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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.

La 1 − x Nd x PO 4 monazite-type ceramics were synthesized by precipitation and sintering. The obtained crystalline powders and pellets were structurally characterized by X-ray diffraction, electron microscopy, and infrared and Raman spectroscopy. The determined unit cell constants agree with published values for the pure end-members ranging from LaPO 4 to DyPO 4 , as well as with values for known mixed monazite-type lanthanide orthophosphates, indicating a homogeneous solid solution. However, Raman spectroscopy shows a change in short-range order and the results of high temperature oxide melt solution calorimetry suggest a subregular solid solution, which requires two Margules parameters to calculate the observed enthalpy of mixing Δ H mix for La 1 − x Nd x PO 4 , which is positive and asymmetric and shows a maximum at x = 0.299 ( Δ H mix = 7.53 ± 0.01 kJ mol − 1 ) at 298 K.

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.

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.

The order-disorder phase transition in the Nd x Zr 1−x O 2−0.5x system is studied by complementary techniques which include wet chemical synthesis of a series of compositions with various Nd/Zr ratios with the final annealing at 1873 K, X-ray diffraction, oxide melt solution calorimetry and ab initio thermodynamic modeling. Our structural data indicate the transition from ordered to disordered pyrochlore at x ∼0.31 at a temperature of 1873 K. Our calorimetric data show a transition enthalpy of ∼30 kJ/mol, which corresponds to an entropy of disordering of ∼16 J/K/mol. The latter value is significantly smaller than the configurational entropy of transition computed under the assumption of complete disorder in a fluorite phase, indicating a substantial degree of order remaining in the fluorite phase at the temperature of synthesis. The considered phases are computed ab initio using a series of special quasi-random structures that emulate the complete or partial disorder. The results of our calculations and thermodynamic modeling are in good agreement with the measured lattice parameters, the Nd content at the order-disorder transition, and the measured formation and transformation enthalpies. Thus our combined experimental and modeling results provide valuable insight into the disordering of this pyrochlore phase and of other pyrochlore materials.

Enthalpy of formation (ΔH°f) of single phase orthophosphate solid solutions La1−xLnxPO4 (Ln = Eu, Gd; 0

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 monazite-type solid solution La 1 − x Pr x PO 4 was synthesized by solid-state reaction and extensively investigated using electron microprobe and thermogravimetric analyses, differential scanning and high-temperature oxide melt solution calorimetry, powder X-ray diffraction, infrared and Raman spectroscopy. Lattice parameters and Ln-O bond lengths show a decrease with increasing Pr content. A small excess volume is observed for the solid solution. IR spectra of the solid solution members present no detectable differences, while a blue shift of the PO4-related modes is seen in the Raman data. This shift can be attributed to the lanthanide contraction. Within errors, calorimetry data show no systematic deviation from an ideal behavior, though one might interpret the data as an indication of a slightly asymmetric mixture. All data indicate that deviations from ideality of the solid solution – if present – are very small.

The direct selective separation of the trivalent actinides americium and curium from a simulated Plutonium Uranium Refining by EXtraction (PUREX) raffinate solution by a continuous counter-current solvent extraction process using miniature annular centrifugal contactors was demonstrated on a laboratory scale. In a 32-stage spiked test (12 stages for extraction, 16 stages for scrubbing, and 4 stages for Am/Cm stripping), an extractant mixture of CyMe4BTBP and TODGA in a TPH/1-octanol mixture was used. The co-extraction of some fission and corrosion product elements, such as zirconium and molybdenum, was prevented by using oxalic acid. Co-extracted palladium was selectively stripped using an L-cysteine scrubbing solution and the trivalent actinides were selectively stripped using a glycolic acid-based stripping solution. It was demonstrated that a selective extraction and high recovery of > 99.4% of the trivalent minor actinides was achieved with low contamination by fission and corrosion products. The prod...

During the partitioning of trivalent actinides from High Active Raffinate (HAR) solutions, most processes have to cope with an undesirable co-extraction of some of the fission products. Four hydrophilic complexing agents of the group of polyaminocarboxylic acids, namely EDTA, HEDTA, DTPA, and CTDA were tested and compared for their ability to complex fission products in a simulated PUREX raffinate solution, thereby preventing their extraction into an organic solvent. Several solvents, based on TODGA and the DIAMEX reference molecule DMDOHEMA, that are commonly known to show quite high Zr and Pd co-extraction, were studied. Our investigations ultimately resulted in a substitution of oxalic acid and HEDTA by cyclohexanediaminetetraacetic acid (CDTA). A small addition of this hydrophilic complexing agent to the feed decreased the distribution ratios of Zr from 100 to 90% of the metal retained in the feed solution. The extraction of trivalent...

Within the framework of our research activities related to the partitioning of spent nuclear-fuel solutions, the direct selective extraction of trivalent actinides from a simulated PUREX raffinate was studied using a mixture of CyMe4BTBP and TODGA (1-cycle SANEX). The solvent showed a high selectivity for trivalent actinides with a high lanthanide separation factor. However, the coextraction of some fission product elements (Cu, Ni, Zr, Mo, Pd, Ag, and Cd) from a simulated PUREX raffinate was observed, with distribution ratios up to 30 (Cu). The extraction of Zr and Mo could be suppressed using oxalic acid but the use of the well-known Pd complexant N-(2-Hydroxyethyl)-ethylendiamin-N,N',N'-triacetic acid (HEDTA) was unsuccessful. During screening experiments with different amino acids and derivatives, the sulfur-bearing amino acid L-Cysteine showed good complexation of Pd and prevented its extraction into the organic phase without influencing the extraction of the trivalent actinides Am (III) and Cm (III). The optimization studies included the influence of the L-Cysteine and HNO3 concentration and the kinetics of the extraction. The development of a process-like extraction series showed very promising results in view of further optimizing the process. A strategy for a single-cycle process is proposed within this article.

The extractant, N,N,N′,N′‐tetraoctyl diglycolamide (TODGA) has been evaluated for the separation of actinides(III) and lanthanides(III) from a high active raffinate (HAR). The effect of oxalic acid and HEDTA complexant on the extraction of actinides(III), lanthanides(III), and important fission products (e.g. Mo, Pd, Sr, Zr, Ru etc.) from synthetic HAR has been studied with 0.2 mol/L TODGA in TPH. With an extractant mixture of TODGA and tributyl phosphate (TBP) the amount of oxalic acid can be reduced to less than 0.3 mol/L for the effective complexation of zirconium, whereas the distribution ratios of actinides(III) and lanthanides(III) are still high for the separation from HAR. Furthermore the maximum loading of lanthanides (e.g. Nd) can be significantly increased by adding TBP to the extractant. However, the extraction of oxalic acid and nitric acid also increased by the addition of TBP, which can lead to problems during back extraction of the loaded extractant. Extraction studies after radio...

A 30% (w/w) N,N,N′,N′-tetraoctyldiglycol amide (TODGA) and 10% (w/w) tributyl-phosphate (TBP)-containing resin was prepared and the batch uptake of several actinides [U(VI), Th(IV), Am(III), Cm(III), Cf(III)] and selected fission products from nitric acid solutions was investigated. Two chromatographic column tests were carried out with a synthetic PUREX raffinate containing nominal amounts of fission products and trace amounts of Am(III), Cm(III) and Cf(III). The results of the separation experiments revealed that actinides and lanthanides could be separated from the bulk of other fission products by extraction chromatography using TODGA and TBP as the stationary phases.

In the context of advanced nuclear fuel cycles including partitioning and transmutation (P&T), actinide co-conversion processes play an important role to close the loop. Therefore, actinides are embedded in ceramics to be used as precursors for the fabrication of fuels. The sol–gel route by internal gelation is a suitable and dust-free conversion method. Furthermore, the required spherical particles are already formed at the gel stage. Pure UO 2 microspheres and U/Nd oxide particles with a variable content of Nd (5–40%) were fabricated by internal gelation. Acid deficient uranyl nitrate and Nd nitrate solutions were used as precursor for the microsphere fabrication. Neodymium is used as a surrogate for trivalent actinides, such as americium. Mass- and diameter-distributions were studied and particle densities were calculated. Finally, the microspheres were thermally treated in a reducing H 2 :Ar gas atmosphere at 1573 K. The solid solutions of UO 2 and Nd 2 O 3 were investigated by scanning electron microscopy and X-ray powder diffraction. The obtained reflexes are indicating one cubic phase. Lattice parameter calculations were performed, the results depend linear on the Nd content up to ≈ 30%. Particle compositions containing more Nd are showing an unexpected behavior.

Conversion of actinides into oxide precursors for innovative fuel fabrication

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.