Mineralogical Study for a Safer and Cleaner World: The Study of Thermodynamics and Kinetics of Crystalline Nuclear Waste Forms

The energy and environmental issues are two challenges we are facing today, and will face in future. New basic research for advanced nuclear energy systems again is being promoted by the US Department of Energy. Nuclear energy is a safe and clean energy form if we can master energy conversion system and nuclear waste well. Thermodynamic evaluations of the crystalline forms of nuclear wastes for uranium and plutonium indicate that titanate-based pyrochlore phases are stable for U- and Pu-dominated phases with respect to their oxide components, and can also incorporate Gd and Hf (neutron absorbers). However, PuSiO4 with zircon structure is thermodynamically unfeasible and un-stable with respect to silica and PuO2 even at very high temperature. Because both zirconolite and pyrochlore will experience incongruent melting at high temperature, sintering below peritectic temperature is feasible for fabricating homogeneous U- and Pu-loaded zirconolite and pyrochlore. Synthesis experiments indicate there is a miscibility gap between zirconolite-structure phase (Zr- and/or Hf-dominated) and pyrochlore-structure phase (U and/or Pu dominated). Both phases can form a composite with lamella-like intergrowth texture. The sub-micron to nano-meter scale lamellae also preserve roles of Gd (rich in pyrochlore lamellae) and Hf (rich in zirconolite lamellae): absorbing neutrons.

Long-term (950 days) leaching experiments (chemical durability tests) show incongruent leaching (dissolution) of the waste in flow-through experiments. TEM and results indicate titania-rich layers form on the surface of the waste during the leaching process. The layers potentially serve as a protective shield, limiting the long-term leaching and mobilization of U and Pu in the waste. Natural analogue study of U- and REE-bearing pyrochlore shows that radiation-induced amorphous (metamict state) of the pyrochlore will enhance oxidation and leaching of Ce and U. It is proposed that the pyrochlore / zirconolite composite-based ceramic waste will be safe and stable in non-oxidizing geological repository environments.