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The meltdown started when water to cool the reactors fell to dangerously low levels four hours after the fourth-largest recorded earthquake rattled the Fukushima Daiichi nuclear power plant. Five out of six of its reactors lost electricity when a 14-meter tall tsunami swept in 40 minutes later. Backup diesel generators lost their fuel tanks and died. Cooling water pumps failed. Nuclear fuel rods began melting and volatile hydrogen gas built up. Subsequent explosions and fire spewed 15,000 terabecquerels of radioactive cesium 137 alone, enough so that officials created an "exclusion zone" of 20 kilometers around the plant that persists today. (A becquerel is a unit of the rate of radioactive decay—or radiation emitted by a substance.) As a result, the emergency at Fukushima Daiichi that began on March 11, 2011, is only the second nuclear accident to merit the most severe international crisis rating, joining the reactor that exploded at the Soviet Union's Chernobyl nuclear facility in Ukraine April 1986.
But the disaster was no surprise given the type of reactors at Fukushima. In fact, nuclear power experts, computer models and other analyses have consistently shown for decades that a problem in the older boiling-water reactors employed at Fukushima Daiichi would become disastrous because of a flawed safety system that houses the nuclear fuel, known as the Mark I containment. It is "the worst one of all the containments we have"—and in a complete blackout, "you're going to lose containment," noted U.S. Nuclear Regulatory Commission (NRC) Deputy Regional Administrator Charles Casto on March 16, 2011, who was in Japan to assist, according to transcripts of internal meetings released by the NRC. "There's no doubt about it."
The U.S. has 23 reactors with the same kind of safety systems—and the same risky placement of pools for spent nuclear fuel, namely, alongside the main reactor in the top of the reactor building. Would U.S. reactors perform any better than Japan's in a crisis? And what lessons does Fukushima hold for reactor safety worldwide?
Off the Mark
The Mark I containment is a doughnut-shaped structure beneath the reactor itself that is partially filled with water. In the event of a breakdown of pumps that supply the reactor with fresh cooling water, the torus design is supposed to provide additional cooling. Steam created by the still fissioning fuel floods into the torus and is cooled by the supplemental water there. That additional cooling would limit the pressure created by any steam buildup, theoretically allowing the reactor's designers to employ less strength in other parts of the safety system.
Unfortunately, any additional cooling provided by the torus did not last as long as the loss of electricity at Fukushima. As a result, the nuclear rods heated their zirconium cladding along with the remaining water to steam. At high heat, the cladding interacts with the surrounding water vapor, binding tightly to the oxygen and freeing the hydrogen, which escapes as a gas. If allowed to accumulate, the hydrogen can burn with an invisible flame as it did at Three Mile Island (which had a different containment system) or, as appears to be the case at Fukushima, explode. As much as 1,000 kilograms of hydrogen may have been generated at the complex this way, according to the Japan's Nuclear and Industrial Safety Agency. In fact, the nuclear fuel in Unit 3 produced enough hydrogen to cause the explosion in Unit 4 next door via a shared exhaust stack.