According to reports, the employees were taken ill after drinking contaminated water in the plant.
When the employees were rushed to hospital and tested, the samples confirmed radioactivity.
Heavy water molecules are extremely toxic and can prove fatal if consumed.
"The contents of the water in the cooler are being investigated," a source said on condition of anonymity.
Scientists carried out immediate checks for radioactivity at the plant site but no leakage was detected, they said.
A probe was being carried out to find out how the water in the cooler was contaminated, the sources added.
Kaiga is one of India's newer nuclear plants and has three atomic power units of 220 MW each.