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. 
 
 Photo 
 
 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. 
 
 Advertisement Continue reading the main story 
 
 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. 
 
 Photo 
 
 “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. 
 
 Newsletter Sign Up Continue reading the main story Please verify you're not a robot by clicking the box. Invalid email address. Please re-enter. You must select a newsletter to subscribe to. Sign Up You agree to receive occasional updates and special offers for The New York Times's products and services. Thank you for subscribing. An error has occurred. Please try again later. View all New York Times newsletters. 
 
 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. 
 
 Advertisement Continue reading the main story 
 
 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. 
 
 Photo 
 
 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. 
 
 Advertisement Continue reading the main story 
 
 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. ||||| Authorities battling the nuclear disaster at the Fukushima Daiichi power plant have doubled the number of workers on the site to 100 in an effort to continue cooling the three reactors and the spent fuel pools but have abandoned — at least temporarily — plans to use helicopters to dump water on the pools because of the radiation danger. Police may now use water cannons to spray the pools.An earlier version of the article incorrectly spelled the first name of Yukia Amano, chief of the International Atomic Energy Agency , as Yukio.Workmen are also nearly finished stringing a new power line from the electrical grid to the power plant, which could ease the situation somewhat by providing a stable source of power for valves and controls at the plant and for pumps supplying water to the spent fuel pools.It is too late for the electricity to do much good for the three endangered reactors, however, because their cooling systems all appear to have been damaged by the hydrogen explosions that have wracked the plants. Only external pumps can now be used for those reactors.The status of reactor No. 3 at the site was not clear, with some reports saying that the reactor containment vessel may have been breached and was releasing radioactivity and others saying that it was still intact.Authorities had feared a breach of the reactor vessel after the eruption of a plume of smoke from the reactor on Wednesday. The most recent statement from Chief Cabinet Secretary Yukio Edano, however, indicated that the smoke was probably steam from a spent fuel pool at that reactor building and that the reactor vessel itself had not been breached.Officials are particularly concerned about reactor No. 3 because it is the only reactor at the facility that is fueled with what is known as a mixed oxide fuel. The pellets in mixed oxide fuel contain both plutonium and uranium. Plutonium is highly carcinogenic in small quantities, and its release into the environment would be very dangerous.The containment vessel at reactor No. 2 has previously been breached, and it appeared to be leaking small amounts of radioactivity.As the crisis at the power plant entered its sixth night, the situation appeared to be deteriorating. One sign was that the Japanese government increased the maximum radiation dose that workers could be exposed to from 100 millisieverts to 250 millisieverts, describing the move as "unavoidable due to the circumstances."The workers are all wearing full protective gear and working only in short shifts, but it is believed they are still being exposed to significant doses of radiation.All of the workers were pulled out of the plant for nearly an hour Wednesday when radiation levels spiked, but pumps continued to inject seawater into the reactors in their absence, according to Tokyo Electric Power Co., which owns the plant. The workers returned as soon as the radiation levels had subsided. The radiation spike was believed to result from the release of steam from reactor No. 2.Authorities estimated that about 70% of the fuel rods in reactor No. 1 had been damaged and about 30% of the rods in reactor No. 2. Those were merely estimates, however, because they have no direct way to measure the extent of damage.Military helicopters were seen flying over the reactor buildings Wednesday night to measure radiation levels. Authorities had planned to use helicopters to drop water on the spent-fuel cooling ponds on the roofs of the buildings, which also have lost their emergency cooling systems.The need was especially critical at the pool for reactor No. 4, which has caught fire twice and where the cooling water was boiling and believed to be reaching dangerously low levels. Authorities concluded, however, that radiation levels were too high to allow the water drops.That spent fuel pool contains about 135 tons of uranium fuel rods, according to Marvin Resnikoff, a nuclear physicist who studies disposal of spent fuel. Some reports suggest that the spent fuel in that pool has now boiled off all the cooling water and the fuel rods are exposed to the air and heating up. That has not been confirmed.Resnikoff also noted that, in addition to the spent fuel pools on the roofs of each reactor, there is a larger stand-alone fuel pool at ground level. "We are not certain what happened to this in the tsunami," he said.Authorities said the police may now attempt to use water cannons, normally used to quell riots, to spray water into the pools. Workers will first have to remove some debris from near the buildings so the apparatus can get close enough.As a further indication of the seriousness of the problem, the United Nations nuclear agency will call an emergency meeting to discuss the problem. Yukia Amano, chief of the International Atomic Energy Agency, is flying to Japan on Wednesday to consult with Japanese authorities about the crisis. U.S. military authorities on Wednesday night (Japanese time) warned all military not to go within 50 miles of the stricken plant without special authorization. The U.S. Embassy in Tokyo also cautioned all U.S. citizens to observe the same restriction.The good news is that the reactors should be undergoing a certain amount of cooling on their own. When an operating reactor is shut down, it continues to produce a large amount of heat, known as decay heat. Within the first week after a shutdown, that decay heat declines by about 50%, experts said, so that the reactors require less external cooling.
– 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."