1The term ‘facility’ as used in this Safety Guide means a facility with its associated land, buildings and equipment in which radioactive material is used, processed, handled or stored on such a scale that consideration of safety is required (Ref. [1], para. 1.1).

2The term ‘decommissioning’ as used in this Safety Guide refers to the administrative and technical actions taken to allow the removal of some or all of the regulatory controls from a facility (except for a repository, for which the term ‘closed’ and not ‘decommissioned’ is used) (Ref. [1], para. 1.1).

3The term ‘accident’ as used in this Safety Guide means any unintended event, including operating errors, equipment failures and other mishaps, the consequences or potential consequences of which are not negligible from the point of view of protection or safety.

4The term ‘risk’ used in this Safety Guide means a multi-attribute quantity expressing hazard, danger or chance of harmful or injurious consequences associated with actual or potential exposures. It relates to quantities such as the probability that specific deleterious consequences may arise, and the magnitude and character of such consequences [3].

5The term ‘authorization’ means the granting by a regulatory body or other governmental body of written permission for an operator to perform specified activities

6The term ‘asphyxiants’ as used in this Safety Guide means gases which, when present in an atmosphere in high concentrations, lead to a reduction of the oxygen concentration by displacement or dilution (e.g. acetylene nitrogen).

7The term ‘initiating event’ means an identified event that leads to anticipated operational occurrences or accident conditions and that challenges safety functions.

8The term ‘defence in depth’ means a hierarchical deployment of different levels of diverse equipment and procedures to prevent the escalation of anticipated operational occurrences and to maintain the effectiveness of physical barriers placed between a radiation source or radioactive material and workers, members of the public or the environment, in operational states and, for some barriers, in accident conditions. The International Nuclear Safety Advisory Group (in INSAG-10 [18]) defines five levels of defence in depth: (a) Level 1: Prevention of abnormal operation and failures; (b) Level 2: Control of abnormal operation and detection of failures; (c) Level 3: Control of accidents within the design basis; (d) Level 4: Control of severe plant conditions, including prevention of accident progression and mitigation of the consequences of severe accidents; and (e) Level 5: Mitigation of radiological consequences of significant releases of radioactive material.

9The term ‘safety function’ as used in this Safety Guide means a specific purpose that must be accomplished for safety. Its use here is more general than the three main safety functions for a nuclear power plant (control of reactivity, cooling of radioactive material and confinement of radioactive material), to reflect the wider range of hazards and scenarios that are relevant to decommissioning activities. Examples of safety functions during decommissioning include, in addition to these three main safety functions, shielding, radiation detection and actuation of alarms, fire suppression and ventilation.

10The term ‘model verification’ as used in this Safety Guide is the process of determining whether a computational model correctly implements the intended conceptual model or mathematical model. In relation to a computer code and other modelling tools, model validation is the process of determining whether a model is an adequate representation of the real system being modelled, by comparing the predictions of the model with observations of the real system.