Cos'è la Fusione Nucleare? Possiamo usarla?

Hi guys, I'm Marco for friends il Cuso, welcome to a new episode of the physics that you do not expect. First of all, I remind you that if you do not want to miss the contents of the channel, you should subscribe and activate the bell, but the safest way is to subscribe to the newsletter. In the description you will find more information. And secondly, I have a message to make, but I refer you to the end of the video. Since nuclear fusion is starting to be talked about very often, and many rightly consider it an extremely interesting source of energy, even if at the moment it is still in the potential state, I decided to update an old video on the subject. And to do this I asked the engineer Carlo Carrelli, who works in England on the EUDEMO project, for help. Here in the description I leave you two laughs and two to present it to you. Today we will see how fusion works, why it happens and why. Fortunately and unfortunately it is not so easy to obtain it here on Earth. In future videos we will talk instead of the state, art and technology in this sector and we will probably also have a surprise or two, but for today, fusion. The term speaks for itself. Nuclear fusion is that process by which two nuclei merge together to give a larger nucleus, usually with the production of other fragments or radiations. But before falling into misunderstandings, in this video I will talk about the so-called nuclear fusion, hot, of processes that generally require very high temperatures to occur. If you are interested in knowing something about cold fusion, I refer you to a video from some time ago. Nuclear fusion is an extremely frequent process in the universe, since it is the main energy source of the stars, at least up to a certain point in their development. Our sun works thanks to the so-called proton-proton chain, but we will go deeper later. To understand why nuclear fusion occurs and why it interests us a lot as a source of energy, we have to take a step back and ask ourselves another question. Why do the nuclei stay together? Yes, because atomic nuclei, including radioactive ones, remain united for periods of time on average long enough, if we exclude the ultra-heavy isotopes that are artificially created in the particle accelerators and whose life is so short that it is doubtful sometimes of their very existence. Nuclei contain protons and neutrons. The first have a positive electric charge, so they repel each other for electrostatic repulsion, the second have no electric charge and therefore are indifferent to this particular interaction. But if a nucleus stays together, there must be some attractive force that exceeds that of electrostatic intensity. If you have followed the series on the Standard Model, you will already know the answer to our question. But in three words. Strong nuclear interaction. Protons and neutrons are made of quarks. Three each. Among these quarks there is a really very intense attractive force, the strong nuclear interaction . However, its range of action is very short, but large enough to make it operate even among nucleons. General term for protons and neutrons. So there is a force that keeps the nucleus together. Consequently, we can define a certain binding energy, called B, from the English Binding, which is given by the difference between the energy of the free components and the energy of the same in the bound system, that is to say the nucleus. I remind you that Z indicates the number of protons, N the number of neutrons, A is their sum, it is called the number of atomic mass, and that the number of electrons of an atom is typically equal to the number of protons. Often it is preferred to calculate not so much the total binding energy of a nucleus, but the binding energy per nucleon, simply obtained by dividing B by A. The progress of this average energy per nucleon is shown in this graph, which we should keep in mind at least in its general foundation. This graph should be read as follows. On the X axis you find the number of atomic mass A, while on the Y axis there is no average binding energy . Obviously in this graph there are not all isotopes, there would be too many, but there are enough to allow us to observe some things. First of all, the curve has a maximum, which corresponds to a number of atomic mass of about 60. Among the most common isotopes in this area we find iron-56, which is usually considered as the watershed between two areas, that of light nuclei, that of heavy nuclei. Before iron-56, the average energy increases as A increases and, moreover, increases very rapidly, even if with an important exception given by Helium-4, which has a higher value than the isotopes immediately nearby. After iron-56, the energy decreases with A, although rather slowly. Virtually all the nuclei in the second part of the graph have an average energy of about 8 mega electron volts per nucleon. This already suggests one thing to us. The higher the binding energy, the more favorable the existence of that nucleus will be. Remember that the binding energy is defined as the difference between the energy of the free components and the energy of the same once bound, and therefore the greater this difference, the more favorable the formation of the bound state will be. So, for light atoms, the fusion is a process that will increase the binding energy and will be favorable from an energetic point of view. For heavy atoms, on the other hand, it will be the fission that will lead to the same result. In both cases, the corner of the curve rises, pointing to the edge. And speaking of fusion, we see that the fusion of really light nuclei, like the isotopes of hydrogen, leads us to go up just the steepest part of the curve, which implies the release, in some way, of a huge amount of energy. Yet, if we look around, fusion is not exactly what we would define as a process common in our daily life. After all, it is a very rare and extremely difficult phenomenon to obtain. And luckily, because if the atoms of hydrogen and other elements such as oxygen, nitrogen and carbon preferred to melt their nuclei instead of bonding to form molecules such as water, sugars and a whole myriad of other fundamental molecules for life, the Earth would have a very different look. But why is it so difficult? Because for nuclear fusion to happen, it must overcome a couple of non-negligible obstacles. Electrons. It is the strength of Coulomb. The vast majority of atoms present on Earth are in neutral form, that is, as many electrons as protons. Even that minority portion of ionized atoms has lost at most one electron. And if we exclude hydrogen, this means that there are still others around the nucleus. When two atoms get close to each other, the first thing one sees of the other is the electrons. The wave functions of the valence electrons touch, interact and at this point, depending on a whole series of factors that need no further explanation, decide whether to share one or more electrons, or if an atom subtracts one from the other, or if the two atoms simply ignore each other. But in no case do the nuclei have the opportunity to get so close to allow the strong nuclear interaction to do its job and lead to nuclear fusion. And then we remove the neutrons, completely ionizing the atoms and making them interact in the plasma state. Note that the term plasma generically identifies a gas in which an unusually high number of atoms is ionized at least once, which makes it susceptible to magnetic fields among other things. But we here want to exaggerate. The atoms must be completely ionized. We need naked nuclei. But even so we have a problem. The nuclei are no longer neutral, but they have positive electric charge and therefore they repel each other by electrostatic repulsion. Coulomb's force has an infinite radius of action, unlike the strong nuclear interaction, which has a very short radius. And therefore the nuclei, to merge, must be very, very close. And these two conditions in the stars are achieved with temperatures and pressures beyond our daily experience. The nucleus of the sun reaches temperatures estimated at about 15 million degrees, and at these levels it makes no difference if we talk about degrees, centigrades or Kelvin, and pressures of the order of 260 billion times the atmospheric pressure at sea level. And we would like to take advantage of a phenomenon like this to produce energy? Yes, but no. Yes, we want to take advantage of nuclear fusion to produce energy, but not in these conditions. First of all, let's see how nuclear fusion works in the nucleus of the stars, at least the youngest ones like our sun. These stars consume hydrogen-1 to produce helium-4, with a process called chain-proton-proton. It is a relatively simple process. The nucleus of a young star is tremendously hot and the pressure is very high, as we said before. It is also mainly made up of hydrogen-1, the most common element in the universe. The nuclei of hydrogen-1 are simple protons. When two protons merge, they form a nucleus in which a proton quickly transforms into a neutron. This beta-plus decay leads to the formation of a nucleus of hydrogen-2, or deuterium, with the emission of a positron and an electron neutrino. If you don't remember why these particles are emitted, I suggest you watch my video on nuclear decay. Deuterium can merge with a new proton and this time the result is a nucleus of helium-3, which would be a stable nucleus, it simply has too much energy, so it decays producing a gamma radiation photon, but it does not change its shape. Two of these nuclei of helium-3 can finally merge, producing a nucleus of helium-4 and two protons. There are other possible chains, some involving heavier nuclei, such as CNO, which involves atoms of carbon, nitrogen and oxygen, but they are much more unlikely, at least in relatively small stars like the sun. No! There is one thing to say about this reaction. It is extremely unlikely that the first step, the fusion of two protons to produce deuterium, will take place. The probability that the impact section will take place, in fact, is very low. Despite the high temperatures and pressures, it is estimated that a proton can wait about 9000 years before being able to merge with a similar one. A time span completely negligible from the point of view of the life of a star. But we don't have time to waste. It would make no sense to use an improbable reaction even in extreme conditions like those in the nucleus of a star to produce energy here on Earth, we must choose a reaction with a due section greater, much greater, even if this should require extremely complicated reactors. The best reaction we can choose in terms of impact section, potentially extractable energy and relative ease of fuel recovery is the deuterium-tritium reaction. Instead of melting the most common isotope of hydrogen, let's choose the other two. Moreover, tritium is radioactive and has a rather short life span of about a dozen years, so we will have to produce it. But this reaction promises really well. And then it's really simple simple. The two nuclei merge together, producing an helium-4 nucleus and a neutron. Moreover, we will be able for the first time to produce helium continuously. Did you know that at the moment our only source of helium are the mines? The only problem is to obtain the conditions for this reaction. And it's not a trivial thing, since we need higher temperatures than the solar nucleus, because we won't be able to reach the same pressures. The nucleus of ITER will be heated up to something in the order of 150 million degrees, ten times hotter than the nucleus of the Sun. In this way, the kinetic energy of the nuclei will be such that they will be effectively counteracted by the fusion reaction. But wait a minute. Why, if the fusion reaction in the Sun takes place at about 15 million degrees, do we have to heat the plasma of ITER at about 150 million degrees? The answer takes the name of the triple product. The triple product is a parameter that allows us to evaluate the shock section of the reaction, that is, the probability that it will occur and how it varies depending on some parameters. It is called that because it is the product of three terms, temperature, density of the plasma and time of confinement of the plasma. Since, unfortunately, we cannot rely on the gravitational collapse of the gas in the reactor and we are actually forced to work at a very low density because the entire chamber is under vacuum rather than pushed, the only solution we have to obtain an interesting shock section is to work at a high temperature. This is because it is easier to heat a gas at 150 million degrees than to bring its density to various tens of grams per cubic centimeter. The density of the nucleus of the Sun is in fact 150 grams per cubic centimeter. Well, we have chosen our nuclear reaction that we will try to use to produce energy in the reactors. But how will we evaluate the efficiency of a reactor? When can we say we have a useful reactor? The reactor will be useful if the energy produced by the reaction will be enough to produce the plasma, to maintain the reaction, to maintain the system that supports the reaction and maybe advance a little bit to turn on at least a light bulb. For nuclear reactions we can define a parameter called Q-Value, pedisseque and horrendous translation of Q-Value in English, so you can't tell me I use too many Anglicisms. Ah, by the way, moment, it's a Latin word. Q-Value is defined as the ratio between the power produced by the fusion reaction and the power needed to keep the plasma at the correct temperature. Note that the amount of energy produced does not coincide with the amount of energy we can extract from the fusion itself. Part of that energy is unusable, for various reasons. For example, about 3.5 megaelectronvolts constitute the kinetic energy of the nuclei of Helium-4 or Alpha Particles and are not usable. But we'll be back soon. For this reason, even if the Q-Value of a reactor was equal to 1, it would still be necessary to provide energy from the outside to keep the plasma hot. It is estimated that the Q-Value of about 5 is reached, but to have an interesting reactor from an economic point of view, it is necessary to aim at values ​​not less than 15, or even better at 20. And speaking of the energy that remains in the Alpha Particles produced by the fusion, part of it is given to the plasma. So the fusion itself warms the plasma a bit. In theory, the deuteriotry reaction can arrive at a wonderful condition called, in jargon, ignition, in which the frequency of reactions is quite high enough to make the heat given by the Alpha Particles enough to keep the plasma hot. No longer having to provide energy from the outside for this purpose, the Q-Value in this case is worth more than infinite and would be an optimal condition, although technologically it is even more difficult to obtain compared to the objectives we have set ourselves. Be careful, Q-Value Infinite does not mean infinite energy, it just means that the reactor can self-sustain without the need to rotate some of the energy we extract to the heating system. In short, we can use, for other purposes, all the energy we can capture. But the reactor needs fuel, and it consumes it, but we will talk about it better in the video in which we will describe ITER. The fact that the fuel that is at the base of fusion reactors like ITER is widely available, more or less, does not make it an inexhaustible source of energy. It will be, we hope, an efficient production system, it will allow to produce huge amounts of energy, but I emphasize, not infinite. Infinite energy, alas, does not exist. And in a next video I will try to update you on the state of the art of nuclear fusion. When will we have the first reactors attached to the morse codes of the electric current? Well guys, this video ends here. If you liked it, thumbs up and share it with your friends. And now, the promised communication at the beginning of the video. Italia per il Nucleare, in collaboration with Associazione Italiano Nucleare, Associazione Nucleare Ragione, L'Avvocato dell'Atomo, and obviously us others, has decided to launch a national investigation into the perception of nuclear energy in our country. The survey will be commissioned by experts from the SVG company and will allow us to evaluate how the opinion of Italians on nuclear energy has changed 10 years from the last referendum on this form of energy. To do this, we are collecting the necessary funds and if you want to give us a hand, in the description you will find more information. Having said that, I remind you that on Docety you can find my introductory course on quantum mechanics and on Quiproquark the video lessons of physics for high school. Thanks again to Carlo Carrelli for the supervision, thanks to you for the viewing, I'll meet you at the next video and be sure, be curious, physics is where you don't expect it.