Effect of oxygen-to-metal ratio on properties of corium prepared from UO 2 and zircaloy-2 Shun Hirooka ⇑ , Masato Kato, Kyoichi Morimoto, Akira Komeno, Teppei Uchida, Masatoshi Akashi Advanced Nuclear System R&D Directorate, Japan Atomic Energy Agency, 4-33 Muramatsu, Tokai-mura, Naka-gun, Ibaraki 319-1194, Japan article info Article history: Received 5 November 2012 Accepted 26 January 2013 Available online 8 February 2013 abstract The UO 2 and zircaloy system has been studied in severe accident research because these materials form most of a molten corium. In addition, during core meltdown, it is expected that water vapor and hydro- gen which is produced in the H 2 O–zircaloy reaction fill the reactor core space. In this study, simulated corium specimens were prepared considering the ambient atmosphere in an accident, and adjustment of the oxygen-to-metal (O/M) ratio. Phase observations by XRD and EPMA and measurement of thermal conductivity were then done on the specimens. It was confirmed that the prepared O/M ratio had hardly any effect on melting temperature although UO 2+x has obvious dependency on the O/M ratio. Thermal conductivity of the molten specimens showed little dependency on the O/M ratio and temperature. It appeared that the thermal conductivity had already been significantly decreased by the solution of U and Zr. Microstructure information as lattice constant and phase segregation was obtained by XRD and EPMA observation. These properties will be basic data for the development of conditioning techniques for an actual corium. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction In the core meltdown accident of LWRs, UO 2 , zircaloy (Zry), B4C and SUS are mixed as metal and/or oxide forms because of steam oxidation to make the corium of complex compositions [1,2]. Among the materials in the corium, UO 2 and Zry represent about 90% of them and their thermal properties, such as melting temper- ature and thermal conductivity, are important in analyzing the mechanism of core meltdown behavior and developing corium treatment techniques. High-temperature steam and hydrogen produced in the reac- tion between claddings and steam fill the reactor core space in the meltdown accident. Phase state of corium is decided from the oxygen potential which is decided from the partial pressure of H 2 to partial pressure of H 2 O ratio (P H2 /P H2O ). For the core melt- down accident at the Three Mile Island Unit-2 (TMI-2), the ratio was analyzed from corium obtained after the accident [1]. In the corium, for example, Sn and Ni were observed as metals. Mo and In were observed as metals or oxides. From the data, the P H2 / P H2O was expected to be 0.1–10 in the TMI-2 damaged core. In that atmosphere, the oxygen-to-metal (O/M) ratio of uranium oxide would range from 2.00 to 2.05. It was reported that this deviation can affect corium properties. Therefore, in research on corium, it is important to prepare cor- ium specimens in ambient atmosphere conditions, but few data have been reported so far. In this research, simulated corium spec- imens from UO 2 and Zry-2 were prepared based on measurement of melting temperature. Before measurement of melting tempera- ture, heat treatment was carried out with reference to the ambient atmosphere conditions in the TMI-2 accident. Phase observations by XRD and EPMA and measurement of thermal conductivity were done on specimens of the simulated corium. The data are expected to be fundamental data for analyses of the mechanism of core meltdown behavior and development of a conditioning technique for corium. 2. Experiment Fig. 1 outlines the procedure for specimen preparation. Starting materials were UO 2 and Zry-2 powders, and they were blended at a metal percentage of 50% each. Table 1 shows the simulated corium composition ratio. The blended powder was pelletized for better handling and heat-treated in 0.05%H 2 /Ar with 10,000 ppm H 2 O at 1923 K for 3 h to become oxidized with a thermo-gravimeter. Sin- tering condition such as temperature and moisture had been pre- viously examined to get enough oxidation of zircaloy-2 in the specimen. In the sintering condition, weight increase by oxidation of zircaloy-2 in the specimen came to equilibrium. Large swelling was observed due to the oxidation. Thus, the pellets were crushed, 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.01.334 ⇑ Corresponding author. E-mail address: hirooka.shun@jaea.go.jp (S. Hirooka). Journal of Nuclear Materials 437 (2013) 130–134 Contents lists available at SciVerse ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat