Search

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.