Zirconolite as a matrix for immobilization of high-level radioactive wastes

The term zirconolite embraces three polymorphic varieties of a mineral with composition (Ca, Na, REE, Th)(2)Zr-VIII(2)VII(Ti, Nb)(3)(VI)(Fe,Ti)O-V,IV(14) and orthorhombic (zirconolite-30), trigonal (zirconolite-3T), and monoclinic (zirconolite-2M) crystal lattices. The highest concentrations of isomorphic components in natural and synthetic zirconolites (as recalculated to oxides) are 20-25 wt % of actinides, 15-18 wt % of combined Nh and Ta, 12-15 wt % of REE, 11 wt % of Pu etc. The cations with diverse charges (+1 to +5) and ionic radii (0.4 to 1.4 Angstrom) can be incorporated in the zirconolite structure. Such parameters are typical of most radionuclides that remain in the high-level radioactive wastes (HLW) after removal of Cs and Sr isotopes. High resistance of zirconolite to radioactive and hydrothermal attacks, wide solid solution limits, and well-developed production technology make this mineral the best waste form. for immobilisation of actinides of high-level radioactive wastes, as well as weapon plutonium.