Transient interactions of boron carbide with molten uranium oxide

Abstract

• Two transient experiments performed of boron carbide quenching in uranium dioxide melts under inert atmosphere. • XRD and SEM-EDS analysis conducted for samples of the melt, aerosolized particles and the solidified ingot. • Experimental observations compared with Thermocalc/NUCLEA thermodynamic equilibrium calculations. • Ingot samples imply the coexistence of highly heterogeneous oxidation environments in the melt. • Significant boron volatilization observed during both tests and supported by thermodynamic calculations. Thermodynamic equilibrium calculations have shown that, at the elevated temperatures anticipated during severe accidents, uranium oxide-boron carbide interactions are likely to result in significant boron volatilization and the formation of uranium borides. This interaction is further investigated through transient experiments whereby boron carbide pellets are submerged in uranium dioxide melts held at elevated temperatures beneath inert atmospheres. X-ray diffraction and electron-microscopy analyses revealed the aerosols generated by the interaction to be rich in boron sesquioxide (B 2 O 3 ) and uranium oxide. The solidified melt ingots recovered post-test exhibited highly heterogeneous chemistry with some samples richer in UO 2 and others richer in U 3 O 8 . Thermodynamic equilibrium calculations indicate that this could be explained by heterogeneity in redox environment. The second, larger-scale, interaction, with an increased boron inventory, indicated uranium oxide-boron carbide reaction formed either uranium boride or uranium borocarbide.