Thermal Conductivities of Cs- M -O ( M = Mo or U) Ternary Compounds
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Thermal Conductivities of Cs-M-O (M = Mo or U) Ternary Compounds Kazuyuki TOKUSHIMA1 Kosuke TANAKA2 Ken KUROSAKI1 1 1 Hiromichi GIMA Hiroaki MUTA Masayoshi UNO3 1,3 Shinsuke YAMANAKA 1 Division of Sustainable Energy and Environmental Engineering, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita 565-0871, Japan 2 Japan Atomic Energy Agency, Narita-cho 4002, Oarai-machi, Higashiibaraki-gun, Ibaraki, 311-1393, Japan 3 Research Institute of Nuclear Engineering, Fukui University, 3-9-1 Bunkyo, Fukui 910-8507, Japan
ABSTRACT Thermal conductivities of Cs-M-O (M = Mo or U) ternary compounds, observed in the pellet-cladding gap region and in the pellet periphery in irradiated oxide fuels with high oxygen potentials, were investigated. Bulk samples of Cs2MoO4 and Cs2UO4 were prepared by hot pressing or spark plasma sintering, and their thermal diffusivities were measured by the laser flash method from room temperature to 823 K for Cs2MoO4 and to 900 K for Cs2UO4. The thermal conductivities were evaluated from the thermal diffusivity and bulk density, and the specific heat capacity values available in the literature. The thermal conductivities of Cs2MoO4 and Cs2UO4 were quite low compared with UO2 (e.g. 0.5 Wm-1K-1 at 800 K for Cs2MoO4). INTRODUCTION It is important to understand the behavior of fission products (FPs) to evaluate fuel performance. For example, in high burn-up oxide fuels, some FPs dissolve in the fuel matrix and others form oxide or metallic inclusions, both of which would affect the physical and chemical properties of the fuels. Among FPs, cesium is one of the most important due to its high fission yield in mixed oxide (MOX) fuel when burned in the fast reactor. In the post irradiation examinations of the Phenix fuel pins, accumulation of cesium molybdate in the fuel-cladding gap was observed [1]. Cesium uranates were generally observed in the fuel pellet periphery. In order to improve the accuracy of prediction of the thermal performance of the MOX fuel to high burnup, it is indispensable to measure the thermal conductivity of these cesium compounds. Many compounds have been identified in the Cs-Mo-O and Cs-U-O system and their crystal structures and thermochemical data have been reviewed and summarized [2]. Experimental thermal conductivity data of their compounds is restricted however to Cs2MoO4 [3,4] and Cs2UO4 [5] .
The objective of the present study was to acquire data on thermal conductivities of Cs2MoO4 and Cs2UO4 using samples fabricated by hot press and spark plasma sintering (SPS) technique.
EXPERIMENTAL Powders of Cs2MoO4 (Mitsuwa Chemicals; purity 99 %), Cs2CO3 (Kishida Chemicals; purity 99.99 %) and U3O8 were prepared as raw materials. Cs2UO4 powder was obtained by the following procedure: Cs2CO3 and U3O8 powders were weighed out as their hyperstoichiometrical amounts (U3O8/Cs2CO3 molar ratios were 1/6) and mixed thoroughly in an agate mortar with a pestle. The mixture was pressed into disks at 150 MPa pressure and room temperature for 5 min. The disks were
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