Hildegard Curtius

Thermodynamics of formation of coffinite, USiO4

Significance Coffinite, USiO4, is an important alteration mineral of uraninite. Its somewhat unexpected formation and persistence in a large variety of natural and contaminated low-temperature aqueous settings must be governed by its thermodynamic properties, which, at present, are poorly constrained. We report direct calorimetric measurements of the enthalpy of formation of coffinite. The calorimetric data confirm the thermodynamic metastability of coffinite with respect to uraninite plus quartz but show that it can form from silica-rich aqueous solutions in contact with dissolved uranium species in a reducing environment. These constraints on thermodynamic properties support that coffinitization in uranium deposits and spent nuclear fuel occurs through dissolution of UO2 (often forming hexavalent uranium intermediates) followed by reaction with silica-rich fluids. Coffinite, USiO4, is an important U(IV) mineral, but its thermodynamic properties are not well-constrained. In this work, two different coffinite samples were synthesized under hydrothermal conditions and purified from a mixture of products. The enthalpy of formation was obtained by high-temperature oxide melt solution calorimetry. Coffinite is energetically metastable with respect to a mixture of UO2 (uraninite) and SiO2 (quartz) by 25.6 ± 3.9 kJ/mol. Its standard enthalpy of formation from the elements at 25 °C is −1,970.0 ± 4.2 kJ/mol. Decomposition of the two samples was characterized by X-ray diffraction and by thermogravimetry and differential scanning calorimetry coupled with mass spectrometric analysis of evolved gases. Coffinite slowly decomposes to U3O8 and SiO2 starting around 450 °C in air and thus has poor thermal stability in the ambient environment. The energetic metastability explains why coffinite cannot be synthesized directly from uraninite and quartz but can be made by low-temperature precipitation in aqueous and hydrothermal environments. These thermochemical constraints are in accord with observations of the occurrence of coffinite in nature and are relevant to spent nuclear fuel corrosion.

Energetics of metastudtite and implications for nuclear waste alteration

Significance Uranium peroxides, metastudtite and studtite, can be formed on exposure of UO2 based nuclear fuels to water during geological disposal or as a result of reactor accidents. We report detailed structural and thermochemical analysis of the metastudtite decomposition process. The thermodynamic data confirm the irreversible transformation from studtite to metastudtite and show that metastudtite can be a major oxidized corrosion product at the surface of UO2 and contribute a significant pathway to dissolution. The prevalence of metastudtite may require additional tailoring of waste forms to minimize this dissolution pathway for uranium. Metastudtite, (UO2)O2(H2O)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 UO3 to crystallize orthorhombic α-UO2.9; and reduction to crystalline U3O8. 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.

Characterisation of secondary products of uranium–aluminium material test reactor fuel element corrosion in repository-relevant brine

Abstract Corrosion experiments with non-irradiated uranium–aluminium fuel elements were performed in MgCl 2 -rich brine. Distribution analysis of corroded material showed that about 90% of the initially available metallic U and Al precipitated. Investigations of these secondary corrosion products provided that one component is a Mg–Al–Cl–hydrotalcite.

Preparation, characterization and thermodynamic properties of Zr-containing Cl-bearing layered double hydroxides (LDHs)

Abstract Zr-containing layered double hydroxides (LDHs) with variable xZrsolid = Zr/(Zr + Al) mole fractions were synthesized by a co-precipitation method at ambient conditions. The chemical compositions of samples and corresponding aqueous solutions after syntheses were analyzed by ICP-OES, EDX (Mg, Al, Zr) and ion chromatography (Cl–). Results of PXRD technique demonstrated that solids with 0 ≤ xZrsolid ≤ 0.5 show only X-ray reflexes typical for pure LDH compositions, while products of syntheses with xZrsolid > 0.5 display additional patterns attributed to brucite. ICP-OES and EDX techniques shown that in pure Zr-containing LDHs the Mg/(Al + Zr) ratio is reducing with increase of xZrsolid and the stoichiometry of brucite-like layers corresponds to [Mg3–2xAl1–xZrx]. This fact may indicate that the incorporation of 1 Zr-containing specie results in the removal of 1 Al- and 2 Mg-containing species from the pure Mg-Al-composition. Such mechanism may be confirmed by the observation that measured a0 = b0 distances are generally consistent with theoretical estimates obtained from [Mg3–2xAl1–xZrx]-stoichiometry. The presence of predominant Mg2+, Al(OH)4– and Zr(OH)5– complexes in aqueous solutions after syntheses was established in thermodynamic calculations by applying GEMS-Selektor v.3. code and, therefore, the reaction: Mg3Al1(OH)8Cl1 + Zr(OH)5– = Mg1Zr1(OH)5Cl1 + Al(OH)4– + 2 Mg2+ + 4 OH– can describe a mechanism of Zr-substitution. Estimates of the molar Gibbs free energies of Zr-containing LDHs with 0 ≤ xZrsolid ≤ 0.5 show that the incorporation of Zr into the LDH increasing significantly their aqueous solubility. Thus, it is not possible to neglect that Zr can be partly localized as Zr(OH)5–-ligands in the interlayer space of the LDH structure.

Spent UO2 TRISO coated particles – instant release fraction and microstructure evolution

Abstract The impact of burn-up on the instant release fraction (IRF) from spent fuel was studied using very high burn-up UO2 fuel (∼ 100 GWd/t) from a prototype high temperature reactor (HTR). TRISO (TRi-structural-ISO-tropic) particles from the spherical fuel elements contain UO2 fuel kernels (500 μm diameter) which are coated by three tight layers ensuring the encapsulation of fission products during reactor operation. After cracking of the tight coatings 85Kr and 14C as 14CO2 were detected in the gas fraction. Xe was not detected in the gas fraction, although ESEM (Environmental Scanning Electron Micoscope) investigations revealed an accumulation in the buffer. UO2 fuel kernels were exposed to synthetic groundwater under oxic and anoxic/reducing conditions. U concentration in the leachate was below the detection limit, indicating an extremely low matrix dissolution. Within the leach period of 276 d 90Sr and 134/137Cs fractions located at grain boundaries were released and contribution to IRF up to max. 0.2% respectively 8%. Depending on the environmental conditions, different release functions were observed. Second relevant release steps occurred in air after ∼ 120 d, indicating the formation of new accessible leaching sites. ESEM investigations were performed to study the impact of leaching on the microstructure. In oxic environment, numerous intragranular open pores acting as new accessible leaching sites were formed and white spherical spots containing Mo and Zr were identified. Under anoxic/reducing conditions numerous metallic precipitates (Mo, Tc and Ru) filling the intragranular pores and white spherical spots containing Mo and Zr, were detected. In conclusion, leaching in different geochemical environments influenced the speciation of radionuclides and in consequence the stability of neoformed phases, which has an impact on IRF.

Corrosion of non-irradiated UAlx-Al fuel in the presence of clay pore solution: a quantitative XRD secondary phase analysis applying the DDM method

Abstract Corrosion experiments with non-irradiated metallic UAlx–Al research reactor fuel elements were carried out in autoclaves to identify and quantify the corrosion products. Such compounds, considering the long-term safety assessment of final repositories, can interact with the released inventory and this constitutes a sink for radionuclide migration in formation waters. Therefore, the metallic fuel sample was subjected to clay pore solution to investigate its process of disintegration by analyzing the resulting products and the remnants, i.e. the secondary phases. Due to the fast corrosion rate a full sample disintegration was observed within the experimental period of 1 year at 90°C. The obtained solids were subdivided into different grain size fractions and prepared for analysis. The elemental analysis of the suspension showed that, uranium and aluminum are concentrated in the solids, whereas iron was mainly dissolved. Non-ambient X-ray diffraction (XRD) combined with the derivative difference minimization (DDM) method was applied for the qualitative and quantitative phase analysis (QPA) of the secondary phases. Gypsum and hemihydrate (bassanite), residues of non-corroded nuclear fuel, hematite, and goethite were identified. The quantitative phase analysis showed that goethite is the major crystalline phase. The amorphous content exceeded 80 wt% and hosted the uranium. All other compounds were present to a minor content. The obtained results by XRD were well supported by complementary scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) analysis.

Research reactor fuel element corrosion under repository relevant conditions: separation, identification, and quantification of secondary alteration phases of UAlx-Al in MgCl2-rich brine

Abstract The corrosion of the UAlx-Al research reactor fuel type in synthetic MgCl2-rich brine (static batch-type experiments) was investigated with respect to the long-term safety of directly disposed research reactor fuel elements in salt formations. During corrosion, crystalline secondary phases were formed, which may serve as a barrier against radionuclide migration. For an optimized identification and quantification of the secondary phases using X-ray diffraction, a sample treatment to separate and enrich the secondary phases is necessary. A grain size fractionation was carried out in iso-propanol. A chemical composition and phase characterization of the secondary phases was accomplished. The results of the chemical investigations reveal that only traces of Al and U were dissolved. The separation and enrichment of secondary phases were carried out reproducible and successfully. Due to the phase characterization by scanning electron microscopy/energy dispersive X-ray spectroscopy and X-ray diffraction the following secondary phases were unambiguously identified: Mg-Al-Cl layered double hydroxide, lesukite, Fe layered double hydroxide (green rust), lawrencite, Fe (elemental), and traces of uncorroded fuel (UAl4). The quantitative analysis showed that LDH compounds and lesukite are the major crystalline phases. All other observed compounds were only present in trace amounts, i.e. constituting accessories. The Rietveld analysis also revealed the high content of amorphous phases of approximately 30%, which are expected to include the uranium as U(OH)4.

Synthesis, characterization and stability properties of Cl-bearing hydrotalcite-pyroaurite solids

Abstract A layered double hydroxides (LDH) hydrotalcite-pyroaurite solid solution series (Mg1−xFe(II)x)3Al1Cl1·nH2O with variable xFesolid = Fe2+/(Fe2++Mg2+) iron mole fractions were studied in co-precipitation experiments at T = 25, 40, 45, 50, 55 and 60 ºC and pH = 10.00 ± 0.05. The compositions of the solids and reaction solutions were determined using ICP-OES, EDX (Mg, Al, Fe) and TGA techniques (Cl−, OH−, H2O). Powder X-ray diffraction was applied for phase identification and determination of unit-cell parameters ao = bo and co from Bragg evaluation. Syntheses products containing xFesolid > 0.13 display additional X-ray patterns attributed to the mixture of iron oxides and hydroxides. On the other side, precipitates with 0 ≤ xFesolid ≤ 0.13 show only X-ray reflexes typical for pure LDH compositions. Moreover, in this case unit-cell parameters ao = bo as a function of xFesolid follow Vegards law corroborating the existence of a continuous solid solution series. TGA data demonstrated the temperatures at which interlayer H2O molecules and Cl−-anions are lost, and at which temperatures dehydroxylation of brucite-like layer occurs. Based on detailed analyses of TGA curves it was established that the increase of xFesolid does not result in a visible change of the thermal stability of hydrotalcite-pyroaurite solids. From the chemical analyses of both the solids and the reaction solutions after syntheses, preliminary Gibbs free energies of formation were estimated by using GEMS-PSI code package. Values of Gºf (Hydrotalcite) = −3619.04 ± 15.27 kJ/mol and Gºf(Pyroaurite) = −2703.61 ± 191.93 kJ/mol were found at 298.15 K. A comparison of our estimate with Gºf value −3746.90 ± 11.00 kJ/mol for CO32−-bearing hydrotalcite presented in our previous studies, denotes the effect of intercalated anion on the aqueous solubilities of LDH when Cl-containing solids have to be more soluble than CO32−-bearing substances. Estimation of the standard molar entropy of the hydrotalcite end-member by applying Helgesons methods and using results of co-precipitation experiments at variable temperatures let us to conclude that derivation of more precise Sºf values would require calorimetric measurements.

Analysis of the Secondary Phases Formed by Corrosion of U3Si2-Al Research Reactor Fuel Elements in the Presence of Chloride Rich Brines

Corrosion experiments with non-irradiated U3Si2-Al research reactor fuel samples were carried out in synthetic MgCl2-rich brine to identify and quantify the secondary phases because depending on their composition and on their amount, such compounds can act as a sink for the radionuclide release in final repositories. Within the experimental period of 100 days at 90 °C and anoxic conditions the U3Si2-Al fuel sample was completely disintegrated. The obtained solids were subdivided into different grain size fractions and non-ambient X-ray diffraction (XRD) was applied for their qualitative and quantitative phase analysis. The secondary phases consist of lesukite (aluminum chloro hydrate) and layered double hydroxides (LDH) with varying chemical compositions. Furthermore, iron, residues of non-corroded nuclear fuel (U3Si2), iron oxy hydroxides and chlorides were also observed. In addition to high amorphous contents (>45 wt %) hosting the uranium, the quantitative phase analysis showed, that LDH compounds and lesukite were the major crystalline phases. Scanning electron microscopy (SEM) and energy dispersive -Xray spectroscopy (EDS) confirmed the results of the XRD analysis. Elemental analysis revealed that U and Al were concentrated in the solids. However, most of the iron, added as Fe(II) aqueous species, remained in solution.