Abstract <p>This paper presents a comparative analysis of results of computational studies of thermodynamics of austenitic ChS68-ID and ferritic-martensitic EP823-Sh steels during their interaction with carbon and oxygen. Particular attention is paid to the behavior of unbound chromium depending on the content of oxygen and carbon impurities, since it is unbound chromium that is apparently the main protector of intergranular corrosion. According to the results obtained, EP823-Sh ferritic-martensitic steel appears to be significantly less sensitive to the introduction of carbon impurities into fuel compared to ChS68-ID austenitic steel. A possible reason for the relative stabilization of the unbound Cr concentration in the EP823-Sh steel cladding is the redistribution of carbon between chromium and silicon. It was found that the influence of oxygen on the unbound chromium content in the “EP823-Sh steel cladding + fuel” and “EP823-Sh steel cladding + oxygen” systems differs significantly. According to the calculations, owing to the oxidation of chromium silicide in the “EP823-Sh steel cladding + oxygen” system, a lot of unbound Cr is formed, and oxygen combines with silicon. In the “EP823-Sh steel cladding + fuel” system, the oxygen combines with fuel components and Cr is not released in the cladding.</p>