1.1.8 The prevention principle To anticipate any environmental damage, the prevention principle, defined in Article 3 of the Environment Charter, stipulates the implementation of rules and measures, which must take account of “the best available technology at an economically acceptable cost”. In the nuclear field, this principle underpins the concept of “Defence in Depth”, presented below. 1.2 Some aspects of the safety approach The safety principles and approaches presented below were gradually implemented and incorporate the lessons learned from accidents. Absolute safety can never be guaranteed. Despite all the precautions taken in the design, construction and operation of nuclear facilities, an accident can never be completely ruled out. Willingness to move forward and to create a continuous improvement approach is thus essential if the risks are to be reduced. 1.2.1 Safety culture Safety culture is defined by the International Nuclear Safety Advisory Group (INSAG), an international consultative group for nuclear safety reporting to the Director General of the IAEA, as that complete range of characteristics and attitudes in organisations and individuals which establishes that, as an overriding priority, nuclear plant safety issues receive the attention warranted by their significance. Safety culture therefore determines the ways in which an organisation and individuals perform their duties and assume their responsibilities with respect to safety. It is one of the key fundamentals in maintaining and improving safety. It commits organisations and individuals to paying particular and appropriate attention to safety. At the individual level it must be given expression by a rigorous and cautious approach and a questioning attitude making it possible to both obey rules and take initiatives. In operational terms, the concept underpins daily decisions and actions relating to activities. 1.2.2 The “Defence in Depth” concept The concept of “Defence in Depth” consists in implementing a series of levels of defence based on the intrinsic characteristics of the installation, material, organisational and human measures and procedures designed to prevent accidents and then, if this fails, to mitigate their consequences. “Defence in Depth” is a concept which applies to all stages in the lifetime of a facility, from design to decommissioning. These levels of defence are consecutive and independant, in order to prevent an accident from developing. An important element for the independence of the levels of defence is the use of different technologies (“diversified” systems). The design of nuclear installations is based on a “Defence in Depth” approach. For example, the following five levels are defined for nuclear reactors: Level 1: Prevention of abnormal operation and system failures This is a question firstly of designing and building the facility in a robust and conservative manner, integrating safety margins and planning for resistance with respect to its own failures or to hazards. It implies conducting the most exhaustive study possible of normal operating conditions to determine the severest stresses to which the systems will be subjected. It is then possible to produce an initial design basis for the facility, incorporating safety margins. The facility must then be maintained in a state at least equivalent to that planned for in its design through appropriate maintenance. The facility must be operated in an informed and careful manner. Level 2: Keeping the installation within authorised limits Regulation and governing systems must be designed, installed and operated such that the installation is kept within an operating range that is far below the safety limits. For example, if the temperature in a system increases, a cooling system starts up before the temperature reaches the authorised limit. Condition monitoring and correct operation of systems form part of this level of defence. Level 3: Control of accidents without core melt The aim here is to postulate that certain accidents, chosen for their “envelope” characteristics (the most penalising in a given family), can happen and to design and size backup systems to withstand those conditions. Such accidents are generally studied with pessimistic hypotheses, that is to say the various parameters governing this accident are assumed to be as unfavourable as possible. In addition, the single failure criterion is applied, in other words we postulate that in the accident situation and in addition to the accident, there will be the most prejudicial failure of one of the components used to manage this situation. As a result of this, the systems brought into play in the event of an accident (“safeguard” systems ensuring emergency shutdown, injection of cooling water into the reactor, etc.) comprise at least two redundant and independent channels. THE FUNDAMENTAL SAFETY PRINCIPLES The International Atomic Energy Agency (IAEA) defines the following ten principles in its “Fundamental principles of safety” publication, IAEA Safety Standards Series – No. SF-1: 1. Responsibility for safety must rest with the person or organisation responsible for facilities and activities that give rise to radiation risks. 2. An effective legal and governmental framework for safety, including an independent regulatory body, must be established and sustained. 3. Effective leadership and management of safety must be established and maintained in organisations concerned with radiological risks, and in facilities and activities that give rise to such risks. 4. Facilities and activities that give rise to radiation risks must yield an overall benefit. 5. Protection must be optimised to provide the highest level of safety that can reasonably be achieved. 6. Measures for controlling radiation risks must ensure that no individual bears an unacceptable risk of harm. 7. People and the environment, both present and future, must be protected against radiation risks. 8. All practical efforts must be made to prevent and mitigate nuclear or radiation accidents. 9. Arrangements must be made for emergency preparedness and response for nuclear or radiation incidents. 10.Protective actions to reduce existing or unregulated radiological risks must be justified and optimised. 122 ASN Report on the state of nuclear safety and radiation protection in France in 2021 02 – THE PRINCIPLES OF NUCLEAR SAFETY AND RADIATION PROTECTION AND THE REGULATION AND OVERSIGHT STAKEHOLDERS
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