The main radioactive element released from the Fukushima nuclear disaster since March equals 168 Hiroshima World War II bombs, a news report said Thursday.

Japan estimates the amount of radioactive isotope caesium-137 released by the Fukushima nuclear accident, which has seen continuous radiation leak, however, resulted in few deaths so far. The government's nuclear experts said the World War II bomb blast and the accidental reactor meltdowns at Fukushima were beyond comparison, according to the Telegraph.

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

But the uranium bomb called Little Boy, dropped by the United States on the western Japanese city on Aug. 6, 1945, released 89 tera becquerels, the report said.

The estimate was submitted by Prime Minister Naoto Kan's cabinet to a lower house committee on promotion of technology and innovation, the Tokyo Shimbun said.

Caesium-137, the main radioactive element thrown out during the various explosions, has a half-life of 30 years, and it is not going to disappear overnight, a French nuclear safety expert, Didier Champion, told Le Figaro newspaper.

However, the Japanese government argued that the comparison was not valid. While the Hiroshima bomb claimed most of its victims in the intense heatwave of a mid-air nuclear explosion and the highly radioactive fallout from its mushroom cloud, no such nuclear explosions hit Fukushima.

In Fukushima, the radiation has seeped from molten fuel inside reactors damaged by hydrogen explosions.

An atomic bomb is designed to enable mass-killing and mass-destruction by causing blast waves and heat rays and releasing neutron radiation, the Tokyo Shimbun daily quoted a government official as saying. It is not rational to make a simple comparison only based on the amount of isotopes released.

The blinding blast of the Hiroshima bomb killed some 140,000 people, either instantly or in the days and weeks that followed as high radiation or horrific burns took their toll.

Japan declared a 20-kilometre (12 mile) evacuation and no-go zone around the Fukushima plant after the March 11 quake and tsunami that triggered the world's worst nuclear accident since Chernobyl 25 years ago.

A recent government survey showed that some areas within the 20-kilometer zone are contaminated with radiation equivalent to more than 500 millisieverts per year -- 25 times more than the government's annual limit.