Effect of temperature on studtite stability: Thermogravimetry and differential scanning calorimetry investigations

Rey, Alexandra; Casas, I.; Giménez, Javier; Quiñones, J.; Pablo, Joan de

doi:10.1016/j.jnucmat.2008.12.045

Cited by 25 publications

(32 citation statements)

References 25 publications

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“…This exothermic enthalpy of the transformation of a supposed low temperature phase (studtite) to its less hydrous high temperature phase assemblage (metastudtite plus water) confirms that studtite is a metastable phase. Thus, studtite is unlikely to have a thermodynamic stability field and its dehydration to metastudtite is irreversible, as has been generally observed (2,17,(19)(20)(21)(22)(23)(24). The temperature at which studtite is observed to dehydrate is thus controlled by kinetics.…”

Section: Discussionmentioning

confidence: 88%

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Energetics of metastudtite and implications for nuclear waste alteration

Guo

Ushakov

Labs

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

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Metastudtite, (UO 2 )O 2 (H 2 O) 2 , is one of two known natural peroxide minerals, but little is established about its thermodynamic stability. In this work, its standard enthalpy of formation, −1,779.6 ± 1.9 kJ/mol, was obtained by high temperature oxide melt drop solution calorimetry. Decomposition of synthetic metastudtite was characterized by thermogravimetry and differential scanning calorimetry (DSC) with ex situ X-ray diffraction analysis. Four decomposition steps were observed in oxygen atmosphere: water loss around 220°C associated with an endothermic heat effect accompanied by amorphization; another water loss from 400°C to 530°C; oxygen loss from amorphous UO 3 to crystallize orthorhombic α-UO 2.9 ; and reduction to crystalline U 3 O 8 . This detailed characterization allowed calculation of formation enthalpy from heat effects on decomposition measured by DSC and by transposed temperature drop calorimetry, and both these values agree with that from drop solution calorimetry. The data explain the irreversible transformation from studtite to metastudtite, the conditions under which metastudtite may form, and its significant role in the oxidation, corrosion, and dissolution of nuclear fuel in contact with water. Metastudtite is the main natural peroxide mineral (1) due to the irreversible dehydration of studtite (2). Both these phases may be found in spent nuclear fuel exposed to water (3-5). Burns et al. (6), by single crystal diffraction of a natural sample, determined the crystal structure of studtite to be monoclinic with space group C2/c. The structure consists of edge-sharing UO 8 -polyhedra chains. The water molecules are located on two different positions between these chains. Although there are different, yet very similar, models for metastudtite, thus far no structure has been determined (7,8). It is highly probable that metastudtite, like studtite, consists of chains of edge-sharing UO 8 polyhedra with the water molecules located between the chains. To better depict the bonding situation and the different oxygen atoms, it is proposed to write the sum formulas as (UO 2 )(O 2 )(H 2 O) 2 ·2H 2 O for studtite and (UO 2 )(O 2 )(H 2 O) 2 for metastudtite [in analogy to Burns et al. (6)].Studtite and other polyoxouranylates have a potentially important role as alteration phases in geological repositories for nuclear waste, specifically regarding the interactions of nuclear waste with the aqueous environment (9, 10). They have also been proposed as corrosion products in sea water after the FukushimaDaiichi nuclear plant accident (10-12).In a nuclear waste repository, the high alpha dosage will be the dominant factor after the first thousand years of storage (13). In combination with groundwater, the alpha radiation will lead to formation of H 2 O 2 (3, 4, 9, 14). These very localized oxidative conditions could trigger the genesis of studtite or metastudtite even if the overall conditions are reducing (14). The formation of metastudtite on UO 2 samples as a direct effect of alpha radiolysis of water w...

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Section: Discussionmentioning

confidence: 88%

“…Thus far, no mechanism for dehydration has been proposed. The intermediate amorphous phase forming during the first transition (studtite → metastudtite), observed in previous studies (2,(19)(20)(21)(22)(23)(24), has not been well investigated. Its identity and following intermediate phases, especially regarding structure, remain unclear.…”

mentioning

confidence: 89%

Energetics of metastudtite and implications for nuclear waste alteration

Guo

Ushakov

Labs

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

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“…Solid phase UO 2 O 2 ·4H 2 O(s) was precipitated by mixing a uranium nitrate solution with a hydrogen peroxide solution according to the experimental methodology previously developed [20], based on the reaction [21]:…”

Section: Methodsmentioning

confidence: 99%

“…This characterization was carried out because of the possible transformation of studtite to metastudite or schoepite [20]. The results showed that studtite was stable and no phase transformation took place during the experiments.…”

Section: Methodsmentioning

confidence: 99%

Sorption of strontium on uranyl peroxide: Implications for a high-level nuclear waste repository

Sureda¹,

Martínez‐Lladó

Rovira

et al. 2010

Journal of Hazardous Materials

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“…Studtite was precipitated by mixing a uranium nitrate solution with a hydrogen peroxide solution [16]. The yellow solid obtained was dried and characterized as pure studtite by X-ray diffraction.…”

Section: Methodsmentioning

confidence: 99%

The role of uranium peroxide studtite on the retention of Cs, Sr and Se(VI)

et al. 2009

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The formation of uranyl secondary solid phases onto the spent nuclear fuel surface might influence the radionuclide concentration in solution via, among others, sorption processes. In this work, the incorporation of some radionuclides onto the uranium peroxide studtite, UO 2 O 2 ·4H 2 O, has been tested.The study was carried out in batch experiments where a known amount of studtite (0.05 g) was put in contact with 20 cm 3 of radionuclide solution. Once equilibrium was reached, radionuclide concentrations in solution were determined by ICP-MS. The radionuclide amount attached to the solid was calculated from the mass balance. The S/V values of the experiments were also determined from BET specific solid surface area measurements.In this work, data on sorption of caesium, strontium, and selenium as a function of pH are presented. The behaviour of caesium and strontium are similar: a relatively high amount of radionuclide is sorbed at neutral to alkaline pH while there is almost no sorption at acidic pH. On the other hand, in the case of selenium, the sorption maximum occurs at acidic pH and there is almost no sorption at alkaline pH. The different behavior of the radionuclides is related to the element speciation in solution and the surface charge of the solid. Strontium and caesium are sorbed at alkaline pH because they are positively charged in solution and the surface of the studtite is negatively charged (>Ogroups) while selenium(VI) sorbs at acidic pH because the surface of the studtite is positively charged, and the predominant selenium(VI) species in solution is anionic.These preliminary data indicate that the sorption capacity of uranyl secondary solid phases such as studtite is an important process to be considered when establishing the migration of different radionuclides released from spent nuclear fuel.

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Effect of temperature on studtite stability: Thermogravimetry and differential scanning calorimetry investigations

Cited by 25 publications

References 25 publications

Energetics of metastudtite and implications for nuclear waste alteration

Energetics of metastudtite and implications for nuclear waste alteration

Sorption of strontium on uranyl peroxide: Implications for a high-level nuclear waste repository

The role of uranium peroxide studtite on the retention of Cs, Sr and Se(VI)

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