Bangalore, Nov 30 (IANS) About 50 employees of the state-run Kaiga atomic power plant in coastal Karnataka are being interrogated by investigation agencies to identify the suspect who might have contaminated the drinking water dispenser with the radioactive isotope Tritium, a senior official said Monday.

"I have given the names of persons who were present in the first reactor unit Nov 24 when the incident occurred to the central and state investigation agencies, which are conducting the inquiry to identify who could have committed the mischief," Kaiga director J.P. Gupta told reporters at the plant facility, about 500 km from here.

Besides the state-run Nuclear Power Corporation of India Ltd (NPCIL) and the Intelligence Bureau (IB), the investigation is being carried out by the Kaiga atomic team and the state police, with whom a complaint (FIR) had been formally registered.

"We know the antecedents of the radiation employees who were working in that shift. We have their records in the form of biometrics and photo identification cards. It is for the investigators to find out the suspect(s)," Gupta said.

Clarifying that Tritium contamination was not fatal and that the affected employees were not exposed to excess radiation, Gupta said of the total radiation workers, only one person was found to be near the permissible limit of (30ml) while the rest of them were far below the limit approved by the Indian Atomic Energy Regulation Board (AERB).

"Within hours after the affected employees displayed abnormal symptoms, we administered medicine to flush out the contamination in the drinking water by giving a stomach-wash to each of them," Gupta said.

Though the 220MW first unit has remained shutdown for biennial maintenance since Oct 20, the radiation workers who were carrying out the work were found to be affected after they consumed the contaminated water from the dispenser.

"It is for the investigation agencies to find out how the vials of heavy water (Tritium) from the reactor found their way into the water cooler and who could have been behind it," Gupta said.

Noting that the sensitive plant was safe and secure from intrusion or outside threat, Gupta said the unit had fool-proof security and stringent procedures are followed to screen the employees and contract workers when they enter, during their working hours and when leaving the premises.

"Thorough checks are conducted on everyone when entering and leaving the premises to ensure no radioactive or any other object is taken out," Gupta added.

An official at the plant had earlier told IANS on condition of anonymity that the total number of permanent workers at all the four units is 1,689. "There will be around 5,000 contract workers engaged in various activities," the official said.

"Contract workers are of two categories -- those who are engaged on a regular basis at the plant and those belonging to the contractors who have undertaken the shutdown-related maintenance work," the official added.