 Hi, I'm Zor. Welcome to Engineering Education. Today we will continue talking about certain things related to splitting nuclei of certain elements, which is called fission. Now, the previous lectures were basically kind of preparation to this one. We were talking about mass defect. So that's a very important part and I will definitely talk about this today. Now this lecture is part of the course called Physics for Teens presented on Unisor.com. I suggest you to watch this lecture from the website because you might actually find it somewhere else on YouTube, Google, whatever. But the website actually has a lot of advantages. Number one is completely free. There are no advertisements, no strings attached. You don't even have to sign in. So everything is completely at your disposal. Secondly, every lecture has textual description basically, which is like a textbook. So you can listen to, you can watch the lecture, your video presentation, and you can read about the same thing as in the textbook. Then certain parts of the course have problems to solve organized as exams and you can take these exams as many times as you want. Well, and then there is a prerequisite course called Math for Teens on the same website. Mathematics is extremely important and whatever I'm using in this course of physics, whatever mathematics I'm using in the course of physics is all there. So I mean, if you know it, that's great. If you don't, you can take the course called Maths for Teens on the same website. Okay, back to fission. So it was noticed that certain elements are radioactive, which means they emit certain alpha, beta or gamma rays, which we were talking about in the previous lectures. And certain elements are actually so unstable that if affected in some way, they might actually split into two other elements. And that's what we're talking about, mass defect. But in this case, it's more significant because we're talking about a forceful splitting of certain nuclei. Now, in particular, the special isotope of uranium, uranium 235, which means it has 92 protons and whatever it's 143 neutrons. So the atomic number is 235. That's total number of particles in the nucleus, protons plus neutrons. And this is the protons, this is the positive basically charge, which has the nucleus. So this particular isotope of uranium, which is rare, much more frequently occurring is uranium 238. With more neutrons, the same 92 protons, but more neutrons. So this particular isotope of uranium, if it's bombarded with neutrons, well, bombarded, maybe not exactly the good word, if it absorbs a neutron. Now, neutron has atomic number one, because it's a one particle and no zero charge, electric charge. So what happens in this particular case, the nucleus of this particular element, if it consumes one neutron, becomes unstable. Now, I mean, you can actually, at the moment of absorption, you can say it's uranium 236, because it adds one to the atomic number. But it's unstable. Uranium 238 is stable. Uranium 236 is not. Why? It's a different story and I don't know really quite frankly. But anyway, it's unstable and it splits into two halves. Well, not exactly equal halves and not exactly always the same way, but it splits somehow. So one of the possible splitting is as follows. It produces birium, which has one 39 atomic number and 56 protons plus krypton, what is it, 95? 36 plus two neutrons, three. Now, let's just count the number of particles, 235 plus one, that's 236. 139 plus 95, it's 244 plus 236. Okay, so number of particles is okay. Now the electric charge, 92 plus zero is 92. 56 plus 36 is also 92. So we have balance. I mean, nothing disappeared, actually. Electric charge is the same, which means number of protons, basically. And total number of particles is also the same, so number of neutrons is the same. Everything is great. Now, okay, fine. Now let's talk about mass. Now, when you take the mass of these particles, and I have it written, it's 235.043928 plus 1007 plus neutrons. These are masses. Now, if you take these, you have 139. something. I have exact numbers, 94 something and two something. And the result would be 235.863. The result of here is 236.051. So I have exact numbers in the text part of this lecture in the website. Doesn't matter. What matters is there is no equality in mass. That's the most important part of the fission process. It's fine, number of particles is the same, number of positive particles, protons is the same. But the mass is not the same. So we have a difference in this case, 188. Now, when I'm talking about these numbers, what are these masses? These are atomic mass units or dilatons sometimes. That's the name of the physicists. Atomic mass unit is basically approximate mass of one proton or one neutron, kind of an average between them. So they have decided the mass of carbon is 12 particles divided by 12. So this is atomic mass unit. I didn't talk about this before, but anyway, that's how we measure masses. And obviously, it has its equivalent in kilograms, obviously, in the C system. All right, so in any case, we have a difference in mass, whether it's measured in atomic mass unit or in kilogram, whatever it is. And so the question is, where the mass goes? Well, by now, we all know that mass and energy are related. And there is a very important equality, which I did touch before. The most famous, I would say the most famous equation in all the physics, the relationship between energy and mass. And this is the speed of light in vacuum. So if we will multiply the amount of mass, which is the deficiency between left and right side by square of C, square of speed of light, we will get the energy, which is basically supposed to be released during this process of fission. If this mass is greater than this, then the axis of mass is something which was converted into energy, and the amount of energy is this. Now, the calculations show it's a huge amount of energy. Now, in particular, if you take one gram, one gram of uranium 235, this one, and if you assume that all its nuclei are split, so that would give you amount of energy, which can be released if all the nuclei of one gram of uranium 235 is under the process of fission. The amount of energy would be equivalent to 5,000 gallon of gasoline. So the amount of energy which can be released by burning 5,000 gallons of gasoline is equivalent to amount of energy which can be released if all the nuclei of one gram of uranium is split. Okay, that's huge, and that is the nuclear energy, which is the basis for nuclear power stations and atomic bomb. Now, what's very important is that the fission process doesn't really occur always the same way. It's not organized in some way. It's much more complex. First of all, we have to somehow find neutrons to attack the atoms of uranium 235. So the more neutrons we are supplying, well, the more atoms become excited and split. Now, splitting is also not always such an organized thing. For example, under a certain condition, it can go into this barium and kryptonium. Under some other condition, it can be something else. For example, I have written here barium 516144. It's a different isotope of barium plus kryptonium 3689 plus 3 neutrons. So different isotopes of these two elements can be not necessarily. It can be zirconium 4094 plus tellerium 139 plus 3 neutrons. Or it can be, I think it's called luteinium or something like this, 57 if I'm not mistaken plus molybden 9542 plus 2 neutrons plus 7 electrons, which means in this particular case, a certain number of neutrons converted, 7 actually neutrons were transformed into proton plus electron. Electron released and protons are here. So it all depends on conditions. And quite frankly, sometimes it depends just like an accident, whatever happens happens. You never know how the atom might actually split. It depends on so many different conditions that we can just say it's random. In any case, there are different fission reactions based on only one particular thing, how uranium 235 absorbs the neutron and then becomes so agitated that nucleus splits. And in all these cases, there are certain energy released. Now another element which which has exactly the same property is uranium, not uranium sorry, plutonium 239. So plutonium 239, I think it's 94. I don't remember, I think it's 94 protons. What's important is it actually has exactly the same quality absorbed. If it absorbs neutron, it becomes unstable and splits. And it also releases energy, etc., etc. Now everything is fine so far. And that was until people realized, physicists realized. Look, this is two neutrons. This is one. These are three. Now what happens if just having one particular nucleus absorbs one particular neutron? We produce two. These two might actually go to other nuclei which are nearby and these nuclei will absorb these two neutrons. Each one of them produces another two or three neutrons, depending on how they split. Etc. So we will have a chain reaction. Is it possible? Yes, just possible. Under what circumstances we will or will not get the chain reaction? Well let's just think about physics of this. You have a neutron which somehow goes into the mass of uranium 235. Now if it hits the nucleus, then nucleus will absorb it and split. But it might not hit a nucleus. Why? Because the atoms are, generally speaking, empty. So the boundaries of the atoms relative to the boundaries of the nuclei are extremely large, which means atom is practically empty. There are some electrons in orbits and the nucleus is in the middle of this and nucleus is very small relative to the size of the atom itself. So neutron can just pass. Now how to increase the probability of neutron to be absorbed by some nucleus? Well the more massive this mass of uranium 235 is the more probability that neutron on its way will hit some nucleus. So obviously the mass is important and it's not only with the primary neutron, it's also for all subsequent one. Because whenever you have one particular nucleus split, these two electrons are going into some random direction. So it all goes to all the different directions and again the more massive amount of uranium we have, or plutonium doesn't matter, the more massive we have, the more the probability is of chain reaction to start. Because after it starts, after the amount will be sufficient enough to create the mass where the neutron will definitely hit something, that is the mass which might actually start the chain reaction. The amount of mass which is necessary to start the chain reaction is called critical mass. Now it's very important in this case that the critical mass of uranium 235 is about 47 kilograms. The critical mass for plutonium is about 10 kilograms, much lighter. So we need less plutonium. Why? It's not such an easy question. It's something related to density probably and some other factors. I'm not sure myself, but that's the basically experimental fact. So that's why actually the whole physics of nuclei started with uranium, but then they realized that with plutonium they have better results with a less amount of plutonium. So that's very important and obviously it depends on not only this particular weight, but it also depends on purity because you see uranium 235 or plutonium 239 are not just pure. In nature we have uranium 238 primarily and only tiny fraction with this 235. So we need somehow to separate this. So we don't really necessarily have a pure uranium 235. Now if for example instead of this pure 235 we have 20 percent uranium 235 and the rest 80 percent is uranium 238. Now in in nature actually the percentage is much smaller than one percent, but if we are enriching this by basically separating 235 from 238, if we have reached the purity of at least 20 percent, then we need more mass of uranium to start critical to start chain reaction. So the critical mass of 20 percent uranium is about 400 kilograms, so we need much more. So that's why uranium which is basically extracted from Earth should be enriched, which means the percentage of 235 should be enriched, should be greater than whatever it is and it's just a fraction of a percent. Okay so we talked about critical mass. Now we are moving towards two different kind of approaches. Peaceful and not so peaceful usage of chain reaction. Now the not so peaceful chain reaction was the first actually, that's the bomb. So if we just don't do anything just have the critical mass of uranium somehow and let's say it's pure uranium or it's a plutonium, whatever it is, we have a sufficient amount, sufficient mass of this I would say nuclear fuel, uranium, plutonium, whatever it is. So if we have it and we just somehow hit one of the atoms with a neutron, it can start chain reaction and if we don't do anything chain reaction will just go and go and go. Amount of energy released will be huge as you understand because I was telling you about one gram of uranium releasing as much as 5000 gallons of gasoline burned. So imagine if we have something like 50 kilograms. So that would be the way how atomic bomb actually works. It's uncontrolled chain reaction. All you need is sufficient amount of fuel which means uranium or plutonium, sufficient means greater than the critical mass, depending on the purity obviously. And you need somehow to, the first only one actually, neutron which starts the chain reaction. So that was the first unfortunate and very, not very peaceful application of this particular process. And again we're using this mass defect, the difference in mass between this and this and Einstein's formula of converting mass into energy. So Einstein was not actually involved in creation of the atomic bomb, but he knew all the physicists actually in a so-called Manhattan project in Los Alamos in the United States of America. They came up with the first atomic bomb and he was kind of politically involved. He wrote a letter to President Roosevelt about German physicists also going to the same way. And if they were the first to develop the atomic bomb, that would be a completely different story in World War II. So that was a very important letter and that was the beginning of the Manhattan project, the result of which was the first atomic bomb. In the Soviet Union they were doing exactly the same thing, plus they were using a lot of developments which were done in the United States. The spies were working over time obviously, all this time. So the first atomic bomb was in America and soon after the Soviet Union had the same. And by the way, many German physicists after the war were cooperating with either United States or the Soviet Union to improve basically the process of creation of nuclear weapon. Now, that's a non-peaceful usage. How about the peaceful usage? Well, the peaceful usage is we have to somehow control these neutrons, so they don't really go uncontrollable. So how to do that? Well, very simply, if the mass is significant to create the critical mass, basically, but we will somehow take out the neutrons from the game. And what it's basically done is, this is the mass of, let's say, plutonium or uranium 235. If we will put something like graphite or I think boron rods inside this body of the uranium, this is uranium, and this is a graphite. This is a carbon. This carbon has a property of basically absorbing neutrons without any kind of reaction. So that deletes from these equations the neutrons, so they do not go further. And by putting these rods down or lifting them up, we can regulate the flow of the neutrons. And that's how the nuclear power station is working. So what's important is to check the temperature of the whole process here. And if the temperature is rising above certain limit, we will put the rods down and they will absorb extra neutrons and it will slow down the nuclear fission reaction. So that's how we control it. And obviously, the temperature is used to, let's say, heat up the steam and it goes to turbines and it develops electricity, etc., etc. And by the way, something like 75% of electricity in France is produced by atomic stations. In the United States, it's about 20% of electricity is produced by these nuclear power stations. And that's what it is, basically. So I do suggest you to read the notes for this lecture. There are some calculations with mass, etc., more detailed than I have just suggested. Plus, there are a couple of nice pictures about how this chain reaction goes. Other than that, thanks very much and good luck.