Miniature nuclear reactors, once the stuff of science fiction, soon may be coming to a town near you — that is, if terrorists don't pick them off on the way.

Reactors being developed by Hyperion Power Generation of Santa Fe, N.M.; NuScale Power of Corvallis, Ore., and the giant Japanese conglomerate Toshiba use different nuclear fuels, but all rely on the same basic design: a self-contained cylindrical nuclear reactor that is factory-sealed and produces electricity for years without any human oversight or maintenance.

Each reactor would be transported to a site, buried underground, hooked up to a power grid and started up.

After five to 20 years, depending on the design, the nuclear fuel would exhaust itself and the cool reactor would be dug up and shipped back to the manufacturer.

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The companies hope to have their minireactors on the market and running within the next decade, marking what could be the beginning of a nuclear-energy renaissance.

But critics say there are safety and security risks, as well as the possibility that the reactors could fall into the hands of terrorists. And those risks, they say, outweigh any benefits the minireactors may bring.

"Our concern is that it really takes a concerted effort to protect a nuclear power plant from terrorist attack," said Edwin Lyman, a senior scientist with the Union of Concerned Scientists. "It's just not plausible that you could deploy these small reactors widely to communities and the developing world with no infrastructure and no experience with operating and protecting a nuclear reactor."

Michael Greenberger, professor at the University of Maryland School of Law and the director of the University of Maryland Center for Health and Homeland Security, said the minireactors' size will make them attractive to terrorists.

"Anything that's portable, provides technology, would assist terrorists in their goal to perfect a nuclear weapon, and it's very dangerous to the United States," Greenberger said.

But the companies that are designing the minireactors say they will be safe in every way.

Lawrence Livermore National Laboratory in Livermore, Calif., operated by the University of California and the Department of Energy, expects to have a prototype of its SSTAR — for "small, sealed, transportable autonomous reactor" — built by 2015.

The lead-cooled fast reactors will be about 3 yards wide, 13 yards high and weigh less than 500 tons.

The Toshiba 4S — "super safe, small and simple" — will be about 72 feet tall and buried about 100 feet underground. The remote Alaska town of Galena is considering obtaining a 4S to provide its electricity.

Hyperion's Power Reactor would be about the size of a hot tub, encased in concrete and buried underground, which Hyperion says would minimize health and security risks.

The design is licensed from Los Alamos National Lab in New Mexico, also run by the Department of Energy.

It runs on "a natural heat-producing process that occurs with the oscillation of hydrogen in uranium hydride," Hyperion says on its Web site.

Hyperion's CEO John Deal insists the reactors will be safe.

"Our reactor has been optimized for safety and security and it lends itself to deployment in remote areas, which are our initial markets," he told FOXNews.com. "We have a very aggressive schedule. We have about 100 confirmed orders from around the world."

The company says six reactors will be sold to a Czech energy company, to be operational in 2013. Each will cost about $25 million and will be able to provide power for about 20,000 homes.

The NuScale reactor, based on technology developed by Oregon State University, is a light-water reactor that produces 45 megawatts of electricity, enough to power about 35,000 homes, which is about 5 percent the output of a full-scale reactor.

At 60 feet in length, 40 feet across and about 300 tons, it will be larger than the Hyperion design, and it is intended for standard civilian use; the company says its reactors can be grouped to serve communities of different sizes, with up to 24 reactors at a location.

"They can be shipped to site by rail, by truck or by barge. They can all be fabricated within the United States," said spokesman Bruce Landry.

But Greenberger says transporting the reactors generates opportunities for terrorists.

"The reactor in and of itself has technology that's useful to the final goal of those that want to procure a nuclear weapon," he said.

Lyman agrees.

"They are vulnerable to being hijacked, sabotaged," he said. "Look at the pirate situation now off the coast of Somalia. ... Even with substantial security, they can't seem to be able to deter these pirate attacks."

"Any nuclear power plant, no matter how small, is vulnerable to radiological sabotage," Lyman says. "That is, a terrorist who knows what to do can cause an event where a core might overheat, and if you breach containment, then radioactive material can be released."

But the companies say the terrorism threat is just talk.

The danger of a terrorist snatching a reactor is "extremely unlikely," says NuScale's Landry. "Our plants are not designed to be mini-nukes that you put downtown. They're built and sited the same way existing nuclear power plants would be."

Hyperion's John Deal said his company's reactors don't pose a threat and that media reports that they would be plopped down in back yards were untrue. They'll be connected to power-generating plants and industrial installations, he said.

"Like all nuclear power plants, our plants are shielded [by] many feet of concrete. They're guarded. They're observed," Deal said. "But you have to do that with gas power plants too. The terrorist is more likely to go after a soft target, not something that's highly guarded like a nuclear plant."

"Only the United States is so wealthy and so pacified that they have not even bothered to learn the science around nuclear energy," Deal said. "People still treat radiation like they treated witches and witchcraft back in the 17th century."