Video: Nuclear Fusion

In this lesson, we will learn how to describe the process of nuclear fusion and the advantages and disadvantages of nuclear fusion reactors.

14:50

Video Transcript

In this video, our topic is nuclear fusion. This is a process that takes place all throughout the universe, in the core of active stars. Nuclear fusion is behind all the light and heat that the Earth receives from the Sun in our solar system. From this fact, we get a sense for just how much energy is available through this phenomenon.

Talking about this topic, let’s start out with the definition. Nuclear fusion is the joining of more than one atomic nucleus to create a single nucleus. This basic idea then is that if we have two separate atomic nuclei and they come together and form a third combination nucleus, then that’s fusion. Let’s look at an example of this.

Say that we have a hydrogen atom nucleus. That’s right here, where the blue dot represents a proton and the green dot represents a neutron. So this is our nucleus. And if we were to write this down as an atomic isotope using symbols, then we would write it as capital H, since this is hydrogen which has an atomic number of one. And it has a mass number of two, since there are a total of one plus one protons and neutrons. So that’s our first nucleus. And say that the second nucleus that will come together with this one to fuse is identical to it. It’s also hydrogen two. In other words, it’s a hydrogen nucleus that has one neutron in it. So its mass number is again two.

Now let’s say it happens that these two hydrogen nuclei collide with one another. This is actually harder than it may at first seem. After all, these two nuclei both have a net positive charge. So they’ll resist being pushed together. But if we’re able to overcome that repulsion and actually get the two nuclei to collide and fuse, then here’s what can happen. The two hydrogen nuclei come together, join up, and create a third fused nucleus, now with two protons and one neutron in it. And then along with this fused nucleus, there’s a free neutron that’s released.

Considering this fused nucleus, since it has two protons in it, that must mean that it has an atomic number of two. And as we look that value up on the periodic table, we see that it corresponds to helium. So these two hydrogen nuclei have come together to form a totally new element, helium. And then in addition to that fused nucleus, there’s a neutron that’s released.

Now that we have this fusion reaction, let’s consider the atomic numbers as well as the mass numbers on either side of it. We can see that the atomic number of each one of these hydrogen nuclei is one. So if we total them together, we get one plus one, two. Then looking on the other side of the equation, we have the two protons in the helium nucleus and then no protons in the neutron. This tells us that, in this reaction, atomic number is conserved from the beginning to the end.

Now what about the mass number, the number of protons plus neutrons in each of these constituents? The two hydrogen nuclei both have mass numbers of two. So that gives us a total of two plus two, or four. And then on the right-hand side, the helium nucleus has a mass number of three. And if we add that to the mass number of the neutron, we once again get four. So mass number as well as atomic number is conserved across this reaction. Okay, so that’s true. But here’s where things get interesting.

If we were able to measure the total mass, not mass number, but mass on either side of this equation — say we were to put the two constituents on either side onto a scale — then we would find that the total mass of the two hydrogen nuclei before the fusion occurred is greater than the mass of the products of that fusion, the helium nucleus and the neutron. But wait! How could that be? Because we just counted up the number of protons and neutrons and found they’re consistent on either side.

Well, it turns out that some of the mass in an atomic nucleus, like the nuclei of our two hydrogen atoms or the nuclei of this helium atom, is used up — we could say — as a glue that holds the nucleus together. For example, say that you had a whole lot of small wooden balls and you wanted to find a way to keep them all attached together. One great way to do that would be to glue all these wooden balls together. Now of course the glue itself has some mass. So if we were to calculate the total mass of this collection of wooden balls, we would include the mass of the glue along with the balls.

It’s a similar idea over here with our hydrogen nuclei. When they fuse together into one combined nucleus, the helium nucleus, some of the glue — we could call it — that kept these two hydrogen nuclei together is not needed to glue together this resulting helium nucleus. To make this fused helium nucleus, we need less glue than we needed for the two hydrogen nuclei. And that’s why if we were to weigh out the reactants of this process against the products of the process, we would find the total reactant mass is greater.

By the way, the technical term for this glue that holds nuclei together is binding energy. So interestingly, just for nucleons, protons, and neutrons in order to be able to stick together, that takes a little bit of energy in and of itself. When fusion takes place, the binding energy that previously went into holding these two hydrogen nuclei together that isn’t needed to hold the helium nucleus together is released.

Just to show that, we could add on an energy term on the product side of this reaction. This energy is the binding energy that’s no longer needed to fuse together this resulting nucleus, in this case our helium three nucleus. It’s because of this energy released that the process of fusion is so useful at generating energy. Fusion is the core process that takes place in our Sun. It’s the reason behind all the light and heat we receive from the Sun.

Now at this point, it’s worth saying a word about what nuclear fusion is not, because there’s actually a nuclear process which sounds similar but is quite the opposite. As we’ve seen, nuclear fusion involves the joining of more than one atomic nucleus to create a single resulting one. This is in contrast to the process known as nuclear fission, which involves the splitting of a single nucleus into multiple smaller ones. So if we take a large atomic nucleus and break it up into smaller pieces, that’s fission. But if we take small atomic nuclei and fuse or join them together to make a larger one, that’s fusion.

Since both these processes involve atomic nuclei and they’re both used to generate energy, it can be confusing to keep the two separate. One way to do this is to realize that the word “fusion” means to fuse together or join separate parts and that this is the opposite of fission, which involves splitting apart.

Now if we look up this process of nuclear fusion online, one of the things we’ll find is that, even though this process occurs regularly in the cores of stars, finding a way to bring the process down to Earth, so to speak, has been quite a technological challenge. Say that we wanted to build a facility where we could have nuclear fusion going on for the purpose of generating power. For a few reasons, this seems like a really great idea. First, this process obviously works. Consider all the energy created by our Sun, for example. And also the ingredients — we could call them — the elements involved in this process, hydrogen and helium, are very common on Earth. This means that it shouldn’t be hard to find fuel for a fusion reaction. And it also means that the products of that reaction will be easy to work with. They won’t be dangerous or radioactive or need very special handling.

All in all, there are a lot of great advantages to the process of nuclear fusion as an energy supply source. But to get a sense for the challenges involved in making this process work on Earth, consider where fusion happens now. It happens in the core of stars, specifically where temperatures are in the tens of millions of degrees Celsius. This high-temperature, high-energy environment is no accident.

Remember, we said that, in order for fusion to occur, say for our two hydrogen nuclei to come together and to fuse into one nucleus, it’s necessary to overcome their mutual repulsion, since after all these two nucleus have an overall positive charge and therefore push one another apart. From that perspective, we could say that fusion doesn’t want to happen. Electrically, these nuclei want to repel each other. In order to make fusion happen, we need to put so much energy in the environment of these nuclei that that energy is able to overcome this repulsion. And that’s why fusion only happens in places where the temperature and therefore the energy is very high.

So in order to make fusion work on Earth, we need to somehow create an environment that’s able to handle temperatures in the tens of millions of degrees. Various ideas for how to do that exist. And it’s an ongoing process. We’re still figuring it out. For our purposes though, we want to focus on what fusion is and how it works. To better understand that, let’s consider this example.

Say that we, once again, have two hydrogen nuclei. And as before, the blue dots represent protons and the green dots represent neutrons. In our earlier example, both our hydrogen nuclei had one neutron. But now one of them has a single neutron and the other has two of them. If we write out the symbols representing these hydrogen isotopes, one would be hydrogen two and the other would be hydrogen three.

Now before we go further with this fusion reaction, it’s helpful to realize that these particular isotopes of hydrogen come with special names. We can call them according to their mass number, hydrogen two and hydrogen three, respectively. But it turns out that these particular isotopes of this particular element have the names deuterium and tritium, respectively. There’s nothing wrong with calling them hydrogen two and hydrogen three instead. But if you come across these names, just know that they refer to the same things. And a helpful way to remember which name goes with which isotope is to know that tritium has this prefix “tri” meaning three and deuterium has the prefix “deu” meaning two. So anyway, those are names for these hydrogen isotopes we may sometimes encounter.

So let’s say we take these two nuclei and we fuse them together. In other words, we put them in an environment where, instead of repelling one another, they actually join to create a new combined nucleus. Now in this reaction, one free neutron is released, like we saw in the previous fusion reaction. But in addition to that, there’s also the main fused nucleus that results as a product. The question in this example is, “What is that main fused nucleus? How do we represent it as a symbol?”

To figure this out, to see what atomic isotope is formed in this fusion reaction, we can use the fact that atomic number is conserved on either side of the reaction, as is mass number. In other words, the total atomic number on the left side of the reaction equals the total atomic number on the right side, and the same thing for the mass number. This is an equivalence between the product side and the reactant side of a nuclear reaction that we can generally assume.

So if we start with atomic number, on the left-hand side of the reaction, we have a total atomic number of one plus one, two. Now on the product side, our neutron has an atomic number of zero, which means that, whatever our fused nucleus is, it must have an atomic number of two. That’s to make the total atomic number on this side of the equation agree with the total on the other.

Now if we look up on the periodic table of elements what element is number two, that is, has two protons in its nucleus, we see that the answer is helium, symbolized He. So our fused product nucleus is a helium atom. And we now just wanna figure out how many neutrons are in the nucleus of that atom. To solve for that, we’ll balance the mass number on either side of this equation. On the left-hand side, our total mass number is two plus three, or five. And on the right-hand side, our total mass number is one plus the mass number of this helium atom. The number we need to add to one in order to raise it to five is four. Therefore, that’s the mass number of this helium nucleus.

So we’ve answered the question of what atomic element and what isotope of that element is formed in this fusion reaction. Like the reaction we saw earlier, we took hydrogen and fused hydrogen nuclei together to create helium plus a free neutron. This reaction form, adding hydrogen to hydrogen to create helium, is very common in fusion processes. The reason for this is that, by doing the fusion reaction this way, combining hydrogen to make helium, we get the largest energy yield from the fusion that goes on. So whenever we see a nuclear reaction where hydrogen is used to create helium, it’s a good guess that this is a fusion process we’re seeing. To get just a bit more practice with these ideas, let’s try another example.

The following nuclear equation shows two hydrogen nuclei fusing to form a helium nucleus. What is the value of 𝑚 in this equation? What is the value of 𝑛 in this equation?

Taking a look at the nuclear equation, we see these two hydrogen nuclei, which are fusing, we’re told, to create helium plus the release of energy. We also see that the atomic numbers as well as the mass numbers of these hydrogen nuclei are shown, whereas the atomic number of helium and its mass number are not shown. It’s those values we want to solve for. And we’ll do it by using the fact that atomic number and mass number is conserved in this reaction. That means that if we add together all the atomic numbers on the left side of the equation, that sum will equal the sum of the atomic numbers on the right side, and same thing with mass number.

Summing the values on the left side will equal the sum of the values on the right. Now on the right-hand side, since our only products are a helium nucleus plus energy, we know that only the helium nucleus will contribute in terms of mass number and atomic number. The energy that’s released in this fusion reaction has no charge and it has no mass. This means that when it comes to answering our first question, what is the value of 𝑚, the mass number of helium, we can write that the sum of the mass numbers on the left-hand side of our equation one plus two is equal to 𝑚, the mass number of the helium atom. And this tells us that 𝑚, the mass number of that atom, is three.

Then moving on to solve for the value of 𝑛, the atomic number of helium, there are a couple of ways we could do this. One is to look up helium on the periodic table of elements and see what element number it is. Another way to solve for 𝑛 is to realize that it must equal the sum of the total atomic numbers on the left-hand side or the reactant side of this equation. So the atomic number of our first hydrogen atom plus the atomic number of our second is equal to 𝑛, the atomic number of helium. And we find that 𝑛 is equal to two, a result we could find using either one of these two methods, either using the periodic table or the fact that atomic number is conserved in this equation.

Let’s summarize now what we’ve learned in this lesson about nuclear fusion. We’ve seen that nuclear fusion is the joining of more than one atomic nucleus together to create a single resulting nucleus. This is the opposite, we noted, of the similarly named process of nuclear fission, where atomic nuclei are split apart. The energy generated by fusion comes from what’s called the binding energy that exists between protons and neutrons in an atomic nucleus. As we studied fusion reactions, we learned that the most common form is where hydrogen nuclei fuse together — they join — to create helium. Along with all this, we saw that while nuclear fusion is an ongoing constant process in the core of our Sun, we haven’t yet found a practical way of reproducing this process on Earth in a nuclear reactor. Nonetheless, nuclear fusion shows great promise as an energy supply source.

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