Stress corrosion cracking of a low alloy steel in high purity steam

Abstract

The catastrophic failure of a steam turbine at Hinkley Point “A” Nuclear Power Station resulted from stress corrosion cracking of the discs on the low pressure (L.P.) turbine rotor. This paper describes investigations carried out within the South Western Region of the C.E.G.B. to determine the mechanism and the environment responsible for cracking. The results show that stress corrosion cracks can be initiated and propagated during normal turbine operation and furthermore they suggest that cracking is due to the action of wet, high purity steam on stressed metal. The chemical quality of the steam was found to be very high, the impurities were well within the normal range associated with power plant operation. The results are in agreement with the recent findings of more widespread cracking at Hinkley Point and elsewhere. It is considered that as the steam is inherently aggressive then the measures currently being taken by C.E.G.B., i.e. the removal of the highly stressed keyways, limiting steam access to regions of high stress and the use of improved alloys, should prevent further failures of this type.