Government nuclear experts, however, said the World War II bomb blast and the accidental reactor meltdowns at Fukushima, which has seen ongoing radiation leaks but no deaths so far, were beyond comparison.

The amount of caesium-137 released since the three reactors were crippled by the March 11 quake and tsunami has been estimated at 15,000 tera becquerels, the Tokyo Shimbun reported, quoting a government calculation.

That compares with the 89 tera becquerels released by “Little Boy”, the uranium bomb the United States dropped on the western Japanese city in the final days of World War II, the report said.