An estimated 900,000 terabecquerels of radioactive substances were released into the atmosphere in March 2011 by the triple meltdowns at the Fukushima No. 1 nuclear plant, Tokyo Electric Power Co. said Thursday.
The figure is higher than projections previously released by the government's nuclear bodies, but less than a fifth of the 5.2 million terabecquerels thought to have been emitted by the 1986 Chernobyl disaster.
Some 4,600 terabecquerels of radioactive substances were discharged when an explosion ripped through the reactor 1 building March 12, and another 1,060 following an explosion at reactor 3 two days later, according to Tepco's study of the fallout emitted by the plant's three crippled units between March 12 and March 31.
The emissions peaked March 15 and 16, possibly because the primary containment vessels of the three reactors degraded due to high temperatures and released massive amounts of fallout and steam from their upper part.
In the following months, however, the amount of radioactive substances spewed by the three reactors was less than 1 percent of the levels seen in March, Tepco's study showed.
Reactors 1 to 3 at the Fukushima No. 1 power station suffered meltdowns after the March 2011 quake and tsunami knocked out almost all of the facility's power sources, preventing Tepco from maintaining the fuel inside them in a cool state.
Hydrogen explosions erupted in all three units in the early days of the crisis, blowing away their outer walls and roofs and releasing radioactive fallout into the environment.
The government last year acknowledged that the severity level of the Fukushima nuclear crisis registers a maximum 7 on the International Atomic Energy Agency's scale based on the amount of radioactive materials released — a ranking equivalent to Chernobyl.