Description

The alteration of the waste package by chemical processes such as dissolution, leaching, chloride and sulphate attack, carbonation and polymer degradation.

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.

“Alteration” is a change in the chemical or physical form(s) of one or more solid materials within a waste package. Many alteration processes will impact upon the functions of waste package components, such as their ability to provide containment (e.g. corrosion of a steel waste container) their ability to provide mechanical support (e.g. corrosion of iron inserts in copper containers for spent fuel) or their ability to provide chemical buffering (e.g. carbonation of a cementitious encapsulant). Waste form alteration (e.g. degradation of organic waste forms) may be accompanied by the release of radionuclides and other contaminants. Alteration of solid materials may involve the consumption or production of gases (including water vapour), liquid water or organic liquids. These processes could impact upon the mobilisation of radionuclides and other contaminants because the evolved or consumed phase may be able to transport these contaminants and / or because these processes will tend to cause pressure gradients. Alteration processes may also cause the volumes of waste package components to change. There could be consequent pressure changes, which may impact upon the migration of radionuclides and other contaminants. A net change in the volume of a waste package may result from alteration processes, potentially impacting upon the surrounding engineered and / or natural barriers. For example, expansive corrosion might result in cracking of a surrounding cementitious backfill.

Leaching can result in the reduction of pH in concrete in the long-term. Chloride attack can increase concrete porosity by increasing the leaching of portlandite and reduce concrete strength. Sulphate attack depletes the reservoir of alkalinity (calcium hydroxide) in the concrete and can also result in a reduction in concrete strength. Whilst carbonation reduces the ability of concrete to impose high pH conditions on water by reacting with calcium hydroxide, it heals cracks, sealing them to ingress by water through the production of calcium carbonate which has low solubility. Thus, carbonation can counter the effects of leaching and chloride/sulphate attack. Polymer degradation can lead to the generation of gases (e.g. carbon dioxide and methane) and the loss of the integrity of polymeric packaging material. Volatile compounds can be formed with the rate being controlled by changes in pressure, temperature and concentration of volatiles in the waste package

2000 List

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

2.1.09

Related References