This would make it more difficult to nail the man who had added radioactive Tritium to the water, he added. Forty-five employees were affected and had to be hospitalised Nov 24 after drinking water from the dispenser, which was located inside a laboratory.

There were 1,689 permanent employees and around 5,000 contract workers, all with access to the area where the Tritium had been stored as well as to the dispenser, the official said, speaking on condition of anonymity.

Atomic Energy Commission Chairman Anil Kakodkar had said the contamination could have been an inside job.

"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 said.

"Out of the affected eight are permanent workers and the rest are contract employees. The matter came out into the open when a worker who did not enter the reactor main area was found to have been exposed to high levels of radiation Nov 23 evening. It was then that the water cooler was zeroed in as the source of contamination," the official said.

Tritium can cause cancer if ingested. The affected workers are from Unit 1 of Kaiga plant, located around 500 km from Bangalore and owned by Nuclear Power Corporation of India Ltd (NPCIL).

They were found to have received high dosage of radiation during urine analysis Nov 24. However, many of the workers taken to hospital have been discharged.

NPCIL Chairman and Managing Director S.K. Jain had Sunday said that a probe has been launched into the incident.

He also pointed out that Unit 1 of Kaiga has been shut down for annual maintenance since Oct 20. Around 500 workers are involved in the maintenance work. Many of them are contract workers.

A union official said: "Atomic power plants unlike thermal plants are of vital nature. Outsourcing should be restricted to non-core activities."

There is talk that NPCIL's newly introduced performance-linked incentive scheme could have caused some heartburn among workers.

But Kaiga Station Director J.P. Gupta ruled out such issues at the plant.

"We have a list of the people who had entered the reactor building that day. The investigating agencies are analysing the data," Gupta said.

At Kaiga there are four reactors each with a capacity to generate 220 MW. While three units are operational, the fourth one is expected to go on stream soon.