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Zhou L, Li F, Liu JX, Sun SK, Liang Y, Zhang GJ. High-entropy A 2B 2O 7-type oxide ceramics: A potential immobilising matrix for high-level radioactive waste. JOURNAL OF HAZARDOUS MATERIALS 2021; 415:125596. [PMID: 33725552 DOI: 10.1016/j.jhazmat.2021.125596] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/18/2020] [Revised: 02/15/2021] [Accepted: 03/03/2021] [Indexed: 06/12/2023]
Abstract
The sustainable development of civil nuclear energy requires the fabrication of the durable nuclear wasteforms, in particular for high-level radioactive waste, which involves the design of the composition and microstructure. Herein, we demonstrated that high-entropy ceramics (Eu1-xGdx)2(Ti0.2Zr0.2Hf0.2Nb0.2Ce0.2)2O7 are the potential candidate as immobilizing hosts for high-level radioactive waste. The static aqueous leaching test indicates that the normalized leaching rates for the simulated radionuclides Ce (LRCe) and Gd (LRGd) in as-prepared high-entropy ceramics are approximately 10-6~10-8 g·m-2·d-1 after 42 days testing, much lower than those reported values in doped-Gd2Zr2O7 (10-6~10-3 g·m-2·d-1). The excellent chemical durability is mainly due to the synergistic effects of the compositional complexity and severe lattice distortion. Compared to their ternary oxides, the low oxygen vacancy concentration slows down the migration and diffusion of cations. Moreover, the lattice distortion increases the lattice potential energy, also inhibiting the migration of cations. This study provides a strategy for the development and application of high-entropy ceramics as the wasteforms.
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Affiliation(s)
- Lin Zhou
- State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, College of Science, Institute of Functional Materials, Donghua University, Shanghai 201620, China
| | - Fei Li
- State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, College of Science, Institute of Functional Materials, Donghua University, Shanghai 201620, China.
| | - Ji-Xuan Liu
- State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, College of Science, Institute of Functional Materials, Donghua University, Shanghai 201620, China
| | - Shi-Kuan Sun
- Immobilisation Science Laboratory, Department of Materials Science and Engineering, University of Sheffield, Sheffield S1 3JD, United Kingdom; School of Material Science and Energy Engineering, Foshan University, Foshan, Guangdong 528000, China
| | - Yongcheng Liang
- State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, College of Science, Institute of Functional Materials, Donghua University, Shanghai 201620, China
| | - Guo-Jun Zhang
- State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, College of Science, Institute of Functional Materials, Donghua University, Shanghai 201620, China.
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Aldridge JD, Womick JM, Rosmus KA, Weiland A, Aitken JA, Polvani DA. Phase identification and structure investigation of novel quaternary rare-earth substituted titanates. J SOLID STATE CHEM 2017. [DOI: 10.1016/j.jssc.2017.08.030] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/19/2022]
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Evaluation of sorption capability and aqueous stability of Ba2TiSi2O8 (BTS). J Radioanal Nucl Chem 2017. [DOI: 10.1007/s10967-016-5066-0] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/20/2022]
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Latta DE, Pearce CI, Rosso KM, Kemner KM, Boyanov MI. Reaction of U(VI) with titanium-substituted magnetite: influence of Ti on U(IV) speciation. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2013; 47:4121-4130. [PMID: 23597442 DOI: 10.1021/es303383n] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/02/2023]
Abstract
Reduction of hexavalent uranium (U(VI)) to less soluble tetravalent uranium (U(IV)) through enzymatic or abiotic redox reactions has the potential to alter U mobility in subsurface environments. As a ubiquitous natural mineral, magnetite (Fe3O4) is of interest because of its ability to act as a rechargeable reductant for U(VI). Natural magnetites are often impure with titanium, and structural Fe(3+) replacement by Ti(IV) yields a proportional increase in the relative Fe(2+) content in the metal sublattice to maintain bulk charge neutrality. In the absence of oxidation, the Ti content sets the initial bulk Fe(2+)/Fe(3+) ratio (R). Here, we demonstrate that Ti-doped magnetites (Fe3 - xTixO4) reduce U(VI) to U(IV). The U(VI)-Fe(2+) redox reactivity was found to be controlled directly by R but was otherwise independent of Ti content (xTi). However, in contrast to previous studies with pure magnetite where U(VI) was reduced to nanocrystalline uraninite (UO2), the presence of structural Ti (xTi = 0.25-0.53) results in the formation of U(IV) species that lack the bidentate U-O2-U bridges of uraninite. Extended X-ray absorption fine structure spectroscopic analysis indicated that the titanomagnetite-bound U(IV) phase has a novel U(IV)-Ti binding geometry different from the coordination of U(IV) in the mineral brannerite (U(IV)Ti2O6). The observed U(IV)-Ti coordination at a distance of 3.43 Å suggests a binuclear corner-sharing adsorption/incorporation U(IV) complex with the solid phase. Furthermore, we explored the effect of oxidation (decreasing R) and solids-to-solution ratio on the reduced U(IV) phase. The formation of the non-uraninite U(IV)-Ti phase appears to be controlled by availability of surface Ti sites rather than R. Our work highlights a previously unrecognized role of Ti in the environmental chemistry of U(IV) and suggests that further work to characterize the long-term stability of U(IV) phases formed in the presence of Ti is warranted.
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Affiliation(s)
- Drew E Latta
- Biosciences Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.
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Odoh SO, Pan QJ, Shamov GA, Wang F, Fayek M, Schreckenbach G. Theoretical Study of the Reduction of Uranium(VI) Aquo Complexes on Titania Particles and by Alcohols. Chemistry 2012; 18:7117-27. [DOI: 10.1002/chem.201101197] [Citation(s) in RCA: 24] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/18/2011] [Revised: 12/20/2011] [Indexed: 11/06/2022]
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Abstract
Summary
The aqueous dissolution of synthetic brannerite (UTi2O6) in an open atmosphere has been investigated. Previous data in the literature have been combined with new experimental work, dealing with the release of uranium from brannerite as a function of solution pH and aqueous carbonate species, in oxygenated solutions. From these data we have developed a conceptual model for uranium release from brannerite consisting of two reaction steps: oxidation of surface uranium(IV) atoms, and subsequent detachment of U(VI) atoms into solution, which is catalysed by surface coordination with protons (acidic media) or carbonate species (alkaline media in equilibrium with the atmosphere). A kinetic rate law is derived for this simple reaction mechanism and fitted to experimental data. The resulting predictive equation for uranium release qualitatively describes the pH-dependent behaviour observed in experiment, and quantitatively gives an upper limit for uranium release from brannerite over a range of conditions and experiment types.
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Xu H, Wang Y, Zhao P, Bourcier WL, Van Konynenburg R, Shaw HF. Investigation of pyrochlore-based U-bearing ceramic nuclear waste: uranium leaching test and TEM observation. ENVIRONMENTAL SCIENCE & TECHNOLOGY 2004; 38:1480-1486. [PMID: 15046350 DOI: 10.1021/es030582v] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
A durable titanate ceramic waste form (Synroc) with pyrochlore (Ca(U,Pu)Ti2O7) and zirconolite (CaZrTi2O7) as major crystalline phases has been considered to be a candidate for immobilizing various high-level wastes containing fissile elements (239Pu and 235U). Transmission electron microscopy study of a sintered ceramic with stoichiometry of Ca(U(0.5)Ce(0.25)Hf(0.25))Ti2O7 shows the material contains both pyrochlore and zirconolite phases and structural intergrowth of zirconolite lamellae within pyrochlore. The (001) plane of zirconolite is parallel to the (111) plane of pyrochlore because of their structural similarities. The pyrochlore is relatively rich in U, Ce, and Ca with respect to the coexisting zirconolite. Average compositions for the coexisting pyrochlore and zirconolite at 1350 degrees C are Ca(1.01)(Ce3+(0.13)Ce4+(0.19)U(0.52)Hf(0.18))(Ti(1.95)Hf(0.05))O7 (with U/(U + Hf) = 0.72) and (Ca(0.91)Ce(0.09))(Ce3+(0.08)U(0.26)Hf(0.66)Ti(0.01))Ti(2.00)O7 (with U/(U + Hf) = 0.28), respectively. A single pyrochlore (Ca(U,Hf)Ti2O7) phase may be synthesized at 1350 degrees C if the ratio of U/(U + Hf) is greater than 0.72, and a single zirconolite (Ca(Hf,U)Ti2O7) phase may be synthesized at 1350 degrees C if the ratio of U/(U + Hf) is less than 0.28. The synthesized products were used for dissolution tests. The single-pass flow-through dissolution tests show that the dissolution of the U-bearing pyrochlore is incongruent. All the elements are released at differing rates. The dissolution data also show a decrease in rate with run time. The results indicate that a diffusion-controlled process may play a key role during the release of U. TEM observation of the leached pyrochlore directly proves that an amorphous leached layer that is rich in Ti and Hf formed on the surface after the ceramic was leached in pH 4 buffered solution for 835 days. The thickness of the layer ranges from 6 to 10 nm. A nanocrystalline TiO2 phase also forms in the leached layer. The U leaching rate (g/(m2 day)) in acidic solutions can be expressed as log(NR) = -5.36-0.20 pH, where NR is the normalized rate. Conservative leaching rates of uranium [log(NR)] for the U-bearing ceramic at pH 2 and pH 4 solutions are -5.76 and -6.16 g/(m2 day), respectively. The results show that the U release rate of the ceramic waste is 10 times slower than that of defense high-level waste glass and about 1000 times slower than that of spent fuel. The pyrochlore-based ceramic is an ideal waste form for immobilizing long-lived radionuclides of 239Pu and 235U due to the Ti- and Hf-rich leached layer that forms on the ceramic surface. The leached layer functions as a protective layer and therefore reduces the leaching rate as thickness of the leached layer increases.
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Affiliation(s)
- Huifang Xu
- Transmission Electron Microscopy Laboratory, Department of Earth and Planetary Sciences, The University of New Mexico, Albuquerque, New Mexico 87131, USA.
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Lumpkin GR, Smith KL, Gieré R, Williams CT. Geochemical behaviour of host phases for actinides and fission products in crystalline ceramic nuclear waste forms. ACTA ACUST UNITED AC 2004. [DOI: 10.1144/gsl.sp.2004.236.01.06] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/04/2022]
Abstract
AbstractA number of polyphase or single-phase ceramic waste forms have been considered as options for the disposal of nuclear waste in geological repositories. Of critical concern in the scientific evaluation of these materials is their performance in natural systems over long periods of time (e.g., 103 to 106 years). This paper gives an overview of the aqueous durability of the major titanate host phases for actinides (e.g., Th, U, Np, Pu, Cm) and important fission products (e.g., Sr and Cs) in alternative crystalline ceramic waste forms. These host phases are compared with reference to some basic acceptance criteria, including the long-term behaviour determined from studies of natural samples. The available data indicate that zirconolite and pyrochlore are excellent candidate host phases for actinides. These structures exhibit excellent aqueous durability, crystal chemical flexibility, high waste loadings, and well-known processing conditions. Although both pyrochlore and zirconolite become amorphous due to alpha-decay processes, the total volume swelling is only 5–6% and there is no significant effect of radiation damage on aqueous durability. Hollandite also appears to be an excellent candidate host phase for radioactive Cs isotopes. Brannerite and perovskite, on the other hand, are more prone to alteration in aqueous fluids and have a lower degree of chemical flexibility. With the exception of hollandite, many of the properties of these potential host phases have been confirmed through studies of natural samples.
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Affiliation(s)
- Gregory R. Lumpkin
- Cambridge Centre for Ceramic Immobilisation, Department of Earth Sciences, University of Cambridge
Downing Street, Cambridge CB2 3EQ, UK
- Australian Nuclear Science and Technology Organisation
PMB 1, Menai, NSW 2234, Australia
| | - Katherine L. Smith
- Australian Nuclear Science and Technology Organisation
PMB 1, Menai, NSW 2234, Australia
| | - Reto Gieré
- Department of Earth and Atmospheric Sciences, Purdue University
West Lafayette, IN 47907-1397, USA
Institut für Mineralogie, Petrologie und Geochemie, Universität Freiburg
Albertstrasse 23B, D-79104 Freiburg, Germany
| | - C. Terry Williams
- Department of Mineralogy, The Natural History Museum
Cromwell Road, London SW7 5BD, UK
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