The Japanese government has sought to play down fears of a meltdown at the Fukushima plant.
It says radiation levels around the stricken plant have now fallen.
A huge rescue and relief operation is under way in the region after the earthquake and subsequent tsunami, which are thought to have killed more than 1,000 people.
Tokyo Electric Power said four of its workers had been injured in Saturday's blast at Fukushima, 250km (155 miles) north of Tokyo, but that their injuries were not life-threatening.
An evacuation zone around the damaged nuclear plant has been extended to 20km (12.4 miles) from 10km, and a state of emergency declared.
An estimated 200,000 people have been evacuated from the area, the International Atomic Energy Authority says.
Government spokesman Yukio Edano said the force of the explosion had destroyed the concrete roof and walls of a building around the plant's number one reactor, but a steel container encasing the reactor had not been ruptured.
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