 This is the first part of this unit on nuclear safety. We will see first some definition and element of basic nuclear physics. And in the second part, you will see some description of the main component of PWRs. But it is first important to give some definition of the main concept which are used here. And to make an distinction between nuclear safety, radiation protection, nuclear security and industrial safety. Nuclear safety covers technical and organizational measures related to design, construction, operation and decommissioning of nuclear facilities, as well as transport of radioactive substances, defined to prevent accidents and mitigate the consequences should they happen. Radiation protection against ionizing radiations covers regulation and processes to avoid or to reduce direct or indirect harmful impacts of radiation on peoples and environment. Nuclear security covers technical and organizational measures to prevent malevolent actions on nuclear installation, as well as diversion of radioactive materials. Eventually industrial safety covers non-nuclear risk in a nuclear facility. You will find this definition in a textbook, but it's important to know that depending on the country, these different concepts are not covering exactly the same thing. So let's begin now with a description of the atom structure. We will use the Bohr atomic model, in which an atom is modeled as a positively charged nucleus with negatively charged electron surrounding it. The nucleus itself is composed of nutrients and protons, and this figure provides a schematic representation of an atom. This implies that any nucleus could be identified by the couple NZ, with N representing the number of neutrons, and then Z representing the number of protons. As an example, the hydrogen nucleus made of one neutron and one proton corresponds to the couple 1-1. Let's define now the concept of radioactivity. Most nuclei are stable, but some are not. For some nuclei, there is a probability of spontaneous disintegration. That means that their structure suddenly changes to reach a stable form with a modification of mass and hence an exchange of energy with the environment. These take the form of radiation emission, which is a characteristic of radioactive substance. An element is radioactive if it can disintegrate spontaneously with an emission of radiation. There are different types of radiation that are different types of particles to be emitted. The first is the gamma ray, which is an electromagnetic radiation. A second type of radiation is called beta and is composed of electrons. A third one is called alpha and alpha particles are actually helium nucleus. And then there are the neutrons. Let's introduce now the concept of radioactive alpha-life. The natural radioactivity of unstable nuclei consists in its disintegration through radioactive radiation. Let us consider a population of nuclei or radioactive elements. We can observe a natural decrease with time in the population due to natural radioactivity. As we can see in this figure, there is a decrease by half of the number of nuclei after a period of time named radioactive alpha-life. We are coming now to the core of the physics of a nuclear reactor, which is the fission reaction. Some heavy nuclei like uranium-235 or plutonium-239 are called fissile material. Because if they are eaten by a neutron, they can split into two or more products. At the same time, emitting a lot of energy and a certain number of neutrons in average 2.5. These neutrons in turn could eat the fissile nuclei and produce new fissions. This is a continuing process that we call the chain reaction. You will see now in this small video to come more detail about this concept of fission and the nuclear chain reaction and the way to control it. Neutrons play a key role in the chain reaction. When a neutron hits a nucleus of an atom, it either just bounce off corresponding to elastic collision or be absorbed, in which case a capture reaction takes place, frequently associated with the emission of a gamma ray. Another possibility, in the case of certain rare heavy nuclei such as uranium-235, is a fission reaction. This releases energy and two, three or four neutrons corresponding to 2.5 on average. And it is this that makes a chain reaction possible. If, at a given moment in time, a thousand fissions occur in the reactor core, this results in the emission of 2,500 neutrons. After a certain number of collisions, these neutrons can disappear in three ways. Fission, capture by different nuclei or escape from the reactor, which is referred to as leakage, in what proportions? Appropriate arrangements are made to obtain exactly 1,000 fissions when the fission rate is maintained at a constant power, around 1,450 captures and 50 escapes. This is the relative proportions of fission, capture and leakage if the chain reaction is to be exactly self-maintaining. When a thousand fissions give a thousand fissions, the multiplication factor k, by which the number of fissions is multiplied from one generation to the next, is equal to one, in which case the reactor is said to be critical. If the number of fissions is then made to slightly increase relative to the captures, and if the number of fissions is increased, for instance, from 1,000 to 1,001, the multiplication factor exceeds one, in which case the reactor is said to be supercritical. The power level then increases from generation to generation. If, conversely, the number of captures is made to slightly increase relative to the fissions, and if the number of fissions is decreased, for instance, from 1,000 to 998, the multiplication factor is then less than one, in which case the reaction is said to be subcritical, and the power level decreases. It will later be seen how it is possible to vary the multiplication factor by using the control rods, or the boron present in the primary coolant, to adjust the reactor power as required. For a nuclear reactor to operate there need to be three main components, the fuel where the fission reaction will occur, then a moderator, because the neutron emitted by the fission reaction have too high speed to be able to produce another fission reaction. So they have been to be slowed down. And the third element is a coolant, because the quantity of energy that is produced by the fission reaction need to be extracted and to be used afterwards. And so these three components are essential. The fuel is azur uranium, and especially the U235 isotopes, which is the only one to be fissile, and the plutonium 239. Uranium could be used azur on its natural form, or it could be enriched in its isotopes 235. And this fuel could take the form of azure metal, oxide, carbide, nitride, ion molten salt. The different moderator use have been graphite, deuterium, or water, the simplest one. And as coolant we can use water, CO2, helium, or liquid metals, such as sodium or lead. So these components could be combined in different ways, and they are, let's give, different types of reactors. So if we categorize the reactors by moderator, the first moderator use was heavy water. And the coolant associated with the moderator could be CO2, heavy water itself, or nitrogen, and in these reactors the fuel is on the oxide form. Another type of moderator was graphite, and the coolant could be used with this reactor are azure air, water, CO2, or helium, and the fuel could be on different form, metal, oxide, or molten salt. And the most common type of moderator is just water, which is in the same time the coolant. The fuel is on an oxide form. And the last category of reactor has no moderator. The coolant, because of the high density of this reactor, a very efficient coolant should be used, and it's usually liquid metal or a molten salt, but the fuel could be either in the metal, oxide, carbide, or nitride form. Several types of reactors have been experienced during the research period, but we are only now going to see the main ones. So the first type of reactors use natural uranium as fuel, and the first category use gas as coolant and graphite as moderator. They are called MAGNOX. This is a type of reactor that has been built in the UK, or uranium, natural, and gas graphite in France. Another type of reactor using natural uranium has heavy water as both coolant and moderator. This reactor has been built mainly in Canada, and they are called KANDU. Another type of industrial reactor use unreached uranium, and they are the most common. We can see the gas graphite, which has been developed in the UK, the AGR. The water as coolant and graphite as moderator, this is a Russian type of reactor, a kind of reactor of Chernobyl, which are called Airbnb. And the most common type of reactor using unreached uranium that use water both as a coolant and as a moderator. And among this family, there are the pressurized water reactors. The boiling water reactor or the VVER, which is a sort of pressurized water reactor, but with a sort of specific design developing in Russia. Another type of industrial reactor, which is not yet fully industrial, are the fast breeder, which have no moderator and which could use liquid metal, mainly the sodium or lead. And the high-temperature gas-cooled reactor, we used helium as a coolant. This figure gives an evolution of the reactor types and what we call reactor generation. It's a development along the age. The first generation was built in the 50s and 60s. They were early prototype reactors. The second generation was designed during the 60s, 70s, and it covers the commercial power reactor already operating in this category. You have the light-water reactor, I mentioned, the Kandu, the VVER and the RBMK. We are now beginning to develop and to build reactors that are falling in what we call the generation three. They are advanced light-water reactors like the ABWR by General Electric or the AP600 developed by Westinghouse or the European Pressurized Reactor developed by Arriva. Some efforts have been made to run this type of reactor a little bit more economical and they fall in this category of gen three plus, which will be built in the coming years. At the end, this is this generation four reactor where the objective had to be highly economic to have a better safety level to minimize the waste and this type of reactor is more resistant to proliferation.