The big fear at the Fukushima nuclear complex, 240 km north of Tokyo, is of a major radiation leak. The complex has already seen explosions at two of its reactors on Saturday and on Monday, which sent a huge plume of smoke billowing above the plant.
Jiji news agency said fuel rods at the No 2 reactor had been entirely exposed and a fuel rod meltdown could not be ruled out. The plant operator confirmed there was little water left in the reactor, adding that the fuel rods may have been exposed.
The rods have now been partially covered by sea water, the reactor's operator said.
There were earlier partial meltdowns of the fuel rods at both the No 1 and the No 3 reactors, where the explosions had occurred.
A meltdown raises the risk of damage to the reactor vessel and a possible radioactive leak. Levels of cooling sea water around the reactor core had been reported as falling earlier in the day. Jiji said the pump had run out of fuel.
Crucially, officials said the thick walls around the radioactive cores of the damaged reactors appeared to be intact after the earlier hydrogen blast.
But the government warned those still in the 20-km evacuation zone to stay indoors. The plant operator, Tokyo Electric Power Co (TEPCO) , said 11 people had been injured in the blast.
Kyodo said 80,000 people had been evacuated from the zone, joining more than 450,000 other evacuees from quake and tsunami-hit areas in the northeast.
“Everything I've seen says that the containment structure is operating as it's designed to operate. It's keeping the radiation in and it's holding everything in, which is the good news,” said Murray Jennex, a nuclear expert at San Diego State University.
“This is nothing like a Chernobyl ... At Chernobyl (in Ukraine in 1986) you had no containment structure — when it blew, it blew everything straight out into the atmosphere.”
Nuclear experts said it was probably the first time in the industry's 57-year history that sea water has been used to cool the fuel rods, a sign of how close Japan may be to a major accident.
“Injection of sea water into a core is an extreme measure,” Mark Hibbs of the Carnegie Endowment for International Peace. “This is not according to the book.”
Nuclear fuel accounts for 30 per cent of Japan's electricity. Of Japan's 54 reactors, 11 have been shut down by the quake.
“If cooling water is not returned, the core should melt in a matter of hours,” said Edwin Lyman, senior scientist for global security programs at the Union of Concerned Scientists which lobbies for stronger security and safety measures at nuclear plants.
A Japanese official said before the blast that 22 people were confirmed to have suffered radiation contamination and up to 190 may have been exposed. Workers in protective clothing used hand-held scanners to check people arriving at evacuation centres. US warships and planes helping with relief efforts moved away from the coast temporarily because of low-level radiation. The US Seventh Fleet described the move as precautionary.
South Korea, Hong Kong, Singapore and the Philippines said they would test Japanese food imports for radiation.
Almost 2 million households were without power in the north. There were about 1.4 million without running water. Tens of thousands of people are missing. Roads and rail, power and ports have been crippled across much of Japan's northeast and estimates of the cost of the multiple disasters have leapt to as much as $170 billion.
“It’s a scene from hell, absolutely nightmarish,” said Patrick Fuller of the International Red Cross Federation from the coastal town of Otsuchi.
