Failure analysis of branch connection on the reactor primary pipeline

A shut down event occurred in one pressurized heavy water reactor unit due to primary loop heavy water leakage. Failure analysis was hence performed in order to develop the following pertinent actions. The sampling plan was developed via visual inspection and analysis of the defective connection. Physical and chemical tests were conducted by means of stereo microscope, metallurgical microscope, scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The results revealed that the main crack initiated at the weld root inside the connection and eventually progressed throughout the pipe wall. The fracture and metallographical morphology showed the typical character of stress corrosion cracking. It is concluded that the failure was caused by transgranular stress corrosion cracking (SCC) on the stainless steel side of the dissimilar metal weld. The root causation could be bad weld forming due to inappropriate welding process.