Corrosion Fatigue of a Pump Bearing Journal after Exposure to Two-Phase Flow

Abstract

A failure of a pressure relief valve in one of the CANDU (acronym for CANadian Deuterium Uranium Reactors, a registered trademark of AECL) reactors during routine testing resulted in the Heat Transport (HT) pumps operating under a Loss Of Coolant Accident (LOCA) condition for a short time. It took 50 min of operation before the reactor could be safely shutdown. During a part of this 50 min period, HT pumps were exposed to two-phase flow conditions. When the reactor was restarted following this incident, one of the HT pumps was operating with high shaft and frame vibrations that reached the shut down limit in a very short time. The reactor was shut down and a detailed inspection of the pump showed that the pump bearing journal on one of the 4 HT pumps had failed and had a large crack extending across its entire length. This paper summarizes the results of extensive investigations carried out to establish the root cause of the journal failure. The failure analysis included a finite-element analysis of the shrink fitted journal, metallurgical analysis, scanning electron fractography of the failed journal, and a design review of the assembly fits. Based on these investigations, the journal failure was determined to be due to corrosion fatigue. The corrective actions implemented to prevent the susceptibility of this component to future failures were detailed and required significant effort and expertise to develop and implement.