 So, beside the safety function that we have seen, there is another important safety concept which is the defense in depth. Defense in depth is not something specific to the nuclear industry because it can be used in any kind of activity in other industries, but it has known some specific development in the nuclear industry. So basically, defense in depth is to imagine any potential equipment failures or organizational or human errors and to implement several successive line of protection. If something goes wrong, you will have a first barrier or the first way of avoiding the consequence and if this first barrier or this first system fails, you will have a second line of protection and even a third line of protection. So in that case, with that kind of configuration, you will need two, three or four successive failures from an initial problem to get any significant consequences. We consider three categories of line of defense that we call prevention, monitoring and mitigation and these are developed in five levels at the design stage. So the first level is prevention of anomaly. This is done through the quality of the products, the quality of the design, the margin taken in the designing of the various components and in operation to some limit of operation, limits in the domain of operation. The second level of defense in depth is monitoring and detection of anomaly. Three, this is done through several surveillance or instrumentation to be able to follow the various parameters. The third level is serve as mitigating the consequence of design-based accidents through various safety system or emergency system in order to even if the accident occurs, the consequence will be very limited. The fourth level, if the accident occurs and that the safety system designed to control them fails or not sufficiently efficient, there is a fourth level to control the development of severe accidents that is to control the accident that I turn in core melting and to avoid the release of activities outside the containment. The fifth and last level of the defense in depth is to protect the environment. If all the first, the fourth level have failed, the protection of the environment and the public is obtained through some sort of emergency planning such as the evacuation of the population of confinement of the population around the plant. So this is another way to represent the defense in depth concept and it's five level. So the first one is a prevention of abnormal operation and failure of the component. That is achieved through a very conservative design and the high quality of construction and operation of course. So the second level, the purpose is to monitor the system and control the development of any abnormal operation and detection of failures. This is done through a lot of instrumentation surveillance feature and in order to control the various transients in some case to include some limitation. So the third level is the control of accidents which are considered in the design basis. This if an accident occurs such as a loss of current accident that we will see a bit later. Some safety system such as safety injections or containment system or emergency boration systems or these systems are deemed to limit the consequence of these accidents and to remain the plants in the design basis without consequence. But if these fail, we go to the fourth level which is the severe accident where there might be some fuel degradation and even some core melting. And so the purpose here is to prevent any accident progression and to mitigate the consequence of that through the containment. So the main role here and the main action is to manage the accidents ensuring that the containment remain leak tight and that there is no release. And if these fail, we go to the fifth level which is try to mitigate the gradual consequence of the accidents and the release of radioactive material. And this is the main role of the emergency response organization both on site and off site with the public authority. So you have here the five level of the defense in depth, the condition and the various features taken to prevent, monitor and mitigate the consequence of accidents. Another way of presenting the defense in depth concept is what we call the 3E barrier model. The idea is to have between the radioactive products inside produce by efficient reaction inside the fuel and their environment, three different leak tight barrier. The first being the fuel rod itself and its cladding. The second being the reactor cooling system and the third one the containment. Another way of presenting that is this Russian doll which insert one in each other and you again see here the fuel rod cladding inside then the reactor cooling system pressure boundary and the containment. You will see another illustration of this 3E barrier model in the next small video. Containment of the reactivity depends on the integrity of the three containment barriers, the three successive walls between the fission products and the environment. The first barrier is the cladding of the fuel rods. The second barrier is the pressure boundary of the reactor cooling system. The third barrier is the containment vessel. Each has its own particularities. The fuel rod cladding is in direct contact with the radioactive material to be contained. This is a multiple barrier depending on the power rating of the reactor there are between 40,000 and 50,000 rods. The first barrier is subjected to a number of challenges, thermal effects at the boundary between the fuel material and the cooling water, mechanical effects including pressure and vibration and also the effect of irradiation as intense neutron and gamma radiation is continuously passing through it. To preserve the cladding it must first be ensured that heat is removed at all times and as effectively as possible otherwise the result will be melting of this first barrier. The second barrier which is the reactor cooling system pressure boundary is a complex arrangement of large chambers and pipes such as the primary loops. It also comprises a number of ancillary systems connected to the loops consisting of many pipes of different diameters. Here the challenge is the extremely high pressure to which a system is subjected requiring thick walls. There is a danger of pressure excursions well beyond the normal operating levels as well as eventual fatigue of the materials as a result of the pressure and the associated risk of leakage. The third barrier consisting of the containment vessel is extremely large and of apparently simple geometry but it is reversed by numerous pipes and cables which can constitute weak points where leakage can occur. Here the risk is associated with contingent thermal stresses and over pressure in the event of the sudden release or large amount of steam in the reactor building. Special care is needed to ensure that the penetrations are leak tight as the third barrier represents the last line of defence protecting the environment against the dispersal of radioactive material in the event of a severe accident. It must be borne in mind that the three barriers are not totally independent and the failure of one can have repercussions for the others. For instance in the event of failure of the large pipe making up one of the loops and hence loss of the second barrier leakage of coolant would compromise cooling of the fuel resulting in a major risk for the first barrier while flashing of the water at 300 degrees to steam in the reactor building would also challenge the third barrier by exposing it to extreme heat and pressure. It is also because the three barriers are not totally independent that so much care has to be paid to them. There is a tricky point on the third barrier because there is an extension. Actually if you look at the steam generator we are here inside the reactor building so inside the third barrier and here outside the reactor building so outside the containment and there is this steam piping penetration of the containment. And so actually that means as there is a safety valve here actually that's to the fact that the outside of this part of the steam generator could communicate with the outside the containment. So that means at this level the steam generator tubes which constitute the barrier the limit of the reactor cooling system of the primary system of the second barrier is at the same time the limit between the second and the third barrier. So at the level of the steam generator tubes the second and the third barrier are combined. It's the same components that play the two roles. This underlies the importance of these tubes because the two barriers are combined at this level and so it's very important to first to design with adapted material these tubes and then during operation to control NA leaks and to control periodically the health condition of this material through non-destructive examination and this is an important part of the maintenance during shutdown of the reactor. So to summarize defense in depth it's a very important tool of safety analysis it could be applicable not only during the design phase of a reactor but also during all this operation and also during the decommissioning. It could be applied not only on technical issues but also on non-technical issues as some organizational aspect or human behavior aspects and even in day-to-day life for instance when you want to cross a street you have a look on the left and on the right to see if you can go. And so this looks is a sort of element of defense in depth.