Thermodynamic modelling of the C–U and B–U binary systems

Abstract The thermodynamic modelling of the carbon–uranium (C–U) and boron–uranium (B–U) binary systems is being performed in the framework of the development of a thermodynamic database for nuclear materials, for increasing the basic knowledge of key phenomena which may occur in the event of a severe accident in a nuclear power plant. Applications are foreseen in the nuclear safety field to the physico-chemical interaction modelling, on the one hand the in-vessel core degradation producing the corium (fuel, zircaloy, steel, control rods) and on the other hand the ex-vessel molten corium–concrete interaction (MCCI). The key O–U–Zr ternary system, previously modelled, allows us to describe the first interaction of the fuel with zircaloy cladding. Then, the three binary systems Fe–U, Cr–U and Ni–U were modelled as a preliminary work for modelling the O–U–Zr–Fe–Cr–Ni multicomponent system, allowing us to introduce the steel components in the corium. In the existing database (TDBCR, thermodynamic data base for corium), Ag and In were introduced for modelling AIC (silver–indium–cadmium) control rods which are used in French pressurized water reactors (PWR). Elsewhere, B 4 C is also used for control rods. That is why it was agreed to extend in the next years the database with two new components, B and C. Such a work needs the thermodynamic modelling of all the binary and pseudo-binary sub-systems resulting from the combination of B, B 2 O 3 and C with the major components of TDBCR, O–U–Zr–Fe–Cr–Ni–Ag–In–Ba–La–Ru–Sr–Al–Ca–Mg–Si + Ar–H. The critical assessment of the very numerous experimental information available for the C–U and B–U binary systems was performed by using a classical optimization procedure and the Scientific Group Thermodata Europe (SGTE). New optimized Gibbs energy parameters are given, and comparisons between calculated and experimental equilibrium phase diagrams or thermodynamic properties are presented. The self-consistency obtained is quite satisfactory.