Underpinning the use of indium as a neutron absorbing additive in zirconolite by X-ray absorption spectroscopy.

Indium (In) is a neutron absorbing additive that could feasibly be used to mitigate criticality in ceramic wasteforms containing Pu in the immobilised form, for which zirconolite (nominally CaZrTi2O7) is a candidate host phase. Herein, the solid solutions Ca1-xZr1-xIn2xTi2O7 (0.10 ≤ x ≤ 1.00; air synthesis) and Ca1-xUxZrTi2-2xIn2xO7 (x = 0.05, 0.10; air and argon synthesis) were investigated by conventional solid state sintering at a temperature of 1350 °C maintained for 20 h, with a view to characterise In3+ substitution behaviour in the zirconolite phase across the Ca2+, Zr4+ and Ti4+ sites. When targeting Ca1-xZr1-xIn2xTi2O7, single phase zirconolite-2M was formed at In concentrations of 0.10 ≤ x ≤ 0.20; beyond x ≥ 0.20, a number of secondary In-containing phases were stabilised. Zirconolite-2M remained a constituent of the phase assemblage up to a concentration of x = 0.80, albeit at relatively low concentration beyond x ≥ 0.40. It was not possible to synthesise the In2Ti2O7 end member compound using a solid state route. Analysis of the In K-edge XANES spectra in the single phase zirconolite-2M compounds confirmed that the In inventory was speciated as trivalent In3+, consistent with targeted oxidation state. However, fitting of the EXAFS region using the zirconolite-2M structural model was consistent with In3+ cations accommodated within the Ti4+ site, contrary to the targeted substitution scheme. When deploying U as a surrogate for immobilised Pu in the Ca1-xUxZrTi2-2xIn2xO7 solid solution, it was demonstrated that, for both x = 0.05 and 0.10, In3+ was successfully able to stabilise zirconolite-2M when U was distributed predominantly as both U4+ and average U5+, when synthesised under argon and air, respectively, determined by U L3-edge XANES analysis.

Phase Evolution in the CaZrTi2O7-Dy2Ti2O7 System: A Potential Host Phase for Minor Actinide Immobilization.

Zirconolite is considered to be a suitable wasteform material for the immobilization of Pu and other minor actinide species produced through advanced nuclear separations. Here, we present a comprehensive investigation of Dy3+ incorporation within the self-charge balancing zirconolite Ca1-xZr1-xDy2xTi2O7 solid solution, with the view to simulate trivalent minor actinide immobilization. Compositions in the substitution range 0.10 ≤ x ≤ 1.00 (Δx = 0.10) were fabricated by a conventional mixed oxide synthesis, with a two-step sintering regime at 1400 °C in air for 48 h. Three distinct coexisting phase fields were identified, with single-phase zirconolite-2M identified only for x = 0.10. A structural transformation from zirconolite-2M to zirconolite-4M occurred in the range 0.20 ≤ x ≤ 0.30, while a mixed-phase assemblage of zirconolite-4M and cubic pyrochlore was evident at Dy concentrations 0.40 ≤ x ≤ 0.50. Compositions for which x ≥ 0.60 were consistent with single-phase pyrochlore. The formation of zirconolite-4M and pyrochlore polytype phases, with increasing Dy content, was confirmed by high-resolution transmission electron microscopy, coupled with selected area electron diffraction. Analysis of the Dy L3-edge XANES region confirmed that Dy was present uniformly as Dy3+, remaining analogous to Am3+. Fitting of the EXAFS region was consistent with Dy3+ cations distributed across both Ca2+ and Zr4+ sites in both zirconolite-2M and 4M, in agreement with the targeted self-compensating substitution scheme, whereas Dy3+ was 8-fold coordinated in the pyrochlore structure. The observed phase fields were contextualized within the existing literature, demonstrating that phase transitions in CaZrTi2O7-REE3+Ti2O7 binary solid solutions are fundamentally controlled by the ratio of ionic radius of REE3+ cations.

Solubility, speciation and local environment of chlorine in zirconolite glass-ceramics for the immobilisation of plutonium residues.

The immobilisation and disposal of fissile materials from civil and defence nuclear programmes requires compatible, passively safe and proliferation resistant wasteforms. In this study, we demonstrate the application of an albite glass-zirconolite ceramic material for immobilisation of chloride contaminated plutonium oxide residues in the United Kingdom. The chlorine solubility limit in the albite glass phase was determined to be 1.0 ± 0.1 wt%, above the maximum envisaged chorine inventory of 0.5 wt%, attainable at a 20 wt% PuO2 incorporation rate within the ceramic. Cl K-edge of X-ray Absorption Near Edge Spectroscopy (XANES) was exploited to confirm partitioning of Cl to the glass phase, speciated as the chloride anion, with exsolution of crystalline NaCl above the chlorine solubility limit. Combinatorial fitting of Cl XANES data, utilising a library of chemically plausible reference spectra, demonstrated the association of Cl with Na and Ca modifier cations, with environments characteristic of the aluminosilicate chloride minerals eudialyte, sodalite, chlorellestadite and afghanite. Adventitious incorporation of Ca, Zr and Ti within the albite glass phase apparently assists chlorine solubility, by templating a local chemical environment characteristic of the mineral reference compounds. The partitioning of Ce, as a Pu analogue, within the glass-ceramic was not adversely impacted by incorporation of Cl. The significance of this research is in demonstrating the compatibility of the glass-ceramic wasteform toward Cl solubility at the expected incorporation rate, below the determined solubility limit. Thus, an upstream heat treatment facility to remove chloride contamination, as specified in the current conceptual flowsheet, would not be required from the perspective of wasteform compatibility, thus providing scope to de-risk the technology roadmap and reduce the projected capital and operational plant costs.