Blast heard at Fukushima's No.2 reactor:

READ MORE: An explosion was heard early Tuesday morning at the troubled No. 2 reactor of the quake-hit Fukushima No. 1 nuclear power plant, the government said, about five hours after work resumed to inject seawater into it to prevent overheating of exposed fuel rods.

Shortly after the 6:10 a.m. incident, the radiation level exceeded the legal limit to reach 965.5 micro sievert per hour, and it is feared that the reactor's pressure-suppression system was damaged, the Nuclear and Industrial Safety Agency.