Bruce D. Begg

Heavy-ion irradiation effects in Gd2(Ti2−xZrx)O7 pyrochlores

Gd 2 (Ti 2-x Zr x )O 7 samples with 0≤x≤1.5 were single-phase and pyrochlore structured after sintering at 1600°C in air. The Gd 2 Zr 2 O 7 (x = 2) end member predominantly displayed an anion deficient-fluorite structure. Raman spectroscopy indicated that the level of short-range fluorite-like disorder in the unirradiated Gd 2 (Ti 2-x Zr x )O 7 samples increased significantly as Zr was substituted for Ti, despite the retention of a long-range pyrochlore structure for samples with 0≤x≤ 1.5. Glancing-incidence X-ray diffraction indicated that pyrochlores with an ionic radii ratio r A /r B ≤ 1.52(x≥ 1.5) were transformed into a radiation resistant defect-fluorite structure after irradiation at room temperature with 2 MeV Au 2 to a fluence of 5 ions/nm 2 . As the ionic radii ratio of the pyrochlore increased beyond r A /r B > 1.52, the defect-fluorite structure became increasingly unstable with respect to the amorphous state under identical irradiation conditions.

The incorporation of plutonium and neptunium in zirconolite and perovskite

Abstract The incorporation of Pu and Np in one zirconolite and a number of perovskite formulations has been investigated under oxidising and reducing conditions. The Pu valence in both perovskite and zirconolite was found to vary independently of the crystal chemical design by altering the annealing atmosphere, from Pu 4+ in air to Pu 3+ in 3.5% H 2 /N 2 . For both atmospheres, only tetravalent Np was found in the zirconolite. Tetravalent Np was observed in perovskites fired in air, but trivalent Np was stabilised in suitable perovskite formulations annealed under a reducing atmosphere. A trivalent Np phosphate, NpPO 4 was also prepared under reducing conditions.

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.

Enthalpies of Formation of Gd2(Ti2-xZrx)O7 Pyrochlores

A calorimetric investigation of the enthalpies of formation of Gd 2 (Ti 2-x Zr x )O 7 , where 0≤ × ≤ 2 is underway. All samples exhibit pyrochlore (Fd3m) peaks in their XRD patterns. However, where x=2 significant local disorder is observed in the Raman spectra. Preliminary data for the enthalpies of formation from the oxides in kJ/mol are: x=0, ΔH f = -113.4±2.7; x=0.5, ΔH f = -94.0±3.0; x=1.0, ΔH f = -74.2±4.9; x=1.5, ΔH f = -64.5±2.0; x=2, ΔH f = -52.2±4.8. Two additional samples, Gd 1.80 Zr 2.15 O 7.00 (pyrochlore) and Gd 2.15 Zr 1.87 O 7.00 (fluorite), were also studied. Their enthalpies of formation from the oxides in kJ/mole are -50.9±3.3 and -46.4±3.4 respectively. Replacing Ti with Zr, i.e. when x=2, destabilizes the pyrochlore in enthalpy by approximately 60 kJ/mol. The ΔH mix for the Gd 2 (Ti 2-x Zr x )O 7 solid-solution series is positive and can be described by a regular solution formalism with an estimated interaction parameter, ŝ = +20 kJ/mol. The results of this study suggest that the pyrochlore to fluorite transition enthalpy in Gd 2 Zr 2 O 7 is small, of the order of the configurational entropy contribution due to cation disorder at the transition temperature, TΔS conf. ≍ 10 kJ/mol.

Computer simulation of Pu3+ and pu4+ substitutions in zircon

Energy minimization methods were used in atomistic computer simulations to determine the energetics of Pu3+ and Pu4+ substitutions and interstitials in zircon, including the effect of ion size. The lowest energy was found for Pu4+Zr substitutions. The lowest energy for Pu3+ substitutions was found for the defect cluster 2Pu3+′Zr+VO••. Mean field calculations of unit-cell volumes for 8% Pu substitutions were in agreement with X-ray diffraction (XRD) data.

Plutonium and neptunium incorporation in zirconolite

The incorporation of Pu and Np in zirconolite (CaZrTi 2 O 7 ) has been investigated over a range of redox conditions. Zirconolite formulations designed to favour either trivalent or tetravalent Pu and Np were prepared by limiting the amount of charge compensating additives available to maintain electroneutrality. From near-edge X-ray absorption spectroscopy the Pu valence state was found to vary with the processing atmosphere, from completely tetravalent when fired in air, and located on either the Ca or Zr sites, to trivalent, when substituted on the Ca site after annealing in 3.5% H 2 /N 2 . Np was predominantly tetravalent over the range of redox conditions examined and was readily incorporated on either of zirconolites Ca or Zr sites. The charge compensation mechanisms at work in different zirconolites are also discussed.

Zirconolite-rich titanate ceramics for high-level actinide wastes

New diffraction and microstructural data on the incorporation of rare earths, Np and Pu in nominally single-phase zirconolite (CaZrTi{sub 2}O{sub 7}) are presented. Such data are vital for the crystal-chemical design of zirconolite-rich titanate ceramics of the Synroc family, for the immobilization of up to 30 wt% of actinide wastes from advanced reprocessing/partitioning schemes. Some trial zirconolite-rich ceramics were made with U and Nd simulating tetravalent and trivalent actinides respectively. Some preliminary results to examine chemical flexibility with regards to variations in waste/precursor are given, for different actinide waste compositions.

Near-edge x-ray absorption fine-structure study of ion-beam-induced phase transformation in Gd2(Ti1−yZry)2O7

The structural and electronic properties of Gd2(Ti1−yZry)2O7 (y=0–1) pyrochlores following a 2.0-MeV Au2+ ion-beam irradiation (∼5.0×1014Au2+∕cm2) have been investigated by Ti2p and O1s near-edge x-ray absorption fine structure (NEXAFS). The irradiation of Gd2(Ti1−yZry)2O7 leads to the phase transformation from the ordered pyrochlore structure (Fd3m) to the defect fluorite structure (Fm3m) regardless of Zr concentration. Irradiated Gd2(Ti1−yZry)2O7 with y⩽0.5 are amorphous, although significant short-range order is present. Contrasting to this behavior, compositions with y⩾0.75 retain crystallinity in the defect fluorite structure following irradiation. The local structures of Zr4+ in the irradiated Gd2(Ti1−yZry)2O7 with y⩾0.75 determined by NEXAFS are the same as in the cubic fluorite-structured yttria-stabilized zirconia (Y–ZrO2), thereby providing conclusive evidence for the phase transformation. The TiO6 octahedra present in Gd2(Ti1−yZry)2O7 are completely modified by ion-beam irradiation to TiOx poly...

Further Studies of Synroc Immobilization of HLW Sludges and Tc for Hanford Tank Waste Remediation

Synroc/glass composites were designed for simulated Hanford HLW sludges containing U (the current “All-Blend” formulation). The composite contained ∼ 50 wt% of simulated HLW (oxide equivalent), to which ∼ 6, 10, 10, and 24 wt% of CaO, Al 2 O 3 , TiO 2 and SiO 2 were added, and melted under an argon atmosphere at 1350°C. The phase assemblage consisted of zirconolite, perovskite, spinel, nepheline, whitlockite and glass as major phases. Seven-day PCT tests yielded values of 2 for all elements studied. The PCT results were tolerant to changes of ∼ 20% of the inventories of the additives, and to variations in redox conditions. Technetium separated out during decontamination of liquid Hanford wastes can be incorporated as metal in hot-pressed Synroc prepared under reducing conditions, and its leach resistance is good (∼ 10 ∼3 g/m 2 /day at 70°C), and can be improved by alloying with iron group metals. With a choice of “neutral” (P(O 2 ) ∼ 10 −4 atm., near the Ni/NiO buffer) hot-pressing conditions, Tc can also be incorporated as Tc 4+ , substituting for Ti 4+ in the ceramic phases, and in this form, it should be highly leach resistant also.