Description
The possibility and effects of spontaneous nuclear fission chain reactions within the repository. Criticality requires a sufficient concentration and localised mass (critical mass) of fissile isotopes (e.g. U-235, Pu-239) and also presence of neutron moderating materials in a suitable geometry; a chain reaction is liable to be damped by the presence of neutron absorbing isotopes (e.g. Pu-240).
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.
Were it to occur, criticality could impact upon the physical, chemical and biological characteristics of the EBS and the adjacent geosphere. Potentially, the performance of the engineered and natural barriers could be affected. Heat generation, radiation damage and radiolysis could all potentially contribute to chemical and physical changes in these barriers that might produce pathways through them, through which fluids (such as liquid water, non-aqueous liquids and gases) might flow. These fluids could in turn potentially transport radionuclides and other contaminants.
Criticality might influence the identities and proportions of different immobile and potentially mobile fluid phase present. For example, heat generated by spontaneous fission could lead to breakdown of organic materials and the generation of gas. Such heating would also affect the densities of solid and fluid phases. Chemical conditions might be affected by criticality, as a result of this process affecting chemical reactions among chemical species present in the engineered and natural barriers.
Criticality might therefore influence forces driving advection of fluids and chemical gradients driving diffusion.
Due to criticality, the identities and proportions of radionuclides present in the repository would change. Criticality would produce fission products, many of which would then undergo radioactive decay to produce daughter isotopes. Thus, criticality has the potential to influence the radiation doses to which receptors in the biosphere could be exposed, should radionuclides be transported from the repository to the biosphere.
2000 List
A reference to the related FEP(s) within the 2000 NEA IFEP List.