The explosion early Saturday ripped through one of the buildings on the Fukushima Daiichi plant, injuring four workers.
Radiation levels were high before the explosion — at one point releasing as much radiation every hour as a person would normally absorb from the environment in a year — but radiation outside the plant started decreasing after the blast and the pressure inside the reactor was also dropping, the spokesman said.
A government spokesman said the blast did not damage the nuclear reactor itself, which would cause radioactive material to leak out.
But Ryohei Shiomi with Japan’s Nuclear Safety Commission said a meltdown was possible.
The evacuation radius has now been expanded to 20 kilometers (12 miles).
The majority of the 51,000 people living near the danger zone have already been evacuated, according to Shiomi.
Three evacuees have been exposed to radiation, but have not shown signs of illness, a disaster official told The Associated Press.
Japanese authorities say they have plans to distribute iodine to residents in the area around both the Fukushima Daiichi and nearby Fukushima Daini plants.