Photo/Illutration The No. 1 reactor building of the Fukushima No. 1 nuclear plant was severely damaged by a hydrogen explosion in 2011. (Asahi Shimbun file photo)

The water level at a damaged reactor building of the stricken Fukushima No. 1 nuclear power plant continued to decline after Tokyo Electric Power Co.’s initial report following the March 16 earthquake.

The utility said on March 17 that the water level in the No. 1 reactor containment vessel had dropped by 20 centimeters possibly because of something related to atmospheric pressure.

But TEPCO said on March 22 that the water level had fallen by another 20 cm, possibly because earlier damage to the structure was expanded in the March 16 quake.

The company said the further decline cannot be explained by atmospheric pressure alone.

It said radioactive water has not leaked outside the reactor building, and that it can continue to cool the melted nuclear fuel within the vessel.

A robotic probe on March 22 found the water level in the containment vessel was 40 cm lower than usual level of 2 meters.

TEPCO said it will add more cooling water if the level drops further.

The No. 1 reactor was one of three at the plant that melted down after the magnitude-9.0 Great East Japan Earthquake and the ensuing tsunami damaged the facility on March 11, 2011.

The No. 1 reactor building was also hit by a hydrogen explosion in the chaotic early stages of containing the nuclear disaster.

The magnitude-7.4 earthquake on March 16 struck in the same offshore area and measured a lower 6 on the Japanese seismic intensity scale of 7 around the nuclear plant.

Water levels also dropped at the No. 1 and No. 3 reactors following a powerful quake in February last year. TEPCO had to pump in additional water to keep the nuclear debris cool.

(This article was written by Yu Fujinami and Tsuyoshi Kawamura.)

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