Phase studies on Np-Zr-O system

It has been known for long time that actinide dioxide ThO2, UO2, PuO2 and AmO2 form extensive series of mixed crystals with ZrO2 above 1000 deg C, which have tetragonal structure at low concentration of actinide oxide and cubic fluorite structure at lower content of zirconium oxide. The solid solubility of ZrO2 in actinide oxide, forming fluorite type solid solutions ZryM1-yO2, increases with decrease in size of actinide ion (M). The solid solubility of ZrO2 in ThO2, UO2, PuO2 and AmO2 is 5, 15, 80 and 82 mol% at 1400 deg C in air, respectively. Our recent studies, on ZrO2-UO2 system, have shown that the solubility of ZrO2 in UO2 matrix can be increased up to 35 mol% at 1400 deg C by slightly modifying the preparation conditions. Fluorite type zirconium based solid solutions with actinide oxides are known for their high thermal and irradiation stability and chemical durability under repository conditions