BANGALORE-- Workers at a nuclear power plant in southern India were treated for poisoning after drinking water was deliberately spiked with radiation, senior government officials said Sunday.

Routine tests showed 55 employees from the plant in Kaiga in the state of Karnataka had increased levels of the radioactive element tritium, which is used in nuclear reactors.

B. Bhattacharjee, a member of the National Disaster Management Authority, said someone had inserted contaminated water into a water cooler, according to the Press Trust of India.

The employees had not suffered any ill effects and had returned to work, plant officials told AFP.

Atomic Energy Commission chairman Anil Kakodkar, speaking on the Headlines Today television network, blamed "an insider who has played mischief".

Kakodkar said security was "fool-proof" and there was no chance of an outsider gaining access to the facility.

State ministers assured local residents that their health was not at risk.