Cyclic Crack Growth Behavior of Reactor Pressure Vessel Steels in Light Water Reactor Environments

Abstract

During normal operation light water reactor (LWR) pressure vessels are subjected to a variety of transients resulting in time varying stresses. Consequently, fatigue and environmentally assisted fatigue are growth mechanisms relevant to flaws in these pressure vessels. In order to provide a better understanding of the resistance of nuclear pressure vessel steels to flaw growth process, a series of fracture mechanics experiments were conducted to generate data on the rate of cyclic crack growth in SA508-2 and SA533B-1 steels in simulated 550°F Boiling Water Reactor (BWR) and 550°F Pressurized Water Reactor (PWR) environments. Areas investigated over the course of the test program included the effects of loading frequency and R ratio (Kmin/Kmax) on crack growth rate as a function of the stress intensity factor (ΔK) range. In addition, the effect of sulfur content of the test material on the cyclic crack growth rate was studied. Cyclic crack growth rates were found to be controlled by ΔK, R ratio, and loading frequency. The sulfur impurity content of the reactor pressure vessel steels studied had a significant effect on the cyclic crack growth rates. The Higher growth rates were always associated with materials of higher sulfur content. For a given level of sulfur, growth rates were higher in a 550°F simulated BWR environment than in a 550°F simulated PWR environment. In both environments cyclic crack growth rates were a strong function of the loading frequency. Further, the loading frequency at which the highest cyclic crack growth rate was observed was found to be a function of the applied ΔK level. In most cases, all cyclic crack growth rates were on or under the ASME Section XI high R water reference flaw growth line and above the Section XI air reference flaw growth line, supporting the position of these lines on the growth rate–ΔK level graph.