Long-term corrosion behavior of borated stainless steel in a simulated spent fuel pool environment

Borated stainless steel (BSS) is widely utilized as a neutron absorber material for criticality control in spent nuclear fuel pools, which use borated water to cool spent nuclear fuel to room temperature. By incorporating boron into SS304, BSS exhibits a higher neutron absorption cross section than other austenitic stainless steels. Boron in BSS has a low solubility in the austenite structure, leading to the formation of a secondary phase, (Fe, Cr) 2 B, upon alloying. Given that BSS is intended for long-term use in spent nuclear fuel pools, it is important to evaluate its long-term integrity. This paper investigates the long-term corrosion behavior of BSS along with its oxide microstructure through an accelerated corrosion experiment simulating spent nuclear fuel pool conditions. The 2-year experiment was conducted at elevated temperatures based on the Arrhenius equation with temperature as a variable. Detailed microstructural analysis employed electron microscopy, energy-dispersive X-ray spectroscopy, electron probe microanalysis, and image analysis. According to the results, upon oxidation, hematite oxide film was formed and shallow, non-propagating incipient localized attack was obserbed on the substrate; the features were typically ≈1–3 μm deep and accounted for <0.1 % of the cross-sectional thickness. Incipient localized attack from the relatively low Cr content in BSS compared to conventional stainless steel. Dissolution of Cr and B was observed from the secondary phase (Fe, Cr) 2 B, indicating that B dissolution is caused by oxidation. • Long-term corrosion of BSS evaluated under simulated spent fuel pool conditions. • Selective oxidation of boride phases observed, forming porous corrosion structures. • Oxide film on secondary phase thicker than on substrate due to enhanced diffusion. • Oxidation sequence followed B .→ Cr → Fe based on Gibbs free energy • Provides insight into BSS degradation mechanism for nuclear storage system safety.