At 15:36 JST on 12 March, there was an explosion in the reactor building at Unit 1 while employees and the contracted workers who were engaged in the water injection work were outside the vehicle to refuel the fire engine. The workers fell down in surprise. When they looked up, debris was spreading all over the sky and then descending. Workers moved near the tank besides the nearby building to shelter themselves from the flying debris. They walked away from the spot carrying a worker with them who was unable to stand up due to the shock incurred from the explosion.
TEPCO estimates the nuclear fuel was exposed to the air less than five hours after the earthquake struck. Fuel rods melted away rapidly as the temperature inside the core reached 2,800 °C within six hours.
Officials have said that radiation levels at Fukushima were elevated before the blast: At one point, the plant was releasing each hour the amount of radiation a person normally absorbs from the environment each year. The explosion was preceded by puff of white smoke that gathered intensity until it became a huge cloud enveloping the entire facility, located in Fukushima, 20 miles (30 kilometers) from Iwaki and near the port city of Sendai. After the explosion, the walls of the building crumbled, leaving only a skeletal metal frame.
The side walls of the upper level were blown away, leaving in place only the vertical steel framed gridworks. Video of the explosion shows that it was primarily directed sideways, and an increase in radiation levels was confirmed following the explosion. The pressure in the reactor was also decreasing after the blast, according to Edano.