Description

The possibility and effects of spontaneous nuclear fission chain reactions within the waste package. Criticality requires a sufficient concentration and localised mass (critical mass) of fissile isotopes (e.g. U-235, Pu-239) and is more likely to occur in the presence of neutron moderating materials, such as water, in a suitable geometry; a chain reaction is less likely to occur in 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.

Spontaneous nuclear fission chain reactions within the waste package would produce a wide range of fission products, and ultimately their daughter isotopes, which would need to be taken into account in any assessment of post-closure criticality scenarios. Criticality would generate power and in an increase in temperature. Such power and temperature excursions may affect the chemical and physical properties of the waste form, any other materials in the waste package, such as encapsulant, and the container itself. These changes may impact the rate at which radionuclides and other contaminants might be released from the waste package, which may be damaged by the criticality event. However, criticality will continue until negative feedback mechanisms, such as a decrease in moderator density associated with heating or depletion of the fissile material, cause it to shut down.

2000 List

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

2.1.14

Related References