima — the water boils off, leaving nothing to stop a meltdown.

Unlike water, salt wouldn’t boil off, so even if operators switched off safety systems and walked away, the salts would keep cooling the system, Irish said. Salts heat up and expand, pushing uranium atoms apart and slowing down the reaction (the farther apart the uranium atoms, the less likely a flying neutron will split them apart, triggering the next link in the chain reaction).

“It’s like your pot on the stove when you are boiling pasta,” Irish said. No matter how hot your stove, your pasta will never get hotter than 212 degrees Fahrenheit unless the water boils off. Until it’s gone, the water is just circulating and dissipating heat. When you replace water with liquid salt, however, you have to get to 2,500 degrees Fahrenheit before your coolant starts to evaporate.

This stuff can sound like science fiction — but it’s real. Russia has been producing electricity from an advanced reactor that burns up radioactive waste since 2016. China has built a “pebble bed” reactor that keeps radioactive elements locked inside cue ball-sized graphite spheres.

In 2015, to keep track of the startups and public-sector projects working on trying to provide low-carbon energy with safer, cheaper, and cleaner nuclear power, the centrist think tank, Third Way, started mapping all of the advanced nuke projects across the country. There were 48 dots on the first map, and now there are 75, spreading like a candy-colored case of measles.

“In terms of the number of projects, the number of people working on it, and the amount of private financing, there isn’t anything to compare it to unless you go back to the 1960s,” said Ryan Fitzpatrick who works on clean energy for Third Way.

Back then, just after Walt Disney released the film “Our Friend the Atom” promoting nuclear energy, when the futuristic notion of electricity “too cheap to meter” seemed plausible, electric utilities had plans to build hundreds of reactors across the United States.

Found Image Holdings / Corbis via Getty Images

Why is this all happening now? After all, scientists have been working on these alternative types of reactors since the beginning of the Cold War, yet they’ve never caught on. The history of advanced reactors is littered with the carcasses of failed attempts. A salt-cooled reactor first ran successfully back in 1954, but the United States opted to specialize in water-cooled reactors and defunded other designs.

But something fundamental has changed: Previously, there was no reason for a nuclear company to pony up the billion dollars needed to get a new design through the federal regulatory process because conventional reactors were profitable. That’s not true anymore.

“For the first time in half a century, the incumbent nuclear players are in financial distress,” Irish said.

Recently, the United States’ bet on conventional water-cooled reactors has been going bad in very expensive ways. In 2012, South Carolina Electric & Gas got permission to build two huge conventional reactors to generate 2,200 megawatts, enough to power 1.8 million homes, promising to have them up and running sometime in 2018. Electricity users saw their bills jump 18 percent to pay for the construction, which soon ran into delays. Last year, after sinking $9 billion into the project, the utility gave up.

“The most recent builds in the United States have been a disaster, largely due to poor on-sight construction practices,” said John Parsons, codirector of MIT’s Low-Carbon Energy Center for Advanced Nuclear Energy Systems.

Similar stories have played out abroad. In Finland, construction of a new reactor at the Olkiluoto power plant is eight years behind schedule and $6.5 billion over budget.

In response, these nuclear startups are designing their businesses to avoid horrible cost overruns. Many have plans to build standardized reactor parts in a factory, then put them together like Legos at the construction site. “If you can move construction to the factory you can drive costs down significantly,” Parsons said.

New reactors could also reduce costs by being safer. Conventional reactors have a fundamental risk of meltdown, largely because they were designed to power submarines. It’s easy to cool a reactor with water when it’s in a submarine, underwater, but when we lifted these reactors onto land, we had to start pumping water up to cool them, Irish explained. “That pumping system can never, ever break, or you get a Fukushima. You need safety system on top of safety system, redundancy on top of redundancy.”

Oklo, a Silicon Valley startup, based its reactor design on a prototype that isn’t susceptible to meltdowns. “When engineers shut off all the cooling systems, it cooled itself and then started back up and was running normally later that day,” said Caroline Cochran, Oklo’s cofounder. If these safer reactors don’t require all those backup cooling systems and concrete containment domes, companies can build plants for much less money.

Technologies often fail for a long time before succeeding: 45 years of tinkering passed between the first electric light and Thomas Edison’s patent for an incandescent bulb. It can take decades for the engineering to catch up to the idea. Others have tried seemingly every idea for advanced nuclear in the past, Parsons said. “But science has moved forward,” he said. “You have much better materials than you did a few decades ago. That makes it believable these things could work.”

A recent study from the nonprofit Energy Innovation Reform Project estimated that the latest batch of nuclear startups could deliver electricity somewhere between $36 and $90 a megawatt hour. That’s competitive with any power plant that runs on natural gas (which runs between $42 to $78), and would provide a viable alternative to fossil fuels.

In a best-case scenario, nuclear power could be even cheaper. There are projections a study like this can make based on, say, an improved design that cuts construction costs, but it can’t anticipate revolutionary advances.

“Hopefully these designers will come up with much more radical reductions in cost — you would like energy to be more accessible to a billion more people — so that nuclear becomes a cheap alternative that can beat natural gas even if there’s no carbon price,” Parsons said. “That’s just a hope, but that’s what entrepreneurs are supposed to do.”

Matthew Bunn, a nuclear expert at Harvard, said that if nuclear power is going to play a role in fighting climate change, these advanced nuclear companies will have to scale up insanely fast. “To supply a tenth of the clean energy we need by 2050, we have to add 30 gigawatts to the grid every year,” he said.

That means the world would have to build 10 times as much nuclear power as it was before the Fukushima disaster in 2011. Is that even realistic?

“I think we ought to be trying — I’m not optimistic,” Bunn said, noting that the pace at which we’d need to build solar and wind to quit fossil fuels is just as daunting.

Big barriers remain in the way of a nuclear renaissance. It takes years to test prototypes and get approval from federal regulators before a company can even start construction. “In order for advanced nuclear technologies to play a role in deep decarbonization over the next several decades,” the United States would need to overhaul the way it’s rolling out the technology, according to a study published earlier this month in the Proceedings of the National Academy of Sciences.

Experts point to many of the same steps to give advanced nuclear a fighting chance: Making regulations more friendly to innovation, instead of favoring conventional reactors. Creating incentives to reward utilities for buying low-carbon power. And a lot more funding.

A rendering of an advanced nuclear-gas hybrid reactor. Hybrid Power Technologies LLC

The people behind the new crop of nuclear companies think they can get to market much faster with the right help. Oklo is shooting to have a commercial reactor online before 2025.

“Can we decarbonize quickly with nuclear? France did it, it can be done,” Cochran from Oklo said. “Our reactors are 500 times smaller than the [latest conventional reactors], they have all these inherent safety characteristics, and they can consume nuclear waste. Will our application process be any shorter?”

Lowering these barriers would be cheaper than letting the government pick one promising idea and coddle it like a privileged child, which is the way we’ve treated conventional nuclear in the past, said Jessica Lovering, who studies nuclear power at the Breakthrough Institute, a pro-technology environmental think tank.

“We could pick one idea, spend a lot of money helping it become commercial, and then subsidize every project for even more money,” Lovering said. “Or, we could invest a much smaller amount of money across the entire innovation system.”

Still, it could easily take the advanced nuclear projects 30 years to get through regulatory review, fix the unexpected problems that crop up along the way, and prove that they can compete, said Dan Kammen, who studies clean energy at the University of California Berkeley. And by then Kammen thinks there will be other options in competition: Electric storage is getting better, and fusion could have a breakthrough.

“Ultimately on a planet with 10 billion people, some amount of large, convenient, affordable, safe baseload power — like we get from nuclear fission, or fusion — would be just hugely beneficial,” Kammen said. “There are other competitors in view on the straight solar side that 10 years ago sounded like science fiction — space-based solar, transparent solar films on every window. That world works, too.”

At this point in history, everything is a longshot. We’ve got to completely replace our energy system on the fly. To do that, people are planting a lot of different seeds. It’s still a long time until harvest, but we’re seeing a flush of new sprouts from the advanced nuclear section of the garden.

This new flush of nuclear possibility has excited young people who see nuclear as a way to shift away from fossil fuels. College students are gravitating toward nuclear engineering. The number of students studying the subject cratered when the nuclear industry collapsed in the late 1970s (the Three Mile Island accident in 1979 didn’t help), but it has been creeping steadily higher since the early 2000s.

The number of degrees earned in nuclear engineering since 1966. Oak Ridge Institute and Science and Education

Some of those students are going on to start their own advanced nuclear companies. David Schumacher, a documentary filmmaker, met some of these young people and became so infected with their enthusiasm that he made a documentary about them, The New Fire, which came out last year.

“They are truly idealistic young people trying to save the planet by doing something really important but really unpopular,” Schumacher said. “They could be making a lot of money elsewhere, but instead they are starting these nuclear companies, knowing they are going to be maligned.”

It’s a feeling Simon Irish, at Terrestrial Energy, is familiar with. “The views on nuclear are so negative,” he said. “The great win is simply to persuade busy people to listen.”

While Terrestrial battles public opinion, Irish said his company has been hitting every milestone on time. Canadian regulators announced last year that Terrestrial had completed the initial stage of its design review — the first step toward approval in that country. Irish has already selected sites in Ontario where Terrestrial could build the first reactors.

Although Irish was mum on Terrestrial’s other milestones, he did describe an experience that he said gives him more confidence in the company’s prospects than any of its other accomplishments so far.

Last August, he found himself in the office of a prominent New York investor, a major contributor to environmental organizations. Getting the meeting had been a challenge — again because of the controversy around nuclear. But by the end, Irish had convinced the businessman that renewables and nuclear could not just coexist but compliment each other.

In Irish’s telling, he was in the middle of explaining Terrestrial’s reactor design when the man stopped him and said, “‘Hold on, this can deliver heat! The industrial sector needs heat, and wind and solar aren’t making any dent in that at all.’

“As far as he was concerned,” Irish said, “this was the great missing piece.”