High-Temperature Water Effects on the Fracture Behaviour 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. High-temperature water (HTW) and hydrogen absorbed from environment, in synergy with irradiation embrittlement, dynamic strain aging (DSA), environmentally-assisted cracking (EAC) or temper embrittlement (TE) may reduce the fracture resistance of RPV steels. The fracture behaviour in the upper shelf region of low-alloy RPV steels with different microstructures and DSA, EAC and TE susceptibilities in various simulated LWR environments was evaluated by elastic-plastic fracture mechanics tests.

In the reference RPV steels with low sulphur and phosphorus contents and low DSA, EAC and TE susceptibilities, environmental effects on fracture resistance are absent or marginal in both oxygenated and hydrogenated HTW. However, moderate but clear reduction of fracture initiation resistance occurred in: a) Simulated coarse grain heat-affected zone material with high yield stress, since a higher yield stress facilitates the hydrogen enrichment in the fracture process zone; b) Low-sulphur RPV steels with high DSA susceptibility, where the reduction of fracture initiation resistance increased with decreasing loading rate at 288 °C and was most pronounced in hydrogenated HTW due to the localization of plastic deformation by the interaction between DSA and hydrogen; c) High-sulphur RPV steels with high EAC susceptibility in aggressive occluded crevice environment (oxygenated HTW with addition of impurities) with preceding EAC crack growth, resulting from the enhancement of hydrogen availability and uptake; and d) High-phosphorus RPV steel with high TE susceptibility, where the reduction of fracture initiation resistance was most pronounced in hydrogenated HTW, indicating that TE effect dominates over effects of occluded crevice chemistry.

In hydrogenated HTW, the reduction of fracture resistance correlated fairly well with the DSA susceptibility of different steels and sulphur content played no or a minor role. In oxygenated HTW, the reduction of fracture resistance increased with steel sulphur content for steels with low DSA susceptibility. Stable ductile transgranular tearing by micro-void coalescence dominates in both air and HTW environments. The fracture surface of specimens tested in air was mainly ductile. In contrast, specimens tested in HTW environments, varying amounts (a few %) of secondary cracking, macro-voids, quasi-cleavage and intergranular cracking were observed on fracture surface. The observed fracture modes and morphology suggest a combination of hydrogen-enhanced local plasticity and hydrogen-enhanced strain-induced vacancies mechanisms with minor contributions of hydrogen-enhanced decohesion embrittlement mechanism.

The main reason for the moderate degradation effects is the low hydrogen availability in HTW together with a high density of (fine-dispersed and strong) hydrogen traps in RPV steels. In addition, the environmental reduction and softening by hydrogen and HTW environments was partially compensated by the toughness increase due to DSA. Although the environmental effects are moderate, effects can be more critical for RPV materials with low initial upper shelf toughness under unfavourable combinations (high sulphur content, increased strength, high EAC, TE and DSA susceptibilities) or old plants with small margins regarding irradiation embrittlement.