Republicans and many Democrats have high hopes for small nuclear reactors as an answer to climate change, but the nascent technology faces daunting regulatory and economic obstacles.

"I am not optimistic that any kind of nuclear reactors will have any significant impact on climate change for the next 20 or 30 years," said Allison Macfarlane, a former chairman of the Nuclear Regulatory Commission who is now a professor at George Washington University. "It's really hard to make the economics of nuclear power work."

The Trump administration is betting on nuclear, even as it opposes most government-led initiatives to curb climate change. It is investing money into the research and development of new reactors and fuels, promoting faster permitting approvals, and even signing up to buy power from NuScale, the company racing to be the first to operate a small modular nuclear reactor.

"I am very optimistic this can be a low-carbon tool moving forward," Rita Baranwal, the Energy Department's assistant secretary of energy for nuclear energy, told the Washington Examiner in an interview. "It is essential to any country or community looking to minimize their carbon footprint."

While many environmentalists have been skeptical of nuclear energy on safety and cost grounds, some Democrats embrace nuclear energy, which provides most of America's zero-carbon power.

Presidential candidates Joe Biden and Andrew Yang are running on investment in small modular nuclear reactors. Bernie Sanders, by contrast, has said he would look to shut down existing nuclear plants, along with stopping new ones.

"Bernie Sanders is the only candidate who doesn't seem to have updated his positions to reflect the realities of climate change," said Josh Freed, who runs the clean energy program at the center-left think tank Third Way. "What we are seeing is the equivalent of updating nuclear for the era of iPhones."

The United States can't reach net-zero greenhouse gas emissions by midcentury without advancements in nuclear energy, energy analysts say, since wind and solar cannot deliver 24-hour electricity.

"There is no chance of meeting a net-zero goal without everybody on deck, including advanced nuclear," Ernst Moniz, who was energy secretary during the Obama administration, said at the U.S. Chamber of Commerce's Energy Innovation Summit last year.

In the early 2010s, the Obama administration aimed to facilitate the construction of as many as 50 small modular reactors a year by 2040 or sooner.

The 1979 accident at the Three Mile Island nuclear plant in Pennsylvania, along with the 1986 Chernobyl disaster, had turned public sentiment against the large nuclear plants first built in the 1950s. Only one new reactor has been completed in the last three decades in the U.S., with recent projects canceled or delayed because of mounting expenses.

But a growing awareness of climate change, and a decline of the existing nuclear fleet due to competition from natural gas and renewables, sparked a wave of newer, less established companies to conceive of creating smaller reactors that would be easier and cheaper to build, safer to run, and more flexible to use.

Obama energy secretary Steven Chu announced a partnership with Babcock & Wilcox in 2012 to split costs on a small modular reactor, saying that the reactors would be operating by 2022.

That vision has not proceeded as planned. Babcock & Wilcox later withdrew its application due to high costs.

"That's when I became a little more skeptical. What's wrong? This is not working," Macfarlane said.

NuScale, one of the original new companies to crop up, is still standing.

The Energy Department has invested more than $300 million into the Oregon-based technology firm since 2014. Chief Strategy Officer Chris Colbert said NuScale's survival shows the government's investment has been worth it.

It is hoping by 2020 to be the first company to obtain a license to operate a small reactor in the U.S. and expects to have its reactors in commercial use by 2026.

"Every year, it's been a challenge of making the case that the money being allocated to us is being wisely spent," Colbert told the Washington Examiner. "We have demonstrated through our success in the NRC licensing process that it's been a wise investment."

NuScale has lined up its first customer, Utah Associated Municipal Power Systems, a group of utilities in six Western states that are purchasing power from 12 individual 60-megawatt reactors being built at the Energy Department's Idaho National Laboratory. The Energy Department plans to buy power from two of the reactors.

NuScale is further ahead in the permitting process than other companies because it is designing light-water reactors, the most common type used in traditional plants, meaning regulators are accustomed to it.

Its smaller, 75-foot-tall reactors are about one-third the size of traditional reactors. They sit in huge underground pools of water, using it as a coolant. About 12 of them can be installed at one site.

Other types of advanced reactors have faced setbacks. Megabillionaire Bill Gates, who co-founded the nuclear technology firm TerraPower, recently warned that the obstacles are "daunting."

TerraPower, which employs a traveling-wave reactor that uses depleted uranium as fuel, had to scrap its first demonstration project in China because of the Trump administration's trade policies.

Even if NuScale's reactors begin operating on schedule, some nuclear experts doubt they would be widely adopted.

While the smaller reactors have lower capital costs, they produce less electricity than a traditional reactor, meaning they don't enjoy the same economy of scale.

New nuclear reactors could have a hard time competing in power markets with cheap gas and renewables, a disadvantage that could be counteracted if the federal government penalized carbon pollution, which would make emissions-free nuclear power relatively more attractive.

"The most important thing for advanced nuclear as for any other zero-carbon technology is to have a set of policies that acknowledge climate change and help drive demand," said Freed.

Colbert of Nuscale countered that the company could compete in states that require utilities to procure power from lower-carbon sources (more than half of states do). NuScale has also signed agreements with Canada, Romania, Jordan, and other countries that are considering deploying the company's small nuclear reactors once they are proven in the U.S.

"We don't expect to sell small modular actors in every single market," Colbert said. "To be successful, we don't have to."

Some climate hawks argue the money invested in nuclear energy would be better spent expanding renewable energy sources, which they say can replace nuclear power.

"There is an argument of lost opportunities," said Matthew McKinzie, the director of the nuclear program at the Natural Resources Defense Council.

Supporters, however, note that companies are exploring different types of small reactors to spread out the risk and increase the chance of success.

The Energy Department, with the backing of Congress, is supporting the development of nonwater advanced reactors, such as liquid metal-cooled fast reactors, that would use sodium or lead as a coolant instead of water, operating at a higher temperature and enabling them to provide power in other applications, such as heating for industrial purposes.

Congress's 2019 end-of-year spending bill included $300 million to create a first-ever advanced nuclear reactor demonstration program to spur cost-sharing projects with private companies.

There are currently 25 different advanced reactors proposed by U.S.-based companies in various stages of development, according to the Energy Department.

The agency is also helping to develop different types of fuels to power advanced reactors to address a shortage of domestic supply.

And the Trump administration, at the direction of Congress, has directed the Nuclear Regulatory Commission to streamline approvals for small reactors. Nuclear takes longer to permit than other technologies, such as wind and solar, because of the safety risk.

"Nuclear takes a long time to permit," Macfarlane said. "It's complicated, and you don't want to get it wrong."

Some critics fear the push to speed safety reviews could come back to bite the nuclear industry, which is still struggling to enhance its public image.

The Nuclear Regulatory Commission is considering a proposal by companies such as NuScale that would allow them to reduce the size of the emergency planning zone around a plant, given the fact that smaller reactor cores produce less radioactive material that could be released in an accident.

"That is premature at best," said Edwin Lyman, director of nuclear power safety at the Union of Concerned Scientists. "These reactors are still on paper. You need to build a few, test, and demonstrate them before having confidence they will be safe. Being too overconfident in the systems you have is the kind of complacency that brought the world Fukushima."