TOKYO - An official with Japan's nuclear safety commission says that a meltdown at a nuclear power plant affected by the country's massive earthquake is possible.
Ryohei Shiomi said Saturday that officials were checking whether a meltdown had taken place at the Fukushima Daiichi power plant, which had lost cooling ability in the aftermath of Friday's powerful earthquake.
If the fuel rods melted or are melting, a breach could develop in the nuclear reactor vessel and the question then becomes one of how strong the containment structure around the vessel is and whether it has been undermined by the earthquake, experts said.
Shiomi said that even if there was a meltdown, it wouldn't affect humans outside a six-mile (10-kilometer) radius.
Japan earlier Saturday declared states of emergency for five nuclear reactors at two power plants after the units lost cooling ability in the aftermath of Friday's powerful earthquake. Thousands of residents were evacuated as workers struggled to get the reactors under control to prevent meltdowns.
Operators at the Fukushima Daiichi plant's Unit 1 scrambled ferociously to tamp down heat and pressure inside the reactor after the 8.9 magnitude quake and the tsunami that followed cut off electricity to the site and disabled emergency generators, knocking out the main cooling system.
Some 3,000 people within two miles (three kilometers) of the plant were urged to leave their homes, but the evacuation zone was more than tripled to 6.2 miles (10 kilometers) after authorities detected eight times the normal radiation levels outside the facility and 1,000 times normal inside Unit 1's control room.
The government declared a state of emergency at the Daiichi unit — the first at a nuclear plant in Japan's history. But hours later, the Tokyo Electric Power Co., which operates the six-reactor Daiichi site, announced that it had lost cooling ability at a second reactor there and three units at its nearby Fukushima Daini site.
The government quickly declared states of emergency for those units, too, and thousands of residents near Fukushima Daini also were told to leave.
Japan's nuclear safety agency said the situation was most dire at Fukushima Daiichi's Unit 1, where pressure had risen to twice what is consider the normal level. The International Atomic Energy Agency said in a statement that diesel generators that normally would have kept cooling systems running at Fukushima Daiichi had been disabled by tsunami flooding.
Officials at the Daiichi facility began venting radioactive vapors from the unit to relieve pressure inside the reactor case. The loss of electricity had delayed that effort for several hours.
Plant workers there labored to cool down the reactor core, but there was no prospect for immediate success. They were temporarily cooling the reactor with a secondary system, but it wasn't working as well as the primary one, according to Yuji Kakizaki, an official at the Japanese nuclear safety agency.
TEPCO said the boiling water reactors shut down at about 2:46 p.m. local time following the earthquake due to the loss of offsite power and the malfunction of one of two off-site power systems. That triggered emergency diesel generators to startup and provide backup power for plant systems.
About an hour after the plant shut down, however, the emergency diesel generators stopped, leaving the units with no power for important cooling functions.
Nuclear plants need power to operate motors, valves and instruments that control the systems that provide cooling water to the radioactive core.
The race to restore the reactors' cooling systems before the radioactive fuel was damaged sent ripples of concern across Pacific, where scientists on both sides of the U.S. debate over the safety of nuclear power acknowledged that the company was facing a serious situation.
Edwin Lyman, a senior scientist with the Union of Concerned Scientists, which opposes nuclear energy, told msnbc.com that TEPCO was facing a potential catastrophe.
'It's just as bad as it sounds' "It's just as bad as it sounds," he said. "What they have not been able to do is restore cooling of the radioactive core to prevent overheating and that's causing a variety of problems, including a rise in temperature and pressure with the containment (buildings).
"What's critical is, are they able to restore cooling and prevent fuel damage? If the fuel starts to get damaged, eventually it will melt through the reactor vessel and drop to the floor of the containment building," raising the odds that highly radioactive materials could be released into the environment.
But Steve Kerekes, spokesman for the U.S.-based Nuclear Energy Institute, said that while the situation was serious, a meltdown remains unlikely and, even if it occurred would not necessarily pose a threat to public health and safety.
"Obviously that wouldn't be a good thing, but at Three Mile Island about half the core melted and, at the end of the day … there were no adverse impacts to the public," he said.
Experts also downplayed the seriousness of the trace levels of radiation detected at the Fukushima Daiichi plant.
Japan's Asahi Shimbum newspaper reported that radiation levels per hour in the area near the front entrance of the No. 1 Fukushima plant reached 0.59 micro Sievert, which is eight times the normal levels. The central control room of the reactor recorded radiation levels 1,000 times the normal level, which would be approximately 70 microsieverts per hour, or 7 millirems, according to calculations by msnbc.com.
Health effects unlikely Generally it would take much higher levels of outside exposure to cause health problems in humans. Radiation exposure is often measured in units called "millirem," which is 1/1000 of a rem. The average American is exposed to about 620 millirem each year, with about half from natural sources and half from manmade sources, according to the Nuclear Regulatory Commission.
Exposures of less than 50 millirem typically produce changes in blood chemistry, but no symptoms, according to the U.S. Environmental Protection Agency.
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