Japanese media say officials have detected caesium, one of the elements released when overheating causes core damage, around reactor at Fukushima No 1 plant in Futuba

This article is more than 9 years old

This article is more than 9 years old

Workers are battling to stave off a possible nuclear meltdown at a plant in north-eastern Japan as the country struggles with the aftermath of Friday's enormous earthquake and tsunami.

Japanese media said officials had detected caesium, one of the elements released when overheating causes core damage, around the reactor at Fukushima No 1 plant in Futuba, 150 miles (240km) north of Tokyo.

The Tokyo Electric Power Company (Tepco) said it did not believe a meltdown was under way, but Ryohei Shiomi, an official with Japan's nuclear safety commission, said that it was possible.

Experts and authorities played down the dangers of a Chernobyl-style disaster, saying they believed a partial meltdown was controllable. The government urged people to remain calm.

Officials had earlier evacuated 20,000 residents living within 6 miles of the plant on the orders of the prime minister, Naoto Kan, who had inspected it via helicopter. Experts told Associated Press that the risk area was 4 miles.

The crisis began when the 8.9 magnitude shock cut off power, turning off the water supply needed to cool the system. The tsunami is thought to have cut off the backup diesel generator an hour later, leading to pressure rising rapidly within the reactor.

Broadcaster NHK said attempts to vent radioactive gas to lower the pressure had been suspended because the radiation level on one valve was higher than expected, heightening the risk of exposing workers to radiation.

Earlier in the day a Japanese nuclear safety panel said radiation levels were 1,000 times higher than normal in a control room and eight times higher than normal just outside the plant. Workers were changing shifts frequently.

Tepco has also declared an emergency at the Fukushima No 2 plant after reporting problems with a reactor there.

The chief cabinet secretary, Yukio Edano, said the pressure control system was not functioning at the plant's three reactors. Officials were evacuating residents within a 2 mile radius and had ordered those with 6 miles to stay indoors, NHK said.

Naoto Sekimura, a professor at the University of Tokyo, told AP a major radioactive disaster was unlikely.

"No Chernobyl is possible at a light water reactor. Loss of coolant means a temperature rise, but it also will stop the reaction," he said.

"Even in the worst-case scenario, that would mean some radioactive leakage and equipment damage, but not an explosion. If venting is done carefully, there will be little leakage. Certainly not beyond the 3km radius."

A partial meltdown in one of the light water reactors at Three Mile Island in 1979 resulted in the release of radioactive gases in the most serious incident in the history of the US nuclear power industry. The reactor was eventually brought under control despite a series of errors.

Across the worst hit area of Japan, the north-east Tohoku region, at least 630 people are dead and a similar number missing, according to police sources, with 1,128 injured. The country has mobilised 50,000 rescuers but they have yet to reach the most severely affected areas.

Residents woke up after a freezing night on rooftops and in emergency shelters to a sea of mud, water and debris. Earthquakes continued to rock the north-east coast overnight, although some said the worst tremors appeared to be subsiding.

Japan downgraded tsunami warnings in most areas but Tohoku remained on high alert for waves up to 10 metres high.

The tsunami has reached countries across the Pacific region but there were no reports of major damage outside Japan.

Images shot from helicopters showed many people still crowding the rooftops of buildings surrounded by water and mud. Some, including children, were winched to safety by rescuers.

Other footage showed the letters SOS spelled out on the roof of a hospital in Iwanuma, Miyagi prefecture.

Photographs from Sendai – one of the worst hit cities – showed families crammed into schools.

"The flood came in from behind the store and swept around both sides. Cars were flowing right by," said Wakio Fushima, who owns a convenience store in Sendai, which has around 1 million inhabitants and is 80 miles from the quake's epicentre.

Witnesses said the tsunami had reached about six miles inland.

"The tsunami was unbelievably fast. Smaller cars were being swept around me and all I could do was sit in my truck," said driver Koichi Takairin, 34, who was trapped in his four-tonne vehicle by the torrent.

Hundreds queued outside supermarkets for basic supplies and petrol stations were crowded with cars.

Authorities warned citizens in northern Japan to be prepared for severe power cuts due to the shutdown of nuclear plants, which provide about 30% of the country's electricity.

More than 1 million households are without water.

Phone voice services are also down across much of the north-east, although data services seemed to be working sporadically.