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The Thermal Decomposition of Studtite: Analysis of the Amorphous Phase

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The Thermal Decomposition of Studtite: Analysis of the Amorphous Phase This is a repository copy of The thermal decomposition of studtite: analysis of the amorphous phase. White Rose Research Online URL for this paper: http://eprints.whiterose.ac.uk/173163/ Version: Published Version Article: Thompson, N.B.A., Frankland, V.L., Bright, J.W.G. et al. (4 more authors) (2021) The thermal decomposition of studtite: analysis of the amorphous phase. Journal of Radioanalytical and Nuclear Chemistry, 327 (3). pp. 1335-1347. ISSN 0236-5731 https://doi.org/10.1007/s10967-021-07611-4 Reuse This article is distributed under the terms of the Creative Commons Attribution (CC BY) licence. This licence allows you to distribute, remix, tweak, and build upon the work, even commercially, as long as you credit the authors for the original work. 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/ Journal of Radioanalytical and Nuclear Chemistry (2021) 327:1335–1347 https://doi.org/10.1007/s10967-021-07611-4 The thermal decomposition of studtite: analysis of the amorphous phase Nathan B. A. Thompson1 · Victoria L. Frankland2 · Joshua W. G. Bright2 · David Read2,3 · Matthew R. Gilbert4 · Martin C. Stennett1 · Neil C. Hyatt1 Received: 29 November 2020 / Accepted: 15 January 2021 / Published online: 15 February 2021 © The Author(s) 2021 Abstract Studtite is known to exist at the back-end of the nuclear fuel cycle as an intermediate phase formed in the reprocessing of spent nuclear fuel. In the thermal decomposition of studtite, an amorphous phase is obtained at calcination temperatures between 200 and 500 °C. This amorphous compound, referred to elsewhere in the literature as U2O7, has been characterised by analytical spectroscopic methods. The local structure of the amorphous compound has been found to contain uranyl bonding by X-ray absorption near edge (XANES), Fourier transform infrared and Raman spectroscopy. Changes in bond distances in the uranyl group are discussed with respect to studtite calcination temperature. The reaction of the amorphous compound with water to form metaschoepite is also discussed and compared with the structure of schoepite and metasch- oepite by X-ray diffraction. A novel schematic reaction mechanism for the thermal decomposition of studtite is proposed. Keywords Studtite · Thermal decomposition · Spectroscopy · Uranium oxide Introduction In the reprocessing of spent nuclear fuel (SNF), ura- nium can be dissolved as uranyl nitrate hexahydrate * Nathan B. A. Thompson (UO2(NO3)2·6H2O, UNH), and precipitated with hydro- [email protected]; gen peroxide (H 2O2) to form uranyl peroxide tetrahydrate, [email protected] also known by the mineral name, studtite ([(UO2)(O2) Victoria L. Frankland (H2O)2]·2H2O) (Fig. 1). It has previously been shown that [email protected] studtite dehydrates into uranyl peroxide dihydrate (metas- Joshua W. G. Bright tudtite; [(UO2)(O2)(H2O)2]·2H2O) when heated to 100 °C [email protected] [1–4]. By 200 °C, metastudtite decomposes into an amor- David Read phous phase before it is calcined further to α-UO3 (Fig. 1, [email protected] right) and α-U3O8 at temperatures above 500 °C [1–5]. Matthew R. Gilbert X-ray diffraction (XRD) analysis has previously [email protected] revealed that studtite consists of chains of edge-sharing Martin C. Stennett hexagonal bipyramids containing two peroxide groups [email protected] (O–O; Fig. 1, left) [6]. Both O atoms in the two perox- Neil C. Hyatt ide groups are bonded to the central uranium atom with [email protected] the remaining two vertices in the bipyramid occupied 1 Department of Materials Science and Engineering, by water molecules. A further two water molecules are University of Sheffield, Sheffield S10 2TN, UK located between the chains; it is the loss of these water 2 School of Chemistry and Chemical Engineering, University molecules that results in the formation of metastudtite. of Surrey, Guildford GU2 7XH, UK [1–4] In addition to its occurrence in spent fuel repro- 3 National Physical Laboratory, Teddington TW11 0LW, UK cessing, both studtite and metastudtite are formed in nat- ural uranium deposits and during nuclear waste storage 4 AWE, Aldermaston, Reading RG7 4PR, UK Vol.:(0123456789)1 3 1336 Journal of Radioanalytical and Nuclear Chemistry (2021) 327:1335–1347 Studte α-UO3 Fig. 1 Schematic image of the crystal structures for studtite, ([(UO2) U, O and H are shown in blue, red and white, respectively. [8, 9] (O2)(H2O)2]·2H2O), left) and α-uranium trioxide (α-UO3, right). Both Water O atoms are light blue are viewed along the b-axis, perpendicular to the c-axis. The elements through the build-up of hydrogen peroxide produced by The mechanism by which studtite decomposes to form α-radiolysis of water [7]. XRD analysis of α-UO3 reveals the amorphous phase has not yet been reported. Given its that this species is formed of hexagonal sheets bonded existence as an intermediate in the thermal decomposition together by axial O-atoms (Fig. 1, right) [8, 9]. of studtite, it is important that the compound is well charac- The thermal decomposition of studtite has been character- terised in order to understand the mechanism for transforma- ised by the use of spectroscopic techniques including XRD, tion of studtite to α-UO3. This paper aims to characterise the Fourier transform infrared (FTIR) and Raman spectros- amorphous phase spectroscopically, within the wider context copy [1, 2]; however, the amorphous phase has only briefly of the thermal decomposition of studtite and its products of been alluded to in the literature [2]. In addition to methods thermal decomposition. employed previously, this study makes use of X-ray absorp- tion near edge spectroscopy (XANES), Raman and fluores- cence spectroscopy, to build on current understanding of the Experimental methods local structure and bonding in the amorphous compound. Some recent studies have addressed the characterisation of Sample preparation this material, reporting it either to be stoichiometric U 2O7 or an amorphous UO3 hydrate. Both computational and exper- Studtite was precipitated by reaction of uranyl nitrate imental efforts have aided in elucidating the short-range hexahydrate (BDH Chemicals, 0.5 M) with hydrogen structure, although there has been little agreement on either peroxide (Sigma Aldrich® reagent grade, 30 w/w%). its structure or stoichiometry [2, 10, 11]. It has been dem- The precipitate was filtered under reduced pressure and onstrated by density functional theory (DFT) and neutron allowed to dry in air for 48 h, before being heated to 100, scattering analysis that the compound may contain uranyl 150, 250, 350, 450, 500, 550, 650 and 1050 °C in a nitro- 2+ −1 cation units (UO 2 ) and peroxide groups (O–O), although gen atmosphere at a rate of 10 °C min . A Netszch TG further exploration of the amorphous structure is required. 449 F3 Jupiter simultaneous thermal analyser was used As the material is X-ray amorphous, the long-range structure for accuracy of heating temperature (± 0.01 °C). Studtite of this material is difficult to determine, and is highly likely and metastudtite were pale yellow in colour, previously to be disordered. assigned by objective colour analysis as Pantone™ P 8–9 1 3 Journal of Radioanalytical and Nuclear Chemistry (2021) 327:1335–1347 1337 C and P 19–9 C, respectively [12]. The amorphous phase Fourier transform infrared spectroscopy samples (250–500 °C) were bright orange (250 °C P24-13 U; 350 °C P17-16 U; 450 °C 7572 XGC; 500 °C P25-6 Fourier transform infrared spectroscopy (FTIR) was per- C), while α-UO3 was brown (550 °C 18–0521 TCX). The formed using a Perkin Elmer Frontier FTIR spectrometer sample phases were confirmed by XRD (Supplementary with an attenuated total reflectance (ATR) crystal. Samples Information, Figure S1). were prepared with potassium bromide (KBr) and pelleted Those samples heated between 30 and 550 °C were into discs for spectroscopy. Spectral data were collected characterised by FTIR, Raman and fluorescence spec- over a range of 600 to 3800 cm−1 and processed in Origin troscopy at room temperature. XANES analysis was also software (Origin(Pro) version 2017, OriginLab Corporation) performed on the 650 and 1050 °C samples to confirm the [16]. oxidation state of uranium. Roughly 50 mg amorphous powder was placed in a small plastic weighing boat and Raman spectroscopy mixed with 5 mL deionised water and allowed to dry, pro- ducing a yellow powder. For Raman and TRLFS characterisation, each sample was placed into a groove between two quartz slides. A Ren- ishaw InVia Reflex Raman Microscope was used to collect X‑Ray absorption near edge spectroscopy Raman spectra from each sample with a diode 785 nm laser [17]. The microscope was calibrated using silicon to ensure X-ray absorption spectroscopy was carried out on beam- peak position accuracy of ± 1 cm−1. Spectra were collected line BL-27B at The Photon Factory (PF), High Energy between 4000 to 50 cm−1 from a least three different loca- Accelerator Research Organisation (KEK), Japan [13]. The tions to confirm sample homogeneity. PF operates at 2.5 GeV with a ring current of 300 mA and supplies X-rays in the energy range 4 – 20 keV into Fluorescence spectroscopy beamline BL-27B, via a bending magnet. The optics hutch contains a double crystal Si (111) monochromator and a Fluorescence spectra were obtained using the time- bent plane SiO2 mirror. Spectra were collected in transmis- resolved laser fluorescence spectroscopy (TRLFS) appara- sion mode on finely ground powder specimens dispersed tus described elsewhere [18].
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