Many of the remaining coal-fired power plants in the United States are getting ready for retirement. They’re old, costly to build and in dwindling demand, as the domestic market for coal has declined.

But a Portland-based company has a plan to repurpose closed coal plants—large sources of carbon dioxide emissions when they were operating—to generate carbon-free energy in the form of nuclear power.

Last month, U.S. officials approved NuScale Power’s designs for 12 small nuclear reactors to be built in Boise, Idaho. The reactors could make use of the water, transmission lines and general infrastructure of former coal-powered plants in the West to produce clean energy, said Jose Reyes, co-founder of the company.

NuScale said the energy produced by its reactors would generate enough electricity to power about 50,000 homes across six Western states. The Utah Associated Municipal Power Systems, an energy cooperative, would be the first to build the reactors on a federal site at the Idaho National Laboratory.

But many environmental advocates oppose the project on safety grounds, and some scientists say that by the time the reactors are built, they will not offer much help in staving off the effects of climate change.

Since 2013, the Department of Energy has backed NuScale’s initiative, providing more than $300 million to help finance the project and cover the costs of delays. Last week, the Trump administration took another significant step to support the project, pumping $1.3 billion in financing over 10 years to help build the company’s reactors.

The goal is to have the first small, modular reactor up and running by 2029, with 11 more to follow in 2030.

The timeline for the reactors has been delayed several times. The new schedule allows for the project to be “further de-risked” and “provides time for regulatory, engineering and licensing review of project features,” said Kelly Conroy, a spokeswoman for NuScale.

“This is the first small modular reactor to gain approval by the United States. It’s a big event,” said Neil Todreas, a professor of nuclear science and engineering at the Massachusetts Institute of Technology, who was not involved in designing the reactors.

Nuclear power has a long history of controversy within the environmental movement, and many advocates object to its inclusion as part of a clean energy transition. The NuScale Power initiative has met with opposition from local environmental groups, who say that nuclear power is a dangerous and unsustainable energy source.

In addition, the highly radioactive waste from nuclear reactors must be securely stored indefinitely to prevent accidents, and contains plutonium and uranium that can be reprocessed into nuclear weapons. “We see this project as a way to create a whole new generation of high level radioactive waste,” said Scott Williams, executive director of Healthy Environment Alliance of Utah, a nuclear watchdog.

Todreas said that what distinguished the NuScale reactors was their safety. In an emergency situation, the reactors would shut off by themselves, without requiring water, power or intervention by an operator, he said. The reactors would be built as a set of 12, 60-megawatt modules immersed in a giant pool of water to prevent overheating.

The designs underwent a public health and safety review by the Nuclear Regulatory Commission. But some scientists think they still aren’t safe enough. In a public statement, Edwin Lyman, director of nuclear power safety with the Union of Concerned Scientists, cited a report by a senior engineer at the Nuclear Regulatory Commission expressing concern that the cooling process might inadvertently cause “catastrophic” core damage to the reactors.

Other scientists worry that NuScale may be getting ahead of itself by not having a planning protocol for a radioactive emergency that affects areas around the site.

“In the event of an accident, the people around there will not have rehearsed how to do an evacuation,” said M.V. Ramana, a professor in the School of Public Policy and Global Affairs at the University of British Columbia.

Too Late in a Climate Crisis?

The municipal power systems cooperative still needs to obtain a license to build and begin operating the reactors. To do so, the project will undergo an additional site-specific review to consider the potential ecological, geographic and residential impact the technology may have on the area, said George Griffith, lead technician at the Idaho National Laboratory.

The delay means that while NuScale will be ready to manufacture modular reactors by around 2024, it will take an additional five to six years for them to be operational at the Idaho site, said Reyes.

Some experts, however, question whether 2029 is too late for the technology to be relevant in a time of climate crisis. “2030, yes, it’s great, but we’re probably going to need to start deploying these plants sooner than that,” said Jacopo Buongiorno, a professor of nuclear sciences at Massachusetts Institute of Technology.

Reyes conceded that 2029 is late, but said that the company is currently negotiating with other potential clients who might want modular reactors before then.

Despite the slow progress and controversy, Buongiorno said he thinks there are important reasons why small modular reactors are a promising new technology. For one thing, each modular reactor can be switched on and off flexibly, meaning that the reactors can be paired as an energy source with the intermittent power generation of renewables like solar and wind, he said.

An all-renewable future might seem attractive, Buongiorno said, but he believes it’s also unrealistic.”If you want to decarbonise the power grid, what you’re going to need is a combination of nuclear and renewables,” he said. “Otherwise you have to overbuild both renewables and energy storage, which cost a fortune and may not be either reliable or resilient.”

He added that another advantage of NuScale Power’s design is that the reactors would be manufactured in a factory rather than on-site, lowering the costs of production. The Utah municipal power cooperative estimates that while the total cost of the 10-year project would be about $6.1 billion, the price for customers would ultimately be about $55 per megawatt-hour, making the reactors competitive with natural gas, said LaVarr Webb, a spokesman for the cooperative.

But not everyone is convinced by these numbers. “There’s a lot of uncertainties, and so I would be surprised if they could get it coming in at that price,” said Joshua Rhodes, a research associate at the Energy Institute at the University of Texas in Austin. Rhodes said such prices would only be possible if the initiative was going to be heavily subsidized by the government, avoided getting caught-up in expensive legal battles and was able to produce the reactors at an industrial scale.

Such prices also assume a high demand for the reactors—something that can’t be guaranteed in advance, he added.

Ramana, of the University of British Columbia, said, “While the overall capital cost [for small modular reactors] might be smaller, they also generate smaller amounts of electricity.” He outlined his concerns in a report released in September urging the Utah energy cooperative to “end their pursuit of small modular reactors.”

Ramana made clear that while devastating incidents associated with nuclear power plants might seem unlikely, we need to remain cautious.

“The lesson we should learn from all the many nuclear and other accidents that have happened with hazardous technologies, is a little bit of humility,” he said.