Evaluation of long-term exposure of 310S and 800H under conditions of SCW-SMR

Austenitic stainless steels 310S and Alloy 800H are considered promising candidates for fuel cladding in Small Modular Reactors cooled by Supercritical Water due to their high corrosion resistance and favourable mechanical properties. To evaluate their long-term behaviour in supercritical water environments, full-length tube samples were exposed to supercritical water at 500 °C, 25 MPa, and 150 ppb dissolved oxygen for up to 10,000 h. Weight gain measurements revealed a decelerating oxidation rate over time, with low cumulative mass increases indicative of excellent corrosion resistance. Conservative extrapolation suggests that the wall penetration depth is projected to remain below 5 μm after 30,000 h for both alloys, supporting the expectation of long-term structural integrity. Surface roughness measurements corroborated these trends: 310S showed a gradual increase from 0.18 μm (as received) to 0.29 μm after 10,000 h, whereas 800H exhibited minimal change, attributed to its initially higher surface roughness. Post-exposure characterization by Scanning Electron Microscope with Energy-Dispersive X-ray Spectroscopy and Transmission Electron Microscope confirmed the formation of compact, chromium-rich oxide layers (Cr₂O₃) on both materials, with underlying Cr-depleted zones. Microstructural analysis revealed that 310S developed thicker oxide layer with larger grains and Cr-Ni-rich phases, whereas 800H exhibited finer oxide grains in a thinner layer, with occasional localized corrosion – nodules. These differences underscore the role of alloy composition in oxidation behaviour under supercritical water conditions. Overall, both 310S and 800H demonstrate excellent oxidation resistance and microstructural stability, reinforcing their applicability as fuel cladding materials in Small Modular Reactors cooled by Supercritical Water designs. • Cr 2 O 3 -rich oxide layers formed on 310S and 800H after 10,000 h in SCW. • Oxidation rates peaked early; wall penetration depth after 30,000 h < 5 μm. • SEM/TEM showed varied oxide grain sizes and chromium-depleted subsurface zones. • 800H oxide layer showed localized corrosion with nodule formation. • Both alloys show strong long-term oxidation resistance in SCW.