The Prospect for Safe Nuclear PowerBy
Three Mile Island, Chernobyl, Fukushima. First the accident, then the predictable allegations in the postmortem: The design was flawed. Inspections were inadequate. Lines of defense crumbled, and reliable backups proved unreliable. Planners lacked the imagination or willpower to prepare for the very worst.
There's a way to break out of this pattern. Nuclear power plants will never be completely safe, but they can be made far safer than they are today. The key is humility. The next generation of plants must be built to work with nature—and human nature—rather than against them. They must be safe by design, so that even if every possible thing goes wrong, the outcome will stop short of disaster. In the language of the nuclear industry, they must be "walkaway safe," meaning that even if all power is lost and the coolant leaks and the operators flee the scene, there will be no meltdown of the core, no fire in the spent fuel rods, and no bursts of radioactive steam into the atmosphere.
For inspiration, consider the manhole. If manholes were square, a cover that got jounced around by a passing vehicle could fall diagonally through the open hole. That's why manholes aren't square—they're round, because no matter how you rotate a round cover, it can never fall through a round hole. The solution is brilliant in its simplicity, and cheaper than hiring armies of inspectors to go around making sure square covers are correctly aligned on square holes.
The beginnings of manhole wisdom are incorporated into new nuclear power plants now under construction in China, India, and probably soon in the U.S. The plants use "passive" safety features, a label that sounds underwhelming until you consider its implications. Passive means that the reactor's safety doesn't depend on active interventions, such as operators flipping the correct switches or sensors and actuators working properly. The safety depends, rather, on physics.
The new Westinghouse AP1000 (the AP stands for Advanced Passive), for example, has a huge emergency water reservoir above the reactor vessel that's held back by valves. If the cooling system fails, the valves open and a highly reliable force takes over: gravity. Water pours down to cool the outside of the containment vessel. Then another highly reliable force, convection, kicks in. As the water turns to steam, it rises. Then it cools under the roof, turns back into a liquid, and pours down again. Westinghouse estimates that the pool contains enough water to last three days, after which pumps operated by diesel generators are supposed to kick in and add water from an on-site lake.
This isn't an idea on the drawing board. Westinghouse, which is majority owned by Japan's Toshiba, is about halfway to completion of one of the plants at Sanmen in China's Zhejiang province. It's supposed to go online in 2013. In Vogtle, Ga., excavation and non-safety-related construction has begun for two AP1000 reactors. Southern Co. (SO), the largest utility in the U.S. by market capitalization, has 1,400 workers on the site and is expecting a combined construction and operating license for them later this year. France's Areva, a General Electric (GE)-Hitachi (HIT) venture, and others are lined up behind Westinghouse in getting Nuclear Regulatory Commission certification for their own new, safer designs.
Critics continue to argue that there will never be a truly safe nuclear power plant, and it's true: Any time you split atoms, there's risk. But nuclear plants have one thing going for them that hasn't changed since the leak at Fukushima. They generate badly needed electricity without creating greenhouse gases that cause global warming. As long as nukes are going to be part of the world's energy mix, it makes sense to have the safest ones possible.
In his State of the Union address in January, President Barack Obama called for "building a new generation of safe, clean nuclear power plants in this country." In his fiscal 2012 budget request in February, Obama asked for $36 billion in government-backed loan guarantees for new nuclear reactors. Obama also asked for $125 million for what the Energy Dept. calls Generation IV nuclear reactors—futuristic ones that the DOE says "will feature advances in safety and reliability to improve public confidence in nuclear energy while providing enhanced investment protection for plant owners." Nuclear power faces all sorts of challenges, including the multibillion-dollar construction cost of each new plant. Fukushima is a reminder that making the plants simpler and safer is the biggest challenge of all.
In late 1942, Enrico Fermi—the Italian physicist who was one of the key figures in the development of the atomic bomb—came up with nuclear power's first safety mechanism: a man with an ax. In case of a runaway reaction, he decided, the ax man's job was to cut a rope, dropping cadmium rods that would absorb neutrons and halt the reaction. "The ax man received his hand-signaled instructions from Fermi, who stressed the speed necessary by holding one hand flat and depicting a chopping motion with the other," recalled Edwin Blackburn, a millwright who was on the scene, in an Oak Ridge National Laboratory publication.
The ax man's skills were never needed, but in the decades that followed, nuclear energy suffered a series of stigmatizing accidents. One reason for that is the almost total reliance on one particular design, the light water reactor, which has some inherent problems. The light water's prominence dates to a fateful 1950s choice by Admiral Hyman Rickover, father of the nuclear Navy. Rickover decided that the first nuclear submarine, the USS Nautilus, which was launched in 1955, would be powered by solid uranium oxide and would use water as both a coolant and a moderator. (Technical digression: A coolant carries heat from the reactor to produce power; a moderator slows down the neutrons emitted by the fuel so they have a better chance of interacting with other fissile materials to keep the reaction going. There's no requirement that water perform either of those functions, but that's the way light water reactors work.)
Rickover went with the design because it was likely to be ready soonest and because it produced plutonium-239, a bomb-making material, as a byproduct. But it has some big drawbacks as well, as explained by physicist Robert Hargraves and nuclear engineer Ralph Moir in a 2010 article in the American Scientist. The bundles of fuel rods are quickly damaged by heat and radiation. Short-lived byproducts such as xenon-135, which poisons the fission process, are tricky to manage—and contributed to the instability that led to the Chernobyl explosion. Long-lived byproducts are abundant and highly toxic. The water is corrosive and radioactive. To raise its boiling point, it must be pressurized to 150 times atmospheric pressure. That necessitates a costly network of vessels, pipes, and valves—and raises the risk of an explosive release of radioactive steam to the atmosphere. Overall, the design is the precise opposite of passive safety.
Rickover's choice had lasting consequences. The first commercial nuclear power plant in Shippingport, Pa., used a design similar to that of the Nautilus, setting a pattern that endures to today. "Maddeningly," write Hargraves and Moir, "historical, technological, and regulatory reasons conspire to make it hugely difficult to diverge from our current path of solid-fuel, uranium-based plants."
New reactors such as the Westinghouse AP1000 and Areva's EPR don't diverge from Rickover's path—they too have the solid-fuel cores and high-pressure water cooling systems. But they are substantially safer than older light water reactors. Areva says its EPR has four redundant safety systems instead of the two or three found on most current reactors. Westinghouse says the AP1000 is 100 times as safe as current plants. Those claims, while subject to debate, are not mere corporate puffery. They're based on engineering analyses that have been reviewed by the Nuclear Regulatory Commission as part of the plant certification. The Energy Dept. is also supporting development of small "plug-and-play" modular reactors like the 125-megawatt mPower design from Babcock & Wilcox (BWC) and Bechtel.
The case for these new, safer plants doesn't come through clearly because their existence conflicts with the agendas of both pro- and anti-nuke organizations. Utilities and manufacturers don't want to imply that the older designs now in service are unsafe. In fact, Westinghouse points out in its marketing materials for the AP1000 that even the older reactors are twice as safe as the NRC requires. "The plants we're operating now are extremely safe as well," says Southern Co. spokeswoman Beth Thomas.
Nuclear opponents have even less reason to point out safety advances. "My shortest answer is that we shouldn't build more nuclear plants," says Charles Perrow, a retired Yale University sociology professor who wrote extensively about the partial meltdown in 1979 at Three Mile Island nuclear power plant near Harrisburg, Pa. "There's no way to make any system free of a disaster."
If worse comes to worst, as it so often seems to, the safety systems that give the most confidence are the passive ones. The International Atomic Energy Agency has created an elegant hierarchy of these passive systems. The ones at the highest level, Category A, require no signal inputs, no external power sources or forces, no moving mechanical parts, and no moving working fluid. Example: really thick concrete walls. (Our fortress-building ancestors understood Category A.) Westinghouse's reservoir above the AP1000's containment vessel, clever as it is, doesn't fit into Category A because it involves moving fluids and valves.
Generation IV reactors will be a much bigger departure. Many will do away with water, using elements such as helium or liquid sodium as a coolant. Most also get rid of solid uranium-235 as a fuel, relying instead on different uranium isotopes, or liquid uranium mixtures, or even thorium as the primary fuel. The profusion of creativity in nuclear design recalls the early days of the automobile, when the Stanley Steam Car co-existed with the Columbia electric buggy and internal combustion engines running on gasoline and diesel.
The comparison with cars goes deeper. Just as electric vehicles are staging a minor comeback after their early demise, some long-since-rejected concepts for nuclear energy are getting a second look as the faults of the incumbent technology become clearer. The only requirement for a nuke design is that somehow a nuclear chain reaction must occur: A fissile material absorbs a neutron and splits, releasing energy and more neutrons that carry the process on. There are probably 1,000 conceivable combinations of the basic choices, an Oak Ridge official once calculated.
The new design that's closest to commercial electricity generation is the pebble bed reactor, which has been under development for decades in Germany, then South Africa, and now China and the U.S. Its uranium fuel is encased in more than 300,000 tennis-ball-sized "pebbles," each one containing thousands of tiny graphite-coated fuel seeds, like a metal pomegranate. The radioactive fission products are absorbed in the coatings, and the fuel doesn't get hot enough to melt down even if the plant loses all its cooling for days.
China is closest to commercializing the pebble bed, pushing forward where the South Africans left off for financial reasons. Tsinghua University, working with Massachusetts Institute of Technology, already has a 10-megawatt experimental reactor in operation. It is building a 200-megawatt plant in Shidaowan in Shandong province with a unit of China Huaneng Group, the nation's largest power group. The Energy Dept. is leading a U.S. initiative, based at Idaho National Engineering and Environmental Research Laboratory in Idaho Falls, called the Next Generation Nuclear Plant. It, too, is a pebble bed design. Energy Secretary Steven Chu has appointed a review committee to decide later this year whether to move on to the second phase of the project, which could lead to an operating pebble bed plant in the U.S. by around 2025, says David A. Petti, director of the Very High Temperature Reactor Technology Development Office at Idaho National Laboratory.
Pebble bed reactors don't scale up well. Above 600 megawatts they lose their safety advantage over reactors with ordinary fuel rods, says Petti. At that scale the pebbles can get so hot that they can't shed heat fast enough and can melt down just like any other uranium fuel, releasing radioactivity. The sweet spot for pebble bed reactors is 250 to 600 megawatts, Petti says. He envisions them being located next to industrial plants, where their excess energy could be used to heat facilities and produce petrochemicals.
Liquid fuels are getting another look, too. As early as 1944, Fermi tested the world's first liquid-fuel reactor in Chicago. It used uranium sulfate dissolved in water. In 1965, Oak Ridge National Laboratory in Tennessee achieved criticality with a reactor using a more advanced liquid fuel: molten fluoride salt with uranium dissolved in it. The experiment ran for five years, then stopped when the money ran out. Now, scientists in India, France, and the U.S. are experimenting with another variety: liquid fluoride thorium reactors (known as "lifters").
Perhaps the farthest-out design comes from a spinoff of Intellectual Ventures, a company headed by former Microsoft (MSFT) chief scientist Nathan Myhrvold and funded in part by Microsoft co-founder Bill Gates. TerraPower, as the spinoff is known, used massive computing power to design a reactor that could run for decades on an isotope of uranium that is today considered waste. The concept, first proposed in the 1950s, is to set up a slow-moving wave in which neutrons transmute inert, nonfissile fuel such as uranium-238 into fissile isotopes such as plutonium-239 that can split and throw off energy. TerraPower says its spent fuel would not be useful for making weapons. The company, chaired by Gates, has been seeking a production partner and a host country. So far, no takers.
Technological breakthroughs such as the traveling wave reactor are part of the answer to nuclear safety, but not the answer in its entirety. Better management is crucial as well. For many people, the most disturbing thing about the Fukushima disaster is that it occurred in a nation renowned for quality control. Japan invented poka-yoke, a design approach that is supposed to remove the possibility of doing things the wrong way. (It means mistake-proof. Round manhole covers are poka-yoke in spirit.)
The trouble with poka-yoke is that it lends itself mainly to preventing day-to-day types of mistakes, not strategic errors. On that score, Japanese regulators and officials of Tokyo Electric Power (Tepco) appear to have underestimated the risk that a huge tsunami would overwhelm Fukushima's defenses. Katsuhiko Ishibashi, Japan's leading seismologist and an adviser to the government, warned in 2007 of what he called genpatsu-shinsai—a coinage meaning a quake-caused meltdown. The oldest of the six reactors on the site was recently given a 10-year license extension in spite of regulators' findings of inadequate management maintenance and Tepco's spotty history of compliance with regulation. (In 2003, regulators forced it to suspend operations briefly at 10 reactors after evidence emerged that it had falsified inspection records.)
In fairness to Tokyo Electric Power, no system can be made safe against every risk. "It's a very difficult question," says Tepco spokesman Hiro Hasegawa. "Maybe we should have prepared for the worst, worst, worst, worst, worst. But we can't do anything if we consider every risk." Hasegawa says the utility did reinforce the Fukushima Dai-Ichi reactors after the 2007 quakes. And he says the 2003 tussle with regulators was caused in part by their overstrictness about a hairline crack in one key part.
There's nothing particularly Japanese about what went wrong at Fukushima. It's natural to minimize the risk of catastrophic events when you have no clear idea about how to deal with them, says J. Edward Russo, a management professor at Cornell University's Johnson Graduate School of Management. "It's human nature to want to lower the probability of a particular disaster. They say, 'We can't plan for and pay for everything, so let's knock that off the list.' " Homer Simpson, the ultimate everyman-nuclear technician, would approve.
To be truly safe, the reactors of the future will have to withstand the worst that the Homer Simpsons of the world can throw at them. Remember the cynical adage: Whoever invented the term "foolproof" underestimated the ingenuity of fools. Actually, it's worse than that. In a genuine emergency, even calm, seasoned pros can make terrible decisions. At Three Mile Island, operators refused to believe a dial that told them the reactor was boiling dry, preferring to trust another dial that was giving a different (and incorrect) interpretation, says Perrow, the Yale sociologist, who studied the accident in a 1984 book called Normal Accidents.
That's where passive safety comes in. The ideal system is one that's inherently stable, like a ball at the bottom of a bowl. However it's shaken, the ball tends to roll back to the bottom. An inherently unstable system is the opposite, like a ball balanced on top of another ball. In some special cases, such as fighter aircraft, inherent instability is an asset. Fighter jets are built to perform incredible dogfighting maneuvers, but the price for achieving that airborne maneuverability is radical instability: The onboard computer must send adjustment signals as many as 40 times a second to the engines and various wing surfaces to keep the plane flying true.
A nuclear plant should be like a ball in a bowl, not a jet fighter. In other words, stable and safe. For future plants, safety can be designed in as a basic feature. Not so easy for existing plants, some of which may be with us for decades to come. For them, the solution is retrofitting where possible, greater vigilance, and creative thinking about what might go wrong.
A "premortem" might help. Gary Klein, a consultant to the Air Force Research Laboratory, advises institutions that deal with risk to analyze a failure before it even occurs. Often, Klein says, planners will underestimate dangers by assuming they will be clever enough to surmount any difficulty that arises. Klein, who is senior scientist of MacroCognition in Yellow Springs, Ohio, removes that overconfidence bias in sessions with clients by asking participants to assume that a terrible accident, however unlikely, has already occurred. The challenge is to explain how it happened. "You show you're smart by trying to identify things that are plausible and worrisome," says Klein. "It almost gets into a competition."
To say nuclear power can be made much safer is not to say it can be made safe enough. That's a political judgment, not a question for nuclear engineers. If Japan's radiation nightmare continues to worsen, the outcry against nukes will be hard to ignore. Some are already calling nuclear energy dead. Although that's premature, NRG Energy (NRG) said on Mar. 21 that it was slowing work at its South Texas nuclear plant expansion because of possible regulatory changes stemming from the Fukushima disaster. Those plants are based on evolutionary, Generation III+ designs. On the other hand, Chinese executives confirmed on Mar. 22 that they are going ahead with building their Generation IV pebble bed reactor. "Japan's Fukushima plant was using old technology while Chinese reactors are more advanced," Cui Shaozhang, deputy general manager at Huaneng Nuclear Power Development, told Bloomberg News.
Those are the two faces of the nuke debate. Fukushima could bring reactor development to a halt. Or it might stimulate the demand for safer designs. The future of atomic energy is at stake.