Nuclides can move with groundwater either as solutes or colloids,where the latter mechanism generally results in much shorter traveling time as the nuclides interact strongly with solid phases,such as actinides.In the performance assessment,it is therefore essential to assess the relative importance of these two transport mechanisms for different nuclides.The relative importance of colloids depends on the nature and concentration of the colloids in groundwater.Plutonium(Pu),neptunium(Np),uranium(U) and americium(Am) are four nuclides of concern for the long-term emplacement of nuclear wastes at potential repository sites.These four actinides have a high potential for migrating if attached to iron oxide,clay or silica colloids in the groundwater.Strong sorption of the actinides by colloids in the groundwater may facilitate the transport of these nuclides along potential flow paths.The solubility-limited dissolution model can be used to assess the safety of the release of nuclear waste in geological disposal sites.Usually,it has been assumed that the solubility of the waste form is constant.If a nuclide reaches its solubility limit at an inner location near the waste form,it is unlikely that the same nuclide will reach its solubility limit at an outer location unless this nuclide has a parent nuclide.It is unlikely that the daughter nuclides will exceed their solubility limit due to decay of their parent nuclide.The present study investigates the effect of colloids on the transport of solubility-limited nuclides under the kinetic solubility-limited dissolution(KSLD) boundary condition in fractured media.The release rate of the nuclides is proportional to the difference between the saturation concentration and the inlet aqueous concentration of the nuclides.The presence of colloids decreases the aqueous concentration of nuclides and,thus,increases the release flux of nuclides from the waste form.
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