March 12, 2011
Explosion Rocks Japan Nuclear Plant After Quake
By MATTHEW L. WALD
WASHINGTON — An explosion at a nuclear power plant in northern Japan on Saturday blew the roof off one building, brought down walls and caused a radiation leak of unspecified proportions, Japanese officials said, after Friday’s huge earthquake caused critical failures in the plant’s cooling system.
Television images showed a huge cloud of white-gray smoke from the explosion. Soon afterward, government officials said an evacuation zone around the plant had been doubled, to 12 miles. The chief cabinet secretary, Yukio Edano, confirmed earlier news reports of an explosion at the Fukushima Daiichi nuclear plant, 15o miles north of Tokyo, saying: “We are looking into the cause and the situation and we’ll make that public when we have further information.” He was speaking amid fears that a disastrous meltdown could be imminent because of critical cooling failures at that plant and another nearby, Daini, after both were shut down.
Images on Japanese television showed that the walls of one building had crumbled, leaving only a skeletal metal frame standing with smoke billowing from the plant. The Associated Press reported that the damaged building housed a nuclear reactor, though that report was not immediately verified by nuclear officials. The cause of the explosion was unclear, with some experts speculating that it may have resulted from a hydrogen build-up.
There was no immediate confirmation of news reports that the container of the nuclear reactor itself had escaped damage.
Bloomberg News quoted Tokyo Electric Power Co., the plant’s operator, as saying the explosion happened “near” the No. 1 reactor at around 3:40 p.m. Japan time on Saturday. Four people were reported injured. The explosion came roughly 26 and a half hours after an 8.9-magnitude earthquake caused a deadly tsunami that killed hundreds and caused both plants to be shut down. Authorities issued broad evacuation orders on Saturday for people living near the plants and warned that small amounts of radioactive material were likely to leak out.
Officials said even before the explosion that they had detected cesium, an indication that some of the fuel was already damaged.
In the form found in reactors, radioactive cesium is a fragment of a uranium atom that has been split. In normal operations, some radioactivity in the cooling water is inevitable, because neutrons, the sub-atomic particles that carry on the chain reaction, hit hydrogen and oxygen atoms in the water and make those radioactive. But cesium, which persists far longer in the environment, comes from the fuel itself.
Naoto Sekimura, a professor at Tokyo University, told NHK, Japan’s public broadcaster, that “only a small portion of the fuel has been melted. But the plant is shut down already, and being cooled down. Most of the fuel is contained in the plant case, so I would like to ask people to be calm.”
Failure of the cooling systems allowed pressure to build up beyond the design capacity of the reactors. Small amounts of radioactive vapor were expected to be released into the atmosphere to prevent damage to the containment systems, safety officials said. They said that the levels of radiation were not large enough to threaten the health of people outside the plants, and that the evacuations had been ordered as a precaution.
Nuclear safety officials focused initially on the Daiichi plant. But by Saturday morning Japan had declared states of emergency for five reactors at the two plants, an escalation that added to worries about the safety of nuclear facilities in the quake-prone Japanese islands.
The Daiichi and Daini plants are 10 miles apart in Fukushima Prefecture, close to the quake’s epicenter off the coast.
The plants’ problems were described as serious but were far short of a catastrophic emergency like the partial core meltdown that occurred at the Three Mile Island plant near Harrisburg, Pa., in 1979.
A Japanese nuclear safety panel said the radiation levels were 1,000 times above normal in a reactor control room at the Daiichi plant. Some radioactive material had also seeped outside, with radiation levels near the main gate measured at eight times normal, NHK quoted nuclear safety officials as saying.
The safety officials said there was “no immediate health hazard” to residents from the leaks, which they described as “minute,” and people were urged to stay calm.
The emergency at the Daiichi plant began shortly after the earthquake struck on Friday afternoon. Emergency diesel generators, which had kicked in to run the reactor’s cooling system after the electrical power grid failed, shut down about an hour after the earthquake. There was speculation that the tsunami had knocked the generators out of service.
Twenty hours later, the plant was operating in a battery-controlled cooling mode. Tokyo Electric said that by Saturday morning it had installed a mobile generator at Daiichi to ensure that the cooling system would continue operating even after reserve battery power was depleted. Even so, the company said it was considering a “controlled containment venting” in order to avoid an “uncontrolled rupture and damage” to the containment unit.
“With evacuation in place and the oceanbound wind, we can ensure the safety,” a nuclear safety official, Yukio Edano, said at a news conference early Saturday.
It was not clear, however, how long the cooling systems could continue to function in emergency mode or when normal power supplies could be restored.
Two workers were reported missing at the Daiichi plant, but the company did not explain what might have happened to them.
A pump run by steam, designed to function in the absence of electricity, was adding water to the reactor vessel, and as that water boiled off, it was being released. Such water is usually only slightly radioactive, according to nuclear experts. As long as the fuel stays covered by water, it will remain intact, and the bulk of the radioactive material will stay inside. If the fuel is exposed, it can result in a meltdown.
The reactors at the two plants initially shut down when the earthquake began at 2:46 p.m. Friday. But emergency diesel generators at the Daiichi plant went down a little less than an hour later, and pressure began to rise in the reactor, leading operators to vent it.
During much of the early morning on Saturday, safety officials focused on getting emergency power supplies to the Daiichi plant to restore the normal cooling function.
Secretary of State Hillary Rodham Clinton, speaking in Washington, said that American military planes had already delivered “coolant.” But American military officials indicated that while they were prepared to help Japan grapple with any problems related to its nuclear facilities, they had not been asked to do so.
Japan relies heavily on nuclear power, which generates just over one-third of the country’s electricity. Its plants are designed to withstand earthquakes, which are common, but experts have long expressed concerns about safety standards, particularly if major quake hit close to a reactor.
One major concern is that while plant operators can quickly shut down a nuclear reactor, they cannot allow the cooling systems to stop working. Even after the plant’s chain reaction is stopped, its fuel rods produce about six percent as much heat as they do when the plant is running. The production of heat drops off sharply in the following hours, but continued cooling is needed or the water will boil away and the fuel will melt, releasing the uranium fragments inside.
Heat from the nuclear fuel rods must be removed by water in a cooling system, but that requires power to run the pumps, align the valves in the pipes and run the instruments. The plant requires a continuous supply of electricity even after the reactor stops generating power.
With the steam-driven pump in operation, pressure valves on the reactor vessel would open automatically as pressure rose too high, or could be opened by operators. “It’s not like they have a breach; there’s no broken pipe venting steam,” said Margaret E. Harding, a nuclear safety consultant who managed a team at General Electric, the reactors’ designer, that analyzed pressure buildup in reactor containments. “You’re getting pops of release valves for minutes, not hours, that take pressure back down.”
Civilian power reactors are designed with emergency diesel generators to assure the ability to continue cooling even during a blackout. Many reactors have two, assuring redundancy; some have three, so that if one must be taken out of service for maintenance, the plant can still keep running.
It was not immediately clear how many diesel generators there are at Daiichi, but the operators reported earlier in the day that they were not working, prompting the evacuation.
Daiichi, which is formally known as Fukushima Daiichi Nuclear Power Station, was designed by General Electric and entered commercial service in 1971. It was probably equipped to function for some hours without emergency diesel generators, said David Lochbaum, who worked at three American reactor complexes that use G.E. technology.
Mr. Lochbaum, who also worked as an instructor for the Nuclear Regulatory Commission on G.E. reactors, said that such reactors were equipped to ride out interruptions in electrical power by using pumps that could be powered by steam, which would still be available in case of electric power failure. Valves can be opened by motors that run off batteries, he said. Plants as old as Fukushima Daiichi 1 generally have batteries that are large enough to operate for four hours, he said.
After that, he said, the heat production in the core is still substantial but has been reduced. The heat would boil away the cooling water, raising pressure in the reactor vessel, until automatic relief valves opened to let out some of the steam. Then the valves would close and the pressure would start building again.
If the cooling system remains inoperative for many hours, the water will eventually boil away, he said, and the fuel will begin to melt. That is what happened at Three Mile Island. In that case, the causes were mechanical failure, operator error and poor design, according to government investigators.
Yasuko Kamiizumi contributed reporting from Tokyo, Alan Cowell from Paris and Ken Belson from New York.
http://www.nytimes.com/2011/03/13/wo...13nuclear.html
8.9 magnitude earthquake near Japan, enormous tsunami hitting surrounding areas
They have put up a list of when the tsunami will hit other countries too, here:
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Originally Posted by LittleFuzzy
