Eutectic melting and relocation behavior of B4C pellet-stainless steel under radiative heating

Evaluating severe accidents in Generation IV sodium-cooled fast reactors (SFRs) is challenging due to the eutectic reaction between boron carbide (B4C) and stainless steel (SS), which results in boron migration and enhanced neutron absorption. A novel quantitative visualization method using radiative heating to observe eutectic behavior and resulting melt structure during boron migration. Experiments, replicating real-scale B4C-SS control rods, were conducted in an integrated test facility under core disruptive accidents (CDAs) conditions, introducing a new approach. This study explores the long duration melting (candling phenomenon) of boron within stainless steel, shedding light on relocation behavior not previously studied. We identified two distinct failure mechanisms: the separation of SS from the B4C pellet, resulting in the formation of a later melting drop, and the fracture of the B4C pellet into multiple pieces, possibly due to thermal stress. The visualization technique and thermal interfacial resistance analysis precisely captured the eutectic temperature, aligning well with prior research. Our study yield insights into eutectic melt relocation behavior by systematically characterizing relocated solidified eutectic melt with a cooling rate of 50 °C/s in stagnant argon environment via scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), and microhardness testing. Our study shows that due to high chromium content, (Cr,Fe)2B phases are observed in XRD analysis instead of just Fe2B phase. Other phases present in our solidified eutectic melt include γ-Fe (fcc), (Cr,Fe)23(B,C)6 (cubic), and (Cr,Fe)5B3. The atomic composition within these zones was ascertained through SEM-EDS analysis. Our study shows varying hardness values in distinct phases, reflecting differing boron concentrations within micrograph zones. Chromium promotes (Cr,Fe)2B and (Cr,Fe)5B3 boride formation, transforming the structure from tetragonal to orthorhombic with increased chromium content, thereby increasing hardness. This validates B4C-SS eutectic mixture relocation and diverse boride phase formation under extreme reactor conditions.