SYDNEY - The impact of any meltdown in Japanese nuclear reactors damaged by the recent earthquake will be minor compared to the damage caused by the quake itself and the subsequent tsunami, Australia's well-known nuclear power expert said on Monday.

Former Australian Nuclear Science and Technology Organization (ANSTO) Chairman Ziggy Switkowski said a significant build-up of radiation was unlikely.

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"The contribution, if any, to this (disaster) from the nuclear fleet, I expect even under worst case scenarios is going to be small," he told Australia's Fairfax Radio Network on Monday.

"That's not to deny that people are always concerned and justly concerned about the integrity of the nuclear reactor network," Switkowski said.

"The Japanese reactors are probably as good as you can find around the world, but this magnitude 9 earthquake may well have tested the limits of their design."

However, he said the nuclear situation in Japan was obviously very serious, ranking with the Three Mile Island disaster in the US in 1979 where a reactor was destroyed after the core melted.

"The Japanese reactors are not quite at that point. But there are obviously serious concerns about ensuring that the core of one of these reactors where the cooling isn't working remains under control and does not melt and does not create radiation leaks," he said.

The risk of an uncontrolled loss of containment of the core, releasing large amounts of radiation, was believed to be small, and the radiation would be unlikely to spread very far, he said.

It was reported that there was a large explosion inside a concrete reactor building at the Fukushima No. 1 power station about 220 kilometers north of Tokyo after the 9 magnitude earthquake struck Northern Japan on Friday and radioactivity had risen 20-fold.