The Next Nuclear Plant
The first commercial “pebble bed” reactor–nearing approval in South Africa–may revive nuclear power.
Except for the genets, caracals and other exotic wildlife ranging in the surrounding nature preserve, the pair of nuclear power reactors at Koeberg, South Africa, look much like their squat, domed counterparts dotting the United States. The basic technology is the same: beneath steel-reinforced concrete domes, the fission of uranium fuel generates turbine-driving steam.
Just like the United States, South Africa is also weighing difficult choices about how to meet future energy needs. The African nation doesn’t have much hydroelectric or natural-gas capacity. Coal is plentiful but dirty to mine and burn–and most of it is located too far from coastal population centers to make economic sense. And as in the U.S., many in South Africa believe nuclear power has a place in the future mix.
But there’s a key difference. So far, Washington is only talking about a nuclear rebirth; South Africa is getting ready to build. And Koeberg, on the Atlantic coast about 30 kilometers north of Cape Town, is emerging as the epicenter of a technology initiative that sponsors claim could usher in a new era of safe, inexpensive nuclear power using a long-studied and promising design: the pebble bed modular reactor. A construction decision by an industrial consortium is expected sometime this year. South Africa’s regulatory apparatus will get final say, but momentum behind the proposal is remarkably strong. “I believe that we will resolve all issues outstanding and this will get built,” says David Nicholls, CEO of the consortium, Pebble Bed Modular Reactor.
The consortium says the technology–rooted in research and prototype reactors that date to the 1960s–has matured to the point of achieving the twin goals that have eluded the nuclear industry in the post-Three Mile Island and Chernobyl era: affordability and inherent safety. First, a pebble bed reactor is relatively simple to build and inexpensive to operate; the consortium says construction and operating costs are expected to be “competitive” with those of coal and natural-gas plants. Second, and perhaps more crucial, they say, it is immune to today’s worst-case scenario: a loss of coolant in the reactor core that would lead to a melting of uranium fuel and a catastrophic release of radiation. That’s because the fuel is encased in billiard-ball-sized graphite “pebbles” that can’t get hot enough to melt. What’s more, this encasement may make the spent pebbles more rugged in long-term storage.
The fuel design isn’t the only thing that makes this reactor fundamentally different from the more than 430 commercial nuclear power reactors worldwide, nearly a quarter of which are in the United States.The pebble bed reactor is cooled with helium gas instead of water, operates at higher, more efficient temperatures and–thanks to the inherent safety claimed by its builders–dispenses with the containment dome and regional evacuation plan now required of U.S. nuclear facilities. Individual pebble-bed plants would also have a smaller footprint than today’s plants and produce a mere 100 megawatts or so of electrical power–a tenth as much as today’s typical nuclear behemoth. This modest scale limits the early financial losses many large plants incur by initially glutting the market with electricity, and gives utilities the option of building just what’s needed at first and then adding units later if demand warrants it.
Though pebble beds have advocates in utility boardrooms, their case is not yet proved in the eyes of the U.S. Nuclear Regulatory Commission. The agency hasn’t signed off on the safety of any of the plant’s features, from the fuel design to the lack of containment. And these reactors don’t solve the same two basic problems that dog the entire nuclear industry: they create highly radioactive fuel waste and are potentially vulnerable as terrorist targets. Nevertheless, the South African partners believe they can resolve regulatory questions and bring a pebble bed juggernaut to the United States. The consortium includes not only the South African government’s utility and industrial agency but also Chicago-based utility conglomerate Exelon and British Nuclear Fuels, owner of Pittsburgh-based reactor builder Westinghouse Electric. Already, Exelon is the largest U.S. nuclear-plant operator; its 17 reactors include the still-operating reactor at Three Mile Island near Harrisburg, PA, whose twin partly melted down in 1979, crippling the nuclear industry.
Exelon in particular sees pebble bed technology as the breeze blowing the nuclear industry out of its doldrums. Though it hasn’t yet made a construction proposal in the United States, the company envisions erecting clusters of pebble beds next to existing nuclear plants. “It is considered a very safe technology; it is relatively simple, it’s very efficient,” says Jim Muntz, vice president for nuclear projects. Asked whether Exelon wants to construct a pebble bed reactor in the United States, he answers unabashedly, “We want to build 40 or 50 of these. This isn’t about building one; this is about building a lot of them.”
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.
The growing interest in the pebble bed design comes amid changes in the U.S. regulatory apparatus designed to make it easier to build and operate nuclear plants of any approved type. The Nuclear Regulatory Commission has reduced bureaucratic obstacles to nuclear plants by allowing for a combined construction and operating license. This eliminates the two-step process that produced the debacle in Shoreham, NY, where a nuclear power plant was fully built but never allowed to operate because of fears the region’s population couldn’t be evacuated in case of accident. The regulatory commission also has begun approving new reactor designs. In recent years, it has signed off on upgraded versions of existing water-cooled designs said to be significantly simpler, safer and more efficient than their predecessors. Regulators and Exelon have already begun preliminary reviews of the safety of the pebble bed design.
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