Colloids radionuclide interaction

The potentially important role of colloidal species in the geochemical behaviour of the polyvalent actinides has nevertheless been stated by various authors (e.g., Kim 1991 Kersting et al. 1999). The present paper discusses the role of colloids on the release of radionuclides from a nuclear waste repositoiy with regard to the processes leading to (1) colloid generation and stability (2) radionuclide interaction with aquatic colloids and (3) colloid-borne radionuclide migration. 

Formation of intrinsic colloids in natural waters can be excluded for radioisotopes of elements of groups 0, I and VII, and the probability that they may be formed is small for radioisotopes of elements of other groups as long as the concentration of the elements is low. In general, formation of carrier colloids by interaction of radionuclides with colloids already present in natural waters is most probable. Thus, clay particles have a high affinity for heavy alkali and alkaline-earth ions, which are bound by ion exchange. This leads to the formation of carrier colloids with Cs, Ra and °Sr. Formation of radiocolloids with hydrolysing species has already been discussed (section 13.4). 

Dearlove JLP, Longworth G, Ivanovich M, Kim Jl, Delakowitz B, Zeh P (1991) A study of groundwater colloids and their geochemical interactions with natural radionuclides in Gorleben aquifer systems. Radiochim Acta 52/53 83-89... 

Moore WS, Demaster DJ, Smoak JM, McKee BA, Swarzenski PW (1996) Radionuclide tracers of sediment-water interactions on the Amazon shelf. Cont Shelf Res 16 645-665 Moran SB, Moore RM (1989) The distribution of colloidal aluminum and organic carbon in coastal and open ocean waters offNova Scotia. Geochim Cosmochim Acta 53 2519-2527 Nozaki Y (1991) The systematics and kinetics of U/Th decay series nuchdes in ocean water. Rev Aquatic Sci 4 75-105... 

The reliable long-term safety assessment of a nuclear waste repository requires the quantification of all processes that may affect the isolation of the nuclear waste from the biosphere. The colloid-mediated radionuclide migration is discussed as a possible pathway for radionuclide release. As soon as groundwater has access to the nuclear waste, a complicated interactive network of physical and chemical reactions is initiated, and may lead to (1) radionuclide mobilization (2) radionuclide retardation by surface sorption and co-precipitation reactions and (3) radionuclide immobilization by mineralization reactions, that is, the inclusion of radionuclides into thermodynamically or kinetically stabilized solid host matrices. 

Much stronger kinetic stabilization can be expected for processes leading to the inclusion of radionuclide ions into the colloid structure (Fig. 7, lower part). Spectroscopic indications for such processes have indeed been found again by TRLFS for the Cm(III) interaction with colloidal and particulate amorphous silica, calcite and CSH phases (Chung et al. 1998 Stumpf Fanghanel 2002 Tits et al. 2003). The incorporation of actinide ions into colloidal precursor clay phases has been recently investigated as a possible mechanism in natural... 

Mechanisms that may lead to the quasi irreversible binding of radionuclides to colloids belong to the key uncertainties of the assessment of the colloid problem. The kinetics of the dissociation of colloid-bound radionuclides are not yet understood. Radionuclide incorporation into stable colloids may enhance the colloid-mediated radionuclide release considerably. It is clear that only the investigation of the interaction mechanisms by spectroscopic methods is able to unravel the relevance of such processes. In order to allow the description of colloid-facilitated radionuclide migration, it is furthermore required to improve our understanding of the colloid interaction... 

If colloids generated by waste package component interactions readily flocculate or are otherwise removed from solution soon after formation, they may not represent a waste management problem because colloidal transport of radionuclides would be limited. 

In the previous interaction experiments (1-3), no evidence for flocculation or precipitation was reported over the pH range 6 to 9.5, which implies that the colloids remain as sols and could potentially be transported. If transport is possible, then it is desirable to know the extent to which sorbed radionuclides could also be transported. 

Adsorption of complexes of radionuclides with inorganic or organic ligands (in particular complexes with humic substances) and of colloidal species of radionuclides may also markedly influence the migration behaviour. The predominant kind of interaction is physical adsorption. 

Ivanovich, M. Longworth, G. Hasler, S. E. Gardiner, M.P. In Colloid migration in Groundwaterss geochemical interactions of radionuclides with natural colloids Kim, I. J. and Delakowitz B, Eds. Second Progress Report RCM 00692,1992. 

The colloid problem is composed of a number of elements, including (i) the identification, isolation, and characterization of colloidal materials from target environments (ii) the development of predictive models for colloid release, transport, and deposition (iii) the characterization of interactions between radionuclides and colloids (sorptive processes for pseudocolloids, radionuclide solubility for intrin-... 

Figure 7-12 is a comparison of the U(VI) sorptive behavior of three surface types. Sorption data are normalized to surface sites P = [U(VI)sorbed]/ [U(VI)soiu-tion] [colloid sites]. Note that (i) the surfaces dominate the sorption of U( VI) at different pH values and (ii) the surfaces are markedly different in their ability to sorb U(VI). Because the relative ability of a surface to compete against other sorbents for a radionuclide is a function of the product of the interaction parameter (e.g., P or and the site concentration (e.g., Eq. [2]), Fig. 7-12 suggests, for example, that Desulfovibrio vulgaris could be an effective colloidal transport agent at substantially lower concentrations than in the cases of the NOM and hematite. 

To ensure that the GDF is fit for purpose, we need to understand the long term effects of irradiation on materials as well as the interactions between the various components of a facility and the wastes. Radiochemical research will play an important role in helping us luiderstand these processes and will enable us to implement the best strategies for long term disposal. Studies on radionuclide release from spent fuel and vitrified wastes, the effects of complexants on radionuclide behaviour, the impact of microbes on the mobility of radionuclides and the behaviour of colloids (see Box 1) and non-aqueous phase liquids (see Box 2) - all of these pose questions that need to be answered as we develop the safety case for a geological disposal facility. 

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