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 the threat of ongoing radiation leaks prompted them to expand the evacuation area around the facility to a 12-mile radius. (Watch: Japan, the day after)
Although safety officials described the release of radioactive materials as small, they also told the International Atomic Energy Agency that they were making preparations to distribute iodine, which is used to help protect from radiation exposure, to people living near two nuclear plants that suffered damage in the quake. Japanese news media said three workers at the Daiichi plant had suffered radiation exposure.
Government officials and executives of Tokyo Electric Power, which runs the plant, gave confusing accounts of the causes of the explosion and the damage it caused. Late Saturday night, officials said that the explosion occurred in a structure housing turbines near the No. 1 reactor at the plant rather than inside the reactor itself. (Watch: Japan: The day earth shook)
The blast, apparently caused by a sharp build-up of pressure after the reactor's cooling system failed, destroyed the concrete structure surrounding the reactor but did not collapse the critical steel container inside, they said. They said that raised the chances they could prevent the release of large amounts of radioactive material and could avoid a core meltdown at the plant.
"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 on 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."
The company also said its workers also added boric acid to the containment vessel on Saturday night to slow down the nuclear reaction.
Mr. Edano said radioactive materials had leaked outside the plant before the explosion, but that the explosion did not worsen the leak and that, in fact, measured levels of radioactive emission had been decreasing. He did not specify the levels of radiation involved. (Read: Japan tsunami - Toll could rise to more than 1,300)
Officials said even before the explosion that they had detected cesium, an indication that some of the nuclear fuel in the reactor was already damaged. That suggests that the plant experienced a partial meltdown. But officials insisted the fuel damage was contained and that the prospect of more radioactive leaks had receded.
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."
The crisis at the aging plant confronted Japan with its worst nuclear accident -- and one of the biggest malfunctions 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 had suffered a complete core meltdown, which they said it had not. But the problems at the plant are likely to increase 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. (See Pics: Japan earthquake triggers tsunami)
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.
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 plant, which ignited and exploded Saturday afternoon, government officials said. They said the explosion itself did not increase 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. (Watch: Japan - It's all rubble now)
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.
David Lochbaum, who worked at three reactors in the United States similar to the Fukushima design, and who was later hired by the Nuclear Regulatory Commission to teach its personnel about that technology, said that from pictures he had seen of the stricken plant, the explosion appeared to have occurred in the turbine hall, which is usually a large, rectangular building adjacent to a smaller, taller reactor building.
The technology used at Fukushima is called a boiling water reactor, in which the reactor, inside a containment, sends its steam out of containment to a turbine. The turbine converts the steam's energy into rotary motion, which turns a generator and makes electricity.
But as the water goes through the reactor, some water molecules break up into hydrogen and oxygen. A system in the turbine hall usually scrubs out those gases. Hydrogen is also used in the turbine hall to cool the electric generator. Hydrogen from both sources has sometimes escaped and exploded, he said, but in this case, there is an additional source of hydrogen: interaction of steam with the metal of the fuel rods.
Earlier 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. (Watch: Japan begins evacuation drive)
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.
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