Interactions between U(VI) doped CSH phases and high saline brines


Interactions between U(VI) doped CSH phases and high saline brines

Wolter, J.-M.; Schmeide, K.; Stumpf, T.

Cementitious materials will be used for the geotechnical barrier of a deep geological repository where long-lived radioactive waste like used fuel rods will be stored [1]. Calcium silicate hydrate (CSH) phases, as main component of hardened cement paste, are known for their retention potential for radionuclides like U which is the main component of used fuel rods [2].

The formation water of North German clay stone formations is characterized by high ionic strengths [3], which potentially lead to a corrosion of concrete. Thus, with respect to long-term safety assessment of cement containing repositories chemical alteration processes at high saline conditions have to be studied. Therefore, the leaching of U(VI) doped CSH phases in high ionic strengths electrolyte solutions is studied in batch experiments in combination with spectroscopic methods.

The approach of this study is to get a molecular understanding of interactions between U(VI) doped CSH phases and high ionic strengths electrolyte solutions. For this, batch leaching experiments were performed in combination with spectroscopic methods.

The formation of CSH phases is confirmed by XRD and IR / Raman spectroscopy. The TRLFS spectra obtained for U(VI) doped CSH phases are comparable to those reported by Tits et al. [4]. Thus, an incorporation of U(VI) in the CSH phases is verified. The results of leaching experiments are exemplarily shown for Ca for the CSH phase with C/S ratio of 1.6 for in Figure. 1. In presence of NaCl and Na2SO4, Ca is released into the supernatant solution. This mainly can be attributed to a release of Ca from the interlayers of CSH phases. Simultaneously, the pH values of the supernatant solutions increase to values between 11.2 and 12, depending on the C/S ratio. In the presence of NaHCO3, the Ca concentration in solution is much lower due to precipitation of CaCO3. In this case, pH values between 11.4 and 10.1 are observed.

In the presence of NaCl and Na2SO4, the release of Si and U is minimal. In the presence of NaHCO3, however, the Si and U concentration in solution is increased. Since Si can only be released from the complex layers of CSH phases, the release of Si and simultaneously, the release of U can be related to a decomposition of the CSH phases in the presence of carbonate. Spectroscopic investigations also confirmed the decomposition of CSH phases and the release of U due to carbonate.

Keywords: CSH phases; uranium; North Germany; clay stone; cement; retention; leaching; PXRD; TRLFS; IR; RAMAN; saline; salt

  • Contribution to proceedings
    MIGRATION 2017, 10.-15.09.2017, Barcelona, España
    Interactions between U(VI) doped CSH phases and high saline brines
  • Contribution to proceedings
    ABC Salt V, 26.-28.03.2017, Ruidoso, New Mexico, USA
    Proceedings of ABC Salt V
  • Poster
    ABC Salt V Actinide and Brine Chemistry in a Salt Repository Workshop (V), 26.-28.03.2017, English, USA
  • Poster
    Migration 2017 16th International Conference on the Chemistry and Migration Behaviour of Actinides and Fission Products in the Geosphere, 10.-15.09.2017, Barça, España

Permalink: https://www.hzdr.de/publications/Publ-25238
Publ.-Id: 25238