The formation of PuSiO<sub>4</sub>under hydrothermal conditions

Estevenon, Paul; Welcomme, E.; Tamain, Christelle; Jouan, Gauthier; Szenknect, Stéphanie; Mesbah, Adel; Poinssot, Christophe; Moisy, Philippe; Dacheux, Nicolas

doi:10.1039/d0dt01183e

Cited by 14 publications

(23 citation statements)

References 72 publications

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“…Furthermore, zircon has been reported to readily allow for the substitution of Pu into its structure and has been identified as a key mineral phase for Pu in the "lavas" at the Chernobyl Nuclear Power Plant 21,[30][31][32][33] . The synthesis of a pure Pu endmember orthosilicate (PuSiO 4 ) possessing the zircon structure was reported in the literature by Keller in 1963 through hydrothermal synthetic techniques 34 , however questions still remain about the overall purity of this zircon and the extreme difficulty of such a synthesis 35 . Also, there could be an miscibility gap within the (Zr,Pu)SiO 4 system, as is evidenced by the substitution of Pu into zircon becoming increasingly more difficult when exceeding 10 wt% Pu 15,36,37 .…”

Section: Introductionmentioning

confidence: 99%

Thermodynamics of CeSiO₄: Implications for Actinide Orthosilicates

et al. 2020

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The mineral zircon (ZrSiO 4 : I4 1 /amd) can accommodate natural actinides, such as thorium and uranium. The zircon structure has also been obtained for several of the end member compositions of other actinides, such as plutonium and neptunium. However, the thermodynamic properties of these actinide zircon structure-types are largely unknown due to the difficulties in synthesizing these materials and handling transuranium actinides. Thus, we have completed a thermodynamic study of cerium orthosilicate, stetindite (CeSiO 4), a surrogate of PuSiO 4. For the first time, the standard enthalpy of formation of CeSiO 4 was obtained by high temperature oxide melt solution calorimetry to be-1971.9 ± 3.6 kJ/mol. Stetindite is energetically metastable with respect to CeO 2 and SiO 2 by 27.5 ± 3.1 kJ/mol. The metastability explains the rarity of the natural occurrence of stetindite and the difficulty of its synthesis. Applying the obtained enthalpy of formation of CeSiO 4 from this work, along with those previously reported for USiO 4 and ThSiO 4 , we developed an empirical energetic relation for actinide orthosilicates. The predicted enthalpies of formation of AnSiO 4 are then made with a discussion of future strategies to efficiently immobilize Pu or minor actinides in the zircon structure.

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Section: Introductionmentioning

confidence: 99%

Thermodynamics of CeSiO₄: Implications for Actinide Orthosilicates

et al. 2020

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“…Even if the formation of plutonium silicate complexes and solid plutonium silicate species (or that of other actinide or surrogate elements) has been reported, [5][6][7][8][9][10][11][12] plutonium (IV) chemistry in silicate environment remains poorly understood. Among them, Pu(OSi(OH) 3 ) 3+ was prepared between pH 0.3 and 1.4 in diluted aqueous plutonium and silicate solution and a log(β°) = 11.8 (1) value was evaluated according to equation (1).…”

Section: Introductionmentioning

confidence: 99%

Formation of plutonium(iv) silicate species in very alkaline reactive media

et al. 2021

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“…The formation of coffinite is a mechanism that reduces the amount of uranium released from the SNF, especially in the event of an increase of the redox potential of the groundwater. This mechanism may also reduce the release of tetravalent actinides, such as plutonium, which form solid solutions with tetragonal structure of coffinite 34,35 but it could also trigger the release of other radionuclides which do not fit into the coffinite structure, as demonstrated by the release of radiogenic Pb occuring during coffinitization 3 .…”

Section: Resultsmentioning

confidence: 99%

Coffinite formation from UO2+x

Szenknect

Alby

García

et al. 2020

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Most of the highly radioactive spent nuclear fuel (SNF) around the world is destined for final disposal in deep-mined geological repositories. At the end of the fuel's useful life in a reactor, about 96% of the SNF is still UO 2. Thus, the behaviour of UO 2 in SNF must be understood and evaluated under the weathering conditions of geologic disposal, which extend to periods of hundreds of thousands of years. There is ample evidence from nature that many uranium deposits have experienced conditions for which the formation of coffinite, USiO 4 , has been favoured over uraninite, UO 2+x , during subsequent alteration events. Thus, coffinite is an important alteration product of the UO 2 in SNF. Here, we present the first evidence of the formation of coffinite on the surface of UO 2 at the time scale of laboratory experiments in a solution saturated with respect to amorphous silica at pH = 9, room temperature and under anoxic conditions. Uraninite, UO 2+x is the most common U 4+ mineral in nature followed by coffinite, USiO 4 , which is found as a primary phase or an alteration product in many uranium deposits. Coffinite, tetragonal, is isostructural with zircon (ZrSiO 4) and thorite (ThSiO 4); however, coffinite can contain some water either as H 2 O or OH groups 1. Altered uraninite and coffinite have been documented from Oklo, Gabon 2-5 , deposits in the Athabasca Basin 4,6,7 and Elliot Lake, Canada 8. Other examples include Jachymov, Czech Republic 9 or La Crouzille district, France 10. For many years, coffinite had gone unrecognized in most uranium deposits, particularly uranium roll-front deposits, as a distinct phase because of its fine grain size and intimate association with uraninite 5,6,11,12. The alteration of uraninite to coffinite is a key event for UO 2 in nature and UO 2 in spent fuel in a geologic repository. Coffinite, being a U 4+-silicate, is associated with reducing environments, with sulphides and organic matter 1 , where it likely precipitated from neutral to weakly alkaline fluids. Coffinite formation in sedimentary uranium deposits is associated with relatively low temperatures, 80-130 °C. A detailed investigation of meteoric roll-front deposits in the Athabasca basin, suggest an estimated temperature of coffinite precipitation in the uranium front of no greater than 50 °C 13. Even though laboratory experiments report coffinite formation at 150-250 °C 14,15 , it appears that these elevated temperatures are not required to form coffinite in nature. Although coffinite is abundant in uranium ore deposits, its synthesis has been a major challenge since its initial description as a mineral in 1955 16. A number of investigators have sought to obtain pure synthetic coffinite, but only a few have succeeded 14,15,17-21. Synthetic coffinite was always obtained under hydrothermal conditions. Systematically, the samples obtained were a mixture of phases, mainly composed of fine grains of USiO 4 , nanoparticles of UO 2 and amorphous SiO 2. All of these attempts to synthesize coffinite indicate that there i...

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The formation of PuSiO₄under hydrothermal conditions

Abstract: This study reports an innovative way of synthesis of PuSiO4 by hydrothermal in situ oxidation of solid Pu(iii) silicate precursors. It also identifies how representative Th-, U- and Ce-surrogates are of Pu chemistry in silicate ions rich media.

Cited by 14 publications

References 72 publications

Thermodynamics of CeSiO₄: Implications for Actinide Orthosilicates

Thermodynamics of CeSiO₄: Implications for Actinide Orthosilicates

Formation of plutonium(iv) silicate species in very alkaline reactive media

Coffinite formation from UO2+x

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The formation of PuSiO4under hydrothermal conditions

Abstract: This study reports an innovative way of synthesis of PuSiO4 by hydrothermal in situ oxidation of solid Pu(iii) silicate precursors. It also identifies how representative Th-, U- and Ce-surrogates are of Pu chemistry in silicate ions rich media.

Cited by 14 publications

References 72 publications

Thermodynamics of CeSiO4: Implications for Actinide Orthosilicates

Thermodynamics of CeSiO4: Implications for Actinide Orthosilicates

Formation of plutonium(iv) silicate species in very alkaline reactive media

Coffinite formation from UO2+x

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The formation of PuSiO₄under hydrothermal conditions

Thermodynamics of CeSiO₄: Implications for Actinide Orthosilicates

Thermodynamics of CeSiO₄: Implications for Actinide Orthosilicates