Photograph courtesy Tokyo Electric Power Company via Kyodo/AP
Published March 23, 2011
For a world that was on the brink of a major expansion in nuclear power, a key question raised by the Fukushima Daiichi crisis is this: Would brand-new reactors have fared better in the power outage that triggered dangerous overheating at one of Japan's oldest power plants?
The answer seems to be: Not necessarily.
The nuclear industry has developed reactors that rely on so-called "passive safety" systems that could address the turn of events that occurred in Japan—the loss of power to pump water crucial to cooling radioactive fuel and spent fuel. But these designs are being deployed in only four of the 65 plants under construction worldwide. (Four reactors that are in the site-preparation phase and still awaiting regulatory approval in Georgia and South Carolina in the United States would make that eight of 69 plants.)
The vast majority of plants under construction around the world, 47 in all, are considered Generation II reactor designs—the same 1970s vintage as Fukushima Daiichi, and without integrated passive safety systems.
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Nuclear plant operators are quick to point out that even if passive safety was not integrated into reactor design at the outset, these and other improvements have been added to existing reactors or to blueprints for ones under construction. For example, at the San Onofre Nuclear Generation Station on the southern California coast, modifications have been made that allow the operators to use a gravity-driven system to circulate the water to cool down the plant for a period of time upon loss of power, according to the Nuclear Energy Institute (NEI), a U.S. industry trade group.
But there are limits to such retrofits. "This is a huge volume of water," says Adrian Heymer, executive director of strategic programs for the NEI. "What happens to that tank in an earthquake?" That's why there's been an effort to integrate a fully passive system from the get-go of the design process, he said.
There is no ready reference list of which plants around the world have been modified with gravity-driven or other safety features. And as for new nuclear plants with integrated passive safety systems, deployment is slow. Nuclear plants require long lead times to gain government approval, obtain financing, and complete planning and construction—making them an anomaly in a world accustomed to lighting-fast changes in technology. In many cases, nuclear plants today are being built with much of the basic technology developed three or four decades ago.
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At Fukushima Daiichi, five of the six reactors use a General Electric design called BWR-3 (a boiling-water reactor), with a Mark 1 containment system. As many as 92 plants operating globally have been built using GE's boiling-water reactor design, and 32 of them feature the Mark 1 containment system.
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The Fukushima reactors, like all of the reactors in the top two nuclear countries—the United States and France—are considered "Generation II" reactors, under the somewhat loose categories recognized by the industry and regulators around the world. First-generation reactors were developed in the 1950s and '60s, according to the World Nuclear Association (WNA), an industry group based in London. The United Kingdom is the only place where any are still running; the Wylfa Nuclear Power Station in North Wales is one example.
Heymer says the 1979 nuclear crisis at Three Mile Island forced a reassessment of risk and safety in nuclear plant designs. "What if you lost all station electrics?" he asks. "You want to be in a position to restore power before the point where you would damage the fuel."
The use of gravity to move cooling water into the reactor vessel is a relevant example, in light of the cooling problems at Fukushima Daiichi. Think of it like a water tank on the roof of a house, where gravity could send the water gushing down the pipes and into the kitchen sink when a faucet is opened. In the reactor, the heat of the fuel then heats the air and water, causing them to rise up through a pipe to a heat exchanger.
With the loss of cooling systems in plants of a design and vintage similar to Fukushima Daiichi, said Heymer, "everything just slowly grinds to a halt" when grid power is lost. A passive system could buy time for a plant operator to "find a generator large enough to recharge a series of batteries, and pump cool water into the tank."
Technology's Slow March
But only 15 of the 442 nuclear reactors operating in the world are considered Generation III reactors—designs that have begun to integrate some "passive" or "inherent" safety systems. (Japan and Korea each have four; Canada, China, and Romania each have two; and Argentina has one.) Another 14 Generation III reactors are under construction in Japan, China, Taiwan, Korea, Finland, France, and Russia. And as the Gen III reactors now in operation date anywhere from 1982 and 2007, according to a list provided by NEI, they aren't exactly the leading edge in nuclear technology.
That next advance would be the Generation III-plus design, a plant that relies completely on passive safety systems in the case of an accident, as the U.S. Department of Energy categorizes it.
Gravity, natural convection, and conduction instead of grid-powered, diesel-fueled, or battery back-up electricity, are among the key strategies that have emerged for tackling the challenge of nuclear plants' constant need for cooling power. Rather than relying on the proper functioning of engineered components, auxiliary power and operator control, passive systems depend "only on physical phenomena," explains the World Nuclear Association.
"These are designs that have fully functional passive safety systems that have the ability to function at least 72 hours without AC [alternating current] electrical power or external cooling water," says Heymer.
The world's first four Generation III-plus units are under construction now in China, with the reactor at the Sanmen nuclear power plant in east China's Zhejiang province expected to come online in 2013. Another four Generation III-plus reactors are in the site-preparation phase in the United States: two units at Southern Company's Vogtle plant near Waynesboro, Georgia, and two at South Carolina Electric and Gas's Virgil C. Summer nuclear station near Jenkinsville, South Carolina.
All eight of these first Gen III-plus reactors would be Westinghouse's AP1000 design, which circulates cool outside air around a steel containment vessel, and drains water by gravity from a tank positioned atop the vessel. The system can provide cooling for up to 72 hours, according to Westinghouse spokesman Scott Shaw.
After that, a small diesel generator is meant to supply power to pump water from an onsite storage container into the reactor core and spent fuel pool at 100 gallons per minute for up to four days. According to Shaw, Westinghouse "jumped" from a Generation II design to Generation III-plus with the AP1000.
Planning for Power Loss
David Lochbaum, who directs the Union of Concerned Scientists' Nuclear Safety Project, agrees that passive safety systems can "give workers more time to deal with situations, and most often, more time translates into greater success of meeting a challenge, given the odds." But his watchdog organization remains concerned about a situation where normal cooling is out of commission for longer than 72 hours: "Then you have the same problem of how do you get water back up into the tank to replenish it?" he asks. At Fukushima, he noted, 72 hours would have afforded workers more time, "but perhaps still not enough time."
Heymer, however, says the system could be replenished by adding water with a fire truck and pump. (That approach doesn't work with the Generation II Fukishima Daiichi plant, because cooling there still relies on active operation of the plant's own pumping system.)
Another Generation III-plus reactor design is GE-Hitachi's ESBWR (Economic and Simplified Boiling Water Reactor), which employs a gravity-driven system for keeping the core covered with cool water, said Heymer.
Prior to the earthquake and tsunami in Japan, he said, the design was "on the verge of having licenses issued here [in the United States] in the next nine months." On March 9—two days before the natural disasters hit Japan—the U.S. Nuclear Regulatory Commission issued a final safety evaluation and design approval for the ESBWR. But the ruling is not scheduled to take effect until the fall. And given the still-evolving crisis at Fukushima Daiichi, the timeline is now difficult to predict, Heymer said. GE-Hitachi Nuclear Energy spokesperson Michael Tetuan expects final certification from the NRC in the fall. This is the company's first design with a passive safety system, he said.
According to Tetuan, the ESBWR is slated for construction (pending regulatory approval) in India, where the government has selected two sites. "We'll get one, Westinghouse gets the other," he said. In the United States, electric utility Detroit Edison selected GE's ESBWR back in 2008 for a potential new reactor at its Fermi 2 Power Plant on the shore of Lake Erie.
Whereas Fukushima's backup systems did not survive the tsunami, newer reactors such as the ESBWR and the Westinghouse AP1000 could be better equipped to handle this type of event, said Heymer. "They don't rely on an engine that has to start." And where the workers at Fukushima "have to manage many, many valves," some newer systems could have only two or three, said Heymer.
Advanced passive designs (once approved by regulators) could make boiling-water nuclear reactors 10 to 100 times safer than their active predecessors, said Heymer, based on the core damage frequency metric, a calculation of the likelihood that an accident could cause the fuel in a given reactor to melt.
But with so many Generation II reactors in operation and under construction, regulators and operators often focus on how retrofits can make older designs safer. In a U.S. Senate Environment and Public Works hearing this month, Nuclear Regulatory Commission Chairman Gregory B. Jaczko compared the process of keeping nuclear reactors up to date to that of "upgrading and modifying" 20-year-old airplane systems "as we better understand what can go wrong."
For example, all boiling-water reactors in the United States that use a similar design to the Fukushima Daiichi reactors have been outfitted since the 1990s with what's called a hardened vent, said Heymer. This allows the reactor to vent steam and pressure "straight to the atmosphere," with a filter intended to remove radioactivity. In older designs, the vent ran from the reactor and into the reactor building, where hydrogen can build up.
At this point, the NEI has "struggled" to get a firm answer from Japanese officials about whether the troubled Fukushima reactors have hardened vents, Heymer said. "They have so many things going on, they may not be able to get information out to us." But explosions at four of the Fukishima's six reactors indicate that hydrogen did in fact build up in the reactor buildings, he said.
Quenchers, Deflectors, and Saddles
Facing growing scrutiny as to the safety of its Mark 1 containment system, General Electric released a document March 16 emphasizing that the 40-year-old technology has "continued to evolve."
For example, the company devised a "quencher" system for reducing pressure in the large donut-shaped suppression chamber (called a "torus") that surrounds the base of the reactor core in a boiling-water reactor. Steam bubbles go underwater in the torus to help remove heat. According to GE, the quencher system breaks up large bubbles into smaller ones, producing rapid condensation and reducing pressure.
Also in the torus, GE has installed devices called deflectors, which are designed to break up a pressure wave created when steam enters the pool and raises the water level. GE says it has also fortified the construction of saddles—the leg-like structures on which the torus sits.
The U.S. NRC required this reinforcement of the torus, as well as the improved venting system, for all U.S. plants with the GE Mark 1 systems. "We shared that with our customers overseas," Tetuan said in a story by the news agency AFP, "but I can't tell you if they did indeed retrofit."
According to Ellen Vancko, nuclear energy and climate change project manager for the Union of Concerned Scientists, "There are going to be a lot of lessons learned that we're going to have to take from this. We don't know what they all are yet."
Whether safety procedures were not followed at Fukushima, or if they were followed and they did not help, said Lochbaum, "either way, we are going to have to beef up the procedures . . . so people take the right paths when they reach a crossroads or develop new crossroads that will lead you in the right direction."
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