C i n flowing dry hydrogen. Homogeneous fuel pellets with densities in e x c e s s of 97 per cent of theoretical were obtained for property studies. of the UO2-5O mole per cent Z r 0 2 composition was found considerably lower than that of UO, but higher than that of ZrOp. roughly paralleled that of pu.re U 0 2 at a slightly higher value, over the temperature range measured. Electrical resistivity decreased with increasing temperature, a s is typical of a ceramic material. In t e s t s of UO2-2O to 50 mole per cent Z r 0 2 compositions, in general, both room-temperature and high-pressure hardness values increased a s the ZrOz content increased. Homogeneous UO2-5O mole per cent ZrO, pellets were more resistant to cracking under thermal cycling than were inhomogeneous pellets of the same composition. Phase relationships in the UOp-ZrOp s y s t e m up to 50 mole per cent ZrOq were evaluated to determine a s a function of temperature the equilibrium phases present and the solubility limits of Z r 0 2 in UOz. At 1000 C or above it was found that, at low ZrOp contents, a singlephase face-centered-cubic solid-solution regior) i s present. A t Zr02 contents greater than 13.5 mole per cent a two-phase face centered cubic-face centered tetragonal region e x i s t s at 1000 C.

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