Description

The temporal evolution of the waste package’s redox state (as represented by parameters such as redox potential relative to the standard hydrogen electrode, Eh) from its initial state. This evolution depends on a number of factors, including the Eh conditions of the surrounding water and the consumption rate of any available oxygen. Oxygen-deficient (anaerobic) conditions promote the formation of lower, and often less soluble, oxidation states of elements, promote relatively slow corrosion and microbial processes, and minimise the rate of gas generation.

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

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.

The evolution of redox conditions (as represented by a redox potential relative to the standard hydrogen electrode, Eh) may be affected by many chemical processes within a waste package, or at its outer surface, that involve chemical species in differing oxidation states (e.g. oxidation of metals, reduction of Fe-oxides). The overall evolution of Eh will reflect the couplings between these processes and will probably be heterogeneous within the waste package.

The evolution of redox conditions will influence/reflect the evolution pathways and evolution rates of waste package components. Consequently, the integrity of the waste package and the potential for radionuclides or other contaminants to be released from it may be affected. Redox-dependent processes may cause a package that initially offers containment to lose its integrity (e.g. corrosion of steel canisters). Alternatively, where a package does not offer full containment initially (e.g. because the package is vented), the ability of the package to resist migration of radionuclides or other contaminants may be affected.

Where a waste package does not provide containment, the oxidation state in which radionuclides and other contaminants are released and migrate from it will be influenced by Eh within the waste package. In such a case, the ability of the waste package components to retard the migration of radionuclides and other contaminants may be affected (e.g. radionuclides in different oxidation states may sorb to different extents).

Redox conditions within the waste package may be coupled to chemical processes outside the waste package, where the package lacks integrity. Redox conditions at the outer surface of a waste package will be coupled to chemical processes in the surrounding natural and / or engineered barriers. These couplings may cause the redox conditions at the outer surface of the waste package and possibly within the waste package (if the waste package lacks integrity) to also influence the chemical evolution of the surrounding barriers.

2000 List

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

2.1.09

Related References