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 six workers. It was not immediately clear how much — if any — radiation had been released.
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.
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 six 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.
Earlier, Tokyo Electric Power Co., which operate the reactor, said three workers were injured and seven missing.
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 (40 kilometers) north of the reactor. They reported feeling the faint rumble a blast and the ground shaking.
At the time, sirens were wailing as rescue workers were in the midst of evacuating all those in the city
KORIYAMA, Japan -- Japan's nuclear crisis intensified Sunday as authorities raced to combat the threat of multiple reactor meltdowns and more than 170,000 people evacuated the quake- and tsunami-savaged northeastern coast where fears spread over possible radioactive contamination.
Nuclear plant operators were frantically trying to keep temperatures down in a series of nuclear reactors - including one where officials feared a partial meltdown could be happening Sunday - to prevent the disaster from growing worse.
If a full-scale meltdown were to occur, experts interviewed by The Associated Press said melted fuel would eat through the bottom of the reactor vessel, then through the floor of the containment building. At that point, the uranium and dangerous byproducts would start escaping into the environment.
Eventually, the walls of the reactor vessel - six 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 that would enhance the spread of radioactive contaminants.
If the reactor core became exposed to the outside, 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, told reporters.
At that point, Bradford added, "many first responders would die."
Chief Cabinet Secretary Yukio Edano also said Sunday that a hydrogen explosion could occur at Unit 3 of the Fukushima Dai-ichi nuclear complex, the latest reactor to face a possible meltdown. That follows a blast the day before in the power plant's Unit 1, as operators attempted to prevent a meltdown there by injecting sea water into it.
"At the risk of raising further public concern, we cannot rule out the possibility of an explosion," Edano said. "If there is an explosion, however, there would be no significant impact on human health."
More than 170,000 people had been evacuated as a precaution, though Edano said the radioactivity released into the environment so far was so small it didn't pose any health threats.
"First I was worried about the quake," Kenji Koshiba, a construction worker who lives near the plant. "Now I'm worried about radiation." He spoke at an emergency center in Koriyama town near the power plant in Fukushima.
Edano said neither Fukushima Dai-ichi reactor was near the point of complete meltdown, and he was confident of escaping the worst scenarios.
A complete meltdown - the collapse of a power plant's systems and its ability to keep temperatures under control - could release uranium and dangerous contaminants into the environment and pose major, widespread health risks.
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. The severity of their exposure, or if it had reached dangerous levels, was not clear. They were being taken to hospitals.
Edano said operators were trying to cool and decrease the pressure in the Unit 3 reactor, just as they had the day before at Unit 1.
"We're taking measures on Unit 3 based on a similar possibility" of a partial meltdown, Edano said.
Japan struggled with the nuclear crisis as it tried to determine the scale of the Friday disasters, 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.
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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 could eventually top 10,000.
The scale of the multiple disasters appeared to be outpacing the efforts of Japanese authorities to bring the situation under control more than two days after the initial quake.
Rescue teams were struggling to search hundreds of miles (kilometers) of devastated coastline, and hundreds of thousands of hungry survivors huddled in darkened emergency centers cut off from rescuers and aid. At least 1.4 million households had gone without water since the quake, and food and gasoline were quickly running out across the region. Large areas of the countryside were surrounded by water and unreachable. Some 2 million households were without electricity.
Japanese Trade Minister Banri Kaeda warned that the region was likely to face further blackouts, and power would be rationed to ensure supplies to essential facilities.
The government doubled the number of troops pressed into rescue and recovery operations to about 100,000 from 51,000, as powerful aftershocks continued to rock the country. Hundreds have hit since the initial temblor.
Unit 3 at the Fukushima plant is one of the three reactors that had automatically shut down and lost cooling functions necessary to keep fuel rods working properly due to power outage from the quake. The facility's Unit 1 is also in trouble, but Unit 2 has been less affected.
On Saturday, an explosion destroyed the walls of Unit 1 as operators desperately tried to prevent it from overheating and melting down.
Without power, and with its valves and pumps damaged by the tsunami, authorities resorted to drawing sea water mixed with boron in an attempt to cool the unit's overheated uranium fuel rods. Boron disrupts nuclear chain reactions.
The move likely renders the 40-year-old reactor unusable, said a foreign ministry official briefing reporters. Officials said the sea water will remain inside the unit, possibly for several months.
Robert Alvarez, senior scholar at the Institute for Policy Studies and former senior policy adviser to the U.S. secretary of energy, told reporters that the sea water was a desperate measure.
"It's a Hail Mary passIt's a Hail Mary pass," he said.
He said that the success of using sea water and boron to cool the reactor will depend on the volume and rate of their distribution. He said the dousing would need to continue nonstop for days.
Another key, he said, was the restoration of electrical power, so that normal cooling systems can operate.
Edano said the cooling operation at Unit 1 was going smoothly after the sea water was pumped in.
Operators released slightly radioactive air from Unit 3 on Sunday, while injecting water into it hoping to reduce pressure and temperature to prevent a possible meltdown, Edano said.
He said radiation levels just outside the plant briefly rose above legal limits, but since had declined significantly. Also, fuel rods were exposed briefly, he said, indicating that coolant water didn't cover the rods for some time. That would have contributed further to raising the temperature in the reactor vessel.
At an evacuation center in Koriyama, about 40 miles (60 kilometers) from the troubled reactors and 125 miles (190 kilometers) north of Tokyo, medical experts had checked about 1,500 people for radiation exposure in an emergency testing center, an official said.
On Sunday, a few dozen people waited to be checked in a collection of blue tents set up in a parking lot outside a local gymnasium. Fire engines surrounded the scene, with their lights flashing.
Many of the gym's windows were shattered by the quake, and glass shards littered the ground.
A steady flow of people - the elderly, schoolchildren and families with babies - arrived at the center, where they were checked by officials wearing helmets, surgical masks and goggles.
Officials placed five reactors, including Units 1 and 3 at Dai-ichi, under states of emergency Friday after operators lost the ability to cool the reactors using usual procedures.
An additional reactor was added to the list early Sunday, for a total of six - three at the Dai-ichi complex and three at another nearby complex. Local evacuations have been ordered at each location. Japan has a total of 55 reactors spread across 17 complexes nationwide.
Officials began venting radioactive steam at Fukushima Dai-ichi's Unit 1 to relieve pressure inside the reactor vessel, which houses the overheated uranium fuel.
Concerns escalated dramatically Saturday when that unit's containment building exploded.
Officials were aware that the steam contained hydrogen and were risking an explosion by venting it, acknowledged Shinji Kinjo, spokesman for the government's Nuclear and Industrial Safety Agency, but chose to do so because they needed to keep circulating cool water on the fuel rods to prevent a meltdown.
Officials insisted there was no significant radioactive leak after the explosion.
Yamaguchi reported from Tokyo. Associated Press writers Tomoko A. Hosaka in Tokyo and Jeff Donn in Boston contributed to this report.