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Characterization and thermomechanical properties of Ln2Zr2O7 (Ln=La, Pr, Nd, Eu, Gd, Dy) and Nd2Ce2O7

Toshiaki Kawano1, Hiroaki Muta1, Masayoshi Uno2, Yuji Ohishi1, Ken Kurosaki1 and Shinsuke Yamanaka1, 2 1

Division of Sustainable and Environmental Engineering, Graduate School of Engineering,

Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan 2

Research Institute of Nuclear Engineering, Fukui University, Bunkyo 3-9-1, Fukui-shi, Fukui

910-8507, Japan ABSTRACT Pyrochlore type compound Nd2(Zr,Ce)2O7 is considered to precipitate in ThO2-based fuel, that is not observed in irradiated UO2. In order to evaluate the influences on fuel properties, thermomechanical properties of the pyrochlore type compounds, Ln2Zr2O7 (Ln=La, Pr, Nd, Eu, Gd, Dy) and Nd2Ce2O7 were investigated. We synthesized the samples by solid-state reaction and pelletized by spark plasma sintering to make high density (≥ 90 %T.D.) pellets. The phase states and lattice parameters were examined by using X-ray diffraction and SEM/EDX analysis. The lattice parameters of Ln2Zr2O7 depended on the ionic radii of lanthanide ions. The heat capacity, thermal conductivity, linear thermal expansion coefficient, and elastic constants were also measured. It was confirmed that the thermal conductivities for Ln2Zr2O7 were lower than that for ThO2 and depended on Ln ionic radii. The values of elastic constants tended to increase with increasing the Ln ionic radii, corresponding to the thermal conductivity. INTRODUCTION Thorium fuel cycle has many advantages such as higher resistance for nuclear proliferation, less TRU nuclide production and better thermomechanical properties as compared with uranium-plutonium fuel cycle [1,2]. As for fuel form, Thorium dioxide, ThO2 are considered to be used in existing light/heavy water reactor. In high burnup ThO2-based fuel, a number of fission products (FPs) are produced in common with UO2. Several FP elements are known to be soluble in fuel matrix and others form precipitates. These FPs change the physical properties of fuel, therefore it is required to understand the effects of FPs for the utilization of nuclear fuel. However, on ThO2-based fuel, there are few reports focusing on the FPs effects. To investigate the chemical form of FP elements in ThO2-based fuel, Ugajin et al. synthesized ThO2-based SIMFUEL which included simulated FP elements of U, Y, Zr, La, Ce, Nd, Pr, Sr, Ba, Mo, Ru and Pd [3]. In the SIMFUEL, the precipitates, Nd2(Zr, Ce)2O7, (Ba, Sr)MoO4 and Ru-Pd alloy were observed. Nd2(Zr, Ce)2O7 which has the pyrochlore-type structure have not been found in irradiated UO2 and (U, Pu)O2. In this study, thermomechanical properties of pyrochlore type compounds Ln2Zr2O7 (Ln=La, Pr, Nd, Eu, Gd, Dy) and Nd2Ce2O7 which can be formed in the ThO2-based fuel were evaluated to understand influences of FP precipitates on fuel.

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EXPERIMENT Sample preparation Ln2Zr2O7 (Ln=La, Pr, Nd, Eu, Gd, Dy) and Nd2Ce2O7 were synthesized by means of solid-state reactions using powders of ZrO2, La2O3, Pr2O3, Nd2O3, Eu2O3, Gd2O3,