Tokyo – An explosion at a crippled nuclear power plant in northern Japan on Saturday blew the roof off one building and caused a radiation leak of unspecified proportions, escalating the emergency confronting Japan’s government a day after an earthquake and tsunami devastated parts of the country’s northeastern coast.
Japanese television showed a cloud of white-gray smoke from the explosion billowing up from a stricken reactor at the Fukushima Daiichi Nuclear Power Station Saturday afternoon, and officials said leaks of radiation from the plant prompted them to expand the evacuation area around the facility to a 12-mile radius.
Government officials said that the explosion, caused by a build-up of pressure in the reactor after the cooling system failed, destroyed the concrete structure surrounding the reactor but did not collapse the critical steel container inside. They said that raised the chances that they could prevent the release of large amounts of radioactive material and could avoid a core meltdown at the plant.
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“We’ve confirmed that the reactor container was not damaged. The explosion didn’t occur inside the reactor container. As such there was no large amount of radiation leakage outside,” Japan’s Chief Cabinet Secretary Yukio Edano said in a news conference Saturday evening. “At this point, there has been no major change to the level of radiation leakage outside, so we’d like everyone to respond calmly.”
Tokyo Electric Power, which operates the plant, which is located 160 miles north of Tokyo, now plans to fill the reactor with sea water to cool it down and reduce pressure. The process would take five to 10 hours, Mr. Edano said, expressing confidence that the operation could “prevent criticality.”
But the crisis at the aging plant confronted Japan with its worst nuclear accident – and perhaps the biggest mishap at a nuclear plant since the Chernobyl nuclear disaster in 1986.
Japanese nuclear safety officials and international experts said that because of crucial design differences the release of radiation at the Fukushima plant would likely be much smaller than at Chernobyl even if the Fukushima plant has a complete core meltdown, which they said it had not. But the problems at the plant are certain to worsen concerns about the safety record and reliability of Japan’s extensive nuclear power facilities, which have been criticized for major safety violations in the past.
The vulnerability of nuclear plants to earthquakes was also underscored by ongoing problems at the cooling system of reactors at a second nearby plant, known as Daini, which prompted a smaller evacuation from surrounding communities.
Tokyo Electric Power said the explosion happened “near” the No. 1 reactor at Daiichi at around 3:40 p.m. Japan time on Saturday. It said four of its workers were injured in the blast.
Officials said even before the explosion that they had detected cesium, an indication that some of the nuclear 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.”
Both the Daiichi and Daini plants were shut down during Friday’s earthquake. But the loss of power in the area and damage to the plant’s generators from the subsequent tsunami crippled the cooling systems, which need to function after a shut down to cool down nuclear fuel rods.
Malfunctioning cooling systems allowed pressure to build up beyond the design capacity of the reactors. Early Saturday officials had said that small amounts of radioactive vapor were expected to be released into the atmosphere to prevent damage to the containment systems and that they were evacuating tens of thousands of people living around the plants as a precaution.
Those releases apparently did not prevent the buildup of hydrogen inside the reactor, which ignited and exploded Saturday afternoon, government officials said. They said the explosion itself probably did not result in dramatic increases in the amount of radioactive material being released into the atmosphere, but they expanded the evacuation area around the Daiichi plant from a six-mile radius to a 12-mile radius.
Safety officials continued to insist that the levels of radiation were not large enough to threaten the health of people outside the plants, but they also told people living in the vicinity to cover their mouths and stay indoors.
Earlier on Saturday, before the explosion, 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 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 flooded the generators and knocked them out of service.
For some time after the quake, the plant was operating in a battery-controlled cooling mode. Tokyo Electric said that by Saturday morning it had also 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 needed to conduct “controlled containment venting” in order to avoid an “uncontrolled rupture and damage” to the containment unit.
Why the controlled release of pressure on Saturday did not succeed in addressing the problem at the reactor was not immediately explained. Tokyo Electric and government nuclear safety officials also did not explain the precise sequence of failures at the plant.
Daiichi and other nuclear facilities are designed with extensive backup systems that are supposed to function in emergencies to ensure the plants can be shut down safely.
At Daiichi, 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. But if fresh water cannot be pumped into the containment vessel and the cooling water evaporates, the nuclear fuel is exposed, which can result in a meltdown.
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.
This article “Explosion Rocks Japan Nuclear Power Plant After Quake” originally appeared at The New York Times.
© 2011 The New York Times Company
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