Tokyo Electric Power (TEPCO) warned that the number three reactor at Fukushima No. 1 plant was overheating and that so much of the cooling water had briefly evaporated that mixed oxide fuel rods were exposed to the air, Kyodo news said.

A TEPCO spokesman later told AFP that the rods were covered again.

An explosion that destroyed the building around the No. 1 reactor at the same plant Saturday came despite the operator's efforts to control high temperatures and growing pressure.