Lesson Video: Nuclear Fission Physics

Video Transcript

In this video, our topic is nuclear fission. As we’ll see, this is a process where atomic nuclei, the cores of atoms, split into smaller pieces. Nuclear fission happens spontaneously in nature. And it also takes place in a controlled way in nuclear power stations in order to generate energy.

To begin understanding how nuclear fission happens, let’s consider the nucleus of an atom. We know that atomic nuclei are made up of neutrons, what we’ve colored here in green, and protons that we’ve colored here in blue. It’s also the case that as nuclei increase in size, they become less and less stable. When a nucleus reaches a certain level of instability, it becomes a candidate for nuclear fission.

Fission can take place in one of two different ways. First, it can happen spontaneously. This is where an atomic nucleus becomes large and unstable enough that it simply splits into two smaller chunks. This is an unpredictable process because for a nucleus that does split, we never know exactly when that will happen. This is known as spontaneous nuclear fission.

But there’s a second way that fission can happen. In this process, a free neutron comes along and attaches itself to a large atomic nucleus. The addition of this neutron pushes the nucleus over the edge from an energy perspective. It causes the already-unstable nucleus to split apart. And often times, in addition to the chunks of protons and neutrons that are released from this split, free neutrons are given off as well. This fission process is known as induced nuclear fission. It’s called that because we could say that the incoming neutron induced the split. And we saw that that split happened when this particular atomic nucleus received the neutron.

It’s quite possible, though, for there to be more than one such atomic nucleus in the environment. For example, say that there’s another nucleus over here that has the same number of protons and neutrons as this original one. That means that this neutron that was released from the original fission process, if it continues on moving in a line like this, will attach to the second nucleus. And then, that nucleus will experience induced nuclear fission, being split apart into two larger chunks and also releasing a couple of free neutrons.

And then, we can see that if either one of these neutrons runs into another such atomic nucleus, the process will continue. When this happens, when one fission event leads to another which leads to another which leads to another and so on and so forth, we’ve generated what is called a chain reaction. We saw a picture of just such a reaction on the opening screen. And this is the kind of reaction that takes place in a very carefully controlled way in a nuclear reactor facility.

In order for induced nuclear fission to take place, and for that event to lead to a chain reaction, very specific atomic nuclei are required. One of the most common nuclear isotopes used for fission is uranium, represented using the letter capital U, and its isotope 235. This number, called the mass number of a nucleus, refers to how many protons it has plus how many neutrons it has. So, uranium — which is atomic element number 92, meaning it has a 92 protons in its nucleus — can be called the isotope uranium-235 when it also has 143 neutrons in its nucleus as well. And this particular uranium isotope is a prime candidate for nuclear fission.

One way to indicate fission happening is using a sketch like we have over here. But another way is through writing out what’s called a nuclear equation. In a nuclear equation, we have what are called the reactants on the left-hand side and the products on the right. To see an example of this, we’re going to write a nuclear equation for the fission of uranium-235. One thing to keep in mind, though, is that when this isotope goes through fission, the products, the elements we’ll find on the right-hand side of this equation, aren’t always the same. That said, there are some fairly standard products from this reaction that we can write down.

Now from our sketch, we saw that we started out with an atomic nucleus, and then a neutron was added to this nucleus to lead to induced fission. So, in our nuclear equation, on the reactant side, we’ll add a neutron to uranium-235. This will represent mathematically the collision of a free neutron with this atomic isotope. When a neutron is added to uranium-235, it becomes unstable and splits apart.

A common product of this reaction is barium, barium-144, along with krypton, krypton-89. Along with these isotopes, this fission reaction often produces a number of neutrons. We know that this is the symbol for a single neutron. And then, the number we multiply that symbol by indicates the total number of neutrons produced. One important fact about nuclear equations is that they’re always balanced. This means if we count up the number of protons and the number of neutrons on one side of the equation, then those values will equal the number of protons and neutrons, respectively, on the other side.

Another way to say this is that the total atomic number on the left is equal to the total atomic number on the right and the total mass number on the left is equal to the total mass number on the right. This is always the case for a correctly written nuclear equation. Let’s check this equation to see that it follows this rule.

Starting out with atomic number, on the left-hand side, we have an atomic number of 92 and of zero. So, our total is 92. Then, on the right, we have an atomic number of 56 and one of 36. Note that our three neutrons, because their atomic number is zero, don’t contribute to this sum. If we take 56 and add it to 36, we find a result of 92, which means that we have the same total atomic number on the left as we do on the right.

We can follow a similar process for the mass number, the number written to the upper left of these symbols. On the left-hand side of our equation, the total mass number is 235 plus one, or 236. And then, on the right, we have 144 plus 89 plus three times one. The reason we multiply one by three in this last term is because each individual neutron contributes one mass number and there are three of them, so three times one. If we add 144, 89, and three times one, we get a result of 236, which matches the value we found on the left-hand side. We see then that this nuclear equation is balanced; the atomic numbers and mass numbers on the left equal the atomic numbers and mass numbers on the right.

Now, if these three products, barium, krypton, and a few neutrons, were the only products from this fission reaction, then fission would be much less useful to us than it is. But actually, there’s another product, not one that contributes any atomic number or mass number but an important product nonetheless. Energy is also released in the fission process. So, if we imagine a uranium-235 chain reaction, then each one of those fission events that takes place will release some energy. And as the reaction grows and multiplies, more and more energy will be released.

Capturing that energy released in a nuclear fission chain reaction and converting it to a useful form is the basic idea behind a nuclear power station. Nuclear power stations or power plants use the process of induced nuclear fission. This all takes place in a chamber called a nuclear fission reactor. Inside these chambers are rods called fuel rods. These rods are made of whatever material will go through atomic fission; often it’s uranium-235 or sometimes it’s an isotope of plutonium, plutonium-239.

Whatever the fuel rods are made of, they respond to incoming neutrons by splitting apart and releasing energy. But there’s a condition required in order for this to happen, in order for the incoming neutron to actually bond with the fissionable material. For that to take place, the neutron has to be moving fairly slowly. Otherwise, it will pass right through the nucleus and not be captured. In order to slow neutrons in the reactor core down enough so that they’re able to bond with the fissionable material, a substance called a moderator is inserted into the reactor core.

Water is a fairly common moderator material. And its job is to slow neutrons in the reactor core down enough so that they’re able to bind with the nuclei that will split, that will go through fission. So, when our reactor core has fuel rods, free neutrons, and a moderator material, a chain reaction is able to begin.

But let’s think about this chain reaction for a moment. Looking at our nuclear equation, we see that one neutron and one uranium-235 nucleus are needed in order to produce a fission event. But then, it’s possible for that one event to produce three free neutrons. And each one of these three free neutrons might run into another uranium-235 isotope somewhere else in the reactor core.

We could think of it this way. First, we have one fission event, our first neutron running into our first uranium-235 isotope. But then, if each of the three neutrons produced there also lead to fission, then we go from one to three fission events. And if each one of these three fission events also produces three free neutrons which also all lead to three more fission events each, then we’ll have nine uranium nuclei splitting. And if each one of those produces three free neutrons which also leads to fusion [fission], then we go up to 27.

What we’re seeing then is not only a chain reaction, but a growing chain reaction, one with an increasing rate. This is good up to a point. But beyond that point, this can actually be dangerous if our reaction rate gets too high. Recall that each one of these fission events releases some amount of energy. And if the number of events that take place gets out of control, so will the energy generated. This could lead to a dangerous condition for our nuclear reactor. In order to guard against this, to control the rate of our chain reaction, all nuclear reactors are equipped with what are called control rods.

The job of control rods is to absorb free neutrons in the reactor core and slow down the reaction rate that way. The idea is if free neutrons are taken out of the equation, then there’s nothing to induce fission in the uranium-235 that’s in the fuel rods. The control rods exist on a moveable platform that can be moved into or out of the reactor core. The farther into the core the control rods are pushed, the more neutrons they soak up and the slower the reaction rate becomes. On the other hand, if a higher reaction rate is desired, then the control rod assembly can be retracted from this core. Regarding the material the control rods are made of, often they’re made of the element boron. Boron is effective at absorbing free neutrons.

Now, in a nuclear power facility, every fission event produces some amount of energy. The effect of the energy generated in the reactor core is to heat up the water in the core. This heated water is given a pathway to escape from the core. And sometimes, depending on the reactor design, this hot water is passed by a second isolated stream of water. When these two streams are passed by one another and come in thermal contact, the hot water from the core gives heat energy to the cooled water passing by in the other direction and thereby heats this other water while cooling down itself.

The water that’s heated in this exchange then goes to produce steam, which is used to turn turbines, which produces electricity. In this way, the thermal energy generated from nuclear fission is exchanged from one flow of water to another and then used to produce steam ultimately leading to the production of electrical energy. In this reactor core, water is not only the moderator material that slows down neutrons so that fission can take place, but it also functions as a coolant, taking heat energy produced in the reactor core and removing it from that environment.

In general terms, this is how a nuclear fission reactor works. Let’s summarize now what we’ve learned about nuclear fission.

Starting off, we saw that nuclear fission, sometimes called fission for short, is the process of an unstable atomic nucleus splitting into smaller parts. Fission can be either spontaneous or induced. Spontaneous fission is when an atomic nucleus splits apart in an unpredictable way. And induced fission is when a free neutron collides with a nucleus and causes it to split.

Induced fission, we saw, can create a chain reaction that releases energy. And we also learned that fission events can be described using what’s called a nuclear equation. These equations show the reactants on the left-hand side and the products on the right side.

And lastly, we looked at the basic workings of a nuclear reactor core. We saw that the core includes fuel rods which provide the fissionable material, a substance called a moderator, often this is water, which is used to slow down free neutrons moving around the core. And there are also control rods, often made of boron, which are used to slow down the reaction rate. Along with this, we saw the nuclear reactor cores require a coolant material, something that will transport the heat energy generated from the nuclear reactions out of the core. This is a summary of nuclear fission.