Ming Tang

Radiation damage effects in uranium-bearing delta-phase oxides

Actinide oxides have been used as nuclear fuel since the first controlled chain reaction in 1942. Today, the DOE is facing the disposal of vast quantities of spent uranium-oxide (UO₂) fuel that have accumulated over fifty years or more. Simultaneously, the DOE is positioning itself to coordinate the development of advanced nuclear fuels that may help to mitigate both nuclear materials disposal and proliferation problems.

More recently, the DOE has expressed an interest in the performance of non-metallic materials in hostile environments, especially the behavior of advanced ceramic nuclear fuel forms. UO₂ and mixed oxide (MOX) fuels, which crystallize in the fluorite structure, exhibit exceptional resistance to irradiation damage and are therefore the predominant fuel for thermal power reactors. 

My research involves the synthesis and fabrication of complex ceramic oxide samples, which are fluorite structural derivatives of compounds with stoichiometries near to M₇O₁₂, namely, the so-called delta (δ) phase, made from mixtures of uranium oxide (both UO₂ and UO₃) and sesquioxide compounds such as Y₂O₃. In addition, I am investigating the radiation damage behavior of these MOX fuels to determine the viability of these compounds for application as actinide-host nuclear fuel and waste forms.

My colleagues and I have recently shown that certain δ-phase oxides are extraordinarily radiation tolerant; in particular, they are especially resistant to amorphization transformations. Delta-phase compounds are also spacious (fluorite-derivative-structured) compounds that can readily accommodate actinides in their crystal lattices. The ability of the d phase to accommodate actinides is illustrated in the UO₂-UO₃-Y₂O₃ phase diagram. In that system, the δ phase exists from a fully oxidized stoichiometry of Y₆³⁺U₁⁶⁺O₁₂^2- to a fully reduced stoichiometry of Y₄³⁺U₃⁴⁺O₁₂^2- (the red line in the phase diagram below).

The importance of compounds containing a mixture of U⁶⁺ and U⁴⁺ species is that such compounds are surrogates for waste forms or fuel forms that may contain a mixture of uranium and higher actinides. We envision that the δ phase may be used to incorporate a variety of actinides to produce compounds such as Y₆³⁺(U_x⁶⁺Pu_y⁴⁺Am_z⁴⁺)₁O₁₂^2-.

One goal of this research is to understand trends in radiation damage behavior as one progresses from the fully oxidized δ-phase composition, Y₆³⁺U₁⁶⁺O₁₂^2-, to the fully reduced composition, Y₄³⁺U₃⁴⁺O₁₂^2-, to explain the paradoxical distinction between the radiation damage responses of seemingly similar materials. Another goal is to establish criteria that will enable scientists to predict the radiation-damage tolerance (or damage sensitivity) of specific ceramic compounds. Ultimately, my colleagues and I hope to discover new radiation-tolerant ceramics and develop these materials for application in hostile radiation environments, especially as actinide hosts for advanced nuclear fuels or wastes.

This three-component phase diagram shows the phase stability regions for mixtures of UO2-UO3-Y2O3. The red line indicates the compositional range for the M7O12 delta (δ)-phase structure. The label in red above the diagram indicates the many names that have been ascribed to the δ phase in the literature over the years.

Next: Probing the nature of uranium-ligand multiple bonds