Experimental contribution to the corium thermodynamic modelling – The U–Zr–Al–Ca–Si–O system

•Two mixture within the UO2–ZrO2–Al2O3–CaO–SiO2 system were studied.•The final microstructure of the samples is affected by the SiO2 content.•The SiO2-rich sample is characterised by a miscibility gap in the liquid phase.•Segregation of U in one of the two immiscible liquids. During a severe accident in a nuclear reactor, extreme temperatures may be reached (T>2500K). In these conditions, the nuclear fuel may react with the Zircaloy cladding and then with the steel vessel, forming a mixture of solid–liquid phases called in-vessel corium. In the worst scenario, this mixture may penetrate the vessel and reach the concrete underneath the reactor. In order to develop the TAF-ID thermodynamic database (www.oecd-nea.org/science/taf-id) on nuclear fuels and to predict the high temperature behaviour of the corium+concrete system, new high temperature thermodynamic data are needed. The LM2T at CEA Saclay centre started an experimental campaign of phase equilibria measurements at high temperature (up to 2600K) on interesting corium sub-systems. In particular, a heat treatment at 2500K has been performed on two prototypic ex-vessel corium samples (within the U–Zr–Al–Ca–Si–O system) with different amounts of CaO and SiO2. The results show that depending on the SiO2-content, the final configuration of the samples can be significantly different. The sample with the higher CaO-content showed a dendritic structure representative of a single quenched liquid phase, whilst the sample richer in SiO2 exhibited a microstructure which suggests the presence of a liquid miscibility gap. Furthermore a new laser heating setup has been conceived. This technique allows very high temperature measures (T>3000K) limiting the interactions between the sample and the surroundings.