TOKYO (Reuters) - Japan on Monday expanded the evacuation zone around its crippled nuclear plant because of high levels of accumulated radiation, as a strong aftershock rattled the area one month after a quake and tsunami sparked the worst nuclear crisis since Chernobyl.
A magnitude 6.6 tremor shook buildings in Tokyo and a wide swathe of eastern Japan on Monday evening, knocking out power to 220,000 households and causing a halt to water pumping to cool three damaged reactors at Fukushima.
The epicentre of the latest quake was 88 km (56 miles) east of the plant and stopped power supply for pumping water to cool reactors No. 1, No. 2 and No. 3.
The aftershock also forced engineers to postpone plans to remove highly contaminated water from a trench at reactor No. 2.
The government announced earlier that because of accumulated radiation contamination, it would encourage people to leave certain areas beyond its 20 km (12 mile) exclusion zone around the plant.
Children, pregnant women, and hospitalised patients should stay out of some areas 20-30 km from the nuclear complex, Chief Cabinet Secretary Yukio Edano told reporters.
"These new evacuation plans are meant to ensure safety against risks of living there for half a year or one year," he said. There was no need to evacuate immediately, he added.
The move comes amid international concern over radiation spreading from the six damaged reactors at Fukushima, which engineers are still struggling to bring under control after they were wrecked by the 15-metre tsunami on March 11.
TEPCO President Masataka Shimizu visited the area on Monday for the first time the disaster. He had all but vanished from public view apart from a brief apology shortly after the crisis began and has spent some of the time since in hospital.
"I would like to deeply apologise again for causing physical and psychological hardships to people of Fukushima prefecture and near the nuclear plant," said a grim-faced Shimizu.
Dressed in a blue work jacket, he bowed his head for a moment of silence with other TEPCO officials at 2:46 p.m. (0546 GMT), exactly a month after the earthquake hit.
Fukushima Governor Yuhei Sato refused to meet him, but the TEPCO boss left a business card at the government office.
Engineers at the damaged Daiichi plant north of Tokyo said they were no closer to restoring the plant's cooling system which is critical to bring down the temperature of overheated fuel rods and to bringing the six reactors under control.
In a desperate move to cool the highly radioactive fuel rods, operator Tokyo Electric Power Co (TEPCO) 9501.t has pumped water onto reactors, some of which have experienced partial meltdown.
But the strategy has hindered moves to restore the plant's internal cooling system as engineers have had to focus how to store 60,000 tonnes of contaminated water.
They have been forced to pump low-level radioactive water, left by the tsunami, back into the sea in order to free up storage capacity for highly contaminated water from reactors.
China and South Korea have both criticised Japan for pumping radioactive water into the sea, with Seoul calling it incompetent, reflecting growing international unease over the month-long atomic disaster and the spread of radiation.
TEPCO hopes to stop pumping radioactive water into the ocean on Monday, days later than planned.
Engineers are also pumping nitrogen into reactors to counter a build-up of hydrogen and prevent another explosion sending more radiation into the air, but they say the risk of such a dramatic event has lowered significantly since March 11.
The triple disaster is the worst to hit Japan since World War Two after a 9.0 magnitude earthquake and a huge tsunami battered its northeast coast, leaving nearly 28,000 dead or missing and rocking the world's third-largest economy.
Concern at Japan's inability contain its nuclear crisis is mounting with Prime Minister Naoto Kan's ruling party suffering embarrassing losses in local elections on Sunday.
Voters vented their anger at the government's handling of the nuclear and humanitarian crisis, with Kan's ruling Democratic Party of Japan losing nearly 70 seats in local elections.
In a statement marking one month since the earthquake, On behalf of the people of Japan, Kan expressed "heartfelt thanks" for assistance and support from 130 nations around the world.
"Japan will certainly repay, through our contributions to the international community, the cordial assistance we have received from around the world," he said.
The unpopular Kan was already under pressure to step down before March 11, but analysts say he is unlikely to be forced out during the crisis, set to drag on for months.
"The great disaster was a double tragedy for Japan. The first tragedy was the catastrophe caused by the earthquake, tsunami and the nuclear accident. The other misfortune was that the disaster resulted in prolonging Prime Minister Kan's time in office," Sankei newspaper said in an editorial on Monday.
Additional reporting by Issei Kato, Shinichi Saoshiro, Chisa Fujioka, Elaine Lies, Masahiro Koike and Linda Sieg in Tokyo; Writing by Michael Perry; Editing by Jonathan Thatcher and Alex Richardson