Computational discovery of novel U-N-C ternary phases as promising accident-tolerant fuels

Recently, uranium carbonitride solid solution has been recognized as a promising nuclear reactor fuel due to its excellent physicochemical properties. However, due to the complexity of U-N-C ternary system, the research on the structure and properties of uranium carbonitride is still insufficient. In this work, a series of promising accident-tolerant ternary compounds in the U-N-C system were predicted through a combination of first-principles calculations with an evolutionary algorithm. Fifteen dynamically and mechanically stable novel U-N-C ternary compounds with seven chemical compositions, including U-N-C solid solutions and new intermetallic structures, were suggested to be promising for experimental synthesis. For U-N-C solid solutions with rock-salt structures, it was revealed that the lattice expansion and bond length elongation caused by the substitution of carbon atoms for nitrogen atoms can lead to a decrease in elastic moduli and melting point and an increase in toughness. Moreover, two newly identified structures P6 ₃/ mcm-U ₃N ₂C and P2/ m-U ₄ NC were considered to be very promising accident-tolerant fuels due to their high mechanical strength, excellent melting point, low thermal expansion coefficient, and high thermal conductivity. The larger atomic number density of P6 ₃/ mcm-U ₃N ₂C enables a higher elastic modulus and a satisfactory melting point than those of rock-salt structures. While for P2/ m-U ₄ NC, its unique shorter and stronger U-N/C and U-U bonding leads to its highest mechanical strength and appropriate melting point. Furthermore, the linear thermal expansion coefficients of P2/ m-U ₄NC and P6 ₃/ mcm-U ₃N ₂ C are comparable to those of UN and UO ₂ , respectively, and their thermal conductivities increase with temperature, much higher than that of UO ₂.