 So we will see now the description of the main component of a pressurized water reactor. This is the type of reactor which is the most common worldwide and this is the reason why I chose to focus on this one. You have here a sketch of a pressurized water reactor. Here is a vessel with inside the core of the reactor. The steam generator where the coolant of the primary system exchange is heat with the secondary system. The secondary system is water entering the steam generator which is transformed in steam exchanging the heat from the primary system and the steam goes to the turbine which generates the generator. The steam is then condensed in the condenser which is cooled by an open circuit either a cooling tower or the river water. We will see in more detail the different elements of a nuclear reactor especially in the coming small video you will have a look on the main cooling circuits. The heat released by the fuel in the reactor vessel is removed by cooling loops. Each loop includes a steam generator and a pump. The water leaves the reactor vessel through a large pipe called the hot leg and enters the steam generator. Here it is cooled by the secondary water which is vaporized then is sucked into the intermediate leg by the main coolant pump and discharged into the reactor vessel through the cold leg. Depending on the power rating of the reactor the reactor coolant system consists of three loops as is the case in Electra 1 or four loops as is the case in Electra 2. To prevent the primary water from boiling although it is at a temperature of 320 degrees centigrade on leaving the reactor vessel a pressurizer is provided which maintains a high pressure in the system. The bottom of the pressurizer is connected to one of the loops by a pipe which after a number of expansion bends joins the hot leg of the loop. This constitutes the pressurizer expansion line. These are the main components of the reactor coolant system. This is an overview of a 1300 megawatt pressurized water reactor. You find here in the middle the containment and the primary system. The reactor vessel here. You can see the steam generator. And beside this is the fuel building where the fresh fuel is inserted in the pool and before being loaded in the reactor vessel. And when the fuel is used, we call that a span fuel, it is even back in the pool here to be cooled up to the time that it could be transported outside of the plant. Here is a turbine building where you can see here the main turbine and the generator at the end. Of course there is a lot of auxiliary facility around there, such as some tanks here to store liquid waste or elements to make up the water inside the primary or the secondary system. Let's discuss now about the fuel itself. The basic element is a fuel assembly, which is shown here. It's for this type of reactor, a 17 by 17 assembly of fuel rods. You can see here a vertical view of the fuel assembly and with all the small rods. Inside the rods you have some pellets and the fuel itself is in the form of pellets which are piled up inside these fuel rods. To maintain the rods you have at various elevations some grids that maintain the fuel rods. On some position inside the fuel assembly there is no fuel but there are some holes for the incision of control rods that will allow to control the level of power of the reactor. In a reactor core, typical, there is something like 200 such fuel assembly and the length from the top to the bottom is about 4.2 meters. Of course, at the two extremities of the fuel assembly there are elements to allow the transport of the fuel assembly when we are loading or defueling the assembly from the core. You will see now a small video giving more detail about the fuel. The enriched uranium in which the energy is produced is contained in long thin tubes some 4 meters in length and less than 1 centimeter in diameter. These are the fuel rods. Each rod contains a stack of cylindrical pellets made of uranium dioxide. The pellets are around 8 millimeters in diameter and 13.5 millimeters high. There are around 300 in each rod. The metal tube forming the outer shell of the rod referred to as the cladding is made of an alloy of zirconium. End caps are welded to the ends of the rods. At the top there is an empty space referred to as a plenum that contains a spring to receive the gases released by the fuel while it is in the reactor. These rods are combined in sets to form fuel assemblies. The top of the fuel assembly or upper nozzle is designed to facilitate handling. The bottom of the fuel assembly or the lower nozzle ensures accurate positioning on the support plate in the reactor vessel. The fuel assembly is rigidified by 25 guide tubes which link the two end fittings. Grids attach the guide tubes received for the fuel rods of which there are 264 in each fuel assembly. These grids are in the form of a square. Each side being approximately 20 centimeters long and having 17 rod positions. They maintain the rods at a regular spacing and prevent them from vibrating while facilitating water flow. A range of devices can be inserted into the guide tubes, notably the control rods that are inserted into designated fuel assemblies which play an important role in controlling the chain reaction. This is the bottom of the vessel with some penetration. The water enters on the top side of the vessel and exits. You have here a more detailed view. So the water entering this penetration first goes down to the bottom of the vessel, then enters the core and goes up through the fuel assembly to prove it, is a steam generator. This component comes out of the vessel and enters what we call here the hotbox of the steam generator. Above these very thick plates, you have hundreds of new tubes, hot water entering the hotbox, goes up inside the tubes and then getting down and exits through this core box. The secondary water, what we call the feed water, enters the steam generator at the top, goes down between these two shell plates, up cooling, exchanging the heat from the inside of the tube, and then it is transformed into steam, which is separated the steam from the droplets in this part. Then there are some dryers here and the steam goes out of the steam generator through this core here, going to the top. You will have in the next video details about this steam generator. In each of the reactor-coullent system loops, there is a steam generator. This constitutes the means of exchanging heat between the primary and the secondary systems. The primary-coullent from the reactor vessel enters at more than 320 degrees, circulates through a bundle of U-tubes, where it gives up its heat as it is cooled by some 40 degrees, then leaves towards the pump. Heat is exchanged via these hot tubes. The secondary side water enters via the top of the steam generator, is channeled down to the bottom along the periphery, then rises up the center between the tubes, in contact with which it is vaporized. The steam, thus produced, leaves from the top of the steam generator, and apart from being used to generate power, plays an important safety role by removing the heat produced in the core of the reactor. The third important component of a nuclear reactor, at least for the pressurizer water reactor, is the pressurizer, because the water should remain in the liquid form in the system, and even though the pressure is around 140 bars, so the pressurizer serves at maintaining this pressure and sort of regulating the pressure inside the system. For this purpose, you have at the bottom some heaters, electric heaters, and if they are on, of course the temperature will increase in the pressurizer, and so the pressure in the system. On the other hand, you have at the top a spring nozzle where cold water could be injected, condensing the steam which is on the top of the pressurizer, and so reducing the pressure of the system. Another important element of the pressurizer is the safety relief valve, which is here at the top, which is set at certain set points in order to avoid over pressurization of the system. The pressure increases too much, the safety valve will open, and the steam will be discharged in a tank around there. So this safety relief valve played an important role in the accident at Srimal Island that we will describe later. So this is a sort of summary of what we have seen inside the reactor building. You can see here the main component, again the vessel here, the force steam generator, the pressurizer, and the main coolant pump you have done here. We will discuss perhaps a little bit later about these tanks, or the accumulators, which serve an important safety role, in case there is a break in the system. So this concludes this sub-part. We have seen the main component of a nuclear reactor, and we will see now in the next sub-parts how we designed and how the safety is analyzed. First at the design step, and then of course during operation.