A considerable amount of the melted fuel is thought to have penetrated the reactor's pressure vessel and fallen to the bottom of the containment vessel, which includes the pressure vessel, TEPCO said.

If it is assumed that all of the fuel at the No. 1 reactor fell to the containment vessel, this could have eroded the vessel's 100-centimeter-thick concrete bottom by 65 centimeters, it said. But the fuel would not penetrate the containment vessel, it added.

At the No. 2 and No. 3 reactors where cooling continued for a certain period of time, about 60 pct of fuel is estimated to have melted. If all of the melted fuel is assumed to have fallen to the reactors' containment vessels, their bottoms could have been eroded by at the most 12 centimeters and 20 centimeters, respectively.