During the past forty years, the materials science of nuclear waste forms has focused on the stability and long-term behaviour of nuclear waste glasses and used nuclear fuels, mainly UO₂. During this same period, substantial quantities of Pu, now more than 2,000 metric tones, have accumulated, either still in the used nuclear fuel or chemically separated for weapons or energy applications. This "excess" plutonium, as well as associated "minor" actinides (Np, Am and Cm) offer a new, but seldom pursued, opportunity for the safe geologic disposal of transuranium elements. A variety of materials, with mineral analogues, including oxides, silicates and phosphates, have been investigated because of their high capacity to incorporate actinides, their chemical durability, and in some cases, their resistance to the radiation-induced transformation to the aperiodic state. There has been substantial interest in isometric pyrochlore, A₂B₂O₇ (A= rare earths, actinides; B = Ti, Zr, Sn, Hf), for the immobilization of actinides, particularly plutonium. Systematic studies 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 have been observed: i) radiation-induced amorphization, ii) an order-disorder transformation and iii) phase decomposition. The radiation stability of these derivatives of the fluorite structure-type is closely related to the structural distortions caused by compositional variations that affect electronic structure and bond-type. Based on this very fundamental understanding of the radiation response, durable, actinide waste forms can be designed for specific temperature and radiation dose conditions, such as those found in very deep boreholes.
¹R.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.
²F.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.
³M. Lang, F.X Zhang, R.C. Ewing, J. Lian, C. Trautmann, and Z. Wang (in press) Structural modifications of GdZr_2-xTi_xO₇ pyrochlore induced by swift heavy ions: Disordering and amorphization. Journal of Mater. Res.