Review of zirconolite crystal chemistry and aqueous durability

Abstract

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.

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

“…This seems reasonable given the Ti site in zirconolite can accommodate cations with radii ranging from 0.0535 to 0.083 nm and the B site in pyrochlore can similarly accommodate cations with radii ranging from 0.060 to 0.083 nm 35 . Further, an extensive solubility of Mg 2+ , Al 3+ , Ti 3+ , Fe 3+ , and Nb 5+ species within the Ti 4+ site of zirconolite has also been demonstrated 20 .…”

“…Zirconolite (general formula, CaZrTi 2 O 7 ) is one of the key mineral phases in the original titanate‐based Synroc formulation and has long been accepted as a radiation‐tolerant and chemically durable phase to host actinides, such as U, Np, and Pu 7,18–20 . In zirconolite, Ca‐ and Zr cations are eight‐ and sevenfold coordinated, respectively.…”

“…In zirconolite, Ca‐ and Zr cations are eight‐ and sevenfold coordinated, respectively. There are three unique Ti coordination polyhedra, encompassing two fully occupied octahedral sites (TiO 6 polyhedra) and 0.5 occupancies at trigonal bipyramidal (TiO 5 ) site 20 . Trivalent and tetravalent early actinide ions can be incorporated on either the Ca or Zr site 4 .…”

“…The ideal zirconolite comprises planes of corner and edge sharing CaO 8 and ZrO 7 polyhedra, interleaved by hexagonal tungsten bronze (HTB) type Ti–O layers along (001) and is referred to as zirconolite‐2M. The zirconolite polytypes are then characterized by the variation in a stacking sequence of adjacent Ca/Zr and HTB‐type Ti–O layers 20 . A cubic pyrochlore structure is produced when high levels of actinide (such as Pu) and REE are incorporated 21,22 .…”

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

“…The ideal zirconolite unit cell is composed of planes of corner-sharing CaO 8 and ZrO 7 polyhedra, interleaved by hexagonal tungsten bronze (HTB)-style motifs along the [001] direction. 13 Ti 4+ cations are distributed across three distinct sites in the HTB plane: two Ti 4+ sites are arranged in TiO 6 octahedra and one Ti 4+ is 5-fold coordinated in a 50% statistically coordinated trigonal bipyramidal site. In this idealized structure description, cation and HTB layers are integrated 1:1 along [001], related by a 180° rotation along the c * axis.…”