A hole in reactor one of the Fukushima Dai-ichi plant may be behind a dramatic drop in water levels inside the pressure vessel, operator Tokyo Electric Power Company said Thursday.
TEPCO officials said the amount of water inside the vessel was much lower than previously thought, fully exposing the fuel rods and melting them.
The pressure and temperature inside held steady, an indication that the fuel rods were now being cooled in water at the bottom of the vessel, TEPCO said.
The latest discovery comes after workers fixed a water gauge inside reactor one.
TEPCO has been pouring 150 tons of water a day to cool the reactor, since an earthquake and tsunami crippled it on March 11.
When water levels failed to rise, workers suspected a faulty water gauge.
On Thursday morning, they discovered that the water was more than 3 feet below the bottom of the fuel rods, leading them to conclude a possible leak.
"The important point is that this is news that TEPCO has found that a measurement they have been making for several weeks was incorrect," said Per Peterson, a professor of nuclear engineering at the University of California at Berkeley. "It does not mean that anything new is happening in the unit one reactor now."
Peterson said that steady pressure and temperatures indicated the fuel was being adequately cooled in its current configuration.
TEPCO workers have been struggling to stabilize nuclear reactors at the Fukushima plant, since a magnitude 9.0 quake and tsunami triggered the world's second worst nuclear crises.
About 80,000 evacuees living near the plant have been displaced because of radiation concerns.
Company officials have said they plan to bring the reactors to a state of cold shutdown in nine months.