Examination of simulant corium (U,Zr)O2 structures before and after laser heating melting: An XRD, XANES and Raman examination

This paper presents surface and bulk characterization results obtained on a series of (U, Zr)O 2 samples, simulating the type of material produced in a nuclear reactor core during the first stages of a meltdown accident. Ceramic mixed oxide pellets were produced by sol-gel mixing and traditional or spark-plasma sintering. They were analysed by classical metallography, SEM-EDX, X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES) and Raman spectroscopy. In addition to the analyses performed on the as-fabricated material, a characterisation campaign was carried out on re-solidified samples after melting them by laser heating, as part of a project aimed at examining in detail the corium structure. Metallography and X-ray diffraction reveal the coexistence, in as-fabricated samples, of various species, essentially Zr-rich monoclinic or tetragonal phases and U-rich face-centred cubic phases. Such observations, in line with existing studies, are extended by Raman spectroscopy and XANES. XANES corroborates the phase analysis by revealing the oxidation states of uranium ad zirconium in the investigated samples. Raman spectroscopy is useful as a quite flexible and a non-invasive material characterisation technique that may be used also for the ex-situ analysis of corium. Laser-molten and refrozen samples present features related to the conditions under which the samples were melted. Recent results show that samples melted in argon and quenched to room temperature mainly preserve Zr 4+ and U 4+ compounds. Phase demixing seems to occur upon rapid quenching, whereby a tetragonal solid solution was observed by XRD. The formation of a tetragonal ZrO 2 –like phase can also be inferred from a Raman investigation. Samples melted in air and quenched to room temperature essentially contain Zr 4+ and a mixture of U 4+ , U 5+ and U 6+ , as expected due to the strongly oxidizing conditions. In these latter samples, monoclinic and tetragonal Zr-rich phases were only observed in Zr-rich samples, whereby cubic mixed phases were observed for U-rich materials. Other uranium oxides (U 4 O 9 , U 3 O 8 ) could be assumed based on Raman spectroscopy besides UO 2 in U-rich samples whereas the corresponding XRD patterns showed additional peaks but these could not be easily related to higher uranium oxides. Raman spectroscopy served to perform a local analysis on the micrometric scale, where it helped to identify the presence of different non-symmetric phases in quenched samples, not observed by XRD. This reveals the complexity of the refrozen material. This observation can be useful for possible applications of this technique in ex-situ measurement of corium samples, and in general in order to develop an efficient experimental approach to the analysis of corium.