Lesson video

Hello, my name's Dr.

George and this lesson is called Nuclear Fission and Fusion.

It's part of the unit, nuclear physics.

The outcome of the lesson is, I can explain how energy can be transferred from the nuclear of atoms to cause heating and these are the key words which I'll introduce as we go along.

You can come back to this slide later if you need to remind yourself of the definitions.

The lesson has three parts.

They're called nuclear fission, nuclear chain reactions and nuclear fusion.

So we'll start with nuclear fission.

You may already know about radioactive decay, a process in which a nucleus becomes more stable by emitting radiation.

There's alpha decay, beta decay, and it's also possible for a nucleus to emit a neutron, but a nucleus can also change by two other processes that are rather more dramatic.

There's nuclear fission, which is the splitting of a large nucleus into two.

A nuclear fusion, the merging together of two small nuclei.

We'll focus first on nuclear fission.

A very large nucleus can sometimes split into two separate nuclei that are called daughter nuclei.

Here's an example, uranium 235.

In this example, it splits into a nucleus of barium and one of krypton and there are also a couple of neutrons left over.

We could write an equation for this reaction like this, and the process I'm describing here is called spontaneous nu nuclear fission.

It's actually very rare for a nucleus to do this for no apparent reason.

Now in this lesson, we're going to be using nuclear notation, so let's see whether you know how to read it.

Which of these three shows an isotope with 94 protons and 136 neutrons? And each time I ask a short question like this, I'll wait five seconds, but if you need longer, just press pause and press play when you have your answer ready.

The correct answer is B.

The lower number should be the atomic number.

That's the number of protons in the nucleus, and the upper number should be the mass number or nucleon number, it's the total number of protons and neutrons, which is 230.

Pu is the element symbol for plutonium.

by the way.

A is wrong, because the two numbers are the wrong way round and C is wrong because the number of neutrons has been written at the top and it should be the total number of nucleons.

Make sure you know how to write this kind of nuclear notation, we'll be using it later in the lesson.

A very large nucleus such as uranium or plutonium can absorb a neutron, and if it does, that makes it much more unstable.

It then splits into two daughter nuclei.

Here's an example.

In this case, it's a uranium nucleus splitting into a barium nucleus and a krypton nucleus and there are some neutrons left over and here's a word equation for this process.

This is nuclear fission again, but it's called induced nuclear fission, induced means made to happen.

It's been made to happen by a neutron sticking to the original parent nucleus.

This can happen naturally, it can happen that there is a free moving neutron or it can be made to happen.

You can get a better idea of fission by looking at an animation.

Here you'll see a neutron colliding with a uranium 235 nucleus.

When that happens, it briefly becomes uranium 236 and then dramatically splits into two pieces with some neutrons flying off as well.

Let's watch that again.

The daughter nuclei and the neutrons produced in nuclear fission travel very quickly.

They have a large amount of kinetic energy for their size and they can transfer this energy in collisions, causing heating.

The process of fission also releases gamma radiation.

We wouldn't describe this as gamma decay, because it's not simply an excited nucleus emitting gamma radiation.

Gamma radiation is being emitted here as part of nuclear fission.

Which of the following are produced by nuclear fission processes? The answer is gamma radiation and free neutrons, free meaning they're not stuck within a nucleus.

The daughter nuclei produced by the fission of a particular parent nucleus aren't always the same, but one daughter nucleus will be slightly more massive than the other and several different combinations are possible for any particular parent nucleus.

One thing that is always the same, is that the total number of protons and neutrons must be the same before and after the fission.

Here's an equation for a nuclear fission that's possible.

The total number of protons on the left is 92 and it's the same on the right and the total number of nucleons on both sides is 236.

Notice that there are two free neutrons on the far right.

This equation shows a fission that isn't possible.

There are different numbers of nucleons on the left and right.

There are 236 nucleons on the left as shown by the top numbers, but on the right there are 237 nucleons, because of those three neutrons, this would be possible with just two neutrons.

Now I'd like you to work out which of these three nuclear equations show a possible nuclear fission reaction and the answer is C.

It's the only one in which the total nucleon number is 236 on both sides and the total proton number is 92 on both sides.

In the equation for A, the total numbers of neutrons don't match, the nucleon numbers aren't the same on both sides, although the proton numbers are, and in B, the total numbers of protons don't match.

There are 92 protons on the left and 56 plus 37, which is 93 protons on the right.

Well done if you got that one right and now a couple more questions for you about nuclear fission.

Press pause while you write down your answers and press play when you're ready to check them and here are the answers.

Similarities and differences between spontaneous nuclear fission and induced nuclear fission.

A similarity is that both involve splitting of a very large nucleus to produce two smaller daughter nuclei and some free neutrons and here's a difference, induced fission requires a neutron to collide with the nucleus to make it more unstable and cause it to fission.

Spontaneous fission doesn't require the initial neutron collision and for question two, here are the complete equations with the numbers of neutrons shown and now let's move on to the second part of the lesson, nuclear chain reactions.

A single nuclear fission of a uranium 235 nucleus releases about 3.

2 times 10 to the minus 11 joules of energy.

That's 3.

2 divided by 10, 11 times.

That's a very small amount of energy, so you'd need to split 30,000 million or 30 billion nuclei to release a single joule of energy.

That may seem like a large number of nuclei, but actually there are about 2.

56 times 10 to the 21, the number shown in the bracket, nuclei in just one gramme of uranium 235 and that means that if you could make every nucleus in a gramme of uranium 235 undergo fission, that would release over 80 billion joules of energy.

For comparison, if you burned one gramme of carbon based fuel such as coal, that releases 33 kilojoules of energy, 33,000 joules.

The energy there for uranium is about two and a half million times the energy for the coal.

Now, it wouldn't be practical for us to try to induce fission, individually, nucleus by nucleus to try to get a lot of energy, but it turns out that we can get large numbers of nuclei to undergo fission in what's called a nuclear chain reaction.

We've already seen that the fission of one nucleus can be caused by the absorption of a neutron, but that fission releases two or three more neutrons and if those released neutrons reach other uranium 235 nuclei, they can cause the nuclei to split.

This process can continue with neutrons hitting nuclei, which undergo fission, which send out more neutrons, which hit more nuclei and so on in a chain reaction.

Here's a visual model of a chain reaction.

There's a key at the bottom showing that we have a neutron approaching a uranium 235 nucleus and that causes it to split into daughter nuclei and here there are three neutrons.

Let's say two of those hit other uranium 235 nuclei, causing them to undergo fission and so the process goes on.

It's easier to understand a chain reaction by seeing an animation.

Imagine we could fire a neutron from a neutron gun, although there isn't exactly such a thing and here we have a load of uranium 235 nuclei.

The first neutron set off a chain reaction so that in the end, all of those nuclei have undergone fission.

We can watch that again.

So it's not the daughter nuclei that are causing other nuclei to undergo fission, it's the neutrons that have been released.

If each step in a chain reaction cause two more nuclei to split, then the reaction would double at every step.

So we can start with a single fission and in the next step there'd be two, then four, then eight, 16, 32 and so on and after 30 steps, we have over 500 million fissions.

In practise, each step takes a very short period of time, so within a few microseconds, a few millions of a second, trillions of fissions can occur.

Nuclear chain reactions can become very large very quickly for this reason, if they're not controlled in some way.

In an uncontrolled chain reaction, the reaction is allowed to use up all of the fuel in a fraction of a second.

So all of the fissions occur very quickly.

There's a huge release of energy in a short time, and that's an explosion, we'd call it a nuclear explosion, but in a controlled chain reaction, the reaction is kept going at a steady rate.

Controlled chain reactions can be used to produce electricity, the similar way to using coal.

The fuel produces heat, although in a different way here and that heat can be used to make steam, which turn turbines, which are attached to generators which generate electricity.

Now have a look at these examples.

Can you say which of them are examples of chain reactions? A line of dominoes knocking each other over can be described as a chain reaction, because when one domino falls, it makes the next one fall and that makes the next one fall and so on and if you heat the end of a metal rod, it makes particles at the end vibrate and those vibrations get passed along the rod from particles to their neighbours.

The other two aren't chain reactions, they aren't examples of one thing happening causing the next, which causes the next and so on.

Here's a longer task for you.

In question one, a nuclear fission process releases three neutrons at each stage, I'd let you to complete the table to show the maximum number of fissions at each of the stages and then in two, I'd like you to think about using a large set of dominoes to model chain reactions.

How could they be used to model a controlled chain reaction and how could they be used to model an uncontrolled chain reaction? Press pause while you write down your answers and press play when you're ready to check them.

So here are the answers.

What you should do for the table is multiply by three every time.

In the first step there's a single fission, but that releases three neutrons, so in the next step there could be as many as three fissions.

If each of those releases three neutrons and each of those sets off another fission, there'll be nine in the next step and so on.

Keep multiplying by three.

Now, how could we use dominoes to model controlled nuclear fission? Here's a possible answer, although it may not be exactly the same as yours, arrange the dominoes in a single row.

So one domino knocks over one more when they fall, the rate of reaction, that's the rate of dominoes falling will be constant once the reaction has started and will continue until all the dominoes have fallen.

An uncontrolled nuclear reaction could be modelled like this.

Arrange the dominoes so that when one falls, it knocks over two others, which each knock over two more and so on.

The rate of reaction will increase every step and all the dominoes will fall over in a very short time.

Your answers don't have to be exactly the same, but check that your answer to A describe something that would happen at a steady rate, whereas your answer in B describes something that would happen faster and faster.

Now let's take a look at nuclear fusion.

Nuclear fission was the breaking up of nuclei.

Nuclear fusion is the joining of two small nuclei together to make a large one.

The word fusion is occasionally used in everyday life to mean joining together.

So here's a visualisation of that, a small nucleus plus another small nucleus joining together to make a larger nucleus and this process releases energy.

I'm not going to go into detail about why it releases energy, but I will say that the mass of the particles produced turns out to be slightly less than the total mass of the original particles.

That results in a release of energy, and that's to do with Einstein's famous equation, E = mc squared, which relates energy to mass.

An example of fusion is that a hydrogen-2 nucleus and a hydrogen-1 nucleus can collide at very high speeds.

We can write them like this.

Hydrogen-2 means a hydrogen nucleus with two nucleons and hydrogen-1 has just one nucleon, which will be a proton.

We get this helium-3.

So the two nuclei have sympathy joins together.

Here's another example.

Hydrogen-2 can merge with hydrogen-3 and if they do, turns out slightly different thing happens.

We get a larger nucleus, helium-4, but there's a neutron leftover and which of the following are possible nuclear fusion reactions? These small numbers are easier to check than the larger numbers for fission.

A and C are possible, simply because the number of nucleons are the same on the left and right, and the number of protons are the same left and right and therefore the number of neutrons are the same on both sides as well.

What's wrong with B? The mass number, the nucleon number on the left is eight, but on the right it's 12 and there are four protons on the left, but six on the right, that's not possible.

A and C actually describe processes that happen inside stars.

Nuclei of beryllium and carbon are produced in stars in this way.

There's something else that can happen during fusion, which is protons can change into neutrons.

Here's an example in which a pair of hydrogen-1 nuclei collide of very high speeds.

During this fusion, one of the protons converts a neutron and so hydrogen-2 is produced.

Another particle, a positron, which is actually the antimatter version of an electron is also produced, because the proton converts into a neutron and a positron.

As you probably know, protons are positively charged and that means there's large repulsive forces between protons when they get close together, so nuclei don't merge easily.

For fusion to happen, the nuclei have to be moving towards each other at very high speeds, so that they don't simply repel each other and move apart, before they're able to fuse.

That requires very high temperatures.

When the temperature's very high, particles move fast.

So why is it difficult to merge nuclei together? It's because the nuclei repel each other strongly and they repel because protons are positively charged.

Now, nuclear fusion is the source of power in stars.

The core, the centre of a star like the sun contains hydrogen nuclei and the pressure and temperature in the core are so high that hydrogen nuclei can fuse together and release energy and stars like the sun can fuse hydrogen for billions of years, so the sun isn't on fire, it's not like burning wood on earth.

The process that's happening is nuclear fusion.

Scientists are trying to produce controlled nuclear fusion on earth, because that could be used to generate electricity.

The hydrogen isotopes that you would need to do this can be found on earth in water and the oceans contain enough of this kind of fuel for many millions of years, but it's been difficult to produce the high temperature pressure needed to keep fusion going.

What do oceans contain that's a source of fuel for nuclear fusion reactions? And it's hydrogen isotopes.

Now, can you produce a single page summary using what you've learned to compare the processes of nuclear fission and nuclear fusion? Your summary should contain a description of both processes, a diagram showing an example of each process and an example nuclear equation for each process.

Press pause when you do this and press play when you're ready and I'll show you an example answer.

So here's a possible way of answering the question.

Nuclear fusion is splitting of a large nucleus into two smaller daughter nuclei and some free neutrons.

It can be induced, caused by absorbing a neutron or it can be spontaneous and here's an example diagram and an equation.

Nuclear fusion is the joining of small nuclei to form larger ones, and it happens at very high temperature and pressure, for example, inside stars and here's an example of a nuclear fusion reaction.

Well done if you describe both processes correctly and your equations don't have to be the same as these.

There are many different fission equations and quite a number of different fusion equations that you could write and make sure you don't confuse nuclear fission with nuclear fusion and that you don't confuse either of them with radioactive decay.

They're all nuclear processes, but they're quite different from other and now we've reached the end of the lesson.

So here's a summary for you.

Large nuclei can split in nuclear fission.

This produces two daughter nuclei, several neutrons and gamma radiation.

These transfer energy from the original nuclei.

The neutrons released from nuclear fission can cause a nuclear chain reaction, which can be controlled or uncontrolled.

Small nuclei can merge together in nuclear fusion.

This process transfers energy from the original nuclei, because nuclear repel each other strongly, nuclear fusion requires very high temperatures like those in the core of stars.

Well done for working through this lesson and I hope you found it interesting.

I hope to see you again in a future lesson.

Bye for now.