The US wants to build a different kind of nuclear reactor from a decades-old concept that could power ships and other off-the-grid locations

• Molten salt reactors were first built in the 1950s but haven't been used in the US since the 1970s.
• The US Nuclear Regulatory Commission recently issued a permit to build a molten salt nuclear plant.
• Kairos Power is heading the project, which it hopes to finish by 2027.

The US has issued a permit to construct a different kind of nuclear reactor.

Known as a molten salt reactor, it could lead to smaller, easier-to-build nuclear power plants in the future that could end up powering ships and other off-the-grid locations.

What makes this reactor different is how it will cool its core, using molten salt instead of water.

Water vs. salt

Almost all the nuclear reactors working today use water to cool down. Their cores can reach temperatures of 572 degrees Fahrenheit, far hotter than water's boiling point of 212 degrees.

To prevent the water from evaporating and keep it a liquid at such high temperatures requires a lot of pressure, which in turn costs additional technology, space, and money.

Some salts, on the other hand, have much higher boiling points, so they wouldn't require the same expensive, high-pressure environments.

"You can utilize it at these high temperatures, and it doesn't boil," Nicholas V. Smith, project director of the molten chloride reactor experiment at the Idaho National Laboratory, told Business Insider. "You don't have to have big, thick pressure vessels to contain coolant."

The first molten salt reactor tested in the 1950s, for example, was small enough to fit on a plane whereas the portion of the Diablo Canyon Nuclear Power Plant in California that generates energy takes up 12 acres of land, according to Berkeley Engineering.

That benefit, and others, is why the US Nuclear Regulatory Commission just issued the first permit to construct a non-water-cooled nuclear plant.

It will be the first since 1968, Mike Laufer, Kairos Power's chief executive officer, told Bloomberg.

Kairos Power is the company that plans to build a test plant it calls Hermes, which will be cooled by molten fluoride salt in Oak Ridge, Tennessee by 2027.

The first version of the plant won't provide electricity, but the company hopes its successor, Hermes 2, will by 2028.

Why is molten salt worth re-exploring?

Molten salt reactors have been around since the 1950s but the US mostly abandoned them in the 1970s in favor of water-cooled reactors, many of which had already been built.

Recently, however, companies, including Kairos, and laboratories are looking into salt-cooled reactors again.

"Salt as a coolant is just superior to water, once you've got the engineering details hammered out," Smith told BI.

Because molten salt reactors don't need those thick pressure vessels to keep water a liquid at high temperatures, there's more design flexibility, Smith said.

Reactors can be smaller than water-cooled options and built in a wider variety of locations, for example.

"Molten salt sort of opens up a lot of design options that you simply can't get to without it," Smith said. "As you move into that low-pressure paradigm, manufacturing just gets a lot more straightforward."

"I see molten salt reactors as being prolifically deployed in all areas," from remote locations to shipping vessels to large power plants, he added.

How Hermes will operate

Hermes will operate at temperatures up to 1,200 degrees Fahrenheit. But its molten salt coolant — which is made of a mixture of lithium fluoride and beryllium fluoride known as FLiBe — boils at about 2,606 degrees Fahrenheit, well above the reactor core's temperature.

Therefore, the FLiBe will remain a liquid at those high temperatures without additional pressure. That should make the reactors easier and less expensive to build, Smith said.

Kairos Power's proposed fuel is also different from a typical nuclear reactor. The company plans to use TRISO, or TRi-structural ISOtropic particle fuel.

It can withstand extreme temperatures better than current fuels, making it less likely to release radioactive fission products, according to the US Office of Nuclear Energy.

Challenges remain

Some challenges include limiting corrosion.

"Oxygen is sort of the driving force for corrosion in molten salt," Smith said.

The challenge is to limit the salt's exposure to oxygen. "It's not the exact same processes, but it's the same principles as any other coolant," he said. "You've got to control the chemistry."

Molten salt reactors do have drawbacks. For example, they "would produce several different waste streams, all of which would require extensive processing and would face disposal-related challenges," physicist M.V. Ramana wrote in 2022.

One study suggested that the reactors could produce more nuclear waste than current systems and that they "will use highly corrosive and pyrophoric fuels and coolants that, following irradiation, will become highly radioactive."