Glossary of terms - Safety Assessment Principles (SAPs)

A

Accident
Any unintended event, including operator 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 (source of description: IAEA ‘Nuclear Safety and Security Glossary’, [1]).

Accident management
The strategies which are developed to reduce the risks arising from accidents, and bring the facility to a safe, controlled state.

Alarm
An automatic visual or audible indication to personnel of when a specific plant variable or condition has reached a pre-set limit or state.

Availability
The fraction of time for which a system is capable of fulfilling its intended purpose [1].

B

Barrier
A barrier means to: prevent the further progression of a fault; prevent or inhibit the movement of people or radioactive material, or some other phenomenon (for example, fire); and/or provide shielding against radiation.

Best estimate
When used to describe analysis, this refers to an approach expected to provide the most accurate description that could be achieved within the limitations of the analytical model employed and the knowledge of the analysts, without any deliberate bias being introduced. When used to describe the data (for example, from experiment or operating experience), it refers to the unbiased estimate of the variable that minimises its variance.

Bounding case
A single situation used to represent a wider class of situations that is more extreme than any member of the class in all important respects.

C

Capability
The description in qualitative and quantitative terms of the complete function(s) provided by a component, sub-system or system, including information on (a) the operating limits and conditions within which the function(s) can be sustained; and (b) the circumstances beyond which permanent degradation of functions should be assumed.

Care and maintenance
Care and maintenance is a planned and substantiated stage of deferred decommissioning, in which a facility or site is maintained in a quiescent and passively safe condition for a defined period of time, often several decades, until final decommissioning to the defined end-state is commenced.

Commissioning
The process of bringing systems and components into operation and verifying they meet design and performance requirements.

Common cause failure
Failures of two or more structures, systems or components due to a single specific event or cause [1].

Competence
Possession of the necessary skills, knowledge, experience and behavioural attributes to safely and effectively fulfil one's duties.

Conservative
In analysis, an approach where the use of models, data and assumptions would be expected to lead to a result that bounds the best estimate (where known) on the safe side. The degree of conservatism should be proportionate to both the level of uncertainty and the overall significance of the estimate to the safety case.

Containment
Methods or physical structures designed to prevent the dispersion of radioactive material (description based on ‘Containment’ in [1]).

Contractors
All references to 'contractors' include proportionate consideration of the whole contracting and supply chain, whether for the provision of goods and services to the dutyholder. This includes designers, vendors, suppliers, manufacturers and so on as appropriate.

Criticality incident
The accidental occurrence of a fission chain reaction.

D

Decommissioning
Administrative and technical actions taken to reduce hazards progressively, usually with the intent to allow the removal of some or all of the regulatory controls from a plant, facility or site or to allow the plant or facility or the land it is on to be reused. Decommissioning is assumed to start on cessation of operations and continue until the defined end-state has been demonstrably achieved.

Decommissioning strategy
A document providing an overview of the approach to the decommissioning of a site (or a group of similar sites) encompassing all existing and proposed new facilities, setting down the overall decommissioning objectives including the target end-state(s) and timescales for achieving them, taking account of relevant factors, and integrated with other relevant strategies.

Design Authority
The defined function of a dutyholder’s organisation with the responsibility for, and the requisite knowledge to maintain the design integrity and the overall basis for safety of, its nuclear facilities throughout the full lifecycle of those facilities.

Design basis
The range of conditions and events taken explicitly into account in the design.

Design basis fault
A design basis fault (or design basis event) is a specific, postulated accident scenario that a facility should be designed to withstand without unacceptable consequences.

Design extension conditions
Design extension conditions are postulated accident conditions associated with multiple failures or very low predicted sequence frequencies that are considered in the design of the facility through less conservative analysis, down to best estimate if appropriate.

Design life
The period of time during which a facility or component is expected to perform according to the technical specifications to which it was produced [1].

Design intent
The fundamental criteria and characteristics (including reliability levels) that need to be realised in a facility, plant or SSC in order that it achieves its operational requirements and safety functions.

Detailed emergency planning zone
The defined zone surrounding an installation, within which emergency arrangements to protect the public are planned in detail (refer to The Radiation (Emergency Preparedness and Public Information) Regulations 2019 (REPPIR), Regulation 9 and associated guidance).

Diversity
The presence of two or more systems or components to perform an identified function, where the systems or components have different attributes so as to reduce the possibility of common cause failure, including common mode failure [1].

Dose
Refer to ‘Effective dose’.

Dutyholder
A person or corporate body who has a duty in law.

E

Effective dose
The quantity obtained by multiplying the equivalent dose to various tissues and organs by a weighting factor appropriate to each and summing the products. Total effective dose includes the effective dose from external radiation and 50-year committed effective dose from internal radiation. Measured in Sieverts (Sv). (Refer to HSE ‘Approved Code of Practice: Working with ionising radiation’ (Appendix 2) for more information [2]). ‘Effective dose’ is frequently abbreviated to ‘dose’.

Emergency exposure situation
Refer to ‘Accident’.

Emergency preparedness
The capability to take actions that will effectively mitigate the consequences of an emergency for human health and safety, quality of life, property, and the environment [1].

Employees working with ionising radiation
The term ‘employees’ is used in IRR17. Working with ionising radiation has the same interpretation as in IRR17, namely work involving the production, processing, handling, use, holding, storage, transport or disposal of radioactive substances. For the purposes of assessment, employees can be regarded as the same as workers.

Essential services
Essential services are all those resources necessary to maintain the safety measures in an operational state at all times and may include electricity, gas, water, compressed air, fuel and lubricants.

Equivalent dose
The quantity obtained by multiplying the absorbed dose by a factor to allow for the differing effectiveness of the various ionising radiations in causing harm to tissue. Measured in Sieverts (Sv). (Refer to Appendix 2 of [2] for more information).

Extended Enterprise
The totality of the supply chain to ensure the licensee or dutyholder can deliver its objectives.

F

Facility
A part of a nuclear site identified as being a separate unit for the purposes of nuclear or radiological risk. A facility may, for example, be a single reactor, a group of processing plants as on a nuclear fuel-cycle facility or a dock and its support systems containing a naval reactor plant. The term encompasses both the terms ‘nuclear installations’ as defined in the Nuclear Installations Act 1965 (NIA), and the term ‘plant’ as used in nuclear site licences.

Failure
When a structure, system or component no longer meets its safety function, or malfunctions.

Fault
Any unplanned departure from the specified mode of operation of a structure, system or component due to a malfunction or defect within the structure, system or component or due to external influences or human failures.

Fault conditions
Unplanned operation of a facility beyond normal operations arising from a fault and with the potential to lead to an accident. Fault conditions include faults with consequences that have not (or cannot) be justified within the safety case as acceptable for normal operations.

Fault sequence
A combination of an initiating fault and any additional failures, faults and internal or external hazards which have the potential to lead to an accident.

G

Government
For the purposes of this document, the term ‘the Government’ means ‘the UK Government and/or the Devolved Administrations, as appropriate’.

Geometrical constraint
In criticality safety, control of the geometrical configuration in such a way that the neutron leakage of the system is sufficient to prevent criticality.

H

Hazard
The potential for harm arising from an intrinsic property or disposition of something to cause detriment (source: HSE ‘Reducing risks, protecting people - R2P2’ [3]). Hazard is also used in the terms internal hazard and external hazard, where it refers to an initiating or consequential event that challenges the safety of a nuclear facility (such as a fire, flood, or earthquake).

Human actions impacting safety
A human action, which if completed incorrectly or omitted, could contribute to an incident and/or accident.

Human based safety claim
A human action which is an important contributor to the delivery of a safety function.

Hypothetical person
An individual who is in some fixed relation to the (radiological) hazard, for example, the person most exposed to it, or a person living at some fixed point or with some assumed pattern of life [3].

I

Incident
An undesired circumstance or ‘near miss’, for example, an initiating event or a fault condition, that has the potential to cause an accident.

Inherent safety
Preventing harm through design or arrangements that make it physically impossible, such as a criticality-safe vessel. Inherent safety exceeds passive safety by ensuring the harm cannot occur.

Initiating fault/event
The starting point of a fault sequence. This may be an internal failure, or caused by an internal or external hazard or by human failure, or a combination of these. The definition does not include pre-existing latent failures that may be revealed once safety measures are called upon to function during a fault sequence.

Intelligent customer
The capability of an organisation to understand where and when work is needed; specify what needs to be done; understand and set suitable standards; supervise and control the work; and review, evaluate and accept the work carried out on its behalf.

Ionising radiations
For the purposes of radiological protection, radiation capable of producing ion pairs in biological materials [1].

L

Licensed site
A site in respect of which a Nuclear Site Licence has been granted under the NIA (as amended), whether or not that licence remains in force.

Licensee
The body corporate that has been granted a Nuclear Site Licence under the NIA (as amended), which permits it to carry out a defined scope of activities on a delineated site.

Lifecycle
All the stages in the life of a facility from conception through to delicensing. This includes design, build, commissioning, operation, maintenance, closure, decommissioning, disposal of waste and the return of a site to a safe state.

M

Management system
A set of interrelated or interacting elements that organizations use to establish policies, objectives, and processes to achieve those goals. It forms a structured framework ensuring consistency, quality, and continual improvement in products, services, and operations.

N

Normal operation(s)
The intended state of a facility or activity within specified operational limits and conditions. Normal operations include all the operating modes permitted at the facility, for example, start-up and shutdown states and temporary situations arising due to maintenance and testing. Normal operations also include minor deviations from desired operating conditions including anticipated operational occurrences justified within the safety case.

Nuclear matter
Nuclear matter is defined in the NIA section 26 and in Licence Condition (LC) 1. In the context of these SAPs, the term nuclear matter is not directly aligned to this legal definition and instead is taken more generally to include any substance on a nuclear licensed site which is radioactive, as it is the inherent radioactive nature of such substances which determine the principles for their safe management. For the SAPs, nuclear matter is considered to include fissile material (uranium or plutonium metal, alloy or chemical compound), nuclear fuel (including spent fuel), radioactive material and radioactive waste, unless stated otherwise.

O

Operating modes
The states that the facility may be in during the course of normal operations. Refer also to ‘Normal operations’.

Operating rule
Any condition or limit in place at a nuclear facility through which a dutyholder demonstrates compliance with its safety case. An operating rule is any limit or condition necessary in the interests of safety defined in the safety case, and not just those that the dutyholder terms ‘operating rules’ – refer to LC23.

P

Passive safety
Safety measures can be passive, active or manual, to different degrees. Moving towards the passive end of this scale means reducing or removing the dependence on human intervention, external power supplies and moving parts (refer to SAP KP.7). Refer also to ‘Inherent safety’: a passive safety system is not necessarily inherently safe.

Procedure
A series of specified actions conducted in a certain order or manner [1].

Protection system
A system that monitors the operation of a facility and, on sensing an abnormal condition, automatically initiates actions to prevent an unsafe or potentially unsafe condition (description based on [1]).

Protective actions
An action for the purposes of avoiding or reducing doses that might otherwise be received in an emergency exposure situation or an existing exposure situation (source: IAEA GSR Parts 3 and 7 [4, 5]).

Q

Qualification
The process of demonstrating that a structure, system or component is fit for its intended purpose. The generation and maintenance of evidence to ensure that equipment will operate on demand, under specified service conditions, to meet system performance requirements.

Quality management
Co-ordinated activities to direct and control an organisation with regard to quality (that is, ensuring that requirements are fulfilled). Direction and control with regard to quality generally includes quality policy, quality objectives, quality planning, quality control, quality assurance and quality improvement. LC17 requires adequate quality management arrangements in respect of all matters which may affect safety. Such matters include those derived from the safety case, facility design and LCs.

R

Radioactively contaminated land
Land containing radioactive contamination at levels that would preclude its delicensing.

Radioactive material
Radioactive material is defined in LC1 and is also used as an undefined term in NIA (as is the term “radioactive matter”). In the context of these SAPs the meaning of the term radioactive material is not directly aligned to the definition in LC1 or the undefined terms in NIA, and instead is taken more generally to mean any nuclear matter that is not considered to be waste.

Radioactive waste
Radioactive waste is defined in NIA Sections 3 and 4 and in LC1. In the context of these SAPs the meaning of the term radioactive waste is not directly aligned to these definitions and instead is taken more generally to mean any nuclear matter that is considered to be waste.

Redundancy
Provision of alternative (identical or diverse) structures, systems or components, so that anyone can perform the required function regardless of the state of operation or failure of any other [1].

Reliability
The likelihood a system or component meets its performance requirements when needed.

Remediation
Remediation means any measures that may be carried out to reduce risks from existing contamination. Typically this term refers to measures carried out to reduce the radiation exposure from radioactively contaminated land, but it can also refer to reducing risks from other radiological and non-radiological contamination such as asbestos or chemicals.

Risk
Risk is the chance that someone or something is adversely affected in a particular manner by a hazard [3].

S

Safety
In this document, ‘safety’ refers to the safety of persons in relation to radiological hazards.

Safety actuation system
The collection of equipment required to accomplish the necessary safety actions when initiated by the protection system [1].

Safety case
A safety case is a structured argument, supported by evidence, to justify safety during each lifecycle stage of the installation. It should demonstrate the nuclear safety of the relevant facilities and activities within its scope.

Safety culture
The assembly of characteristics and attitudes in organisations and individuals which establishes that, as an overriding priority, protection and safety issues receive the attention warranted by their significance [1].

Safety function
A specific purpose that should be accomplished for safety [1].

Safety measure
Anything that delivers, or contributes to the delivery of, a safety function. This covers all levels of defence in depth, and may include, for example, design features, SSCs (inherently safe, passive or active), and human actions.

Safety-related system
An item important to safety that is not part of a safety system [1]. Safety-related systems are therefore systems in place to perform an operational function, but which also provide a safety benefit. This is distinct from safety systems, which are systems which do not perform any operational functions and are included solely because of the safety functions they perform.

Safety system
A system that acts in response to a fault to protect against a radiological consequence.

Segregation
Depending on the context: 1. physical separation of structures, systems, or components by distance or barrier to reduce the risk of common cause failures; or 2. separating waste or materials by radiological, chemical, or physical properties to aid handling or processing.

Service
An organisation or utility supporting the safety of one or more facilities. Examples include health physics or emergency services, or utilities such as steam, electricity, water, nitrogen or compressed air required for safety.

Severe accident
An accident with off-site consequences with the potential to exceed 100 mSv, or to a substantial unintended relocation of radioactive material within the facility that places a demand on the integrity of the remaining physical barriers.

Shielding
A structure or material placed around a source of radiation to reduce the radiation dose rate in the vicinity.

Sievert
The unit of equivalent dose and its derivatives; for example, effective dose and committed effective dose.

Societal effects
Quantifiable aspects of an accident with widespread adverse repercussions regionally or nationally, for example, numbers of deaths or injuries, numbers of people evacuated, area of land contaminated or general economic loss.

Societal risk
The risk of an accident with societal effects causing the deaths of a specified number of people in a single event from a single major industrial activity, i.e. an activity from which risk is assessed as a whole and is under the control of one company in one location, or within a site boundary.

Source term
The types, quantities, and physical and chemical forms of the radionuclides present in a nuclear facility that have the potential to give rise to exposure to radiation, radioactive waste or discharges, during all operational states and in abnormal or accident conditions. Typically, more than one source term needs to be defined to cover all these different situations. Source term(s) are used to understand and control the hazards posed by a nuclear facility and inform radiological consequence calculations.

Stable, safe state
The state of the facility once stabilisation of any transient or fault has been achieved; that is, the facility is subcritical, adequate heat removal is ensured and continuing radioactive releases are limited (description based on international standard, IEC61226 [6]). A transient or fault is considered to be stabilised when for all safety-significant parameters, the margins (for example, between heat removal capacity and heat generation) are either stable or increasing, or sufficient margin remains to cover all expected physical processes, and there is high confidence that this stability will be maintained.

Structure, system and/or component
An item important to safety within the facility design which provides a safety function.

T

Task analysis
An assessment of how a task is to be accomplished, including a suitably detailed decomposition of manual and mental activities and identification/analysis of task-specific factors which could impact human performance.

U

Unmitigated consequences
The potential radiological consequences of a fault or accident evaluated assuming all safety measures are absent or fail to operate. This excludes passive safety features such as walls or pipes, unless the fault or accident affects that feature.

V

Validation
The process of confirming, for example, by use of objective evidence, that the outputs from an activity will meet the objectives and requirements set for that activity.

Verification
The process of confirming, for example, by use of objective evidence, that an activity was carried out as intended, specified or stated.

References

1. IAEA, “IAEA Nuclear Safety and Security Glossary,” 2022 (interim edition). [Online]
2. HSE, “ACoP - L121: Working with ionising radiation. Ionising Radiations Regulations 2017,” March 2018 (second edition). [Online]
3. HSE, “Reducing risks, protecting people (R2P2): HSE’s decision-making process,” 2001. [Online]
4. IAEA, “Safety Standards No. GSR Part 3 - Radiation Protection and Safety of Radiation Sources: International Basic Safety Standards,” 2014. [Online]
5. IAEA, “Safety Standard No. GSR Part 7 - Preparedness and Response for a Nuclear or Radiological Emergency,” 2015. [Online]
6. IEC, “IEC 61226:2020 - Nuclear power plants - Instrumentation, control and electrical power systems important to safety - Categorization of functions and classification of systems,” 2020. [Online].