Description
The physical, chemical and biological characteristics and properties of the buffer/backfill at the time of waste emplacement in the repository.
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.
- Feature
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.
Buffer and backfill are sometimes used synonymously. In some HLW/spent fuel concepts, the term buffer is used to mean material immediately surrounding a waste package and having some chemical and/or mechanical buffering role, whereas backfill is used to mean material used to fill other underground openings. However, in some ILW/LLW concepts the term backfill is used to describe the material placed between waste packages which may have a chemical and/or mechanical role. Buffer/backfill materials may include clays, cement and mixtures of cement with aggregates, e.g. of crushed rock.
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.
A buffer / backfill, if present, will affect the rate and nature of waste package evolution. The buffer/backfill may therefore influence whether waste package integrity is lost and if so the time following repository closure at which this occurs. Depending upon its design, a buffer / backfill will influence to some degree the mechanical forces upon a waste package, the rate of fluid flow around the waste package (such as liquid water, non-aqueous phase liquids and gas), and the biological /chemical environment of the waste package. These factors will affect the potential for the waste package to undergo mechanical deformation and to evolve chemically (e.g. corrode, in the case of metallic overpacks, or leach, in the case of cementitious containers).
Should release from waste packages occur, the rate at which radionuclides and other contaminants are able to leave a repository will depend upon the physical, chemical and biological characteristics of any buffer or backfill that is present. The chemical characteristics of the buffer / backfill will influence the chemical and physical forms in which radionuclides or other contaminants migrate (e.g. chemical speciation, partitioning between aqueous and gaseous forms).
The porosity and permeability of the buffer / backfill will influence the pressures that are attained within the repository post closure, should gas be generated by the wastes and / or engineered barrier components. The pressure evolution could in turn impact the integrity and effectiveness of repository seals. The presence and mechanical characteristics of a buffer / backfill will influence post-closure deformation of the surrounding geosphere and possibly evolution of potential pathways for fluid flow within it. For example, backfill with swelling properties may exert a pressure on the wall rocks of sealed tunnels that tends to decrease apertures of fractures in the excavation disturbed zone.
2000 List
A reference to the related FEP(s) within the 2000 NEA IFEP List.
Related References
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Bond AE, Egan MJ, Metcalfe R, Robinson PJ and Towler G (), Understanding Controls on the Performance of Engineered Barrier Systems in Repositories for High-level Radioactive Waste and Spent Fuel, Environment Agency Science Report SC060055, Environment Agency, Rio House, Waterside Drive, Aztec West, Almondsbury, Bristol, BS32 4UD, 200, http://www.gov.uk/government/uploads/system/uploads/attachment_data/file/291238/scho0910bsze-e-e.pdf
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IAEA (), Scientific and Technical Basis for the Geological Disposal of Radioactive Wastes, IAEA Technical Report Series, International Atomic Energy Agency, Vienna, 413, 80, http://www-pub.iaea.org/MTCD/publications/PDF/TRS413_web.pdf
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NEA (), Engineered Barrier Systems (EBS) in the Safety Case Design Confirmation and Demonstration - Workshop Proceedings, Tokyo, Japan, 12-15 September 2006, OECD, 150, http://dx.doi.org/10.1787/9789264040885-en
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Sellin P (), BEACON – Bentonite mechanical evolution: full project report that includes the synthesis report from the WP1-6. Deliverable D7.8., Euratom research and training programme 2014-2018 under grant agreement No 745 942., https://www.beacon-h2020.eu/wp-content/uploads/2022/09/D7.8-Full-project-report-that-includes-the-synthesis-report-from-the-WP1-6.pdf
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Sellin P, Westermark M, Leupin O, Norris S, Gens A, Wieczorek K, Talandier J, Swahn S (), BEACON: bentonite mechanical evolution, EPJ Nuclear Science and Technology, 6, 23, https://doi.org/10.1051/epjn/2019045