Guest blog: A beginner’s guide to nuclear fission

Nuclear fission is a renewable energy source, and approximately 21% of the UK’s electricity comes from nuclear reactors! So why not know a little bit more about where this electricity comes from? Fission may also have piqued your interest if you have watched the hit show Chernobyl! Find out why fission can be dangerous and help understand the science behind the show. Both nuclear fission and fusion have bad reputations for being complicated and scary, but their basic principles are fairly simple! I will attempt to write this blog in the same manner I present my videos – simple, understandable physics. By the end of this post, you will know the process of fission, how it happens, why it can be dangerous, and how we can produce and extract energy from it!

What is an atom?

To start, I should explain that an atom is the smallest form of an element – and elements are the substances you can find in the periodic table. So, take an element like chlorine (random example), but an atom of chlorine is the smallest piece of chlorine you can get. An atom can then be broken up into particles, like electrons, neutrons and protons. And finally, the nucleus is the centre of the atom. The nucleus is made up of the bound protons and neutrons, and the atom is made up of the nucleus plus the electrons. Does this make sense? Oh, and then a molecule is two or more atoms joined together. Anyway, back to nuclear fission!




A visual representation of an atom! See the nucleus in the centre made up of protons and neutrons, with the electrons around the outside of the nucleus.

The nuclear fission reaction

To begin nuclear fission, you need an atom with a heavy atomic nucleus – so an atom with a high atomic mass. I’ll explain more on atomic mass later but for now, it’s the combined number of protons and neutrons in the nucleus. The more protons and neutrons in the nucleus, the heavier the atom is! This is why most examples, and most fission reactors, use uranium. Uranium has the highest atomic mass in the periodic table. So, you start with your heavy atom which in our example is Uranium-235 (where 235 is the atomic mass). This heavy atom then needs to be hit by a slow-moving neutron. This neutron needs to be slow so that it has more chance of being captured by the heavy atom! If it is moving too quickly then it will just whizz by. When the slow-moving neutron hits the Uranium-235 atom, the atom absorbs the neutron. This then produces a Uranium-236 atom, so it is now even heavier. This makes sense because if 235 is the total number of protons and neutrons, then adding 1 neutron will increase the atomic mass by 1 – to 236. As a result of this added neutron in the uranium atom’s nucleus, the nucleus becomes unstable.

The uranium becomes unstable because atoms naturally like to have a certain number of neutrons in order to be stable. These stable atoms are the ones described in the periodic table. Therefore, by absorbing the additional neutron, the uranium becomes unstable. This instability within the nucleus causes the atom to oscillate and vibrate heavily. The repulsive forces between the protons and neutrons within the nucleus are now so strong that they begin to separate the atom, and the uranium atom splits into two lighter atoms.

As well as two lighter atoms being produced – you may hear these being referred to as daughter atoms, or daughter nuclei – three neutrons are also produced as a result of the reaction. Once again, this is all about the atoms trying to become stable. The Uranium releases three neutrons so that the daughter nuclei will be stable (and have the correct number of neutrons in their nucleus). The final product from this reaction is a burst of energy! This is the energy that fission reactors harness for the grid.




Nuclear fission reaction, showing a heavy atom absorbing a slow-moving neutron. This results in three neutrons, two daughter atoms and some energy being produced.

Let’s create the same image, but for our uranium example.




Uranium-235 fission reaction, producing barium, krypton, three neutrons and some energy.

How do nuclear fission reactors work?

To continue, and provide some context, let’s discuss how nuclear fission reactors work. Fission reactions are a chain reaction. This means that the product of the first reaction, can be used to stimulate the next reaction, and so on, and so on, and so on (you get the idea). Due to the first reaction producing the two daughter products plus the three neutrons, these three neutrons can then go on to collide with other heavy atoms, causing more fission reactions!

Fission chain reaction!

In our example, the three neutrons produced can go on to collide with other Uranium-235 atoms. However, let’s not forget that the emitted neutrons need to be slowed down first, as otherwise they have no (much less) chance of being absorbed into the uranium’s nucleus.

A fission reactor has rods that contain the uranium fuel. These then get inserted into a ‘core’ that facilitates the neutrons striking more uranium atoms – by slowing the neutrons down. This core is generally made out of graphite or water. The uranium fuel rods are inserted into the graphite or water core – and nuclear fission occurs!! The core slows down the neutrons enough that they can strike more of the uranium contained in the rods. With every fission reaction, energy is emitted; this begins to heat up the core. The core is therefore surrounded by a coolant – normally water or a gas is used. This coolant surrounds the core, and therefore the heat being generated in the core transfers into the coolant. The coolant is then transported away from the core and is transformed into steam. This steam drives the turbines that are used to create the energy that we harness for the grid! There you have it, a beginner’s guide to nuclear fission reactors.

The nuclear fission equation

Now, let’s analyse a nuclear fission equation. We can write the reaction out as an equation:

As with any equation, both sides must equal each other. The uranium atom  has an atomic mass (A) of 236 and an atomic number (Z) of 92. The atomic number of an element is how many protons are in one of the atoms, and the atomic mass of an element is the combined number of protons and neutrons in one atom of that element. Therefore, you can calculate the number of neutrons in an atom by the following equation: Number of Neutrons = Atomic Mass – Atomic Number. See the image below for my chlorine example from the periodic table.

This may get a bit confusing, as the values in the periodic table are displayed as above. However, when writing out the elements in an equation, the atomic mass and atomic number are written as:

Hopefully that’s clearer, now back to the fission equation!

On the left hand side of the equation – if you add together the neutron and the Uranium-235’s atomic masses it makes 236 (the product’s atomic mass); and if you add together the neutron and the Uranium-235’s atomic numbers they equal 92, the product’s atomic number. As you can see, both the top and the bottom numbers have to align on both sides of the equation. Now we can do the same with the numbers on the right-hand side of the equation.

As we start with          , we know that the products will have to have a total atomic mass of 236, and a total atomic number of 92. If we start along the top we calculate: 92 + 141 + 1+ 1+ 1 = 236, which equals the Uranium-236 atomic mass. So, we know that the atomic masses in this equation line up. When we do the same for the bottom numbers we calculate: 36 + 56 + 0 + 0 + 0 = 92, which equals the Uranium-236 atomic number. Hurrah! This is one of the reasons why we can’t forget about the three neutrons in fission reactions, because otherwise the equation would not align on both sides!!

Where does the energy come from?

I explained earlier that energy was also produced as part of a fission reaction, and it is important we know how this energy is actually produced. Without dipping into too much chemistry – atoms have a Nuclear Binding Energy. The binding energy is how strongly the nuclei are held together. This results in the mass of a nucleus being less than the total mass of the individual nucleons (protons and neutrons) combined. See image below – hopefully it helps paint the picture in your head.

So, if both sides of this image have to equal each other, e.g. Nucleus + Binding Energy = Separated Nucleons, then this must mean that the binding energy has a mass and that the mass of the separated nucleons is larger than the mass of the nucleus. To help visualise this principle, try and think of the binding energy in terms of glue and the nucleons as building blocks! If you built a cube out of building blocks, you would have to use glue to stick the blocks together. Therefore – if the glued together brick formation had to have the same mass as the separated bricks, then this means that the cube of bricks would have to have a smaller mass to account for the mass of the glue.

To add to this, a large nucleus (like uranium) will have a greater binding energy than two smaller nuclei (like the barium and krypton daughter products). To continue with the building block example – say uranium was made up of 100 blocks, barium of 60, and krypton of 40. You would need more glue to stick together all 100 blocks in one cube than you would need for two separate cubes of 60 and 40 blocks. Therefore, a uranium atom will have a greater binding energy than barium and krypton combined. When we apply this principle to fission, we can work out that the energy released by the reaction is caused by a change in binding energy.

[U-235 + n] Total Binding Energy > [Ba-141 + Kr-92 + 3n] Total Binding Energy

Therefore, during the reaction there has been a loss of binding energy, aka loss of mass (remember the glue has a mass); there is a mass defect. Thanks to Einstein and his E = mc2 equation (where E is energy, m is mass and c is the speed of light), we now know that this loss of mass results in a burst of energy. Now we can re-write the above equation to equal it on both sides:

[U-235 + n] Total Binding Energy = [Ba-141 + Kr-92 + 3n] Total Binding Energy + Energy

As shown earlier through the Fission equation, the atomic masses are equal on both sides. The energy in the reaction is produced by the loss of mass from the loss of binding energy. There you have it, hopefully a simple and clear explanation of nuclear fission reactions!


Thank you so much for reading my post on nuclear fission! I hope you can now understand a bit more about how it happens, how we harness the energy produced for the grid, and even how energy is produced from the reaction! In the UK, about 45% of power generation came from renewables, and 15% from nuclear plants. This leaves 40% having to be generated through other methods – most of which will be fossil fuels. Finding new ways to create greener energy is the key to our future. Raising awareness about our current methods and their pros and cons is important. So thank you again for reading and increasing your own understanding of one of our energy sources – fission. If you enjoyed reading this then please check out my YouTube Channel – Science with Bexy. I create short science videos that are simple, understandable and hopefully fun as well. You can find the video about nuclear fission here.


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