So B&W has designed an integral pressurized-water reactor that it can manufacture in a factory and ship to a site. "We wanted to be able to put it on a rail car," Mowry says, and so the self-contained reactor is 25 meters tall and 4 meters wide. Operating at high pressure and temperature, such reactors are not truly novel, having propelled the first commercial nuclear ships, such as the nuclear-powered cargo ship named Savannah, in past decades.
Shrinking the reactor and putting its parts (such as the control rods that regulate the fission process) inside the reactor vessel, also reduces the need for redundant safety systems to deal with problems such as leaky pipes. The pipes in and out of the reactor sit above the nuclear fuel rods themselves, ensuring that any leaks do not result in uncovered fuel. In addition, the pipes are much smaller than those required for a larger reactor. "You have a hell of a lot more water in the reactor relative to the size of the breaks that can happen," Mowry explains. External tanks hold enough additional water to cool the reactor for two weeks in the event of a loss of power as well. In addition, the reactor—in essence, a nuclear battery because it is largely self-contained—is buried, rendering it "immune from external events like tornadoes, hurricanes or tsunamis."
The smaller reactor uses the same nuclear fuel rods—albeit slightly shorter in length to fit—but fissions them more slowly, operating for four years before fresh fuel is needed. A test facility near Lynchburg, Va. is up and running to ensure that what looks good on paper will also work in practice and B&W already has one potential customer in the U.S.—the TVA—expressing an interest in building as many as six of the small modular reactors at its Clinch River site, former home of a failed effort to build a fast breeder reactor in the 1970s.
The B&W mPower reactor is not the only such small modular design moving forward: Westinghouse Electric, NuScale and Holtec—a company better known for making the hulking concrete and steel casks to store used nuclear fuel—have similar designs of varying sizes. "The advantage of the smaller one is: even if you need 1000 megawatts you can put investment in piecemeal and generate money while the next unit is put in," says Westinghouse CEO Aris Candris.
But multiple reactor sites proved problematic at Fukushima Daiichi, where an accident in one rapidly became a crisis for multiple reactors and spent fuel pools. "If you're going to have multiple reactors, are you going to gain in safety or lose in safety?" asks physicist M.V. Ramana of Princeton University. "We don't know."
"Early in the discovery of any new technology you have this rosy picture that is formed," Candris admits of small modular reactors. "In the early days of nuclear, there were people out there saying it would be too cheap to meter. We found out otherwise."
Small modular reactors may help with two of the biggest challenges facing the nuclear industry: the growing stores of waste from existing reactors and residue from the mass production of nuclear weapons as well as the overall safety of nuclear power. GE's PRISM fast reactor, General Atomic's helium-cooled fast reactor, or Hyperion Power's liquid lead-bismuth cooled reactor could all turn waste into fuel. Hyperion hopes to demonstrate its reactor, capable of generating 25 megawatts of electricity, at the Savannah River National Laboratory in South Carolina. The site has also signed memorandums of understanding to host prototypes of the NuScale and Holtech reactors.