Reuters

By Shinichi Saoshiro and Chisa Fujioka

TOKYO (Reuters) – Japan faced a potential catastrophe on Tuesday after a quake-crippled nuclear power plant exploded and sent low levels of radiation floating toward Tokyo, prompting some people to flee the capital and others to stock up on essential supplies.

Prime Minister Naoto Kan urged people within 30 km (18 miles) of the facility -- a population of 140,000 -- to remain indoors amid the world's most serious nuclear accident since the Chernobyl disaster in Ukraine in 1986.

Officials in Tokyo -- 240 km (150 miles) to the south of the plant -- said only minute levels of radiation had been detected so far in the capital, which were "not a problem."

Radiation levels in the city of Maebashi, 100 km (60 miles) north of Tokyo, and in Chiba prefecture, nearer the city, were up to 10 times normal levels, Kyodo news agency said. Foreign experts disagreed on whether this was harmful or not.

Around eight hours after the explosions, the U.N. weather agency said winds were dispersing radioactive material over the Pacific Ocean, away from Japan and other Asian countries. The Geneva-based World Meteorological Organization added that weather conditions could change.

As concern about the crippling economic impact of the nuclear and earthquake disasters mounted, Japan's Nikkei index fell as much as 14 percent before ending down 10.6 percent, compounding a slide of 6.2 percent the day before. The two-day fall has wiped some $620 billion off the market.

Two of the reactors exploded on Tuesday at the Fukushima Daiichi plant after days of frantic efforts to cool them. Kyodo news agency said the nuclear fuel pool at the No. 4 reactor may be boiling, suggesting the crisis is far from over at the plant.

"The possibility of further radioactive leakage is heightening," a grim-faced Kan said in an address to the nation. "We are making every effort to prevent the leak from spreading. I know that people are very worried but I would like to ask you to act calmly."

Levels of 400 millisieverts per hour had been recorded near the No. 4 reactor, the government said. Exposure to over 100 millisieverts a year is a level which can lead to cancer, according to the World Nuclear Association.

The plant operator pulled out 750 workers, leaving just 50, and a 30-km no-fly zone was imposed around the reactors. There have been no detailed updates on what levels the radiation reached inside the exclusion zone where people live.

"Radioactive material will reach Tokyo but it is not harmful to human bodies because it will be dissipated by the time it gets to Tokyo," said Koji Yamazaki, professor at Hokkaido University graduate school of environmental science. "If the wind gets stronger, it means the material flies faster but it will be even more dispersed in the air."

Despite pleas for calm, residents rushed to shops in Tokyo to stock up on supplies. Don Quixote, a multi-storey, 24-hour general store in Roppongi district, sold out of radios, flashlights, candles and sleeping bags.

In a sign of regional fears about the risk of radiation, China said it would evacuate its citizens from areas worst affected but it had detected no abnormal radiation levels at home. Air China said it had canceled flights to Tokyo.

Several embassies advised staff and citizens to leave affected areas. Tourists cut short vacations and multinational companies either urged staff to leave or said they were considering plans to move outside Tokyo.

"I'm scared. I'm so scared I would rather be in the eye of a tornado," said 10-year-old Lucy Niver of Egan, Minnesota, who was on holiday in Japan. "I want to leave."

"WHAT THE HELL IS GOING ON"

Japanese media have became more critical of Kan's handling of the disaster and criticized the government and nuclear plant operator Tokyo Electric Power Co (TEPCO) for its failure to provide enough information on the incident.

Kan himself lambasted the operator for taking so long to inform his office about one of the blasts, demanding to know "what the hell is going on?," Kyodo reported.

Kyodo said Kan had ordered TEPCO not to pull employees out of the plant.

"The TV reported an explosion. But nothing was said to the premier's office for about an hour," a Kyodo reporter quoted Kan telling power company executives.

Lam Ching-wan, a chemical pathologist at the University of Hong Kong, said the blasts could expose the population to longer-term exposure to radiation, which can raise the risk of thyroid and bone cancers and leukemia. Children and fetuses are especially vulnerable, he said.

"Very acute radiation, like that which happened in Chernobyl and to the Japanese workers at the nuclear power station, is unlikely for the population," he said.

Chief Cabinet Secretary Yukio Edano, talking of levels of radiation at the Fukushima Daiichi plant's No. 4 reactor, said: "There is definitely a possibility that this could affect people's bodies."

There have been a total of four explosions at the plant since it was damaged in last Friday's massive quake and tsunami. The most recent were blasts at reactors No. 2 and No. 4.

There was a real possibility of a leak in the No.4 reactor container, which houses the nuclear fuel rods, according to Murray Jennex, a professor at San Diego State University in California.

Concerns center on damage to a part of the reactor core known as the suppression pool, which helps cool and trap the majority of cesium, iodine, strontium in its water. The nature of the damage was unclear, as was its impact on the containment structure, a thick steel vessel that surrounds the core.

Jennex said the crisis in Japan, the only nation to have suffered a nuclear attack, was worse that the Three Mile Island disaster of 1979.

"But you're nowhere near a Chernobyl ... Chernobyl there was no impediment to release, it just blew everything out into the atmosphere," he said. "You've still got a big chunk of the containment there holding most of it in."

Authorities had previously been trying to prevent meltdowns in the complex's nuclear reactors by flooding the chambers with sea water to cool the reactors down.

A sudden drop in cooling water levels when a pump ran out of fuel had fully exposed the fuel rods for a time, an official said. TEPCO had resumed pumping sea water into the reactor early on Tuesday.

U.S. warships and planes helping with relief efforts moved away from the coast temporarily because of low-level radiation. The Seventh Fleet described the move as precautionary.

South Korea, Thailand, Hong Kong, Singapore and the Philippines said they would test Japanese food imports for radiation.

VILLAGES AND TOWNS WIPED OFF THE MAP

The full extent of the destruction from last Friday's 9.0-magnitude earthquake and tsunami that followed it was still becoming clear, as rescuers combed through the region north of Tokyo where officials say at least 10,000 people were killed.

Whole villages and towns have been wiped off the map by Friday's wall of water, triggering an international humanitarian effort of epic proportions.

About 850,000 households in the north were still without electricity in near-freezing weather, Tohuku Electric Power Co. said, and the government said at least 1.5 million households lack running water. Tens of thousands of people were missing.

Toshiyuki Suzuki, 61, has a heart pacemaker and takes seven kinds of medicine a day. He lost all of them when the waves swept away his home, along with his father and son.

He cannot go to hospitals because there is no gasoline at local fuel stations. "I am having problems with walking and with my heartbeat. I absolutely need medicine."

Kan has said Japan is facing its worst crisis since World War Two.

Hiromichi Shirakawa, chief economist for Japan at Credit Suisse, said in a note to clients that the economic loss will likely be around 14-15 trillion yen ($171-183 billion) just to the region hit by the quake and tsunami.

Even that would put it above the commonly accepted cost of the 1995 Kobe quake which killed 6,000 people.

The earthquake has forced many firms to suspend production and global companies -- from semiconductor makers to shipbuilders -- face disruptions to operations after the quake and tsunami destroyed vital infrastructure, damaged ports and knocked out factories.

"The earthquake could have great implications on the global economic front," said Andre Bakhos, director of market analytics at Lec Securities in New York. "If you shut down Japan, there could be a global recession."

(Additional reporting by Nathan Layne, Linda sieg, Risa Maeda, and Leika Kihara in Tokyo, Chris Meyers and Kim Kyung-hoon in Sendai, Taiga Uranaka and Ki Joon Kwon in Fukushima, Noel Randewich in San Francisco, Tan Ee-lyn in Singapore and Miyoung Kim in Seoul; Writing by Nick Macfie; Editing by John Chalmers and Dean Yates)