Tokyo Electric Power Co (TEPCO) said Saturday it had found radioactive water leaking into the sea from a cracked concrete pit at its No.2 reactor in Fukushima. Russian expert Natalia Mironova: Fukushima 'much bigger than Chernobyl.'

The radiation in the pit was measured 1,000 millisieverts per hour, TEPCO said in a statement adding that they were planning to pour concrete into the pit to seal the crack, the Reuters reported.

"With radiation levels rising in the seawater near the plant, we have been trying to confirm the reason why, and in that context, this could be one source. We are testing samples of water from the pit and from the sea near the plant, and we cannot really say for certain until we've studied the results," Hidehiko Nishiyama, deputy director-general of the Nuclear and Industrial Safety Agency said.

Exposure to 500 millisieverts over a short period of time can increase the long-term risk of cancer.

Meanwhile, the Russian nuclear energy expert, Natalia Mironova, has assessed that Japan's unfolding nuclear disaster is "much bigger than Chernobyl" and could rewrite the international scale used to measure the severity of atomic accidents.

"Chernobyl was a dirty bomb explosion. The next dirty bomb is Fukushima and it will cost much more" in economic and human terms, Mironova was cited by the agencies.

Mironova is a thermodynamic engineer who became a leading anti-nuclear activist in Russia in the wake of the accident at the Soviet-built reactor in Ukraine in 1986.

The International Atomic Energy Agency (IAEA) called Chernobyl "the most severe in the history of the nuclear power industry" and ranked it a seven on the International Nuclear Event Scale (INES), and “it had only one reactor and lasted only two weeks. We have now three weeks (at Fukushima) and we have four reactors which we know are in very dangerous situations," she warned.

IAEA says the Fukushima is ranked five on the INES scale.