Description
The processes by which deposited sediments undergo physical, chemical and biological alteration during compaction, cementation and crystallisation, leading to the formation of sedimentary rocks. Diagenesis occurs at relatively low pressure and temperature, under conditions of temperature and pressure normal to the upper few kilometres of the Earth’s crust.
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
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.
There is no universally accepted distinction between Diagenesis (This FEP, 1.2.9) and Metamorphism (FEP 1.2.6). However, FEP 1.2.9 applies only to sedimentary rocks, whereas FEP 1.2.6 may be applied to any kind of rock. Additionally, FEP 1.2.6 extends to higher pressures and temperatures than FEP 1.2.9.
FEP 1.2.9 differs from FEP 4.2.1 (Thermal processes [Geosphere]), FEP 4.2.2 (Hydraulic processes [Geosphere]), FEP 4.2.3 (Mechanical processes [Geosphere]) and FEP 4.2.4 (Geochemical processes [Geosphere]), which do not involve fundamental lithological changes. That is, FEP 1.2.9 involves the formation of a rock from sediment, FEP 4.2.1, FEP 4.2.2, FEP 4.2.3 and FEP 4.2.4 may involve changes to the properties of a particular lithology (e.g. a decrease or increase in its porosity), but the affected rock type remains unchanged (e.g. if they affect a shale, then the shale does not transform into another rock type).
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.
Diagenesis affects sediments and sedimentary rocks. Thus, diagenesis is relevant to repository performance and safety where sedimentary rocks host a repository and / or where sediments and sedimentary rocks occur in the wider groundwater flow system within which the repository resides or might reside in the future. Diagenesis 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 surrounding rock formations. During diagenesis there may be, depending upon the nature of the sediments or sedimentary rock types, a wide range of organic and inorganic chemical reactions. These reactions will influence the chemistry of groundwater, other liquids and gases in these rocks. Diagenesis will also influence the fluid pressures and hence the potential for fluids to flow, by generating or consuming fluids and changing the porosity of the rock. Mineral transformations (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. Diagenesis in the past may have influenced the physical and chemical characteristics of these rocks. 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 wider area around the repository that were acquired during past diagenesis will influence the future evolution of the rock during the post-closure period. Future diagenesis may occur in repository host rocks and / or surrounding rocks, potentially within timescales that are typically considered by performance assessments (often c.1 Ma).
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