The amount of radioactive caesium that has leaked from the Fukushima Dai-ichi nuclear power plant is roughly 168 times that released by the atomic bomb the US dropped on Hiroshima in 1945, according to Japan's nuclear agency.
The Nuclear and Industrial Safety Agency said the privately owned and operated plant has released 15,000 tera-becquerels of caesium-137, which lingers for decades and could cause cancer.
That compares with 89 tera-becquerels released by the US "Little Boy" uranium bomb.
The agency supplied the estimate at a parliamentary panel's request.
The US dropped "Little Boy" on Hiroshima on August 6 1945, destroying most of the city and killing around 140,000 people.
The atomic bombing of Nagasaki three days later killed tens of thousands more.
The Hiroshima bomb claimed most of its victims in the intense heatwave and neutron rays from the explosion and the highly radioactive fallout.
No one has died from radiation leaks from the Fukushima plant, which was damaged by the earthquake and tsunami that struck Japan on March 11.
Explosions from hydrogen buildup damaged reactor buildings but did not involve reactor cores.
The report estimated that iodine-131, another isotope that accumulates in the thyroid gland, and strontium-90, which has a 28-year half-life and could accumulate in bones, leaked from the plant in amounts about equal to 2.5 of the Hiroshima bomb.
The National Institute for Environmental Studies said its simulation of aerial flow, diffusion and deposition of the two isotopes released from the plant showed their impact reached most of Japan's eastern half, ranging from Iwate in the north to Tokyo and central prefecture of Shizuoka.
Both Iwate and Shizuoka are more than 180 miles away from the plant.
Around 100,000 people have been forced to leave their homes due to radiation threats from Fukushima.
About 30 residents from the immediate neighbourhood of the plant were allowed to briefly return home on Friday to get clothes and other necessities they left early in the crisis. But officials have said their area may remain off-limits for decades.
Plant owner and operator Tokyo Electric Power Co and the government aim to bring the reactors to stable cold shutdowns by early January.
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