Radio New Zealand - Te Reo Irirangi o Aotearoa

Twenty workers have been accidentally exposed to radiation at the Three Mile Island nuclear power plant in the American state of Pennsylvania.

It happened while maintenance work was being carried out at the plant, the site of the worst nuclear accident in US history, when a reactor core partially melted in 1979.

Three Mile Island spokesperson Ralph DeSantis says the workers were cutting pipes when a radiation alarm sounded.

The leak was only a small one, he says, and the exposed workers have since been decontaminated. All 150 staff who work in the affected unit were sent home.

The US Nuclear Regulatory Commission also says that the amount of leaked radiation detected at the plant is not significant.

"It's a minor incident," says commission spokeswoman Diane Screnci, adding that the leak, which occurred at 4pm Saturday local time, was not a public health risk.

The unit has been shut for refuelling and maintenance since 26 October.

AAP reports specialist John White as telling ABC News on Sunday that there is no indication that radiation at the plant exceeded or even approached regulatory limits.

Nuclear energy supplies 20% of power in the US, with 104 reactors in operation, but no new plants have been built since the 1979 accident.