Failure analysis on premature fracture of boric acid recycle pump shaft in 1000 MW nuclear power plant

Boric acid recycling pump is used to maintain the circulation of boric acid solution for the need of nuclear safety protection. In a boric acid recycling pump, the pump shaft is an important com- ponent. As it is exposed to relatively harsh work environment, the pump shaft may encounter severe failures. In this paper, premature fracture of the pump shaft, happened after the pump serving only one year in a 1000 MW nuclear power plant, was carefully investigated. In order to find out the root cause of the failure, material identification, metallographic inspection, me- chanical property test, wear and fractographic observation, micro-zone analysis and mechanism analysis were carried out. The obtained results indicated that triple stress concentration, caused by the small transition arc radius, surface defects and the aggregation of inclusions, was the primary reason. It resulted in the nucleation and coalescence of micro-cracks and consequently lead to the premature fracture. Countermeasures were then put forward to solve corresponding problems.