Description

The migration of reactants into and reaction products from the repository. Chemical species can migrate into and out of the repository and its various components by advection (head/pressure gradient driven) and diffusion (chemical potential gradient driven). Where solutions with different concentrations are separated by a semi-permeable membrane there may be osmotic transport of water. Chemical concentration gradients in the repository and its various components could be caused by various factors. Migration of chemical species by advection may cause concentration gradients that drive diffusive transport. Temperature gradients may produce concentration gradients by influencing the chemical reactions that occur between fluids (e.g. liquid water, non-aqueous liquids and gases) and between solids and fluids, and the rates of these reactions. Temperature gradients may also lead to advective (convective) migration. heterogeneities in the spatial distribution of waste packages and repository materials.

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.

The migration of chemical species into and out from the repository will influence the chemical evolution of the EBS and the surrounding geosphere. This migration may also influence biological processes within the repository, since nutrients and energy sources required for microbial activity may be among the migrating chemical species.

The chemical evolution of the EBS and surrounding geosphere may be coupled to its physical evolution; pathways for movement of fluids (such as water, non-aqueous liquids or gases) can form, be enhanced, partially seal or fully seal due to chemical processes. The formation of chemical concentration gradients may lead to the dissolution and precipitation of chemical compounds with subsequent opening or plugging of flow paths. The presence of certain species, such as chloride, sulphide, sulphate and potassium, can affect the evolution of the repository and its seals, for example through promoting the corrosion of metals (high chloride/sulphide concentrations), the degradation of cement (high sulphate concentrations) and the illitisation of bentonite (high potassium concentrations).

Among the migrating chemical species may be radionuclides and other contaminants originating in the wastes, should the waste packages release them. Migrating chemical species may include ligands that can complex with radionuclides and other contaminants, thereby influencing their mobility and retardation. The chemical speciation of the radionuclides and other contaminants will depend partly upon the migration of chemical species to the repository. This chemical speciation will influence the partitioning of the radionuclides and other contaminants among different solid phases and mobile phases (such as water, non-aqueous liquids and gases).

2000 List

A reference to the related FEP(s) within the 2000 NEA IFEP List.

2.1.09