Failure Mechanism of the High Temperature Sampling Cooler in a Nuclear Power Plant

The chemical composition and macroscopical & microscopical corrosion morphology of a leaky high temperature sampling cooler in a nuclear power plant were analyzed and observed by chemical analysis, stereoscopic microscopy and SEM/EDS. The mechanism of coiled pipe corrosion failure was put forward combining with consideration for working condition of the cooler. EDS analysis of corrosion products showed that large amounts of Fe and P as well as trace Na existed in failure positions of the coiled pipe, indicating that pitting corrosion and perforation were caused by phosphate induced alkaline corrosion. The outer wall of coiled pipe was overheated. Then phosphate and impurities reached saturation under the condition of local liquid boiling, and were precipitated onto the tube wall. On the one hand, phosphate reacted with oxide on the coiled pipe surface; on the other hand, the concentration and deposition of media made heat transfer deteriorate. Phosphate hiding and alkaline corrosion occurred, and then pitting corrosion and perforation formed.