Radionuclide speciation modelling

For experimental studies, a chemical thermodynamic modelling approach could theoretically reduce unnecessary experimental effort and hence the overall cost of a research project. Once experiments are underway, the computer simulation should also offer valuable assistance in the interpretation of results. Modelling techniques with particular reference to radionuclide speciation have been discussed by Cross and Day (1986) who pointed out that models are only as good as the thermodynamic data upon which they are based. For example, formation constants (a prerequisite for chemical modelling) are invariably generated in idealised laboratory conditions and their use seldom reflects the natural environment... 

Tipping, E., Woof, C., Kelly, M., Bradshaw, K., and Rowe, J.E. (1995b) Solid-solution distributions of radionuclides in acid soils applications of the WHAM chemical speciation model. Environ. Sci. Technol. 29, 1365-1372. 

Turner D. R., Griffin T., and Dietrich T. (1993) Radionuclide sorption modeling using the MIN-TEQA2 speciation code. Mater. Res. Soc. Symp. Proc. 783-789. 

Wanner, H. 1985. Modelling radionuclide speciation and solubility limits in the near-field of a deep repository. Scientific Basis for Nuclear Waste Management, IX, 509-516. 

Bradbury Baeyens (2002a, b) provided sorption data bases for Opalinus Clay and MX-80 bentonite, respectively, based on numerous experiments with varying aqueous solution compositions. Since sorption can be influenced by the complexa-tion of radionuclides with various ligands, corrections had to be made to account for the differences in speciation between the experimental solutions and the Opalinus Clay and bentonite porewaters. For this purpose, the Nagra/PSI TDB 01 /01 was used to model the speciation of all safety-relevant radionuclides in the experimental solutions and in the Opalinus Clay and bentonite porewaters. 

This second edition retains the structure of the first edition. It aims to provide a comprehensive review of the current science of speciation, covering relevant methodological, analytical and modelling aspects as well as giving an overview of recent work on speciation in various spheres. It is divided into two parts, the first dealing with the more methodological aspects and the second with specific compartments of the environment, with the speciation of radionuclides, and presents a review of current trends and developments. 

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). 

A second obvious line of research for the future must be that related to the development and improvement of computer-based simulation of long-term environmental behaviour of radionuclides. Most currently available models are still comparatively simple compared with the physical, chemical and biological complexity of environments they purport to represent but, as noted in Section 13.5, our ability to construct ever more complex conceptual models for predicting the future behaviour of radionuclides is improving. However, the more complex the model, the more demands it places on the basic thermodynamic data and knowledge of likely speciation. The challenge for the future will therefore be to produce high-quality data for model construction and to devise realistic ways to validate those models once produced. 

Cross, J.E. and Day, J.R (1986) Marine speciation of some effluent radionuclides inferences from an empirical model for transport and estuarine deposition. In Speciation of Fission and Activation Products in the Environment (eds Bulman, RA. and Cooper, J.R.). Elsevier Applied Science, London, pp. 339-342. 

In the late 1990s and early 2000s certain attempts were made to expand Langmuir and Freundlich-type models to describe two- or multi-metal ions biosorption system to more accurately represent the nature of real effluents. These empirical models hardly reflect the sorption mechanism as they do not take into account all the various processes and parameters which influence the retention of metals and radionuclides by algal cells. Furthermore, a number of attempts were made to consider other mechanism in metal biosorption, e.g. ion exchange between protons in the biomass and/or complexation. Those models considered the sorbate speciation in solution pH and even electrostatic attraction however are not yet widely accepted in the scientific community... 

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