The three were randomly chosen for examination out of some 90 bedridden patients moved from a hospital in the town of Futaba-machi, public broadcaster NHK reported.

The patients had waited for rescuers on a school ground, spending a long time outside and then being moved by helicopter at the time when an explosion hit the ageing Fukushima No. 1 plant, NHK said, citing the Fukushima local government.

The explosion sent plumes of smoke spewing from the nuclear power plant today, raising fears of a possible meltdown a day after a massive quake struck the facility's cooling system.

The government declared an atomic emergency and said tens of thousands of people living within 20 kilometres (12 miles) of the plant should leave.

The operator of the nuclear plant said the container surrounding the reactor was not damaged despite the explosion.