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SCC of a series of AFA stainless steels with different Al and Nb contents were studied in supercritical CO2 by SSRT. The results show that Nb element plays a precipitation strengthening on the mechanical properties, while it shows few effects on the corrosion properties. The surface oxide film of the Al-free material only consisted of amorphous Cr2O3 and Cr-rich spinel. With the addition of Al, the Al2O3 layers are formed and significantly decreases the element diffusion, thus inhibiting the initiation of SCC. Fe3O4 fills the interior of cracks of both Al-free and Al-containing materials. The Al2O3 layer is formed at the crack tip of Al-containing materials. Because the matrix grains are large, the protective Al2O3 layer can only be formed at the crack tip, which cannot completely hinder the outward diffusion of ions on the crack walls and its protective effect on the crack propagation is limited.

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In this paper, the performance degradation behavior of fuel cladding material 20Cr25NiNb for the British Advanced Gas Reactor (AGR) is reviewed in detail, which is a strong guideline for the material selection of supercritical carbon dioxide cooled reactors. The degradation behavior during in-core service mainly includes high-temperature creep, thermal aging and mechanical property degradation caused by neutron irradiation (fission gas products, helium embrittlement and irradiation sensitization) and CO2 (oxidation and carburizing). Long-term service in AGR leads to coarsening of the second phase and precipitation of harmful phases such as σ, leading to performance degradation of the cladding. A point that should require special attention is that intergranular stress corrosion cracking (IGSCC) and intergranular attack (IGA) problems occur during wet storage of spent fuel.