SOMA, Japan -- The second hydrogen explosion in three days rocked Japan's stricken Fukushima Dai-ichi nuclear plant Monday, sending a massive column of smoke into the air and wounding 6 workers. The plant's operator said radiation levels at the reactor were still within legal limits.
The explosion at the plant's Unit 3, which authorities have been frantically trying to cool following a system failure in the wake of a massive earthquake and tsunami, triggered an order for hundreds of people to stay indoors, said Chief Cabinet Secretary Yukio Edano.
Tokyo Electric Power Co. said radiation levels at Unit 3 were 10.65 microsieverts, significantly under the 500 microsieverts at which a nuclear operator must file a report to the government.
The blast follows a similar explosion Saturday that took place at the plant's Unit 1, which injured four workers and caused mass-evacuations.
Japan's nuclear safety agency said 6 workers were injured in Monday's explosion but it was not immediately clear how, or whether they were exposed to radiation. They were all conscious, said the agency's Ryohei Shomi.
The reactor's inner containment vessel holding nuclear rods was intact, Edano said, allaying some fears of the risk to the environment and public. TV footage of the building housing the reactor appeared to show similar damage to Monday's blast, with outer walls shorn off, leaving only a skeletal frame.
More than 180,000 people have evacuated the area in recent days, and up to 160 may have been exposed to radiation.
Earlier Monday, pressure had jumped inside Unit 3, forcing the evacuation of 21 workers. But they returned to work after levels appeared to ease.
Associated Press journalists felt the explosion in the tsunami-devastated port town of Soma, some 25 miles north of the reactor. They reported feeling the faint rumble of a blast and the ground shaking.
Four nuclear plants in northeastern Japan have reported damage, but the danger was greatest at Fukushima's Dai-ichi plant. Operators have lost the ability to cool three reactors at Dai-ichi and three more at another nearby complex using usual procedures, after the quake knocked out power and the tsunami swamped backup generators.
Operators have been dumping seawater into units 1 and 3 in a last-ditch measure to cool the reactors. They were getting water into the other four reactors with cooling problems without resorting to corrosive sea water, which likely makes the reactors unusable.
Edano said radioactivity released into the environment so far was so small it didn't pose any health threats.
Such statements, though, did little to ease public worries.
"First I was worried about the quake," said Kenji Koshiba, a construction worker who lives near the plant. "Now I'm worried about radiation." He spoke at an emergency center in Koriyama, about 40 miles from the most troubled reactors and 125 miles north of Tokyo.
At the makeshift center set up in a gym, a steady flow of people -- mostly the elderly, schoolchildren and families with babies -- were met by officials wearing helmets, surgical masks and goggles.
About 1,500 people had been scanned for radiation exposure, officials said.
Up to 160 people, including 60 elderly patients and medical staff who had been waiting for evacuation in the nearby town of Futabe, and 100 others evacuating by bus, might have been exposed to radiation, said Ryo Miyake, a spokesman from Japan's nuclear agency. It was unclear whether any cases of exposure had reached dangerous levels.
A foreign ministry official briefing reporters said radiation levels outside the Dai-ichi plant briefly rose above legal limits, but had since declined significantly.
Edano said none of the Fukushima Dai-ichi reactors was near the point of complete meltdown, and he was confident of escaping the worst scenarios.
Officials, though, have declared states of emergency at the six reactors where cooling systems were down -- three at Dai-ichi and three at the nearby Fukushima Daini complex. The U.N. nuclear agency said a state of emergency was also declared Sunday at another complex, the Onagawa power plant, after higher-than-permitted levels of radiation were measured there. It said Japan informed it that all three reactors there were under control.
A pump for the cooling system at yet another nuclear complex, the Tokai Dai-Ni plant, also failed after Friday's quake but a second pump operated normally as did the reactor, said the utility, the Japan Atomic Power Co. It did not explain why it did not announce the incident until Sunday.
Edano denied there had been a meltdown in the Fukushima Dai-ichi complex, but other officials said the situation was not so clear.
Hidehiko Nishiyama, a senior official of the Economy, Trade and Industry Ministry, indicated the reactor core in Unit 3 had melted partially, telling a news conference, "I don't think the fuel rods themselves have been spared damage," according to the Kyodo News agency.
A complete meltdown -- the melting of the radioactive core -- could release uranium and dangerous contaminants into the environment and pose major, widespread health risks.
The steel reactor vessel could melt or break from the heat and pressure. A concrete platform underneath the reactor is supposed to catch the molten metal and nuclear fuel, but the intensely hot material could set off a massive explosion if water has collected on the platform. Radioactive material also could be released into the ground if the platform fails.
The explosion that destroyed the walls and ceiling of Dai-ichi Unit 1's containment building was much less serious that a meltdown would be -- in fact, it was operators' efforts to avoid a meltdown that caused it.
Officials vented steam from the reactor to reduce pressure, and were aware that there was an explosion risk because the steam contained hydrogen, said Shinji Kinjo, spokesman for the government's Nuclear and Industrial Safety Agency. The explosion occurred when hydrogen reacted with oxygen outside the reactor.
It is unclear how far the impact of a meltdown might reach. In the United States, local communities plan for evacuation typically within 10 miles of a nuclear plant. However, states must be ready to cope with contamination of food and water as far as 50 miles away. Radioactivity can also be carried to faraway places by the winds, as it was in the 1986 Chernobyl disaster, though it will become increasingly diffuse. Acute radiation deaths would normally be expected only much closer to the plant.
The reactor that exploded at Chernobyl, sending a cloud of radiation over much of Europe, was not housed in a sealed container as those at Dai-ichi are. The Japanese reactors also do not use graphite, which burned for several days at Chernobyl.
Japan's nuclear crisis was triggered by twin disasters on Friday, when an 8.9-magnitude earthquake, the most powerful in the country's recorded history, was followed by a tsunami that savaged its northeastern coast with breathtaking speed and power.
More than 1,400 people were killed and hundreds more were missing, according to officials, but police in one of the worst-hit areas estimated the toll there alone was more than 10,000.
All of the reactors in the region shut down automatically when the earthquake hit. But with backup power supplies also failing, shutting down the reactors is just the beginning of the problem, scientists said.
"You need to get rid of the heat," said Friedrich Steinhaeusler, a professor of physics and biophysics at Salzburg University and an adviser to the Austrian government on nuclear issues.
"You are basically putting the lid down on a pot that is boiling."
"They have a window of opportunity where they can do a lot," he said, such as using sea water as an emergency coolant. But if the heat is not brought down, the cascading problems can eventually be impossible to control. "This isn't something that will happen in a few hours. It's days."
Japan's nuclear safety agency said 1,450 workers were at the Dai-ichi plant on Sunday, its usual staffing. The workers were in protective gear and were taking shorter turns than usual in units 1 and 3 to limit their exposure, agency spokesman Yoshihiro Sugiyama said.
The emergencies at the nuclear plants have led to an electricity shortage in Japan, where nearly 2 million households were without power Sunday. Starting Monday, power will be rationed with rolling blackouts in several cities, including Tokyo.
Japan has a total of 55 reactors spread across 17 complexes nationwide.