Technicians are battling to stabilise a third reactor at a quake-stricken Japanese nuclear plant, which has been rocked by a second blast in three days.
The Fukushima Daiichi plant's operators have resumed pumping seawater into reactor 2 after a cooling system broke.
They warned of a possible meltdown when the fuel rods became exposed after the pump stopped as its fuel ran out.
A cooling system breakdown preceded explosions at the plant's reactor 3 on Monday and reactor 1 on Saturday.
The latest hydrogen blast injured 11 people, one of them seriously, and sent a huge column of smoke billowing into the air.
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image of Richard Black Richard Black Environment correspondent, BBC News
The fuel rod exposure at Fukushima Daiichi number 2 reactor is potentially the most serious event so far at the plant.
A local government official confirmed the fuel rods were at one point largely, if not totally exposed; but we do not know for how long.
Without coolant around the rods, temperatures can rise to hundreds of degrees Celsius, almost certainly resulting in some melting.
This opens the possibility of a serious meltdown - where molten, highly radioactive reactor core falls through the floor of the containment vessel and into the ground underneath.
However, engineers appear to have restored some water flow into the reactor vessel and if they are successful, temperatures will begin to fall again rapidly.
The Tokyo Electric Power Co (Tepco) is playing down any health risk, saying thick containment walls shielding the reactor cores have remained intact.
Experts say a disaster on the scale of Chernobyl in the 1980s is highly unlikely because the reactors are built to a higher standard and have much more rigorous safety measures.
But the US said it had moved one of its aircraft carriers from the area after detecting low-level radiation 160km (100 miles) offshore.
Tens of thousands of people have been evacuated from a 20-km exclusion zone around the Fukushima Daiichi plant.
The relief operation is continuing after Friday's magnitude 8.9 quake, which triggered a tsunami that devastated swathes of the north-eastern coast of the country.
The death toll remains unclear - officials in Miyagi estimate 10,000 people died in that prefecture alone.
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Chief Cabinet Secretary Yukio Edano said the core container at the reactor was still intact
Tens of thousands of relief workers, soldiers and police have been deployed to the disaster zone.
The government asked people not to go to work or school on Monday because the transport network would not be able to cope with demand.
The capital, Tokyo, is still experiencing regular aftershocks, amid warnings that another powerful earthquake is likely to strike very soon.
The disaster is a huge blow for the Japanese economy - the world's third largest - which has been ailing for two decades