The fuel rods in the number two reactor at a quake-damaged nuclear power plant in Japan are again "fully exposed", officials said, boosting fears of an eventual partial meltdown.
Air pressure inside the reactor at the Fukushima No. 1 plant, located 250 kilometres (155 miles) north of Tokyo, rose suddenly when the air flow gauge was accidentally turned off, operator Tokyo Electric Power Co. (TEPCO) said.
That blocked the flow of cooling water into the reactor, leading to full exposure of the rods at around 11:00 pm Monday (1400 GMT), TEPCO said.
"We are not optimistic but I think we can inject water once we can reopen the valve and lower air pressure," a TEPCO official told reporters.
Japan has been grappling with a nuclear emergency since a massive earthquake and tsunami battered its northeast coast Friday.
The nuclear plants shut down automatically, as they are designed to do. But the loss of power in the area and tsunami damage to back-up generators apparently crippled reactor cooling systems.
Explosions created by hydrogen blew apart the buildings housing the number one and number three reactors, but did not pierce the reactors' steel and concrete containment vessel.
Chief government spokesman Yukio Edano said that radiation around the Fukushima plant was at a tolerable level for humans.
AIRCRAFT CHECKED FOR RADIATION
Meanwhile, German airline Lufthansa is scanning aircraft that return from Japan for radioactivity but has not detected any yet.
"This is a precautionary measure for us," a Lufthansa spokesman told AFP.
Aircraft that flew to Lufthansa hubs in Frankfurt and Munich were checked by airport fire services but no radioactivity has been detected so far, he added.
A spokesman for the Frankfurt airport said that only Lufthansa had taken the measure so far.
"As far as I know Japanese airlines have not" asked that their planes be checked as well, and "no official directive" has been issued regarding the measure, the airport spokesman said.
A US aircraft carrier deployed off tsunami-hit Japan for relief efforts has repositioned after detecting low-level radiation from malfunctioning nuclear power plants, a US statement said Monday.
"The source of this airborne radioactivity is a radioactive plume released from the Fukushima Dai-Ichi Nuclear Power Plant," said the US statement.
The ship was operating at sea about 160 kilometres (100 miles) northeast of the power plant at the time, but it is not known how high or how far the plume had spread.