0:00:00.360,0:00:01.800 - [Lecturer] An atomic bomb 0:00:01.800,0:00:03.330 and a nuclear power plant 0:00:03.330,0:00:05.640 works on the same basic principle, 0:00:05.640,0:00:07.770 nuclear fission chain reactions. 0:00:07.770,0:00:09.810 But what exactly is this? 0:00:09.810,0:00:11.280 And more importantly, 0:00:11.280,0:00:13.650 if the same thing is[br]happening inside both a bomb 0:00:13.650,0:00:15.030 and a nuclear reactor, 0:00:15.030,0:00:18.150 then why doesn't a nuclear[br]reactor just explode like a bomb? 0:00:18.150,0:00:19.950 What's the difference? 0:00:19.950,0:00:21.840 Well, let's find out. 0:00:21.840,0:00:24.090 So what is nuclear fission? 0:00:24.090,0:00:26.220 Well, the word fission means breaking. 0:00:26.220,0:00:29.100 So nuclear fission is a nuclear reaction 0:00:29.100,0:00:33.510 in which a heavy nucleus[br]breaks into smaller nuclei. 0:00:33.510,0:00:35.400 But how does it break exactly? 0:00:35.400,0:00:38.010 Well, one way is for it[br]to break spontaneously. 0:00:38.010,0:00:39.570 It can happen all by itself 0:00:39.570,0:00:41.280 without us having to do anything. 0:00:41.280,0:00:43.260 But we usually call that radioactivity, 0:00:43.260,0:00:46.230 or we sometimes also call[br]it spontaneous fission. 0:00:46.230,0:00:48.420 But when we usually say nuclear fission, 0:00:48.420,0:00:50.460 we're talking about the[br]ones where we break it 0:00:50.460,0:00:55.050 by specifically bombarding[br]it with a neutron. 0:00:55.050,0:00:58.290 Think about it, this[br]nucleus is already unstable. 0:00:58.290,0:01:00.690 Now you add another neutron to it, 0:01:00.690,0:01:02.040 it makes it more unstable, 0:01:02.040,0:01:03.540 kind of like pushing it over the edge 0:01:03.540,0:01:06.240 and then it breaks into smaller nuclei. 0:01:06.240,0:01:07.980 And here when it breaks, 0:01:07.980,0:01:10.350 you also end up getting a few neutrons. 0:01:10.350,0:01:11.183 You get somewhere between 0:01:11.183,0:01:13.200 one to three neutrons usually out. 0:01:13.200,0:01:14.850 So let's take an example. 0:01:14.850,0:01:19.850 If you take Uranium 235,[br]an isotope of uranium, 0:01:20.070,0:01:22.680 and if you bombarded with a neutron, 0:01:22.680,0:01:27.680 then it can break into[br]Strontium 94 and Xenon 140. 0:01:27.930,0:01:30.150 We don't have to remember[br]the numbers or anything, 0:01:30.150,0:01:31.230 don't worry about it. 0:01:31.230,0:01:32.910 But my question would be, can we predict 0:01:32.910,0:01:35.040 how many neutrons we'll get over here? 0:01:35.040,0:01:35.910 Well, we can. 0:01:35.910,0:01:36.750 All we have to do, 0:01:36.750,0:01:38.100 just like any nuclear reaction, 0:01:38.100,0:01:40.200 is to keep track of protons and neutrons. 0:01:40.200,0:01:41.880 So if I keep track of protons, let's see, 0:01:41.880,0:01:44.340 I have 92 protons on the left hand side. 0:01:44.340,0:01:46.200 How many protons do I have[br]on the right hand side? 0:01:46.200,0:01:48.117 Well, eight plus four is two, so 12. 0:01:48.117,0:01:49.080 So five plus three. 0:01:49.080,0:01:50.433 I get 92 over here. 0:01:51.420,0:01:53.280 But what about the total[br]number of particles? 0:01:53.280,0:01:56.640 Well, I have 235 plus one that is 230... 0:01:56.640,0:02:00.750 Oops, that is 236 on the left hand side. 0:02:00.750,0:02:02.880 But over here, 94 plus 140. 0:02:02.880,0:02:04.590 So I get four. 0:02:04.590,0:02:06.120 Nine plus four is 13. 0:02:06.120,0:02:08.640 So one carry over, I get 234. 0:02:08.640,0:02:11.070 So there are only 234 particles over here, 0:02:11.070,0:02:13.440 which means two particles[br]must have been released. 0:02:13.440,0:02:15.180 And these must be two neutrons 0:02:15.180,0:02:17.850 because we've already[br]accounted for all the protons. 0:02:17.850,0:02:19.050 So that's how I know 0:02:19.050,0:02:23.280 that there must be two[br]neutrons released over here. 0:02:23.280,0:02:25.500 But you know what's cool about[br]nuclear fission reactions? 0:02:25.500,0:02:26.700 For the same reactants, 0:02:26.700,0:02:29.430 you could get completely[br]different products altogether. 0:02:29.430,0:02:32.940 For example, if we take[br]another uranium 235 0:02:32.940,0:02:34.410 and bombard it with another neutron, 0:02:34.410,0:02:36.930 look exactly the same reactance, 0:02:36.930,0:02:40.320 but this time you might get[br]completely different products. 0:02:40.320,0:02:44.280 You might get Barium[br]141 and say Krypton 92. 0:02:44.280,0:02:46.800 Again, we'll get some amount of neutrons, 0:02:46.800,0:02:48.090 when you pause the video over here 0:02:48.090,0:02:49.650 and try it yourself to figure out 0:02:49.650,0:02:52.410 how many number of neutrons[br]we should be getting here. 0:02:52.410,0:02:54.000 Alright, again, we can see 0:02:54.000,0:02:56.550 the number of protons is balanced. 0:02:56.550,0:02:59.490 You have 56 plus 36 is 92. 0:02:59.490,0:03:01.350 But how many total particles we have? 0:03:01.350,0:03:03.570 We have 236 here again, 0:03:03.570,0:03:05.494 this time we have one plus 2, 3, 0:03:05.494,0:03:07.280 14 plus nine is 23. 0:03:07.280,0:03:09.810 So you get 233, which means look, 0:03:09.810,0:03:11.670 three particles are missing. 0:03:11.670,0:03:15.630 So this time we'll get three neutrons. 0:03:15.630,0:03:17.700 And just like with the fusion reactions, 0:03:17.700,0:03:21.060 we will see even here,[br]some energy is released 0:03:21.060,0:03:24.030 and energy is released[br]usually as kinetic energy 0:03:24.030,0:03:26.460 of the products and the neutrons. 0:03:26.460,0:03:28.890 And because energy is released 0:03:28.890,0:03:31.680 and remember that energy[br]and mass are equivalent, 0:03:31.680,0:03:34.500 we will find that the mass of the products 0:03:34.500,0:03:38.370 will be smaller than the[br]mass of the reactants. 0:03:38.370,0:03:41.760 And just by figuring out[br]the difference in the mass, 0:03:41.760,0:03:43.800 you can figure out how[br]much energy was released. 0:03:43.800,0:03:45.360 That difference in the mass is basically 0:03:45.360,0:03:46.740 what got released as energy. 0:03:46.740,0:03:48.330 Again, something that we've seen before 0:03:48.330,0:03:50.520 in the nuclear fusion[br]reactions, very similar. 0:03:50.520,0:03:53.790 Now, can any heavy nucleus[br]give you fission reactions? 0:03:53.790,0:03:55.860 No, that can't happen. 0:03:55.860,0:03:58.605 The ones that do, we[br]call them fissile nuclei. 0:03:58.605,0:04:01.230 So uranium 235 is fissile 0:04:01.230,0:04:03.390 because it does undergo fission reaction 0:04:03.390,0:04:05.430 and gives you energy. 0:04:05.430,0:04:08.970 But if you consider[br]another isotope of uranium, 0:04:08.970,0:04:13.920 which is say Uranium 92, 238, 0:04:13.920,0:04:16.593 turns out it is non-fissile. 0:04:18.030,0:04:21.510 It does not undergo[br]nuclear fission easily. 0:04:21.510,0:04:23.100 And if you're wondering why certain nuclei 0:04:23.100,0:04:26.640 are fissile and others are not, 0:04:26.640,0:04:27.630 well, it has something to do 0:04:27.630,0:04:29.460 with energy and stability. 0:04:29.460,0:04:31.320 Well, turns out for uranium, 0:04:31.320,0:04:32.850 when it undergoes fission, 0:04:32.850,0:04:34.710 you end up getting more stable products 0:04:34.710,0:04:36.630 and therefore energy is released. 0:04:36.630,0:04:39.180 Turns out that's not the[br]case for Uranium 238, 0:04:39.180,0:04:41.100 or at least that's not[br]very easy to happen. 0:04:41.100,0:04:43.350 But of course we'll not[br]dive too much into it. 0:04:43.350,0:04:45.390 But a big question now[br]we could ask ourselves is 0:04:45.390,0:04:47.670 how much energy do we get out of it? 0:04:47.670,0:04:50.100 Well, if you look at a single reaction, 0:04:50.100,0:04:52.590 of course we'll get a[br]tiny amount of energy. 0:04:52.590,0:04:55.290 But if you want to get usable amount, 0:04:55.290,0:04:58.680 then we will require lots[br]and lots of reactions. 0:04:58.680,0:05:00.870 But how do we do that practically? 0:05:00.870,0:05:02.730 Because nuclear fission 0:05:02.730,0:05:07.620 requires you to bombard[br]a nucleus with neutron. 0:05:07.620,0:05:09.390 So how do we ensure we get lots 0:05:09.390,0:05:11.730 and lots of reactions like this? 0:05:11.730,0:05:13.980 Well, the answer is right in front of us. 0:05:13.980,0:05:15.690 Since each nuclear fission reaction 0:05:15.690,0:05:17.400 gives us a few neutrons, 0:05:17.400,0:05:19.230 if we can ensure that these neutrons 0:05:19.230,0:05:23.400 go and hit other uranium[br]235 atoms, nuclei, sorry, 0:05:23.400,0:05:25.560 then they will again undergo fission 0:05:25.560,0:05:26.970 and give you more neutrons 0:05:26.970,0:05:29.340 and each cause even more fission reaction. 0:05:29.340,0:05:30.780 Here's the way we can show that. 0:05:30.780,0:05:33.300 So let me just go to the next page. 0:05:33.300,0:05:34.290 Here we go. 0:05:34.290,0:05:36.600 So if you have one neutron that bombards 0:05:36.600,0:05:40.770 with a uranium 235 giving[br]you energy, fission reaction, 0:05:40.770,0:05:42.690 giving you energy and some neutrons. 0:05:42.690,0:05:44.970 Now if these neutrons could go 0:05:44.970,0:05:49.230 and hit even more of these urine 235, 0:05:49.230,0:05:51.180 then you'll get even more energy 0:05:51.180,0:05:53.065 and this thing can keep on going 0:05:53.065,0:05:54.872 and you can see very quickly 0:05:54.872,0:05:56.730 this will keep increasing. 0:05:56.730,0:05:59.310 You'll have one fission,[br]then you have three fission, 0:05:59.310,0:06:01.620 and then you'll have nine[br]and so on and so forth. 0:06:01.620,0:06:03.120 So the amount of fission[br]happening per second 0:06:03.120,0:06:04.680 would just keep increasing. 0:06:04.680,0:06:09.420 This is what we call a chain reaction. 0:06:09.420,0:06:12.090 Nuclear chain reactions[br]can be quite devastating. 0:06:12.090,0:06:14.220 You start with very few[br]reactions per second, 0:06:14.220,0:06:17.940 but very quickly, very[br]rapidly, that number increases. 0:06:17.940,0:06:19.680 And within a short amount of time, 0:06:19.680,0:06:22.890 you can release tremendous[br]amount of energy. 0:06:22.890,0:06:27.390 That is the whole idea[br]behind atomic bombs. 0:06:27.390,0:06:29.910 What makes atomic bombs[br]so much more devastating 0:06:29.910,0:06:32.010 compared to traditional regular bombs 0:06:32.010,0:06:33.990 is that we are dealing[br]with nuclear energy, 0:06:33.990,0:06:35.700 which is hoarders of magnitude higher 0:06:35.700,0:06:37.320 than the chemical energy 0:06:37.320,0:06:38.700 that we get from traditional bombs. 0:06:38.700,0:06:40.590 So a small amount of fissile material 0:06:40.590,0:06:44.880 can give you a lot of[br]energy, but that's not it. 0:06:44.880,0:06:45.780 That's not it. 0:06:45.780,0:06:47.730 You see, the products of[br]nuclear fission reactions 0:06:47.730,0:06:50.100 are usually radioactive, 0:06:50.100,0:06:52.590 which means even after[br]the explosion is done, 0:06:52.590,0:06:54.360 the whole area is contaminated 0:06:54.360,0:06:56.070 with radioactive isotopes now, 0:06:56.070,0:06:58.530 which can further cause[br]damage for ears to come, 0:06:58.530,0:07:00.480 making that whole area inhabitable. 0:07:00.480,0:07:03.480 So yeah, atomic bombs[br]are really destructive. 0:07:03.480,0:07:05.370 But on the flip side, if you're using this 0:07:05.370,0:07:07.170 to generate electricity, let's say, 0:07:07.170,0:07:09.300 then we'll get way more energy 0:07:09.300,0:07:11.400 compared to what we get from fossil fuels. 0:07:11.400,0:07:13.770 Because again, there we are[br]dealing with chemical energy. 0:07:13.770,0:07:16.410 And of course, another advantage[br]of using nuclear energy 0:07:16.410,0:07:18.300 is that in fossil fuels, 0:07:18.300,0:07:20.160 because you're using combustion reactions, 0:07:20.160,0:07:23.130 there is CO2 that is[br]released into the atmosphere. 0:07:23.130,0:07:24.480 None of that happens over here. 0:07:24.480,0:07:26.490 But now this brings us[br]to the original question. 0:07:26.490,0:07:28.320 How do we use chain reactions 0:07:28.320,0:07:29.850 in nuclear power reactors 0:07:29.850,0:07:31.080 to generate electricity? 0:07:31.080,0:07:34.320 Wouldn't they just explore[br]just like an atomic bomb? 0:07:34.320,0:07:36.270 So what's the big difference? 0:07:36.270,0:07:39.480 Well, the big difference is over here, 0:07:39.480,0:07:40.620 when it comes to bombs, 0:07:40.620,0:07:42.420 we are using uncontrolled chain reaction. 0:07:42.420,0:07:44.340 Whatever we just saw right now, 0:07:44.340,0:07:46.440 it's a about uncontrolled chain reaction. 0:07:46.440,0:07:48.090 But when it comes to power... 0:07:48.090,0:07:49.650 When it comes to nuclear reactors, 0:07:49.650,0:07:51.840 we use controlled chain reactions. 0:07:51.840,0:07:54.000 How do you control chain[br]reactions, you ask? 0:07:54.000,0:07:55.710 Well, one of the most common ways 0:07:55.710,0:07:58.500 is by absorbing a lot of neutrons. 0:07:58.500,0:08:01.380 So imagine we absorbed a[br]lot of neutrons like this. 0:08:01.380,0:08:03.420 Then look, by absorbing neutrons, 0:08:03.420,0:08:05.580 you are controlling how many further 0:08:05.580,0:08:07.230 fission reactions are happening. 0:08:07.230,0:08:09.360 This way you can control it, 0:08:09.360,0:08:13.860 you can ensure that the energy[br]is released in a steady rate. 0:08:13.860,0:08:17.670 And that's how you can get[br]controlled chain reaction. 0:08:17.670,0:08:19.380 But there's another major difference. 0:08:19.380,0:08:20.580 Remember how we said earlier 0:08:20.580,0:08:23.280 that uranium 238 is non fissile? 0:08:23.280,0:08:25.500 Well, it turns out if[br]you take a uranium ore 0:08:25.500,0:08:28.293 then most of it is actually uranium 238. 0:08:29.130,0:08:31.950 That means you cannot[br]directly use a uranium ore 0:08:31.950,0:08:35.400 either as a bomb or as a[br]fuel for nuclear power plant. 0:08:35.400,0:08:37.500 This means we have to[br]take it through a process 0:08:37.500,0:08:40.440 where we increase the[br]amount of fissile material. 0:08:40.440,0:08:42.540 And this process is called enrichment. 0:08:42.540,0:08:44.100 And the big difference is 0:08:44.100,0:08:46.680 if you're using a fuel for... 0:08:46.680,0:08:48.570 You're using it for a bomb, 0:08:48.570,0:08:51.240 then we would want a lot of enrichment. 0:08:51.240,0:08:53.940 In fact, we'd want about 90% enriched. 0:08:53.940,0:08:54.870 And that makes sense 0:08:54.870,0:08:56.527 because you would want 0:08:56.527,0:08:57.360 as many fission reactions happening 0:08:57.360,0:08:58.770 as possible per second 0:08:58.770,0:09:01.410 so that the whole thing[br]explodes immediately. 0:09:01.410,0:09:03.210 But when it comes to nuclear reactors, 0:09:03.210,0:09:04.380 nuclear power plants, 0:09:04.380,0:09:06.930 you see we have only about[br]three to 5% enrichment. 0:09:06.930,0:09:08.760 That means a single Uranium 235 0:09:08.760,0:09:11.970 is surrounded by a lot[br]of non-fissile materials. 0:09:11.970,0:09:13.598 That's why you will... 0:09:13.598,0:09:18.180 That's why the nuclear fuel[br]will not explode like a bomb 0:09:18.180,0:09:20.190 because it's not enriched as much 0:09:20.190,0:09:21.600 as you would need for a bomb. 0:09:21.600,0:09:24.630 So anyways, by using[br]controlled chain reaction, 0:09:24.630,0:09:27.960 we get energy as the kinetic[br]energy of these products, 0:09:27.960,0:09:29.820 which is then used to heat up water. 0:09:29.820,0:09:31.860 And then the process is very similar 0:09:31.860,0:09:33.960 to how any other power plant works. 0:09:33.960,0:09:37.110 The heated water produces[br]high pressure steam 0:09:37.110,0:09:38.730 that turns turbines, 0:09:38.730,0:09:40.950 and that's how you[br]eventually get electricity. 0:09:40.950,0:09:42.750 And then that hot steam 0:09:42.750,0:09:45.150 is cooled in a cooling tower. 0:09:45.150,0:09:48.300 And in the process a lot[br]of water vapor is produced 0:09:48.300,0:09:50.010 and that is released over here. 0:09:50.010,0:09:52.050 I'm mentioning this[br]because I used to think 0:09:52.050,0:09:54.270 that this itself was a nuclear reactor 0:09:54.270,0:09:56.340 and it was producing a lot of smoke, 0:09:56.340,0:09:58.800 radioactive smoke,[br]which could be dangerous 0:09:58.800,0:10:00.240 because it's going into the atmosphere. 0:10:00.240,0:10:01.920 But none of that 'cause first of all, 0:10:01.920,0:10:03.270 this is just a cooling tower, 0:10:03.270,0:10:05.430 and what is it releasing is water vapor. 0:10:05.430,0:10:07.200 And that water never comes in contact 0:10:07.200,0:10:09.060 with any of the radioactive material 0:10:09.060,0:10:10.050 that you have over here. 0:10:10.050,0:10:11.220 So it's not dangerous, 0:10:11.220,0:10:15.480 but there will be radioactive[br]products left over, 0:10:15.480,0:10:18.660 radioactive waste inside[br]the nuclear power plants, 0:10:18.660,0:10:21.000 and that needs to be safely disposed. 0:10:21.000,0:10:22.950 And that is a challenge that scientists 0:10:22.950,0:10:25.683 and engineers are[br]actively working on today.