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Review of Zirconolite Crystal Chemistry and Aqueous Durability This is a repository copy of Review of zirconolite crystal chemistry and aqueous durability. White Rose Research Online URL for this paper: https://eprints.whiterose.ac.uk/174554/ Version: Published Version Article: Blackburn, L.R. orcid.org/0000-0002-5889-2035, Bailey, D.J. orcid.org/0000-0002-0313- 8748, Sun, S.K. et al. (4 more authors) (2021) Review of zirconolite crystal chemistry and aqueous durability. Advances in Applied Ceramics, 120 (2). pp. 69-83. ISSN 1743-6753 https://doi.org/10.1080/17436753.2021.1877596 Reuse This article is distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs (CC BY-NC-ND) licence. This licence only allows you to download this work and share it with others as long as you credit the authors, but you can’t change the article in any way or use it commercially. More information and the full terms of the licence here: https://creativecommons.org/licenses/ Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing [email protected] including the URL of the record and the reason for the withdrawal request. [email protected] https://eprints.whiterose.ac.uk/ Advances in Applied Ceramics Structural, Functional and Bioceramics ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/yaac20 Review of zirconolite crystal chemistry and aqueous durability Lewis R. Blackburn, Daniel J. Bailey, Shi-Kuan Sun, Laura J. Gardner, Martin C. Stennett, Claire L. Corkhill & Neil C. Hyatt To cite this article: Lewis R. Blackburn, Daniel J. Bailey, Shi-Kuan Sun, Laura J. Gardner, Martin C. Stennett, Claire L. Corkhill & Neil C. Hyatt (2021) Review of zirconolite crystal chemistry and aqueous durability, Advances in Applied Ceramics, 120:2, 69-83, DOI: 10.1080/17436753.2021.1877596 To link to this article: https://doi.org/10.1080/17436753.2021.1877596 © 2021 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group Published online: 05 May 2021. Submit your article to this journal Article views: 48 View related articles View Crossmark data Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=yaac20 ADVANCES IN APPLIED CERAMICS 2021, VOL. 120, NO. 2, 69–83 https://doi.org/10.1080/17436753.2021.1877596 REVIEW Review of zirconolite crystal chemistry and aqueous durability Lewis R. Blackburn , Daniel J. Bailey, Shi-Kuan Sun, Laura J. Gardner , Martin C. Stennett, Claire L. Corkhill and Neil C. Hyatt Immobilisation Science Laboratory, Department of Materials Science and Engineering, University of Sheffield, Sheffield, UK ABSTRACT ARTICLE HISTORY Zirconolite (CaZrTi2O7) has been identified as a candidate ceramic wasteform for the Received 18 November 2020 immobilisation and disposal of Pu inventories, for which there is no foreseen future use. Revised 16 December 2020 Here, we provide an overview of relevant zirconolite solid solution chemistry with respect to Accepted 8 January 2021 Ce, U and Pu incorporation, alongside a summary of the available literature on zirconolite KEYWORDS aqueous durability. The zirconolite phase may accommodate a wide variety of tri- and Wasteform; zirconolite; tetravalent actinide and rare-earth dopants through isovalent and heterovalent solid ceramics; plutonium; solution, e.g. CaZr1–xPuxTi2O7 or Ca1–xPuxZrTi2–2xFe2xO7. The progressive incorporation of immobilisation 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. Introduction corrosion products (Mn, Ni, Cr), and entrained U/Pu. This effluent is referred to as high level liquid The resurgence of nuclear power, as a driver towards waste (HLLW) and is stored on the Sellafield site, cleaner energy production, will necessitate the before blending and calcination, prior thermal con- implementation of advanced spent fuel management ditioning. The HLLW remains highly radioactive due fl 237 strategy, and development of advanced nuclear to the long hal ife of certain elements (e.g. t1/2 Np 6 129 7 materials capable of safely conditioning highly radio- = 2.1 × 10 y, t1/2 I = 1.57 × 10 y). The current base- active waste [1–3]. After nuclear fuel is removed line thermal treatment for HLLW is vitrification in from a reactor, nation states have the option to chemi- alkali borosilicate glass. In the vitrification process, cally recover a significant portion of the fissile inven- HLLW is calcined and melted with glass forming addi- tory, or treat the fuel as waste for disposal. These fuel tives, allowing complete dissolution of waste species cycle options are considered closed or open respect- into the vitrified network via incorporation into the ively; the unit operations associated with these are glass forming structure; incorporation as network illustrated in Figure 1. For many years the U.K. has modifiers; and incorporation by encapsulation [4]. operated a closed fuel cycle, in which a PUREX (pluto- Although borosilicate glasses can incorporate a wide nium-uranium-reduction-extraction) reprocessing variety of elements, and vitrification is a well-estab- step is implemented, with the primary motive to lished process that remains relatively insensitive to recover U/Pu from spent fuel. In the PUREX process, variations in feedstock chemistry, it is not the optimal nuclear fuel pins are stripped of cladding and dissolved choice for waste streams consisting of high actinide in 9M HNO3; the aqueous nitric solution is then con- fractions, such as waste PuO2. Actinides have exhibited tacted with tri-butylphosphate (TBP). U6+ and Pu4+ low solubility in borosilicate glass matrices, alongside form TBP complexes and are extracted to the organic leach rates that are considerably higher than alternate phase; U6+ and Pu4+ are converted to oxides and cal- wasteforms such as crystalline ceramics. The Pu4+ cined before storage. The remaining aqueous nitrate solubility in the French R7T7 glass has been limited solution is comprised predominantly of high fission at around 1.5 wt-% [5]. Several notable publications products and metalloids (Cs, I, Sr, As, Nd, Pd, Pr, have indicated the solubility of plutonium can be Eu, La, Gd), minor actinides (Cm, Am, Np, Th), increased to 4 wt-% when reduced to the Pu(III) CONTACT Lewis R. Blackburn lewis.blackburn@sheffield.ac.uk Immobilisation Science Laboratory, Department of Materials Science and Engin- eering, University of Sheffield, Sir Robert Hadfield Building, Mappin Street, S1 3JD, UK; Neil C. Hyatt n.c.hyatt@sheffield.ac.uk Immobilisation Science Laboratory, Department of Materials Science and Engineering, University of Sheffield, Sir Robert Hadfield Building, Mappin Street, S1 3JD, UK © 2021 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by- nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or built upon in any way. 70 L. R. BLACKBURN ET AL. Figure 1. Illustration of unit operations associated with open and closed nuclear fuel cycles. Orange arrows indicate closed fuel cycle operations, while blue arrows represent those for an open fuel cycle (© IOP Publishing. Reproduced with permission. All rights reserved. [10]). species [5–7]. A series of borosilicate glasses containing waste based on ceramic systems [9]. The SYNROC for- 1 wt-% PuO2 were fabricated Wellman et al. in order to mulation comprises an assemblage of chemically dur- elucidate the effect of self-irradiation on the elemental able titanate crystalline phases (zirconolite, dissolution of the glass phase. Although it was deter- hollandite, perovskite, pyrochlore), based on natural mined that the release rate of Pu into the extraction mineral hosts that have demonstrated resistance to phase was insensitive to dose rate (measured by weathering on geological timescales; these can act as 238Pu/239Pu ratio), temperature, and pH, was of the dedicated hosts for specific elements via accommo- order 10−3 gm−2d−1 [8]. The development of SYNROC dation in specific lattice sites in the host phase, provid- technology (synthetic-rock) in the 1980s has led to ing a marked increase in solubility and chemical development of alternative wasteforms for nuclear durability. The host phase for actinides is zirconolite – nominally CaZrTi2O7. The aim of this work is to pro- vide an extensive literature review into the suitability of the zirconolite phase as a host for PuO2. An assessment of the current UK situation regarding Pu will first be outlined, followed by a critical assessment of zircono- lite as a ceramic host phase for Pu. Actinide immobilisation in ceramic materials Owing to their relatively long half-lives and radiotoxi- city, radionuclides must be separated from the bio- sphere and permanently disposed. The current intended disposal route for many countries
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