The Japanese government formally has asked the U.S. Nuclear Regulatory Commission for help in stabilizing its troubled nuclear reactors in the wake of the country's massive earthquake and tsunami.
The NRC sent two boiling water reactor experts to Japan as part of a team of aid workers to help in the recovery efforts.
A number of nuclear reactors continue to deteriorate at the Fukushima Daiichi plant, raising worries of a nuclear meltdown.
Officials had grown increasingly worried about Fukushima Daiichi nuclear plant unit 2 after two hydrogen explosions in three days occurred at the plant, and the unit lost its ability to cool.
The fuel rods on unit 2 became fully exposed for the second time Monday, a dangerous development in the effort to stop the reactor from melting down.
Japanese officials said a closed steam vent caused a dip in the water levels, allowing the rods to be exposed, The Associated Press reported.
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The exposure of the fuel rods means that the temperature in the reactor is likely to rise, allowing steam to form. The steam could lead to the creation of hydrogen, which is what caused the explosions at reactors 1 and 3.
Knowing how long the fuel rods have been exposed is key to understanding if there is a real chance of a meltdown, said Dr. Peter Hosemann, a nuclear energy expert and professor at the University of California at Berkeley.
"Having too much of the fuel rods exposed for too long of a time can lead to the core melt. Again, if a core melt happens, the reactor pressure vessel and the containment are designed to contain it," Hosemann said.
Japanese officials acknowledged that the fuel rods appeared to be melting inside all three of the reactors at the Fukushima plant.
"Although we cannot directly check it, it's highly likely happening," Edano told the Associated Press.
Experts said that the melting of the fuel rods should not be seen as an indication of imminent danger.
"The melting of the fuel rods in and of itself is not an immediate threat to the life and health of the public…there's at least three layers [of protection]: 1. The fuel is inside cladding; 2. The fuel and cladding are inside the pressure vessel; and 3. The fuel and pressure vessel are inside a containment building and that containment building is holding up well and the pressure vessel is holding up well," University of California at Berkeley nuclear engineering professor Edward Morse said.
Officials first became concerned about unit 2 at the plant after pressure began rising in the reactor. Officials from the Tokyo Electric Power Co. told NHK News that the explosion at unit 3 might have damaged unit 2's cooling system.
Workers began pumping sea water into the reactor following the explosion Monday morning. The system pumping the sea water experienced a fuel loss, causing a dip in the water levels around the rods, NHK News reported. This led to the first exposure of the rods.
"They've had trouble with getting the pumps working, with site power in general," Morse said. "They've shipped in extra diesel generators and they may have to do some extra retrofit plumbing."
Workers had returned to pumping sea water when the fuel rods were exposed for a second time.
While unit 1, the first reactor to explode at the plant, appeared to be stable, unit 3, which exploded early Monday morning in Japan, reportedly had a leak in its bottom.