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 B_0.9Cr_0.9Fe_1.1, (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.