abnormal operation:
See anticipated operational occurrence.

acceptable limit
See limit.

acceptance criteria:
Specified bounds on the value of a functional or condition indicator used to assess the ability of a structure, system or component to perform its design function.

1.   Any unintended event, including operating errors, equipment failures or other mishaps, the consequences or potential consequences of which are not negligible from the point of view of protection or safety. [40]

! This is not explicitly stated to be a definition of nuclear accident, but it is derived from the statement of the scope of application in Article 1 of the Convention on Early Notification of a Nuclear Accident, and therefore may be regarded as such a definition. For the purposes of Agency publications, however, the definitions of this and related terms (e.g. radiological emergency) are currently under review.

2.  See INES.

accident with off-site risk:

accident without significant off-site risk:

accident conditions:
See plant states.

accident management:
See plant states.

accident precursor:
An initiating event that could lead to accident conditions.

action level:
See level.

1.   The quantity A for an amount of radionuclide in a given energy state at a given time, defined as:

where dN is the expectation value of the number of spontaneous nuclear transformations from the given energy state in the time interval dt. [40]
The rate at which nuclear transformations occur in a radioactive material.  The equation is sometimes given as

, where N is the number of nuclei of the radionuclide, and hence the rate of change of N with time is negative.  Numerically, the two forms are identical.
The SI unit of activity is the reciprocal second (s-1), termed the becquerel (Bq). [40]
Formerly expressed in curie (Ci); activity values may be given in Ci (with the equivalent in Bq in parentheses) if they are being quoted from a reference which uses that unit.

  • specific activity: Of a radionuclide, the activity per unit mass of that nuclide.  Of a material, the activity per unit mass or volume of the material in which the radionuclides are essentially uniformly distributed. [41]
    The distinction in usage between specific activity and activity concentration is controversial.  Some regard the terms as synonymous, and may favour one or the other (as above).  ISO 921 [45] distinguishes between specific activity as the activity per unit mass and activity concentration as the activity per unit volume.  Another common distinction is that specific activity is used (usually as activity per unit mass) with reference to a pure sample of a radionuclide or, less strictly, to cases where a radionuclide is intrinsically present in the material (e.g. carbon-14 in organic materials, uranium-235 in natural uranium), even if the abundance of the radionuclide is artificially changed.  In this usage, activity concentration (which may be activity per unit mass or per unit volume) is used for any other situation, i.e. when the activity is in the form of contamination in or on a material.
    In general, the term activity concentration is more widely applicable, is more self-evident in meaning, and is less likely than specific activity to be confused with unrelated terms (such as ‘specified activities’).  Activity concentration is therefore preferred to specific activity for general use in safety related Agency publications.

2.   See facilities and activities.

activity concentration:
See specific activity.

actuated equipment:
An assembly of prime movers and driven equipment used to accomplish one or more safety tasks.

actuation device:
A component that directly controls the motive power for actuated equipment.
Examples of actuation devices include circuit breakers and relays that control the distribution and use of electric power and pilot valves controlling hydraulic or pneumatic fluids.

acute exposure:
See exposure situations.

The movement of a substance or the transfer of heat by the motion of the air or water in which it is present.
Sometimes used with the more common meaning — transfer of heat by the horizontal motion of the air — but in Agency publications is more often used in a more general sense, particularly in waste safety, to describe the movement of a radionuclide due to the movement of the water in which it is dissolved or suspended.
Usually contrasted with diffusion, where the radionuclide moves relative to the carrying medium.

General process in which characteristics of a structure, system or component gradually change with time or use.
Although the term ageing is defined in a neutral sense — the changes involved in ageing may have no effect on protection or safety, or could even have a beneficial effect — it is most commonly used with a connotation of changes that are (or could be) detrimental to protection or safety, i.e. as a synonym of ageing degradation.

  • non-physical ageing: The process of becoming out-of-date (i.e. obsolete) owing to the evolution of knowledge and technology and the associated changes in codes and standards.
    Examples of non-physical ageing include unavailability of qualified spare parts for old equipment, incompatibility between old and new equipment, and outdated procedures or documentation (e.g. which do not comply with current regulations).
    Strictly, this is not always ageing as defined above, because it is sometimes not due to changes in the structure, system or component itself. Nevertheless, the effects on protection and safety, and the solutions that need to be adopted, are often very similar to those for physical ageing; the management of non-physical ageing is therefore often addressed within the same programme as that for the management of physical ageing.
    The term technological obsolescence is also used.
  • physical ageing: Ageing of structures, systems and components due to physical, chemical and/or biological processes.
    Examples of physical ageing include wear, heat or radiation damage, and corrosion.
    The term material ageing is also used.

ageing degradation:
Ageing effects that could impair the ability of a structure, system or component to function within acceptance criteria.
Examples include reduction in diameter from wear of a rotating shaft, loss in material strength due to fatigue or thermal ageing, and loss of dielectric strength or cracking of insulation.

ageing management:
Engineering, operations and maintenance actions to control within acceptable limits ageing degradation of structures, systems or components.
Examples of engineering actions include design, qualification, and failure analysis.  Examples of operations actions include surveillance, carrying out operational procedures within specified limits, and performing environmental measurements.
Life management (or life cycle management) is the integration of ageing management with economic planning to: (1) optimize the operation, maintenance and service life of structures, systems and components; (2) maintain an acceptable level of performance and safety; and (3) maximize return on investment over the service life of the facility.

ALARA (as low as reasonably achievable):
See optimization (of protection).

See emergency class.

ambient dose equivalent:
See dose equivalent quantities.

Often used interchangeably with assessment, especially in more specific terms such as safety analysis. In general, however, analysis suggests a more narrowly technical process than assessment, aimed at understanding the subject of the analysis rather than determining whether or not it is acceptable. Analysis is also often associated with the use of a specific technique. Hence, one or more forms of analysis may be used in assessment.

  • safety analysis: Evaluation of the potential hazards associated with the implementation of a proposed activity.
    Safety analysis is typically more limited in scope than safety assessment, and less focused upon quantifying the overall radiological impact or safety of a facility or activity.

annual dose:
See dose concepts.


anticipated operational occurrence:
See plant states.

A legal person who applies to a regulatory body for authorization to undertake specified activities.
Strictly, an applicant would be such from the time at which an application is submitted until the requested authorization is either granted or refused. However, the term is often used a little more loosely than this, particularly in cases where the authorization process is long and complex.

The granting of consent by a regulatory body.
Typically used to represent any form of consent from the regulatory body that does not meet the definition of authorization.


1.   The process, and the result, of analysing systematically the hazards associated with sources and practices, and associated protection and safety measures, aimed at quantifying performance measures for comparison with criteria.
In Agency publications, assessment should be distinguished from analysis. Assessment is aimed at providing information that forms the basis of a decision whether something is satisfactory or not. Various kinds of analysis may be used as tools in doing this. Hence an assessment may include a number of analyses.

2.   Activities carried out to determine that requirements are met and that processes are adequate and effective, and to encourage managers to implement improvements, including safety improvements.
This usage originated in quality assurance and related fields.
Assessment activities may include reviewing, checking, inspecting, testing, surveillance, auditing, peer evaluation and technical review.  These activities can be divided into two broad categories: independent assessment and self-assessment.

  • independent assessment: Assessment activities conducted by an independent organizational unit, to determine the effectiveness of management processes, the adequacy of work performance and the quality of items and services.
    Independent assessment activities include internal and external audit, surveillance, peer evaluation and technical review, which are focused on safety aspects and areas where problems have been found.
  • self-assessment: A routine and continuing process conducted by management at all levels to evaluate the effectiveness of performance in all areas of their responsibility.
    Self-assessment activities include review, surveillance and discrete checks, which are focused on preventing, or identifying and correcting, management problems that hinder the achievement of the organization’s objectives, particularly safety objectives.
    The term management self-assessment is also used, notably in the Agency’s safety standards on quality assurance in nuclear power plants.

atmospheric dispersion:
See dispersion.

A documented activity performed to determine by investigation, examination and evaluation of objective evidence the adequacy of, and adherence to, established procedures, instructions, specifications, codes, standards, administrative or operational programmes and other applicable documents, and the effectiveness of implementation.

The granting by a regulatory body or other governmental body of written permission for an operator to perform specified activities.
Authorization could include, for example, licensing, certification, registration, etc.
The term authorization is also sometimes used to describe the document granting such permission.
Authorization is normally a more formal process than approval.

authorized activity:
See facilities and activities.

authorized facility:
See facilities and activities.

authorized limit:
See limit.

The fraction of time during which a system is capable of performing its intended purpose.
The reliability represents essentially the same information, but in a different form.

averted dose:
See dose concepts.