SKIN

Slow processes in close-to-equilibrium conditions for radionuclides in water/solid systems of relevance to nuclear waste management

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 Obiettivo del progetto (Objective)

'Solid/liquid chemical equilibrium hypotheses (sorption, solubility, solid-solution formation) are key concepts in the assessment of nuclear waste safety. For radionuclides at trace concentrations this corresponds to constant solution concentrations, or solid/liquid distribution ratios, if environmental conditions remain constant. However, these concepts do not account for irreversible incorporation of radionuclides in the solid phases. Indeed, there is often a gradual and very slow transition from simple adsorption processes to incorporation of trace elements in the surface structure of solid phases. For certain tetravalent actinides apparent solubility equilibrium applies to only the surface without bulk phase equilibrium. This can lead to very large uncertainty in solubility values and derived thermodynamic constants. Equilibrium concepts are characterized by a dynamic state of equal forward and backward reaction rates, under conditions where phase compositions remain constant. Most of the problems arise from a lack of understanding of the dynamics of slow processes close to equilibrium, specifically in the coupling of sorption with other surface equilibrium reactions such as dissolution/precipitation, recrystallisation, isotopic exchange and with the bulk phase equilibrium. The project intends to assess the effect of surface properties on apparent solubility as well as the kinetics of incorporation of radionuclides in the structure of a solid phase, and the associated reaction mechanisms for various solids in a systematic manner, using isotope exchange under close-to-equilibrium conditions. The project results will impact strongly (1) the use/misuse of solubility data for thermodynamics; (2) the understanding of affinity/rate relations close to equilibrium; (3) the inclusion of irreversibility in models on the long-term mobility of radionuclides; and (4) the coupling of radionuclide chemistry with main element chemistry in the repository environment.'

Introduzione (Teaser)

For radionuclides at trace concentrations, there can be a very gradual and slow transition from reversible surface sorption to irreversible incorporation into solids. A new EU-funded study has shed important light on such mechanisms.

Descrizione progetto (Article)

Solid/liquid equilibrium concepts (for example, the same amount of a substance dissolving as becoming incorporated into the solid phase) are key to assessment of nuclear safety. Standard concepts do not account for slow equilibrium processes and this can lead to overly conservative or, in some cases, overly optimistic evaluations of risk.

The EU-funded project http://www.emn.fr/z-subatech/skin/index.php/Main_Page (SKIN) set out to clarify the issue for more effective use of solubility data in the context of nuclear waste management. The focus was on the tetravalent actinides (An(IV)) often considered environmentally immobile due to their low solubilities. Detailed data describing the slow thermodynamic processes close to equilibrium are lacking.

SKIN carried out a large number of experiments on these systems. Results are directly relevant to characterising solubility controls that influence maximum groundwater concentration and associated calculated doses. Investigations included the study of dynamic isotope exchanges and spectroscopic studies on radionuclide incorporation. The latter evaluated aspects of reversibility or irreversibility that relate to the amounts of free radioisotopes able to diffuse into groundwater or soil.

Sorption/desorption to and from the surface of materials is typically reversible, whereas incorporation into the solid phase is considered irreversible. There are some exceptions and one important one has been the irreversibility of sorption/desorption of caesium into pure illite. In contrast, SKIN showed that, in the case of interstratified illite, the process is reversible.

This could be because the interstratification blocks diffusion into the bulk or that the experimental conditions do not approximate the very slow, long-term diffusion processes. Current geochemical sorption/desorption models are not yet able to describe such long-term evolutions.

A very important contribution of the project is thus the comparison of three existing models and the development of a new model of irreversible trace mineral uptake. Understanding the temporal evolution of solubility and sorption is critical to safety assessments.

SKIN has developed a scientific methodology to quantify the degree of irreversible incorporation of radionuclides in mineral phases following initial surface adsorption. This will help qualify the degree of conservatism in safety assessments, and support the safe and widespread uptake of clean and cost-effective nuclear power.