C.J. Ball

Actinide and rare earth incorporation into zirconolite

Abstract Nd 3+ and Ce 3+ can substitute for about 65% of the Ca in monoclinic zirconolite, using AI as a charge compensator in a Ti site. Further substitution up to 85% of Nd 3+ and Ce 3+ produces an orthorhombic structure, while more than 85% substitution produces additional phases. Substitution of Ce 4+ in the Zr site appeared to be quite limited. Incorporation of U 4+ into the Ca and Zr sites in zirconolite gave results which were similar to those observed by others. Both trivalent and tetravalent Np and Pu can be substituted in the Ca and Zr sites, respectively, under oxygen partial pressures of 0.2−1 × 10 −5 atm, provided appropriate charge compensators are present. The implications of these results for formulating actinide-bearing zirconolite-rich ceramics are discussed briefly.

Zirconolite-Rich Ceramics for Actinide Wastes

New X-ray diffraction and scanning electron microscopy data are given for the incorporation of Np and Pu in zirconolite, at levels of tens of percent. The actinide valences and the cations they replace are deduced from the microanalysis of the zirconolite compositions, and X-ray absorption data are used to obtain more direct information on the valences of Ce and Nd, which are used as simulants of Pu and trivalent actinides respectively. Trivalent rare earths and actinides have extensive solid solubility in zirconolite, mainly but not exclusively in the Ca site. Tetravalent rare earths and actinides have considerable solid solubility in the Zr site of zirconolite, and some solubility in the Ca site, but the strong tendency of zirconolite with ions substituted in the Zr site to undergo phase separation complicates structural interpretation. In zirconolite-rich Synroc-type ceramics designed to immobilise waste actinides, the target actinide waste loading has been set at 20 wt% and early leach results indicate the durability is at least as good as that of Synroc-C.

Crystallisation of zirconolite from an alkoxide precursor

Abstract Crystallisation of zirconolite (CaZrTi2O7) from a stoichiometric alkoxide precursor was studied by X-ray diffraction, differential thermal analysis and high resolution electron microscopy. The X-ray amorphous oxide formed by drying mixed ethanolic solutions crystallised to a disordered fluorite structure on heating to ~ 700 °C. Zirconolite with a pseudo-trigonal structure formed at ~ 900 °C, and at higher temperatures gradually transformed to fully ordered monoclinic zirconolite. Activation energies for the amorphous to fluorite and fluorite to pseudo-trigonal zirconolite transformations were obtained from the DTA measurements by the Ozawa method, but were not consistent with the rates of transformation observed by X-ray diffraction in isothermal measurements. The pseudo-trigonal zirconolite was found by high-resolution electron microscopy to be highly disordered monoclinic zirconolite.

High-temperature study of CaZrTi2O7

Abstract Previously reported anomalous thermal expansion effects in the 1200–1500°C range for hot-pressed CaZrTi 2 O 7 were shown to be due to irreversible bloating effects, from occluded gases. X-ray diffraction and differential thermal analysis of ordered CaZrTi 2 O 7 did not reveal evidence of a solid-state transformation at temperatures up to 1450°C.

Thermal expansion coefficients of zirconolite (CaZrTi2O7) and perovskite (CaTiO3) from X-ray powder diffraction analysis

Abstract The thermal expansion coefficients of zirconolite, both ordered and disordered, and perovskite, were measured by X-ray powder diffraction analysis in the temperature range 25–1200°C. The principal results are: (1) The mean thermal expansion coefficients ( K −1 ) of ordered zirconolite are:α a = (11.35±0.25) × 10 −6 ;α b = (8.72± 0.22) × 10 −6 ;α c = (10.0 ±0.5) × 10 −6 . (2) The mean thermal expansion coefficients ( K −1 ) of disordered zirconolite areα ∥ = (9.89±0.15) × 10 −6 in the (001) plane and α ⊥ = (9.37±0.14) × 10 −6 normal to (001). (3) The mean thermal expansion coefficients ( K −1 ) of perovskite are:α a = (7.86±0.30) × 10 −6 ;α b = (13.46±0.17) × 10 −6 ;α c = (16.55±0.26) × 10 −6 .

Ionic Size Limits for A Ions in Brannerite (ATi 2 O 6 ) and Pyrochlore (CaATi 2 O 7 ) Titanate Structures ( A = tetravalent rare earths and actinides)

The lower limit of the size of the octahedral A 4+ ion in the ATi 2 O 6 brannerite structure is just smaller than that of Ce/Pu. Attempts to expand the A ion size beyond that of Th by (a) substituting a Ba ion plus two U 5+ ions for three A ions or (b) substituting one Ba plus one hexavalent ion for two A ions did not succeed. Ge, Sn and Zr substitutions in the Ti site of ThTi 2 O 6 do not exceed 0.2 formula unit in ceramic preparations. These and other coupled substitutions in the B site of ThTi 2 O 6 showed that the average B site size could tolerate deviations of 4+ is unusually stabilised in air atmospheres at temperatures close to the melting point of 1400°C in the A site of brannerite. Lattice parameter data on different endmember ATi 2 O 6 brannerites are given. The lower and upper size limits for the eightfold A ions in the pyrochlore structure are around 0.100 and 0.117 nm respectively. A BaUTi 2 O 7 stoichiometry did not produce a pyrochlore structure, and when fired in either argon or air yielded a mixture of BaUTiO 6 , whose structure is still uncertain, plus brannerite and rutile.

Interface phenomena in synroc, a titanate-based nuclear waste ceramic

Abstract Several aspects of Synroc which fall into the broad class of interface phenomena are discussed. These are radiation damage processes which give rise to interfaces between damage tracks and neighbouring unirradiated material, intergranular films which have deleterious effects on chemical durability, and aqueous leaching of Synroc which takes place primarily at the interface between the solid and groundwater.