Like today’s commercial nuclear power plants–which produce about 20 percent of U.S. electricity–the pebble bed reactor uses uranium as its power source. Under the right conditions, uranium atoms split, or “fission,” throwing off energetic neutrons and other particles that break up still more uranium atoms in a chain reaction that generates enormous amounts of heat. In today’s plants, the heat boils water to create steam to drive turbines and create electricity. In the pebble bed reactor, the nuclear reactions heat helium gas, which spins turbines as it expands.
The more fundamental departure from conventional nuclear design, though, rests in the physical configuration of the uranium fuel. Today’s reactors use uranium pellets embedded in metal rods and bathed in cooling water. In contrast, the pebble bed’s fuel kernels are encased in billiard-ball-sized graphite pebbles filling a doughnut-shaped reactor core lined with graphite (see “Heart of the Pebble Bed Reactor” below). The graphite lining “moderates” the nuclear reaction, slowing neutrons and reflecting them back to the pebbles to keep the fission process humming. (In conventional reactors, the cooling water doubles as the moderator.)
While the pebble bed reactor has yet to make its commercial debut, the design isn’t new. Over the past four decades, nuclear engineers have been tinkering with gas-cooled reactors, some using pebbles, with varying degrees of success. Early gas-cooled plants operated in Pennsylvania (Peach Bottom 1, in service from 1967 to 1974) and Colorado (Fort St. Vrain, 1974 to 1989) but encased the kernels in stationary graphite blocks. The West German government spearheaded the pebble concept, funding one prototype that operated from 1966 to 1989, as well as an updated version that opened in 1983 near Hamburg. The new plant, though, suffered from design flaws; its boron carbide control rods occasionally smashed fuel pebbles, and some of the reactor’s gas ductwork broke, leading to shutdowns and poor efficiency. Amid hard economic times for nuclear power and worldwide safety fears brought on by the April 1986 disaster at the Soviet Union’s Chernobyl nuclear reactor, West Germany decided in 1990 to shut the plant rather than fix it. But in 1999, the South African utility licensed the German technology. The utility says it has eliminated the flaws that plagued the German reactor and is now resolving final design details.
The South Africans were not alone in seeing value in the technology. In 2000, the Chinese government began operating a small, 10-megawatt pebble bed research reactor, also based on the German design. And in recent years, a research program has grown up around the concept at MIT. The two decades of German operating experience “formed the basis for our feeling that this would actually work,” says nuclear engineering professor Andrew Kadak. “The fundamentals of the technology were thoroughly demonstrated.” In 1998, Kadak joined fellow MIT nuclear engineering professor Ronald Ballinger in launching a next-generation pebble bed design, one Kadak says was developed from the public record and from the freely shared experience of pebble bed researchers. The MIT design calls for standardized, readily replaceable parts that Kadak says should make the plants easier to build and less expensive to maintain than the South African version.
Whatever upgrades might come to pebble bed, though, none will solve the question of what to do with waste. Resolution of this issue may prove decisive for the future of any reactor technology. Spent fuel is dangerously radioactive for 10,000 years, and all fuel waste from the nation’s existing nuclear plants remains stored at the plants themselves, awaiting the federal government’s decision on opening a repository at Nevada’s Yucca Mountain or elsewhere (see “Whose Nuclear Waste?”).
The design of the pebble bed fuel, though, may provide at least a partial solution. Kadak believes that the silicon carbide and graphite coatings will stand up better over the millennia than the zirconium-alloy jackets of conventional fuel rods. He says that some studies show that the pebbles would provide excellent resistance against the chief nemesis of long-term storage: corrosion from water. “We don’t need a 10,000-year container, because we’ve already got it in the ball,” Kadak asserts.
No matter how rugged such pebbles may be, they’ll still require long-term storage, says Richard Lester, another MIT nuclear engineering professor and director of the school’s Industrial Performance Center. And the same layering that provides protection also adds volume to the waste, requiring a larger repository. Finally, he adds, no amount of shielding can protect the industry from public fears. “I don’t think it’s the case,” he says, “that a nuclear-waste repository will be more acceptable to folks who live in the area because you’ve got pebbles.”
Heart of the Pebble Bed Reactor
In a pebble bed modular reactor, uranium fuel contained inside graphite “pebbles” slowly flows, gumball-style, through a helium-cooled reactor lined with graphite.
The lining, along with plain graphite pebbles moving through the center, reflect and slow the uranium’s neutrons to keep the energy-producing fission process humming. Helium heated by the energetic fuel pebbles expands to spin a turbine (not shown), which generates electricity.
As pebbles flow out of the reactor bottom, automated systems discard broken pebbles and send plain graphite balls back to the top, propelled by a pressure differential.
Intact fuel balls are checked for power levels. Reusable fuel is sent back to the reactor–a trip that it will make, on average, ten times in three years. Spent fuel is discarded into a container and replaced with fresh fuel.