Description
The establishment of unsaturated conditions in the repository during the construction and operation phases, and the subsequent return to saturated 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
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 degree to which the EBS and rock surrounding a repository are water-saturated will influence: 1) the chemical, physical and biological evolution of the EBS; and 2) the mobility of radionuclides and other contaminants, which may originate in the wastes, should there be breaches in the waste packages.
Changes in water saturation (desaturation and resaturation) may cause changes in the chemical reactions that occur in the EBS and rock of the excavation disturbed zone and affect their physical properties. Water is necessary for many chemical reactions that may affect the EBS and the rock immediately surrounding the repository. Flowing water may supply dissolved reactants and remove reaction products. Microbes, which may mediate many chemical reactions, may be supplied by water, while water is necessary for the metabolism of microbes; the presence of water-saturated or unsaturated conditions will influence the kinds and abundances of microbes that occur. Cracking and fracture dilation may accompany desaturation. Swelling and fracture sealing may occur as clay-rich (smectite-bearing) rocks and EBS components (e.g. bentonite buffer, backfill) resaturate.
The air in the desaturated rock around excavated cavities (tunnels, waste emplacement rooms, waste emplacement holes etc.) at the time of closure may cause oxidation of reduced minerals in the rock. Under initially under-saturated conditions, air will also be present in many of the EBS components (e.g. bentonite, backfill), with the potential for oxidation of reduced solid phases present. When the excavation disturbed zone and/or EBS components are unsaturated with water, their effective permeabilities with respect to non-aqueous liquids and gases will be higher than those in the presence of liquid water.
2000 List
A reference to the related FEP(s) within the 2000 NEA IFEP List.