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Terminology to Describe Radiation Incidents

Key definitions: incident vs. event

What is the difference between an event and an incident using current US terminology?

  • Event (planned event)
    • Examples: a scheduled nonemergency activity (e.g., sporting event, concert, parade, training exercise, large convention, fair, large gathering, etc.)
  • Incident (unplanned event)
    • Examples: An occurrence or event, natural or manmade that requires a response to protect life or property. Incidents can include major disasters, emergencies, terrorist attacks, terrorist threats, civil unrest, wild land and urban fires, floods, hazardous materials spills, nuclear accidents, aircraft accidents, earthquakes, hurricanes, tornadoes, tropical storms, tsunamis, war-related disasters, public health and medical emergencies, and other occurrences requiring an emergency response.
  • Source: Introduction to the Incident Command System (ICS 100.c), Student Manual, November 2018, (PDF - 8.99 MB). See glossary. This is part of the National Incident Management System (NIMS).

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International Nuclear and Radiological "Event" (Severity) Scale (INES)

  • Developed by International Atomic Energy Agency (IAEA) and partners in 1990 and it has undergone revisions.
  • The diagram below is the current version.
  • Severity scale ranges from 1 (least severe) to 7 (most severe)
  • The severity of an event is about 10 times greater for each increase in level on this scale.
  • Events without safety significance are called "deviations" and are classified as Below Scale/Level 0.
  • See INES information
  • Note nomenclature difference between IAEA and US
    • IAEA nomenclature references "event".
    • US nomenclature references "incident" as in the NIMS.
  • Examples
    • IAEA characterized the March 2011 Fukushima Nuclear Accident in Japan as an INES Level 7 accident
    • IAEA characterized the 1986 Chernobyl nuclear reactor accident in the Ukraine as an INES Level 7 accident.

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Incident's Phases and Timelines

  • "The Radiological/Nuclear Incident Annex to the Response and Recovery Federal Interagency Operations Plans" (PDF - 3.38 MB) (US Government Interagency, October 2016) describes 3 main operational phases for the response to and recovery from a nuclear/radiological incident.
  • The phases vary based upon the size, scope, and complexity of the incident.
  • The operational phases identified in the Response and Recovery Federal Interagency Operations Plans serve as the default posture for achieving Nuclear/Radiological Incident Annex response and recovery objectives.
  • Catastrophic nuclear/radiological incident response and recovery activities are interdependent and often concurrent.
  • Decisions made and priorities set early in the response will have a cascading effect on the nature and speed of recovery
  • Figure 2 from that document, shown below, provides an overview of this default posture.
    Default Phased Posture for the Nuclear/Radiological Incident Annex


  • HHS has used the following nomenclature (see below) to describe the chronological phases of an incident.
  • It is valuable for all agencies to work from the same or similar nomenclature and timeline, as they respond to an incident.
  • Illustration of the current nomenclature for the timeline and its phases.

    Phases of an incident for planning and response


  • Expected Timeline of Events for a 10-Kiloton Improvised Nuclear Device Detonation
    expected timeline of events for a 10-Kiloton Improvised Nuclear Device Detonation

  • The EPA PAG Manual 2017 says that emergency planners divide responses to radiological incidents into three phases of activity.
    • Early Phase - The beginning of a radiological incident for which immediate decisions for effective use of protective actions are required and must therefore be based primarily on the status of the radiological incident and the prognosis for worsening conditions. When available, predictions of radiological conditions in the environment based on the condition of the source or actual environmental measurements may be used. Protective actions based on the PAGs may be preceded by precautionary actions during the period. This phase may last from hours to days.
    • Intermediate Phase - The period beginning after the source and releases have been brought under control (has not necessarily stopped but is no longer growing) and reliable environmental measurements are available for use as a basis for decisions on protective actions and extending until these additional protective actions are no longer needed. This phase may overlap the early phase and late phase and may last from weeks to months.
    • Late Phase - The period beginning when recovery actions designed to reduce radiation levels in the environment to acceptable levels are commenced and ending when all recovery actions have been completed. This phase may extend from months to years. A PAG level, or dose to avoid, is not appropriate for long-term cleanup.
  • The phases cannot be represented by precise periods of time – and may even overlap - but to view them in terms of activities, rather than time spans, can provide a useful framework for emergency response planning.
  • Diagram showing the Recovery Continuum timelines and tasks and considerable overlap among recovery tasks during the short term, intermediate term and long term phases of an incident. National Disaster Recovery Framework (PDF - 11 MB) (DHS/FEMA, September 2011) See page 8, Figure 1.
    • The EPA PAG Manual 2017 has a radiation specific update to this recovery framework in Table 4-2, page 50 "Reentry Matrix: Quick Reference to Operational Guides".
  • 2008 description of phases of an RDD and IND incident has been superseded by the nomenclature above.