"Even if the leakage was very small, this is not permissible after an earthquake," said Gensuikin, a group representing victims of the nuclear attacks on Hiroshima and Nagasaki.

"It clearly shows how the current safety calculations were based on over-optimistic thinking," it said in a statement, saying that the standards "should be overhauled completely."

Japan has been sensitive about nuclear issues due to the US atomic attacks at the end of World War II, which killed more than 210,000 people instantly. Tens of thousands later died from radiation and horrific burns.

But Japan's government has embraced atomic energy. Despite being one of the most earthquake-prone nations, the world's second largest economy has virtually no oil or gas resources.

Tokyo Electric Power Co (TEPCO), operator of the world's largest nuclear plant, has acknowledged it had not prepared for an earthquake as forceful as Monday's, which killed 10 people and destroyed hundreds of buildings.

The company said that radioactive material leaked from the massive facility northwest of Tokyo but that the amount was far too small to pose any health risks.

Japan relies on nuclear energy for about one-third of its needs, the highest level among Group of Eight industrial nations except for France.

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