Japanese officials were trying frantically to thwart partial meltdowns presumed under way Sunday at two earthquake-stricken nuclear reactors in Japan.
Fuel rods were briefly exposed and radiation levels briefly rose above the legal limit at the second reactor Sunday, said Chief Cabinet Secretary Yukio Edano.
A partial meltdown in the unit is "highly possible," he told reporters. "Because it's inside the reactor, we cannot directly check it but we are taking measures on the assumption of the possible partial meltdown."
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He also said a hydrogen explosion could occur at the reactor, Unit 3 of the Fukushima Daiichi nuclear complex. That would follow a Saturday blast at Unit 1 of the same complex as operators attempted to prevent a nuclear meltdown by injecting sea water into it.
Japan's largest electric utility Sunday released what it said was slightly radioactive air at Unit 3.
Both reactors are among three shut down Friday at the Fukushima Daiishi plant when their cooling functions stopped after the area was jolted by a magnitude 8.9 earthquake.GE-designed reactors in Fukushima have 23 sisters in U.S.
Tokyo Electric Co., or TEPCO, Saturday began pouring seawater and boric acid into its Fukushima Daiichi power plant Unit 1 reactor, whose core partially melted. On Sunday TEPCO released air containing radioactive materials for more than 2 hours and injected water at the Unit 3 nuclear reactor container vessel to reduce pressure and temperature to save the reactor from a possible meltdown.
Critical core cooling systems failed at both reactors. The Unit 2 reactor, though shut down by the quake, was not in the same trouble.
Released steam raised radiation levels above safety limits outside the Unit 3 reactor Sunday, TEPCO officials said, adding they informed the government of an "emergency situation." Still, they said, there was no immediate threat to human health.
Edano said the radiation levels later fell.
A similar radiation increase was seen Saturday during venting at the No. 1 reactor before it exploded. The blast destroyed the exterior walls of a building, but did not breach the steel housing enveloping the reactor there, officials said.
Radioactive cesium and iodine were detected near the facility Saturday, indicating that the melting had occurred, Kyodo News Service reported.
Officials said that at one point, the Unit 1 was releasing each hour the amount of radiation a person normally absorbs from the environment each year.
It is the first time a Japanese nuclear plant has ever experienced any level of reactor core melting.
TEPCO on Sunday also said it was preparing pressure reducing measures for its nearby Fukushima Daini Nuclear Power Station, where four reactors were shut down due to earthquakes. Releases of air with radioactive material were being considered for Daini units 1, 2, and 3.
Earlier Edano said TEPCO has begun new cooling operations to fill the Daiishi No. 1 reactor where the melting occurred with seawater and pour in boric acid, which absorbs neutrons, an operation expected to take several hours, Kyodo reported.
Filling the entire reactor container with seawater will take about 10 days, Edano said. It is likely that the reactor will have to be decommissioned because of the contamination by salts and other substances, experts said, according to Kyodo.
Edano on Sunday said the cooling operation at Unit 1 was going smoothly.
Robert Alvarez, senior scholar at the Institute for Policy Studies and former senior policy adviser to the U.S. secretary of energy, said in a briefing for reporters that the seawater was a desperate measure.
"It's a Hail Mary pass," he said.
He said that the success of using seawater and boron to cool the reactor will depend on the volume and rate of their distribution.
The evacuation zone around the facilities was doubled in size from 6 miles to 12 miles. More than 210,000 people had been evacuated from the area by early Sunday morning, and evacuations were continuing, the safety agency said.
Footage on Japanese TV showed the explosion had crumbled the building's walls, leaving only a skeletal metal frame standing. Its roof had also been blown off. Plumes of smoke spewed out of the plant, 20 miles from Iwaki.
The Japanese authorities have classified the event at Fukushima Daiichi Unit 1 as a level 4 "accident with local consequences" on the International Nuclear and Radiological Event Scale (INES). The scale is used to consistently communicate the safety significance of events associated with sources of radiation. The scale runs from 0 (deviation -- no safety significance) to 7 (major accident).
The 1979 Three Mile Island accident in Pennsylvania was a level 5 ("accident with wider consequences"). The 1986 Chernobyl disaster was a level 7 ("major accident).
Four workers were injured by the blast at Fukushima Unit 1, officials said.
Edano said the radiation around the plant had not risen after the blast, but had in fact decreased. He did not say why that was so.
Area residents were told to stay indoors, not to drink tap water and to cover their faces with masks or wet towels, according to Britain's Sky News.
Japanese authorities also told the U.N. atomic watchdog that they were making preparations to distribute iodine to people living near nuclear power plants affected by Friday's earthquake.
Iodine can help protect the body from radioactive exposure.
Japan has 55 reactors spread across 17 complexes nationwide.
Road to meltdown
Officials began venting radioactive steam at Fukushima Daiichi's Unit 1 to relieve pressure inside the reactor vessel, which houses the overheated uranium fuel. Officials declined to say what the temperature was inside Unit 1.
Officials were aware that the steam contained hydrogen, acknowledged Shinji Kinjo, spokesman for the government nuclear safety agency. More importantly, they also were aware they were risking an explosion by deciding to vent the steam.
The significance of the hydrogen began to come clear late Saturday:
— Officials decided to reduce rising pressure inside the reactor vessel, so they vented some of the steam buildup. They needed to do that to prevent the entire structure from exploding, and thus starting down the road to a meltdown.
— At the same time, in order to keep the reactor fuel cool, and also prevent a meltdown, operators needed to keep circulating more and more cool water on the fuel rods.
— Temperature in the reactor vessel apparently kept rising, heating the zirconium cladding that makes up the fuel rod casings. Once the zirconium reached 2,200 degrees Fahrenheit (1,200 Celsius), it reacted with the water, becoming zirconium oxide and hydrogen.
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— When the hydrogen-filled steam was vented from the reactor vessel, the hydrogen reacted with oxygen, either in the air or water outside the vessel, and exploded.
A similar "hydrogen bubble" had concerned officials at the 1979 Three Mile Island nuclear disaster in Pennsylvania until it dissipated.
If the temperature inside the Fukushima reactor vessel continued to rise even more — to roughly 4,000 degrees Fahrenheit (2,200 Celsius) — then the uranium fuel pellets would start to melt.
According to experts interviewed by The Associated Press, any melted fuel would eat through the bottom of the reactor vessel. Next, it would eat through the floor of the already-damaged containment building. At that point, the uranium and dangerous byproducts would start escaping into the environment.
At some point in the process, the walls of the reactor vessel — 6 inches (15 centimeters) of stainless steel — would melt into a lava-like pile, slump into any remaining water on the floor, and potentially cause an explosion much bigger than the one caused by the hydrogen. Such an explosion would enhance the spread of radioactive contaminants.
If the reactor core became exposed to the external environment, officials would likely began pouring cement and sand over the entire facility, as was done at the 1986 Chernobyl nuclear accident in the Ukraine, Peter Bradford, a former commissioner of the U.S. Nuclear Regulatory Commission, said in a briefing for reporters.
Another expert, Ken Bergeron, a physicist and former Sandia scientist, added that as a result of such a meltdown the surrounding land would be off-limits for a considerable period of time, and "a lot of first responders would die."Video: Expert: Already 'one of the worst nuclear disasters' (on this page)
'Not a fast reaction like at Chernobyl'
TEPCO, which operates the six-reactor Daiichi site, said Friday that it had also lost cooling ability at a second reactor there and three units at its nearby Fukushima Daini site.
The government declared a state of emergency at all those units.
Kinjo acknowledged there were still fears of a meltdown.Interactive: How a nuclear plant works
A "meltdown" is not a technical term. Rather, it is an informal way of referring to a very serious collapse of a power plant's systems and its ability to manage temperatures.
Yaroslov Shtrombakh, a Russian nuclear expert, said a Chernobyl-style meltdown was unlikely.
"It's not a fast reaction like at Chernobyl," he said. "I think that everything will be contained within the grounds, and there will be no big catastrophe."
In 1986, the Chernobyl nuclear reactor exploded and caught fire, sending a cloud of radiation over much of Europe. That reactor — unlike the Fukushima one — was not housed in a sealed container, so there was no way to contain the radiation once the reactor exploded.
Edwin Lyman, a senior scientist with the Union of Concerned Scientists, which opposes nuclear energy, told msnbc.com Friday that TEPCO was facing a potential catastrophe.
"What's critical is, are they able to restore cooling and prevent fuel damage? If the fuel starts to get damaged, eventually it will melt through the reactor vessel and drop to the floor of the containment building," raising the odds that highly radioactive materials could be released into the environment, he said.
But Steve Kerekes, spokesman for the U.S.-based Nuclear Energy Institute, said that while the situation was serious, a meltdown remains unlikely and, even if it occurred would not necessarily pose a threat to public health and safety.
"Obviously that wouldn't be a good thing, but at Three Mile Island about half the core melted and, at the end of the day … there were no adverse impacts to the public," he said.Video: Japan in shock: 'Nothing compares to what happened' Open Channel blog: 2007 Japan quake was wake-up call on nuclear safety
The Daiichi site is located in Onahama city. The 460-megawatt Unit 1 began operating in 1971 and is the oldest at the facility. It is a boiling water reactor that drives the turbine with radioactive water, unlike pressurized water reactors usually found in the United States. Japanese regulators decided in February to allow it to run another 10 years. The reactors were manufactured by GE. (Msnbc.com is a joint venture between NBC Universal and Microsoft. GE is a part owner of NBC Universal.)
The Fukushima Daiichi Nuclear Power Station has six reactors. Units 1, 2 and 3 were shut down due to the earthquake and units 4 to 6 were shutdown earlier for regular inspection, TEPCO said.
At Fukushima Daini Nuclear Power Station, the units 1 to 4 were shut down due to the earthquake, TEPCO said. A trapped crane operator there died Sunday, TEPCO said.
In a statement released Saturday, The Citizens' Nuclear Information Center said, "This could and should have been predicted ... We warned that Japan's nuclear plants could be subjected to much stronger earthquakes and much bigger tsunamis than they were designed to withstand."
Greenpeace called for the existing reactors to be phased out, saying in a statement, "How many more warnings before we finally grasp that nuclear reactors are inherently hazardous?"
NBC News, msnbc.com staff, Reuters and The Associated Press contributed to this report.