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Zirconolite (CaZrTi 2 O 7 ) has been identified as a candidate ceramic wasteform for the immobilisation and disposal of Pu inventories, for which there is no foreseen future use. Here, we provide an overview of relevant zirconolite solid solution chemistry with respect to Ce, U and Pu incorporation, alongside a summary of the available literature on zirconolite aqueous durability. The zirconolite phase may accommodate a wide variety of tri-and tetravalent actinide and rare-earth dopants through isovalent and heterovalent solid solution, e.g. CaZr 1-x Pu x Ti 2 O 7 or Ca 1-x Pu x ZrTi 2-2x Fe 2x O 7 . The progressive incorporation of actinides within the zirconolite-2M parent structure is accommodated through the formation of zirconolite polytypoids, such as zirconolite-4M or 3T, depending on the choice of substitution regime and processing route. A variety of standardised durability tests have demonstrated that the zirconolite phase exhibits exceptional chemical durability, with release rates of constituent elements typically <10 −5 gm −2 •d −1 . Further work is required to understand the extent to which polytype formation and surrogate choice influence the dissolution behaviour of zirconolite wasteforms.

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A fraction of the UK Pu inventory may be immobilised in a zirconolite ceramic matrix prior to disposal. Two zirconolite compositions, targeting CaZr0.80Ce0.20Ti2O7 and CaZr0.80U0.20Ti2O7, were fabricated by hot isostatic pressing, alongside a reformulated composition, nominally Ca0.80Zr0.90Ce0.30Ti1.60Al0.40O7, with an excess of Ti and Zr added to preclude the formation of an accessory perovskite phase. Materials were subjected to accelerated leaching in a variety of acidic and alkaline media at 90 °C, over a cumulative period of 14 d. The greatest Ce release was measured from CaZr0.80Ce0.20Ti2.00O7 exposed to 1 M H2SO4, for which 14.7 ± 0.2% of the original Ce inventory was released from the wasteform into solution. The extent of Ce leaching into the solution was correlated with the quantity of perovskite present in the wasteform, and associated with the incorporation and preferential dissolution of Ce3+. CaZr0.80U0.20Ti2.00O7 exhibited improved leach resistance relative to CaZr0.80Ce0.20Ti2.00O7, attributed to the decreased proportion of accessory perovskite, with 7.1 ± 0.1% U released to in 8 M HNO3 after 7 d. The Ca0.80Zr0.90Ce0.30Ti1.60Al0.40O7 composition, with no accessory perovskite phase, presented significantly improved leaching characteristics, with < 0.4%Ce released in both 8 M HNO3 and 1 M H2SO4. These data demonstrate the need for careful compositional design for zirconolite wasteforms with regard to accessory phase formation and surrogate choice.

The United Kingdom is in the process of consolidating all civil Pu inventories at the Sellafield site, in an effort to satisfy security and proliferation requirements, presenting a significant decommissioning challenge in terms of materials degradation. 1 The Nuclear Decommissioning Authority (NDA) is liable for the management and disposition of all nuclear wastes under UK safeguards, and has commissioned a credible options analysis to identify technically mature reuse and disposition options, one of which is a strategy of immobilization and disposal. 2 Zirconolite (nominally CaZrTi 2 O 7 ) is the proposed candidate ceramic wasteform for the immobilization of Pu oxides. [3][4][5][6][7] The zirconolite-2M parent structure is derived from the anion-deficient fluorite unit cell, and is closely related to the A 2 B 2 O 7 pyrochlore structure (Fd-3m) by a compression along the (111) direction. Furthermore, a range of polytypical zirconolite structures (e.g., 4M, 3O, 3T) have been reported, the formation of which is controlled by cation substitution and processing conditions. [8][9][10] The zirconolite-2M unit cell is considered the archetype, with observed stability over the compositional range CaZr x Ti 3-x Ti 2 O 7 (0.8 < x < 1.3), with a lamellar unit cell crystallizing in the space group C2/c. 11 The nomenclature "2M" is a