JAPAN'S unfolding nuclear disaster is "much bigger than Chernobyl" and could rewrite the international scale used to measure the severity of atomic accidents, a Russian expert says.

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

Ms Mironova is 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.

"Fukushima is much bigger than Chernobyl," she said, adding that the Japanese nuclear crisis was likely to eclipse Chernobyl on the seven-point international scale used to rate nuclear disasters.

Chernobyl, which a 2005 report by UN bodies including the International Atomic Energy Agency (IAEA) called "the most severe in the history of the nuclear power industry", was ranked a seven on the International Nuclear Event Scale (INES).

But Japan's ongoing crisis, triggered by a massive earthquake and tsunami three weeks ago which took down the main electricity and back-up power supplies needed to power cooling systems at several reactors at Fukushima, could be "even higher" on the INES scale, she said.

"Chernobyl was level seven 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 said.

Japan's nuclear safety agency has maintained its rating of the Fukushima accident at four, while a French watchdog has upgraded it to six.

Chernobyl's death toll is hotly debated. UN agencies estimate up to 9000 people could be expected to die as a direct consequence of the accident, and the disaster will end up costing hundreds of billions of dollars.

Environmental groups such as Greenpeace say up to 100,000 people could die.

Ms Mironova is touring the United States with other Russian anti-nuclear activists, including Tatiana Muchamedyarova and Natalia Manzurova, who worked as a "liquidator", or emergency clean-up and recovery worker, at Chernobyl.

Their visit was originally planned to coincide with the 25th anniversary of the Chernobyl meltdown, which occurred on April 26, 1986.

But in the wake of the disaster in Japan, Ms Mironova and her colleagues rewrote their presentations to compare the accident at Chernobyl with Fukushima.