Crack Growth Behavior of Ferritic Pressure Vessel Steels in Oxygenated High Temperature Water Under Transient Loadings

Abstract

The assessment of the influence of the LWR coolant environment and postulated chloride transients on the crack growth is of importance for ageing management with regard to safety and reliability. Aim of the investigations was to determine cyclic crack growth rates at LWR conditions and to study possible size effects and the impact of chlorides on environmentally assisted cracking.Crack growth experiments were performed with fracture mechanics specimens of different size in simulated BWR water of high purity and under the effect of chloride transients with RPV steel 22NiMoCr3-7.Subsequent to a phase of cyclic loading, the specimens were exposed to static load, interrupted by partial unloadings.All cyclic crack growth rates da/dN vs. ∆K in high purity water were in good agreement with ASME XI water curves.No significant influence of specimen size on the crack growth behavior and with regard to SCC could be detected in high purity water environment. Cyclic induced crack propagation immediately stopped when turning to static load. Under static load the chloride transients did not cause crack initiation by SCC.Load transients in chloride containing environment initiated significant SCC-induced crack growth. A "chloride memory effect" with regard to a preceding chloride transient at static load, leading to SCC-induced crack propagation during subsequent load transients in high purity water environment did not arise.

The influence of ppb levels of chloride impurities on the stress corrosion crack growth behaviour of low-alloy steels under simulated boiling water reactor conditions

“…Similar observations on the effect of chloride transients with the RPV steel D (0.007 % S, low DSA susceptibility, lowest EAC susceptibility in PSI investigations) were obtained by M. Herbst et al [26,27] and Herter et al [28] in a very limited number of tests at slightly lower ECPs. In case of dormant cracks under pure constant load, no acceleration of SCC was observed even for prolonged and severe chloride transients of 50 ppb [27,28] and up to 150 ppb [28]. A single partial un-/reloading cycle immediately induced fast SCC during chloride transients with similar CGRs as in the PSI investigations.…”

“…In case of a sharp and very slowly growing crack under pure constant load, acceleration of SCC already occurred 3 hours after the addition of 50 ppb of chloride [27]. On the other hand, 100 h after a 100 h chloride transient of 50 ppb in case of a dormant crack, three partial un-& re-loading cycles did not induce accelerated SCC crack growth [28], indicating that the original crack crevice chemistry already had re-established at this time, as expected. It is stressed that RPV steel D showed the lowest EAC susceptibility in PSI investigations in comparison to the other tested materials.…”

“…Under steady state conditions, the migration flux of chloride towards the crack-tip and back diffusion flux out of the crack due to the chloride concentration gradient built-up in the crevice are equal. This typically results in a chloride and anion enrichment by a factor 10 to 50 (typically 20 to 30) depending on the DO content [1,2,25,28]. Chloride, sulphate and other anion enrichments typically occur within some few to some few ten hours after anion addition.…”