The fuel rods at a third nuclear reactor at the Fukushima No.1 nuclear power plant have been fully exposed to air, allowing them to heat up and raising the risk of a meltdown, according to officials of the Tokyo Electric Power Co, which owns the plant.
Engineers had begun pumping seawater into the reactor at the facility, the third reactor to receive the last-ditch treatment, after the plant's emergency cooling system had failed and the fuel rods had been partially exposed to the air.
But apparently something went wrong and the injection of water failed.
Workers were scrambling to re-immerse the fuel assembly before more damage is done to the reactor core.
No one knows how much damage has been done to the fuel rods, either in this reactor, No.2, or in reactors No.1 and No.3, where engineers began pumping in seawater..
Officials have called the situation a partial meltdown because they have detected minute quantities of radioactive caesium and iodine - byproducts of the nuclear fission that powers the reactor - outside the plant.
That may mean simply that the zirconium cladding that sheathes the uranium fuel pellets has cracked due to heat from being exposed to the air, allowing small quantities of the radionuclides to escape, or it may mean that the fuel pellets themselves have partially melted.
As long as the reactor containment vessel remains intact, however, no one will know until workers can physically examine the fuel rods for damage.
Officials of the Tokyo Electric Power Co, which owns the plant, said conditions are stable at reactors No.1 and No.3 and that the cooling seems to be working.
An explosion at reactor No.3 destroyed the outer building at the reactor and injured 11 workers, but did not damage the reactor containment vessel.
A similar explosion at reactor No.1 earlier in the weekend damaged that building and released what is said to be small amounts of radiation into the environment.
Three workers at the plant have been hospitalised for radiation exposure, but it is not clear how much radiation they received.
At least 20 civilians have had radiation detected on their clothes, but US experts believe their exposure was minimal.
A US warship sailing off the coast of Japan reported that it sailed through a small plume of radiation from the plant, but has successfully decontaminated both the ship and sailors.
Japanese authorities have so far reported no radiation release from the explosion at the No.3 reactor.
Authorities fear that the injection of seawater into the No.2 reactor and the exposure of the fuel rods to air could lead to a buildup of hydrogen gas and an explosion at that plant as well.
