 So we have seen the main risk of nuclear installation. Let's touch now on a very important principle of safety analysis, which is the concept of safety function. So what should be prevented and mitigated? As we have seen in the section on the main risk of accident, we should avoid the failure to control the nuclear reaction, to avoid the kind of power excursions that occur at Chernobyl. And failure to remove the heat, and for instance a loss of cooling of the reactor, is something essential, not only during the operation of the reactor, but also after shutdown. Because these two kinds of events could result in a core damage, failure of the containment and potentially radioactive releases. So the fundamental safety objective is to keep permanently three main safety functions. First, the control of core reactivity. Second, core cooling and removal of residual power. And the containment of radioactive products. So the first safety function is how to control reactivity. You see again here this picture of the vessel with the control rods that could insert in the reactor. So these control rods can fall by gravity. And another way of controlling reactivity is to inject boron in the water of the reactor, because the boron is a nucleus that captures neutron. But to control this reactivity you need also nuclear instrumentation and nuclear detectors to know exactly what is the level of flux and the sort of population of neutron in the core. You will see now a small video that tells you more about how to control reactivity. Control of the chain reaction requires careful control of the reactivity present in the reactor core. Using two complementary means of action consisting of the control rod clusters and the boron concentration in the primary coolant. Before describing their roles it is necessary to note that there are two categories of control rod clusters. The black ones that strongly absorb neutrons and the grey ones that contain a far less powerful absorbent. Shown here in a thin outline. And that it is also possible to vary the boron concentration in the primary coolant as desired. These means of action are used to fulfill three main functions. The shutdown function by means of the black control rod clusters which are highly absorbent and must be entirely withdrawn from the core to obtain maximum effect when rapidly introduced to stop the chain reaction. The compensation function which consists in approaching criticality when k equals one mainly by adjusting the boron concentration but also by control rod cluster movement. The adjustment or control function corresponding to final adjustment of the multiplication factor with k nearly equal to one to change or stabilise the heat output of the reactor. This is done using the grey control rod clusters. Let's now see how the 53 control rod clusters of Electro-1 are used which are not individually controlled but move in banks of 4 or 8 clusters positioned symmetrically. There are one group designated R consisting of eight adjustment control rod clusters which are of the grey type. Two groups designated G1 consisting of four control rod clusters and one group designated G2 consisting of eight grey control rod clusters. Two groups designated N1 and N2 of eight black control rod clusters each and 17 shutdown or safety control rod clusters. In Electro-2 whose core is larger there are 65 control rod clusters i.e. 12 more specifically 9 adjustment control rod clusters instead of 8 the same number of grey control rod clusters the same number of black control rod clusters and 28 shutdown control rod clusters. It is to be noted that the movements of the control rod clusters are made with special care being paid to the heat distribution in the core. Observation of the manner in which the heat due to fission varies vertically for instance along the core axis shows that far less heat is released near the top and the bottom as a result of leakage of neutrons most is released at mid height in the core but this pattern can be disturbed by a number of effects. For instance if the highly absorbent black control rod clusters are introduced less heat is produced in the upper part and more in the lower part with a substantial risk of local overheating at a constant overall heat output if on the other hand the grey control rod clusters are introduced the effect is far more diffuse and therefore acceptable but the ideal approach is to vary the boron concentration in the water as neutron absorption is then uniform and there are no disturbances in the core for this reason when operating at full power the boron concentration is adjusted so that all the control rod clusters are in the up position except for a few grey control rod clusters that are assigned to control and which are only inserted a small distance into the core furthermore there is a device that constantly compares the amount of heat released in the upper half of the core to that released in the lower half so as to be able to detect any major imbalance in the heat distribution this is a safety concern. So the second safety function is to ensure permanently cooling of the system so again here we have here the sketch of the main cooling system this is related to the EPR but typical of most PWR reactors so in normal operation again you have here the main primary system which cools the core goes to the steam generator and the pressurizer here and the steam generator itself is cooled by the main feed water that conforms heat but in case you lose this secondary system you have an emergency feed water system that could compensate especially during shutdown if this system becomes unavailable you have here a system which is capable of controlling the concentration of boron inside the primary system and also to remove the purity through the filters during shutdown period of the reactor you got reactor heat removal system which is able at low power to remove the heat of the residual power and this reactor heat removal system is itself cooled by a component cooling water and then an essential service water which is water for the river you have also here a system which is an emergency operating system if there is a problem of reactivity in the core we can then just large quantity very rapidly of boron to stop the reactor system and then inside the containment you have here large quantity of water that is able to serve a safety injection system if there is a bridge on one of these loops in order to compensate and to continue cooling the overall system the next video will show you a little bit more on this different cooling system than cooling function one of the three nuclear safety functions is cooling of the fuel it is necessary to perfectly control the removal of the heat released in the reactor core which requires excellent functioning of the reactor cooling system and its ancillary systems three basic conditions must be fulfilled in normal operation the presence of coolant i.e. the primary cooling water at the required pressure circulation of the coolant by the action of the main coolant pumps and removal of heat from the reactor coolant system as the primary loops operate in the closed circuit mode it is necessary for all the heat produced in the reactor core to be removed by the steam generators this cooling system must be highly effective in the reactor vessel the water rises up inside the fuel assemblies at high speed passing from bottom to top in less than one second it is necessary to extract around 20 megawatts from each fuel assembly when the reactor is operating at full power and around 100,000 watts from each fuel rod let's get an idea of the temperature in the rods at full power the water removing the heat that circulates in contact with the rods is at a temperature of around 300 degrees Celsius whereas the cladding of the rods is at around 350 degrees but the temperature increases over a few millimeters to around 1000 degrees as the center of the fuel pellets that constitute the heat source all the parameters indicating proper cooling of the fuel must be carefully monitored to be able to act quickly and effectively at the slightest incident if anything wrong is detected the reactor is immediately scrammed but this does not completely solve the problem as what is referred to as decay heat or residual heat remains due to the radioactivity of the fission products when the chain reaction is stopped the power level rapidly drops to 7% of the operating level then drops more slowly thus the power level is around 7% immediately after shutdown 5% after one minute 1.5% after one hour 0.6% after one day and continues to drop increasingly slowly it is to be noted that the level of 5% after one minute corresponds to around 140 megawatts in the case of Electra 1 and 200 megawatts in the case of Electra 2 and that after one hour it is still at 40 megawatts in the case of Electra 1 and 60 megawatts in the case of Electra 2 removal of the decay heat must be continued well after normal shutdown of the reactor or after a scram caused by a fault this is one of the crucial problems that must be solved to guarantee that the insulation is safe ok as was already mentioned it's very important not only to cool the reactor during normal operation but also after shutdown and this picture give you an idea of the power after shutdown that needs to be extracted from the system in order to control the temperature so starting from 100% power at the initial that's one second after the shutdown you still have 7% of the thermal power after one hour you have 1.5% so this is one year after you have 0.6% of the initial power so this is very important and usually the problem is not so much during the cooling function is not so much during power operation but it's after shutdown to ensure that there is always the capacity to extract DC you will see now in the next video a little more detail than that also and so the last safety function is the containment function so through that we can avoid the radioactive release even if there is an accident inside the reactor and that some contamination or even friction products is released in the containment so the containment could be either passive through big buildings which is leak tight and there are for instance two kinds of passive containment one with only one building which has a liner inside so it's very leak tight and the pressure can sustain this type of containment in the order of 5 bars and another way of ensuring the containment function is to have two two buildings with a space between both buildings maintain under pressure through a ventilation and filtration system so in a leakage occurring through the first containment is taken with this ventilation system and filtered before release so these are the three main safety functions that should be ensured permanently control of the reactivity control of the cooling and control of the containment