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The inclusions formed in 304L stainless steel for nuclear power produced by the electric arc furnace (EAF)-argon oxygen decarburization furnace (AOD)-ladle furnace (LF)-continuous casting (CC) process were investigated by thermodynamics calculations and experimental results. The results showed that the inclusions after AOD and LF refining were almost the same as the slag composition. The types of inclusions (sizes larger than 5 µm) were mainly CaSiO₃ with high SiO₂ content at the end of AOD, and Ca₂SiO₄ with high CaO content at the end of LF. The Al₂O₃ and MgO content of inclusions increased from AOD to LF. There were two types of inclusions in the tundish: CaO-SiO₂-Al₂O₃-MgO and CaO-SiO₂-Al₂O₃-MgO-MnO inclusions with MgO·Al₂O₃ spinel precipitation. The content of Al₂O₃ in the inclusions increased rapidly with the decrease in temperature from the end of LF refining to continuous casting, as calculated using FactSage6.3 software. The rapid increase of Al₂O₃ in the CaO-SiO₂-Al₂O₃-MgO-(MnO) inclusions promoted the precipitation of MgO·Al₂O₃ spinel in continuous casting tundish, suggesting mechanisms for the formation of inclusions in the 304L stainless steel.