We have looked at two sources of renewable energy, solar and wind. We're now moving to another source of energy, which is sometimes proposed and sometimes opposed, as a possible solution to the climate warming problem. This is nuclear energy. Nuclear energy is highly controversial, and hand on my heart, as I've told you at the introduction of these series of talks, in my academic background, my first doctorate was in nuclear engineering. Despite there for knowing about nuclear energy probably more than the average person, I am still of two minds about the desirability of nuclear power in order to tackle climate change. However, probably the reasons why I am less than fully convinced that nuclear power is a strong candidate as a solution; the reasons for my perplexity are probably not the same reasons that probably shared by you or by most of people who look at this problem. I will present the facts, I will present as usual numbers, data, and I'll try to keep my opinions by myself. Then you'll be able to draw your own conclusions and to see whether your conclusions coincide with mine or not. In order to understand what is the promise of nuclear energy, it is important to understand, in its essence, what are the principles behind fission and fusion. Again, the idea is not to turn you into nuclear engineers, but to give you the tools to appreciate the principles behind nuclear fission, to weigh the opportunities and dangers of nuclear fission, the real dangers and the real opportunities and to create an informed opinion about the pros and cons on nuclear fission and fusion in the context of global warming. What is the big idea behind fission and fusion? Well, we can start by thinking of what nuclear is made out of, and this will give us a way to understand both fission and fusion at the same time. We all know that an element is made up of electrons in, let's say ''the outside'' and a nucleus. The nucleus always has a number of protons equal to the number of electrons. It is always the case. However, in the nucleus there are not only protons, but also neutrons. Neutrons and protons are collectively known as nucleons. Now, the atomic element as a whole, is electrically neutral because it has many protons as electrons. However, the nucleus itself is full of protons which are positively charged and neutrons. You might ask yourself, how does the nucleus stick together? Why does the electrostatic repulsion amongst the positively charged protons? Why does it just break apart the nucleus? Well, the reason is in addition to the electromagnetic force, there is one other important force at play, which is called the strong force. The strong force that I show graphically here, is much shorter range than the Coulomb force. If you try to squeeze two nucleons, whether they are protons or neutrons together, there is a very, very strong repulsive force, almost like a wall, but there is an attractive bottom of the well. Remember, physical systems always try to go to a condition of minimum energy. You might try to think of a system trying to find the position of the bottom of this well, which is very, very steep on one side and more shallow on the other side. Notice that the slope of this well gives me the force, the force which is repulsive, if I try to get too close and attractive, if I try to get too far. But the range of this force is very, very short. Look on the x-axis, the units there are Fermis. One Fermi is 10^ minus 15 meters. For an order of magnitude, the hydrogen is 10^ minus 10 meters. We're talking about really, really short range forces of much shorter range, and this is crucial, than the Coulomb forces of repulsion between protons. The idea is that the stability of the nucleus comes from a tug of war between the long-range, weaker repulsive Coulomb force and the strong range, stronger nuclear force. Neutrons do not repel other protons, but actually attract them, but they can only attract them over a very, very short range. Therefore, neutrons act as a short distance glue, overcoming via the strong force, the Coulomb force of repulsion. But exactly because the Coulomb force is longer range than the strong force, when I begin to pack more, and more, and more protons together, I cannot stack enough neutrons in a suitable way in order to overcome these repulsion forces, exactly because the strong force is so short ranged. Therefore, there is a maximum number of protons that I can stack together with neutrons, beyond which the system becomes unstable. Another very important thing is that there is an ideal number of protons and neutrons, whereby the resulting nucleus is most stable. We can see this in this picture here. The maximum of stability occurs approximately around 56 for iron. You see that I have a curve that is coming down strongly on one side. What this curve shows is the binding energy. Even if it looks a bit like the picture we saw before, it is a completely different picture. There is the binding energy per nucleon NIF. On the very far left, I have hydrogen, helium, and the light elements. At the very far right, I have the heavy elements. You can see that the highest stability occurs in the middle. When I go from a energetically higher to an energetically lower configuration, I have release of energy. This picture tells us immediately that I can obtain energy in two way, either by putting together light elements, hydrogen, for instance, they become Cerium, or by splitting heavy elements, for instance, uranium, that becomes lanthanum and bromine, for instance. As I go down these two slope, there is a release of energy. Now we can begin to understand how nuclear reactors work. Nuclear reactors in operation generate energy through fission, which is the process by which a big heavy nucleus is split into two smaller nuclei. These smaller nuclei are neutron rich, in the sense that they have more neutrons than they need. Why do they have more neutrons than they need? Because all those neutrons were required in order to hold together the big nucleus they came from, which had lots and lots of protons. Now it has been split, and therefore there are 'too many' neutrons. These neutrons are released, and as they hit other big atoms, the original nuclei they came from, they create in turn further fission. In addition, we know that we have gone to a more energetically favorable state. It is this difference in energy which is released as the energy produced by a nuclear reactor. In order to harness this energy, one has to achieve a controlled chain reaction. When a uranium, let's take uranium as a typical nuclear fuel, is split, the unstable fragments produce two or more or what are called prompt neutrons. Neutrons that can be absorbed by other uranium-235 nuclei, causing them to undergo fission as well. More neutrons are released and continuous fission is achieved. How this process works is clearly shown in this picture and in the following one. In this picture, we have a nucleus at the top, which is made up of neutrons, which are the green balls, and protons, which are the red balls, and we have a neutron arriving colliding with the nucleus. The collision causes the nucleus to deform and if there has been enough, and not just enough energy, but energy in the right energy band, which is conveyed to the nucleus, the nucleus will deform so much that in extremely short time, in 10 to the minus 14 seconds, splits into two smaller nuclei. As I've explained, the smaller nuclei do not need as many neutrons to hold together the protons, and therefore there are further neutrons, which are the green balls here, that are released. In a very, very short time, about 10 to the minus 12 seconds, the fission fragments, which are the two smaller nuclei, lose their kinetic energy and come to rest by emission of Gamma rays, which are the yellow arrows. Over a longer period of time, that could be seconds, but it could be as long as years, the fission products lose their excess energy by a radioactive decay, emitting particles over a longer period of time. This last picture here shows something which is very important. I have the neutron arriving on the nucleus. Noticing the nucleus, there are 92 protons and 143 making up of it, 235, uranium 235. Look how many more neutrons are required to hold this bulky, huge nucleus together. When the big uranium 235 nucleus is broken up, the neutrons emitted a very high energy. This high energy is not the right energy to split another uranium-235 atom. These neutrons have to be slowed down via a moderator. When we discuss what happened at Chernobyl, understanding the role of the moderator is absolutely key. This is what the moderator does, it slows down the neutrons. I have, it could be graphite, it could be different elements, they have to be light elements because they have to be roughly of the same mass as the fast neutrons to exchange energy efficiently with the neutrons and slow them down. They have to be slowed down exactly in the energy range, will become efficient to splitting another nucleus, as we see here. That in terms produces more fast neutrons, and then get moderated, and that is the chain reaction going on and on.
