THE EFFECT OF TRANSIENTS ON THE CRACK GROWTH BEHAVIOR OF LOW ALLOY STEELS FOR PRESSURE BOUNDARY COMPONENTS UNDER LIGHT WATER REACTOR OPERATING CONDITIONS

摘要

A European research project (CASTOC) was launched to investigate some of the remaining open issues regarding the environmentally assisted cracking of low alloy steels in high-temperature water. The materials of concern comprised low alloy steels for pressure boundaries of both Western type boiling water reactors (BWR) and Russian type pressurized water reactors (WER). The environments were based on BWR normal water chemistry and WER primary water chemistry. Besides studies under steady-state conditions (purely static or cyclic load and constant chemistry), special emphasis was put on mechanical transients (e.g. periodic partial unloading) and chemical transients (e.g. sulfate, chloride). At relevant stress intensity factors, it could be shown that immediate cessation of growing cracks occurs after changing from cyclic to static load in high purity oxygenated BWR water and oxygen-free VVER water, corresponding to conditions for steady-state reactor operation. Susceptibility to environmentally assisted cracking under static load was observed for a weld heat affected zone in base material in oxygenated high purity water, and also in normal base materials during a chloride transient representing BWR water conditions between Action Levels 1 and 2 of the EPRI Water Chemistry Guidelines Constant load crack growth was also observed in one Russian type base material in oxygenated WER water and in one Western type base material in oxygenated high purity BWR water at stress intensity factors above the validity limit for linear elastic fracture mechanics. The results provide an important contribution to the mechanistic understanding of crack growth behavior in the corrosion system studied. This is key to the evaluation of transient events which may occur in a plant during service.