Influence of accessory phases and surrogate type on accelerated leaching of zirconolite wasteforms

Blackburn, Lewis R.; Crawford, Rachel; Walling, Sam A.; Gardner, Laura J.; Cole, Max R.; Sun, Shi‐Kuan; Gausse, Clémence; Mason, Amber R.; Stennett, Martin C.; Maddrell, Ewan R.; Hyatt, Neil C.; Corkhill, Claire L.

doi:10.1038/s41529-021-00171-8

Cited by 13 publications

(18 citation statements)

References 31 publications

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“…As with HIP-SS, a complete incorporation of plutonium within the ceramic was confirmed as no bright regions indicative of unreacted PuO 27 as well as for mixed U 4+/5+ have been measured during the HIP consolidation of U-bearing zirconolites and pyrochlore, 13,25 the valence state being influenced by several aspects such as crystal chemistry, charge compensation mechanisms, and consolidation conditions. It was observed that the majority of the actinides were incorporated within the pyrochlore structure (An = 0.81 f.u.)…”

Section: Hip-ni Compositionmentioning

confidence: 75%

“…The elemental analysis suggests that at least part of the U and/or Pu is present in higher oxidation state than U 4+ and Pu 3+ (again assuming Ca 2+ , Gd 3+ , Th 4+ , Zr 4+ , Ti 4+ , Al 3+ , and Ni 2+ ). It is noted that experimental evidence for U 4+ , 27 as well as for mixed U 4+/5+ have been measured during the HIP consolidation of U‐bearing zirconolites and pyrochlore, 13,25 the valence state being influenced by several aspects such as crystal chemistry, charge compensation mechanisms, and consolidation conditions. It was observed that the majority of the actinides were incorporated within the pyrochlore structure (An = 0.81 f.u.)…”

Section: Resultsmentioning

confidence: 99%

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Profiling hot isostatically pressed canister–wasteform interaction for Pu‐bearing zirconolite‐rich wasteforms

et al. 2022

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Zirconolite‐rich full ceramic wasteforms designed to immobilize Pu‐bearing wastes were produced via hot isostatic pressing (HIP) using stainless steel (SS) and nickel (Ni) HIP canisters. A detailed profiling of the elemental compositions of the major and minor phases over the canister–wasteform interaction zone was performed using scanning electron microscopy combined with energy‐dispersive X‐ray spectroscopy (SEM‐EDS) characterization. Bulk sample analyses from regions near the center of the HIP canister were also conducted for both samples using X‐ray diffraction and SEM‐EDS. The sample with the Ni HIP canister showed almost no interaction zone with only minor diffusion of Ni from the inner wall of the canister into the near‐surface region of the wasteform. The sample with the SS HIP canister showed ∼100–120 μm of interaction zone dominated by high‐temperature Cr diffusion from canister materials to the wasteform with the Cr predominantly incorporated into the durable zirconolite phase. We also examined, for the first time, changes to the HIP canister wall thickness caused by HIPing and demonstrated that no canister wall thinning occurred. Instead, in the areas examined, the canister wall thickness was observed to increase (up to ∼20%) due to the compression occurring during the HIP cycle. Further, only sparse formation of (Cr, Mn)‐rich oxide particles were noted within the HIP canister inner wall area immediately adjacent to the ceramic material, with no evidence for reverse diffusion of ceramic materials. Though the HIP canister–wasteform interaction extends to ∼120 μm when using an SS HIP canister for the system investigated, this translates to <<1 vol.% for an industrial scale HIPed wasteform. Importantly, the HIP canister–wasteform interactions did not produce any obviously less durable phases in the wasteform or had any detrimental impact on the HIP canister properties.

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Section: Hip-ni Compositionmentioning

confidence: 75%

Section: Resultsmentioning

confidence: 99%

Profiling hot isostatically pressed canister–wasteform interaction for Pu‐bearing zirconolite‐rich wasteforms

et al. 2022

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Section: Introductionmentioning

confidence: 99%

“…Zirconolite structured materials have been widely studied for the immobilization of actinide-rich radioactive waste streams, due to excellent chemical alteration resistance and radiation tolerance. − This includes use as a major constituent of the various SYNROC assemblages for the disposition of high-level actinide-rich wastes derived from nuclear fuel reprocessing. , The CaZrTi 2 O 7 parent structure has been shown to accommodate U, Pu, Np, and Cm and is therefore a suitable host matrix for minor actinide (MA) species such as Am. − Natural analogue specimens have also been shown to retain ∼20 wt % U/Th over geological timescales . The parent structure ABC 2 O 7 is derivative of the anion deficient fluorite structure type and is closely related to the pyrochlore A 2 B 2 O 7 family of minerals.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2022

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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 Dy 3+ incorporation within the self-charge balancing zirconolite Ca 1– x Zr 1– x Dy 2 x Ti 2 O 7 solid solution, with the view to simulate trivalent minor actinide immobilization. Compositions in the substitution range 0.10 ≤ x ≤ 1.00 (Δ x = 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 L 3 -edge XANES region confirmed that Dy was present uniformly as Dy 3+ , remaining analogous to Am 3+ . Fitting of the EXAFS region was consistent with Dy 3+ cations distributed across both Ca 2+ and Zr 4+ sites in both zirconolite-2M and 4M, in agreement with the targeted self-compensating substitution scheme, whereas Dy 3+ was 8-fold coordinated in the pyrochlore structure. The observed phase fields were contextualized within the existing literature, demonstrating that phase transitions in CaZrTi 2 O 7 –REE 3+ Ti 2 O 7 binary solid solutions are fundamentally controlled by the ratio of ionic radius of REE 3+ cations.

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“…Ce 3+ can be readily incorporated into a Ce-bearing perovskite phase, and with the exception of sample A, the samples with the highest perovskite concentration were clearly correlated with the highest proportion of Ce 3+ . Precluding the formation of additional perovskite phases is beneficial, as this phase is known to lower the overall durability of zirconolite-rich waste form compositions when present, even in low quantities [28]. As the XANES technique is probing the global Ce oxidation state, some of the signal will represent Ce present in Ce-bearing perovskite and unincorporated CeO 2 (demonstrated in the SEM image of sample B), which should be taken into account when assessing the data.…”

Section: Oxidation State and Local Environment Of Cementioning

confidence: 99%

Investigation of the Effect of Milling Duration on a Ce-Gd Doped Zirconolite Phase Assemblage Synthesised by Hot Isostatic Pressing

et al. 2023

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Zirconolite is a candidate ceramic wasteform under consideration for the immobilisation of the UK civil PuO2 inventory. In the present work, a baseline dual-substituted zirconolite with the target composition (Ca0.783Gd0.017Ce0.2)(Zr0.883Gd0.017Ce0.1)(Ti1.6Al0.4)O7 was fabricated by hot isostatic pressing (HIPing). In order to optimise the microstructure properties and improve the obtained yield of the zirconolite phase, a range of planetary ball milling parameters were investigated prior to consolidation by HIP. This included milling the batched oxide precursors at 400 rpm for up to 120 min, the pre-milling of CeO2 (PuO2 surrogate) to reduce the particle size and using a CeO2 source with finer particle size (<5 µm). The HIPed zirconolite product consisted of both zirconolite-2M and zirconolite-3T polytypes in varying proportions; however, an additional perovskite phase was obtained in varying quantities as a secondary phase. Ce L3-edge X-ray absorption spectroscopy was utilised to determine the Ce oxidation state. In this study, the ideal milling parameter for the fabrication of zirconolite waste forms was defined as 60 min at 400 rpm.

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Influence of accessory phases and surrogate type on accelerated leaching of zirconolite wasteforms

Cited by 13 publications

References 31 publications

Profiling hot isostatically pressed canister–wasteform interaction for Pu‐bearing zirconolite‐rich wasteforms

Profiling hot isostatically pressed canister–wasteform interaction for Pu‐bearing zirconolite‐rich wasteforms

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

Investigation of the Effect of Milling Duration on a Ce-Gd Doped Zirconolite Phase Assemblage Synthesised by Hot Isostatic Pressing

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Influence of accessory phases and surrogate type on accelerated leaching of zirconolite wasteforms

Cited by 13 publications

References 31 publications

Profiling hot isostatically pressed canister–wasteform interaction for Pu‐bearing zirconolite‐rich wasteforms

Profiling hot isostatically pressed canister–wasteform interaction for Pu‐bearing zirconolite‐rich wasteforms

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

Investigation of the Effect of Milling Duration on a Ce-Gd Doped Zirconolite Phase Assemblage Synthesised by Hot Isostatic Pressing

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