The blast occurred at reactor 2 at the Fukushima Daiichi plant, which engineers had been trying to stabilise after two other reactors exploded.
One minister has said it is "highly likely" that the rods might melt. Radiation levels near the plant have risen.
The crisis was sparked by a 9.0-magnitude quake and tsunami on Friday.
Thousands of people are believed to have died, and millions are spending a fourth night without water, food, electricity or gas. More than 500,000 people have been left homeless.
Staff evacuation A fresh explosion rocked reactor 2 at the Fukushima Daiichi plant - 250km (155 miles) north-east of Tokyo - in the early hours of Tuesday.
Some staff have been evacuated, but initial indications suggest it is not on the same scale as the previous blasts.
On Monday, a hydrogen blast at reactor 3 injured 11 people and destroyed the building surrounding it. That explosion was felt 40km (25 miles) away and sent a huge column of smoke into the air.
It followed a blast at reactor 1 on Saturday.
http://www.bbc.co.uk/news/world-12740843
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