Medical officials check for signs of radiation on children from the evacuation area near the Fukushima Daiichi nuclear plant in Koriyama, Japan, on Sunday. Kim Kyung-Hoon / Reuters

Shares shed 4.8 percent in early trading

Possible meltdown at two nuclear reactors

190 people possibly exposed to radiation

Chinese team joins search and rescue efforts

TOKYO - Japanese shares plunged in early trading on Monday as the country fought to avert a meltdown at two earthquake-crippled nuclear reactors.

Japan is struggling to respond to Friday's earthquake and tsunami, which may have killed 10,000 people. Millions are without water or power and whole towns wiped off the map. The Japanese Meteorological Agency upgraded the magnitude of the massive earthquake from 8.8 to 9.0 on Sunday.

The benchmark Nikkei 225 stock average shed 494 points, or 4.8 percent, to 9760.45 shortly after the stock market opened on Monday morning.

A grim-faced Prime Minister Naoto Kan said on Sunday: "The earthquake, tsunami and the nuclear incident have been the biggest crisis Japan has encountered in the 65 years since the end of World War II.

"We're under scrutiny on whether we, the Japanese people, can overcome this crisis."

He said the nation's future will be decided by the choices made by each Japanese person and urged all to join in their determination to rebuild the nation.

Kan also said the world's third biggest economy faced rolling power blackouts when it reopens for business on Monday.

Meanwhile, a 15-member Chinese search and rescue team joined relief work in Ofunato, Iwate, on Monday morning.

The team, which arrived in Ofunato on Sunday night, is the first from overseas to join the relief work in the city, Xinhua reported.

Team leader Yin Guanghui said they have brought along life exploration device and some other equipment to help carry out the rescue.

Team members, including rescue and medical personnel, set off at 7:30 am to the disaster-hit area together with the Japanese rescue teams. The team is expected to be here for seven to 10 days.

Officials are now working desperately to stop fuel rods in the damaged reactors from overheating, which could in turn melt the container that houses the core, or even explode, releasing radioactive material into the wind.

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.

Chief Cabinet Secretary Yukio Edano said on Sunday that a hydrogen explosion could occur at No 3 reactor of the Fukushima Daiichi nuclear complex, the latest reactor to face a possible meltdown. That follows a blast the day before in the power plant's No 1 reactor, and operators attempted to prevent a meltdown there by injecting seawater into it.

"At the risk of raising further public concern, we cannot rule out the possibility of an explosion," Edano said.

He said there might have been a partial meltdown of the fuel rods at the No 1 reactor. Engineers were pumping in seawater, trying to prevent the same happening at the No 3 reactor.

"Unlike the No 1 reactor, we ventilated and injected water at an early stage," Edano told a news briefing.

David Lochbaum, director of the Union of Concerned Scientists Nuclear Safety Project, said the use of seawater means they have "run out of options".

The operator of the nuclear reactors said on Sunday the top of fuel rods at the reactors had been three meters above water, an indication of a possible meltdown.

Tokyo Electric Power warned that the No 3 reactor was overheating and that so much of the cooling water had briefly evaporated that mixed oxide fuel rods were exposed to the air, Kyodo news said.

"Radiation has been released in the air, but there are no reports that a large amount was released," Jiji news agency reported.

A Japanese official said 22 people have been confirmed to has suffered radiation contamination and up to 190 people may have been exposed. Workers in protective clothing used handheld scanners to check people arriving at evacuation centers.

Broadcaster NHK, quoting a police official, said more than 10,000 people may have been killed.

Almost 2 million households were without power in the freezing north, the government said. There were about 1.4 million without running water.

Kyodo news agency said about 300,000 people were evacuated nationwide, many seeking refuge in shelters, wrapped in blankets, some clutching each other, sobbing.

Tokyo Electric Power says it will ration electricity with rolling blackouts in parts of Tokyo and other Japanese cities.

Deng Wei, Chinese embassy spokesman in Tokyo, said on Sunday that there are, so far, no reports of Chinese casualties but Kyodo reported that 40 Chinese trainees sent by a service company in Shandong province have lost contact after the earthquake. Embassy staff members are working hard to track all Chinese citizens in Japan, Deng said.

Authorities in Japan have set up a 20-km exclusion zone around the Fukushima Daiichi plant and a 10-km zone around another plant nearby. About 170,000 people have been moved out, while authorities prepared to distribute iodine to protect people from radioactive exposure.

The nuclear accident, the worst since the Chernobyl disaster in 1986, sparked stinging criticism that authorities were ill-prepared for such a massive quake and the threat that could pose to the country's nuclear power industry.

Kan said on Sunday the crisis was not the same as Chernobyl.

"Radiation has been released in the air, but there are no reports that a large amount was released," Jiji news agency quoted him as saying. "We are working to prevent damage from spreading."

Japanese officials said they had also ordered up the largest mobilization of their Self-Defense Forces since World War II to assist in the relief effort.

The Chinese International Search and Rescue Team arrived in Japan on Sunday - the first time the government has accepted assistance from China for disaster relief.

The 15 team members took with them four tons of materials and equipment for search and rescue, power supply and telecommunication. They began their work in Iwate prefecture. More than 30,000 Chinese nationals are estimated to live in Iwate and the other two most seriously damaged prefectures, Miyagi and Fukujima.