Corium materials characterizations through electron microscopy and X-ray diffraction
Résumé
In case of severe accident conditions in a nuclear reactor, corium, a complex material coming from the melting of the core at a very high temperature (2500-2800 K), is formed. The cooling process of this material from the liquid state can be rapid in the case of contact with water (from a few milliseconds to a few seconds for the so-called Fuel Coolant Interaction (FCI)), or slow in the case of contact with concrete (from a few minutes to a few hours for the so-called Molten Core Concrete Interaction (MCCI)). The knowledge of the structure and microstructure of corium according to the severe accident scenario is fundamental to understand severe accident progression and to guide dismantling operations.
The first studies focused on the investigation of the solidification behavior of a melt representative of the in-vessel conditions of Fukushima Daïchi Unit 1-F2. Boron was initially present in Unit 1-F2 under B4C phase (control rod). Small-scale experiments were carried out at CEA Cadarache –IRESNE (PLINIUS/VITI) to characterize and model the solidification of the melt for materials containing B-C-Fe-O elements. The material were analysed by Electron Probe MicroAnalysis under the wavelength dispersion mode (EPMA /WDS) taking into account the chemical shift of boron and carbon in the different states measured. X-ray diffraction (XRD) was used to determine the nature of the crystalline phases.
The second study was carried out on prototypical corium obtained Fuel Coolant Interaction experiments performed in the KROTOS facility at CEA Cadarache. The fragmented or exploded corium debris under consideration are corresponding to a ternary compound according to the U1-xZrxO2y solid solution. Typical size of 200 µm or less than 50 µm have been studied to determine microstructure, elemental composition and structural state. EPMA was used to determine with high accuracy the local variation of the composition. A specific methodology will be presented and the results will be compared with the phases compositions obtained through Rietveld refinement of both X-rays and neutrons diffraction patterns. These analyses were completed by micro texture analyses with SEM/EBSD and Laue microdiffraction experiments. We observed at local state intra-crystalline local cationic composition fluctuation related to the different cooling processes and the corium particles sizes.
The last study concerns EPMA performed in the frame of a corium round robin on tools and methodology for post test analyses on a sample of typical MCCI experimental tests. The results evidence the ability of the methods that we used to determine the corium global and local compositions characterised by presence of uranium and a large number of major light elements.