Description

The chemical speciation and solubility processes affecting contaminant migration through the repository under prevailing repository conditions. The concentration of an element in aqueous solution at equilibrium with a coexisting solid reflects the solubility of the solid. Factors such as temperature, gas partial pressure, ionic strength, the presence of complexing agents and pH and redox conditions affect solubility. These factors affect the chemical form and speciation of the element. Thus different solids of the same element may have different solubilities in a particular solution.

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 aqueous speciation and solubility of a radionuclide or other contaminant will influence their potential mobility in the aqueous phase, within the EBS and adjacent geosphere. These radionuclides and other contaminants may originate in the wastes, should they be released from a waste package.

Under a given set of temperature, pressure and chemical conditions, at chemical equilibrium the maximum aqueous concentration of a radionuclide or other contaminant contained by a coexisting solid phase is governed by the solubility of that phase. This concentration is termed a “solubility limit”.

The chemical speciation of the radionuclides and other contaminants in the aqueous phase will influence their solubility limits at any given temperature and pressure. However, the chemical speciation may also influence the rates at which solid phases dissolve and precipitate. If dissolution or precipitation reactions are slow (kinetically inhibited), an aqueous solution may not be at equilibrium with coexisting solid phases or may be supersaturated with respect to solid phases that do not occur (i.e. on thermodynamic grounds a solid phase ought to precipitate, but for kinetic reasons it does not do so). Depending upon the chemical speciation, at a given pressure and temperature, the aqueous concentration of a radionuclide or other contaminant may exceed the solubility of a solid phase that, if present and at equilibrium with the water, would control the aqueous concentrations of radionuclides or other contaminants. Alternatively, the aqueous concentration of a radionuclide or other contaminant may be lower than would be expected based on the occurrence of solid phase and the assumption of solubility limitation.

The chemical speciation of a radionuclide or other contaminant in an aqueous solution will also influence the effectiveness of other transport and retardation processes. Partitioning of radionuclides and other contaminants between the aqueous phase and other phases that may occur (solids, non-aqueous liquids and gases) will be influenced by the aqueous speciation of the radionuclides and other contaminants. For example, sorption will depend partly upon the electrical charges of aqueous species. Transport of aqueous species by diffusion may also be affected by chemical speciation. For example, the ability of radionuclides and other contaminants to diffuse from fractures into the surrounding solid matrices may depend partly upon the charges of the aqueous species of the radionuclides and other contaminants.

2000 List

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

3.2.02