Measurement of stress-strain curves for (U, Pu)O2

The plastic deformation of (U, Pu)O2 which results from thermal stress exerts important influence on the fuel performance in fast breeder reactors. Therefore, it is necessary to obtain the stress-strain curves for mixed oxide fuel in the wide ranges of stress and temperature, in order to predict the response of fuel to the variation of reactor power. The purpose of this paper is to describe the results obtained by the compression test of (U, Pu)O2 at a constant strain rate from 0.1 to 10/min in the temperature range from 1000 to 1800 deg C. The method of preparing test specimens and the testing procedure are described. An Instron type testing machine was used, and the stress-strain curves were drawn with an X-Y recorder. The stress-strain curves for the mixed oxide pellets at 0.1/min strain rate and in the range from 1000 to 1800 deg C are shown. The effects of temperature and strain rate on the proportional limit were able to be obtained from the stress-strain curves. The proportional limit drops with temperature rise, and rises with the increase of strain rate. This behavior was in good agreement with that previously reported for UO2. In order to study the deformation mechanism of (U, Pu)O2, the strain rate sensitivity was estimated, and it was 50 at 1200 deg C and 10 at 1800 deg C. The deformation mechanism of (U, Pu)O2 at high strain rate and high temperature seems to be analogous to that of UO2. (Kako, I.)