Level 5 : Mitigation of the radiological consequences of significant releases This requires implementation of the measures provided for in the emergency plans, including measures to protect the general public: shelter, taking of stable iodine tablets to saturate the thyroid and avoid fixation of released radioactive iodine, evacuation, restrictions on consumption of water and of agricultural products, etc. 1.2.3 Positioning of barriers To limit the risk of releases, several barriers are placed between the radioactive substances and the environment. These barriers must be designed to have a high degree of reliability and must be monitored to detect any weaknesses before a failure. There are three such barriers for Pressurised Water Reactors (PWRs): the fuel cladding, the boundary of the reactor primary system, and the containment (see chapter 10). 1.2.4 Deterministic and probabilistic approaches Postulating the occurrence of certain accidents and verifying that, thanks to the planned functioning of the equipment, the consequences of these accidents will remain limited, is known as a “deterministic” approach. This approach is simple to apply in principle and allows an installation to be designed (and its systems to be sized) with good safety margins, by using so-called “envelope” cases. The deterministic approach is however unable to identify the most probable scenarios because it focuses attention on accidents studied with pessimistic hypotheses. The deterministic approach therefore needs to be supplemented by an approach that better reflects possible accident scenarios in terms of their probability, that is to say the probabilistic approach used in the “Probabilistic Safety Assessments” (PSAs). Thus for NPPs, the level 1 PSAs consist in establishing event trees for each “initiating event” leading to the activation of a safeguard system (level 3 of “Defence in Depth”), defined by the failures – or the success – of the actions provided for in the reactor management procedures and the failures – or correct operation – of the reactor’s equipment. The probability of each sequence is then calculated based on statistics on the reliability of systems and on the rate of success of actions (including data on “human reliability”). Similar sequences that correspond to the same initiating event are grouped into families, making it possible to determine the contribution of each family to the probability of reactor core melt. Although the PSAs are limited by uncertainties concerning the reliability data and approximations in the modelling of the facility, they consider a broader set of accidents than the deterministic assessments and enable the design resulting from the deterministic approach to be verified and supplemented if necessary. They are therefore to be used as a complement to deterministic studies and not as a substitute for them. The deterministic studies and probabilistic assessments constitute an essential element in the nuclear safety case that addresses equipment internal faults, internal and external hazards, and plausible combinations of these events. To be more precise, the internal faults correspond to malfunctions, failures or damage to facility equipment, including as a result of inappropriate human action. Internal or external hazards correspond to events originating inside or outside the facility respectively and which can call into question the safety of the facility. Internal faults for example include: ∙loss of the electrical power supplies or the cooling systems; ∙ejection of a rod cluster control assembly; ∙breaking of a pipe in the primary or secondary system of a nuclear reactor; ∙reactor emergency shutdown failure. With regard to internal hazards, the following in particular must be considered: ∙flying projectiles, notably those resulting from the failure of rotating equipment; ∙pressure equipment failures; ∙collisions and falling loads; ∙explosions; ∙fires; ∙hazardous substance emissions; ∙floods originating within the perimeter of the facility; ∙electromagnetic interference; ∙malicious acts. Finally, external hazards more specifically comprise: ∙the risks induced by industrial activities and communication routes, including explosions, hazardous substance emissions and airplane crashes; ∙earthquakes; ∙lightning and electromagnetic interfer- ence; ∙extreme meteorological or climatic conditions; ∙fires; ∙floods originating outside the perimeter of the facility; ∙malicious acts. 1.2.5 Operating Experience Feedback Operating Experience Feedback (OEF), which contributes to “Defence in Depth”, is one of the essential safety management tools. It is based on an organised and systematic collection and analysis of the signals emitted by a system. It should enable acquired experience to be shared so that the organisation can learn (that is through the implementation of preventive measures in a structure that learns from past experience). The first goal of OEF is to understand, and thus ensure progress in technological knowledge and knowledge of actual operating practices, so that whenever pertinent, a fresh look can be taken at the design (technical and documentary). As OEF is a collective process, the second goal is to share the resulting knowledge on the basis of the date of detection and recording of the anomaly, the lessons learned from it and how it was rectified. The third goal of OEF is to act on working organisations and processes, on working practices (both individual and collective) and on the performance of the technical system. OEF therefore encompasses events, incidents and accidents occurring both in France and abroad, whenever their assessment is relevant to enhancing nuclear safety or radiation protection. 1.2.6 Social, human and organisational factors The importance of Social, Human and Organisational Factors (SHOF) for nuclear safety, radiation protection and environmental protection The contribution of humans and organisations to safety, radiation protection and environmental protection is decisive in the design, construction, commissioning, operation and decommissioning of facilities, as well as in the transport of radioactive substances. Similarly, the way in which people and organisations manage deviations from the regulations, from the baseline requirements and from the state of the art, plus the corresponding lessons learned, is also decisive. Therefore, all those involved, regardless of their position in the hierarchy and their functions, make a contribution to safety, radiation protection and environmental protection, owing to their ability to adapt, to detect and correct errors, to rectify degraded situations and to counter certain difficulties involved in the application of procedures. Social, Human and Organisational Factors (SHOF) are defined as being all the aspects of working situations and the organisation which have an influence on the work done by the operators. The elements considered concern the individual (training received, fatigue or stress, etc.) and the organisation within which they work (functional and hierarchical links, joint contractor work, etc.), the technical arrangements (tools, software, etc.) and, more broadly, the working environment with which the individual interacts. The working environment for instance concerns the heat, sound or light environment of the workstation, as well as the accessibility of the premises. The variability in worker characteristics (vigilance varies with the time of day, the level of expertise varies according to the seniority in the position) and in the situations encountered (unexpected failure, social tension) explains that these workers constantly need to adapt how they work in order to optimise effectiveness ASN Report on the state of nuclear safety and radiation protection in France in 2024 127 The principles of nuclear safety and radiation protection and the regulation and oversight stakeholders 02 01 03 04 05 06 07 08 09 10 11 12 13 14 15 AP
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