Description
The diffusion of contaminants from the waste form in a fluid phase. This phase may be liquid water but may alternatively be non-aqueous liquid or a gas. Diffusion results in the net flux of contaminants from a region of higher concentration to one of lower concentration. The rate of diffusion is a function of temperature, viscosity of the fluid and the size (mass) of the associated molecules.
Category
Categorisation as a Feature, Event and/or Process.
Features
are physical components of the disposal system and environment being assessed. Examples include waste packaging, backfill, surface soils. Features typically interact with one another via processes and in some cases events.Events
are dynamic interactions among features that occur over time periods that are short compared to the safety assessment timeframe such as a gas explosion or meteorite impact.- "Processes" are issues or dynamic interactions among features that generally occur over a significant proportion of the safety assessment timeframe and may occur over the whole of this timeframe. Events and processes may be coupled to one another (i.e. may influence one another).
The classification of a FEP as an event or process depends upon the assessment context, because the classification is undertaken with reference to an assessment timeframe. In this generic IFEP List, many IFEPs are classified as both Events and Processes; users will need to decide which of these classifications is relevant to their context and its timeframes.
- Event
- Process
Relevance to Performance and Safety
The “Relevance to Performance and Safety” field contains an explanation of how the IFEP might influence the performance and safety of the disposal system under consideration through its impact on the evolution of the repository system and on the release, migration and/or uptake of repository-derived contaminants.
Given sufficient time, and in the absence of advection, diffusion will tend to remove radionuclides or other contaminants from the surface of a solid waste form, once these constituents have entered a fluid phase. In the absence of significant bulk movement of the fluid (i.e. advection / convection), the rate of diffusion will control the overall rate at which the radionuclides or other contaminants are released from the waste form. In this case, the steeper the chemical potential gradient (approximated by a concentration gradient) from the surface of the waste form into the coexisting fluid, the greater will be the rate at which the radionuclides or other contaminants are released. The degree to which the concentrations of radionuclides or other contaminants are heterogeneous within any fluid phase will depend upon the rates at which these constituents are added to / removed from the liquid phase, the concentration gradients that develop and the diffusion coefficients of the constituents. Typically the fluid phase of concern will be liquid water, with contaminants, including radionuclides, diffusing through it as aqueous species. If the waste package is vented, or is breached later in the post-closure period, then water may enter the package. Then, diffusion in the aqueous phase will be the dominant transport mechanism if pressure / head gradients are sufficiently small that no significant advection occurs. Contaminants, including radionuclides, that are present in other fluids, such as non-aqueous liquids or gases, may also be able to diffuse if chemical potential gradients exist in these phases.
2000 List
A reference to the related FEP(s) within the 2000 NEA IFEP List.
Related References
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Bond AE, Egan MJ, Metcalfe R, Robinson PJ and Towler G (), Understanding Controls on the Performance of Engineered Barrier Systems in Repositories for High-level Radioactive Waste and Spent Fuel, Environment Agency Science Report SC060055, Environment Agency, Rio House, Waterside Drive, Aztec West, Almondsbury, Bristol, BS32 4UD, 200, http://www.gov.uk/government/uploads/system/uploads/attachment_data/file/291238/scho0910bsze-e-e.pdf
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Ahn J and Apted M (Eds) (2010). (), Geological Repository Systems for Safe Disposal of Spent Nuclear Fuels and Radioactive Waste, Woodhead Publishing, ISBN 9781845695422, http://store.elsevier.com/product.jsp?isbn=9781845699789