TOKYO (MarketWatch) — A new danger arose for Japan’s earthquake-damaged nuclear power plant on Sunday as a second reactor there experienced a partial meltdown after the complex lost its emergency cooling system.

Government and power company officials were resorting to an unprecedented measure to stabilize the Fukushima Daiichi reactor that had lost its emergency cooling system earlier in the day by flooding it with seawater.

The Associated Press reported early Sunday, citing comments by the government’s top spokesman, that fuel rods at the reactor had been exposed and the reactor was likely going through a meltdown as a result.

All that was occurring as millions endured another night on Honshu, Japan’s largest and most populous island, without electrical power, water or shelter. Their plight was caused by the magnitude 8.9 earthquake that struck off the east coast of Honshu Friday afternoon and produced a huge tsunami that devastated the island and did damage as far away as California.

Government officials and Tokyo Electric Power Corp., (9501), which runs the Fukushima complex 150 miles north of Tokyo, had worked to stabilize Reactor No. 1. The quake had cut off power to the reactors’ cooling pumps Friday afternoon. An explosion early Saturday damaged the building housing the No.1 reactor and cut off its emergency cooling system. Later in the day, officials resorted flooded that reactor with sea water and said the tactic seemed to be working.

That move was an attempt to prevent the reactor’s fuel rods from overheating to such a high temperature that they would burn through the reactor walls — melting them down — and release potentially deadly radiation into the atmosphere.

But a new problem arose Saturday night when the No. 3 reactor lost its emergency cooling system, renewing fears of a possible meltdown. Japan’s Nuclear and Industrial Safety Agency identified the new problem, according to media reports. and said it was urgently attempting to secure a means to supply to the reactor.

It wasn’t clear if the government and Tepco were planning a move similar to one taken to protect the damaged No. 1 reactor.

The Fukushima Daiichi complex operated by Tokyo Electric Power is the 14th-largest nuclear-power facility in the world and the fourth-largest in Japan, capable of generating 4,700 megawatts when all six reactors are running at capacity. Three generators at the complex were down for regular inspections when the earthquake hit.

At a news conference, Japan’s chief cabinet secretary, Yukio Edano, said the blast didn’t damage the No. 1 reactor container, only the structure outside it, and that there was no major radiation leakage with the explosion.

“I think we can get this under control” by cooling the reactor with seawater, Edano said, a process he said could take 5 to 10 hours.

According to the Wall Street Journal, Edano said the explosion resulted from hydrogen leaking inside the reactor’s containment structure due to the reactor experiencing a high level of vapor pressure. Edano also said that radiation levels outside the reactor rose to 1,015 microsievert, or the equivalent of being exposed to the maximum allowable level of radiation for a full year in a single day.

The government earlier extended a nuclear emergency evacuation order requiring residents within 12 miles of the Daiichi plant to leave the area, up from an earlier radius of 6 miles.

Japan grapples with nuclear crisis

Media reports said about 40,000 people had evacuated from the areas around the Fukushima plants.

Four workers were reported injured at the No. 1 reactor after the explosion. A white cloud of steam or smoke appeared around the plant, visible in televised footage.

NHK earlier cited Fukushima authorities as saying that the ceiling of the reactor building had collapsed. It said radioactivity at the site was rising to 20 times normal levels. Other reports said the walls of the building that housed the reactor had been blown away.

Earlier Saturday, authorities said that Tepco had succeeded in relieving the pressure building up in the reactor.

“If the water level remains at this level, the reactor core might be damaged, but we are now pouring water into the reactor to prevent it from happening,” Dow Jones Newswires quoted a Tepco spokesman as saying.

Japanese media had reported earlier Saturday that the reactor could already be experiencing a meltdown, but the risk of widespread radiation leakage from it would be mitigated if the container of the reactor were to remain intact.

The plant did emit some radiation early Saturday morning, as part of the utility’s controlled release of radioactive steam. That was part of government-mandated steps to relieve the growing pressure in the reactor.

Damon Moglen, director of the climate and energy program at Friends of the Earth, an international network of organizations focusing on environmental issues, said Tepco and the Japanese government are still not being forthcoming enough with information about the scope of the nuclear reactor situation.

“They are handing out nothing and treating it like something,” Moglen said. “Some extremely radioactive elements may have been released and we need more information to say whether we are on the down or up slope of this accident.”

Tepco also said another nuclear-power plant nearby, Fukushima Daini, was experiencing pressure increases inside its four reactors due to cooling-system damage.

Prime Minister Naoto Kan inspected the plant by helicopter on Saturday morning, Kyodo News reported.

“I hope measures (to support the neighboring residents) will be taken at an early stage,” Kan reportedly told a Tepco executive.

Aftershocks

Aftershocks struck Iwate, Miyagi, Aomori and Fukushima late Saturday.

Friday’s earthquake struck at 2:46 p.m. local time Friday near the east coast of Honshu, about 231 miles northeast of Tokyo, according to the U.S. Geological Survey. It lasted about two minutes.

The quake also triggered a tsunami that sent strong waves crashing on Hawaii and also caused minor damage some 5,000 miles across the Pacific Ocean in Oregon and California.

The U.S. Coast Guard was searching for a person who was swept out to sea in Northern California.

According to Japanese media, some 1,600 people are confirmed dead or missing after a tsunami wave struck in coast areas around Sendai near the epicenter.

Across Japan, broadcaster NHK World said there were at least 680 confirmed dead from the earthquake, and that the death toll could rise above 1,000. The toll was expected to climb as many reported missing in coastal areas swept by the tsunami are unlikely to have survived.

Kyodo reported late Saturday that about 9,500 people are unaccounted for in the town of Minamisanriku in Miyagi, citing prefectural officials. The figure is more than half of the town’s population of about 17,000

Sendai, the capital of Miyagi prefecture and home to about 1 million people, is about 115 miles from Tokyo. Its airport was deluged and parts of the tarmac remained flooded Saturday.

Before and after satellite images

Dramatic Japanese television footage showed a helicopter rescuing two people from a rooftop surrounded by water in a tsunami-stricken town. Separate footage showed a hospital and a school with written pleas for help to rescuers.

A ship that was earlier reported missing was found, and all 81 people aboard were rescued, according to an AFP report.

The Tokyo Stock Exchange plans to conduct trading as normal on Monday.