5.1 Glossary

Where terms are not defined in this glossary, we are using them with their normal dictionary meaning.

Accessible environment

Those parts of the environment in contact with or readily available for use by humans and non-human organisms.

Closure

Administrative and technical actions directed at a disposal facility at the end of its operating lifetime — for example, covering of the disposed waste or backfilling and/or sealing shafts and tunnels— and the termination and completion of activities in any associated structures.

Conceptual model

A set of qualitative assumptions used to describe a system, or part of a system, in the real world.

Containment

Methods or physical structures designed to prevent or control the release and the dispersion of radioactive substances. The containment of the radionuclides associated with the waste is through the provision of engineered barriers and natural barriers - including the waste form and packaging, backfill materials, host geology and the wider geological environment.

Criticality

The state of a nuclear chain reacting medium when the chain reaction is just self-sustaining (or critical), i.e. when the reactivity is zero.

Defence in depth

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.

Deterministic effect

A radiation induced health effect for which generally a threshold level of dose exists above which the severity of the effect is greater for a higher dose.

Disposal facility

An engineered facility where waste is emplaced for disposal. Synonymous with repository.

Disposal system

The system of properties of the site for a disposal facility, design of the disposal facility, physical structures and items, procedures for control, characteristics of waste and other elements that contribute in different ways and over different timescales to the fulfilment of safety functions for disposal.

Dose constraint

A prospective and source related value of individual dose that is used in planned exposure situations as a parameter for the optimization of protection and safety for the source, and that serves as a boundary in defining the range of options in optimization.

Dose guidance range

The dose standard against which the radiological consequences of the following are assessed:

• inadvertent human intrusion into the radioactive waste itself after the period of RSR.
• certain scenarios resulting from natural disruptive processes and events after the period of RSR that uncover radioactive waste and could lead to direct encounter with the waste.

It indicates the standard of environmental safety expected but does not suggest that there is an absolute requirement for this level to be met.

Effective dose

Effective dose is a measure of dose designed to reflect the amount of radiation detriment likely to result from the dose.

Emplace or emplacement

The placement of a waste package in a designated location for disposal, with no intent to reposition or retrieve it subsequently.

Environmental safety

The safety of people and the environment both during the period of RSR and afterwards into the indefinite future.

Environmental safety case

A documented set of claims, made by the developer or operator of a disposal facility, to demonstrate achievement of the required standard of environmental safety.

Environmental safety culture

The characteristics and attitudes of organisations and individuals that ensure that the protection of people and the environment receives proper attention.

Environmental safety function

The various ways in which components of the disposal system may contribute towards environmental safety, such as the geology providing a physical barrier function and also having chemical properties that help to retard the migration of radionuclides.

Environmental safety strategy

An approach or course of action designed to achieve and demonstrate Environmental safety.

Equivalent dose

Equivalent dose is a measure of the dose to a tissue or organ designed to reflect the amount of harm caused.

Expected evolution scenario

A scenario that describes how the disposal facility and its environment are expected to develop over time. The expected evolution scenario may be considered to be almost certain or highly likely to occur. This can be contrasted with relatively less likely variants scenarios and highly unlikely ‘what-if’ scenarios.

Exposure (potential)

Prospectively considered exposure that is not expected to be delivered with certainty but that may result from an anticipated operational occurrence or accident at a source or owing to an event or sequence of events of a probabilistic nature, including equipment failures and operating errors.

Exposure scenarios

A postulated or assumed set of conditions and/or events that could result in exposure to ionising radiation.

Fissile content

The component of waste that contains any of the fissile nuclides in sufficient proportion to enable a self-sustained nuclear chain reaction with slow (thermal) neutrons.

Groundwater quality

The chemical, physical, biological, thermal and radiological quality of groundwater within a subsurface aquifer.

Host geology

The geological medium in which a disposal facility is located.

Inadvertent human intrusion

Any human action that unintentionally affects the integrity of a disposal facility and which could potentially give rise to radiological consequences after the release from RSR. Only those human activities (such as construction work, mining or drilling) that could result in direct disturbance of the disposal facility (i.e. disturbance of the waste itself, of the contaminated near field or of materials of the engineered barrier) are included.

Interesting or noticeable objects

A distinct item of waste that, by its characteristics, is recognisable as unusual or not of natural origin and as a result could change the behaviour of those who encounter it (specifically, by attracting people leading to collecting / hoarding and/or further intrusion into the waste and hence increasing exposures). It could be a focus of interest out of general curiosity or because of its potential for collectability, sale, recovery, recycling or re-use should the waste item be exposed after site closure.

Isolation

The physical separation of radioactive waste away from people and from the accessible environment. Isolation of radioactive waste with its associated hazards in a disposal facility involves the minimisation of the influence of factors that could reduce the integrity of the disposal facility and making access to the waste by people without special technical capabilities difficult.

Management system

A set of interrelated or interacting elements (system) for establishing policies and objectives and enabling the objectives to be achieved in an efficient and effective manner.

Multiple barriers

Two or more natural or engineered barriers used to isolate radioactive waste in, and to prevent or to inhibit migration of radionuclides from, a disposal facility.

Natural analogues

A situation in nature used as a model for processes affecting human made systems.

Nuclear licenced site

(a) Any site in respect of which a nuclear site licence is for the time being in force, or

(b) Any site in respect of which, after the revocation or surrender of a nuclear site licence, the period of responsibility of the licensee has not yet come to an end, and “licensee”, when used in relation to a nuclear site, and “period of responsibility” have the same meaning as in the Nuclear Installations Act 1965.

Passively safe

Not placing reliance on safety systems whose functioning depends on an external input, such as a power supply, or human intervention to ensure safety.

Radiation stability

The ability of a material to withstand radiation damage.

Radiological risk

The probability per unit time that an individual will suffer a serious radiation-induced health effect as a result of the presence of a radiation source, for example, a disposal facility. In this context, a serious radiation-induced health effect is a fatal cancer or a severe hereditary defect.

Radionuclide
Any isotope of any element that is radioactive. Some occur naturally in the environment, while others are man-made, either deliberately or as byproducts of nuclear reactions.

Representative person
For the purpose of protection of the public, a hypothetical individual who receives a dose that is representative of the more highly exposed individuals in the population.

Risk coefficient
The lifetime risk or radiation detriment assumed to result from exposure to unit equivalent dose or effective dose

Risk guidance level
A level of radiological risk from a nuclear site which provides a numerical standard for assessing the environmental safety of the site after the release from RSR

Scenario
A postulated or assumed set of conditions and/or events.

Stochastic effects
A radiation induced health effect, the probability of occurrence of which is greater for a higher radiation dose and the severity of which (if it occurs) is independent of dose.

Structures, systems and components
A general term encompassing all of the elements (items) of a facility or activity that contribute to protection and safety, except human factors

Structural integrity
The ability of an engineered structure to function safely and reliability throughout its life

Thermal stability
The ability of a material to withstand damage caused by heat or changes in temperature.

Variant scenarios
Scenarios that describe alternative possibilities for how the disposal facility and its environment may evolve. These can be contrasted with expected evolution and what-if scenarios

Waste acceptance criteria
Quantitative and/or qualitative criteria for solid radioactive waste to be accepted for disposal. These are specified by the operator of a disposal facility and should include any criteria specified in the operator’s permit.

Waste form
Waste in its physical and chemical form after treatment and/or conditioning (resulting in a solid product) but not including the packaging.

Waste consignment
Any waste sent by a consignor to a disposal facility

Waste package
The product of conditioning that includes the waste form and any container(s) and internal barriers (e.g. absorbing materials and liner), as prepared in accordance with requirements for handling, transport, storage and/or disposal.

‘What-if’ scenarios
Scenarios used to explore the consequences of highly unlikely assumptions (e.g. events). What-if scenarios can be used to inform optimisation.

5.2 References

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• International Organization for Standardization (ISO), 2015. Quality management systems ISO 9001:2015
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• Scottish Environment Protection Agency, 2019. Satisfying the optimisation requirement and the role of Best Practicable Means. May 2019 version 2.0. RS-POL-001. Available from https://www.sepa.org.uk/media/101545/satisfying_the_alara_requirement_and_the_role_of_best_practicable_mean.pdf. [Accessed on 14 August 2024] 

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