Tokyo Electric Power Co. said Thursday that the water level in the No. 1 reactor’s pressure vessel at the Fukushima No. 1 power plant is much lower than thought and that some of the fuel rods have melted and sunk to its bottom.

But Tepco also said the temperature in the pressure vessel has been kept at around 100 to 120 degrees, which is cool enough to keep the reactor safe, for now.

The low water level, however, indicates that the pressure vessel could have holes or cracks through which radioactive water is leaking.

Tepco’s latest discoveries were found after workers entered the main building, where they were finally able to check and fix water-level indicators damaged when the March 11 monster earthquake and tsunami crippled the nuclear power plant.

The workers also learned that the water in the containment vessel, which Tepco has kept spraying to cool down the pressure vessel from the outside, is also much lower than earlier believed. There might be some holes or cracks in the vessel or components connected to it, Tepco said.

Any holes or cracks in the pressure and containment vessels are sure to make repair work more difficult.

The utility plans to keep cooling the reactor’s core by attaching air-cooling machines that will recirculate the water inside the containment vessel.

Tepco has poured about 10,000 cu. meters of water into the containment vessel so far. But only about 7,500 cu. meters should be necessary to cool the fuel rods if a way can be found to reuse the water instead of letting it flow away.

“We’re not sure how much of the fuel rods fell down to the bottom and in what form, but the temperature shows that they are being cooled,” Tepco spokesman Junichi Matsumoto said at a news conference.

On March 12, reactor No. 1 suffered a hydrogen explosion after the fuel rods were fully exposed for hours and generated hydrogen, which reacted violently with oxygen.

But Matsumoto said Tepco believes such an explosion at this time is unlikely because nitrogen has already been poured inside the containment vessel to purge explosive gases.

Matsumoto also said that, considering the situation with the No. 1 reactor, the water level data from reactors 2 and 3 may not be credible.

Tadashi Narabayashi, a professor of reactor engineering at Hokkaido University, also said it is likely that the melted fuel rods are being cooled with coolant water at the bottom of the pressure vessel.

According to Narabayashi, the zirconium encasing the fuel rods becomes weak and brittle when it is fully exposed to oxygen and then covered with cold water again.

According to Tepco, the water-level indicators of the pressure vessel had indicated the water surface was about 1.65 meters below the top of the fuel rods. But as of Thursday morning the reading was more than 5 meters below the top. The fuel rods, if undamaged, are only 4.5 meters in height.

Although the water is leaking from somewhere in the pressure vessel, Tepco officials don’t believe the melted fuel has penetrated the bottom of the pressure vessel and dropped into the containment vessel below, given that the thermometer placed at the bottom of the pressure vessel is still working. If a big chunk was missing, it would malfunction, Tepco said.

There are actually no tools specially designed to check the water level in the containment vessel, but Tepco said it made estimates based on other factors, including the pressure in the containment vessel.

information from kyodo added

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