Corrosion and Stress Corrosion Cracking of UNS S31008 and UNS N08810 Alloys in Supercritical Water

Abstract The supercritical water-cooled reactor (SCWR) design requires that fuel core outlet temperature will be up to 650 °C at 25 MPa pressure to achieve optimum thermal efficiency. Selection of suitable fuel cladding materials is a critical issue for SCWR concept design. A number of studies have been conducted over the past years and several top-ranking candidate alloys, including austenitic steels (UNS S31008 and UNS N08810), have been pre-selected for further considerations. To determine which steel can be operated safely under SCWR design conditions, however, more efforts are still needed. Corrosion performance of UNS S31008 and UNS N08810 was tested in an autoclave containing 625°C and 25MPa supercritical water. Post-exposure weight change and microscopy characterizations indicated the steels exhibited good general corrosion resistance in the supercritical water, but a noticeable increase in corrosion was observed after 4th thermal cycle exposure. Stress corrosion cracking (SCC) susceptibility of the steels was studied using slow strain rate test (SSRTs) method in a supercritical water loop with well-controlled water chemistry. The results indicate that there is a threshold strain level, below which stress corrosion cracking could not be developed on the steels.