Pyrochlore: The Elegant Response of a Simple Structure to Extreme Conditions

Featuring Rodney C. Ewing, Ph.D.
Professor, Geological Sciences, Materials Science and Engineering
Nuclear Engineering and Radiological Sciences
University of Michigan

Location:  McDonnell Douglas Auditorium

Abstract:
During the past sixty years, more than 1,800 metric tonnes of Pu, and substantial quantities of the “minor” actinides, such as Np, Am and Cm, have been generated in nuclear reactors. Some of these transuranium elements can be a source of energy in fission reactions (e.g., 239Pu), a source of fissile material for nuclear weapons (e.g., 239Pu and 237Np), and of environmental concern because of their long-half lives and radiotoxicity (e.g., 239Pu and 237Np). In fact, new strategies for the advance fuel cycle initiative (AFCI) are, in part, motivated by an effort to mitigate some of the challenges of the disposal of these long-lived actinides. There are two basic strategies for the disposition of these elements: 1.) to “burn” or transmute the actinides using nuclear reactors or accelerators; 2.) to “sequester” the actinides in chemically durable, radiation-resistant materials that are suitable for geologic disposal.  There has been substantial interest in the use actinide-bearing minerals, such as isometric pyrochlore, A2B2O7 (A= rare earths, actinides; B = Ti, Zr, Sn, Hf), for the immobilization of actinides, particularly plutonium.  Systematic studies1 of rare-earth pyrochlores have led to the discovery that certain compositions (B = Zr, Hf) are stable to very high doses of alpha-decay event damage. Three different processes were observed: i) radiation-induced amorphization, ii) an order-disorder transformation and iii) phase decomposition. The radiation stability of these compositions is closely related to the structural distortions that occur for specific pyrochlore compositions and the effect of electronic structure on bonding. Additionally, in-situ synchrotron X-ray diffraction and Raman spectroscopy measurements were performed on a range of pyrochlore compositions at pressures up to 43 GPa. Similar structural transformations were observed in the pyrochlore structure-type at high pressures. These results demonstrate that there are parallel responses to energetic particle irradiations and high pressure, although the mechanisms of the structural transformations are quite different2. Recent developments in our understanding of the properties of actinide pyrochlore have opened up new possibilities for the design of advanced nuclear fuels and waste forms.

1) R.C. Ewing, W.J. Weber and J. Lian (2004) Pyrochlore (A2B2O7): A nuclear waste form for the immobilization of plutonium and “minor” actinides. (Invited Focus Review) Journal of Applied Physics, vol. 95, 5949-5971.

2) F.X. Zhang, J.W. Wang, J. Lian, M.K. Lang, U. Becker and R.C. Ewing (2008) Phase stability and pressure dependence of defect formation in Gd2Ti2O7 and Gd2Zr2O7 pyrochlore. Physical Review Letters, vol. 199(4), 045503.