The core container was intact, Jiji news agency said, quoting the plant operator, Tokyo Electric Power Co (TEPCO), but the local government warned those still in the 20-km (13-mile) evacuation zone to stay indoors. Seven people, six of them soldiers, were missing in the blast, Jiji said.
A TV station also reported a new tsunami on Monday but it turned out to be a false alarm.
Japan battled through the weekend to prevent a nuclear catastrophe and to care for the millions without power or water in its worst crisis since World War Two, after the huge earthquake and tsunami that likely killed more than 10,000 people.
Kyodo news agency said 2,000 bodies had been found on Monday on the shores of Miyagi prefecture, which took the brunt of the tsunami.
The government had warned of a possible explosion at the No 3 reactor because of the buildup of hydrogen in the building housing the reactor. TV images showed smoke rising from the Fukushima facility, 240 km (150 miles) north of Tokyo.
