Tokyo Electric Power Co., the operator of the crippled Fukushima Daiichi nuclear power plant, revealed Thursday that holes had been created by melted nuclear fuel at the bottom of the No. 1 reactor's pressure vessel.
The company said it has found multiple holes adding up to several centimeters in welded piping. Earlier in the day, it said the amount of water inside the troubled reactor was unexpectedly low -- not enough to cover the nuclear fuel -- hinting that a large part of the fuel melted after being fully exposed.
The finding is raising concerns that the company will face difficulty achieving its plan to bring the damaged reactors to a stable condition known as a "cold shutdown" in about six to nine months, observers said.
But the utility, known as TEPCO, noted that water at the bottom of the reactor pressure vessel holding the fuel is keeping the melted fuel cool, indicating that the company is succeeding in preventing the reactor's fuel from overheating by injecting water from outside.
Based on the latest data after workers adjusted gauges for measuring the water level at the reactor, water could not be confirmed inside the pressure vessel at a point 5 meters below the top of the 4-meter-long fuel rods when they are in their usual position, according to TEPCO.
The water level is far lower than earlier thought. Measurements taken before the gauge adjustment indicated that about 1.5 to 1.7 meters of the fuel rods were exposed and not submerged in water.
Still, the surface temperature of the pressure vessel was relatively low, measuring between 100 C and 120 C, allowing a TEPCO official to say, "We've been successful in cooling (the fuel rods with water)."
The utility had earlier estimated that 55 percent of the reactor core at the No. 1 unit has been damaged. It is unknown how much fuel melted and dropped to the bottom, but the fuel is unlikely to be in its original position.
TEPCO has been trying to check the levels of water inside the No. 1 reactor's pressure vessel and the outer primary container so that it can move ahead with a plan to flood the container with water up to the level above the fuel and create a system to stably keep the fuel cool.
But the latest finding regarding the situation inside the unit -- one of six at the plant crippled by the massive March 11 quake and tsunami -- suggests that a significant amount of the water injected into the reactor core as an emergency measure to keep the fuel cool was leaking out to the primary container.
A TEPCO official said the company will review its plan and map out additional necessary measures to flood the primary container up to the level above the fuel by Tuesday.
Hidehiko Nishiyama, a spokesman for the government's Nuclear and Industrial Safety Agency, said even if the container is not flooded with water, creating a system that would enable coolant to circulate around the reactor is possible by using the water that now exists inside the vessel and primary container.
The review of the flooding plan will include injecting around 8 tons of water per hour to raise the water level in the container and changing the point for the water's removal to make way for the new water circulation system, according to sources familiar with the situation.
The utility will also check the No. 1 reactor vessel to detect from where water is leaking, they added.
TEPCO, however, could face difficulties in accomplishing such missions, considering that the radiation level is high at some locations inside the No. 1 reactor's building, observers said.
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