Fabrication and Characterization of Uranium-Molybdenum-Zirconium Alloys

As part of a global effort to convert reactors that require highly enriched uranium to instead operate with low enriched uranium, monolithic fuel plates consisting of a U-Mo fuel meat with a zirconium foil barrier layer and clad in aluminum are being investigated. Interactions have been noted to occur between the fuel meat and the zirconium foil during fuel plate fabrication. These phases must be characterized as part of the fuel qualification process in order to determine potential behavior in a reactor environment; the observed phases are often too small to fully characterize so this study was initiated to characterize the equilibrium phases present in the U-Mo-Zr ternary alloy system.

Alloys containing molybdenum (7 wt% and 10 wt%) and zirconium (2 wt%, 5 wt%, and 10 wt%) were fabricated and homogenized at 950?C. They were then subjected to annealing at 650?C for periods of 1, 2, and 5 hours in order to study phase characteristics. Characterization was performed using various techniques including electron-probe micro-analyzer, X-Ray diffraction, and differential scanning calorimetry analysis.

Generally, the alloys appeared somewhat similar in terms of microstructure, with the exception of a couple unusual features. A solid solution matrix phase containing intermetallics was typically observed. The density of these intermetallic regions varied with each alloy, but not significantly across varying heat treatments. These intermetallic phases were often observed to have compositions consistent with Mo2Zr, except in U-7Mo-10Zr. In this alloy the precipitates were often consistent with zirconium rich impurity phases; that particular sample could have had an anomalous high impurity composition.

One unusual result of the annealing was observed for the U-7Mo-2Zr alloy in back scattered electron imaging. An area that appeared to be a solid solution was visible along grain boundaries for the homogenized sample. This area appeared to be composed of a very dense region of precipitates for the annealed samples. The phase composition was unable to be analyzed due to the very small size of the precipitates.

The U-10Mo-5Zr and U-10Mo-10Zr alloys exhibited significant depletion of molybdenum from the matrix phase, while the other alloys exhibited a slight increase in molybdenum. This increase in molybdenum content should contribute to the stability of the ?-phase.

The work presented here addresses a knowledge gap in the behavior of the ternary U-Mo-Zr system. Phase evolution and stability of various U-Mo-Zr alloys are examined through casting and heat treating these alloys. Subsequent analyses provide details relevant to the advancement of fuel systems capable of converting many reactors.