IASCC study of proton-irradiated type 316 stainless steels in PWR water

A management of irradiation-assisted stress corrosion cracking (IASCC) of internal components of a nuclear power plants is considered critical for long-term operation of these reactors. Some cracking of the internals in European plants was identified initially in 1985. The cracking mechanism is not fully understood. Among the unrevealed mechanisms, the effect of the dissolved hydrogen (DH) concentration on IASCC propagation has not been studied much. The studies are more limited on the effect of DH concentration on the IASCC initiation process. Stephenson observed a high crack initiation density in a primary water (PW) environment as compared to normal water chemistry (NWC), indicating that the low electrochemical potential in hydrogenated water chemistry (HWC) or PWR primary water suppresses crack growth, possibly facilitating crack initiation. Because such studies are rare, it remains uncertain if a high DH level suppresses crack growth but enhances crack initiation. The objective of the present work is to verify the effect of the dissolved hydrogen concentration on the IASCC initiation behavior of Type 316 stainless steels in PWR water after proton irradiation. The susceptibility of reactor vessel internalsto IASCC initation was studied under simulated PWR conditions. Type 316 stainless steel specimens were irradiated to levels of 1, 3, 5 and 10 dpa with 2 MeV protons at the Michigan lon Beam Laboratory. The grain boundary chemical composition was characterized using TEM and EDS, and the susceptibility to crack initiation was evaluated using SSRT under simulated PWR conditions at various dissolved hydrogen concentrations. Preliminary results for evaluating the DH effect on IASCC initiation under the PWR condition are described. Dose profiles show a plateau up to 25 m deep on proton-irradiated specimens. A micro-void was noted in a 1 dpa sample, and no voids or pores were observed in the 3, 5 and 10 dpasamples. Summation of the total crack length on the side surface appears to be a better procedure for evaluating IASCC initiation susceptibility on irradiated specimens. (authors)