Key facilities at the Fukushima No. 1 nuclear power may have been damaged by the quake itself that day rather than tsunami-caused power loss that failed the reactor's cooling function, Kyodo News quoted a utility source said Saturday.

Data taken by workers entering the No. 1 reactor building at the crippled plant on the night of March 11 showing the radiation level was as high as 300 millisieverts per hour suggest a large amount of radioactive materials from nuclear fuel in the reactor was already released.

The findings may call for a review of preparedness against quakes at various nuclear power stations in Japan as they have primarily focused on securing auxiliary power supplies and embankment enhancement against tsunami after the Fukushima plant crisis, assuming that reactor facilities at the plant were unscathed by trembling.

On March 11, the power plant was shut down automatically just past 2:46 p.m. following the magnitude-9 quake. Within an hour, it was hit by at least two rounds of tsunami waves. The external power supply was then shut down, disabling the emergency core cooling system from injecting water at 4:36 p.m.

It has been thought that power loss failed the cooling system at the No. 1 reactor, releasing highly radioactive steam from the reactor pressure vessel.

Kyodo said a source at TEPCO admitted the possibility of key facilities having been compromised before the tsunami waves, saying, "The quake's trembling may have caused damage to the pressure vessel or pipes."

Japan's Nuclear and Industrial Safety Agency has so far said the reactor withstood shaking but tsunami of an unexpected scale caused power loss, which led to an explosion.