Description

The migration of contaminants in gas or vapour phase or as fine particulate or aerosol in gas or vapour through the geosphere.

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.

Gas-mediated migration may influence the rates at which radionuclides and other contaminants are transported through the geosphere from a repository (if their transport through the EBS is possible) and the pathways followed. Gas migration may directly transport radionuclides and other contaminants occurring within the gas phase. Alternatively, gas migration may indirectly lead to the transport of radionuclides and other contaminants contained in other mobile phases the movement of which is gas-driven. Evolution of gas within the repository may cause a pressure gradient that drives radionuclides in gaseous form away from the repository, through the geosphere.

Radionuclides and other contaminants that leave a repository in and aqueous phase, or in non-aqueous liquids, may subsequently partition into naturally occurring gas that is encountered along the flow path. Migration of the radionuclides and other contaminants in the gaseous phase may then be driven by pressure gradients affecting the gas.

Density contrasts between a gas phase and other fluid phases present may affect the rates and directions of gas movement, and the consequent migration of radionuclides and other contaminants contained in the gas. The buoyancy of gas relative to water may cause radionuclides and other contaminants in gaseous form to migrate generally upwards.

The effective permeability of the rock with respect to gas will depend upon the proportions of gas and other fluids (most likely groundwater, but potentially non-aqueous liquids) that coexist. Interactions between the gaseous phase and any other phases present may influence the partitioning of radionuclides and other contaminants between the gas and the other phases. For example, some gases are more soluble in water than others; under relevant conditions, CO₂ is more soluble in water than CH₄, so that C-14 in the form of CO₂ will tend to partition more into a coexisting aqueous phase than would C-14 in the form of CH₄. These interactions between gases and other phases (including the evolution of gas from other fluids or the dissolution of gas in these fluids) will depend on changes in pressure and / or temperature and / or chemical environment (e.g. pH, salinity) along a migration pathway.

Gases may sorb on the surfaces of solid phases, thereby preventing or retarding the migration of radionuclides or other contaminants that are transported in gaseous form.

2000 List

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

3.2.09