Energy

What We Learned About Nuclear Safety from Fukushima

Reactors must be able to handle the worst if we hope to prevent a repeat of last year’s meltdowns.

A year after Japan’s largest earthquake and most destructive tsunami led to the Fukushima nuclear accident, experts say the industry has moved beyond any claims of absolute safety. As happened after the 2010 BP oil spill in the Gulf of Mexico, experts now recognize that any technology—whether it’s deepwater drilling or nuclear fission—can and will fail, and operators must prepare for the worst.

“Fukushima Daiichi … was not just due to an inadequately sized seawall—that is the wrong way to look at it,” says Edward Blandford, a professor of nuclear security at the University of New Mexico and a postdoctoral fellow at Stanford University’s Center for International Security and Cooperation. “The events at Fukushima Daiichi were due to a series of failures, including failures in plant defensive actions, mitigation efforts, and emergency response. If backup equipment had been stored in waterproof vaults or higher elevations, the accident would have most likely been avoided.”

Nuclear operators and regulators say they embrace the need to anticipate the worst. Nuclear utilities in the U.S. launched a program this winter to stock portable reactor-cooling equipment at regional depots, and last week, the U.S. Nuclear Regulatory Commission (NRC) approved new mandates requiring operators to prepare for events worse than what a reactor is designed to handle—or “beyond design-basis events,” in industry lingo.

Reactors and radioactive materials at Fukushima Daiichi were destabilized by back-to-back beyond design-basis events. First was the magnitude 9.0 earthquake that felled the plant’s power lines, triggering diesel generators to maintain cooling of its reactor cores and spent fuel rods. Less than an hour later, the generators along with some of the plant’s last-resort battery power backup were gone, knocked out by a 14-meter tsunami wave that crested the plant’s seawall.

Human error and design limitations quickly compounded the impact of the loss of power. Operators mistakenly shut down battery-driven cooling on one reactor for three hours, for example. Within 24 hours of the tsunami, nuclear fuel in three reactors was melting down, and superheated fuel was generating hydrogen gas, whose ignition would blow open three reactor buildings in the days ahead, impeding response efforts and exposing elevated pools holding spent nuclear fuel.

While no deaths have yet been attributed to Fukushima’s triple meltdowns, the radioactivity they released prompted a mass evacuation and contaminated an area spanning over 8,000 square miles.

Nuclear experts say the key to controlling future incidents and thus restoring faith in nuclear energy is a “defense in depth” approach to reactor design and emergency preparedness—precisely what was missing at Fukushima. Locating backup air-cooled diesel generators in basements, for example, was a sign that Fukushima wasn’t fully prepared for a tsunami, says Tony Irwin, a lecturer in nuclear technology at the Australian National University who participated in a post-Chernobyl review of operating practices at Russian reactors. “Even if the seawall was not high enough, a proper risk assessment would have identified the need for waterproof rooms, backup pumps on higher ground, etc.,” says Irwin.

The newest generation of reactors features more backup defenses, notes Irwin. Last month, the NRC approved permits for construction of two reactors at Southern Company’s Vogtle, Georgia, nuclear station using the Westinghouse AP1000 design, which has a passive cooling capacity: an elevated reservoir that can be gravity-fed to keep its reactor cool for three days with no electricity. “This 72 hours would have helped greatly [at Fukushima], allowing operators to direct much-needed personnel resources to restoring backup onsite power,” says Blandford.

AP1000’s passive cooling could, of course, fail to operate after a hurricane, tornado, or other calamity greater than the engineers at Westinghouse and the NRC have anticipated. In that event, an AP1000 would have to fall back on conventional power-driven pumps, says Edwin Lyman, a nuclear safety and security expert with the Union of Concerned Scientists. The problem, says Lyman, is that such backup equipment has been winnowed to reduce costs. “If you have a seismic event, for example, that backup may not be available when you need it,” suggests Lyman.

Other experts say the industry’s new voluntary emergency response program will fill such gaps by placing equipment such as portable pumps in regional depots, hopefully beyond the reach of the events that strike a nuclear reactor. “We don’t know what the next rare phenomenon will be, but we’ll be prepared to provide water to the core,” says Andrew Kadak, a professor of nuclear science and engineering at MIT.

But Lyman says NRC audits of voluntary safety upgrades made by the nuclear industry after the September 11 attacks revealed that much of the added equipment was commercial grade, as opposed to higher-grade equipment certified for use in a nuclear plant. “We question whether it would be effective in the heat of an event like Fukushima,” says Lyman. Without greater oversight from the NRC, he says, the industry’s regional depots could similarly fall short of their promise.

In Japan, where nuclear power has, until recently, underpinned the nation’s energy strategy, the inadequate disaster preparation laid bare last year has fueled an antinuclear backlash. If the result is a nuclear phase out, the nuclear industry will have its own propaganda to blame, according to a report on the Fukushima accident’s causes issued last week by an independent commission chaired by Koichi Kitazawa, an expert in materials science and superconductivity and former president of Japan’s Science and Technology Agency.

As the commission’s summary notes, Tepco’s nuclear energy division understood as early as 2006 that some tsunami researchers believed that a tsunami in 869 A.D. would have crested well above Fukushima Daiichi’s seawalls. But the industry judged that raising seawalls and other such high-profile safety upgrades would call its myth of “absolute safety” into question. As the report puts it: “Power companies found themselves caught in their own trap.”

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