Description

The cessation of waste emplacement operations in a repository and the backfilling and sealing of access tunnels, shafts and site investigation/monitoring boreholes.

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 are some similarities between Closure (this FEP 1.1.7), Schedule and planning (FEP 1.1.4) and Operation (FEP 1.1.6). Whereas FEP 1.1.7 concerns final closure of the whole repository, FEP 1.1.6 covers closure of individual sections in sequence. FEP 1.1.4 covers the planning and sequencing of closure, rather than the actual closure itself. FEP 1.1.7 is different from Construction (FEP 1.1.5), which concerns only the development of the repository.

Individual sections of a repository may be closed in sequence (FEP 1.1.4), but, in the present context, closure refers to final closure of the whole repository, (including the sealing of any open site characterisation boreholes) and will probably include removal of surface installations.

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.

Closure activities are undertaken to isolate the waste packages, to prevent human access into and limit the migration of contaminants, including radionuclides from the repository post-closure. Closure of the repository must be done in such a way as to ensure that post-closure migration of water or gas does not compromise repository performance and safety by transporting contaminants, including radionuclides, from the repository to the biosphere. If they are not closed appropriately then boreholes within the repository footprint and / or excavations that are part of the repository (rooms, access tunnels, shafts) could potentially form pathways for this migration to occur. Such pathways could arise due to ineffective seals (e.g. degraded concrete plugs) or due to damaged rock surrounding the excavations. Some of the pathways could connect the wastes directly to the biosphere (e.g. a shaft with ineffective seals) or could connect the waste to a natural pathway (e.g. where an ineffective seal within a tunnel allows water or gas to be transported to a transmissive fault). It may be necessary to examine in the post-closure safety assessment the consequences of the use of poor closure techniques that might not be detected by the quality control programme. It may also be necessary to consider the potential for degraded performance of shaft and borehole seals, particularly over the long time frames over which those seals might be required to contribute to safety.

2000 List

A reference to the related FEP(s) within the 2000 NEA IFEP List.

1.1.04

Related References