S1E4 – Nuclear Power Plants Essentials – Fusion and Radioactive Decay Transcript
This is a transcript of the video S1E4 – Nuclear Power Plants Essentials – Fusion and Radioactive Decay
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 again be looking into nuclear power plants, but this time we are going to spend less time on a part of the system and more at the physical phenomena behind it all. The fission process is what is used to create heat in the nuclear power plant, so we will start with that, before moving onto radioactive decay, which is a side process that reactor designers need to account for. Diving deeper into this area will help us understand the complicated physical process that is going on, so we can make the best engineering plans later to control it all.
At the end of this video you’ll be able to:
- Identify the different components of the fission reaction
- Define the ‘half-life’ of radioactive materials
- Define the differences and similarities between the different atoms and isotopes in the fission reaction
- Cite and quantify key parameters in radioactive reactions, including proportion of energy release, half-life of particles emitted, and radioactive decay rates
- Contrast key considerations in the nuclear power plant, based on key physical phenomena in the fission process
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 it’s time to jump into fission and radioactive decay, so let’s take it apart!
So what we have here is a simple model of an atom which you will all be familiar with, with neutrons and protons in the centre, and electrons flying around the outside. But this is a particularly large atom. Elements low on the periodic table may only have a few of each of the subatomic particles, but this atom has heaps of each.
This atom has 143 Neutrons, 92 Protons and 92 electrons. This is of course the uranium atom, which we spoke about in the last video. This particular uranium atom is called Uranium 235, or simply U-235, because you get 235 if you add the protons and neutrons together. Sometimes atoms have different numbers of neutrons, and these are called isotopes. A common isotope of uranium is uranium is U-238, which, yep you guessed it, has 146 neutrons instead, which is 3 more than U-235.
Uranium is slightly radioactive, which means that sometimes, it will naturally and randomly break up into smaller parts, giving off energy as well. This is called radioactive decay. The smaller parts may be an alpha particle (2 protons and 2 neutrons) or a beta particle (electron or positron), and the energy comes out a gamma rays.
The way we measure how long it takes for something to naturally decay is using the half-life measure. Basically this is the time it takes for half the atoms in a portion of material to decay.
U-238 for example, decays very slowly with a half-life of 4,500M years. So it would take 4500M years for half the atoms in a portion to decay, and another 4500M years for half of that to decay to a ¼ of the initial amount. This long duration means U-238 is barely radioactive, less than other natural isotopes found in rock and sand. It is enough however to heat the earth’s core at a rate of 0.1W/tonne of material.
Uranium is naturally occurring, so it can be dug up from the ground. It’s also one of the heaviest naturally occurring elements, at 18.7 times the density of water. If you had a smartphone sized slab of uranium, it would weigh as much as litre of milk, or 5 times as much as an actual smartphone!
When uranium is mined, 99.3% of it is U-238, which is the one with the extra 3 neutrons. Only 0.7% of natural uranium is U-235, which is the one we need for nuclear reactors.
U-235 is key because it can be used for fission, so we call it fissable. This means the atom can be split giving off a lot of energy. This is different from radioactive decay, as it requires some human intervention to make it happen. U-238 on the other hand is very much more stable.
Fission is the reaction we use to make the heat in a nuclear reactor. Let’s take a look at how it works. First, we will start with the U-235 atom here. If we hit it with a fast-moving neutron, it will capture the neutron, and then split up into smaller atoms, 2 or 3 neutrons, and energy. It’s these extra neutrons that are ejected that can be used to strike the next uranium atom, hence the chain reaction to keep the fission process going.
These smaller atoms, or fission products, are somewhere in the middle of the periodic table, around the 100 and 135 atomic mass units.
About 85% of the energy released in the fission reaction is kinetic energy, as the fission products fly off at incredible speeds. This is turned into heat when these products hit something in the reactor. The rest of the energy is gamma rays.
Most neutrons are released immediately, but less than 1% are delayed, the longest with a half-life of 56 seconds, so it allows for control of the reaction.
U-238 can also capture a neutron to become plutonium Pu239, which act similar to U235
Sometimes Pu239 captures a second neutron and becomes Pu240.
These are the main reactions going on in a reaction, but there are others too, which we won’t cover in detail.
Ok, so the last bit was crazy, so let’s take a break with an 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 downloads section at engineeringallsorts.com.au.
I’m going to present a scenario, and I’ll ask you at the end to fill in a few blanks based on what we have learnt so far. Just pause the video to give yourself enough time, and then come back to us when you are done. Ready? Here we go.
You’ve just designed your very own shiny nuclear power plant. It might be the one that you thought about in video 2. But, it’s currently empty with no fuel in the reactor. You’ll need to answer the following questions correctly to get your plant up and running.
A: The reaction that is planned for the reactor is called the fission reaction, which is when a neutron is captured by a Uranium 235 atom. It produces what, what and, what?
B: The fuel rods are made up of uranium. The proportion of Uranium 235 in the naturally occurring material is how many percent?
C: Before use in the reactor the natural uranium must be made into fuel rods with a greater proportion of Uranium 235. What percent of Uranium 235 is needed for fuel rods? (Careful, this was discussed in the last video!)
Ok, so how did you go? Did your nuclear power plant create any power? Or did it fizzle out? We’ll find out soon enough, when we go over the answers at the end of the video. In the meantime, feel free to share your answers in the comments section below.
Ok, so let’s now put on our engineering hat and look at some of the physical phenomena that need to be considered when designing a nuclear power plant.
Most of the process heat comes from the fission process in the nuclear reactor, but about 6% comes from natural radioactive decay of fission products and transuranic elements (which are particles higher up on the periodic table than uranium). This means that a small amount of heat is continued to be generated even when the reactor is shut down. The reactor designers and operators need to cater for this when designing and running the plant.
After 1 year used fuel can still generate 10kW/tonne of heat decay, and 1kW/tonne after 10 years.
The main transuranic elements are plutonium, curium, neptunium, and americium. These have long half-lives so they need a secure disposal plan extending beyond a few thousand years to stop the radiation escaping into the environment.
Lastly, when neutrons strike the components of the reactor, usually steel, by-products are created called activation products. These range from tritium h3, carbon14, cobolt60, iron55, and nickel63. These activation products need to be handled the same as radioactive waste, so it’s important to treat the demolition of a reactor carefully.
Finally, let’s look at the answers for the activity above and find out how your nuclear power plant went.
A: The reaction that is planned for the reactor is called the fission reaction, which is when a neutron is captured by a Uranium 235 atom. It produces heat, fission products and energy.
B: The fuel rods are made up of uranium. The proportion of Uranium 235 in the naturally occurring material is how many percent? 0.7%
C: Before use in the reactor the natural uranium must be made into fuel rods with a greater proportion of Uranium 235. What percent of Uranium 235 is needed for fuel rods? 3.5%
So, did your nuclear power plant create any power? Well done if you did, and back to the drawing board if not!
In this video we’ve the covered fission reaction and radioactive decay. We looked at the uranium atom and its isotopes U-235 and U-238. We looked at how uranium can be split in the fission process to keep the chain reaction going and generate heat. And we also looked the concepts of radioactive decay and half-life. Lastly we looked at some engineering considerations such as dealing with heat generated by natural radioactive decay after the reactor is turned off, storing the waste fuel, and careful decommissioning of the reactor.
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 at different types of nuclear reactors. I’ll see you there!
This is a transcript of the video S1E4 – Nuclear Power Plants Essentials – Fusion and Radioactive Decay