Phase structure evolution and chemical durability studies of Ce-doped zirconolite–pyrochlore synroc for radioactive waste storage

Meng, Chao; Ding, Xingeng; Li, Wenqi; Zhao, Junchuan; Ye, Hui

doi:10.1007/s10853-016-9822-x

Cited by 39 publications

(34 citation statements)

References 31 publications

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“…Compared to samples U1 and U3 where nearly equal atoms of Y/Eu and U exist in the brannerite phases, sample U2 has much higher Ce content than U. Given U 5+ ions being established in sample U2 (see below uranium valences section), both Ce 3+ and Ce 4+ ions must be present in the sample, consistent with some earlier observations that both Ce 3+ and Ce 4+ ions are present in zirconolite and pyrochlore phases sintered in air . The partition of Y/Ce/Eu/U is mainly toward the brannerite phases with no Y/Ce and ~3.4 wt% of Eu remaining in the residue glasses (Figure ).…”

Section: Resultssupporting

confidence: 84%

“…The EDS analyses also confirmed the dominant brannerite phases ( Figure S11) 3+ and Ce 4+ ions are present in zirconolite and pyrochlore phases sintered in air. 53 The partition of Y/Ce/Eu/U is mainly toward the brannerite phases with no Y/Ce and~3.4 wt% of Eu remaining in the residue glasses ( Figure S12). No U was found in the residue glass for U1, but~20.3 wt% of UO 2 for sample U2 and 5.2 wt% of UO 2 for sample U3 (Figure S12), suggesting that U has been predominantly partitioned toward the durable brannerite phases in U1 and U3.…”

Section: Microstructures and Phase Compositionsmentioning

confidence: 99%

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Structural and spectroscopic investigations on the crystallization of uranium brannerite phases in glass

et al. 2018

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Brannerite‐based glass‐ceramics with relatively high actinide waste loadings are potential waste forms for the immobilization of actinide‐rich radioactive wastes containing process chemicals. In this work, the crystallization of pentavalent uranium brannerite phases in glass with the incorporation of yttrium/cerium/europium has been investigated. The formation of brannerite phases in glass has been confirmed with X‐ray diffraction and Raman spectroscopy. The evolution of microstructures and the development of micro pores in brannerite phases have been revealed by scanning electron microscopy with the nominal formula, Y0.51U0.49Ti2O6, Ce0.65U0.35Ti2O6, and Eu0.53U0.47Ti2O6, being determined with energy‐dispersive spectroscopy. The presence of dominant U5+ species has been proven with both diffuse reflectance spectroscopy and X‐ray absorption near‐edge spectroscopy. In addition, the systematic Raman band shifts with the mean cation radius in the studied brannerite series have been observed and discussed.

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Section: Resultssupporting

confidence: 84%

Section: Microstructures and Phase Compositionsmentioning

confidence: 99%

Structural and spectroscopic investigations on the crystallization of uranium brannerite phases in glass

et al. 2018

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“…In the literature, zirconolite has been intensively studied on the solubility and phase transformation when incorporating different actinides/lanthanides cations . Many researchers focused on doping rare earth elements (REE) (as the simulation of radioactive actinides) on both Ca and Zr site of zirconolite, that is, Ca 1− x REE 2 x Zr 1− x Ti 2 O 7 (REE = Ce, Nd, Gd, Sm, Y) . Their results showed phase evolution from zirconolite‐2M to zirconolite‐4M and zirconolite‐4M to pyrochlore when doping different amounts of REE.…”

Section: Introductionmentioning

confidence: 99%

“…1,[14][15][16][22][23][24][25][26] Many researchers focused on doping rare earth elements (REE) (as the simulation of radioactive actinides) on both Ca and Zr site of zirconolite, that is, Ca 1−x REE 2x Zr 1−x Ti 2 O 7 (REE = Ce, Nd, Gd, Sm, Y). 1,14,[25][26][27][28] Their results showed phase evolution from zirconolite-2M to zirconolite-4M and zirconolite-4M to pyrochlore when doping different amounts of REE. Single zirconolite-2M phase cannot be observed when dopants over 0.3 formula units (f.u.).…”

Section: Introductionmentioning

confidence: 99%

Synchrotron x‐ray spectroscopy investigation of the Ca_1−xLn_xZrTi_2−x(Al, Fe)_xO₇ zirconolite ceramics (Ln = La, Nd, Gd, Ho, Yb)

Liao

et al. 2019

J Am Ceram Soc

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When incorporating actinides into zirconolite for high‐level radioactive waste immobilization, Al3+ and Fe3+ ions generally act as charge compensators. In this study, we rationally designed a series of Ca1-xLnxZrTi2-x)(Al,0.166667emFexnormalO7 (Ln = La, Nd, Gd, Ho, Yb) to unravel the dopant solubility and evolutions of the crystalline phase and local environment of cations through synchrotron X‐ray methods. It was found that single zirconolite phase is difficult to obtain and the fraction of perovskite have an increase with x from 0.1 to 0.9 in Ca1-xLaxZrTi2-x)(Al,0.166667emFexnormalO7. Formation of both zirconolite‐2M and zirconolite‐3O phases was observed in Ca1-xNdxZrTi2-x)(Al,0.166667emFexnormalO7 and Ca1-xGdxZrTi2-x)(Al,0.166667emFexnormalO7. Phase transformation from zirconolite‐2M to 3O occurs at x = 0.7 for Ca1-xNdxZrTi2-x)(Al,0.166667emFexnormalO7 while x = 0.9 for Ca1-xGdxZrTi2-x)(Al,0.166667emFexnormalO7. The solubility of Ca1-xHoxZrTi2-x)(Al,0.166667emFexnormalO7 and Ca1-xYbxZrTi2-x)(Al,0.166667emFexnormalO7 to form single zirconolite‐2M can reach to 0.9 f.u. and 0.7 f.u., respectively. The evolution of lattice parameters of zirconolite in Ca1-xLnxZrTi2-x)(Al,0.166667emFexnormalO7 is greatly related to the ionic radii of cations and substitution mechanism among the cations. X‐ray absorption near edge spectroscopy revealed that Fe3+ ions replace both five‐ and six‐coordinated Ti sites and the ratio of TiO5 to TiO6 decreases when increasing dopant concentration in the Ca1-xHoxZrTi2-x)(Al,0.166667emFexnormalO7. For the local environment of Zr4+, the major form is ZrO7 with a trace of ZrO8.

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“…Although Ce is more stable at oxidation state of 4+ in air, it is well‐known that Ce 4+ has the ability to reduce to Ce 3+ at high temperatures 36,37 . The presence of mixed Ce 3+ and Ce 4+ was observed in several multiphase ceramics sintered in air, containing pyrochlore, zirconolite, and perovskite phases, using either X‐ray photoelectron spectroscopy (XPS) 38,39 or XANES 40,41 method (though the Ce valence state in each individual mineral phase could not be distinguished based on these techniques). This study, however, is able to demonstrate that the pyrochlore phase contains exclusively Ce 4+ , whereas Ce 3+ is responsible for the stabilization of the perovskite phase.…”

Section: Resultsmentioning

confidence: 99%

Partitioning of Ce, as a simulant for Pu, in a multiphase ceramic nuclear waste form

et al. 2020

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A pyrochlore‐rich multiphase ceramic waste form was prepared by the alkoxide‐route and consolidated by sintering in air. Cerium (Ce) was used as a surrogate for plutonium (Pu), which was found in both the major pyrochlore and minor perovskite phases, but not the minor rutile phase. A synchrotron‐based fluorescence X‐ray absorption near‐edge spectroscopy (XANES) imaging technique was employed to determine the Ce oxidation state in each of the two Ce‐containing phases. It was shown that the major pyrochlore phase contains only tetravalent Ce, whereas the minor perovskite phase incorporates exclusively trivalent Ce. The effect of sintering atmosphere was also investigated, which revealed a very different phase assemblage when sintered in argon (Ar). A major perovskite phase was produced under Ar with a minor zirconolite‐4M phase, both containing trivalent Ce. The composition of the minor rutile phase remained the same whether it was sintered in air or Ar. The results demonstrate the importance of controlling the oxidation state of multivalent waste ions in the design of ceramic waste forms.

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Phase structure evolution and chemical durability studies of Ce-doped zirconolite–pyrochlore synroc for radioactive waste storage

Cited by 39 publications

References 31 publications

Structural and spectroscopic investigations on the crystallization of uranium brannerite phases in glass

Structural and spectroscopic investigations on the crystallization of uranium brannerite phases in glass

Synchrotron x‐ray spectroscopy investigation of the Ca_1−xLn_xZrTi_2−x(Al, Fe)_xO₇ zirconolite ceramics (Ln = La, Nd, Gd, Ho, Yb)

Partitioning of Ce, as a simulant for Pu, in a multiphase ceramic nuclear waste form

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Phase structure evolution and chemical durability studies of Ce-doped zirconolite–pyrochlore synroc for radioactive waste storage

Cited by 39 publications

References 31 publications

Structural and spectroscopic investigations on the crystallization of uranium brannerite phases in glass

Structural and spectroscopic investigations on the crystallization of uranium brannerite phases in glass

Synchrotron x‐ray spectroscopy investigation of the Ca1−xLnxZrTi2−x(Al, Fe)xO7 zirconolite ceramics (Ln = La, Nd, Gd, Ho, Yb)

Partitioning of Ce, as a simulant for Pu, in a multiphase ceramic nuclear waste form

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Synchrotron x‐ray spectroscopy investigation of the Ca_1−xLn_xZrTi_2−x(Al, Fe)_xO₇ zirconolite ceramics (Ln = La, Nd, Gd, Ho, Yb)