Environmental Reduction Effect of High Temperature Water and Hydrogen on Fracture Resistance of Low Alloy Reactor Pressure Vessel Steels

The structural integrity of the reactor pressure vessel (RPV) of light water reactors (LWR) is of utmost importance regarding operation safety and lifetime. The fracture behaviour of low-alloy RPV steels with different dynamic strain aging (DSA) & environmentally-assisted cracking (EAC) susceptibilities in simulated LWR environments was evaluated by elastic plastic fracture mechanics (EPFM) tests and various post-test characterizations. Exposure to high temperature water (HTW) environments at LWR operating temperatures only moderately reduces the fracture initiation and tearing resistance of low alloy RPV steels with high DSA or EAC susceptibility, accompanied with a clear change in fracture morphology, which indicates the potential synergies of hydrogen/HTW embrittlement with DSA and EAC under suitable conditions. The most pronounced degradation effects occurred in a) low sulphur RPV steels with high DSA susceptibility, where the fracture initiation and tearing resistance reduction increased with decreasing loading rate and increasing temperature and was most pronounced in hydrogenated HTW, and in b) high sulphur steels with high EAC susceptibility in aggressive occluded crevice environment (oxygenated HTW) with preceding EAC crack growth.