- 5 - 2.4 ASN basic safety rules and guides On a variety of technical subjects concerning nuclear facilities, ASN has in the past drawn up basic safety rules (RFS). These are recommendations which clarify the safety objectives and describe practices that ASN considers to be satisfactory. As part of the current overhaul of the general technical regulations, the RFS are being gradually replaced by "ASN guides". There are at present about forty RFS and other technical rules from ASN, which can be consulted on its website. 3. The nuclear safety approach in France The nuclear safety approach in France is based on: the prime responsibility of the licensee for the safety of its facilities, under the oversight of ASN ; continuous improvement of nuclear safety and radiation protection. The safety principles and approaches presented below were implemented gradually. They included experience feedback from accidents. Safety can never be totally obtained and, despite the precautions taken in the design, construction and operation of nuclear facilities, an accident always remains possible. There must thus be a constant desire to move forwards and to implement a continuous improvement approach in order to reduce the risks. To ensure the safety of nuclear facilities, the French regulations require that they be designed, built and operated to deal with a certain level of risk. These risks in particular comprise natural hazards, such as earthquake and flooding. The regulations also require the implementation of a "defence in depth" arrangement, which consists of a set of redundant, diversified measures (automation, systems or procedures) able to prevent accidents, manage them if they are not preventable or, failing which, mitigate the consequences. These arrangements are regularly checks and systematically reviewed on the occasion of the ten-yearly periodic safety reviews created by article 29 of the Act of 13th June 2006. 3.1 The "defence in depth" concept The main means of preventing and mitigating the consequences of accidents is "defence in depth". This involves a series of consecutive, independent levels of protection. If one level of protection, or barrier, were to fail, the next level would take over. An important aspect in the independence of the levels of defence is the use of technologies of different natures ("diversified" systems). The design of a nuclear facility is based on a defence in depth approach. For example, for nuclear reactors, there are the following five levels: First level: prevention of abnormal operation and system failures This entails choosing a robust and prudent design for the facility, incorporating safety margins, able to withstand its own failures or off-site hazards. This implies conducting a study of the normal operating conditions that is as complete as possible, to determine the most severe constraints to which the systems will be subjected. An initial design of the facility incorporating safety margins can then be established. Second level: Control of abnormal operation and detection of failures This entails designing control and limitation systems which maintain the facility well within its safety limits. For example, if the temperature of a system rises, a cooling system is activated before the temperature exceeds the authorised limit. Monitoring the good condition of the equipment and the correct operation of the systems is part of this level of defence.
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