Description

The temporal evolution of pH within 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.

  • 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 pH within the repository and surrounding geosphere will both influence and reflect the chemical and biological evolution of the EBS and the adjacent natural barrier. The evolution of pH conditions will therefore be related to the physical evolution of these barriers, which is coupled to the chemical evolution. The pH evolution may be coupled with the development or sealing of potential pathways through which fluids (such as liquid water, non-aqueous liquids or gases) may move. If mobile such fluids may in turn transport radionuclides and other contaminants, should these be released from the waste packages.

pH represents the chemical activity of H+ in the water present within the EBS and surrounding geosphere. Some chemical reactions in these barriers will consume H+ (increase pH) while others will generate H+ (decrease pH). Reactions that influence pH in one barrier component, may influence the chemical evolution of an adjacent barrier if fluid is able to move between the components. For example, a high (alkaline) pH developed in the porewater within a cement barrier may passivate the steel forming a waste container in contact with the cement, thereby reducing the rate of steel corrosion.

The pH evolution will affect the chemical speciation of radionuclides and other contaminants and influence their abilities to precipitate / co-precipitate, re-dissolve, sorb and diffuse. The pH will affect the partitioning of the radionuclides and other contaminants among different potentially mobile phases (such as water, non-aqueous liquids or gases). The pH evolution within the repository will therefore influence the release rate of radionuclides and other contaminants from the waste package (should this release occur), and their subsequent migration and retardation within the EBS and surrounding geosphere.

2000 List

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

2.1.09