 We will now begin to look at how nuclear safety is ensured at the design stage. So we will see the different aspects from beginning with the risk, the concept of safety function, a very important principle which is defense in depth, and then the two different approach of the safety analysis, which is a deterministic approach, and the probabilistic approach, including the treatment of hazards. But first let's look at the main risk from nuclear power plants. Of course the first risk is linked with the efficient reaction, but it's important to say right here that nuclear plants cannot be transformed in a atomic bomb because of the physical arrangement of the elements of the core that render that impossible. Then there is the risk from radiation, and we will have a look on the risky normal operation, and of course the risk of accident, which is the major one. So what are the risks from the efficient reaction? Of course the immediate risk is that this reaction goes out of control, so there is the need to control the efficient reaction. That's the immediate risk, but there is also a different risk because even when the efficient reaction has stopped, there is a lot of efficient products, which are radioactive, even if they are decaying in terms of radioactivity, that it remains what we call the residual power, and so there is a team, the need for long-term cooling after the shutdown of the reactor. And another element very important to control is confinement in order to avoid the radioactive products to escape and get to the environment. So now the risk from radiation. Radiation has some biological impact on the cells, either destroying them or modifying their structure. So there are two kinds of consequence of that, an immediate consequence, the destruction of the tissue, that it's exactly like when you got a burn, but there is also a different effect, and the modification of cell could lead to some cancer or some hereditary anomalies. There are two ways of impacting living material from radiation. We call the first one irradiation. It's a situation where the radiation comes from the outside and this could occur without any contact with radioactive products. The way to be protected against irradiation is by putting some screens or increasing the distance between the source and the living material. Another way of being impacted by the radiation is through what we call contamination. This happens when you ingest some contaminating products or inhale, pressuring, or if you are touching some deposit of radioactive products. So if the contamination is outside, this could be eliminated by washing and this is a reason why each time you exit the nuclear plant you have to wash your hands and if you are contaminated you have to go through a shower. And if you happen to inhale or to ingest some contaminated products, the elimination is done through the decay of the radioactivity or through biological processes. So the unit used for measuring the impact of radiation is the gray and the gray is equivalent to one joule per kilo. But the harmful impact on living materials depends on the type of radiation and the type of organ or cell involved and it's not the same for a neutron or a bata-gray. And so we use this concept of effective dose unit and it's the most used and if you hear the media you're talking about sievert or millisievert. So one sievert is equal to one gray for beta and gamma radiation but the impact could be much more important for the neutron radiation and in this case one sievert is equal to 10 or 20 gray. There are strict limits from radiation through regulation and so the standards. For the general public the annual dose is one millisievert per year but for nuclear workers who have specific annual medical follow-up the maximum allowed dose is 20 millisievert per year. We have to compare that to a natural average average radiation which for instance in France is 2.2 millisievert a year and but in some other parts of the world and for instance in some part of India the natural average radiation could reach 10 millisieverts a year and the impacts on radiation depends also on the quantity of the dose and the figure of five sievert is something important because this is the beginning of the point when you have some deterministic impacts that means that it's certain and there is a proof that this kind of dose has a specific impact. Below this value of five sieverts we talk about stochastic effects that means that for this low dose you have a certain probability of having an impact but it's not something certain. Let's have a look now on the the risk in normal operation. First to say that for the public in normal operation the the risk is completely insignificant because of strict authorized limits of both radioactive gaseous and and liquid releases and there is this principle of halera which means as low as reasonably achievable and this is a principle which is applied on all nuclear facilities where if there is something that could reduce the the release either in either form the measure that should be taken to make to implement these elements and reduce again the the dose incurred by the public. For the the workers and specifically for the maintenance staff there is some risk of irradiation and contamination because of course they were closer to the source of radiation so the measure which are taken to reduce the risk is wearing of specific crossing equipment in including some scaphandria. We use also screen lead screen around the piping when there is some contamination inside the the pipe and the the dose incur is also permanently measured through dosimeters. It is also important to limit the time of exposure and for instance before making some control in the steam generator tubes the operates the maintenance staff the persons who make this inspection are trained to jump in the box of the steam generator in order to limit the time it takes to make the job. So the main risk is the risk of accidents and accidents have happened and they could happen and the prevention and mitigation of these accidents is the very purpose of of nuclear safety because the consequence of an accident is to release potentially release contaminated water gas and if there is some fuel degradation there will be also the release of fission products. So we just mentioned here the three major nuclear accidents that have occurred in the world. The first one is the accident at Srimail Island in the US in 1979. There was no significant release and no health impact of this accident because there was a nice reactor around this facility and but this accident has had a large impact and on the way we see nuclear safety and we will come back a little bit on then on on the one of the next parts. Another big accident was Chernobyl in the USSR in 1986. These occur on a very specific design the RBMK. There have been a major release very significant health impact with a lot of people that died from from this accident and again we will come back on that later and the most recent one that I think most of you remind is the Fukushima accident in 2011 in Japan that was a boiling water reactor. There have been some important releases but thanks to a rapid devocation of the population around the plant the health impact of this accident has been very limited and so again we will see some more details in the section on operation.