Japanese officials are struggling with a growing nuclear crisis and the threat of multiple meltdowns, two days after the country's northeastern coast was savaged by a catastrophic earthquake and tsunami.
A partial meltdown was probably under way at one nuclear reactor, a top official said, and operators were frantically trying to keep temperatures down at the power plant's other units and prevent the disaster from growing even worse.
Chief Cabinet Secretary Yukio Edano said a hydrogen explosion could occur at Unit 3 of the Fukushima Dai-ichi nuclear complex, the reactor that could be melting down.
That would follow a blast the day before in another unit at the same power plant, as operators attempted to prevent a meltdown 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, although Edano said the radioactivity released so far into the environment was so small it didn't pose any health threats.
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 medial 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 was not clear. They were being taken to hospitals.
Edano said a partial meltdown in Unit 3 of the Fukushima Dai-ichi power plant was "highly possible."
Asked whether a partial meltdown had occurred, Edano said that "because it's inside the reactor, we cannot directly check it", but authorities were acting on the assumption it had.
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.
At least 1000 people were killed - including about 200 bodies discovered today along the coast - and 678 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 kilometres of devastated coastline, and thousands of hungry survivors huddled in darkened emergency centers cut off from rescuers and aid.
At least a 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. About 2.5 million households were without electricity.
Japanese Trade Minister Banri Kaeda warned that the region was likely to face further blackouts, and that 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, including one today with a magnitude of 6.2 that originated in the sea, about 180km east of Tokyo. It swayed buildings in the capital, but there were no reports of injuries or damage.
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.
Yesterday, an explosion destroyed the walls of Unit 1 as operators desperately tried to prevent it from overheating and melting down.
Without power, and without plant pipes and pumps that were destroyed in the explosion of the most-troubled reactor's containment building, authorities resorted to drawing seawater mixed with boron in an attempt to cool the overheated uranium fuel rods in Unit 1. Boron disrupts nuclear chain reactions.
The move probably renders the 40-year-old reactor unusable, said a foreign ministry official briefing reporters. Officials said the seawater would 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 US secretary of energy, told 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 would 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 today, 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 60km from the troubled reactors and 190km north of Tokyo, medical experts had checked about 1500 people for radiation exposure in an emergency testing centre, an official said.
On Sunday, a few dozen people waited to be tested 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.
"First I was worried about the quake, now I'm worried about radiation. I live near the plants, so I came here to find out if I'm OK. I tested negative, but I don't know what to do next," said Kenji Koshiba, a construction worker.
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 Dai-ichi Unit 1, and four other reactors, 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.
