ICRU sphere:
A sphere of 30 cm diameter made of tissue equivalent material with a density of 1 g/cm3 and a mass composition of 76.2% oxygen, 11.1% carbon, 10.1% hydrogen and 2.6% nitrogen.
Used as a reference phantom in defining dose equivalent quantities.
ICRU, 1980 [52].

The word incident is often used, in INES and elsewhere, to describe events that are, in effect, minor accidents, i.e. that are distinguished from accidents only in terms of being less severe.  This is an arbitrary distinction with little basis in normal usage.  An incident can be minor or major, just as an accident can, but unlike an accident, an incident can be caused intentionally.  The existing misuses of incident, such as INES, cannot be eliminated, but new examples should not be created.


A simple scale, designed for promptly communicating to the public in consistent terms the safety significance of events at nuclear facilities.  The Scale should not be confused with emergency classification systems, and should not be used as a basis for emergency response actions.

! The INES terminology — particularly the use of the term accident — is different from that used in safety standards, and great care should be taken to avoid confusion between the two.  Unless otherwise indicated, in the rest of this glossary the term accident is used with its safety standards meaning (see accident (1)) and the term event is used with its normal English meaning.

initiating event:
>An identified event that leads to anticipated operational occurrences or accident conditions and challenges safety functions.
This term (often shortened to initiator) is used in relation to event reporting and analysis, i.e. when such events have occurred.  For the consideration of hypothetical events considered at the design stage, the term postulated initiating event is used.

  • postulated initiating event: An event identified during design as capable of leading to anticipated operational occurrences or accident conditions.
    The primary causes of postulated initiating events may be credible equipment failures and operator errors (both within and external to the facility), man-induced or natural events.

See initiating event.

An examination, observation, measurement or test undertaken to assess structures, systems and components and materials, as well as operational activities, processes, procedures and personnel competence.

institutional control:
See control (1).

1.   The act or process of taking radionuclides into the body by inhalation or ingestion or through the skin. [40]
2.   The activity of a radionuclide taken into the body in a given time period or as a result of a given event.

  • acute intake: An intake occurring within a time period short enough that it can be treated as instantaneous for the purposes of assessing the resulting committed dose.

! The exposure that results from an acute intake is not necessarily acute exposure.  For a long-lived radionuclide that is retained in the body, an acute intake will result in chronic exposure.

  • chronic intake: An intake over an extended period of time, such that it cannot be treated as a single instantaneous intake for the purposes of assessing the resulting committed dose.
    Chronic intake may, however, be treated as a series of acute intakes.

interim storage:
See storage.

intermediate bulk container (IBC):
A portable packaging that;

  • has a capacity of not more than 3 m3,
  • is designed for mechanical handling,
  • is resistant to the stresses produced in handling and transport, as determined by performance tests, and
  • is designed to conform to the standards in the chapter on Recommendations on Intermediate Bulk Containers (IBCs) of the United Nations Recommendations on the Transport of Dangerous Goods [54]. [2]

Any action intended to reduce or avert exposure or the likelihood of exposure to sources which are not part of a controlled practice or which are out of control as a consequence of an accident. [40]
This definition is somewhat more explicit than (though not necessarily inconsistent with) that of ICRP [48].

iodine prophylaxis:
The administration of a compound of stable iodine (usually potassium iodide) to prevent or reduce the uptake of radioactive isotopes of iodine by the thyroid in the event of an accident involving radioactive iodine.
An urgent protective action.
The term ‘thyroid blocking’ is sometimes used.

ionizing radiation:
See radiation.

irradiation installation:
A structure or an installation that houses a particle accelerator, X ray apparatus or large radioactive source and that can produce high radiation fields. [40]
Irradiation installations include installations for external beam radiation therapy, installations for sterilization or preservation of commercial products and some installations for industrial radiography.