Corrosion of EP-450 ferritic-martensitic steel after neutron irradiation and long-term storage in spent fuel pool

Advanced ferritic-martensitic (F/M) steels are often used in sodium-cooled fast nuclear reactors (SFRs), given their superior resistance to swelling, high-temperature embrittlement, and radiation creep. However, a potential vulnerability of F/M steels lies in their low corrosion resistance to prolonged storage in water-filled spent fuel pools. This poses a significant challenge, especially during fuel storage and reactor decommissioning, increasing the risk of radioactive contamination, safety hazards, and substantial financial costs. Here we show and explain significant corrosion susceptibility of EP-450 ferritic-martensitic steel (1Cr13Mo2NbBV) after irradiation in the BN-350 SFR up to 70 dpa and subsequent long-term (20 years at <50 °C) wet storage in the reactor's spent fuel pool. Corrosion morphologies ranged from oxide films to various forms of localized degradation, including very deep pitting, phase-selective attack, intragranular attack, and interphase corrosion. Deep pits mechanistically form via the preferential dissolution of phase boundaries and the tempered martensitic phase, leading to the eventual detachment of unaffected ferrite grains. The presence and preferential dissolution of the tempered martensitic phase, which is absent in ferritic or austenitic steels, is the dominant factor driving this severe localized corrosion damage in EP-450 steel.