Experimental validation of the TAF-ID: high temperature behaviour of simulated ex-vessel corium and MOX-Lanthanides (Ce, Gd, La, Nd) : JRC in-kind contribution to the TAF-ID Stage 2 Project

Two systems were investigated at the Joint Research Centre (JRC)-Karlsruhe for the experimental validation of the Thermodynamic of Advanced fuels - International Database (TAF-ID). The first system was representative of ex-vessel corium, consisting of nuclear fuel, fuel cladding, steel, and concrete representative materials, while the second consisted of (U, Pu)O2 (MOX) with lanthanides (2.5 at.% each of Ce, Gd, La, and Nd). Experimental observations included laser heating, for measuring liquidus and solidus temperatures; SEM-EDS, EPMA, and XRD for characterising the fuel microstructure, phases composition, and crystal structure; and KEMS, for measuring the vapour pressure of species. The obtained results were compared to thermodynamic calculations using the TAF-ID. Excellent agreement was observed between experimental and calculated results in the UO2-corium system, particularly of the liquidus and solidus temperatures, and the composition of phases. Results obtained on the MOX-corium system were not fully conclusive due to some experimental challenges. They do suggest, however, that some thermodynamic models should be reviewed, particularly those of U-Zr-O and U-Pu-Zr-O in the hypo-stoichiometric region. Regarding the MOX-Lanthanides system, evidence of silicon contamination was observed that led to the formation of a Si-, Pu-, and lanthanide-rich phase in a ratio consistent with M2SiO4. Although significant differences were observed between experimental results and thermodynamic calculations due to this contamination, the miscibility gap in the system has been confirmed experimentally. In addition, this contamination has allowed identifying important knowledge gaps in the database, which will lead to to continued imrpovements.