Description

The dissolution, precipitation and crystallisation of contaminants in the geosphere under prevailing geosphere conditions.

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

Comments

The “Comments” field, when present, contains any additional explanation of the IFEP, beyond that implicit in the FEP's description and provided in the “Relevance to Performance and Safety” field. This additional explanation may include, where appropriate, the IFEPs characteristics, the circumstances under which it might be relevant and its relationship to other (especially similar) IFEPs.

Dissolution is the process by which constituents of a solid, non-aqueous liquid or gas dissolve into liquid water. Precipitation occurs when chemical species in solution produce a solid and are thereby removed from the solution.

Changes in water chemistry, such as may occur when chemically distinct solutions mix, when a solution reacts with a solid phase assemblage, or when gases exsolve, may cause precipitation or dissolution. Temperature or pressure changes may also cause precipitation or dissolution.

Crystallisation is the precipitation of a crystalline solid phase. Solids that are initially precipitated from an aqueous solution may be amorphous or poorly crystalline and then subsequently become more crystalline as they age.

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.

Solid phases that precipitate from water may incorporate radionuclides or other contaminants that have been transported from the repository in the water (if their transport through the EBS is possible). In this way, the radionuclides and other contaminants may be immobilised. In principle, pure phases of the contaminants may possibly precipitate if the chemical conditions are suitable. However, because the contaminants will almost certainly have very low concentrations in the water, it is more likely that they will comprise minor / trace constituents of the solid phase (i.e. co-precipitate with major constituents).

Dissolution of solid phases that have previously precipitated from water carrying radionuclides and other contaminants from a repository, may release these contaminants back to water, thereby remobilising them.

Dissolution of a gas phase by a coexisting aqueous phase may also change the mobility of radionuclides or other contaminants that are transported from the repository in gaseous form. For example, C-14 labelled CO₂ in a buoyant gaseous phase may dissolve in a coexisting aqueous phase, which may be less mobile owing to being less buoyant. Dissolution of a non-aqueous liquid in coexisting water may have a similar effect.

Changes in the crystallinity of a solid phase may influence the partitioning of radionuclides and other contaminants between the solid phase and any coexisting fluid phase. For example, over time, poorly crystalline Fe-oxyhydroxides that precipitate from groundwater may transform to more crystalline Fe-oxyhydroxide phases. As this process occurs, radionuclides and other contaminants that are migrating in the aqueous phase may sorb to the Fe-oxyhydroxides less effectively.

2000 List

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

3.2.01