Description
Alteration, including its evolution in time, of repository materials by chemical processes such as dissolution, leaching, chloride and sulphide/sulphate attack, carbonation, illitisation.
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
Comments
The “Comments” field, when present, contains any additional explanation of the IFEP, beyond that implicit in the FEP's description and provided in the “Relevance to Performance and Safety” field. This additional explanation may include, where appropriate, the IFEPs characteristics, the circumstances under which it might be relevant and its relationship to other (especially similar) IFEPs.
“Dissolution” and “leaching” are closely related terms. “Dissolution” is the dissolving of a solid phase in a fluid phase, typically liquid water. “Leaching” refers to the removal of components of a solid phase by a moving fluid phase, typically liquid water. “Alteration” encompasses these terms and also the transformation of a given assemblage of solid phases to a different assemblage of solid phases. Carbonate minerals dissolving in porewater within a backfill is an example of dissolution. An example of leaching is the removal of soluble components from Ordinary Portland Cement by groundwater flowing through it, leading to a progressive reduction in the pH of cementitious pore fluids in the long-term. Conversion of the smectite in bentonite into illite (illitisation) is an example of alteration involving the transformation of a mineral assemblage.
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.
Alteration may influence the effectiveness of the EBS and the adjacent geosphere barrier. Alteration may change the porosity and permeability distribution of the barriers. Potentially, some alteration reactions could produce pathways through the EBS and adjacent geosphere, via which fluids (such as liquid water, non-aqueous liquids and gases) might flow. Movement of such fluids could transport radionuclides and other contaminants originating in the wastes, should these be released from the waste packages. Other alteration reactions could decrease the porosity and / or permeability of the EBS and / or the adjacent geosphere. Possibly, some alteration reactions could seal previously existing pathways via which fluids might otherwise flow.
Alteration could affect the forces that could potentially drive the flow of fluids through the EBS and adjacent barriers, and possibly transport radionuclides and other contaminants. Alteration could remove solutes from solution, or add solutes to solution, thereby changing the chemical potential gradients that drive diffusion. Such alteration could also affect thermal gradients, reflecting the exothermic character of some reactions and endothermic character of other reactions. Changes in the concentrations and / or temperatures of solutions due to alteration reactions could impact upon the density gradients within fluid phases and consequently upon density-driven fluid flow. Alteration reactions that consume or generate gas and / or that change the porosity could affect gas pressures within the repository, again potentially impacting upon fluid flow.
The swelling pressures exerted by certain barriers (bentonite buffers, bentonite-bearing backfill) could potentially be affected by alteration, again possibly contributing to the development of pathways for fluid flow and the forces driving such flow.
Alteration could impact upon the chemical conditions within the repository (e.g. pH, Eh, concentrations of ligands). These conditions could in turn influence the mobility and retardation of radionuclides and other contaminants that originate in the waste, should they be released from the waste packages. Variations in chemical conditions related to alteration could affect the partitioning of radionuclides and other contaminants among different potentially mobile fluid phases (such as waste, non-aqueous liquids and gases) and between these phases and immobile solid phases.
2000 List
A reference to the related FEP(s) within the 2000 NEA IFEP List.