A total of 55 people, who worked at the Kaiga NPP in southern India were hospitalized with symptoms of radioactive poisoning after drinking contaminated water, Indian media said on Sunday.

Preliminary reports say tritium, a highly radioactive substance, was added to a cooler of the station's first unit. The substance, also known as Hydrogen-3, is used in research, fusion reactors and neutron generators.

"An insider had mixed tritium in drinking water in a cooler kept in the operating island of the first unit," the Hindu cited a top official from the Nuclear Power Corporation of India as saying.

The incident took place on November 25, when the unit with a capacity of 220 MWe was shut down for maintenance.

No leak from the station's reactors was detected. The incident poses no threat to people or environment.