The Importance of Secondary Phase Formation in Natural Aqueous Systems

Irradiated fuel based on UO₂ is expected to be disposed of in an underground repository according to a “once-through” fuel cycle policy. In addition, those countries that have chosen reprocessing their irradiated fuel will also need this kind of repository, as there is international consensus on this option as the best to properly dispose high-level waste. In any case, after storage times of several thousand years, it is foreseen and accepted that groundwater will penetrate and infiltrate in the emplacement, and will eventually interact with the spent fuel.<br/>As a consequence of water radiolysis, uranium is oxidized from its tetravalent state to the hexavalent uranyl ion, being known to be far more soluble in water than uranium (IV). Then, during the oxidative dissolution and depending of the surface/volume ratio, secondary phases containing uranyl cationic UO₂²⁺ will precipitate at the whole pH range. These secondary phases play an essential role on the radionuclide release in the final disposal environment due to its capacity to seize trace radioelements, and then, to reduce radionuclides mobility. In fact, some important radionuclides can precipitate into its inner solid structure (<i>e.g.</i> Schoepite UO₃x2H₂O, Becquerelite Ca(UO₂)₆O₄(OH)₆x8H₂O, Soddyite (UO₂)₂SiO₄x2H₂O, Uranophane (H₃O)₂Ca(UO₂)₂(SiO₄)₂x3H₂O, Studtite UO₄x4H₂O).<br/>The objective of this work is to provide an overview of current knowledge on precipitation and dissolution of potential radionuclide-bearing phases under repository relevant conditions. Different approaches carried out are presented, <i>i.e.</i> long-term behavior of natural analogs such as uraninite [1, 2], short-term laboratory coprecipitation experiments [3] and theoretical studies [4,5]. A review of the analysis methods is also presented.<br/>REFERENCES<br/>[1] L. J. Bonales, C. Menor-Salván, and J. Cobos, "Study of the alteration products of a natural uraninite by Raman spectroscopy," <i>Journal of Nuclear Materials, </i>vol. 462, pp. 296-303, 2015/07/01/ 2015.<br/>[2] L. J. Bonales, J. M. Elorrieta, C. Menor-Salván, and J. Cobos, "The behavior of unirradiated UO₂ and uraninite under repository conditions characterized by Raman," <i>MRS Advances, </i>vol. 1, pp. 4157-4162, 2017.<br/>[3] J. Quiñones, A. González de la Huebra, and A. Martínez Esparza, "Coprecipitation experiments using simulated spent fuel solution in the presence of metallic iron in synthetic bentonitic-granitic water under oxidising conditions," in <i>Scientific Basis for Nuclear Waste Management XXVIII</i>. vol. 824, S. Stroes-Gascoyne, J. Hanchar, and L. Browning, Eds., ed San Francisco. USA: Material Research Society, 2004, pp. 425-430.<br/>[4] F. Colmenero, L. J. Bonales, J. Cobos, and V. Timón, "Study of the thermal stability of studtite by in situ Raman spectroscopy and DFT calculations," <i>Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, </i>vol. 174, pp. 245-253, 2017/03/05/ 2017.<br/>[5] F. Colmenero, V. Timón, L. J. Bonales, and J. Cobos, "Structural, mechanical and Raman spectroscopic characterization of the layered uranyl silicate mineral, uranophane-α, by density functional theory methods," <i>Clay Minerals, </i>vol. 53, pp. 377-392, 2018.