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

Sumita, Takehiro   ; Kitagaki, Toru  ; Takano, Masahide ; Ikeda, Atsushi  

Fundamental understanding of the high-temperature interaction between stainless steel (SUS) and BC 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-BC 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 BC content relevant to the fuel debris composition expected at 1F, in order to elucidate the solidification and re-melting mechanisms. The results indicated that -Fe and (Cr,Fe)B are the major solidified phases when the BC content is below 3 mass%, while (Cr,Fe)(C,B) is formed as an additional third phase when the BC content exceeds 3 mass%. The solidification of molten SUS-BC mixture and re-melting of solidified SUS-BC 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.