Actinides such Pu-239 or Np-237 have low solubility limits, and they are released at a concentration below or equal to their solubility limits (or colloid concentrations), which in turn are determined by the SNF matrix dissolution rate, groundwater flow rate and solubility limit.

Regarding colloid formation, Ahn (1996a) summarized the processes involved. During the dissolution of the SNF matrix, suspended solid parti­cles containing mainly actinides of low solubility may form. The colloids can carry a large amount of actinides compared with dissolved species. The traditional processes of colloid formation (especially in actinide colloids) have been investigated under near-equilibrium conditions. Most studies in this regard pertain to chemical bonding among ions. Extending the chemical bonding process in equilibrium or non-equilibrium states, colloid formation may be described in macroscopic ways by three different processes: (a) condensation, (b) dispersion, and (c) sorption (pseudo-colloid formation). Colloids may form by precipitation in small particles because of supersatu­ration of actinides or the SNF matrix (i. e., condensation). The layer of the precipitated phases on the SNF matrix can be mechanically detached into small suspended particles (i. e., dispersion). Finally, the dissolved pure acti­nide species can be sorbed on the surface of non-radioactive inert ground­water colloids (i. e., sorption). Figure 7.9 shows schematics of these three processes (CRWMS M&O, 2001).