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.
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. It was felt 40km (25 miles) away and sent a huge column of smoke into the air.
The outer building around the reactor was largely destroyed.
But as with the first explosion, Tokyo Electric Power Company (Tepco) said the thick containment walls shielding the reactor cores remained intact. It also said radiation levels outside were still within legal limits.
Mass evacuation Shortly after the blast, Tepco warned that it had lost the ability to cool Fukushima Daiichi's reactor 2.
Hours later, the company revealed that the fuel rods inside had been exposed fully at one point, reportedly for about two-and-a-half hours. It said a fire pump being used to pump seawater into the reactor had run out of fuel.
The company is now trying to inject sea water into the reactor to cover the fuel rods, cool them down and prevent another explosion. Initially, water levels continued to fall despite the efforts, as only one of the five fire pumps was working, officials said. The other four were believed to have been damaged by the blast at reactor 3.
By Monday evening, however, the water level inside the reactor had risen to 2m (7ft), the Kyodo news agency reported.
Exposure for too long a period of time can damage the fuel rods and raise the risk of overheating and possible meltdown.
Japan's Nuclear and Industrial Safety Agency said workers were also battling rising pressure within the reactor. They have opened vents in the containment vessel, which could release small amounts of radiation.
Chief Cabinet Secretary Yukio Edano earlier said the emergency effort to cool reactor 2 would hopefully stabilise the situation, and that radiation around the plant remained at tolerable levels despite the various crises.
Nevertheless, tens of thousands of people have been evacuated from a 20km (12 mile) exclusion zone around the plant.
The US said it had moved one of its aircraft carriers from the area after detecting low-level radiation 160km (100 miles) offshore.
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.
Earlier, Tepco said it had restored the cooling systems at two of the three reactors experiencing problems at the nearby Fukushima Daini power plant, 11.5km (7.1 miles) to the south.
Complete devastation Meanwhile, the relief operation is continuing after Friday's magnitude 9.0 earthquake, which triggered a tsunami that devastated swathes of the north-eastern coast of the country.
About 2,000 bodies had been found washed ashore on beaches in the north-eastern prefecture of Miyagi, police said.
A thousand were found on the Ojika peninsula and another thousand in the town of Minamisanriku, which was flattened by the tsunami.
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