Nuclear fission | Physics | Khan Academy

An atomic bomb and a nuclear power plant work on the same basic principle: nuclear fusion chain reactions. But what exactly is this? More importantly, if the same thing is happening inside both a bomb and a nuclear reactor, then why doesn't the nuclear reactor just explode like a bomb? What's the difference? Well, let's find out.

So, what is nuclear fission? Well, the word fission means breaking. Nuclear fission is a nuclear reaction in which a heavy nucleus breaks into smaller nuclei. But how does it break exactly? One way is for it to break spontaneously; it 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. However, when we usually say nuclear fission, we're talking about the instances where we break it by specifically bombarding it with a neutron.

Think about it: this nucleus is already unstable. Now, you add another neutron onto it; it makes it more unstable, kind of like pushing it over the edge, and then it breaks into smaller nuclei. When it breaks, you also end up getting a few neutrons—usually somewhere between 1 to 3 neutrons.

Let's take an example. If you take uranium-235, an isotope of uranium, and if you bombard it with a neutron, then it can break into strontium-94 and xenon-140. We don’t have to remember the names; the numbers are what matter. My question would be, can we predict how many neutrons we'll get over here? Well, we can! All we have to do, just like any nuclear reaction, is to keep track of protons and neutrons.

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, 94 + 40 gives me 134; hold on, okay, yeah, I get 92 over here. Okay, but what about the total number of particles? I have 235 + 1; that is 236 on the left-hand side. But over here, 94 + 140 gives me 234, so there are only 234 particles over here. This means two particles must have been released, and these must be two neutrons because we already accounted for all the protons.

That’s how I know there must be two neutrons released over here. You know what's cool about nuclear fission reactions? For the same reactants, you could get completely different products. For example, if we take another uranium-235 and bombard it with another neutron, we would see completely different products. You might get barium-141 and say krypton-92.

Again, we'll get some neutrons. Pause the video here and try it yourself to figure out how many neutrons we should be getting. Alright, again we can see the number of protons is balanced; you have 56 + 36, which is 92. But how many total particles do we have? We have 236 here; again this time we have 1 + 2 + 3 + 14 + 9, which gives us 233. This means three particles are missing, so this time we'll get three neutrons.

Just like with the fusion reactions, we will see even here that some energy is released. The energy is released usually as kinetic energy of the products and the neutrons. Because energy is released—and remember that energy and mass are equivalent—we'll find that the mass of the products will be smaller than the mass of the reactants.

By figuring out the difference in the mass, you can determine how much energy is released. That difference in mass is basically what got released as energy, again something that we've seen before in nuclear fusion reactions. Now, can any heavy nucleus give you fission reactions? No, that can't happen. The ones that do we call fissile nuclei.

Uranium-235 is fissile because it does undergo fission reactions and gives you energy. However, if you consider another isotope of uranium, such as uranium-238, it turns out it is non-fissile. It does not undergo nuclear fission easily. If you're wondering why certain nuclei are more fissile than others, it has something to do with energy and stability.

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 it’s not very easy to happen. A big question we could now ask ourselves is: how much energy do we get out of it? If you look at a single reaction, of course, you'll get a tiny amount of energy. But if you want to get a usable amount, then we will require lots and lots of reactions.

So, how do we do that practically? Nuclear fission requires you to bombard a uranium nucleus with neutrons. So how do we ensure we get lots and lots of reactions? 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 nuclei, then they will again undergo fission, giving you more neutrons, which cause even more fission reactions.

Here’s how we can show that. If you have one neutron that bombards a uranium-235 nucleus, it gives you an energy-releasing fission reaction, along with some neutrons. If these neutrons could hit even more of these uranium-235 nuclei, you will generate even more energy. This thing can keep on going, and you can see very quickly that this will keep increasing. You initially have one fission, then three fissions, and then nine, and so on and so forth.

The amount of fission happening per second will just keep increasing. This is what we call a chain reaction. Nuclear chain reactions can be quite devastating. You start with very few reactions per second, but very quickly, that number increases. Within a short amount of time, you can release a tremendous amount of energy.

That is the whole idea behind atomic bombs. What makes atomic bombs so much more devastating compared to traditional bombs is that we're dealing with nuclear energy, which is orders of magnitude higher than the chemical energy that we get from traditional bombs. A small amount of fissile material can give you a lot of energy. However, that's not all. 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.

This can further cause damage for years to come, making that whole area uninhabitable. So yes, atomic bombs are really destructive. On the flip side, if you're using this to generate electricity, then we'll get way more energy compared to what we get from fossil fuels. Again, we're dealing with chemical energy and, of course, another advantage of using nuclear energy is that in fossil fuels, because you're using combustion reactions, CO2 is released into the atmosphere. None of that happens here.

This brings us to the original question: how do we use chain reactions in nuclear power reactors to generate electricity? Wouldn't they just explode like an atomic bomb? So what's the big difference? The big difference is that when it comes to bombs, we are using uncontrolled chain reactions. Whatever we just saw right now is called uncontrolled chain reaction.

But 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. Imagine if we absorbed a lot of neutrons; by absorbing neutrons, you control how many further fission reactions are happening. That way, you can ensure that the energy is released at a steady rate, allowing for the controlled chain reaction.

But there's another major difference. Remember how we said earlier that uranium-238 is non-fissile? It turns out that if you take uranium ore, most of it is actually uranium-238. This means you cannot directly use uranium ore as a bomb or as fuel for a nuclear power plant. Thus, we need to take it through a process to increase the amount of fissile material. This process is called enrichment.

The big difference is that when you're using fuel for a bomb, you would want a lot of enrichment—in fact, about 90% enriched. That makes sense because you would want as many fission reactions happening as possible per second to cause an immediate explosion. However, when it comes to nuclear reactors, we have only about 3 to 5% enrichment. This means a single uranium-235 is surrounded by a lot of non-fissile materials.

That’s why the nuclear fuel will not explode like a bomb: it's not enriched as much as needed for an explosion. By utilizing controlled chain reactions, we get energy as the kinetic energy of these products, which is then used to heat up water. Then, the process is very similar to how any other power plant works. The heated water produces high-pressure steam that turns turbines, generating electricity.

Then that hot steam is cooled in a cooling tower. In the process, a lot of water vapor is produced, which is released. I'm mentioning this because I used to think that this itself was a nuclear reactor and was producing a lot of smoke—radioactive smoke—which could be dangerous due to its release into the atmosphere. But none of that is dangerous. First of all, this is just a cooling tower, and what is being released is water vapor. That water never comes in contact with any of the radioactive material that you have over here, so it’s not dangerous.

However, there will be radioactive products left over—radioactive waste—inside the nuclear power plants, and that needs to be safely disposed of. That is a challenge that scientists and engineers are actively working on today.