Murataite ceramics for the immobilization of actinides

A melted titanate ceramic was synthesized in the Ca-Mn-Ce-Zr-U-Al-Fe-Ti-O s ystem. The ceramic was composed of oxides of the specified elements with a fluorite-type cubic symmetry similar in structure to murataite, a rare natu ral mineral. In addition to the murataite, the following minerals also crys tallize in this system: zirconolite, perovskite, Ca-U-Ti oxide with a pyroc hlore-type structure, loveringite, hibonite, pseudobrookite, and rutile. Th e ceramic is capable of incorporating and securely retaining up to 30 wt % of actinides and their simulators, has low solubility in aqueous solutions, and can be used as a matrix for the efficient immobilization of the actini de portion of the high level radioactive wastes (HLW). The composition of t he starting material defines mineral compositions of this ceramic. For exam ple, zirconolite, rutile, or perovskite exist in the samples with high Zr, Ti, or Ca contents, respectively, while oxides with a pyrochlore structure occur in samples with low Zr and high U concentrations. The elevated Mn con tent in the starting material is an essential condition for the synthetic m urataite formation. The elevated Al content expands the field of the murata ite stability. The synthetic murataite composition corresponding to the gen eral formula A(4)B(2)C(7)O(22), where the A-site is occupied by Ca, Mn, REE , and U; the B-site is engaged by Mn, Ti, Zr, and U and the C-site includes Ti, Al, and Fe. The variations of the murataite composition in components of radioactive wastes is caused by the following isomorphic substitutions: Ca2+ reversible arrow Mn2+, Ti4+ reversible arrow Zr4+ reversible arrow U4, Ca2+ + Ti4+ reversible arrow REE3+ + (Al, Fe, Mn)(3+). Experiments on the murataite synthesis show that several murataite-like phases crystallize in the specified system, and the compositional variation of each phase is res tricted within rather narrow limits. The structural modifications with diff erent unit-cell parameters corresponding to the three-, five-, and eightfol d parameter of the fluorite cell were identified among them. Some ceramics dominated by murataite contain two coexisting murataite-like phases, which are different in cell size, element contents (Ce, Zr, U, and particularly i n Al), and are separated by distinct phase boundaries. The term murataite n ow comprises various mineral species of the murataite group, which have som e similar features in their structures and continuous limited missiblity. I t was found that the murataite-like phases different in compositions and st ructural features are also distinct in their isomorphic capacity for actini des. This raises a problem to design synthesis conditions of murataite with optimal properties.