In a nuclear reactor, certain scenarios can lead to extreme conditions called serious accident conditions with thermal meltdown of the core. In this case, a multiphase mixture, called 'corium' based on nuclear fuel and molten metals, will form at high temperature (>2000°C), such as in the Chernobyl reactors in 1986 and Fukushima-Daiichi in 2011. Knowledge of the thermophysical properties of corium remains limited to date due to the domain of very high temperatures, the multiphasic nature (solids, liquids, gases) and the multiplicity of corium compositions. Knowledge of these properties remains a fundamental issue for the safety assessments of nuclear reactors. Limited current knowledge leads to a lack of robustness of these assessments.
To respond to these fundamental scientific challenges, the CEA has been developing three areas of research for several years:
• a first experimental part concerning the measurement the thermophysical properties - in particular density, surface tension, and viscosity - of corium,
• a second part concerning the parametric modeling of the corium thermophysical properties,
• a third component linked to small-scale modeling using molecular dynamics.
A first thesis has led to the development of an original experimental device on the VITI facility of the PLINIUS severe accidents platform of the IRESNE institute, operated by the LEAG laboratory in Cadarache, capable of measuring the surface tension of corium up to 2700°C using the MBP (Maximum Bubble Pressure) technique. In a complementary approach, a thesis made it possible to develop the ATTILHA installation of the ISAS institute operated by the LM2T laboratory in Saclay, allowing access to precise density measurements at high temperatures (> 2000°C). Finally, a thesis in progress at the LMAG laboratory of the IRESNE institute in Cadarache on the modeling of the interfacial tension of corium using the Calphad approach and Butler formalism has led to first results on certain compositions representative from in-vessel corium.
In continuation of the previous works, the LEAG, LMAG and LM2T laboratories jointly propose a thesis which aims to expand the experimental database by relying on the following approach:
• the first part includes the extension of the characterization tests to an expanded grid (composition, temperature) for various coriums of interest. The PhD student will have to carry out these measurements of properties of uranium samples using the cutting-edge instrumentation implemented on the VITI and ATTILHA experiments. It will also be a matter of improving data post-treatment for test interpretation by improving existing inverse method tools.
• The second part deals with the modeling of these thermophysical properties, in order to explore in a more complete way the (vast) grid (composition, temperature), which will remain partly inaccessible to measurement, due to the difficulty of the latter in extreme conditions. The PhD student's work will focus on improving existing ad hoc tools, based in particular on the Calphad approach.
• The last part involves considerations at the system scale, where the doctoral student will apply reactor calculations to severe accident scenarios, via the PROCOR tool in order to evaluate the sensitivity of accident scenarios to the new correlations obtained for the thermophysical properties of interest.