Development of a new methodology for realistic describing radionuclide retardation in safety assessment

Conceptual model of the new methodology
In previous safety assessments the Kd concept with temporally constant values was applied to describe the radionuclide retardation in the far field of a repository.
Here, the existing transport program r3t [1] used for large model areas and very long time scales has been improved by a new methodology. Implementing the smart Kd concept based on surface complexation allows to consider varying realistic geochemical conditions.
The methodology is based on a description of the sorption of radionuclides as a function of selected, important environmental parameters Ei such as pH, pCO2, ionic strength, concentration of the cations Ca2+ and Al3+ and presence of mineral phases. Exemplarily for a potential repository site in Germany, the Gorleben site has been selected as application case for a proof-of-concept.
Most of the individual parameters Ei are not available in r³t so far. Thus the transport of these influence factors as well as equations describing pH and concentrations of ions as a function of accessible mineral phases had to be implemented. Then the reactive transport model r³t can call for each time-space point pre-calculated Kd values for selected sediments. They are stored in a multidimensional matrix depending on the respective geochemical conditions. Figure 1 shows the frequency distribution of the pre-calculated Kd-values for Am3+ and U6+ and their classification.

Figure 1: Frequency distribution of the pre-calculated Kd values for Am3+ and U6+.

[1] Fein (2004) Report GRS-192.