decay constant, l:
For a radionuclide in a particular energy state, quotient of dP by dt, where dP is the probability of a given nucleus undergoing a spontaneous nuclear transition from that energy state in the time interval dt.

where N is the number of nuclei of concern existing at time t and A is the activity.
Unit: reciprocal second (s-1).
The activity is the decay constant multiplied by the number of nuclei of the radionuclide present.
The decay constant is related to the radioactive half-life, T½, of the radionuclide by the expression:

1.   Administrative and technical actions taken to allow the removal of some or all of the regulatory controls from a facility (except for a repository which is closed and not decommissioned).
The use of the term decommissioning implies that no further use of the facility (or part thereof) for its existing purpose is foreseen.
The actions will need to be such as to ensure the long term protection of the public and the environment, and typically include reducing the levels of residual radionuclides in the materials and the site of the facility so that the materials can be safely recycled, reused or disposed of as exempt waste or as radioactive waste, and the site can be released for unrestricted use or otherwise reused.

! Decommissioning typically includes dismantling the facility (or part thereof), but in the Agency’s usage this need not be the case.  It could, for example, be decommissioned without dismantling and the existing structures subsequently put to another use (after decontamination).

For a repository, the corresponding term is closure.

2.   All steps leading to the release of a nuclear facility, other than a disposal facility, from regulatory control.  These steps include the processes of decontamination and dismantling. [13]

decommissioning plan:
A document containing detailed information on the proposed decommissioning of a facility.

The complete or partial removal of contamination by a deliberate physical, chemical or biological process.
This definition is intended to include a wide range of processes, but to exclude the removal of radionuclides from within the human body, which is not considered to be decontamination.

defence in depth:
1.   A hierarchical deployment of different levels of equipment and procedures in order to maintain the effectiveness of physical barriers placed between a radiation source or radioactive materials and workers, members of the public or the environment, in operational states and, for some barriers, in accident conditions.
The objectives of defence in depth are:

  • to compensate for potential human and component failures;
  • to maintain the effectiveness of the barriers by averting damage to the facility and to the barriers themselves; and
  • to protect the public and the environment from harm in the event that these barriers are not fully effective.

INSAG defines five levels of defence in depth:

  • Level 1: Prevention of abnormal operation and failures;
  • Level 2: Control of abnormal operation and detection of failures;
  • Level 3: Control of accidents within the design basis;
  • Level 4: Control of severe plant conditions, including prevention of accident progression and mitigation of the consequences of severe accidents; and
  • Level 5: Mitigation of radiological consequences of significant releases of radioactive materials.

The levels of defence are sometimes grouped into three safety layers: hardware, software and management control.
In the context of waste disposal, the term multiple barriers is used to describe a similar concept.
See INSAG-10 [9] for further information.
2.   The application of more than one protective measure for a given safety objective, such that the objective is achieved even if one of the protective measures fails. [40]

1.   The process and the result of developing a concept, detailed plans, supporting calculations and specifications for a facility and its parts.5
2.   The description of special form radioactive material, low dispersible radioactive material, package or packaging which enables such an item to be fully identified.  The description may include specifications, engineering drawings, reports demonstrating compliance with regulatory requirements, and other relevant documentation. [41]
This is a much more restricted definition than (1), and is specific to the Transport Regulations.

design basis:
The range of conditions and events taken explicitly into account in the design of a facility, according to established criteria, such that the facility can withstand them without exceeding authorized limits by the planned operation of safety systems.
Used as a noun, with the definition above.  Also often used as an adjective, applied to specific categories of conditions or events to mean ‘included in the design basis’; as, for example, in design basis accident, design basis external events, design basis, earthquake etc.

design basis accident:
See plant states.

design basis external events:
The external event(s) or combination(s) of external events considered in the design basis of all or any part of a facility.

deterministic analysis:
Analysis using, for key parameters, single numerical values (taken to have a probability of 1), leading to a single value of the result.
In nuclear safety, for example, this implies focusing on accident types, releases and consequences, without considering the probabilities of different event sequences.
Typically used with either ‘best estimate’ or ‘worst case’ values, based on expert judgement and knowledge of the phenomena being modelled.
Contrasting terms: probabilistic or stochastic analysis.

deterministic effect:
See health effects (of radiation).

See radiation detriment.


The movement of radionuclides relative to the medium in which they are distributed, under the influence of a concentration gradient.
Usually used for the movement of airborne radionuclides (e.g. from discharges or resulting from an accident) relative to the air, and for movement of dissolved radionuclides (e.g. in groundwater or surface water, from migration following waste disposal, or in surface water from discharges) relative to the water.
Most commonly contrasted with advection, where the radionuclide does not move relative to the carrying medium, but moves with it.

direct cause:
See cause.

directional dose equivalent:
See dose equivalent quantities.

1.   Planned and controlled release of (usually gaseous or liquid) radioactive material to the environment.
Strictly, the act or process of releasing the material, but also used to describe the material released.

2.   A planned and controlled release into the environment, as a legitimate practice, within limits authorized by the regulatory body, of liquid or gaseous radioactive materials that originate from regulated nuclear facilities during normal operation. [13]

The spreading of radioactive material in the environment.
In normal language synonymous with dispersion, but tends to be used in a general sense, not implying the involvement of any particular processes or phenomena, e.g. the uncontrolled spreading of material that has escaped from containment, or as a result of damage to (or the destruction of) a sealed source, special form radioactive material or low dispersible radioactive material.

The spreading of radionuclides in air (aerodynamic dispersion) or water (hydrodynamic dispersion) resulting mainly from physical processes affecting the velocity of different molecules in the medium.
Often used in a more general sense combining all processes (including molecular diffusion) that result in the spreading of a plume.  The terms atmospheric dispersion and hydrological dispersion are used in this more general sense for plumes in air and water respectively.
In normal language synonymous with dispersal, but dispersion is mostly used more specifically as defined above, whereas dispersal is typically (though not universally) used as a more general expression.
See also advection and diffusion.

1.   Emplacement of waste in an appropriate facility without the intention of retrieval.
Some countries use the term disposal to include discharges of effluents to the environment.

! In many cases, the only element of this definition that is important is the distinction between disposal (with no intent to retrieve) and storage (with intent to retrieve).  In such cases, a definition is not necessary; the distinction can be made in the form of a footnote at the first use of the term disposal  or storage (e.g. "The use of the term disposal indicates that there is no intention to retrieve the waste.  If retrieval of the waste at any time in the future is intended, the term storage is used.").

In some States, the term disposal is used administratively in such a way as to include, for example, incineration of waste or the transfer of waste between operators.  In Agency publications, disposal should only be used in accordance with the more restrictive definition given above.

! The term disposal implies that retrieval is not intended; it does not mean that retrieval is not possible.

Contrasted with storage.

2.   The emplacement of spent fuel or radioactive waste in an appropriate facility without the intention of retrieval. [13]
3.   The act or process of getting rid of waste, without the intention of retrieval.
The terms deep sea disposal and seabed disposal do not strictly satisfy definition (1) or (2), but are consistent with the everyday meaning of disposal and are used as such.

  • deep sea disposal: Disposal of waste packaged in containers on the deep ocean floor.<
    As practised until 1982 in accordance with the requirements of the London Convention 1972 [47].
    The commonly used, but informal, term ‘sea dumping’ should not be used in Agency publications.
  • seabed disposal: Emplacement of waste packaged in suitable containers at some depth into the sedimentary layers of the deep ocean floor.
    This may be achieved by direct emplacement, or by placing the waste in specially designed ‘penetrators’ which, when dropped into the sea, embed themselves in the sediment.

disposal facility:
Synonymous with repository.

The presence of two or more redundant systems or components to perform an identified function, where the different systems or components have different attributes so as to reduce the possibility of common cause failure.
Examples of such attributes are: different operating conditions, different working principles or different design teams (which provide functional diversity), and different sizes of equipment, different manufacturers, and types of equipment that use different physical methods (which provide physical diversity).

1.   A measure of the energy deposited by radiation in a target.
For definitions of the most important such measures, see dose quantities and dose concepts.

2.    Absorbed dose, committed equivalent dose, committed effective dose, effective dose, equivalent dose or organ dose, as indicated by the context.

dose concepts:

! This is not, in general, the same as the dose actually delivered during the year in question, which could include doses from radionuclides remaining in the body from intakes in previous years, and could exclude doses delivered in future years from intakes during the year in question.

  • avertable dose: The dose that could be averted if a countermeasure or set of countermeasures were to be applied.
  • averted dose: The dose prevented by the application of a countermeasure or set of countermeasures, i.e. the difference between the projected dose if the countermeasure(s) had not been applied and the actual projected dose.
  • collective dose: The total radiation dose incurred by a population.
    This is the sum of all of the individual doses to members of the population.  If the doses continue for longer than a year, then the annual individual doses must also be integrated over time.  Unless otherwise specified, the time over which the dose is integrated is infinite; if a finite upper limit is applied to the time integration, the collective dose is described as ‘truncated’ at that time.
    Unless otherwise specified, the relevant dose is normally effective dose (see collective effective dose for the formal definition).
    Unit: man sievert (man Sv).  This is, strictly, just a sievert, but the unit man sievert is used to distinguish the collective dose from the individual dose which a dosimeter would measure (just as, for example, ‘man hours’ are used to measure the total effort devoted to a task, as opposed to the elapsed time that would be shown by a clock).
    Contrasting term: individual dose.

! The BSS definition states that the collective dose is "defined as the product of the number of individuals exposed to a source and their average radiation dose."  Strictly, it would be more accurate to say that the collective dose can be expressed in this way, but it is not defined as such — indeed, in order to calculate the average dose it would normally be necessary to calculate the collective dose (by summing the individual doses, and integrating over time if necessary) and divide it by the number of individuals.

  • committed dose:The lifetime dose expected to result from an intake.
    This term partially supersedes dose commitment.
  • dose commitment: The total dose that would eventually result from an event (e.g. a release of activity), a decision or a finite portion of a practice.
    More specific and precise terms, such as committed dose or collective dose should be used as appropriate.
  • individual dose: The dose incurred by an individual.
    For contrast with collective dose.
  • lifetime dose: The total dose received by an individual during his/her lifetime.
    In practice, often approximated as the sum of the annual doses incurred.  Because annual doses include committed doses, some parts of some of the annual doses may not actually be delivered within the lifetime of the individual, and therefore this may overestimate the true lifetime dose.
    For prospective assessments of lifetime dose, a lifetime is normally interpreted as 70 years.
  • projected dose: The dose that would be expected to be incurred if a specified countermeasure or set of countermeasures — or, in particular, no countermeasures — were to be taken.
    The BSS definition [40] refers only to the dose that would be received if no countermeasures were to be taken.
  • residual dose: In a chronic exposure situation, the dose expected to be incurred in the future after intervention has been terminated (or a decision has been taken not to intervene).

dose and dose rate effectiveness factor (DDREF):
The ratio between the risk or radiation detriment per unit effective dose for high doses and/or dose rates and that for low doses and dose rates.
Used in the estimation of risk coefficients for low doses and dose rates from observations and epidemiological findings at high doses and dose rates.
Supersedes dose rate effectiveness factor (DREF).

dose equivalent:
The product of the absorbed dose at a point in the tissue or organ and the appropriate quality factor for the type of radiation giving rise to the dose.
A measure of the dose to a tissue or organ designed to reflect the amount of harm caused.
A quantity used by the International Commission on Radiation Units and Measurements (ICRU) in defining the operational quantities ambient dose equivalent, directional dose equivalent and personal dose equivalent (see dose equivalent quantities).  The quantity dose equivalent has been superseded for radiation protection purposes by equivalent dose. [40]

dose equivalent quantities:

dose limit:
See limit.

dose quantities:

  • absorbed dose, D: The fundamental dosimetric quantity D, defined as:

    where d is the mean energy imparted by ionizing radiation to matter in a volume element and dm is the mass of matter in the volume element. [40]
    The energy can be averaged over any defined volume, the average dose being equal to the total energy imparted in the volume divided by the mass in the volume.
    Absorbed dose is defined at a point; for the average dose in a tissue or organ, see organ dose.
    Unit: J/kg, termed the gray (Gy) (formerly, the rad was used).
  • collective effective dose, S: The total effective dose S to a population, defined as:

    where Ei is the average effective dose in the population subgroup i and Ni is the number of individuals in the subgroup. It can also be defined by the integral:

    where  is the number of individuals receiving an effective dose between E and E+dE. [11]

    The collective effective dose Sk committed by an event, a decision or a finite portion of a practice k is given by:

    where  is the collective effective dose rate at time t caused by k. [40]
  • committed effective dose, E(t): The quantity E(t), defined as:

    where HT(t) is the committed equivalent dose to tissue T over the integration time t and wT is the tissue weighting factor for tissue T.  When t is not specified, it will be taken to be 50 years for adults and to age 70 years for intakes by children. [40]
  • committed equivalent dose, HT(t): The quantity HT(t), defined as:

    where t0 is the time of intake,  is the equivalent dose rate at time t in organ or tissue T and t is the time elapsed after an intake of radioactive substances.  When t is not specified, it will be taken to be 50 years for adults and to age 70 years for intakes by children. [40]
  • effective dose, E: The quantity E, defined as a summation of the tissue equivalent doses, each multiplied by the appropriate tissue weighting factor:

    where HT is the equivalent dose in tissue T and wT is the tissue weighting factor for tissue T.  From the definition of equivalent dose, it follows that:

    where wR is the radiation weighting factor for radiation R and DT,R is the average absorbed dose in the organ or tissue T. [40]
    • The unit of effective dose is J/kg, termed the sievert (Sv).  The rem, equal to 0.01 Sv, is sometimes used as a unit of equivalent dose and effective dose.  This should not be used in Agency documents except when quoting directly from other documents, in which case the value in sieverts should be added in parentheses.
    • Effective dose is a measure of dose designed to reflect the amount of radiation detriment likely to result from the dose.
    • Values of effective dose from any type(s) of radiation and mode(s) of exposure can be compared directly.
  • equivalent dose, HT: The quantity HT,R, defined as:

    where DT,R is the absorbed dose delivered by radiation type R averaged over a tissue or organ T and wR is the radiation weighting factor for radiation type R.  When the radiation field is composed of different radiation types with different values of wR the equivalent dose is:

    • The unit of equivalent dose is J/kg, termed the sievert (Sv).  The rem, equal to 0.01 Sv, is sometimes used as a unit of equivalent dose and effective dose.  This should not be used in Agency documents except when quoting directly from other documents, in which case the value in sieverts should be added in parentheses.
    • A measure of the dose to a tissue or organ designed to reflect the amount of harm caused.
    • Values of equivalent dose to a specified tissue from any type(s) of radiation can therefore be compared directly.
  • organ dose: The mean absorbed dose DT in a specified tissue or organ T of the human body, given by:

    where mT is the mass of the tissue or organ and D is the absorbed dose in the mass element dm.

dose rate:

! Although dose rate could, in principle, be defined over any unit of time (e.g. an annual dose is, technically a dose rate), in Agency documents the term dose rate should be used only in the context of short periods of time, e.g. dose per second or dose per hour.

dose rate effectiveness factor (DREF):
The ratio between the risk per unit effective dose for high dose rates and that for low dose rates.
Superseded by dose and dose rate effectiveness factor (DDREF).