The calculation of phase equilibria of oxide core-concrete systems

Thermodynamic models have been developed to describe the phase equilibria of the oxide system, UO2-ZrO2-SiO2-CaO-MgO-Al2O3, formed during core-concrete interactions. The model has been used to estimate the solidus-liquidus surfaces for oxide compositions of the full system that describe different extents of oxidation of the zirconium in the core debris and various concrete types. The results show that the full oxide system should be treated as a pseudo-eutectic, in contrast to that used in the CORCON-MOD2 code, where the system is treated as a pseudo-ideal liquid and solid solution. A comparison of the calculated values of the solidus and liquidus temperatures with the available experimental data has also been performed for this system. In addition, the model has been used to follow the evolution of the different phases in the system with temperature for the ACE Phase C Test L6. In particular the relative amounts and compositions of the liquid and solid phases have been calculated. Further calculations have been performed to compare the extent of release of the components of the system by vaporization using the model and an ideal solution model. (orig./HP)