Cracking and fracture of valve shafts in the secondary steam circuit of a nuclear power plant. A case investigation.

The failure of a secondary steam circuit valve was investigated. Several fatigue cracks were initiated on valve shafts in the angle of the groove supporting the puppet. Cracks propagated obliquely toward the shaft end. The final shaft fracture occurred by shearing in valve position closed, when the shaft bearing section was diminished sufficiently. The surface of the split ring transmitting the puppet weight to the shaft was worn irregularly. Fatigue cracks initiated and propagated in the split ring groove angle due to resonance stresses at the start of the operation, when the temperature of the valve was low and the shaft steel notch toughness was also low.