The description of the accumulation of radionuclides in some biospheric compartments is in general based on a sorption distribution coefficient Kd. This value is very decisive for the concentration of long-lived radionuclides in reservoirs that are important from the dose point of view.
Sorption is due to several processes such as ion-exchange and a variety of physical and chemical interactions which are difficult to interpret with the current Kd-methodology. In addition, many of the Kd values are obtained from laboratory or geospheric conditions not comparable to conditions prevailing in the biosphere. The main objective with this work is to deepen
the knowledge about the theoretical background of Kd-values.
To achieve this purpose, available theoretical models for ion-exchange and surface-complexation have been adapted for simulation under biospheric conditions.
The elements treated are cesium, radium, neptunium, uranium and
plutonium. The results show that a triple layer surface complexation model may be used in estimating Kd-values for actinides as a function of important chemical parameters such as pH and EH.
It is concluded that by estimating some equilibrium constants and making some careful approximations, surface complexation models can be used for performance assessment of radioactive waste repositories.
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