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A comprehensive analysis of X-ray absorption data obtained at the U L3-edge for a systematic series of single-valence (UO2, KUO3, UO3) and mixed-valence uranium compounds (U4O9, U3O7, U3O8) is reported. High-energy resolution fluorescence detection (HERFD) X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) methods were applied to evaluate U(IV) and U(V) environments, and in particular, to investigate the U3O7 local structure. We find that the valence state distribution in mixed-valence uranium compounds cannot be confidently quantified from a principal component analysis of the U L3-edge XANES data. The spectral line broadening, even when applying the HERFD-XANES method, is sensibly higher (∼3.9 eV) than the observed chemical shifts (∼2.4 eV). Additionally, the white line shape and position are affected not only by the chemical state, but also by crystal field effects, which appear well-resolved in KUO3. The EXAFS of a phase-pure U3O7 sample was assessed based on an average representation of the expanded U60O140 structure. Interatomic U–O distances are found mainly to occur at 2.18 (2), 2.33 (1), and 3.33 (5) Å, and can be seen to correspond to the spatial arrangement of cuboctahedral oxygen clusters. The interatomic distances derived from the EXAFS investigation support a mixed U(IV)–U(V) valence character in U3O7.

and presentation of a contributed talk at the MRS 2022 Spring Meeting in Symposium SF01: Materials Research Needs to Advance Nuclear Fuels, Structural Materials and Wasteforms ---Abstract--- The principal type of nuclear fuel material, which remains widely used today, is uranium dioxide (UO2). After discharge from a nuclear reactor some sort of disposal scenario needs to be considered to manage the spent fuel in a sustainable and safe way during extensive periods of time. However, in most countries which employ(ed) commercial nuclear energy, no political decision has yet been made, leading to prolonged intermediate storage periods up to several decades. As a result, oxidation and corrosion reactions due to accidental interaction between air, water, or moisture and the spent fuel need to be considered and understood. The response of conventional nuclear fuel to oxidation and corrosion is determined mainly by the physico-chemical properties of uranium. The binary uranium-oxygen system displays a very complex landscape of oxide phase fields. At temperatures below about 450 °C two single-valence compounds, uranium(IV) oxide (UO2) and uranium(VI) oxide (UO3), and several mixed-valence compounds (U4O9, U3O7, U3O8) can occur [1-4], characterized by particular associations of oxygen interstitials and vacancies. At higher temperature, long-range correlations among defects become less effective and the phase diagram becomes simpler, displaying a wide phase domain of nonstoichiometric UO2±x [5]. A characteristic feature of these higher oxide compounds lies in their crystallographic modifications, which occur almost exclusively in the anion sublattice, where point defects arrange into larger clusters to form complex long-range ordered structures [6-8]. However, despite sustained efforts for almost a century, the transition and structural relations between the different compounds is yet to be fully understood. New perspectives on the oxidation behavior of UO2 were obtained by investigating the kinetic mechanisms of these materials at the nanoscale, where fundamental differences were reported compared to the macroscale observations. Recent breakthroughs were made possible by using nanoscale imaging techniques like transmission electron microscopy [9], and by evaluating atomistic models of crystal structures [8,10,11]. A key instrument has been the application of neutron scattering techniques, owing to its sensitivity towards the anion sublattice [10], and its selectivity in probing magnetic transitions [12] and the local structure [13]. In this contribution we will report our current understanding of the uranium oxide systems U3O7 and U3O8 in ground state, based on latest neutron scattering data and ab-initio calculations. References [1] Grenthe I., et al., "Uranium", in: Morss L.R., Edelstein N.M., F. J. (Eds.) The Chemistry of the Actinide and Transactinide Elements, Springer, Dordrecht, 2008. [2] K.O. Kvashnina, et al., Phys. Rev. Lett., 111 (2013) 253002. [3] D.A. Andersson, et al., Inorg. Chem., 52 (2013) 2769. [4] S.D. Conradson, et al., Phys. Rev. B, 96 (2017) 125114. [5] P.Y. Chevalier, et al., J. Nucl. Mater., 303 (2002) 1. [6] B.T.M. Willis, J. Chem. Soc., Faraday Trans. II, 83 (1987) 1073. [7] L. Desgranges, et al., Inorg. Chem., 48 (2009) 7585. [8] G. Leinders, et al., Inorg. Chem., 55 (2016) 9923. [9] G. Leinders, et al., Inorg. Chem., 55 (2016) 3915. [10] G. Leinders, et al., Inorg. Chem., 60 (2021) 10550. [11] A. Soulié, et al., Inorg. Chem., 58 (2019) 12678. [12] A. Miskowiec, et al., Phys. Rev. B, 103 (2021) 205101. [13] L. Desgranges, et al., Inorg. Chem., 56 (2017) 321., {"references":["Grenthe I., et al., \"Uranium\", in: Morss L.R., Edelstein N.M., F. J. (Eds.) The Chemistry of the Actinide and Transactinide Elements, Springer, Dordrecht, 2008.","K.O. Kvashnina, et al., Phys. Rev. Lett., 111 (2013) 253002.","D.A. Andersson, et al., Inorg. Chem., 52 (2013) 2769.","S.D. Conradson, et al., Phys. Rev. B, 96 (2017) 125114.","P.Y. Chevalier, et al., J. Nucl. Mater., 303 (2002) 1.","B.T.M. Willis, J. Chem. Soc., Faraday Trans. II, 83 (1987) 1073.","L. Desgranges, et al., Inorg. Chem., 48 (2009) 7585.","G. Leinders, et al., Inorg. Chem., 55 (2016) 9923.","G. Leinders, et al., Inorg. Chem., 55 (2016) 3915.","G. Leinders, et al., Inorg. Chem., 60 (2021) 10550.","A. Soulié, et al., Inorg. Chem., 58 (2019) 12678.","A. Miskowiec, et al., Phys. Rev. B, 103 (2021) 205101.","L. Desgranges, et al., Inorg. Chem., 56 (2017) 321."]}

Autoclave leaching experiments are conducted on three well-characterised, irradiated, and cladded mixed oxide fuel-rod segments with burnups ranging from 29 GWd/tHM to 52 GWd/tHM to investigate the instant release fraction of fission gases and long-lived fission products and to assess the long-term fuel matrix corrosion. The segments are exposed to bicarbonate solutions as reference groundwater at neutral pH and a synthetic young cementitious water at pH 13.5 under reducing atmosphere (4 vol% H2 in Ar at 40 bar pressure), since 2018. The initial leaching results for the fission products caesium and iodine as representative elements of the instant release fraction were found to depend on the leachate composition as well as on the fuel burnup. Graphical abstract

The hydrolysis of uranyl-, Nd- and Ce-cations, induced by thermal decomposition of urea was investigated and the impact of the urea content and the experiment temperature on the reaction kinetics was evaluated. Uranyl precipitated as ammonium diuranate (ADU) with varying stoichiometry. Nd(III) and Ce(III) showed comparable pH evolutions and lanthanide carbonate hydroxide (LnCO3OH) products were identified, whereas Ce(IV) hydrolysed at lower pH and formed CeO2. The precipitation behaviour was confirmed for mixtures of uranyl and the lanthanides. Depending on the urea content, a partial co-precipitation occurred in U/Nd mixtures. The phases formed with Ce(III) and Ce(IV) were also identified in the precipitates of binary U/Ce mixtures. In ternary U/Nd/Ce compositions, a simultaneous precipitation of Nd(III) and Ce(III) was observed and a partial incorporation of the lanthanides into the ADU phase, whereas the precipitation in the presence of Ce(IV)/CeO2 led to the formation of three individual phases.

Theslides werepresented at the 45th Scientific Basis for Nuclear Waste Management SymposiumonOctober 27, 2021. Abstract Disposal in a deep geological repository (DGR) based on a multibarrier concept is considered as the safest and most sustainable option for the management of spent nuclear fuels (SNF) in many countries. Demonstrating the long-term safety of a DGR for SNF over assessment time frames of up to one million years requires a profound understanding of the corrosion behaviour of SNF coming into contact with groundwater, when the waste canister is eventually breached. During the last decades, a number of studies addressed this topic, leading to a good phenomenological understanding of the long-term behaviour of SNF in a DGR. However, various processes contributing to the (radiolytic) matrix corrosion of SNF in the generally reducing repository environment are still not fully understood, and corrosion data on used mixed oxide (MOX) fuels are scarce to date. To investigate the effects of environmental conditions on the corrosion of SNF, the SF-ALE project (Spent Fuel Autoclave Leaching Experiments) was initiated as collaboration between the Belgian Nuclear Research Centre SCKCEN, Forschungszentrum Jülich GmbH, and the Belgian Agency for Radioactive Waste and Fissile Materials (NIRAS/ONDRAF). Within SF-ALE, the release of various radionuclides and fission gases from well-characterised, irradiated MOX fuel segments (burn-up between 29GWd/tHM and 52GWd/tHM) exposed to bicarbonate water as reference groundwater at neutral pH and a synthetic cementitious water at pH13.5 under reducing atmosphere (4vol% H2 in Ar at 40bars pressure) is investigated. The first project phase lasted for about two years and was completed in September 2020. The leaching solutions were renewed and a second leaching phase was started on the same MOX fuel segments, aiming for a total duration of about 3.5years. In this contribution, we report the results obtained during the first phase and describe the transition to phase two of the project.

Atomic arrangements in the mixed-valence oxide U

Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c01212; International audience; Atomic arrangements in the mixed-valence oxide U3O7 are refined from high-resolution neutron scattering data. The crystallographic model describes a long-range structural order in a U60O140 primitive cell (space group P42/n) containing distorted cuboctahedral oxygen clusters. By combining experimental data and electronic structure calculations accounting for spin−orbit interactions, we provide robust evidence of an interplay between charge localization and the magnetic moments carried by the uranium atoms. The calculations predict U3O7 to be a semiconducting solid with a band gap of close to 0.32 eV, and a more pronounced charge-transfer insulator behavior as compared to the well-known Mott insulator UO2. Most uranium ions (56 out of 60) occur in 9-fold and 10-fold coordinated environments, surrounding the oxygen clusters, and have a tetravalent (24 out of 60) or pentavalent (32 out of 60) state. The remaining uranium ions (4 out of 60) are not contiguous to the oxygen cuboctahedra and have a very compact, 8-fold coordinated environment with two short (2 × 1.93(3) Å) “oxo-type” bonds. The higher Hirshfeld charge and the diamagnetic character point to a hexavalent state for these four uranium ions. Hence, the valence state distribution corresponds to 24/60 × U(IV) + 32/60 U(V) + 4/60 U(VI). The tetravalent and pentavalent uranium ions are predicted to carry noncollinear magnetic moments (with amplitudes of 1.6 and 0.8 μB, respectively), resulting in canted ferromagnetic order in characteristic layers within the overall fluorite-related structure.

Technical paper submitted for peer-review, accepted, and presented at the 2021 Topfuel conference (October 24-28, 2021). The conference was hosted as a hybrid event with on-site attendance (Santander, Spain) and remote attendance via a web application. ---Abstract--- In the context of interim or long-term storage of spent nuclear fuel, as well as in final disposal scenarios it is crucial to understand and accurately predict the materials��� behavior, specifically towards the release of radionuclides into the environment. Within the Spent Fuel Autoclave Leaching Experiments (SF-ALE) project at SCK CEN an improved experimental setup for performing leaching studies was developed, and six experiments (three on UOX fuel, three on MOX fuel) are being conducted in parallel over a timeframe of 36 months. To allow an accurate interpretation of the leaching data, depletion calculations were performed to establish the nuclide inventory of the MOX fuel samples at the time of the experiments. Preliminary results of the fission gas release measured during the leaching experiments are reported and discussed., {"references":["B.J. Lewis, W.T. Thompson, F.C. Iglesias, \"Fission Product Chemistry in Oxide Fuels\", in: R.J.M. Konings (Ed.) Comprehensive Nuclear Materials, Elsevier, Oxford, 2012, pp. 515-546.","D. Cubicciotti, J.E. Sanecki, Journal of Nuclear Materials, 78 (1978) 96.","H.J. Matzke, Radiation Effects, 53 (1980) 219.","R.G.J. Ball, W.G. Burns, J. Henshaw, M.A. Mignanelli, P.E. Potter, Journal of Nuclear Materials, 167 (1989) 191.","H. Kleykamp, Journal of Nuclear Materials, 171 (1990) 181.","T.M. Besmann, J.W. McMurray, S. Simunovic, Calphad - Computer Coupling of Phase Diagrams and Thermochemistry, 55, Part 1 (2016) 47.","E.H.P. Cordfunke, R.J.M. Konings, Journal of Nuclear Materials, 152 (1988) 301.","R.W. Grimes, C.R.A. Catlow, Philosophical transactions - Royal Society. Mathematical, Physical and engineering sciences, 335 (1991) 609.","H. Kleykamp, Journal of Nuclear Materials, 131 (1985) 221.","L. Johnson, I. Günther-Leopold, J. Kobler Waldis, H.P. Linder, J. Low, D. Cui, E. Ekeroth, K. Spahiu, L.Z. Evins, Journal of Nuclear Materials, 420 (2012) 54.","E. González-Robles, et al., Journal of Nuclear Materials, 479 (2016) 67.","D. Serrano-Purroy, F. Clarens, E. González-Robles, J.P. Glatz, D.H. Wegen, J. de Pablo, I. Casas, J. Giménez, A. Martínez-Esparza, Journal of Nuclear Materials, 427 (2012) 249.","D. Serrano-Purroy, I. Casas, E. González-Robles, J.P. Glatz, D.H. Wegen, F. Clarens, J. Giménez, J. de Pablo, A. Martínez-Esparza, Journal of Nuclear Materials, 434 (2013) 451.","K. Lemmens, et al., Journal of Nuclear Materials, 484 (2017) 307.","T. Mennecart, G. Leinders, C. Cachoir, G. Cornelis, G. Verpoucke, G. Modolo, D. Bosbach, K. Lemmens, M. Verwerft, \"First phase of the Spent Fuel Autoclave Leaching Experiments (SF-ALE) at SCK•CEN\", Global Top Fuel 2019, Seattle (USA) (2019).","T. Mennecart, K. Lemmens, C. Cachoir, L. Adriaensen, A. Dobney, \"Leaching tests for the experimental determination of IRF radionuclides from Belgian high-burnup spent nuclear fuel: Overview of pre-test fuel characterization experimental set-up and first results\", 2nd Annual Workshop Proceedings of the Collaborative Project \"Fast/Instant Release of Safety Relevant Radionuclides from Spent Nuclear Fuel\", KIT Scientific report KIT- 7676, (2013).","B. Kienzler, V. Metz, L. Duro, A. Valls, \"State of the art report - Update 2013 of the Collaborative Project Fast/Instant Release of Safety Relevant Radionuclides from Spent Nuclear (7th EC FP CP FIRST-Nuclides)\", (2013).","L. Adriaensen, A. Dobney, P. Van Bree, \"Radiochemical burnup analysis of fuel sample FT1X57-D05-BU1\", SCK CEN Restricted contract report R-5640, (2014).","Wieselquist, W.A., THE SCALE 6.2 ORIGEN API FOR HIGH PERFORMANCE DEPLETION (2015)."]}

The structural changes of ammonium diuranate (ADU) microspheres, prepared by the sol–gel route via internal gelation, were investigated during thermal treatments in oxidative and reducing conditions. In particular, in-situ simultaneous thermogravimetric analyses and in-situ high-temperature scanning electron microscopy investigations were carried out on un-doped ADU microspheres, and ADU microspheres doped with 10 mol% neodymium or 10 mol% cerium. Calcination in air caused the surface of the particles to crack, but with increasing temperature up to 900 °C some healing, and additionally, shrinkage occurred. The extent of the fractures and the amount of shrinkage was, however, significantly more pronounced in a particle prepared with a tetravalent Ce precursor, as compared to particles prepared with a trivalent Nd or Ce precursor. The macroscopic behaviour could be related to the release of volatile decomposition products and the calcined compositions were identified as (Ln-doped) α − U 3 O 8 by X-ray powder diffraction. During thermal treatment in reducing conditions a transition from a (Ln-doped) α − U 3 O 8 phase to a (Ln-doped) UO2±x phase was observed. After exposure to a hydrogen containing gas mixture, this transition occurred rapidly in un-doped particles, and in particles prepared with trivalent Nd or Ce dopant precursors at 700 °C. Despite the fast reduction reaction, severe fractures appeared in the particles above temperatures of 850 °C, indicating that such behaviour is mainly attributed to sintering effects and less to the phase transition. In contrast, a more delayed reduction reaction was observed in particles prepared with a tetravalent dopant precursor and the described effects appeared less severe.

[Image: see text] A comprehensive analysis of X-ray absorption data obtained at the U L(3)-edge for a systematic series of single-valence (UO(2), KUO(3), UO(3)) and mixed-valence uranium compounds (U(4)O(9), U(3)O(7), U(3)O(8)) is reported. High-energy resolution fluorescence detection (HERFD) X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) methods were applied to evaluate U(IV) and U(V) environments, and in particular, to investigate the U(3)O(7) local structure. We find that the valence state distribution in mixed-valence uranium compounds cannot be confidently quantified from a principal component analysis of the U L(3)-edge XANES data. The spectral line broadening, even when applying the HERFD-XANES method, is sensibly higher (∼3.9 eV) than the observed chemical shifts (∼2.4 eV). Additionally, the white line shape and position are affected not only by the chemical state, but also by crystal field effects, which appear well-resolved in KUO(3). The EXAFS of a phase-pure U(3)O(7) sample was assessed based on an average representation of the expanded U(60)O(140) structure. Interatomic U–O distances are found mainly to occur at 2.18 (2), 2.33 (1), and 3.33 (5) Å, and can be seen to correspond to the spatial arrangement of cuboctahedral oxygen clusters. The interatomic distances derived from the EXAFS investigation support a mixed U(IV)–U(V) valence character in U(3)O(7).

The sol-gel route via internal gelation was applied for the production of Nd- and Ce-doped uranium dioxide microspheres. Trivalent and tetravalent Ce precursors were used and the influence of the precursors’ oxidation state on the fabrication process and the final product was studied. The successful introduction of the dopant into the 3UO3·2NH3·4H2O matrix of the dried gels, independent of the dopant and the oxidation state of its precursor, was demonstrated for Ln contents up to 30 mol%. Densities of the dried gels were determined and the particle volume shrinkage during the thermal treatment was investigated. X-ray powder diffraction analyses proved the presence of U1−yLnyO2±x single phase solid solutions for the sintered Nd-doped microspheres and Ce-doped microspheres prepared using the tetravalent precursor. For Ce-doped compositions prepared with the trivalent precursor, the presence of two solid solutions was observed for Ce contents > 15 mol%. The lattice parameters determined for the single phase solid solutions follow Vegard’s law and show a decreasing lattice parameter with increasing dopant content. In the case for the Nd-doped material different charge compensation mechanisms, depending on the dopant content, were observed. The conditions applied in this study allow the usage of a solution containing the gelation agents, resulting in a particle fabrication process for the production of Pu and/or minor actinide containing UO2 transmutation fuel, which has benefits in terms of automating and remote handling, leading to a better implementation in glove boxes or hot cells.

A combination of simultaneous thermal analysis, evolved gas analysis and non-ambient XRD techniques was used to characterise and investigate the conversion reactions of ammonium uranates into uranium oxides. Two solid phases of the ternary system NH 3 − UO 3 − H 2 O were synthesised under specified conditions. Microspheres prepared by the sol-gel method via internal gelation were identified as 3 UO 3 ⋅ 2 NH 3 ⋅ 4 H 2 O , whereas the product of a typical ammonium diuranate precipitation reaction was associated to the composition 3 UO 3 ⋅ NH 3 ⋅ 5 H 2 O . The thermal decomposition profile of both compounds in air feature distinct reaction steps towards the conversion to U 3 O 8 , owing to the successive release of water and ammonia molecules. Both compounds are converted into α- U 3 O 8 above 550 °C, but the crystallographic transition occurs differently. In compound 3 UO 3 ⋅ NH 3 ⋅ 5 H 2 O (ADU) the transformation occurs via the crystalline β-UO3 phase, whereas in compound 3 UO 3 ⋅ 2 NH 3 ⋅ 4 H 2 O (microspheres) an amorphous UO 3 intermediate was observed. The new insights obtained on these uranate systems improve the information base for designing and synthesising minor actinide-containing target materials in future applications.

The thermochemical behavior of low-temperature oxidation in fine UO2 powders has been investigated by simultaneous thermogravimetric analysis and differential scanning calorimetry. The evaluation of the thermochemical and kinetic data reveals a complex interplay between different mechanisms. The initial reaction concerns the rapid chemisorption of oxygen gas onto the surface of UO2 grains, having an activation energy of only 13.1 ± 0.6 kJ mol–1. The subsequent oxidation at temperatures between 40 and 100 °C occurs first at the surface via a field-assisted mechanism, which progresses via domain growth into the bulk. At more elevated temperatures, thermally activated diffusion becomes the dominant mechanism.

A wet chemical route (oxalate precipitation) was chosen to prepare gadolinium-doped thoria powders as a feed for pellet fabrication. The oxalate and derived oxide powders were characterized by thermogravimetric analysis coupled to evolved gas analysis, BET, XRD and SEM. Dilatometry was used to study the sintering behavior. From homogeneous solutions containing selected mixtures of gadolinium (III) and thorium (IV) nitrates solid precipitates were obtained, that were composed of two phases. Calcination at 700 °C or 1000 °C was insufficient to obtain single phases for any of the compositions. Rietveld analysis of calcined powders confirmed the presence of both a Th-rich phase (Th x Gd 1−x O 2− x /2 ) and a Gd-rich phase ((Gd y Th 1− y ) 2 O 3+(1− y ) ) in which limited substitution of the foreign element (Gd or Th, respectively) occurred. A fluorite-type solid solution formed during sintering at 1750 °C. The unit cell parameters of sintered pellets were determined for Th x Gd 1− x O 2−(1− x )/2 mixed oxides in the composition range: 0 ≤ x ≤ 0.30 and agree with reported values.

Polycrystalline U3O7 powder was synthesized by oxidation of UO2 powder under controlled conditions using in situ thermal analysis, and by heat treatment in a tubular furnace. The O/U ratio of the U3O7 phase was measured as 2.34 ± 0.01. The crystal structure was assessed from X-ray diffraction (XRD) and selected-area electron diffraction (SAED) data. Similar to U4O9−e (more precisely U64O143), U3O7 exhibits a long-range ordered structure, which is closely related to the fluorite-type arrangement of UO2. Cations remain arranged identical to that in the fluorite structure, and excess anions form distorted cuboctahedral oxygen clusters, which periodically replace the fluorite anion arrangement. The structure can be described in an expanded unit cell containing 15 fluorite-like subcells (U15O35), and spanned by basis vectors A = ap – 2bp, B = −2ap + bp, and C = 3cp (lattice parameters of the subcell are ap = bp = 538.00 ± 0.02 pm and cp = 554.90 ± 0.02 pm; cp/ap = 1.031). The arrangement of cuboctahedra in U3O...

The nanostructure and phase evolution in low-temperature oxidized (40-250 °C), fine UO2 powders (200 nm) have been investigated by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HR-TEM). The extent of oxidation was also measured via in situ thermogravimetric analysis. The oxidation of fine powders was found to proceed differently as compared to oxidation of coarse-grained UO2. No discrete surface oxide layer was observed and no U3O8 was formed, despite the high degree of oxidation (up to O/U = 2.45). Instead, nanosized (5-15 nm) amorphous nuclei (interpreted as amorphous UO3), unmodulated and modulated U4O9, and a continuous range of U3O7-z phases with varying tetragonal distortion (c/a1) were observed. Oxidation involves formation of higher uranium oxides in nanodomains near the grain surface which, initially, have a disordered defect structure ("disordered U4O9"). As oxidation progresses, domain growth increases and the long-period modulated structure of U4O9 develops ("ordered U4O9"). A similar mechanism is understood to happen also in U3O7-z.

Technical paper submitted for peer-review, accepted, and presented at the 2018 Topfuel conference (September 30 - October 04, 2018). The conference was held in Prague, Czech Republic. ---Abstract--- One of the problems for which currently no quantitative answer can be given, are the degradation kinetics of spent nuclear fuel under different atmospheres. In the present contribution, we report on preliminary results after low-temperature moisturized atmosphere oxidation of UO2, and the results are compared to recent results of dry air oxidation. The first results indicate that the addition of moisture results in a slight moderation of the oxidation reaction, when compared to dry oxidizing conditions., {"references":["A. Leenaers, L. Sannen, S. Van den Berghe, M. Verwerft, J. Nucl. Mater., 317 (2003) 226.","R.J. McEachern, P. Taylor, J. Nucl. Mater., 254 (1998) 87.","C.L. Tracy, C.H. Chen, S. Park, M.L. Davisson, R.C. Ewing, J. Nucl. Mater., 502 (2018) 68.","S.B. Donald, M.L. Davisson, Z.R. Dai, S.K. Roberts, A.J. Nelson, J. Nucl. Mater., 496 (2017) 353.","R.J. McEachern, P. Taylor, J. Nucl. Mater., 254 (1998) 87.","C. Rocanière, J.P. Laval, P. Dehaudt, B. Gaudreau, A. Chotard, E. Suard, J. Solid State Chem., 177 (2004) 1758.","D.J.M. Bevan, I.E. Grey, B.T.M. Willis, J. Solid State Chem., 61 (1986) 1.","G. Leinders, R. Delville, J. Pakarinen, T. Cardinaels, K. Binnemans, M. Verwerft, Inorg. Chem., 55 (2016) 9923.","B.O. Loopstra, Acts Cryst., 17 (1964) 651.","G. Leinders, J. Pakarinen, R. Delville, T. Cardinaels, K. Binnemans, M. Verwerft, Inorg. Chem., 55 (2016) 3915.","G. Leinders, T. Cardinaels, K. Binnemans, M. Verwerft, Inorg. Chem., 57 (2018) 4196.","\"Spent Fuel Autoclave Leaching (SF-ALE) project for the release of radionuclides from spent nuclear fuel under representative conditions of water chemistry and oxygen activity Partners: NIRAS/ONDRAF, FZ-Jülich, SCK•CEN\", in, 2017.","G. Leinders, R. Bes, J. Pakarinen, K. Kvashnina, M. Verwerft, Inorg. Chem., 56 (2017) 6784.","G. Leinders, T. Cardinaels, K. Binnemans, M. Verwerft, J. Nucl. Mater., 459 (2015) 135.","C.L. Tracy, C.-H. Chen, S. Park, M.L. Davisson, R.C. Ewing, J. Nucl. Mater., 502 (2018) 68.","H.R. Hoekstra, S. Siegel, J. Inorg. Nucl. Chem., 18 (1961) 154.","P. Taylor, R.J. Lemire, D.D. Wood, Nucl. Technol., 104 (1993) 164."]}

The thermochemical behavior of low-temperature oxidation in fine UO

A fundamental question concerning the chemical state of uranium in the binary oxides UO2, U4O9, U3O7, U3O8, and UO3 is addressed. By utilizing high energy resolution fluorescence detection X-ray absorption near edge spectroscopy (HERFD-XANES) at the uranium M4 edge, a novel technique in the tender X-ray region, we obtain the distribution of formal oxidation states in the mixed-valence oxides U4O9, U3O7, and U3O8. Moreover, we clearly identify a pivot from U(IV)–U(V) to U(V)–U(VI) charge compensation, corresponding with transition from a fluorite-type structure (U3O7) to a layered structure (U3O8). Such physicochemical properties are of interest to a broad audience of researchers and engineers active in domains ranging from fundamental physics to nuclear industry and environmental science.

Polycrystalline U3O7 powder was synthesized by oxidation of UO2 powder under controlled conditions using in situ thermal analysis, and by heat treatment in a tubular furnace. The O/U ratio of the U3O7 phase was measured as 2.34 ± 0.01. The crystal structure was assessed from X-ray diffraction (XRD) and selected-area electron diffraction (SAED) data. Similar to U4O9−e (more precisely U64O143), U3O7 exhibits a long-range ordered structure, which is closely related to the fluorite-type arrangement of UO2. Cations remain arranged identical to that in the fluorite structure, and excess anions form distorted cuboctahedral oxygen clusters, which periodically replace the fluorite anion arrangement. The structure can be described in an expanded unit cell containing 15 fluorite-like subcells (U15O35), and spanned by basis vectors A = ap – 2bp, B = −2ap + bp, and C = 3cp (lattice parameters of the subcell are ap = bp = 538.00 ± 0.02 pm and cp = 554.90 ± 0.02 pm; cp/ap = 1.031). The arrangement of cuboctahedra in U3O...

Polycrystalline U

The paper presents and discusses lattice parameter analyses of pure, stoichiometric UO2. Attention was paid to prepare stoichiometric samples and to maintain stoichiometry throughout the analyses. The lattice parameter of UO2.000±0.001 was evaluated as being 547.127 ± 0.008 pm at 20 °C, which is substantially higher than many published values for the UO2 lattice constant and has an improved precision by about one order of magnitude. The higher value of the lattice constant is mainly attributed to the avoidance of hyperstoichiometry in the present study and to a minor extent to the use of the currently accepted Cu Kα1 X-ray wavelength value. Many of the early studies used Cu Kα1 wavelength values that differ from the currently accepted value, which also contributed to an underestimation of the true lattice parameter. ispartof: Journal of Nuclear Materials vol:459 pages:135-142 status: published

Valence band electronic structure of mixed uranium oxides (UO2, U4O9, U3O7, U3O8, UO3) has been studied by the resonant inelastic X-ray scattering (RIXS) technique at the U M5 edge and by computational methods. We show here that the RIXS technique and recorded U 5f-O 2p charge transfer excitations can be used to proof the validity of theoretical approximations, Chem Commun (2018)

The structural changes of ammonium diuranate (ADU) microspheres, prepared by the sol-gel route via internal gelation, were investigated during thermal treatments in oxidative and reducing conditions. In particular, in-situ simultaneous thermogravimetric analyses and in-situ high-temperature scanning electron microscopy investigations were carried out on un-doped ADU microspheres, and ADU microspheres doped with 10 mol% neodymium or 10 mol% cerium. Calcination in air caused the surface of the particles to crack, but with increasing temperature up to 900 °C some healing, and additionally, shrinkage occurred. The extent of the fractures and the amount of shrinkage was, however, significantly more pronounced in a particle prepared with a tetravalent Ce precursor, as compared to particles prepared with a trivalent Nd or Ce precursor. The macroscopic behaviour could be related to the release of volatile decomposition products and the calcined compositions were identified as (Ln-doped) α-U3O8 by X-ray powder diffraction. During thermal treatment in reducing conditions a transition from a (Ln-doped) α-U3O8 phase to a (Ln-doped) UO2±x phase was observed. After exposure to a hydrogen containing gas mixture, this transition occurred rapidly in un-doped particles, and in particles prepared with trivalent Nd or Ce dopant precursors at 700 °C. Despite the fast reduction reaction, severe fractures appeared in the particles above temperatures of 850 °C, indicating that such behaviour is mainly attributed to sintering effects and less to the phase transition. In contrast, a more delayed reduction reaction was observed in particles prepared with a tetravalent dopant precursor and the described effects appeared less severe.

Theslides werepresented at "Nuclear Fuel Cycle: A Chemistry Conference" (NFC3) onMay 5, 2021 as keynote speech (K1) in the session "Actinide Materials: Nuclear Fuels and Radwaste Matrices" (MAT). Abstract In an advanced nuclear fuel cycle, partitioning and transmutation (P&T) is a key strategy to reduce spent nuclear fuel’s radiotoxicity and heat generation. Long-lived minor actinides (MA) are partitioned from spent nuclear fuel and subsequently converted into fuel or targets materials for use in fast reactor systems, where the actinides are fissioned to short-lived radionuclides. The sol-gel route via internal gelation is a process that is currently explored for the production of such MA containing transmutation fuel, by converting an actinide-containing solution into a homogeneous precipitate with spherical geometry. Those precursors are thermally treated and can be used as particle fuel or compressed into fuel pellets. The advantages of the process are to avoid handling of fine powder and to facilitate automation for the production of nuclear fuel precursors. Within this study, the structural changes in Nd- and Ce- doped ammonium diuranate (ADU) gels, prepared by internal gelation during the conversion to U1-yLnyO2±x were investigated. Nd and Ce molar metal fractions up to 30mol% were introduced, acting as surrogates for the actinides Am and Pu. Nd was used in its trivalent oxidation state, while both trivalent and tetravalent cerium were tested. The dried Ln-doped ADU gels were characterised by optical microscopy and X-ray powder diffraction (XRD). Those precursors were thermally treated in two individual steps: (1) calcination under oxidising conditions at a maximum temperature of 900°C, followed by (2) sintering in reducing atmosphere at a maximum temperature of 1600°C (10h). The behaviour of the dried gels during both thermal treatment steps was studied in-situ by high temperature scanning electron microscopy (HT-SEM) and thermogravimetric analysis coupled with differential scanning calorimetry (TGA-DSC) and evolved gas analysis mass spectrometry (EGA-MS). The material prepared with the trivalent dopant precursors behaved significantly different during the decomposition in air compared to mixtures prepared with Ce(IV), which indicates different crystallinities and/or initial compositions. The calcined products were identified as (Ln-doped) α−U3O8 for dopant contents up to 10mol%. For higher Ln contents, a more complex mixed-phase behaviour was found. The introduction of a reducing atmosphere led to a sudden mass loss. Particle shrinkage occurred significantly later, pointing out that the shrinkage observed within the sintering is controlled by a general densification of the material and insignificantly by the volume change due to the transition of the orthorhombic (Ln-doped) α−U3O8 phase to the cubic (Ln-doped) UO2±x phase. The sintered products were identified as single phase solid solutions according to the formula U1-yLnyO2±x for Nd contents up to 30mol% and Ce contents up to 20mol%. In this contribution we will report and discuss the experimental results.

Theslides werepresented at the Uranium Science ConferenceonJuly 1, 2021 (T21). Abstract Uranium dioxide is used as conventional fuel for the production of energy by nuclear fission.Even though the front-end of the nuclear fuel cycle is well known, studies to investigate alternativefabrication routes to prepare precursors for oxidic uranium-based fuels are ongoing. The precipitation induced by thermal decomposition of urea has been demonstrated for several metals (e.g.Ti, Ni, Cu, Zn, Ce, Th), and a modified hydrothermal approach has been applied to precipitateammonium diuranate (ADU) from a solution containing uranyl ions. Within this study, we investigated the hydrolysis behaviour of uranyl and lanthanide mixturesto support the development of alternative fabrication routes for transmutation fuel, such as sol-gel processes. The lanthanides Nd and Ce acted as surrogates for the actinides Am and Pu,respectively. We specifically sought out parameters for the hydrolysis of uranyl ions induced bythermal decomposition of urea at ambient pressure. Moreover, the hydrolysis behaviour of Nd(III),Ce(III) and Ce(IV), as well as mixtures of the lanthanide- and uranyl ions, was investigated usingthe conditions determined for uranyl. Hydrolysis experiments were carried out at 90 °C and 100 °Cfor n(urea) :n(UO22+) ratios of 26and 52. The solution was sampled during the precipitationreaction to monitor its pH and certain samples were analysed applying UV/VIS spectroscopyand inductively coupled plasma mass spectrometry, while powder X-ray diffraction and scanningelectron microscopy were applied to characterise the precipitates. Uranyl ions hydrolysed between pH 5.1 and pH 5.5 and the experimental conditions impactedthe reaction kinetics significantly. A temperature increase from 90 °C to 100 °C reduced the time tofinish the precipitation by about 75 %, whereas a doubling of the urea content decreased the reactiontime by about 50 %. ADU precipitates of different composition (x UO3 · y NH3 · z H2O) formedunder the applied conditions. For trivalent Nd and Ce, a comparable pH evolution and lanthanidecarbonate hydroxide (LnCO3OH) products were observed, whereas tetravalent Ce hydrolysed at alower pH forming CeO2. The precipitation behaviour was confirmed for solutions containing binarymixtures of uranyl and lanthanide cations, while a simultaneous precipitation of Nd(III) and Ce(III) was observed for ternary U/Nd/Ce compositions. For the latter, a partial incorporation of the Lnphase into the ADU phase was observed, whereas the precipitation in the presence of Ce(IV)/CeO2led to the formation of three separate phases.