SOLID-STATE REACTIONS OF URANIUM OXIDE AND ALKALI-METAL CHROMATES - CHARACTERIZATION OF NEW URANATES

New uranate phases, Rb2U4O11 and Rb2U3O8.5 in the Rb-U-O system and Na2U3O9 and Na(y)U1-yO2-x (y = 0-0.2) in the Na-U-O system were prepared and characterised by X-ray, thermal and chemical methods. Na(y)U1-yO2-x was found to be a solid solution of Na2O in UO2 with fluorite structure with a limited solid solubility of 10 mole% Na2O. These new phases reported could lead, in addition to the modification of the phase diagrams, to a better understanding of the fuel-fission product-coolant-clad chemical interactions. The new phases were identified during studies on solid state reactions of chromates of caesium, rubidium, potassium and sodium with uranium oxides upto 1200-degrees-C in inert atmosphere. These reactions have shown that different uranates are obtained depending upon the O/U of the starting uranium oxide, with chromium separating as chromium oxide (Cr2O3).