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Uranium and uranium-related ceramic oxide fuel pellets are widely used in nuclear power reactors currently in operation for electricity generation. These oxide nuclear fuels developed so far and being used in the reactors have advantages over the metallic fuels; e.g., stable phases with high melting point, good compatibility with coolant and stability against neutron irradiation. Certain disadvantages of these materials are, however, the lower thermal conductivity than that of metallic fuels, which affects and, as a consequence, their thermo-physical and thermo-mechanical properties and their in-reactor performance. Currently, technologies are being developed for the improvement of such properties, especially when these oxide fuels are subjected to higher burn-ups. [1±3] Fuel behavior against thermal shock can be one of the important properties that can give estimations not only for the behavior of the fuels in service during reactor operation but also for the thermo-mechanical properties of the fuel materials, [4] which are affected by material characteristics such as density and microstructure. This paper describes the results of the analyses of the behavior of UO 2 -5 wt.-% CeO 2 , simulating (U-Pu)O 2 fuel, against thermal shock by measuring basic mechanical properties such as Vickers hardness H V , fracture toughness K IC , fracture strength r f and thermal shock resistance DT C for the different material densities before and after the thermal shock by quenching and microscopic observations. An attempt was COMMUNICATIONS 584

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