S1E3 – Nuclear Power Plants Essentials – Reactor and Steam Generator Transcript

This is a transcript of the video S1E3 – Nuclear Power Plants Essentials – Reactor and Steam Generator

G’day and welcome back to the Engineering All Sorts Video Courses – professional development for teachers, helping you to Engineer your Expertise. My name is Daniel, and I’m your navigator of all things Engineering. I’m also the founder of Engineering All Sorts, where we’re all about Engineering for Educators – stem education to help you to be confident in the details – over at engineeringallsorts.com.au.

In this video we’ll be diving deeper into nuclear power plants and look at the Reactor and Steam Generator components. This is where the fission process occurs to create heat, which is used to generate steam for the next step in the process. We’ll also look at this component through the eyes of an engineer to discover the most important engineering considerations, such as safety, physics, and ease of maintenance.

At the end of this video you’ll be able to:

Identify the different components of a nuclear reactor and steam generator in form and function
Describe the flow and conversion of energy through a nuclear reactor and steam generator
Explain the primary reaction of nuclear fission and its by-products
Identify the methods of controlling the nuclear fission reaction in normal operation and in emergencies
Cite and quantify key operating parameters of a nuclear reactor and steam generation, such as neutron speed, number and dimensions of fuel rods, and the dimensions of the heat exchanger
Contrast key considerations in the creation and operation of a nuclear reactor and steam generator, such as safety, physics, and ease of maintenance

For the full list of learning outcomes, transcripts and worksheets for this series, check out the downloads section of the series at engineeringallsorts.com.au

But for now, the reactor and steam generator await, so let’s take it apart!

Here we have a zoomed in picture of the reactor and steam generator, based on the drawing from the core concept video. You’ll recognise the reactor on the left and the steam generator on the right, as well as the first and part of the second water circuits.

The reactor vessels and pipework themselves are very strong steel to withstand the high pressure. In the reactor the uranium is called fuel. Here it has been formed into bars that hang vertically in the reactor. They are usually about 1cm wide and 4m long. I’ve only drawn a few here, but a standard 1000Mw nuclear power plant would have about 51,000 fuel rods, made up of 18million pellets of uranium. Heat is generated in the reactor through a process called fission.

Let’s look at this at the atomic level to see how fission works. This which will be covered in more detail in the next video. We have our uranium 235 atom, which is the key component for fission, which is struck at high speed by a neutron. This causes the atom to break up, giving off heat, more neutrons and some other fission products. These neutrons then fly off at extremely high speed to hit other uranium 235 atoms and hence keep the chain reaction going. This is the basis of fission and where all the heat comes from.

The fuel is surrounded by a moderator to slow the speed of the neutrons coming off the fuel. This allows the chain reaction to continue smoothly, because if the neutrons are going too fast the reaction stops. Water, graphite and heavy water are often used for moderator. Sodium, lead, fluoride and chloride salts can also be used in some special cases.

Neutrons are emitted from the fuel at 7% the speed of light, which is roughly 10 million m/s. It needs to be more like 10,000m/s for the fission reaction to work, so the moderator slows them down with elastic collisions.

The moderator also stops the fuel from exploding. If there ever is a major uncontrolled malfunction, the fuel may overheat and melt, but not explode. The moderator can also have boron added to tweak the absorption rates during normal use, and boron can be dumped in quickly in an emergency.

The reactor also has some other rods inside called control rods. There are made of boron or cadmium and can be raised or lowered to control the reaction. The control rods absorb the neutrons and don’t allow them to continue fission. Lowering the rods into the reactor slows the reaction and pulling them out speeds up the reaction. This allows the power plant operators to generate more or less electricity, to match the load on the system.

You’ll remember that this type of nuclear reactor is called a Pressurised Water Reactor or PWR. The steam generator in a PWR is a heat exchanger, and is basically like your car radiator, allowing two fluids to share their surface area and heat, but never touch

It’s made up of anywhere between 300 and 16,000 tubes about 2cm in diameter, with the water from the reactor inside, and the circuit 2 water flowing over the outside. It can weigh up to 800 tonnes! The circuit 2 water comes in as cool water and boils when it touches the hot steam generator, and leaves as hot steam.

The whole reason the PWR uses a steam generator is so the water coming from the reactor doesn’t have to go into the turbine. The water from the reactor is usually slightly contaminated with a radioactive material called nitrogen 16, which has a half-life of about 7 seconds (more on half-life next video). It doesn’t hang around long, but it’s enough that we don’t want it in the turbine, because then it has to be cleaned regularly.

Around the whole lot is the containment vessel, which is designed to stop radiation from getting out. It’s a meter-thick shell of steel and concrete.

Lastly, we will add a few details such as the pressuriser, which makes sure everything stays at the same pressure, and a pump to keep the water flowing at the right speed.

We spoke earlier about the moderator being in place so the reactor doesn’t explode if it gets uncontrolled, so let’s put on our engineering hat and look at some more safety and design considerations.

There is another reason why the reactor won’t explode, and it’s to do with the mix of Uranium 235 and Uranium 238. As we will find out in the next video, only Uranium 235 can be used for fission, and only a little bit is needed to make a smooth reaction. So the mix of uranium that goes into the power plant is only 2.5% Uranium 235, and the rest is Uranium 238. Meanwhile, to make a warhead, you need something more like 90% Uranium 235 to make it explode. This is what is called ‘enriched uranium’ and has no place in power plants.

Reactors are usually designed with things called negative temperature and void coefficients, which basically means it’s balanced so if something does go wrong and there is excessive temperature, the extra boiling will actually slow the reaction down, as a built in safety control. This is because Uranium 238 absorbs more neutrons as the temperature rises

Also, newer reactors have a ‘core catcher’ which is like a concrete bucket under the reactor to capture it all in the unlikely event that it melts, rather than just letting it all pour over the containment vessel floor.

The steam generator tubes must be designed so the tubes don’t vibrate and fret as the water flows over them, operated to avoid building up of deposits, and chemically maintained to avoid corrosion. They are impossible to clean by hand, as you will know if you’ve ever tried to get the dust out of an air-conditioner radiator. There is almost zero space in there to get a brush in.

If a tube ever fails or leaks, the technicians just plug it up, and the rest take up the slack. Extra tubes have to be installed during construction to cope with this over the years. Leaks can be detected by measuring the amount of nitrogen 16 in the steam in circuit 2.

Let’s finish this video with another quick activity. Grab a pen and paper or open up a text editor on your computer or phone, or download and print the worksheet for this episode from the download section at engineeringallsorts.com.au. Have a think about what we’ve discussed so far, and the drawing of the reactor and steam generator behind me.

What I would like you to do is draw a flow chart that shows the energy transfer from the fuel all the way through to the hot steam exiting this system. It doesn’t have to look exactly like the one we made last episode, but just as long as you understand it and steps created to create hot steam from uranium.

Go ahead and draw this diagram. Just pause the video to give yourself enough time, and then come back to us when you are done. I’ll still be right here!

Ok, so all done with your flow chart of the energy conversion? It would be great if you could share your answers in the comments section below. You won’t be able to post an image, but just writing out the steps would be great.

In this video we’ve covered the reactor and steam generator. In the reactor we have seen the fuel rods, made up of up to 18million pellets of uranium. We have seen the control rods and moderator, which are used to control the speed of the reaction. We saw the steam generator tubes and the fluids inside, where heat is transferred from one water circuit to another. We looked briefly at the fission reaction, and as well as some general construction points, such as the containment vessel, pressuriser and water pump. We also looked at some safety controls and some design considerations for the steam generator, such as the right balance of Uranium types, core catcher, and negative temperature and void coefficients.

Don’t forget you can check out the resources for this series and more professional development for teachers at engineeringallsorts.com.au, including learning outcomes, transcripts and worksheets.

If you haven’t left your comments below from the activity it would be great if you could also share that below, so we can all share our stem education.

Thanks for watching and I hope you’re looking forward to the next video, where we look more closely at the fission reaction and radioactive decay. I’ll see you there!

This is a transcript of the video S1E3 – Nuclear Power Plants Essentials – Reactor and Steam Generator