Characterization and visualization of eutectic melt in B4C powder and pellet-based/304 stainless steel composite control rods under radiative heating

The advancement of Sodium-cooled Fast Reactors (Generation IV) faces a key challenge in managing Core Disruptive Accidents (CDAs) caused by eutectic reactions between boron carbide (B4C) and stainless steel (SS). This reaction causes earlier melting, potentially forming a molten pool containing boron, disrupting neutron balance and causing instability. Therefore, determining boron concentration and distribution in the solidified eutectic pool is critical. This study compares the behavior of B4C in powder and pellet forms within SS hollow claddings, mimicking control rod designs, to confirm the presence of boron in the eutectic composition and characterize its phases upon solidification. Radiative heating, high-resolution visualization, and quantitative techniques were employed over a temperature of 1372 °C. Findings reveal that the eutectic temperature for pellets exceeds that for powder, with pellets showing fragmentation and powder exhibiting sintering during the failure process. SS cladding formed a melt in both cases; however, in the pellet case, it peeled off, while in the powder case, it ruptured. Visualization methods precisely identified the onset of eutectic melting and associated temperatures, differing between pellets and powder. Characterization techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), and hardness testing, confirmed boron ingress and revealed boride phases such as B0.9Cr0.9Fe1.1, (Cr,Fe)2B, γ-Fe, (Cr,Fe)23(B,C)6, and (Cr,Fe)5B3 in the eutectic melt. SEM and XPS analyses reveal that both boron and carbon diffuse and precipitate within the eutectic melt, providing new insights into its behavior and relocation dynamics.