NHK, Japan’s public broadcaster, flashed instructions to evacuees: close doors and windows; place a wet towel over the nose and mouth; cover up as much as possible. At a news conference, Mr. Edano called for calm. “If measures can be taken, we will be able to ensure the safety of the reactor,” he said. 
 
 One result of the venting may have been setting off an explosion, caused by either steam or hydrogen, that tore the outer wall and roof off the building housing reactor No. 1, although the steel containment of the reactor remained in place, officials said. 
 
 Even before the statement on Sunday by Mr. Edano, it was clear form radioactive materials turning up in trace amounts outside the reactors that fuel damage had occurred. The existence or extent of melting might not be clear until workers can open up the reactors and examine the fuel, which could be months. 
 
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 A meltdown occurs when there is insufficient cooling of the reactor core, and it is the most dangerous kind of a nuclear power accident because of the risk of radiation releases. The radiation levels reported so far by the Japanese authorities are far above normal but still too small to pose a hazard to human health if the exposure continued for a brief period. The fear was that more core damage would bring bigger releases. 
 
 The Japanese Nuclear and Industrial Safety Agency said that as many as 160 people may have been exposed to radiation around the plant, and Japanese news media said that three workers at the facility were suffering from full-on radiation sickness. 
 
 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. 
 
 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. 
 
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 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 Daiichi and Daini. 
 
 Worries about the safety of the two plants worsened on Saturday because executives of the company that runs them, Tokyo Electric Power, and government officials gave confusing accounts of the location and 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. But photographs of the damage did not make clear that this was the case. 
 
 They said that the blast, which may have been caused by a sharp buildup of hydrogen when the reactor’s cooling system failed, destroyed the concrete structure surrounding the reactor but did not collapse the critical steel container inside. This pattern of damage cast doubt on the idea that the explosion was in the turbine building. 
 
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 “We’ve confirmed that the reactor container was not damaged,” Mr. 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.” 
 
 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. 
 
 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. 
 
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 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. 
 
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 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. 
 
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 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 quoted nuclear safety officials as saying. 
 
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 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. ||||| Media playback is unsupported on your device Media caption Tepco chairman: "We want to apologise from the bottom of our hearts" 
 
 Japan is to decommission four stricken reactors at the quake-hit Fukushima nuclear plant, the operator says. 
 
 Tokyo Electric Power (Tepco) made the announcement three weeks after failing to bring reactors 1 - 4 under control. Locals would be consulted on reactors 5 and 6, which were shut down safely. 
 
 Harmful levels of radioactivity have been detected in the area. 
 
 More than 11,000 people are known to have been killed by the devastating 11 March earthquake and tsunami. 
 
 Emperor Akihito visited a centre for earthquake and tsunami victims in the Tokyo area on Wednesday. 
 
 Rolling blackouts 
 
 The emperor's visit "gives me strength" said one of the evacuees. 
 
 Amid all the destruction, it is a safe haven Kathy Mueller, International Federation of Red Cross Read Kathy's latest diary 
 
 Japanese experts are considering whether to cover the reactor buildings at the Fukushima Daiichi plant with a special material, to stop the spread of radioactive substances, Chief Cabinet Secretary Yukio Edano says. 
 
 On Wednesday, the government ordered nuclear power plant operators to start implementing new safety measures immediately. 
 
 The steps - to be completed by the end of April - include preparing back-up power in case of loss of power supply. 
 
 Fire trucks will be on standby to intervene and ensure cooling systems for both reactors and pools of used fuel are maintained. 
 
 Minister of Economy, Trade and Industry Banri Kaieda said this did not mean that nuclear plant operations should be halted. 
 
 Tepco's president Masataka Shimizu has been admitted to hospital, suffering from high blood pressure and dizziness. 
 
 Hours later, Tepco chairman Tsunehisa Katsumata spoke to reporters for the first time. 
 
 Analysis The highest radiation level yet in the seawater off Fukushima is the strongest indication so far that highly radioactive water is leaking into the sea. The reading comes as workers at the plant continue the difficult balancing act between pumping in water to keep the reactors cool, and draining pools of radioactive leakage. Japan's Nuclear Safety Agency said the radiation level does not pose an immediate threat to fishery products. But it's likely this latest discovery will increase concern both for the safety of Japanese seafood and its reputation abroad. 
 
 He admitted that the company had not been able to cool the reactors, and pledged maximum efforts to stabilise them. And he added that reactors 1-4 would eventually have to be shut down for good. 
 
 Mr Katsumata said his company was preparing to compensate those suffering damage caused by radiation leaks. 
 
 The chairman also apologised for the inconvenience caused by the rolling blackouts imposed to cope with power shortages. 
 
 The earthquake and tsunami damaged the nuclear plant's power supply, leading to a failure of the cooling systems. 
 
 Since then engineers have been using seawater to cool down the core of the reactors, but the operation has failed to stop radioactive leaks. 
 
 Japan's Prime Minister Naoto Kan has said the country is on "maximum alert". 
 
 Tepco has been accused of a lack of transparency and failing to provide information promptly. 
 
 Seawater radiation 
 
 Seawater near the Fukushima Daiichi nuclear plant has reached a much higher level of radiation than previously reported. 
 
 The new readings near reactor No 1 - 300m (984ft) from the shore - showed radioactive iodine at 3,355 times the legal limit, said Japan's nuclear safety agency. 
 
 Media playback is unsupported on your device Media caption Japan's emperor made a rare public appearance at an evacuation centre in the Tokyo area 
 
 Earlier samples had put the iodine level in the sea at 1,850 times the legal limit. 
 
 Much lower - but still elevated levels - of the same radioactive element have been found in seawater as far as 16km (10 miles) to the south. 
 
 Tepco and the safety agency say the exact source of the leak is unknown. 
 
 "Iodine 131 has a half-life of eight days, and even considering its concentration in marine life, it will have deteriorated considerably by the time it reaches people," Hidehiko Nishiyama, deputy director-general of Japan's nuclear safety agency told a news conference. 
 
 Radioactive materials are measured by scientists in half-lives, or the time it takes to halve the radiation through natural decay. 
 
 Fukushima update (30 March) Reactor 1: Damage to the core from cooling problems. Building holed by gas explosion. Highly radioactive water detected in reactor 
 
 Damage to the core from cooling problems. Building holed by gas explosion. Highly radioactive water detected in reactor Reactor 2: Damage to the core from cooling problems. Building holed by gas blast; containment damage suspected. Highly radioactive water detected in reactor and adjoining tunnel 
 
 Damage to the core from cooling problems. Building holed by gas blast; containment damage suspected. Highly radioactive water detected in reactor and adjoining tunnel Reactor 3: Damage to the core from cooling problems. Building holed by gas blast; containment damage possible. Spent fuel pond partly refilled with water after running low. Highly radioactive water detected in reactor 
 
 Damage to the core from cooling problems. Building holed by gas blast; containment damage possible. Spent fuel pond partly refilled with water after running low. Highly radioactive water detected in reactor Reactor 4: Reactor shut down prior to quake. Fires and explosion in spent fuel pond; water level partly restored 
 
 Reactor shut down prior to quake. Fires and explosion in spent fuel pond; water level partly restored Reactors 5 & 6: Reactors shut down. Temperature of spent fuel pools now lowered after rising high Q&A: Health effects of radiation Q&A: Fukushima radiation alert A new way to look at radiation 
 
 Iodine 131 was blamed for the high incidence of thyroid cancer among children exposed to fallout from the Chernobyl nuclear disaster in 1986. 
 
 Workers at the Fukushima plant are trying to prevent radioactive water from seeping into the sea. 
 
 Highly radioactive liquid has been found inside and outside several reactor buildings. 
 
 Small amounts of plutonium have also been detected in soil at the plant - the latest indication that one of the reactors suffered a partial meltdown. 
 
 But, like the discovery of plutonium, the high levels of radiation found inside and outside reactor buildings are likely to have come from melted fuel rods. 
 
 Theories for the leak centre on two possibilities - steam is flowing from the core into the reactor housing and escaping through cracks, or the contaminated material is leaking from the damaged walls of the water-filled pressure control pool beneath the No 2 reactor. 
 
 The plutonium - used in the fuel mix in the No 3 reactor - is not at levels that threaten human health, officials said.
– If it's been tough to gauge how much risk is posed by Japan's damaged nuclear plants, it's because officials have given what the New York Times describes as "confusing accounts" of exactly what's going on. Safety officials say the explosion at the Fukushima Dai-ichi plant destroyed walls of the building where the reactor is located, but not the metal housing enclosing the reactor, notes AP. The blast released radioactive material, but officials say the amounts are so small the public isn't endangered. Nevertheless, they widened the evacuation zone, warned people to stay inside, and prepared to distribute iodine tablets. Reuters rounds up assessments of experts, but the bottom line seems to be that more information is needed. "We don't know enough about what the status of the fuel is in the reactor core," says one. "The issue is whether the core is uncovered, whether the fuel is breaking up or being damaged, or whether the fuel is melting." The Washington Post talks to one doctor who arrived from Germany to provide relief, but is curtailing plans to go too far north. "I'd rather be working with a pocket knife in Haiti than worrying about the danger of a radioactive cloud."