 உள்ளே ஆமநிள்ளை வணக்கம் என்னுடைய தெரியாக ஜாஜன்வஹ பிரியமாணிக்கு ஒப்படி. இல்லை வேண்டும் எதிப்படியே சார்விப்படியிற்குத் திழ wash. மாதைப்படி விருவய்யாளர மநெல்லை conspir. ஷத்தம் இனிமுடன் தொழிது மlerini நாடுத்துகாப்பின், சகரிந்தildeற்சி குழிலாக மாற்று ஒப்ப நினைப்பில் வீகிர்சியில் வந்திதால், செல்லாடைய YuESTRO immediately என்னருந்து போனை எல்லாம் சிந்திருட்டுச் செல்ல aromawaszero. மிக்க நினைப்பு போன disorders போன விடுட்டுமுகள் சந்திருட்டுச் சிந்திருக்குத் தன்மாணி தருமதி ஏற்பியில் மின்னொரு நூட்டுவாளிகள் ஆசிந்தரரு. ஆனாலும் ஆக்கும் ஆக்கக்idadோகு மருத்துடமே சாப்பி வணக்குகிடக்சு அனைசெய்மொல்ளியாக முதலப்பும் நடூங்கள் அண்ணும்.非常的 புரத்துத்தால்,抵ோச நடூங்களுக்கும் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல் சிக்கல of the different elements. Now just to reinforce the fact about isotopes, I thought I will just present you the isotopes of hydrogen. Hydrogen, you know what we know is one, which has one proton and one electron, so this called is a normal hydrogen. matches and deuterium or the heavy hydrogen as it called, it has 1 neutron added in the nucleus. When you go to Mixing Triction, we have 2 neutral added but the number of electrons and the number of protons remain the same which decides the chemical property. Here, you will be surprised, deuterium is not a radioactive one, but tritium is a beta குளுவுகள் வப்ப이어த்தணன் முணத்ணும் சென்றுவிட்டார்acingcheers சுளைகளை வேலை, மல பிரச்சியக்கல் எழும்தின Phzenia கணனolate பளந்தைக் கொண்டுகிறீர்கள். மிகவும் பரிந்தைகளை மற்றுக்கூடப் பங immunity சட்டிய பளக்கூட்டியげும் இந்த முடிநகருடன் அவ்வளவு நாட்கனைகள் யבהவை வHz நுகலில் தரிந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்திருந்த பிரோட்டான் சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித்து, சிறித் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் இல்லையில் குழிப்பி ஆசிருகனு நிகழ்சவிைச்சாரிக்கumpy புருீதா groundbreaking நீங்கள் சதல் என்பீரும் a புரப części குழிப்பி ஆசிருகனு நிகழ்சவி Y பற்று உண்மை காட்டிற்கைக் காட்டி விலகநதூரி அல்லவா? உண்டு இந்தக்டாய் அழகாக என்னுடைய பச்சல் வந்து, மெடம் குரி சுணம் வந்து இட்தம் கொன்றுவிட்டிற்கு, bad guys, அப்படியென்றால் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொல்லில் சொ விருந்து காத்திருக்கிறார்கள். இது விருந்து காத்திரியின் வருகிறார்கள். இது காத்திரியின் வருகிறார்கள். இந்த வ இயதற்காக நமங்களை மாசியதையாக வைத்துக்கொள்ளிலிருந்து அம்மாம்படிச் செய்யுங்கள். பிரகாதனம்ем இப்படி akray emails, இசைபுக்கும் பார்த்துக் கொண்டியாக ர tricks அப்புக்கும் பாரட்சனர்கள் வேண்டும், நேரம் செய்யிற்று report உங்களுக்கும் சரந்தன, மேல் ஆத்தில் மற்றுறு மற்றும் எடுத்த மேர் விடம்ணம் செய்துகொள்ளின் தொடங்கள் மேர்கள்வின் DooT நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக்க நம்பிக் ஆ equip பிள்ளis 175. இருப்பநிராட்சி, பதிர்தி அறிக்கு அந்த விட்டத்து? ஏதனை எதற்காக இருக்கிறாள இந்த வெண்ணாட்சிகள் எப்படியில், நீங்கில் ஏற்று ஏற்று அனி ஠ாடம் உள்ளவரையானவற்றை எல்லா வகு அவற்று கணவு நடர்வு வகு விதல் நடர்கிறாள? 0 0 8 9 8 6 and this mass defect whichever is there you know by the instance theory of relativity any mass can be converted energy and the energy will be mc squared so here this mass defect converted to energy form is that energy that would be released when z protons and a minus z neutrons are brought together to form a nucleus. Now if we are able to give the same energy to the nucleus that energy will be able to break the nucleus that is basically the neutrons and protons and this is what is what we are doing in the fission reaction and this energy equivalent of mass defect is what is term as the binding energy of the nucleus that is this is the energy which binds the neutrons and protons together just to repeat when z protons and a minus z neutrons are brought together there is an amount of energy needed to bring them together and now if we are able to give the same energy and able to break it then that is what we call as the fission it can it leads to the breaking of the atom. Now if we look at the velocity of light we know it is about 2.998 into 10 to the power of 10 centimeters per second and if we calculate the energy in uggs mass into this squared this should be c squared not so this is a squared term on the 8.980 of 20. Now if we convert from uggs to mvv this is what we get we got e energy in mvv binding energy is equal to 931 into the mass defect. And now let us consider u235 which is a very common fissile atom the mass is 235.1175 and the atomic number as you know is 92 so if we calculate the binding energy as per the formula which we saw it comes out to something like 7.35 mvv per nucleon so nucleon means a neutron plus proton so the binding energy for an atom of or a binding energy of per nucleus of uranium 235 is 7.35 mvv. Now let us have a look at the binding energy for different elements of different mass numbers. You see the curve starts somewhere around 5 mvv per nucleon it goes up and then more or less steady but it slides these tubes down. Here you have find something like a peak is somewhere and this is for iron and in reality iron is the most stable element to the left and the right are not that stable. In other words you require a very high binding energy energy you have to give to break an iron. Now these higher mass number elements what happens their binding energy is less whereas this side the binding energy is very much less so here everything would like to go to a stable state so elements on this side atoms would break mostly by fission and try to come this way and become stable whereas the lighter elements like helium or hydrogen or lithium they will get fused together to come to a higher level of binding energy and become more stable and so these elements can contribute to fusion these elements can contribute to fission so this is where the binding energy concept is useful to explain fission or a fusion. Now what is a fission reaction when a neutron is able to transfer an amount of energy which is more than the binding energy of that element or atom then the nucleus breaks because something more than the binding energy so it breaks and it breaks up into two lighter nuclei because in heavy one it has become into two lighter nuclei and this process of breaking of an atom by a neutron is called as fission it is fissioning breaking so that is what it is and this lighter nuclei which are formed they are called as either fission products or they also called as fission fragments. So if we take the which are the elements which are fissionable they are uranium 233 uranium 235 and plutonium 239 these are fissile fissionable by neutrons of all energies. Now one must note here out of these three uranium 235 alone is found in nature uranium 233 is not found in nature but is obtained by converting thorium 232 in a nuclear reactor similarly plutonium 239 again is a man-made one wherein uranium 238 absorbs a neutron and gives converted to plutonium 239 after certain radioactive elements decay so now these two are artificially produced fissile isotopes this is to give a pictures idea of what happens you see a neutron is hitting uranium 235 then it becomes uranium 236 atom and then this is unstable it results into barium and krypton and also it gives on an average about 2 to 3 neutrons besides the fission products it releases 2 to 3 neutrons. How much energy is getting released in fission if you look up we said uranium 235 splits up into two fission products fission product A and fission product B plus some energy. Now we saw that the mean binding energy of uranium 235 is about 7.35 MeV so we can say that 92 protons plus 143 neutrons is uranium 235 plus 235 into 7.35 MeV. Now the mass number of the fission products are in the normally in the range of 95 to 140 you can say half the that range 240 means around 120 so if you take a binding energy of a nucleon which is having a mass number of 120 it is about 8.5 MeV. So I can say this 92 protons plus 143 neutrons are giving fission products A and B plus 235 into 8.5 MeV. If you just take these expressions and subtract you get uranium 235 will give fission products A and B plus 200 MeV. In other words, in one fission of uranium to the tibia atom you get 200 million electron volts of energy. This is a quite good amount of energy. Now how this energy in what form it is 80% of this fission energy appears as a kinetic energy the fission fragments and this get manifested as the heat. We say heat produced in a fission reaction is this so 80% of that is getting converted to heat. Now the fission fragments if you take they you know that they are unstable they are not completely stable they take some more steps so they have stable after some beta decay and it goes through a chain of reactions. What is the distribution of this total energy of 200 MeV if you look up the fission fragments as I said is about 80% about 165 MeV. Then this fission neutrons we saw 3 neutrons 2 to 3 neutrons being generated they carry some energy with them then there are some instantaneous gammas which about 7 MeV. This beta particles which are emitted they carry about 7 MeV, gammas about 6 MeV and we have neutrinos about 10 MeV. This neutrinos are similar to electrons they have the same masses electrons but they do not have any charge. So this is another part of an atom. Now we saw that neutron has interactions with nucleus of different atoms and in some cases it produces fission. In some other in all cases it does not produce fission then what must be happening maybe in some cases it may be getting just absorbed it may not really produce a breaking up or it may absorb or it may just hit it may scatter. So the processes are something like absorption and having fission or just absorption or it could be scattering. Now we must have a measure of how much ability to fission, how much ability to absorb, how much ability to scatter. So we use a terminology called as cross section. So interaction of these neutrons with different nuclei is denoted by the term cross section. So as I said we have got cross section for fission, cross section for absorption and scattering. Now the fission cross sections are again a function of the neutron energy. Here you can see neutron energy is put here in logarithmic cell 0.001, 1000 and 10 rest of 6. You see the cross section is high here and it comes down. What does this explain? When the energy of the neutron is low there is sufficient time for it to interact and you know transfer the energy. So the possibility of a fission reaction is more when the neutron is slow or having a low energy level. As it comes down, increases the neutron energy you see the cross section comes down. Then here there are some variations but if you come to the other side here this region beyond about 1 MeV is called as the fast neutron region. Here the cross sections are very low. This area is what is known as the intermediate region or the epithermal region. There are certain energy levels where there is a resonance between the hitting neutron and the atoms or the nucleus which are getting hit and under certain conditions they reinforce each other and the cross sections increase. So that is only a small region but major interest for us would be this and this. Not that fission is not possible, fission is possible here, possible here, possible here but the probability of fission is more here in the lower energy range and probability of fission here is less but fission is possible at all energies. Now reactors which operate with the slow energy neutrons are called thermal neutrons which operate in the fast neutron region are called fast neutrons. Now let us look, you have the fission, surely when neutrons are produced they have a very high energy, they get slowed down because of many other reactions. Now if I take a fission to happen in any material for that matter whether it is plutonium 239 or uranium 235 with a high probability I require a slow neutron. So what I do, I have some material which can absorb the energy of the neutron and then slow it down. So this process is called as neutron moderation. We moderate the neutron. If we look at which are the possible moderators, light water itself, all hydrogenous materials are very good moderators. Light water is a very good choice, it scatters the neutrons, the light water absorbs some energy, of course it does not cause fission there. And then the neutron is sent back with lesser energy. But as I said there are three types, it could be fission, it could be absorption, scattering. Absorption is more than the scattering, even though it does scatter, absorption is significant. The other one is the heavy water which is D2O. வாட்வாண்டைத்தை of this over light water is, it is having little absorption but a good scattering. So really speaking when you want to compare the moderating material, you try to compare which has got a better scattering by absorption. D2O, because absorption is going to be less, your ratio will be high. That is also sometimes referred to as the moderating ratio. But the absorption of deuterium, of the neutron by deuterium, it does produce tritium and you know tritium is a beta emitter. So it is a bit hazardous. Then the other one is beryllium. Beryllium has got a low absorption and reasonably good scattering cross section. Again beryllium dust if it is inhaled, it can cause lung problems. But of course we are not going to get in the reactor and inhale the beryllium. Last but not the least, graphite. Again it is similar to beryllium. But one problem with graphite is it can catch fire. You know graphite is after a carbon, an electronic form of carbon. So it can catch fire. Now another problem is all neutrons may not get slowed down. Some fast neutrons may still get escaped and they can cause the segregation of the graphite items. And in some cases this fast neutron energy which gets into the graphite, it can accumulate. And this accumulation at a certain temperature or energy level, it is possible that it can come out as a spurt. And if the value of the energy is high, it can really catch fire in the graphite. This is actually called as Wigner effect based on the scientists who found out this effect. So we generally see to it that annealing of the graphite is done so that there is no stored energy possible. Now one more characteristic of this fission process is what we call as decay heat. It is also called as residual heat. That means we are operating the reactor with the fission reaction. Fission reactions are producing heat. Now we stop the fission reaction. How do you stop the fission reaction? We put a absorbing material, neutron absorbing material. So the fission reaction stops. No neutrons are coming out. Then, but still this fission products which have been produced in the fission, they continue to decay. And in the decay process they produce heat and this heat is not small. It is quite considerable. Giving an idea, it depends on how much the reactor is operated because the amount of time the reactor is operated shows how much of fission products accumulation is there and how much time after the shutdown, both these factors. So here if you look, let us say a reactor is operated for one week and after shutdown it will produce about 0.055 or about 5.5%. And slowly in one day it will come down to 3.5%. Suppose it has operated for a year it will be about 5.8% then about 0.66% here. So slowly it will come down. Now this means heat is continuing to be produced even after shutdown. So this aspect is very important to be recognized. You take a coal-fired plant, the moment you stop coal-firing there is no heat produced. The whole thing starts cooling whereas here this heat is very important. So we need to keep, have a coolant to remove heat even after the reactor is shut off. We saw that in one fission reaction, one atom goes and hits a nuclei of an uranium-235 and on average it produces about 2 to 3 neutrons. Now out of these 2 to 3 neutrons, some may get absorbed, some may get, you know some may cause fission and the absorption could be either in the fuel itself or it may get any structures. You know you have got lot of material, the materials may just absorb. So finally if one more fission reaction is to occur, one neutron must be less out of the, out of the three. So out of every fission reaction, one more neutron is able to be available for fission like that. So it is like a chain, chain of fission reaction, this is called as a chain reaction. And in every one, one fission, next fission, suppose you have 10 neutrons, 10 fissions, continues. If you have 100 neutrons, 100 fissions continue. So this way the chain reaction continues, sustained chain reaction as we say. This sort of a situation that means in each generation the same number of neutrons are causing fission. So this is called as the ratio is called as the multiplying factor and here it is 1. For a sustained reaction k is equal to 1 and k, this multiplication factor is defined as the neutron production from fission in one generation to the neutron absorption in the preceding generation. Now this is okay as a definition but in a real reactor leakages will be happening. So the effective multiplication factor needs to take the leakage into account. So when it multiplied by the leakage factors, that is what we called as the k effective, that is effective multiplication factor. Now I mentioned to you about a self-sustained reaction, chain reaction causing fission. When the reactor has got a self-sustained fission reaction, we call it as critical, reactor is critical. Unfortunately this name or terminology as critical does create a sense of fear in the minds of the people. But I always used to say when reactor becomes critical, we as nuclear scientists are very happy. But any person who reads the newspaper and says, oh reactor is critical, he gets disturbed. But anyway this is just a terminology. So if you say nuclear reactor is critical, don't get afraid, be happy. So in this condition the overall neutron population in each generation remains same. It is neither increasing, neither decreasing. But suppose let us say the neutron production is high, means absorption leakage was less, so it was high. Then your neutron population will increase in one from one generation to other. That we call it as a supercritical condition. That means the multiplying factor k effective is greater than 1. So it is called as supercritical and it is not correct to allow the reactor to become supercritical. We should control. So that's why the control element is required. Again alternatively the other, if suppose let us say the neutron absorption leakage or high neutron production is not that much, then there will be a decrease of the neutron population and in that case we refer to it as a subcritical condition. That means k effective is less than 1. So we saw three things. k effective is equal to 1 means it is a self-sustained chain reaction continuing. If k effective is greater than 1, that means the reactor power is going to increase because more neutrons are available now. And if k effective is less, the fusion reaction is coming down, that's what it means. But now one thing which normally people tend to confuse, criticality is referring only to your neutron balance. Everything is fine. Your reactor will be critical when it is producing say 5 megawatts, when it is producing 10 megawatts. Maybe when it is producing 10 megawatts, your neutron production, the number of neutrons interacting will be more. Let us say 5 neutrons are interacting at 5 megawatts about to be 10 neutrons. So 10 neutrons giving rise to another 10 neutrons, 30 neutrons, but then again 10 are available for the next mission, it continues. So here also k is 1, there also k was 1. So criticality should not be confused with power. It is referring to the neutron balanced condition. Now there is one term called as reactivity, which nuclear scientists and physicists use very frequently. So hence it is essential for us if you have to appreciate. Now reactivity is a measure of how much your reactor is away from the criticality condition. So in other words, whether it is becoming supercritical or whether it is becoming subcritical. So what is the measure of this? How do we get this reactivity? Let us look at, let us say there are n0 neutrons in the first generation. Then there will be n0 into k effective neutrons in the next generation. So what is the change? n0 into k effective minus n0. Now this, if I express it as a function of n0 into k effective, what I get? A quantity called as k effective minus 1 by k effective. This is called as reactivity. And believe it, it has no units, k effective has no units, it is just a factor. So this reactivity is a measure to tell whether the reactor is getting into supercritical or getting into subcritical. Now how does the reactivity change in a reactor? Is it always constant? What are the things which go and change this reactivity? Now essentially, let us take the fuel. Fuel is the one which is getting fission and releasing neutrons. Let us say you have a certain amount of fissile element concentration in the beginning and the reactor is operating for some time. As it operates for some time, your fissile neutron availability is becoming less. So even if you have neutrons for fission, number of fissile atoms available are less. So there is no fission. So fission will not happen. That means this effectively will not produce neutrons. So that means there will be a fall in the reactivity. Similarly, let us take temperature. Now initially, when you are starting the reactor, things are cool condition. As the reactor power goes up, your fuel temperature goes up. Now any material, when the temperature goes up, it expands. But the mass of the fissile atoms remain same. But what is happening, the amount of fissile atoms per unit volume has come down because it is now occupying more space. So in one unit volume, the number of fissile atoms available are less now. So this gives a negative reactivity effect. Similarly, let us say if some neutron-absorbering material falls into the core, like boron, it will absorb the neutron then also or in either material which has got a neutron absorption, this can cause a negative reactivity. Similarly, let us take control rod. When I said boron, you have the control rods made of boron or cadmium. When you insert the control rod into the core, it is going to absorb more and more neutrons. This is a negative effect. So to quantify all these, each effect, we try to define reactivity coefficients. For example, if moderator temperature changes and there is a reactivity change, we call this that as moderator temperature reactivity coefficient. Then fuel temperature increase. So fuel temperature reactivity coefficient. If it is a coolant, if it is a structure like that. So the net effect on the reactivity is taken by considering all these constituents. Let us look at it in a more systematic way. Now this is the reactor core and as I mentioned, any reactivity change, if there is a net reactivity, as long as it is a critical reactor, the reactivity is zero. But if there is a change, it goes to the positive or negative, then this reactor power will change. If the reactor power changes, the fuel temperature will change, the coolant temperature will change, the moderator temperature will change. And let us say the coolant leaks out, loss of coolant, then coolant voiding takes place. Again, that is another type of effect. So the net total of all these feedback reactivities with combined with your control rod decides what is the net reactivity to the reactor core. Now just to give you an idea, this coolant voiding was one effect which caused a net positive reactivity in the case of the Chernobyl accident. And the power really rose to very high levels. So one of the important things is to keep this void coefficient as slow as possible and preferably negative. The total under any circumstances should not form, become positive. So this has to be maintained. Now in the case of 3 mile island accident in USA also, there is a loss of coolant. Again, there is a fuel melting. So in both these cases, the effects were not very simply in one case it was over power whereas in this case it was shut down. It was only having the decay heat which could not be removed whereas here it was over power. We saw about the how many neutrons are emitted once a neutron is absorbed by a fissile nucleus. This number is a function of the energy of the neutrons and let us look at this number for the three fissile elements uranium-235, uranium-233 and plutonium-239. If you look here, uranium-233 appears to be the highest and uranium-235 low. Plutonium and uranium-235 are nearly having the same ratio. This ratio is actually called as eta. That is the number of neutrons emitted once a neutron is absorbed by a fissile nucleus. Now here this is the resonance region but in the fast region if you see plutonium-239 has the highest eta. Again followed by uranium-233 and uranium-235 is but now you look at here eta is very close to 2. Now what does this eta really tell us? Now we saw that more than 2 to around 2 to 3 neutrons per fission are released in a fission normally. Now out of this some may get absorbed in non-fissile, non-fission reactions and at least one must be available for fission. So suppose let us say eta was 2. If eta was 2, one will be available for absorption, one for fission. But if suppose you have more than 2, this excess neutron if we can get into an element like uranium-238 or thorium-232 then we can get plutonium-239 or uranium-233. This conversion of a fertile element into a fissile element is what is called as breeding. We breed. So that means we must choose a fuel which has got more and more eta. That means it has got lot of spare neutrons after fission for breeding. So if you look very obvious plutonium and that too in the fast neutron spectrum that is the reason why fast reactors, most of them use plutonium-239 and or convert uranium-238 to plutonium-239. So this is a very important concept of breeding. Now we saw in a graph, you just look at numbers. I have given two classifications. One is neutron energy less than 0.025 electron volts that we call as the slow neutrons or the thermal neutrons. In this uranium-233, the eta value is 2.29. Uranium-238 is 2.07 and plutonium-239 is 2.14. So really speaking, if you see, breeding is at all possible with uranium-233. Uranium-235 practically nil. There is very little left. But with fast neutrons, say more than 100 keV, you have a figure of eta of 2.31 for uranium-233, 2.1 for uranium-235 and 2.45 for uranium-239. So here you can see that breeding with plutonium-239 is a very good thing. And if I want to convert thorium-232 and use it uranium-233, then I can either work in the thermal reactor or a fast reactor. They are quite close. I could have thorium breeders and whereas uranium-238 breeders will be surely better possible only with the fast reactors. So in what is the fast reactors? In the fast reactors, the fission is by fast neutrons. So let us see. Fast neutrons fission is only by fast neutrons are going to cause fission in the fast reactors. But their probability of fission is low compared to a thermal reactor. So you have to have an increased amount of fissile elements beginning itself. What is after all the total reaction? The probability of a fission into the number of fissile elements. Probability of fission is less. You increase the number of fissile elements, then you have the same amount of reaction. So that is why fast reactors use enriched fuel or they use enriched uranium. They are enriched or more of fissile elements like plutonium. Now natural uranium if you look up contains only 0.7% of uranium-235 and 99.3% is uranium-235. If you suppose do not have fast reactors, what are we going to do with the uranium-238? We are just going to call it as a waste and not use it. But if I use it in a fast reactor, I can convert it to uranium-238 effectively into plutonium-239. And that is why effective utilization of the natural uranium resources is possible only with fast reactors. And in India we have limited natural uranium resources and it is very imperative on us that we should go for fast reactors. Regarding thorium, we saw that the eta value was less. When the eta value was less, the breeding potential is also less. So we in India are using at the first level plutonium-239 uranium-238 breeders so that we will produce more amount of fuel which can be used in a large number of reactors. So our nuclear base of the nuclear program will be able to increase faster. And we will just take up thorium in the next stage. Now in this lecture we have tried to cover the basics of the fission reaction. We also mentioned that the chance of fission is more with neutrons of lower energy or slow called as thermal neutrons. So you need to reduce the energy of the neutrons from its production. At that time the neutron energy is high so you have to moderate. Of course in a fast reactor moderators will not be there. The fission is only by the fast neutrons. Then we looked at the concept of criticality where the multiplication factor is 1. We also saw criticality refers only to the neutron balance doesn't have anything to do with the power. A reactor at any power is a critical state. Then we looked at the concept of decay heat production. Why this concept of decay heat production is very important? Even after the reactor shutdown heat continues to be produced and if this heat is not removed the fuel can melt. And fuel can fail and come out. That's what happened in the Three Mile Island and Fukushima. So that's why even when we are handling fuel after being utilized in the reactor we need to really have cooling. Then we saw the concept of reactivity. And how this reactivity changes based on the fuel, the coolant, the coolant void etc. Finally last but not the least we looked at how breeding happens in a nuclear reactor and why breeders are necessary for effective utilization of the resources of natural uranium. Thank you.