The conversion of ammonium uranate prepared via sol-gel synthesis into uranium oxides

  • Schreinemachers, Christian (Belgian Nuclear Research Centre (SCK.CEN), Institute for Nuclear Materials Science) ;
  • Leinders, Gregory (Belgian Nuclear Research Centre (SCK.CEN), Institute for Nuclear Materials Science) ;
  • Modolo, Giuseppe (Forschungszentrum Julich GmbH, Institute of Energy and Climate Research (IEK)) ;
  • Verwerft, Marc (Belgian Nuclear Research Centre (SCK.CEN), Institute for Nuclear Materials Science) ;
  • Binnemans, Koen (KU Leuven, Department of Chemistry) ;
  • Cardinaels, Thomas (Belgian Nuclear Research Centre (SCK.CEN), Institute for Nuclear Materials Science)
  • Received : 2019.08.22
  • Accepted : 2019.11.04
  • Published : 2020.05.25


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 NH3 - UO3 - H2O were synthesised under specified conditions. Microspheres prepared by the sol-gel method via internal gelation were identified as 3UO3·2NH3·4H2O, whereas the product of a typical ammonium diuranate precipitation reaction was associated to the composition 3UO3·NH3·5H2O. The thermal decomposition profile of both compounds in air feature distinct reaction steps towards the conversion to U3O8, owing to the successive release of water and ammonia molecules. Both compounds are converted into α-U3O8 above 550 ℃, but the crystallographic transition occurs differently. In compound 3UO3·NH3·5H2O (ADU) the transformation occurs via the crystalline β-UO3 phase, whereas in compound 3UO3·2NH3·4H2O (microspheres) an amorphous UO3 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.


  1. F.W. Van Der Brugghen, M.E.A. Hermans, J.B.W. Kanij, A.J. Noothout, T. Van Der Plas, H.S.G. Slooten, Sol-gel Processes for the Preparation of Spherical Thorium-Containing Fuel Particles, Technical Report, Keuring van Electrotechnische Materialen, NV, Arnhem (Netherlands), 1968.
  2. R. Forthmann, Die chemischen Grundlagen des Hydrolyseverfahrens zur Herstellung spharischer Kernbrennstoffteilchen, Technical Report Jul-950-RW, Kernforschungsanlage Julich, Institut fur Reaktorwerkstoffe, 1973.
  3. P.A. Haas, J.M. Begovich, A.D. Ryon, J.S. Vavruska, Chemical flowsheet conditions for preparing urania spheres by internal gelation, Ind. Eng. Chem. Prod. Res. Dev. 19 (1980) 459-467,
  4. R.D. Hunt, J.L. Collins, Uranium kernel formation via internal gelation, Radiochim. Acta 92 (2004) 909-915,
  5. V.N. Vaidya, Status of sol-gel process for nuclear fuels, J. Sol. Gel Sci. Technol. 46 (2008) 369-381,
  6. S. Li, J. Bai, S. Cao, X. Yin, C. Tan, P. Li, W. Tian, J. Wang, H. Guo, Z. Qin, An improved internal gelation process without cooling the solution for preparing uranium dioxide ceramic microspheres, Ceram. Int. 44 (2018) 2524-2528,
  7. M.A. Pouchon, G. Ledergerber, F. Ingold, K. Bakker, Sphere-pac and VIPAC fuel, in: R.J. Konings (Ed.), Comprehensive Nuclear Materials, Elsevier BV, Oxford, 2012, pp. 275-312,
  8. S. Suryanarayana, N. Kumar, Y.R. Bamankar, V.N. Vaidya, D.D. Sood, Fabrication of $UO_2$ pellets by gel pelletization technique without addition of carbon as pore former, J. Nucl. Mater. 230 (1996) 140-147,
  9. A. Kumar, J. Radhakrishna, N. Kumar, R.V. Pai, J.V. Dehadrai, A.C. Deb, S.K. Mukerjee, Studies on preparation of $(U_{0.47},Pu_{0.53})O_2$ microspheres by internal gelation process, J. Nucl. Mater. 434 (2013) 162-169,
  10. G. Ledergerber, F. Ingold, R.W. Stratton, H.-P. Alder, C. Prunier, D. Warin, M. Bauer, Preparation of transuranium fuel and target materials for the transmutation of actinides by gel coconversion, Nucl. Technol. 114 (1996) 194-204,
  11. H. Daniels, Herstellung uranbasierter Keramiken mittels interner Gelierung zur Konversion trivalenter Actinoiden, Ph.D. thesis, RWTH Aachen, 2012,
  12. C. Schreinemachers, A.A. Bukaemskiy, M. Klinkenberg, S. Neumeier, G. Modolo, D. Bosbach, Characterization of uranium neodymium oxide microspheres synthesized by internal gelation, Prog. Nucl. Energy 72 (2014) 17-21,
  13. V.N. Vaidya, S.K. Mukherjee, J.K. Joshi, R.V. Kamat, D.D. Sood, A study of chemical parameters of the internal gelation based sol-gel process for uranium dioxide, J. Nucl. Mater. 148 (1987) 324-331,
  14. J.L. Collins, M.F. Lloyd, R.L. Fellows, The basic chemistry involved in the internal-gelation method of precipitating uranium as determined by pH measurements, Radiochim. Acta 42 (1987) 121-134,
  15. M.H. Lloyd, K. Bischoff, K. Peng, H.-U. Nissen, R. Wessicken, Crystal habit and phase attribution of U(VI) oxides in a gelation process, J. Inorg. Nucl. Chem. 38 (1976) 1141-1147,
  16. A.S. Abdel-Halim, F.A. El-Nour, N. Belacy, H.F. Aly, T. Khalil, Physico-chemical characteristics of uranium oxide microspheres produced by internal gelation, Isotopenpraxis Isotopes in Environmental and Health Studies 26 (1990) 524-529,
  17. E.H.P. Cordfunke, On the uranates of ammonium - I: the ternary system $NH_3-UO_3-H_2O$, J. Inorg. Nucl. Chem. 24 (1962) 303-307,
  18. R. Eloirdi, D. Ho Mer Lin, K. Mayer, R. Caciuffo, T. Fanghanel, Investigation of ammonium diuranate calcination with high-temperature X-ray diffraction, J. Mater. Sci. 49 (2014) 8436-8443,
  19. S. Manna, P. Karthik, A. Mukherjee, J. Banerjee, S.B. Roy, J.B. Joshi, Study of calcinations of ammonium diuranate at different temperatures, J. Nucl. Mater. 426 (2012) 229-232,
  20. A.I. Karelin, A.N. Zhiganov, O.P. Lobas, A.A. Zhiganova, Mechanism and kinetics of thermal-decomposition of ammonium uranates in various gaseous media, Radiokhimiya 31 (1989) 117-124.
  21. P.A. Haas, J.M. Begovich, A.D. Ryon, J.S. Vavruska, Chemical Flowsheet Conditions for Preparing Urania Spheres by Internal Gelation, Technical Report ORNL/TM-6850, Oak Ridge National Laboratory (ORNL), 1979,
  22. I. Grenthe, J. Drozdzynski, T. Fujino, E.C. Buck, T.E. Albrecht-Schmitt, S.F. Wolf, Uranium, in: L.R. Morss, N.M. Edelstein, J. Fuger (Eds.), The Chemistry of the Actinide and Transactinide Elements, Springer Netherlands, Dordrecht, 2011, pp. 253-698,
  23. R.D. Hunt, J.L. Collins, M.H. Lloyd, S.C. Finkeldei, Production of more ideal uranium trioxide microspheres for the sol-gel microsphere pelletization process without the use of carbon, J. Nucl. Mater. 515 (2019) 107-110,
  24. J. Hartwig, G. Holzer, E. Forster, K. Goetz, K. Wokulska, J. Wolf, Remeasurement of characteristic X-ray emission lines and their application to line profile analysis and lattice parameter determination, Phys. Status Solidi 143 (1994) 23-34,
  25. C. Schreinemachers, G. Leinders, Thermal Decomposition Data of Uranium Containing Microspheres Produced via Internal Gelation and Ammonium Diuranate Powder, 2019, (license: CC BY-NC-SA).
  26. L.J. Cabri, J.H.G. Laflamme, Rhodium, platinum, and gold alloys from the Stillwater Complex, Can. Mineral. 12 (1974) 399-403. https://pubs.
  27. P.C. Debets, B.O. Loopstra, On the uranates of ammonium - II: X-ray investigation of the compounds in the system $NH_3-UO_3-H_2O$, J. Inorg. Nucl. Chem. 25 (1963) 945-953,
  28. B.O. Loopstra, On the existence of ${\delta}$-$U_3O_8$; a comment on papers by Karkhanavalla and George, and by Amirthalingam, J. Nucl. Mater. 29 (1969) 354-355,
  29. C. Gueneau, A. Chartier, L.V. Brutzel, Thermodynamic and thermophysical properties of the actinide oxides, in: R.J. Konings (Ed.), Comprehensive Nuclear Materials, Elsevier, Oxford, 2012, pp. 21-59, B978-0-08-056033-5.00009-4.
  30. M.C. Burrell, D.A. Lee, Analysis of $UO_3$ gel Microsphere Decomposition Products by Mass Spectrometry, Technical Report ORNL/TM-6649, Oak Ridge National Laboratory (ORNL), 1979,
  31. D.A. Lee, D.P. Stinton, Evaluation of Urania Gel Pyrolysis by Mass Spectrometry, Technical Report ORNL/TM-6538, Oak Ridge National Laboratory (ORNL), 1978,
  32. B.O. Loopstra, The structure of ${\beta}$-$U_3O_8$, Acta Crystallogr. B 26 (1970) 656-657,