Radiation Heating Effects on B4C-SS Eutectic Melting and Its Relocation Behaviour

The advancement of sodium-cooled fast reactors (generation IV) faces a critical challenge related to core disruptive accidents. A primary issue is the boron carbide (B4C) and stainless steel (SS) eutectic reaction, leading to premature melting and molten pool formation with boron, affecting neutron balance. This study pioneers the use of radiative heating for high-resolution visualisation and quantification of the eutectic melt composition, providing a novel approach to observing the melting and candling phenomena. Experimental investigations were conducted with B4C pellets and powder in SS hollow claddings, replicating control rods, from 1150 to 1372 °C. The eutectic temperature for B4C pellets was found to be higher than that for B4C powder and invariant with the heating rate. Key findings include two distinct failure processes: SS cladding detachment forming molten droplets and fragmentation of B4C pellets, with sintering observed in B4C powder. X-ray diffraction analysis identified boride phases B0.9Cr0.9Fe1.1, (Cr,Fe)2B, γ-Fe, (Cr,Fe)23(B,C)6, and (Cr,Fe)5B3, solidified at 48–50 °C in argon. This novel study elucidates the eutectic behaviour and boride phase formation in severe reactor conditions, providing critical insights for reactor safety.