VIENNA — The tsunami that followed Japan's strongest recorded earthquake exposed nuclear-safety flaws even as an emergency response worker was killed and at least 17 were injured trying to contain radiation from damaged reactors.
Electrical generators intended to run cooling systems in case of an emergency failed at three reactors at Tokyo Electric Power Co.'s Fukushima Dai-Ichi nuclear power plant after the March 11 earthquake and ensuing tsunami. The plant has six reactors, three of which were closed for maintenance when the earthquake struck.
"All systems were working well after the earthquake, but the tsunami destroyed the emergency electrical generators, and an overheating situation has developed," said Robert Kelley, a Vienna-based nuclear engineer who worked in nuclear emergency response during his 30-year career with the Department of Energy. "It appears the redundancy wasn't actually redundant, and that led to the failure."
While the reactor vessels — the metal constructs that house the uranium fuel — withstood the stress of the earthquake and subsequent explosions inside the buildings that house them, Japanese regulators overlooked the magnitude of the tsunami that would follow a 9.0-magnitude quake off the coast.
Japanese authorities designed backup electrical generators to withstand waves 6.3 meters high, below the 7-meter surge that knocked out power at the plant, disabling vital cooling systems, Ian Hore-Lacy, a spokesman for the London-based World Nuclear Association, said in a telephone interview.
"Everybody learns from a tragedy like this," Hore-Lacy said. His association is an advocate for the nuclear energy industry.
Nuclear reactors have layers of safety features designed to prevent radiation from reaching the atmosphere. Uranium fuel rods are sealed with zirconium alloy. Steel ingots are formed to contain the radioactive fuel inside of seam-free containment vessels immune to rupture. A pressure-resistant, airtight containment vessel is built around the reactor.
These "nuclear reactors have multiple safety measures," International Atomic Energy Agency Director General Yukiya Amano said in Vienna. "The nuclear core is contained inside the vessels that is made of solid steel and then that reactor vessel is contained in another primary containment vessel. These vessels withheld."
Because of the reactor design, Amano dismissed the possibility that Japan's nuclear incident could lead to a radiation release on the scale of Ukraine's Chernobyl disaster in 1986. The Chernobyl reactor's heat was modified with graphite rather than water, resulting in a high concentration of radioactive dust when the fuel began to melt.
Radiation dose rates of 400 millisieverts per hour were detected Tuesday at the Japanese site, the IAEA said. That contrasts with readings of 40 microsieverts two miles from the plant Monday. There are 1,000 microsieverts in a millisievert. Normal background radiation is 0.3 microsieverts per hour.
Japan's reactor vessel absorbed 50 percent more pressure than it was designed to withstand, according to a Japanese report to the IAEA in Vienna on March 11.
While most modern nuclear reactor vessels are built to withstand the kind of strong earthquakes that occur once every 1,000 years, the infrastructure surrounding the reactor's radioactive core may still be susceptible to tsunamis, according to the World Nuclear Association.
"Even for a nuclear plant situated very close to sea level, the robust sealed containment structure around the reactor itself would prevent any damage to the nuclear part from a tsunami, though other parts of the plant might be damaged," the association said on its website, citing Japanese regulatory documents.
With assistance from Simeon Bennett in Singapore.