Nuclear fission

0:00- [Lecturer] An atomic bomb and a nuclear power plant works on the same basic principle, nuclear fission chain reactions. But what exactly is this? And more importantly, if the same thing is happening inside both a bomb and a nuclear reactor, then why doesn't a nuclear reactor just explode like a bomb?

0:18What's the difference? Well, let's find out. So what is nuclear fission? Well, the word fission means breaking. So nuclear fission is a nuclear reaction in which a heavy nucleus breaks into smaller nuclei. But how does it break exactly? Well, one way is for it to break spontaneously.

0:38It can happen all by itself without us having to do anything. But we usually call that radioactivity, or we sometimes also call it spontaneous fission. But when we usually say nuclear fission, we're talking about the ones where we break it by specifically bombarding it with a neutron.

0:55Think about it, this nucleus is already unstable. Now you add another neutron to it, it makes it more unstable, kind of like pushing it over the edge and then it breaks into smaller nuclei. And here when it breaks, you also end up getting a few neutrons. You get somewhere between one to three neutrons usually out.

1:13So let's take an example. If you take Uranium 235, an isotope of uranium, and if you bombarded with a neutron, then it can break into Strontium 94 and Xenon 140. We don't have to remember the numbers or anything, don't worry about it. But my question would be, can we predict how many neutrons we'll get over here?

1:35Well, we can. All we have to do, just like any nuclear reaction, is to keep track of protons and neutrons. So if I keep track of protons, let's see, I have 92 protons on the left hand side. How many protons do I have on the right hand side? Well, eight plus four is two, so 12.

1:48So five plus three. I get 92 over here. But what about the total number of particles? Well, I have 235 plus one that is 230... Oops, that is 236 on the left hand side. But over here, 94 plus 140. So I get four. Nine plus four is 13. So one carry over, I get 234.

2:08So there are only 234 particles over here, which means two particles must have been released. And these must be two neutrons because we've already accounted for all the protons. So that's how I know that there must be two neutrons released over here. But you know what's cool about nuclear fission reactions?

2:25For the same reactants, you could get completely different products altogether. For example, if we take another uranium 235 and bombard it with another neutron, look exactly the same reactance, but this time you might get completely different products. You might get Barium 141 and say Krypton 92.

2:44Again, we'll get some amount of neutrons, when you pause the video over here and try it yourself to figure out how many number of neutrons we should be getting here. Alright, again, we can see the number of protons is balanced. You have 56 plus 36 is 92. But how many total particles we have?

3:01We have 236 here again, this time we have one plus 2, 3, 14 plus nine is 23. So you get 233, which means look, three particles are missing. So this time we'll get three neutrons. And just like with the fusion reactions, we will see even here, some energy is released and energy is released usually as kinetic energy of the products and the neutrons.

3:26And because energy is released and remember that energy and mass are equivalent, we will find that the mass of the products will be smaller than the mass of the reactants. And just by figuring out the difference in the mass, you can figure out how much energy was released.

3:43That difference in the mass is basically what got released as energy. Again, something that we've seen before in the nuclear fusion reactions, very similar. Now, can any heavy nucleus give you fission reactions? No, that can't happen. The ones that do, we call them fissile nuclei.

3:58So uranium 235 is fissile because it does undergo fission reaction and gives you energy. But if you consider another isotope of uranium, which is say Uranium 92, 238, turns out it is non-fissile. It does not undergo nuclear fission easily. And if you're wondering why certain nuclei are fissile and others are not, well, it has something to do with energy and stability.

4:29Well, turns out for uranium, when it undergoes fission, you end up getting more stable products and therefore energy is released. Turns out that's not the case for Uranium 238, or at least that's not very easy to happen. But of course we'll not dive too much into it.

4:43But a big question now we could ask ourselves is how much energy do we get out of it? Well, if you look at a single reaction, of course we'll get a tiny amount of energy. But if you want to get usable amount, then we will require lots and lots of reactions. But how do we do that practically?

5:00Because nuclear fission requires you to bombard a nucleus with neutron. So how do we ensure we get lots and lots of reactions like this? Well, the answer is right in front of us. Since each nuclear fission reaction gives us a few neutrons, if we can ensure that these neutrons go and hit other uranium 235 atoms, nuclei, sorry, then they will again undergo fission and give you more neutrons and each cause even more fission reaction.

5:29Here's the way we can show that. So let me just go to the next page. Here we go. So if you have one neutron that bombards with a uranium 235 giving you energy, fission reaction, giving you energy and some neutrons. Now if these neutrons could go and hit even more of these urine 235, then you'll get even more energy and this thing can keep on going and you can see very quickly this will keep increasing.

5:56You'll have one fission, then you have three fission, and then you'll have nine and so on and so forth. So the amount of fission happening per second would just keep increasing. This is what we call a chain reaction. Nuclear chain reactions can be quite devastating.

6:12You start with very few reactions per second, but very quickly, very rapidly, that number increases. And within a short amount of time, you can release tremendous amount of energy. That is the whole idea behind atomic bombs. What makes atomic bombs so much more devastating compared to traditional regular bombs is that we are dealing with nuclear energy, which is hoarders of magnitude higher than the chemical energy that we get from traditional bombs.

6:38So a small amount of fissile material can give you a lot of energy, but that's not it. That's not it. You see, the products of nuclear fission reactions are usually radioactive, which means even after the explosion is done, the whole area is contaminated with radioactive isotopes now, which can further cause damage for ears to come, making that whole area inhabitable.

7:00So yeah, atomic bombs are really destructive. But on the flip side, if you're using this to generate electricity, let's say, then we'll get way more energy compared to what we get from fossil fuels. Because again, there we are dealing with chemical energy. And of course, another advantage of using nuclear energy is that in fossil fuels, because you're using combustion reactions, there is CO2 that is released into the atmosphere.

7:23None of that happens over here. But now this brings us to the original question. How do we use chain reactions in nuclear power reactors to generate electricity? Wouldn't they just explore just like an atomic bomb? So what's the big difference? Well, the big difference is over here, when it comes to bombs, we are using uncontrolled chain reaction.

7:42Whatever we just saw right now, it's a about uncontrolled chain reaction. But when it comes to power... When it comes to nuclear reactors, we use controlled chain reactions. How do you control chain reactions, you ask? Well, one of the most common ways is by absorbing a lot of neutrons.

7:58So imagine we absorbed a lot of neutrons like this. Then look, by absorbing neutrons, you are controlling how many further fission reactions are happening. This way you can control it, you can ensure that the energy is released in a steady rate. And that's how you can get controlled chain reaction.

8:17But there's another major difference. Remember how we said earlier that uranium 238 is non fissile? Well, it turns out if you take a uranium ore then most of it is actually uranium 238. That means you cannot directly use a uranium ore either as a bomb or as a fuel for nuclear power plant.

8:35This means we have to take it through a process where we increase the amount of fissile material. And this process is called enrichment. And the big difference is if you're using a fuel for... You're using it for a bomb, then we would want a lot of enrichment.

8:51In fact, we'd want about 90% enriched. And that makes sense because you would want as many fission reactions happening as possible per second so that the whole thing explodes immediately. But when it comes to nuclear reactors, nuclear power plants, you see we have only about three to 5% enrichment.

9:06That means a single Uranium 235 is surrounded by a lot of non-fissile materials. That's why you will... That's why the nuclear fuel will not explode like a bomb because it's not enriched as much as you would need for a bomb. So anyways, by using controlled chain reaction, we get energy as the kinetic energy of these products, which is then used to heat up water.

9:29And then the process is very similar to how any other power plant works. The heated water produces high pressure steam that turns turbines, and that's how you eventually get electricity. And then that hot steam is cooled in a cooling tower. And in the process a lot of water vapor is produced and that is released over here.

9:50I'm mentioning this because I used to think that this itself was a nuclear reactor and it was producing a lot of smoke, radioactive smoke, which could be dangerous because it's going into the atmosphere. But none of that 'cause first of all, this is just a cooling tower, and what is it releasing is water vapor.

10:05And that water never comes in contact with any of the radioactive material that you have over here. So it's not dangerous, but there will be radioactive products left over, radioactive waste inside the nuclear power plants, and that needs to be safely disposed.

10:21And that is a challenge that scientists and engineers are actively working on today.