Goldschmidt Conference: Hagemann

Water activity and modeling of ternary solutions of the systems CsCl-MCl-H2O and Cs2SO4-MSO4-H2O (M=Na,K,Mg) at 25 to 90 °C
HAGEMANN, S.1*; BISCHOFER, B.1; LU, T.2

1Gesellschaft für Anlagen- und Reaktorsicherheit (GRS),
Theodor-Heuss-Str. 4, 38122 Braunschweig, Germany
(*correspondance: )
2Technische Universität Braunschweig, Universitätsplatz 2,
38106 Braunschweig, Germany

Cesium-137 is a medium-lifetime radionuclide resulting
from fission of nuclear fuel such as uranium-235 and other
isotopes.
Description of its behaviour in the vicinity of geological
repositories for nuclear waste requires a thermodynamic
model that allows the calculation of activity coefficients for
the ion Cs+ at elevated temperatures relevant for the specific
disposal concept (e.g. up to 90°C) and the typical salinity of
the formation waters.
Earlier studies by GRS resulted in a set of Pitzer ion
interaction parameters for the system Cs, Na, K, Mg, Ca, Cl,
SO4-H2O at 25°C [1,2].
Additional isopiestic and calorimetric measurements were
conducted in binary and ternary aqueous solutions containing
cesium at temperatures between 25 and 90°C. The
experiments were designed to cover solution concentrations
up to the saturation of the individual Na, K, and Mg salts but
with restricted Cs content (< 7 mol/kg). Data from the
measurements were combined with literature data and the
THEREDA Pitzer model for the seawater system [3] in order
to provide a model for Cs+ applicable at 25 to 90°C.
Measurements in ternary systems such as CsCl-NaCl-H2O
and CsCl-MgCl2-H2O clearly showed a markable deviation
from ideal mixing. This can be interpreted as evidence for
significant ion pairing or complex formation. Nevertheless,
the developed Pitzer model performs sufficiently well without
explicit consideration of additional Cs species.
This research was funded by the German Federal Ministry
for Economic Affairs and Energy under the contract No. 02 E
11607A.

[1] Scharge et al. (2012) J. Chem. Eng. Data 57, 1637–
1647. [2] Scharge et al. (2013) J. Chem. Eng. Data 58, 187–
188. [3] Moog et al. (2015) Appl. Geochem. 55, 72–84