SOLUBILITY OF MAGNESIUM IN URANIUM-DIOXIDE

The solubility of magnesium in uranium dioxide under low oxygen pressu res was studied at 1200 degrees C. Magnesium was found to dissolve up to y > 0.1 (and below y = 0.15) of the apparent formula, MgyU1-yO2+x(x (less than or equal to)(>) 0) P-O2 = 10(-15) and less than or equal t o 10(-19) atm. The formed solid solution in such a low p(O2) region wa s of the type (MgaU1-a)(Mg-b)O2+c, in which the magnesium atoms partly occupy the interstitial sites together with the substitutional sites for uranium atoms, The ratio of interstitial atoms to the total magnes ium atoms increased from 0.23 (y = 0.05) or 0.39 (y = 0.1) at P-O2 = 1 0(-15) atm with decreasing oxygen partial pressure to 0.62-0.63 (y = 0 .05 and 0.1) at p(O2) less than or equal to 10(-19) atm. The lattice p arameter of less than or equal to the (MgaU1-a)(Mg-b)O2+c solid soluti ons was represented as a linear equation of a, b and c. The interstiti al magnesium caused an increase in the lattice parameter, in contrast to the substitutional magnesium which largely decreases the lattice pa rameter. It is possible that the uranium atoms in the solid solutions prepared at low oxygen partial pressures (less than or equal to 10(-19 ) atm) were reduced to slightly less than the tetravalent state. (C) 1 997 Elsevier Science B.V.