A few years out of college, Robbie Stewart knew he needed to make a career change. It wasn’t that he didn’t love his job working as a mechanical engineer for General Electric – he did. But he also knew, as someone who felt deeply about being a steward of the earth, that he wanted to be part of fighting climate change.

So he thought for a while about how he could best contribute. And then he decided to go into the nuclear industry.

“I saw nuclear as a huge opportunity for decarbonization,” he explains. “And this was where my skillset was.”

Mr. Stewart quit his job, entered a Massachusetts Institute of Technology nuclear science Ph.D. program, and eventually co-founded the company Boston Atomics, which is creating “design to build” nuclear power plants that can be constructed on a quicker schedule and at lower cost than traditional facilities.

“Our entire bet is that the value of low-carbon energy is only going to go up in the future,” he says. “That’s a big part of our value case as a startup.”

A slew of “advanced nuclear” startups have launched recently to bring new, nimble technology to the long-lumbering nuclear industry, often with the goal of reducing global warming. There are companies working to build smaller “micro” reactors the size of shipping containers, some looking to recycle radioactive waste, and some that use materials other than water to cool reactors, among other innovations.

Many of the entrepreneurs are young, and many consider themselves climate activists. They are part of a broader mindset change involving nuclear power overall, in which more environmentalists – although certainly not all – are shedding negative feelings about nuclear and instead embracing the technology.

“There has been a shift where the broader climate community has realized the benefit that nuclear can bring to meet our climate goals,” says Lindsey Walter, deputy director for climate and energy at Third Way, a center-left think tank in Washington. “The biggest issue that we’re trying to address is the climate crisis. And if you’re going to follow the science, if you’re going to follow the evidence, then it’s really quite clear that nuclear has to be part of the solution.”

On climate action, a rising focus on speed
In large part, supporters say, this is because renewable energy alone isn’t ready to speedily free the power grid from carbon. And speed is of the essence, scientists say, as human emissions of greenhouse gases are already altering Earth’s climate.

Wind and solar power are dependent on the weather, and at this point battery storage technology is not advanced enough to smooth the electricity grid’s complicated fluctuations in supply and demand.

A nuclear power plant, on the other hand, can produce energy constantly, without releasing climate warming gasses. And that means that it could provide a key component to a fossil-fuel-free energy system, supporters say.

“We have to radically reduce green gas emissions,” says Judi Greenwald, executive director of the Nuclear Innovation Alliance. “It’s not something like a 10% or 20%, or even a 50% reduction. We have to get as close as we can to zero. And that really makes you have to rethink the whole energy system.”

Still, concerns about nuclear power remain widespread.

Traditionally, environmentalists have considered it dangerous. Accidents at Three Mile Island, Chernobyl, and more recently Fukushima have reinforced this concern – along with lingering questions about what to do with radioactive waste, the environmental and ethical implications of uranium mining, and the risk of nuclear material being used for weapons.

“Nuclear energy is not safe, it’s not economical, and it increases the risk for nuclear weapons proliferation,” says Shaun Burnie, a senior nuclear specialist with Greenpeace Germany who has been working on energy systems for 30 years. “We are in a climate emergency, and we’re wasting our time, and a lot of column inches, talking about a technology that’s essentially irrelevant.”

Beyond the ethical and environmental concerns, he argues that the impracticality of nuclear power should be enough to make people wary of the technology. Traditional nuclear power plants are hugely expensive to build, and they take years, if not decades, to actually come online. In the United States, which currently gets about 20% of its electricity – and about half of its carbon-free electricity – from nuclear, some 21 reactors are in the process of being decommissioned, according to the U.S. Nuclear Regulatory Commission.

“It’s not possible for the nuclear industry to play the essential role for decarbonization over the next decades,” Mr. Burnie says. “It’s a myth.”

But that has not stopped officials worldwide from turning toward it as a possible climate solution.

A “green investment” or not?
Recently, a European Union proposal to classify some nuclear power plants as “green investments” drew both praise and debate. Germany, which has pledged to shutter the last of its nuclear plants by the end of this year, has argued that any process that leaves permanent radioactive waste can’t be labeled “sustainable.” But France, an atomic energy leader, lobbied in favor of the designation.

In the U.S., the bipartisan infrastructure bill passed last year included money to prevent the closing of nuclear power plants, since nuclear power that goes offline is largely replaced with fossil fuels. The bill also has money to support nuclear innovation.

It’s that innovation that is key for many climate activists, says Jessica Lovering, co-founder of the Good Energy Collective, a group that tries to make a progressive political case for nuclear power. Whether through small startups such as Mr. Stewart’s Boston Atomics, or Bill Gates’ company TerraPower, which last year announced plans to build a “next generation” demonstration reactor on the site of a retiring Wyoming coal mine, the climate potential of new nuclear technology offsets old environmental concerns, she says.

“Whatever gets us there”
“A lot of the bigger traditional incumbent environmental groups, founded in the ’60s and ’70s, spun out of anti-war movements, anti-establishment movements,” she says. “They come with a lot of baggage against nuclear. ...

What we’re seeing with these newer climate groups is that they are really pushing for an aggressive standard on decarbonization. ... There’s this pragmatic viewpoint of, ‘well, we’re just trying to reduce emissions as fast as possible as much as possible. So whatever gets us there.’”

From a progressive point of view, Dr. Lovering says, it is also important that the ownership models of nuclear power plants are changing. For years, she says, nuclear power was the purview of large utilities, often with connections to the military. Now, there are more models that give communities more input, with scaled-down footprints. And even in its traditional form, nuclear takes up less land than large solar or wind farms, which can come with their own environmental challenges. While she acknowledges that advanced nuclear is still in the future, it is closer than many people realize, she says, with a number of demonstration technologies launching in the next decade.

And just because the technology doesn’t exist now, she and other supporters say, it doesn’t mean we shouldn’t be working for a future solution. That’s why many scientists are still excited about the long-postponed dream of nuclear fusion as a potentially abundant and clean source of electric power.

In addition to electricity generation, some advanced nuclear technologies – including Boston Atomics – would be able to produce the extreme heat for industrial processes such as chemical production or steelmaking. That emissions-heavy, and varied, industrial sector is widely considered one of the hardest to address as far as greenhouse gasses.

“There’s embodied carbon in everything that we manufacture,” Dr. Lovering says. “I actually was not a supporter of nuclear until I learned more. ... You begin to realize, oh, shoot, we really do need this technology if we’re going to meet these [net zero] goals. I think a lot of others in the climate community that are supportive of nuclear came about it in the same way.”

She concludes: “I’m not for nuclear for the sake of nuclear. I’m purely for nuclear for the sake of climate.”