Structural effect of actinide substitutions on the Ca and Zr sites of zirconolite

Full text: Recently a SYNROC derivative was chosen as the wasteform for the immobilisation of surplus plutonium in the US. The major phase for the immobilisation of the plutonium is a titanate-based pyrochlore. Pyrochlore is stabilised when rare-earth or actinide substitutions in excess of 0.5 formula units are attempted on the Zr site of zirconolite. Substitutions of between 0.15 and 0.5 formula units on the Zr site are known to stabilise the zirconolite-4M polytype. This paper examines both the short and long-range structural impact of substituting Pu and U on the Zr site of zirconolite, leading to the stabilisation of pyrochlore. The impact of tri- and tetravalent Pu substitutions on the Ca site of zirconolite will also be discussed. Pu LIII- and Zr K-edge extended X-ray absorption fine structure (EXAFS) were used to characterise the short-range structures around both the substituted Pu and the Zr host lattice site, whilst X-ray diffraction provided long-range structural information. Copyright (2000) American Chemical Society