Fuel rods in the No. 1 reactor at the Fukushima No. 1 nuclear power plant may have mostly melted and dropped to the bottom of the pressure vessel as the water level in the vessel has been found to be significantly lower than thought, Tokyo Electric Power Co. said Thursday.
The fuel rods in the reactor are believed to be completely exposed, according to TEPCO, the operator of the plant.
So-called water entombment operations to fill the containment chamber with water are continuing in an effort to cool the reactor. But the water level in the chamber cannot be clearly determined, and water is likely leaking from it, the utility said.
TEPCO said the temperature in the pressure vessel is stabilized at 100 C to 120 C but that the water-entombment plan, in which water was expected to be filled to about 1 meter above the top of the fuel rods, needs to be reconsidered. The company is considering increasing the amount of water injected into the pressure vessel, which currently stands at about 8 tons per hour.
TEPCO learned about the water level of the pressure vessel after workers who entered the reactor building beginning Tuesday adjusted a water-level gauge. Previously, the reading of the water level had remained almost unchanged at about 1.6 meters below the top of fuel rods since immediately after the outbreak of the crisis at the plant following the March 11 earthquake and tsunami.
After adjusting the gauge, workers found the actual water level was more than 5 meters below the top of the fuel rods. As the fuel rods are about 4 meters long, they are considered to have been fully exposed above the cooling water, TEPCO said.
It said it believes the fuel rods mostly have fallen to the bottom of the pressure vessel after melting or collapsing.
The Economy, Trade and Industry Ministry's Nuclear and Industrial Safety Agency said the fuel has been cooled with water pooled at the bottom of the pressure vessel as the temperature of the vessel remains relatively low.
At the bottom of the steel pressure vessel, which is 16 centimeters thick, the water level is believed to stand at a maximum of only about 4 meters, TEPCO said. The company believes that most of the 190 tons of water injected every day is leaking from the pressure vessel, which is likely to be damaged more seriously than previously thought.
More than 10,000 cubic meters of water had been injected into the reactor as of Thursday, exceeding the combined 360-cubic-meter capacity of the pressure vessel and the 7,800-cubic-meter capacity of the containment chamber.
It is highly likely that water is leaking from both the pressure vessel and containment chamber and flowing into underground parts of the reactor building and the adjacent turbine building, TEPCO officials said.
To proceed with the water-entombment, it is necessary to accurately grasp the water level in the containment chamber and the conditions of pipes that take in or release cooling water, experts said.