The usefulness and applicability of water indices such as Langelier saturation index, Ryznar stability index and Practical scale index for predicting the corrosive/scale forming tendency of chemically treated open recirculating cooling water has been studied. The study is based on the obtained corrosion and scale data for the period from May 1998 to December 2007, carried out at Fast Breeder Test Reactor, Kalpakkam, India. All the indices are calculated using pH, calcium, total dissolved solids, temperature and alkalinity of cooling water. Practical scale index and Ryznar stability index are found to be more informative than Langelier Saturation Index in keeping track of the corrosive/scaling tendency of the cooling water.