The stability of fissile-fertile oxide solid solutions in air

Abstract

The stability of (UTh)02, (PuTh)02, and (PuUTh)02 solid solutions when heated to high temperatures in air is reviewed and discussed. (PuTh)02 is chemically stable under these conditions, but compositions containing uranium oxidise to give either a non-stoichiometric fluorite phase containing excess oxygen, or to this phase plus orthorhombic U3O8. U3O8 formation is to be avoided if maximum dimensional stability is required. (UTh)02 compositions containing 50 m/o or more of Th02 do not form an orthorhombic phase under any conditions of oxidation. Information on the extent of non-stoichiometry and the effects of excess oxygen on unit cell volume, bulk volume, and the rate of uranium loss at high temperatures is also reviewed.