BBC News

With the loss of power at reactor 3, and with its valves and pumps damaged by the tsunami, emergency workers were pumping in seawater mixed with boron - which disrupts nuclear chain reactions - to cool the rods.

But one report suggested the tops of the rods had briefly been exposed.

Technicians opened valves, allowing small amounts of radioactive vapour to escape in a bid to reduce the pressure in the unit.

They performed a similar operation on the first reactor, hours before the explosion that wrecked the building it was housed in.