High levels of radiation prevented helicopters on Wednesday from dropping water over a quake and tsunami-damaged nuclear reactor in north-eastern Japan, a news report said.

Military helicopters have been unable to pour water over reactor number 3 at a power plant in Fukushima, whose cooling systems failed after Friday’s magnitude-9 earthquake and tsunami, the public broadcaster NHK reported.

The inner shell of the reactor might be damaged and a release of radioactive steam was feared, the government said earlier in the day.

But preparations to cool down the nuclear reactors from land were in their final stages, Chief Cabinet Secretary Yukio Edano said.

The power plant is home to six reactors.

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