Environmentally Assisted Cracking Behavior of Low Alloy Steels in Simulated BWR Coolant Conditions

Abstract

SA533, low carbon and low alloy steel, is most commonly used for nuclear reactor pressure vessels (RPV) whose integrity governs the safety of nuclear power plants. Fatigue is one of the main degradation mechanisms affecting the pressure vessel integrity of pressurized water reactors (PWRs) and boiling water reactors (BWRs) (Shah, 1993;Huang, 2001, 1999, 2007;Seifert, 2001, 2003). Fatigue life can generally be divided into two distinct phases-initiation and propagation. The factors affecting the initiation and propagation lives may not be the same. Recent test data indicate the initiation life is significantly decreased when the applied strain range, temperature, DO level in water, sulfur content in steel and strain rate are simultaneously satisfied(Chopra, 1997). The fatigue crack growth rates of RPV materials in simulated light water reactor (LWR) coolant environments are influenced by sulfur content, sulfide morphology (Huang, 2003, 2004ŻVan Der Sluys, 1985ŻCombrade, 1987) and orientation, water chemistry, loading frequency separately and synergistically (Van Der Sluys, 1985, 1987ŻAmzallag, 1983ŻO’Donnell, 1995Ż Eason, 1993ŻRoth, 2003ŻChopra, 1998ŻAtkinson, 1986ŻShoji, 1986). The sulfur content has been reported to enhance the corrosion fatigue crack growth rates of low alloy steels (Tice, 1986). Hydrogen water chemistry (HWC) has proved to be a powerful method for mitigating environmentally-assisted cracking (EAC) of stainless steel and nickel-base alloy components. Ritter (Ritter & Seifert, 2007) demonstrated HWC resulted in a significant drop in low-frequency corrosion fatigue crack growth rates by at least one order of magnitude with respect to normal water chemistry (NWC) conditions for pressure vessel steels with sulfur content lower than 0.02 wt %. It is of practical and academic interest to study the effect of HWC on the mitigation of corrosion fatigue initiation and propagation for low alloy steels. The slip-oxidation mechanism has been widely accepted to account for the crack propagation of the carbon and low alloy steels in LWR water systems (Ford, 1987). This mechanism relates crack advance to the oxidation rate that occurs at the crack tip. In order to better predict the crack growth rates of low alloy steels, the interaction between the oxide films and sulfur ion dissolved from steels in the oxygenated or HWC water environments should be clarified.