Transport of radionuclides

Heussner S, Cherry RD, Heyraud M (1990) Po-210 and Pb-210 in sediment trap particles on a Mediterranean continental margin. Cont. Shelf Res 10 989-100 Heyraud M, Cherry RD (1983) Correlation of Po-210 and Pb-210 enrichments in the sea-surface microlayer with neuston biomass. Cont Shelf Res 1 283-293 Honeyman BD, Santschi PH (1989)The role of particles and colloids in the transport of radionuclides and trace metals in the oceans. In Environmental particles. Buffle J, van Leewen HP (eds) Lewis Publishers, Boca Raton, p 379-423... 

Kennedy WE, Cadwell LL, McKenzie DH. 1985. Biotic transport of radionuclides from a low-level radioactive waste site. Health Phys 49(1) 11-24. 

Parker, F. L., Churchill, M. A., Andrew, R. W., Frederick, B. J., Carrigan, P. H. Jr., Cragwall, J. S. Jr., Jones, S. L., Struxness, E. G. and Morton, R. J. (1966). Dilution, dispersion and mass transport of radionuclides in the Clinch-Tennessee Rivers, page 35 in Disposal of Radioactive Wastes into Seas, Oceans and Surface Waters, IAEA Publication No. STI/PUB/126 (International Atomic Energy Agency, Vienna). 

In this article we plan to focus on two aspects (i) the transport of radionuclides to the ocean floor and the processes which govern their distribution in deep-sea sediments and (ii) the application of deep-sea sediments to retrieve historical records of large scale phenomena, e.g. long term changes in the rate of production of nuclides by cosmic rays. Even while discussing these aspects, our emphasis will be mainly on the processes rather than on the details of the chronometric method. 

Investigation or transport of radionuclides through natural and agricultural ecosystems to various receptors such as plants, animals and humans ... 

Radioactive waste management is a quite mature field of application of basic geoscientific disciplines. As we will discuss in forthcoming sections, the long-term performance and henceforth the safety of radioactive waste disposal systems, deeply relies on the basic principles that control the release, mobility, and transport of the chemical elements in the geosphere. In the context of radioactive waste disposal, the waste matrix constitutes the innermost of the barriers that may control the release and ulterior transport of radionuclides through the ground-water systems. 

Second, generic and site-specific assessments of near-surface disposal facilities for radioactive waste have shown that allowable doses to hypothetical inadvertent intruders usually are more restrictive in determining acceptable disposals than allowable doses to individuals beyond the boundary of the disposal site. This conclusion is based on predictions that concentrations of radionuclides in the environment (e.g., ground-water) at locations beyond the site boundary usually should be far less than the concentrations at the disposal site to which an inadvertent intruder could be exposed, owing to such factors as the limited solubility of some radionuclides, the partitioning of radionuclides between liquid and solid phases, and the dilution in transport of radionuclides in water or air beyond the site boundary. More people are likely to be exposed beyond the site boundary than on the disposal site, but acceptable disposals of radioactive waste in near-surface facilities have been based on assessments of dose to individuals, rather than populations. 

The chapters of this volume are organized into sections that cover the chemical aspects that are important to understanding the behavior of disposed radioactive wastes. These aspects include radionuclide sorption and desorption, solubility of radionuclide compounds, chemical species of radionuclides in natural waters, hydrothermal geochemical reactions, measurements of radionuclide migration, solid state chemistry of wastes, and waste-form leaching behavior. The information in each of these sections is necessary to predict the transport of radionuclides from wastes via natural waters and thus to predict the safety of the disposed waste. 

The groundwater transport of radionuclides through waterbearing interbed layers in the Columbia River basalt formation will be controlled by reactions of the radionuclides with groundwater and interbed solids. These interactions must be understood to predict possible migration of radionuclides from a proposed radioactive waste repository in basalt. Precipitation and sorption on interbed solids are the principle reactions that retard radionuclide movement in the interbeds. The objective of the work described herein was to determine the sorption and desorption behavior of radionuclides important to safety assessment of a high-level radioactive waste repository in Columbia River basalt. The effects of groundwater composition, redox potential, radionuclide concentration, and temperature on these reactions were determined. 

This equation can be used to describe one-dimensional transport of radionuclides through porous media (e.g. radionuclide elution curves from laboratory columns packed with interbed solids) assuming instantaneous sorption and desorption. Van Genuchten and coworkers have demonstrated the importance of using both sorption and desorption isotherms in this equation when hysteresis is significant. Isotherm data for sorption and desorption reactions of radionuclides with interbed materials are presented in this paper which can be used to predict radionuclide transport. 

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. 

Computer simulation is now used extensively as a tool to help to understand and predict the transport of radionuclides through environmental systems. Most models relate to waste disposal and are based on measured parameters such as water movements, salinity, suspended load and the radionuclide concentration in the solute, suspended particulate matter and bottom deposits. Comparatively few attempts appear to have been made to include chemical speciation into this type of model, presumably because of the added complexity involved. Some modellers have attempted to take into account the characteristics of the major chemical phases such as those present in different particles or coatings (e.g. Martinez-Aquirre et al., 1994). Others have noted the importance of including details of particular chemical species present in industrial waste releases when constructing models to predict dispersion (Abril and Fraga, 1996). 

Baskaran, M., and Santschi, P.H. (1993) The role of particles and colloids in the transport of radionuclides in coastal environments of Texas. Mar. Chem. 43, 95-114. 

Mobility and transport of radionuclides in the geosphere are influenced markedly by their interaction with solids. Migration is retarded, or even stopped, if the interaction is strong, in particular if the radionuclides are incorporated into the solids. Sorption of radionuclides on solids has been investigated extensively for materials in the neighbourhood of planned high-level waste repositories. 

Figure 14 The conceptual hydrogeological model of the Palmottu, Finland research site. The arrows indicate the measured flow directions, the distribution of groundwater types is shown, and some of the measured and inferred geochemical processes are also indicated (Blomqvist et al., 2000) (reproduced by permission of European Commission from The Palmottu Natural Analogue Project, Phase II Transport of Radionuclides in a Natural Flow...

Blomqvist R., Ruskeeniemi T., Kaija J., Ahonen L., Paananen M., Smellie J., Gmndfelt B., Pedersen K., Bmno J., Perez del Villar L., Cera E., Rasilainen K., Pitkanen P., Suksi J., Casanova J., Read D., and Frape S. K. (2000) The Palmottu Natural Analogue Project. Phase II Transport of Radionuclides in a Natural Flow System at Palmottu. Final Report EUR 19611 EN, 174p. European Commission Nuclear Science and Technology. 

Introduction. Colloidal suspensions are defined as suspensions of particles with a mean diameter less than 0.45 p.m, or a size range from 1 nm to 1 p,m. They represent potentially important transport vectors for highly insoluble or strongly sorbing radionuclides in the environment. Colloids are important in both experimental systems and natural settings. In the former, unrecognized presence of colloids may lead to overestimation of the solubility and underestimation of the sorption of radionuchdes if they are included in the estimation of the concentration of radionuclide solution species. In natural systems, they may provide an important transport mechanism for radionuclides not filtered out by the host rock. In fractured rock, local transport of radionuclides by colloids may be important. 

Transport of radionuclides by colloids. Several numerical models have been developed to assess the potential magnitude of colloidal-facilitated transport of radionuclides compared to the transport of dissolved species. Vilks et al. (1998) proposed a simple modification to the standard equation for the retardation factor. The equation applies to the ideal case where colloids are not trapped by the rock matrix and the composition of the colloids and the rock matrix are the same ... 

The following section provides detailed information concerning the transport of radionuclides associated with two very different field analogues the Chernobyl reactor accident and the Oklo Natural Reactor. These examples span wide temporal and spatial scales and include the rapid geochemical and physical processes important to nuclear reactor accidents or industrial discharges as well as the slower processes important to the geologic disposal of nuclear waste. 

Smith P. A. and Degueldre C. (1993) Colloid-facilitated transport of radionuclides through fractured media. J. Contamin. Hydrol. 13, 143-166. 

---