Plate type heat exchanger (PHE) made of type 316 is used in the pool cooling water system of spent fuel storage facility present in Tarapur. Two plates of PHE were resistance welded to form a cassette, which are then vertically stacked one after the other to form PHE. De-mineralized (DM) water which is present in between the plates is cooled using raw water, which flows over the outer surface of these plates. The PHE operates in the temperature range of 32–40 °C. During inspection, a crack was observed in the weld region resulting in intermixing of DM and raw water. The results of failure analysis of this PHE which failed after 15 years of service are presented in this paper. Different characterization techniques like stereomicroscopy, optical microscopy and SEM were used to understand the crack initiation and propagation. Failure of this PHE was attributed to transgranular stress corrosion cracking (TGSCC) which initiated in both the weld and heat affected zone (HAZ) region from the raw water side. These cracks propagated through the thickness resulting in failure of the PHE. Suitable recommendations like de-scaling of cassette plates at frequent intervals to avoid deposit formation were given to mitigate such failure in future.