By Fredrik Dahl and Michael Shields
VIENNA (Reuters) - The head of the U.N. nuclear watchdog said Japan's massive earthquake and tsunami had shaken and flooded nuclear power plants but their reactor vessels remained intact and radiation release was limited.
Yukiya Amano spoke as Japan scrambled to avert a meltdown at a stricken nuclear plant Monday after a hydrogen explosion at one reactor and exposure of fuel rods at another, just days after the devastation that killed thousands.
"The Japanese authorities are working as hard as they can, under extremely difficult circumstances, to stabilize the nuclear power plants and ensure safety," Amano said in a statement to member states of the International Atomic Energy Agency (IAEA) in Vienna.
"The nuclear plants have been shaken, flooded and cut off from electricity. Operators have suffered personal tragedies," he said, according to a copy of his remarks. But "the reactor vessels have held and radioactive release is limited."
Amano, a veteran Japanese diplomat, said Japan had asked the U.N. agency to provide expert missions.
"We are in discussions with Japan on the details," he said.
The Fukushima nuclear complex, 240 km (150 miles) north of Tokyo, has already suffered explosions at two of its reactors on Saturday and Monday, which sent a huge plume of smoke billowing above the plant.
WORST ACCIDENT SINCE CHERNOBYL
The nuclear accident, the worst since the Chernobyl disaster in Ukraine in 1986, sparked criticism that authorities were ill-prepared.
"The Japanese experts and authorities on the ground are doling their best to stabilize the situation," Amano said. "We will not try to second-guess the people on the ground."
The plant operator, Tokyo Electric Power Co (TEPCO), said fuel rods at the plant's No. 2 reactor were fully exposed, which could lead to the rods melting down.
There were earlier partial meltdowns of the fuel rods at both the No. 1 and the No. 3 reactors, where the explosions had occurred, and a TEPCO official described the situation in the No. 2 reactor as even worse than in the other units.
A meltdown raises the risk of damage to the reactor vessel and a possible radioactive leak.
But officials said the thick walls around the radioactive cores of the damaged reactors appeared to be intact after the earlier hydrogen blast, a position Amano confirmed.
Saying the island of Honshu had shifted two and a half meters as a result of the earthquake, he said:
"Japan and all our member states can be assured that all resources put at our disposal are fully mobilized. That will remain the case until this crisis has been resolved."
He said the IAEA's Incident and Emergency Center was working "around the clock" since the quake struck.
(Editing by Mark Heinrich)