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 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.
- 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.
Certain radionuclides and other contaminants can sorb to colloids. Potentially, colloids may also contain radionuclides within their structures. Movement of colloids, in water or non-aqueous fluids, may therefore impact upon the migration rates through the EBS and the surrounding geosphere, of certain radionuclides and other contaminants originating in the wastes (should release from a waste package occur).
Colloids that carry radionuclides and other contaminants may be transported through the repository and possibly into the surrounding geosphere, in water or non-aqueous liquids, if there are physical pathways (e.g. open fractures or connected matrix pores) through the EBS and forces driving fluid flow. Their relatively large sizes may prevent colloids from entering pores in EBS materials and rock that may be accessed by dissolved ions and complexes, which are much smaller than colloids. Consequently, radionuclides and other contaminants that are transported with/as colloids may follow different pathways through the engineered and natural barriers to the pathways followed by dissolved radionuclides and other contaminants. Radionuclides and other contaminants that are transported with/as colloids and in solution may be retarded differently.
2000 List
A reference to the related FEP(s) within the 2000 NEA IFEP List.