Video: Nuclear Fission

In this lesson, we will learn how to describe the process of nuclear fission and how nuclear fission reactors work.

14:18

Video Transcript

In this video, we’re going to be learning about nuclear fission. The first thing to know about this topic comes from the name itself: first, nuclear meaning that it has to do with the nucleus of an atom and then fission which means to split or to break apart. This topic then is all about splitting atomic nuclei.

Now, one of the first things we might wonder is just what is it that would make the nucleus of an atom split anyway. When fission happens, it’s because an atomic nucleus — typically a large one — has become unstable. One way to resolve that instability is for the nucleus to split apart into two smaller nuclei. This is nuclear fission. And it can happen in one of two ways. The first way this happens is called spontaneous nuclear fission. In this case, the nucleus that splits apart or fissions doesn’t have anything that directly causes that event. And at an unpredictable time, it just happens; it splits.

Spontaneous nuclear fission really only happens with very heavy nuclei with atomic numbers of about 232 or greater. This is another way of saying spontaneous fission happens only for very large nuclei. Along with this, it’s the case for spontaneous nuclear fission that this occurs over long time scales on the order of tens, hundreds, or even millions of years. This means that if we had a candidate nucleus that was large enough to spontaneously fission, we still might have to wait a very very long time for that event to occur.

Though it’s rare, there is evidence that spontaneous fission has occurred in the natural world. Currently though, there is a much more likely mechanism for any fission events that occur. This is when nuclear fission is induced. That is it’s deliberately caused by something else. The most common way to induce the fission of an atomic nucleus is to take a neutron — we’ve coloured it as a green dot — and send it into that nucleus. If the neutron is capable of binding with this nucleus, often this is the proverbial straw that breaks the camel’s back or in this case neutron that causes the atom to split.

Now, the reason that we’re so interested in nuclear fission whether induced or spontaneous is because these breakup or splitting events release a large amount of energy. And when we say a large amount of energy, we really do mean it. Compared to the energy generated by a chemical reaction of the same nucleus, the energy coming from this nuclear reaction is literally millions of times greater. Now, if nuclear fission was able to generate energy just on a one-time basis like we see here through this one reaction, it would still be of interest. But actually, there’s something about this reaction that makes it even more useful than what we’ve seen so far.

We’ve seen that in induced nuclear fission, we take a neutron and we send it into the large nucleus expecting it to cause a split. Often, this does indeed happen with the larger nucleus splitting up into two smaller ones and also releasing energy. But it turns out that in many fission events, these are the only things released. It’s also fairly common for a couple of neutrons to be released from this splitting or breaking apart nucleus. Here’s why that’s important. Say that elsewhere in this scenario, we had a nucleus that was just like the big one that split originally. This second nucleus say has the same number of protons and same number of neutrons as the first one. which means that if the second large nucleus gets hit by a neutron say this one released by the first reaction, then that also will cause a nuclear reaction. This recently released neutron may impact the second nuclear causing yet another fission event. The large nucleus splits and the two smaller ones releases energy and then once more releases two or three neutrons.

We can start to see from this how one fission event can cause another, which can cause another, which can cause another, and so on and so forth. When this happens, when the neutrons generated through fission lead to more fission events down the road, it’s called a chain reaction. And notice that each independent fission event releases energy. So when we add all this energy up over many reactions, we get a vast amount.

Since one of the great global needs is for energy, energy to power our homes, our hospitals, our government buildings, energy to turn on the stove, charge our phone, or use our laptop, we can start to see why it is that this process of induced nuclear fission is a primary source of global energy supply. This power generation happens at what is called a nuclear power plant. Many of us have seen the large cooling towers at such a plant. But it turns out that energy generation goes on elsewhere. The nuclear fission events that generate energy at a nuclear power plant often take place in a building that looks like a big concrete dome. If we were to look inside these domed containers, we would get a more detailed perspective on how this power generation process occurs.

When it comes to nuclear fission and to nuclear power generation, one of the most important parts of the whole process is choosing what’s sometimes called the target nucleus. The target nucleus is that large atomic nucleus that we send a neutron into with hope that this will create a fission event. The reason the target nucleus has such a determining influence on the chain reaction leading to energy generation is that it’s the target nucleus that determines what types of fission products are created in this event. We saw from earlier that these products typically involve smaller nuclei, neutrons, and then released energy. But exactly how much energy is released and how many neutrons and what smaller nuclei are created depends on the nature of the target we choose.

After a lot of testing and evaluation, over time two particular isotopes have stood out as very strong candidates to be the target nucleus in a nuclear fission chain reaction. The first and most common target nucleus is a uranium isotope, Uranium-235. Another element sometimes used for a target nucleus is Plutonium-239. For each one of these nuclei, we can reliably bombard them with neutrons, leading to a split and the generation of energy. And also importantly, these isotopes can be induced to split in such a way that more reactions down the line are possible. That is they can be part of a chain reaction.

Now, let’s say that in this case, we choose Uranium-235 to be our target nucleus. So there we have it and our neutron is colliding with this U-235. Just as a side note, this particular isotope of uranium occurs only very rarely in nature. In order to collect enough of it at a high enough concentration so that a chain reaction can be sustained, it’s necessary for naturally occurring uranium to go through a purification process. But anyway, once we get a high enough concentration of Uranium-235 in our sample, we’re able to send neutrons into that sample and reasonably expect that we’ll encounter Uranium-235 atoms which will then split; they will fission.

When this happens, the uranium nucleus will split into two lighter elements, often krypton and barium. But not always these two; sometimes that element varies. In addition to this, three or more commonly two neutrons will be released from the split nucleus. And then along with these products, there is the energy generated by the split.

Like we saw, the idea with a chain reaction is that once we’ve generated these other neutrons, if there’s enough Uranium-235 around, then these neutrons with reasonable probability will also collide with such a nucleus. But this may raise a question, what if each one of these neutrons generated in the first fission event lead to subsequent fission events? So we started out with one split from this original neutron. But then, each of the neutrons generated from that split cause their own fission event. And then what if each of the neutrons generated in those splits led to their own? Well, we can see how this reaction will grow and grow at a faster and faster rate.

In terms of energy generation, we’ll be getting a lot of energy. But over time, the reaction will get out of control. It would generate more energy than we can handle. If we were to describe a continuum of safe nuclear fission operation and we can sketch out their continuum this way, then what we’ve just described is one end of that continuum. It’s where the reaction gets out of control. It grows at a faster and faster rate over time. At one end, the reaction happens too fast because there’s too many successful fission events. A nuclear chain reaction that grows at a faster and faster rate can be very dangerous.

But on the other end of this continuum, it’s also possible to have a reaction that goes too slowly. For example, say that we had a number of fission events that were happening in a chain reaction. But then over time, the neutrons released by these split events weren’t able to interact with Uranium-235 target nuclei. In that case, there will be less and less fission happening over time until finally the reaction would die out. There wouldn’t be any atomic splitting going on. And we wouldn’t be generating any energy.

The nuclear reactor core at a nuclear power plant is designed to avoid both of these extremes but keep the fission reaction happening in a safe regime, one that generates sufficient energy output without getting out of control. Let’s talk for a bit about the way that this happens practically in a reactor core.

If we were able to see inside these concrete chambers, we would get a view into the basic structure of a nuclear reactor. It all starts with what we’re called the fuel rods in the reactor core. These rods which are basically long cylinders are made up of what’s called enriched Uranium-235. It’s that same isotope we saw before as a target nucleus. And it’s called “enriched” because the concentration of this isotope is greater than we would find it in a natural context. The fuel rods aren’t 100 percent Uranium-235, but more like five percent. And that’s enough to get a safe sustainable reaction rate.

Because of how these fuel rods are constructed, if we were to start to send neutrons into them, those neutrons would likely interact with some of the Uranium-235 nuclei and cause fission events. And then as we saw, those fission events will lead to more fission events which will lead to more and so on and so forth. Now, we mentioned that when it comes to reactors, there are two extremes to avoid: one is having a reaction that’s so slow that appears out and the other is having a reaction that’s too fast. And when we say a reaction is too fast, we mean that it increases at an unmanageable rate.

The way the reactor core design deals with that first potential problem of a reaction rate being too slow or not happening at all is to induce these nuclear fission events by inserting a neutron source among the fuel rods. As their name implies, these neutron sources spontaneously release neutrons into the fuel rods to get the reaction going. Now, there’s something interesting about these neutrons and how they interact with the Uranium-235 nuclei. If the neutrons are going too fast, then they’re unable to bind to these nuclei and they’re unable to cause fission. To help with this issue, around the fuel rods, around the centre of the reactor core, are placed what are called moderators.

The job of the moderators is to slow the neutrons in the core down enough so that they can actually attach to the Uranium-235 nuclei and lead them to split. Moderators are often made out of graphite, the stuff in pencils, or even water. Either one of these materials is capable of slowing neutrons down to a speed where they can cause fission interactions.

To this point, everything we’ve talked about in the reactor core — the fuel rods, the neutron sources, and the moderators — is designed to help speed up the reaction rate. But of course, there’s that other reaction rate extreme that we also want to avoid, the extreme of the reaction going too fast. To help guard against that extreme, nuclear reactors are equipped with what’re called control rods. We could think of control rods like sponges for neutrons. When control rods which are commonly made of the material boron are inserted into the reactor core, they absorb neutrons and does prevent them from leading the subsequent fission reactions.

These rods allow for a fairly fine control over the rate of this nuclear reaction. If we want to speed the reaction up, we can raise the control rods up out of the core. But then if the reaction rate starts to get too high, we can lower the control rods down into the core so they more effectively absorb neutrons. If we wanted to completely stop the reaction, we could fully insert the control rods into the fuel assembly. This way, they would soak up the most neutrons and kill off the reaction. On the other hand, if we want to speed up the reaction as much as possible, we will completely remove the control rods from the assembly. During normal operation though, the rods will be somewhere in between these two extremes.

So this is how the rate of the nuclear reaction that goes on is controlled from being either too fast or too slow. But of course, the point of this whole nuclear reactor is to generate usable energy. The way that this energy is harnessed often goes like this. Something else present in the core of a nuclear reactor is a material called a coolant. A common coolant material is water. And it’s this water that absorbs the heat generated from the nuclear fission reaction and then moves it out of the chamber. The heated coolant water is then made to pass through a second chamber. This second chamber contains the water which is heated by the hot coolant which then becomes steam which helps to turn turbines and generate electricity. So the coolant in a nuclear reactor is a vital material for taking the heat generated by the reactor core and transferring it so it becomes useful.

When it comes to generating energy through nuclear fission, all four of these components we’ve talked about, fuel rods, control rods, moderators, and coolants, are necessary. They’re what enable the useful application of this nuclear phenomenon. Let’s take a moment now to summarize what we’ve learned about nuclear fission in this lesson.

First off, we saw that nuclear fission is the splitting of an atomic nucleus. We saw that this split is often brought on by a nucleus getting so big that it’s unstable. Nuclear fission can occur spontaneously although this is a very rare event or it can be induced by sending a neutron to collide with a large nucleus, causing it to split. We learned that induced fission of two particular isotopes, Uranium-235 and Plutonium-239, is useful for controlled energy production. It’s these isotopes which typically are the target nuclei in nuclear power plants. Finally, we saw that in nuclear power plants, the reactor cores where energy is generated involve fuel rods, moderators, control rods, and coolant and that altogether, these components allow for a reaction rate which is both safe and yet also productive of energy.

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