A tiny trend is taking off in nuclear energy, and it just got a big assist from the government.

America's 94 existing nuclear reactors are behemoths, with many of them able to power 1 million homes each. Now companies are looking to miniaturize the industry - building reactors that can fit on the back of a truck and power 500 to 1,000 homes, or industrial and government sites.

So-called microreactors got their latest boost late on Wednesday, when the U.S. Army selected five companies to build mini-reactors at Army bases across the country, with the first one switching on as soon as September 2028.

With support from the Trump administration, the military has been looking at microreactors - none of which have license to operate commercially yet - as a solution to power remote bases, or for backup power in the event of outages on the larger electric grid.

"We are building the energy resilience necessary to project combat power globally, without relying on potentially vulnerable external grids," said Dan Driscoll, Secretary of the Army, in a statement.

The government plans to spend up to $2.2 billion on at least 20 reactors, the Army said in a release. The companies involved include privately held Antares Nuclear, General Atomics Electromagnetic Systems, and Radiant Industries. BWXT Advanced Technologies, part of public company BWX Technologies, was also selected. And Westinghouse Electric, a nuclear developer owned by Cameco and Brookfield Renewable Partners, plans to deploy some too.

Radiant's contract will allow it to build up to 15 microreactors at Fort Benning in Georgia, at a total cost of up to $750 million, the company said. Radiant said it's hoping its Army project will prove that its technology works and open up commercial opportunities.

America has deployed small-scale nuclear energy systems before - most notably in the Navy's nuclear submarines. But the microreactor trend is a new one, and investors are hoping it will spread to commercial uses too. The Army contracts are nice, but they're not going to justify multibillion-dollar valuations for all these firms.

Venture capitalists are pumping money into microreactor startups, betting that they can help power things like AI data centers and industrial sites that need power quickly.

Radiant has already raised more than $500 million from the likes of Andreessen Horowitz and Chevron Technology Ventures. It's valued at $1.9 billion, according to Pitchbook.

Other startups are also raising big sums, at expanding valuations.

Valar Atomics raised $1 billion earlier this month, at a valuation of $6 billion, according to Pitchbook. Antares Nuclear raised $370 million last month at a $2.1 billion valuation.

Another player, Aalo Atomics has raised over $300 million, the company says. A few microreactor companies, including Nano Nuclear, Terra Innovatum and Deep Fission have already gone public. They've attracted some interest but their market caps remain modest, with Nano Nuclear at $1 billion, Terra Innovatum at $625 million and Deep Fission around $400 million.

More seasoned nuclear companies are also experimenting with microreactors. Westinghouse owns the most advanced large-reactor design on the market today, but has also been testing its own microreactor.

Valar, Antares, Westinghouse and Aalo all reached a milestone this year as part of a Department of Energy program designed to help startups turn on their first reactors. They were part of a small handful of firms that built reactors able to reach "criticality," equivalent to getting a car to successfully start running when you turn the key. The reactors didn't generate electricity, but the tests showed that they're on the road to doing so.

Theoretically, microreactors can solve some of the problems of larger reactors. The big machines can take a decade or more to build, and cost over $10 billion each. The smaller ones aren't nearly as powerful as larger reactors, but buyers don't have to take the risk of a massive capital project.

Even if a microreactor project fails, the cost of walking away will be in the millions, not the billions.

"At a minimum, this reactor is not making me bankrupt," said Arthur Hyde, a portfolio manager at hedge fund Segra Capital Management, who focuses on nuclear investments.

The microreactor companies also say they can build their machines much faster than large reactors and have them transported to the site from a factory, instead of having thousands of workers construct the reactors on the site.

The microreactor companies also differ from large-scale reactors in their underlying technology. Radiant's reactors, for instance, operate at high temperatures and are cooled by helium gas instead of the water-cooling systems that most large reactors use. Gas-cooled reactors are meant to have a lower risk of meltdowns, which are considered one of the biggest dangers of large reactors.

That said, microreactors have several potential downsides compared with larger versions.

Nuclear reactors need to be guarded 24 hours a day, with multiple security systems. The cost of security is manageable for a large reactor.

Microreactors might make sense on military bases that already have 24/7 protection. But the idea of putting a microreactor at a remote commercial location might not be cost-effective, Hyde says. Hiring armed guards around the clock in a remote location for a small amount of power wouldn't be cost-effective.

The cost of things like permitting and site-selection for small reactors might also prove to be prohibitive.

"It becomes almost impossible to have that economically make sense," Hyde said. Centrist think tank Third Way has been critical of the government's support for microreactors, arguing they're an inefficient method of meeting America's growing electricity needs.

Hyde still sees promise in some companies, though.

Segra has invested in Valar Atomics and Terra Innovatum. Hyde thinks Valar can deploy at least 30 small or midsize reactors at one site, overcoming some of the efficiency problems associated with deploying a single small reactor in a remote spot.

Valar, like Radiant, uses a gas-cooled technology that can not only produce electricity but gives off heat that can power industrial processes. That expands the markets it can serve, he said.

The military is ready to deploy microreactors. For the industry to succeed, it will have to convince other buyers too.