Japan Floods Nuclear Reactor Crippled by Quake in Effort to Avert Meltdown

TOKYO — Japanese officials took the extraordinary step on Saturday of flooding a crippled nuclear reactor with seawater in a last-ditch effort to avoid a nuclear meltdown, as the nation grappled simultaneously with its worst nuclear accident and the aftermath of its largest recorded earthquake.       
A radiation leak and explosion at the Fukushima Daiichi Nuclear Power Station on Saturday prompted the government to expand an evacuation order to affect 170,000 people in the plant’s vicinity. And the plant’s operator issued an emergency notice early Sunday morning that a second reactor at the same aging plant was also experiencing critical failures of its cooling system, and that a way to inject water into the reactor to cool it was urgently being sought.       
The government said that radiation emanating from the first reactor appeared to be decreasing after the blast on Saturday afternoon destroyed part of the facility, and they said that they had filled it with seawater to prevent full meltdown of the nuclear fuel. That step would be taken only in extreme circumstances because ocean water is likely to permanently disable the reactor.       
The Japanese Nuclear and Industrial safety agency said as many as 160 people may have been exposed to radiation around the plant, and Japanese news media said three workers at the facility were suffering from full-on radiation sickness.       
The handling of the crisis and the vulnerability of Japan’s extensive nuclear facilities to earthquakes and tsunamis will also add to long-simmering grass-roots resistance against nuclear power within Japan, where people have learned to doubt the industry’s reliability as well as anodyne official statements about safety.       
Even if Japan manages to avoid large, uncontrolled releases of radiation that would result from a meltdown, the problems at the Fukushima facility already amounted to the worst nuclear accident in Japan’s history and perhaps the biggest accident at a nuclear plant since the Chernobyl disaster 25 years ago, the worst ever.       
Even before the explosion on Saturday, officials said they had detected radioactive cesium, which is created when uranium fuel is split, an indication that some of the nuclear fuel in the reactor was already damaged — a situation sometimes referred to as a partial meltdown. How much damage the fuel suffered remained uncertain, though safety officials insisted repeatedly through the day that radiation leaks outside the plant remained small and did not pose a major health risk.       
However, they also told the International Atomic Energy Agency that they were making preparations to distribute iodine, which helps protect the thyroid gland from radiation exposure, to people living near the Daiichi (or No. 1) plant and a second nuclear plant that suffered damage in the quake, Daini (or No. 2), about 10 miles away.       
Worries about the safety of the two plants worsened on Saturday because government officials and executives of the company that runs them, Tokyo Electric Power, gave confusing accounts of the causes of the dramatic midday explosion and the damage it caused. Late Saturday night, officials said that the explosion at Daiichi occurred in a structure housing turbines near its No. 1 reactor at the plant, rather than inside the reactor itself.       
They said that the blast — apparently caused by a sharp buildup of hydrogen when the reactor’s cooling system failed after the quake — 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 continue cooling the core, and thereby prevent the release of large amounts of radioactive material and a full core meltdown.       
“We’ve confirmed that the reactor container was not damaged,” Japan’s chief cabinet secretary, Yukio Edano, said in a news conference on Saturday night. “The explosion didn’t occur inside the reactor container. As such there was no large amount of radiation leakage outside. At this point, there has been no major change to the level of radiation leakage outside, so we’d like everyone to respond calmly.”       
Mr. Edano said that, in addition to filling the reactor with seawater, Tokyo Electric Power workers also added boric acid to the containment vessel on Saturday night to interrupt the nuclear chain reaction. Mr. Edano said that the operation could “prevent criticality.”       
He said that radioactive materials had leaked outside the plant before the explosion, but that the blast did not worsen the leak and, in fact, measured levels of radioactive emission had been decreasing. He did not specify the levels of radiation involved.       
On Sunday morning, an official with Tokyo Electric Power said that the emergency cooling system at the No. 3 reactor at Daiichi had stopped working. The official, Atsushi Sugiyama, said that urgent efforts were being made to cool the reactor with water, and that, as with the first reactor, there would be a release of vapor containing trace amounts of radiation to relieve a buildup of pressure.       
Japanese nuclear safety officials and international experts said that because of crucial design differences, the release of radiation at Daiichi would most likely be much smaller than at Chernobyl even if the plant had a complete core meltdown, which they said it had not.       
But the vulnerability of nuclear plants to earthquakes was underscored by the continuing problems with the cooling systems of reactors at the Daini plant, which prompted a evacuation of 30,000 from surrounding communities. Together, the authorities sought to move about 200,000 people around the two plants, an enormous logistical task at a time when rescue workers also sought to help people trapped or injured in the earthquake.       
After a full day of worries about the radiation leaking at Daiichi, Tokyo Electric Power said an explosion occurred “near” the No. 1 reactor at Daiichi around 3:40 p.m. Japan time on Saturday. It said four of its workers were injured in the blast.       
The decision to flood the reactor core with corrosive seawater, experts said, was an indication that Tokyo Electric Power and Japanese authorities had probably decided to scrap the plant. “This plant is almost 40 years old, and now it’s over for that place,” said Olli Heinonen, the former chief inspector for the I.A.E.A., and now a visiting scholar at Harvard.       
Mr. Heinonen lived in Japan in the 1980s, monitoring its nuclear industry, and visited the stricken plant many times. Based on the reports he was seeing, he said he believed that the explosion was caused by a hydrogen formation, which could have begun inside the reactor core. “Now, every hour they gain in keeping the reactor cooling down is crucial,” he said.       
But he was also concerned about the presence of spent nuclear fuel in a pool inside the same reactor building. The pool, too, needs to remain full of water to suppress gamma radiation and prevent the old fuel from melting. If the spent fuel is also exposed — and so far there are only sketchy reports about the condition of that building — it could also pose a significant risk to the workers trying to prevent a meltdown.       
Both Daiichi and Daini were shut down by Friday’s earthquake, but the loss of power in the area and damage to the plants’ generators from the ensuing tsunami crippled the cooling systems. Those are crucial after a shutdown to cool down the nuclear fuel rods.       
The malfunctions 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 people in the area 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. However, safety officials urged people who were not evacuating but still lived relatively nearby to cover their mouths and stay indoors.       
David Lochbaum, who worked at three reactors in the United States with designs similar to Daiichi, and who was later hired by the Nuclear Regulatory Commission to teach its personnel about that technology, said that judging by photographs of the stricken plant, the explosion appeared to have occurred in the turbine hall, not the reactor vessel or the containment that surrounds the vessel.       
The Daiichi reactor is a boiling-water reactor. Inside the containment, the reactor sends its steam out 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, Mr. Lochbaum said, but in this case, there is an additional source of hydrogen: interaction of steam with the metal of the fuel rods. Operators may have vented that hydrogen into the turbine hall.       
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 Daiichi. Some radioactive material had also seeped outside, with radiation levels near the main gate measured at eight times normal levels, NHK, Japan’s public broadcaster, quoted nuclear safety officials as saying.       
The emergency at Daiichi began shortly after the earthquake struck Friday afternoon. Emergency diesel generators, which kicked in to run the 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, knocking them out of service.       
For some time, the plant was able to operate in a battery-controlled cooling mode. Tokyo Electric Power said that by Saturday morning it had also installed a mobile generator 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 did not succeed in addressing the problem was not immediately explained. Tokyo Electric Power 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, the steam 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.       

Michael Wines reported from Tokyo, and Matthew L. Wald from Washington. Martin Fackler contributed reporting from Nakaminato, Japan, and David E. Sanger from Washington.

Copyright. 2011. The New York Times Company. All Rights Reserved

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