Description

The formation of very fine particles (with at least one dimension in the 1 μm to 1 nm range) that can affect the migration of contaminants in the repository. Particles of clay minerals, silica, iron oxy-hydroxides, other minerals, organic and bio-organic macromolecules, and contaminants themselves (e.g. Pu(IV)) may form the colloid phase. Sources can include materials in the waste package itself (e.g. cementitious materials, organic wastes), other repository components (e.g. bentonite and cementitious materials) and inflowing groundwater. Colloid formation may be promoted by steep chemical gradients, such as at an interface where the Eh or pH changes abruptly because of chemical or biological activity. The thermodynamic stability of colloids depends upon factors such as the chemistry and surface charge of the colloid and the chemistry of the dispersion medium. Colloid stability generally decreases as ionic strength (salinity) increases.

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.

  • 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.

Many radionuclides and other contaminants are able to form complexes with colloidal particles and thereafter be transported with the particles. If the colloids are mobile and the radionuclide/contaminant-colloid complexes are strong, then once the waste package is breached, radionuclide/contaminant release from the waste package may be enhanced compared to a case where colloids do not form. On the other hand, under certain circumstances colloids may be less mobile than free complexes in the mobile phase (e.g. aqueous complexes of a radionuclide and bicarbonate). For example, colloids would tend to be even less mobile through compacted bentonite than aqueous complexes which could diffuse.

2000 List

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

3.2.04

Related References