Description

The diffusion of contaminants between a permeable fracture and the network of microfractures and micropores within the adjacent rock matrix. Diffusion is driven by a chemical potential (approximately concentration) gradient between water in the fracture and free water in the rock matrix. Matrix diffusion can operate into / from the rock matrix depending upon the direction of the chemical gradient.

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.

Diffusive transport between groundwater flowing through fractures and porewater in lower permeability wall rocks could affect the rate at which radionuclides and other contaminants migrate from the repository through the geosphere (if radionuclides and other contaminants can travel through the EBS). The migration of radionuclides and other contaminants will be retarded by their diffusion into the rock matrix from a fracture. If environmental conditions change, leading to changed chemical gradients between the water flowing in the fractures and the porewater in the rock matrix, radionuclides that have previously diffused into the rock matrix may diffuse out again and then be transported in the groundwater flowing in the fracture. Radioactive isotopes that have ingrown within the rock matrix may diffuse from the rock matrix into the flowing groundwater in the fracture. Similarly, diffusive transfers between water flowing in a fracture and porewater in the rock matrix may cause the exchange of certain isotopes originating in the repository with different naturally occurring isotopes of the same element. For example, by this mechanism U-235 originating in the repository may exchange with natural U-238 in the rock. This process would change the overall radioactivity of the U being transported in the water in the fracture.

2000 List

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

3.2.07