Thermodynamics of Corium

M. Barrachin
EUROCOURSE 2003 CORIUM : Severe Accident R&D and Nuclear Power Plant Safety Janvier 27-31, 2003 Aix en Provence.

The phase diagrams are of vital importance to determine the behaviour of the mixtures (or corium) resulting of interactions between materials during a severe accident. They could be experimentally determined in specific tests (liquidus and solidus measurements, composition and proportion of the phases…). But the temperature and composition domains relevant for the corium thermodynamics are so large that an exclusive experimental approach could not answer the challenge. So the phase diagrams have to be constructed by calculations performed on the basis of the modelling of the Gibbs energies of the phases able to appear in the corium. One of these approaches, especially well adapted to multicomponent system (as the corium) thermodynamics, is the named CALPHAD (CALculation PHAse Diagram). This approach is presented and the physical description of the different thermodynamical models detailed.

Most of the existing corium databases are built following this scheme. We list them with their content and the status of their validation. The validation mainly depends on the available experimental data since in the CALPHAD approach, the parameters in the phase models are obtained by a self consistent optimisation of the experimental data for both phase boundaries and single phase thermodynamic properties. For example, among all the phase diagrams involved in the corium thermodynamics, many binary and ternary systems are experimentally known, in particular the metallic systems. By contrast, the UO₂ including systems are often unknown. The main relevant uncertainties which may impact an accident scenario are mentioned.

In the last ten years, more fundamental approaches to alloy phase diagram have emerged. In these ab initio calculations of phase diagrams, some or all of the total energy of the alloy, calculated as a function of atomic configuration and volume, is mapped onto the Ising model and sophistical models are used for the evaluation of the configurational partition function. We schematically present these approaches, their advantages and their limitations.