The cooling system at the number two reactor at the quake-damaged Fukushima No. 1 nuclear power plant in Japan has failed, Jiji Press reported Monday, citing operator Tokyo Electric Power Co (Tepco).
A new explosion at the plant hit the country Monday as experts raced to avert a catastrophic meltdown in the wake of the quake and tsunami.
Meanwhle, the crew of the aircraft carrier Ronald Reagan, on a humanitarian mission to Japan, received a month's worth of radiation in about an hour, The New York Times reported.
Citing unnamed government officials, the newspaper said the ship passed through a radioactive cloud from damaged nuclear reactors in Japan when it approached the country on its humanitarian mission.
Japan has been battling to control two overheating reactors at the ageing Fukushima plant after the cooling systems were knocked out by Friday's 8.9-magnitude quake and the resulting tsunami that swallowed up whole towns.
The number one and three reactors at the same plant have experienced the same problem, followed by explosions in the buildings surrounding those reactors.
US helicopters flying missions about 60 miles (96 kilometers) north of the damaged reactors became coated with radioactive dust that had to be washed off, the report said.
Officials at TEPCO were not immediately available to comment.
The plant is located 250 kilometres (120 miles) north of Tokyo.
There was no indication that any of the US military personnel had experienced ill effects from the exposure, the paper said.
But the episodes showed that the prevailing winds were picking up radioactive material from crippled reactors in northeastern Japan, The Times noted.
Water level near empty at Japan nuclear reactor - owner
Water levels inside a quake-stricken Japanese nuclear reactor were almost empty on Monday night, said the power plant's operator, Tokyo Electric Power Co .
Earlier, news agency Jiji said a meltdown of fuel rods inside the Fukushima Daiichi complex's No.2 reactor could not be ruled out.
A meltdown raises the risk of damage to the reactor vessel and a possible radioactive leak, experts say.