Japan faced a fresh radiation threat at an earthquake-crippled nuclear plant on Sunday after the cooling system failed at a second reactor in what could be the world’s worst nuclear disaster in 25 years.

The previous day, thousands were evacuated following an explosion and leak from the facility’s No. 1 reactor in Fukushima, 240 km (150 miles) north of Tokyo.

Strong aftershocks continued to shake Japan’s main island as the desperate search pressed on for survivors from Friday massive earthquake and tsunami and the death toll was expected to rise. Media reports say it is likely to exceed 1,800.

Nuclear plant operator Tokyo Electric Power (TEPCO) said radiation levels around the Fukushima Daiichi plant had risen above the safety limit but it did not mean an "immediate threat" to human health.

It said earlier it was preparing to vent steam to relieve pressure in the No.3 reactor at the plant and the government had warned of a rise in radiation during the procedure.

Thousands spent another freezing night huddled over heaters in emergency shelters along the northeastern coast, a scene of devastation after the 8.9 magnitude quake sent a 10-meter (33-foot) wave surging through towns and cities in the Miyagi region, including its main coastal city of Sendai.

"It’s hard to even imagine the scale of it," a resident said. "I came to Miyagi during the last earthquake as well to help, but once there is water involved, it becomes a whole different story. It’s hard to think about."

Kyodo news agency, which said the number of dead or unaccounted was expected to exceed 1,800, reported that there had been no contact with around 10,000 people in one town, more than half its population.

The government had insisted radiation levels were low following Saturday’s explosion, saying the blast had not affected the reactor core container. The International Atomic Energy Agency (IAEA) said Japan had told it that levels "have been observed to lessen in recent hours".

But Japan’s nuclear safety agency said the number of people exposed to radiation could reach 160. Workers in protective clothing were scanning people arriving at evacuation centers for radioactive exposure.

"They are working on relieving pressure and pumping in water into the No. 3 reactor," Chief Cabinet Secretary Yukio Edano told a news briefing.

"This will result in some radiation leakage, although at a level that won’t affect peoples’ health. It will help stabilize the situation." He also said radiation from the No. 1 reactor was "low enough not to affect people’s health".

Officials ordered the evacuation of a 20-km (12-mile) radius zone around the plant and 10 km (6 miles) around another nuclear facility close by.

Around 140,000 people had left the area, the IAEA said, while authorities prepared to distribute iodine to protect people from radioactive exposure.

"There is radiation leaking out, and since the possibility (of being exposed) is high, it’s quite scary," said Masanori Ono, 17, standing in line on Saturday to be scanned for radiation at an evacuation center in Fukushima prefecture.

TEPCO has been pumping seawater into the No.1 reactor to cool it down.

"The use of seawater means they have run out of options. If they had any other water they would have used it. It likely means the power for their pumps is gone. They must be pumping the seawater in," said David Lochbaum, director of the Union of Concerned Scientists Nuclear Safety Project.

Government Criticised

The government, in power less than two years and which had already been struggling to push policy through a deeply divided parliament, came under criticism for its handling of the crisis.

"Crisis management is incoherent," blared a headline in the Asahi newspaper, charging that information disclosure and instructions to expand the evacuation area around the troubled plant were too slow.

"Every time they repeated ’stay calm’ without giving concrete data, anxiety increased," it quoted an unidentified veteran party lawmaker as saying.

There have been proposal of an extra budget to help pay for huge cost of recovery but the government says there is also a 200 billion yen ($2.4 billion) budget reserve for the current fiscal year which can be used.

In Europe, environmentalists seized on the accident to press for an end to nuclear power. Up to 60,000 protesters formed a 45-km (27-mile) human chain in Germany to denounce the government’s policy of extending the life of nuclear plants.

Before news of the problem with reactor No. 3, the nuclear safety agency said the plant accident was less serious than both the Three Mile Island accident in 1979 and the 1986 Chernobyl nuclear disaster.

An official at the agency said it rated the incident a 4 according to the International Nuclear and Radiological Event Scale (INES). Three Mile Island was rated 5 while Chernobyl was rated 7 on the 1 to 7 scale.

Devastated Coastline

Along the northeast coast, rescue workers searched through the rubble of destroyed buildings, cars and boats, looking for survivors in hardest-hit areas such as Sendai, 300 km (180 miles) northeast of Tokyo.

Aerial footage showed buildings, trains and even light aircraft strewn like children’s toys after powerful walls of seawater swamped areas around Sendai.

In Iwanuma, not far from Sendai, nurses and doctors were rescued on Saturday after spelling S.O.S. on the rooftop of a partially submerged hospital, one of many desperate scenes. In cities and towns, worried relatives checked information boards on survivors at evacuation centers.

Dazed residents hoarded water and huddled in makeshift shelters in near-freezing temperatures.

"All the shops are closed, this is one of the few still open. I came to buy and stock up on diapers, drinking water and cooking," Kunio Iwatsuki, 68, told Reuters in Mito city, where residents queued outside a damaged supermarket for supplies.

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

It said 5.5 million people were without power, while 3,400 buildings had been destroyed or damaged. Four trains were unaccounted for after the tsunami.

In Tokyo, the usually bustling central districts were deserted on Saturday night, and the few in bars and restaurants were glued to television coverage of the disaster.

"Even in the bar we kept staring at the news," said Kasumi, a 26-year-old woman meeting a friend for a drink in the central district of Akasaka. "I looked at the tsunami swallowing houses and it seemed like a film."

Biggest Challenge

Plant operator TEPCO has had a rocky past in an industry plagued by scandal. In 2002, the president of the country’s largest power utility was forced to resign along with four other senior executives, taking responsibility for suspected falsification of nuclear plant safety records.

Many Japanese flooded social networking sites with worries about the plant.

"I can’t trust TEPCO," said a person with the handlename Tanuki Atsushi on mixi, the Japanese social networking site.

The earthquake and tsunami, and now the radiation leak, present Japan’s government with its biggest challenge in a generation.

The disaster struck as the world’s third-largest economy had been showing signs of reviving from an economic contraction in the final quarter of last year. It raised the prospect of major disruptions for many key businesses and a massive repair bill running into tens of billions of dollars.

Foreign countries have started to send disaster relief teams to help Japan, with the United Nations sending a group to help coordinate work.

The earthquake was the fifth most powerful to hit the world in the past century. It surpassed the Great Kanto quake of September 1, 1923, which had a magnitude of 7.9 and killed more than 140,000 people in the Tokyo area.