Solidification and re-melting mechanisms of SUS-B4C eutectic mixture

•Solidified products of molten SUS-B4C mixture were systematically characterized.•γ-Fe and (Cr,Fe)2B are the solidified phases when the B4C content is below 3 mass%.•(Cr,Fe)23(C,B)6 is additionally solidified when the B4C content exceeds 3 mass%.•Solidification and re-melting of molten/solidified SUS-B4C mixture are eutectic. Fundamental understanding of the high-temperature interaction between stainless steel (SUS) and B4C is indispensable for estimating and characterizing the fuel debris generated during severe accidents of boiling water reactors (BWR), such as Fukushima Dai-ichi Nuclear Power Station (FDNPS, also referred to as “1F”) in Japan. This study aims at systematically characterizing the solidified products of molten SUS-B4C mixtures by powder X-ray diffraction (PXRD), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), and thermogravimetry-differential thermal analysis (TG-DTA) with a range of the B4C content relevant to the fuel debris composition expected at 1F, in order to elucidate the solidification and re-melting mechanisms. The results indicated that γ-Fe and (Cr,Fe)2B are the major solidified phases when the B4C content is below 3 mass%, while (Cr,Fe)23(C,B)6 is formed as an additional third phase when the B4C content exceeds 3 mass%. The solidification of molten SUS-B4C mixture and re-melting of solidified SUS-B4C melt are eutectic, which is mainly controlled by the pseudo-binary Fe–B system that is influenced by the C and Cr content and additional minor components such as Mo.