3.12.The design stage may include other tasks, such as a ‘feasibility study’, or a ‘pre-licensing’ step, depending on the national nuclear context (e.g. whether the State already has nuclear installations of the same type).
3.13.If sites and designs are considered separately early in the project to build a nuclear installation, then the regulatory body should establish a definition of ‘generic site’ and a definition of ‘generic design’. A process to ensure that both the site and the design are compatible in the licensing process should also be established. The site evaluation and the environmental impact assessment should be reviewed and, if necessary, enhanced after the bidding process through which the design is selected.
3.14.The regulatory body should review and assess the acceptability of the selected design and should have the authority to approve, agree, comment on, question or reject such designs or parts thereof, as necessary, on the basis of safety concerns.
3.15.The basic design of the proposed nuclear installation should be such that safety requirements can be met in accordance with the design basis. The design basis is the range of conditions and events explicitly taken into account in the design of the nuclear installation, according to established criteria, such that the nuclear installation, through the planned operation of safety systems, can withstand them without exceeding authorized limits [2]. The applicant for authorization for construction should submit a basic design to the regulatory body before construction begins. This basic design can be approved or, depending on regulatory framework, frozen (i.e. no change may be made to the basic design without the regulatory body’s review and approval) or partly frozen with a regulatory instrument upon the review and assessment of the regulatory body.
3.16.During construction and throughout the lifetime of the nuclear installation, parts of the detailed design may be subject to approval or may be frozen. Such approvals or processes for freezing a detailed design should be carried out by means of regulatory instruments, and conditions should be attached, as appropriate. If the licence applications for construction and operation are made concurrently (i.e. a combined licence), parts of the detailed design should then be reviewed by the regulatory body in the course of application for the construction and operation licence.
3.17.Particularly at the design and construction stages, it is important to ensure that structures, systems and components comply with approved or accepted standards, codes and regulatory requirements, including quality assurance requirements. It is also necessary to ensure that construction work at the nuclear installation is undertaken in accordance with design specifications and that sufficient suitably qualified and experienced staff are available for design work, supply and manufacture, and for the control of these activities. The regulatory body should ensure that clear and explicit quality requirements are specified by the licensee or applicant for safety related activities. The regulatory body should check, either through the licensee or directly, depending on national legislation, whether all organizations and contractors involved in design and construction adequately implement these requirements, and should take appropriate actions if necessary.
3.18.Defence in depth is required to be considered in the design and later on, in operation:
“Defence in depth is implemented primarily through the combination of a number of consecutive and independent levels of protection that would have to fail before harmful effects could be caused to people or to the environment. If one level of protection or barrier were to fail, the subsequent level or barrier would be available. When properly implemented, defence in depth ensures that no single technical, human or organizational failure could lead to harmful effects, and that the combinations of failures that could give rise to significant harmful effects are of very low probability. The independent effectiveness of the different levels of defence is a necessary element of defence in depth” [3].
3.19.The objectives of defence in depth are:
“[T]o compensate for potential human and component failures;
[T]o maintain the effectiveness of the barriers by averting damage to the [facility] and to the barriers themselves; and
[T]o protect [workers,] the public and the environment from harm in the event that these barriers are not fully effective.” [9]
3.20.The International Nuclear Safety Advisory Group [9] 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 conditions, including prevention of accident progression and mitigation of the consequences of a severe accident;
Level 5: Mitigation of the radiological consequences of significant external releases of radioactive material.
More details about defence in depth are provided in Ref. [8].
3.21.The following should be verified by the licensee to ensure that safety requirements are met:
Design basis analyses and beyond design basis analyses, fault tree analyses, and probabilistic safety assessments, as appropriate;
Protection against external and internal hazards;
Radiation protection;
Routine radioactive discharges;
Evidence of learning from operating experience and programmes to evaluate human and organizational factors, the management system, etc.;
The main safety functions (i.e. reactivity control or criticality issues, cooling aspects and containment integrity) and the reliability of the associated structures, systems and components.
The results of these verifications should be reviewed and assessed by the regulatory body, as appropriate.
3.22.The approach applied to safety and the safety standards and codes used in the design of the nuclear installation should be proven and appropriate for their purpose.
3.23.Safety analyses of the design should be performed or reviewed by the applicant or the licensee in accordance with its management system and should be used by the applicant or the licensee to specify or improve, as appropriate, the following:
Requirements for commissioning;
Categorization and classification of structures, systems and components (according to safety, quality, seismic qualification, environmental qualification, etc.);
Operational limits and conditions, safety limits, rules and operating procedures;
Requirements for in-service inspection and maintenance;
Radiation protection requirements (for workers, the public and the environment);
Emergency preparedness arrangements;
Physical protection and security requirements, in accordance with national and local regulations;
Human and organizational factors;
Training requirements;
Documented verification and validation activities in design, testing, construction, commissioning, operation, maintenance and ageing management activities to ensure that the qualification of systems, structures and components is valid for life;
The programme for feedback of operating experience;
Procedures and requirements for management of modifications.
3.24.These safety analyses may be reviewed, assessed and, if appropriate, challenged by the regulatory body at an early stage in the licensing process. The vendor can also be involved in this pre-licensing step, if appropriate. Additionally, the operating organization may have an internal process (which could include receipt of independent advice) for review of safety analyses before submission to the regulatory body to ensure that such analyses are appropriate.
3.25.The regulatory body should review, assess and inspect the information on the nuclear installation provided by the applicant or licensee, in particular, documents that form part of the preliminary safety analysis report, including:
Safety analyses of anticipated operational occurrences and postulated initiating events [8], which may be caused by:
External hazards (e.g. tsunamis, flooding, seismic events, volcanic eruptions, aircraft crashes, tornadoes, cyclones, hurricanes, external fires, explosions of gases or liquids);
Internal hazards (e.g. fire, spillages of corrosive material, internal flooding);
Internal events (e.g. mechanical failures, electrical failures, human error);
The assumptions and approximations used in the analyses;
Analyses of combinations of events;
A description, identification, categorization and classification of structures, systems and components important to safety;
Operational limits and conditions, and permitted operational states;
A list of barriers with their relative contributions to confinement of radioactive material and related limits;
The means by which requirements for defence in depth are met;
Planned activities for confirming safety performance;
Analytical methods and computer codes used in the safety analyses and the verification and validation of such codes;
Radioactive discharges and radioactive releases into the environment, and radiation exposure of workers and the public during normal operation and under accident conditions;
Safety criteria for analyses, particularly those relating to common cause events, cross-link effects33 , the single failure criterion, redundancy, diversity and physical separation;
Verification and validation of the safety analyses and evidence of their robustness (e.g. sensitivity studies, research, testing, operating experience in other nuclear installations).
3.26.This demonstration of safety should be carried out in compliance with the regulatory framework, including safety criteria and the applicable international standards, as appropriate, and in accordance with national requirements for the nuclear installation in question. Further details are provided in Refs [8, 10–12].
3.27.The regulatory body should ensure that the applicant has verified the adequacy of design parameters and site specific data in relation to safety criteria of the specified design basis (e.g. hazard protection requirements, cooling requirements).
3.28.The licensee or applicant should develop and implement an appropriate management system to review the detailed design of systems, structures and components important to safety manufactured by designers, vendors and manufacturers. The regulatory body may review, assess and inspect, as appropriate, the activities carried out by the licensee in that respect.
3.29.The licensee should propose arrangements for radioactive waste management. The regulatory body should review, assess and inspect proposals for on-site treatment and storage of radioactive waste, including the management of spent fuel, where appropriate, to ensure that the processed waste and the waste packages will be characterized in a manner compatible with the national strategy for radioactive waste, the applicable waste acceptance requirements for subsequent steps of waste management and regulatory requirements. Specifically, the regulatory body should satisfy itself that the waste or waste packages:
Will be properly characterized and compatible with the anticipated nature and duration of storage pending disposal;
Can be subjected to regular surveillance;
Can be retrieved for further steps of predisposal waste management;
Will be managed such that their volume and activity are kept as low as reasonably achievable.
3.30.The licensee should propose arrangements for managing radioactive discharges (liquid and gaseous and other discharges, including chemical and thermal discharges, as appropriate), which will have to be implemented throughout the lifetime of the nuclear installation. The regulatory body should review, assess and inspect these proposals. Specifically, the regulatory body should satisfy itself that radioactive discharges:
Will be properly characterized and in compliance with national regulations;
Can be subjected to regular surveillance;
Will be kept as low as reasonably achievable.
3.31.In addition, the licensing process should ensure that the following aspects are considered in the design:
The ability to safely transport radioactive and nuclear material to, from and within the nuclear installation.
Safety aspects associated with the replacement of heavy and large components during the operating lifetime of the nuclear installation (e.g. steam generator, reactor pressure vessel heads for a nuclear power plant). The design should take into account:
Buried pipes and conduits;
Openings in structures for access to equipment;
Obstructions.
Access to components important to safety for:
Maintenance;
Inspection and testing, as appropriate;
Replacement;
Future decommissioning.
Minimization of occupational exposure when gaining access to structures, systems and components.
The way the nuclear installation will be decommissioned, and how radioactive waste generated during operation and decommissioning will be managed, according to national strategies.
Minimization of the amount of radioactive waste.
Features dealing with safe shutdown and remote shutdown facility, where appropriate.
In the case of reactors, appropriate arrangements for temporary storage of the spent fuel (including, e.g., criteria for dry storage of spent fuel at reactor sites).
The safe management of the radioactive waste and spent fuel generated throughout the lifetime of the installation.
3.32.Ageing issues should also be addressed in the design and construction phases to anticipate appropriate ageing management actions for the future. Furthermore, ageing considerations during design should also include the actions for ensuring plant integrity until the end of decommissioning.
See also:
SSG-48 Ageing Management and Development of a Programme for Long Term Operation of Nuclear Power Plants
3.33.The licensee should review, audit and be responsible for certifying suppliers and contractors with functions relating to safety. As appropriate, the regulatory body may review, assess and inspect such review, audit and certification processes. The regulatory body may also directly grant certificates or licences to suppliers and contractors in its own State, as appropriate, in accordance with the national regulatory framework.
3.34.Before construction begins, the licensee should set up a configuration management programme44 for updating the design basis of the nuclear installation while ensuring that it remains in compliance with the original agreed or approved design basis.
3.35.More requirements and recommendations on design of nuclear power plants and the safety analysis report can be found in Refs [8, 10].