The steel hull of the reactor in Fukushima, 240 km north of Tokyo, was not damaged, a government spokesman said, citing information from the plant operator, the Tokyo Electric Power Co (TEPCO).
The city is home to 10 reactors at two power plants, and explosions have occurred at two of the reactors since Friday's magnitude-9 earthquake and tsunami. One happened Saturday and the second Monday after a magnitude-6.2 aftershock.
A TEPCO spokesman said radiation was measured at the plant at 20 microsievert per hour 25 minutes after Monday's blast. Japan allows an hourly exposure of 500 microsievert, which is a measure of the biological effects of radiation.