Oxygen Potential of (Th 0.7 Ce 0.3 )O 2-x
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1215-V18-04
Oxygen Potential of (Th0.7Ce0.3)O2-x Masahiko Osaka1, Kosuke Tanaka1, Shuhei Miwa1, Ken Kurosaki2, Masayoshi Uno3 and Shinsuke Yamanaka2 1 Oarai Research and Development Center, Japan Atomic Energy Agency, 4002 Narita-cho, Oarai-machi, Higashiibaraki-gun, Ibaraki, 311-1393, Japan. 2 Division of Sustainable Energy and Environmental Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita-shi, Osaka, 565-0871, Japan. 3 Division of Nuclear Power and Energy Safety Engineering, Graduate School of Engineering, Fukui University, 3-9-1 Bunkyo, Fukui-shi, Fukui, 915-8507, Japan. ABSTRACT Oxygen potentials of (Th0.7Ce0.3)O2-x were experimentally determined by means of thermogravimetric analysis as a function of non-stoichiometry at 1173 and 1273 K. Oxygen potentials of (Th0.7Ce0.3)O2-x at each temperature increased with increase of oxygen-to-metal (O/M) ratio (=2-x) and steep increases of the oxygen potentials when approaching O/M ratio = 2 were observed. These characteristics are typical for non-stoichiometric fluorite-type actinide dioxides. The oxygen potentials of (Th0.7Ce0.3)O2-x were similar to those of CeO2-x when they were plotted as a function of average Ce valence. INTRODUCTION ThO2-based oxides have attracted special attention for use in various nuclear fuel systems owing to their superior properties. For example, a solid solution of (Th,Pu)O2-x is being investigated as a fast breeder reactor fuel in India. ThO2 is also a promising matrix as a radioactive waste form. The present authors have been investigating a ThO2-based oxide as a novel fuel form dedicated to minor actinide incineration in a fast reactor system [1]. On the other hand, ThO2 has been investigated as a solid electrolyte since it was found that notable oxygen ion transportation occurred when aliovalent cations were doped to form oxygen vacancies [2]. Namely, when two trivalent rare earth atoms such as Nd are doped and replaced Th ions, one oxygen vacancy is formed to maintain electrical neutrality. Thus, oxygen non-stoichiometry occurs for the ThO2-based oxides. This is of particular importance for ThO2-based oxide nuclear fuels since large amounts of fission products and actinides can dissolve into the ThO2 matrix to form oxygen non-stoichiometry [3]. Oxygen potential, which is a function of oxygen nonstoichiometry and temperature, of ThO2-based solid solutions should be determined prior to property measurements since the oxygen non-stoichiometry greatly affects various properties that are indispensable for fuel design, such as thermal conductivity, lattice parameter and so on. In this study, oxygen potentials of (Th,Ce)O2-x were experimentally determined at 1173 and 1273 K and were discussed in terms of ThO2 effects on the oxygen potential of CeO2-x. In particular, focus is placed on how the oxygen potential of solute, CeO2-x, was increased or decreased by solving it into ThO2 matrix. Such a characteristic is technologically valuable in terms of not only the extreme importance of oxygen potential itself as a
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