Description
The processes by which rocks are changed by the action of heat and/or pressure at depth (often several kilometres) beneath the Earth’s surface or in the vicinity of magmatic activity or active faulting.
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.
Metamorphism
is not a precisely defined term but refers to the dominantly solid-state alteration of rock under conditions of pressure and/or temperature that are substantially higher than those of the ground surface in areas of normal geothermal gradient (i.e. excepting the effects of near-surface magmatism or hydrothermal activity). Alteration of rocks at relatively low temperatures and/or pressures (but still substantially elevated compared to those near the ground surface) is generally termed metamorphism
if the rocks are igneous or have previously been altered (metamorphosed) at even higher pressures and/or temperatures; alteration of sedimentary rocks under such lower pressure/temperature conditions is termed ‘diagenesis’ (FEP 1.2.9). Some researchers consider hydrothermal alteration to be a form of metamorphism. However, in this FEP list such alteration is covered by FEP 1.2.7 (Hydrothermal activity).
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.
Metamorphism has the potential to influence the performance and safety of a repository by affecting the repository’s chemical environment and the chemical and physical properties of the host rock and / or rocks in the wider groundwater flow system within which the repository presently resides or might reside in future. During metamorphism there may be, depending upon the nature of the affected rock types, a wide range of organic and inorganic chemical reactions. These reactions will influence the fluid chemistry in these rocks. Metamorphism will also influence the pressures of pore fluids and hence the potential for pore fluids to flow, by generating or consuming fluids and changing the porosity of the rock. Mineral reactions (precipitation, dissolution and alteration) may change the connectivity of pore spaces and the nature of mineral surfaces on which migrating radionuclides or other contaminants might sorb. Any past metamorphism of the host rocks or surrounding rocks will have affected their physical and chemical characteristics. Certain of these characteristics (porosity, mineralogy etc.) may influence the transport and retardation of radionuclides and/or other contaminants that were to leave the repository. Any characteristics of a repository’s host rock or rocks in the surroundings of the repository that were acquired during past metamorphism will influence the future evolution of the rock during the post-closure period. At the depths typically proposed for repositories (< 1 km) temperatures and pressures are likely to be relatively low. Consequently, significant metamorphism of rocks at repository depth and shallower will be unlikely to occur during usual timescales considered by performance assessments (often c. 1 Ma) unless there is magmatism (FEP 1.2.5), hydrothermal activity (FEP 1.2.7) or active faulting (FEP 1.2.4). However, ongoing metamorphism at greater depths than a repository could still influence the composition of groundwater, other liquids and gases at repository depths, and the pressure gradients that influence flow of these fluids. For example, metamorphism of limestone at depths substantially greater than a repository could liberate CO₂ which could then be transported to repository depths. Such changes in environmental conditions in and around a repository that are caused by metamorphic processes could influence the migration of radionuclides and other contaminants from the repository.
2000 List
A reference to the related FEP(s) within the 2000 NEA IFEP List.
Related References
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Metcalfe R and Watson SP (), Technical Issues Associated with Deep Repositories for Radioactive Waste in Different Geological Environments, Environment Agency Science Report SC060054/SR1, Environment Agency, Rio House, Waterside Drive, Aztec West, Almondsbury, Bristol, BS32 4UD, 213, http://www.gov.uk/government/uploads/system/uploads/attachment_data/file/291763/scho0809bqvu-e-e.pdf