Description

The precipitation processes, including their evolution in time, affecting repository materials.

Precipitation can be caused by changes in chemical conditions in the repository (particularly pH, Eh and the concentration of complexing ions). These changes can be gradual, or abrupt. The latter occur especially at the interface between different repository components.

Temperature gradients may cause precipitation; some solid phases will precipitate from a fluid phase as temperature decreases, whereas other phases may precipitate as temperature increases.

The kinetics of precipitation reactions will be an important control on whether a solid phase that is over-saturated in a solution (i.e. expected to precipitate on thermodynamic grounds) actually precipitates.

The presence of complexing ions can increase solubility, and cause elements to remain in solution under conditions when they would otherwise be expected to precipitate.

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.

Precipitation of solid phases will tend to decrease the porosity of materials in the EBS and / or the adjacent geosphere. Thus, precipitation may reduce the permeability or even seal potential pathways for the movement of fluids (such as liquid water, non-aqueous liquids or gases) through the EBS or adjacent geosphere. Precipitation of solid phases could therefore decrease the potential for such fluid movements to transport any radionuclides and other contaminants that might be released from the waste packages.

Potentially, radionuclides and other contaminants might be immobilised by being precipitated or co-precipitated in solid phases. Radionuclides and other contaminants might also sorb to the surfaces of precipitated solid phases.

The precipitation of solid phases has the potential to change the chemical conditions (e.g. pH, Eh dissolved chemical species) in the repository. This influence on chemical conditions could in turn affect the chemical speciation of any radionuclides and other contaminants that might be present within the EBS or surrounding natural barrier. Such an influence on chemical speciation could affect the partitioning of the radionuclides and other contaminants between immobile solid phases and potentially mobile fluid 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