Lesson video

Hello, my name is Dr.

George, and this lesson is about nuclear power stations.

It's part of the unit nuclear physics.

The outcome of the lesson is I can describe how a nuclear power station produces energy for heating to generate electricity.

Here are the key words for the lesson, which I'll explain as we go along.

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

The lesson has two parts.

They're called controlling the nuclear chain reaction and generating electricity.

If a nucleus of the isotope uranium-235 happens to absorb a neutron, this makes it unstable.

It quickly splits into two large parts called daughter nuclei and some neutrons.

Here's an example of how that could happen.

And we could write a simple word equation for this process as shown here.

And the process is called induced nuclear fission.

Nuclear fission means the breakup of a large nucleus into two smaller nuclei and induced means it's been made to happen by the absorption of a neutron.

Induced nuclear fission is used in a nuclear power station to generate electricity.

The fission of the nuclear fuel transfers energy to its surroundings by heating, as happens when, for example, coal is burned in a power station.

And this transfer of energy is used to heat water turning it into high-pressure steam, and that steam is used to turn a turbine, which spins an electrical generator, which produces electricity.

Most of the processes that happen in a nuclear power station are the same as in a fossil fuel power station.

What's different is the heat is coming from nuclear fission instead of the burning of fossil fuel.

Uranium-235 is the most common nuclear fuel, but other large radioactive isotopes such as uranium-233 and plutonium 239 can also be used.

In a nuclear power station, the fuel is placed in a fuel rod.

Pellets of uranium dioxide shaped like this are commonly used.

Uranium dioxide is a compound of uranium and it's easier to handle than the pure metal.

The pellets are placed inside long metal rods like this, and then rods are bundled together to make a fuel assembly.

And the part of the power plant that contains the fuel assemblies is called the core.

Now, which of the following types of isotopes are used as fuel in nuclear power stations? And when I ask a question, I'll wait five seconds, but if you need longer, press pause and press play when you have your answer ready.

The correct answer is large unstable isotopes such as uranium-235.

For uranium-235 nucleus to absorb a neutron, the neutron must be travelling at relatively low speed.

The neutrons released during fission are travelling very fast, and that's actually too fast for them to be absorbed, and so, they don't cause fission.

To produce a chain reaction, a situation in which one fission causes more fissions, and they cause more fissions, and so on, these fast neutrons need to be slowed down, so they can be absorbed by nuclei and cause more fission.

The neutron speed is reduced by a material that we call a moderator.

When the neutrons reach the moderator material, they're slowed down in a series of collisions.

The moderator must not absorb the neutrons, otherwise, they won't be able to go on to cause further fission.

To reduce the speed of a neutron without absorbing it, it turns out the moderator nuclei should be small.

The most common moderators are graphite, a form of carbon.

It's solid, so easy to shape, and it was used in most early reactors.

Water, which actually needs to be very pure for this, and which can also be used as coolant in the core.

You'll learn more about the coolant later.

And heavy water, that's water in which the hydrogen atoms are the isotope, hydrogen-2 instead of the common hydrogen-1, is more effective than water, but much more expensive.

Which of the following are properties of the material used as a moderator? As I've said, it has small nuclei and it slows neutrons without absorbing them.

In a nuclear reactor, the rate of the nuclear reaction needs to be carefully controlled.

It should happen at a fairly steady rate rather than increasingly rapidly.

If too much fission's occurring, the reactor become too hot, and if too little fission is occurring, the chain reaction could slow to a stop.

So, the rate of reaction can be altered using control rods.

And these control rods can be inserted into the core and they absorb neutrons.

So, that removes the neutrons from the core, reducing the rate of reaction.

And how fast a control rod absorbs neutrons depends on how far it's lowered into the core.

So, these control rods are movable.

Here's a control rod that's not in the core at all, and neutrons can just pass straight through because the control rod isn't there to absorb them.

Here's a partially inserted control rod.

It will absorb some of the neutrons, so it will slow the reaction without stopping it.

And here's a fully inserted control rod, which will absorb most neutrons.

Now, which of the following is a property of a control rod? And the answer is C.

What they do is absorb neutrons to control the rate of reaction.

And in which of the positions shown will the control rods allow the fastest chain reaction to occur? And the answer is B.

The control rod here is right out of the core, so it won't be absorbing neutrons.

A nuclear reactor is designed so that the fuel rods have control rods and moderator material between each of them.

So, it looks something like this.

Fuel rods, control rods in between, and then surrounded by moderator.

Fission in one fuel rod produces neutrons that pass through the moderator to a different fuel rod to cause more fission.

The reactor core needs to be surrounded by what's called a containment vessel, and this is a thick concrete and steel structure.

The nuclear reactions produce gamma radiation and what we can call neutron radiation.

Fast moving neutrons and the concrete absorbs most of this radiation preventing it from reaching the rest of the power station, including the places where people are working.

Now, can you match these four components of a nuclear reactor to their purposes? And here are the correct answers.

The moderator slows down fast neutrons.

The control rod absorbs excess neutrons.

The fuel rod releases neutrons by fission and the containment vessel prevents radiation escaping from the reactor.

Well done if you match those up correctly.

And now, here's a couple more longer questions for you.

I'd like you to describe the functions of each of the following components of a nuclear reactor and then explain why uranium-235 nuclear fuel is placed in separate fuel rods with moderator material between each.

Press pause while you do this and press play when you have your answers ready.

And here are the answers, although you don't have to word yours in exactly the same way as these.

Fuel rods contain the nuclear fuel, e.

g.

, uranium-235, which undergoes fission and releases energy.

Control rods absorb the excess neutrons to control the rate of reaction.

They can be lowered in to slow it or raised out to speed it up.

A moderator slows the fast neutrons so they can cause more nuclear efficient events and cause a chain reaction.

And a containment vessel is used to absorb the radiation, neutrons and gamma radiation, produced inside the reactor core.

It is made from thick, concrete and steel.

And the answer to question two, why the nuclear fuel is placed in separate fuel rods with moderator material between, the neutrons released by fission of uranium-235 are travelling too quickly to cause other uranium-235 nuclei to split.

They need to be slowed down by travelling through a moderator before they can cause more fission.

Putting the moderator between the fuel rods achieves this.

Well done if you've got those right.

And now, let's move on to the second part of the lesson, generating electricity.

The moderator in a reactor core becomes very hot because the fast moving neutrons that it slows down cause its particles to vibrate more vigorously.

A coolant is used to keep the reactor at the correct operating temperature and of course it's also used to take away the heat to do something useful with it.

The coolant heats up as it flows into the moderator before transferring energy away from it.

In some reactor designs, the moderator is the coolant.

The same material can do both jobs.

The coolant then heats pure water outside of the reactor core to produce high pressure steam in a heat exchanger, cooling down as it does so.

So, here's what that might look like.

The moderator is flowing in and out of the core and inside the heat exchanger, pure water flows through pipes and as it's heated, it turns into high pressure steam.

The water that's turned to steam is kept separate from the reactor core so that it doesn't become radioactive over time.

Now, can you suggest why a liquid water moderator is not used to directly produce steam and turn turbines in a nuclear power station? The correct answer is A, the moderator will become radioactive over time.

Now, you couldn't have known that for sure.

This was a suggest question where you had to look at the three options and see which one could be possible.

The moderator is hot enough to produce steam because it does heat water and turn it into steam, and the moderator certainly can flow through pipes if it's liquid water.

It becomes radioactive over time because the water absorbs neutrons and that can turn some of the atoms in the water into radioactive isotopes.

We've already seen that water gets heated to turn it into high pressure steam and that's used to spin a steam turbine.

The turbines rotate because there's a pressure difference between one side, the high pressure steam, and the other side where the steam is condensed into liquid water.

These work a bit like a wind turbine which turn when wind blows through them.

The water is reused because it's very pure so that it causes less corrosion in the pipes and it's expensive to produce such pure water.

The turbines are connected to an electrical generator and as the generator rotates, there is an induced potential difference.

The generator's connected to a transformer which increases the potential difference so that it can be transmitted through the National Grid electricity network.

All of the steps on this page are the same as in a fossil fuel power station.

And which of the following devices causes a potential difference in a power station? And it's the generator.

A generator has coils of wire and magnets and the relative movement between the magnets and the coil induces a potential difference using what's called the generator effect.

And now a longer task for you.

Can you draw a block diagram, like a flow chart showing the main stages of a nuclear power station and describe what occurs at each stage.

You don't have to include the descriptions within the diagram.

And here are the five parts that your diagram should include.

Press pause while you write down your answer and press play when you're ready to check it.

Here are the correct answers.

The order is reactor core, heat exchanger, steam turbines, electrical generator, and then transformer.

And here are descriptions of these.

The reactor core heats up due to nuclear fission.

Coolant from the reactor core heats pure water in the heat exchanger to produce high pressure steam.

The high pressure steam spins steam turbines due to pressure differences between the steam and the condensed water.

The electrical generator is rotated by the turbines and it causes a potential difference.

The transformer increases p.

d.

for transmitting through the National Grid.

Well done, if you included many or all of those points.

And now we've reached the end of this lesson.

So, here's a summary.

A nuclear power station generates electricity using nuclear fission as a source of heat.

Fuel rods contain nuclear fuel which releases neutrons during fission.

A moderator slows the release neutrons to allow further fission and produce a chain reaction.

Control rods absorb excess neutrons to control the speed of the chain reaction.

A coolant is heated by the reactor core and in turn heats water in a heat exchanger to produce high pressure steam.

Steam turns turbines connected to an electrical generator that produces electricity.

Well done for working through this lesson.

I hope you found it interesting and I hope to see you again in a future lesson.

Bye for now.