KORIYAMA, Japan – Japan's nuclear crisis intensified Sunday as authorities raced to combat the threat of multiple reactor meltdowns and more than 180,000 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.

But hours after officials announced the latest dangers to face the troubled Fukushima Dai-ichi nuclear complex, including the possibility of a second explosion in two days, there were few details about what was being done to bring the situation under control.

Chief Cabinet Secretary Yukio Edano said Sunday that a hydrogen explosion could occur at the complex's Unit 3, the latest reactor to face a possible meltdown. That would follow a hydrogen blast Saturday in the plant's Unit 1, where 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 180,000 people have 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.

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 (60 kilometers) from the troubled reactors and 125 miles (190 kilometers) 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. The severity of their exposure, or if it had reached dangerous levels, was not clear.

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 six reactors — three at Dai-ichi and three at another nearby complex — after operators lost the ability to cool the reactors using usual procedures. Local evacuations have been ordered at each location. The U.N. nuclear agency said a state of emergency was also declared Sunday at another complex 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 reported the incident Sunday.

All of the reactors at the complexes shut down automatically when the earthquake shook the region.

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."

Edano, for his part, 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 collapse of a power plant's ability to keep temperatures under control — could release uranium and dangerous contaminants into the environment and pose major, widespread health risks.

Experts noted, however, that even a complete meltdown would probably be far less severe than the 1986 disaster at Chernobyl, where a reactor exploded and sent a cloud of radiation over much of Europe. That reactor, unlike the ones in Fukushima, was not housed in a sealed container.

The 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.

The scale of the multiple disasters appeared to be outpacing the efforts of Japanese authorities to bring the situation under control.

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. Nearly 2 million households were without electricity.

Starting Monday, power will be rationed with rolling blackouts in several cities, including Tokyo.

The government doubled the number of troops pressed into rescue operations to about 100,000 from 51,000, as powerful aftershocks continued to rock the country. Hundreds have hit since the initial temblor.

On Saturday, an explosion destroyed the walls and ceiling of Fukushima Dai-ichi's Unit 1 as operators desperately tried to prevent it from overheating and melting down by releasing steam.

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 reduce the pressure.

Officials insisted there was no significant radioactive leak after the explosion.

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.

Operators also began using sea water to cool the complex's Unit 3 reactor after earlier attempts to lower its temperature failed, the U.N. Nuclear Agency said.

The move likely renders the 40-year-old reactors unusable, said a foreign ministry official briefing reporters.

He said radiation levels outside the plant briefly rose above legal limits, but had since declined significantly.

Japan has a total of 55 reactors spread across 17 complexes nationwide.

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Yamaguchi reported from Tokyo. Associated Press writers Tomoko A. Hosaka in Tokyo, Tim Sullivan in Bangkok and Jeff Donn in Boston contributed to this report.