Behavior of Molybdenum in Mixed-Oxide Fuel

Metallic molybdenum, Mo-Ru-Rh-Pd alloys, barium, zirconium, and tungsten have been added to uranium and uranium-plutonium oxides by coprecipitation and mechanical mixture techniques. This material has been treated in a thermal gradient similar to that existing in fuel during irradiation to study the behavior of molybdenum in an oxide matrix as a function of the O/(U+Pu) ratio and some added elements. The result of ceramographic and microprobe analysis shows that when the overall O/(U+Pu) ratio is <2, molybdenum and Mo-Ru-Rh-Pd alloy inclusions are present in the uranium-plutonium oxide matrix. If the O/(U+Pu) ratio is >2, molybdenum oxidizes to MoO₂, which is gaseous at a temperature ∼1000°C. Molybdenum oxide vapor reacts with barium oxide and forms a compound that exists as a liquid phase in the columnar grain region. Molybdenum oxide also reacts with tungsten oxide (tungsten is often present as an impurity in the fuel) and forms a compound that contains ∼40 wt% of actinide metals. The apparent solubility of molybdenum in uranium and uranium-plutonium oxides, determined by electron microprobe, was found to be <250 ppm both for hypo- and hyperstoichiometric fuels.