A magnitude 6.6 tremor shook buildings in Tokyo and a wide swath of eastern Japan on Monday evening, killing one man, knocking out power to 220,000 households and causing a brief halt to water pumping to cool three damaged nuclear reactors.
The epicenter of quake, the biggest of several sizable aftershocks on Monday, was 88 km (56 miles) east of the Fukushima Daiichi nuclear complex at the centre of the crisis.
The biggest tremor forced engineers to postpone plans to remove highly contaminated water from one reactor, but nuclear safety officials said work had resumed by nightfall.
The government announced earlier that because of accumulated radiation contamination, it would encourage people to leave certain areas beyond its 20 km 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 05 46 GMT, exactly a calendar 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.
Radioactive water
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) 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 on 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 the 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 was in the final stage of pumping radioactive water into the ocean, Nuclear and Industrial Safety Agency's deputy director-general Hidehiko Nishiyama said.
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 1.
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