The amount of radioactive cesium-137 that has so far been released by the Fukushima Daiichi nuclear disaster is equal to 168 Hiroshima atomic bombs, according to government estimates.
Cesium-137 is a radioactive isotope of cesium, and is the principal source of radiation in the dead zone around the Chernobyl nuclear power plant.
In a report on Thursday, the Japanese daily Tokyo Shimbun said the government calculated that the amount of cesium released in the six months since the three reactors were hit by the earthquake and tsunami is 15,000 tera becquerels.
In comparison, Little Boy, the World War II bomb dropped on the western Japanese city of Hiroshima, released 89 tera becquerels.
The figures were submitted by Prime Minister Naoto Kan's cabinet to a lower house committee.
But despite the estimate, nuclear experts say it is impossible to properly compare the reactor meltdowns at Fukushima with the dropping of an atomic bomb, which was designed to inflict damage.
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Most of Hiroshima's 140,000 victims died in the “intense heatwave of a mid-air nuclear explosion and the highly radioactive fallout from its mushroom cloud”, Agence France-Presse reported.
At the Fukushima plant, about 150 miles north-east of the capital, Tokyo, there were no explosions. Instead, radiation has leaked from molten fuel inside reactors that were damaged by hydrogen explosions.
Japan's Nuclear and Industrial Safety Agency said Fukushima, the world's worst nuclear crisis in 25 years, is likely to have released about 15 percent of the radiation that went into the air in the 1986 Chernobyl accident.