Nuclear Fission - A Definition
- Andrew Crabtree
- Mar 30
- 7 min read
Before diving into nuclear fission, I want to guide you through a mental visualization to help you grasp the incredibly small scale we're discussing.
In order to understand nuclear fission, we'll frequently refer to a nucleus (and its plural, nuclei). A nucleus is extremely tiny, so small that it’s hard to imagine without some perspective.
Let's begin with a grain of sand, a single grain measuring about 1mm from end to end.
We can all envision a grain of sand. Try to set aside thoughts of the beach or your next vacation and concentrate on just that single grain of sand.
Let's gradually enlarge the grain of sand until it reaches the size of a house, then a city, then a country, and eventually the size of the Earth (12,472 km in diameter).
If one grain of sand were as large as the Earth, a uranium atom would be comparable in size to an average two-story house.
Can you imagine this? Great.
If not, visit Google Earth, type in your address, and zoom in and out to grasp the scale. (Earth to house = grain of sand to atom).
We're now considering an atom the size of a house. Even so, from this viewpoint, the atom's nucleus remains invisible to the naked eye. To visualize this, imagine expanding your house-sized atom into a large stadium, like Wembley Stadium.
In this setting, if you look above the center of the pitch, you’ll see a grain of sand—this represents the nucleus of a single proton hydrogen atom. Our uranium atom, containing hundreds of protons and neutrons, is considered a large nucleus. In this scenario, the nucleus of a uranium atom would be the size of a marble.
To complete the exercise, let's zoom back out to regain our initial viewpoint;
Zoom out from the marble-sized nucleus to the uranium atom, which is the size of a stadium.
Reduce the stadium-sized atom to the size of an average double-storey house.
Zoom out from the house-sized atom to a grain of sand the size of Earth.
Now shrink the grain of sand back to its original 1mm size.
There are 43,000,000,000,000,000,000 (43 million million million) atoms in that grain of sand, each with a nucleus the size of a 'marble in a stadium.' When discussing nuclei, we are dealing with incredibly small sizes!
With this understanding of size, we are prepared to explore the process of nuclear fission.
What is Nuclear Fission?
Nuclear fission involves splitting a large nucleus (our marble), releasing a significant amount of energy. This energy can be harnessed to convert water into steam for electricity generation in our grid or can be left uncontrolled to produce the massive explosion seen in a nuclear bomb.
You can find a nice video below explaining nuclear fission from Cognito at the bottom of this article. We have used a couple of screenshots of their video to help us explain.
Fun fact: the term nucleus comes from the Latin word used for the seed inside of a fruit.So, how do we split the nucleus?
To release energy from the nucleus, we can wait for the nucleus to spontaneously split by itself, but this is very rare and happens at such a slow rate, or we can get the process started by firing neutrons at the nucleus, forcing it to split. We'll come back to firing neutrons later.
When the nucleus divides, it creates two nuclei, some neutrons, and a significant amount of energy.
It's an immense amount of energy! In fact, it's a million times more than what is produced by burning fossil fuels. (Remember this fact for later).
The 'daughter' neutrons released during the fission process encounter other nuclei, causing them to split and form more daughter nuclei, which then find additional nuclei to split - you get the idea. Each time a split occurs, a substantial amount of energy is released.
This phenomenon is known as a chain reaction.
Nuclear Bomb
Each of these fissions, accompanied by energy release, occurred in mere fractions of a second.
In the context of a nuclear bomb, it's evident that when the bomb's core is breached, allowing neutrons to interact with nuclei, a rapid chain reaction can occur, resulting in an explosive force millions of times greater than TNT.
If you're interested in learning more about nuclear bombs, we have a separate post that provides detailed information.
Nuclear Energy
If we can control the rate of the chain reaction - accelerating, slowing down or stopping the nuclei from splitting- we can control the energy generated.
Nuclear fission reactors do just this. Using just a small amount of fuel (think our marble-in-a-stadium with a million times more energy than coal), a moderator to slow the speed of the neutrons down (often water) and control rods to govern the rate of fission.
Control rods are made of material that is more likely to absorb neutrons than fuel. Whenever they are placed into the reactor, the neutrons stop nuclei from splitting, slowing down or stopping the chain reactions. With this control, we can control the energy (heat) generated and the amount of steam available to power the electric generator.
To simplify things further, a moderator slows down the neutrons, just like taking a Lamborghini off-roading. And control rods attract the neutrons away from the nuclei, just like... me being distracted by YouTube videos like this for over an hour before finishing this sentence.
You will note that earlier, we stated that a neutron is fired into the first nucleus to start the chain reaction. In fact, by removing the control rods, the neutrons already present in the reactor are allowed to absorb into uranium neutrons in the fuel, becoming unstable and split. Less a firing and more a releasing.
Can a reactor turn into a bomb?
No.
Uranium, when mined naturally, is 99.18% diluted or 0.82% enriched. For a nuclear reactor, you typically need uranium enriched (concentrated) to 4%, and for a nuclear weapon, you are looking for around 90% enrichment - noting that 90% of the effort to getting to 90% is spent getting to 20%.
So, the reason a nuclear reactor cannot turn into a nuclear bomb is that the fuel used for a nuclear reactor is Diet Coke when you need Bourbon to get the party started.
"But I saw the Chernobyl reactor explode on HBO, and I watched the Fukushima reactor building blow up live on the news", I hear you say. And you are not wrong. But these were no atomic explosions.
With Chernobyl, a combination of very hot fuel and cooling water led to fuel damage, followed by a rapid overpressure detaching the cover plate and jamming the fuel rods. This caused intense steam generation that led to a steam explosion. A few seconds later, a second explosion was caused, most likely by the build-up of hydrogen.
The situation was different at the Fukushima plant, with the loss of cooling water causing the hot fuel to melt through the reactor's core. The series of explosions seen on the news were gas and hydrogen explosions from the failure of the equipment surrounding the reactor.
Both situations are very bad and resulted in the loss of life. But neither is an example of a nuclear reactor turning into a nuclear bomb.
Who Discovered Nuclear Fission?
There is some debate over who should be credited with the discovery of nuclear fission.
Rutherford, Curie, and Chadwick are among the prominent figures associated with the nuclear industry. While each contributed significantly to nuclear fission, none of them are credited with its discovery.
It is undisputed that nuclear fission was first accomplished in Berlin in 1938 by a team of scientists led by the German chemist Otto Hahn.
Nevertheless, a Swedish-based female physicist named Lise Meitner, who was part of Hahn's team, discovered that uranium atoms could be split when struck by neutrons. She also coined the term for the process.
Despite this, in 1944, during a time of significant gender inequality, only Hahn received the Nobel Peace Prize for the discovery. Meitner was nominated for a Nobel Peace Prize 48 times but never received the award.
What is the difference between Nuclear Fission and Nuclear Fusion?
Nuclear Fission and Nuclear Fusion are physical processes that produce energy from atoms. Nuclear Fission involves splitting a heavy atom, whereas Nuclear Fusion involves the joining (or fusing) of lighter atoms.
The differences between Nuclear Fission and Nuclear Fusion circle around the fuel used, the by-products generated, the amount of energy released and the ability to commercialise the technology.
Nuclear Fission uses Uranium or Plutonium with a release of energy 1 million times that of fossil fuel sources. This dense atomic energy source leads to much less fuel usage, but the process does result in small amounts of nuclear waste that has been stored and successfully managed for many years.
Nuclear Fusion uses Hydrogen, which results in Helium as a by-product. The energy release is 3-4 times that of nuclear fission. However, although there are many nuclear fusion power plants in development, there are not any currently capable of providing electricity to the grid on a commercial scale.
This was a concise and straightforward overview of nuclear fission, designed to enhance your understanding of the process, its workings, and how nuclear energy technology significantly differs from that of a nuclear bomb.
Without delving too deeply into activism, the information provided should help you appreciate the advantages of nuclear energy based on factual evidence and dispel some myths surrounding the technology. It is inexpensive to operate, requires minimal fuel to produce clean, reliable energy around the clock, and generates very little waste, which can be recycled or easily managed.
As promised, here is the complete YouTube video from Cognito that presents this information in video format.
Nuclear Fission Explained
Three more Get Into Nuclear Definitions
Nuclear Fusion - An explainer similar to this post but about nuclear fusion and how it could be a game changer in the global energy market.
Nuclear Bomb - A definition of a nuclear bomb, their science and which countries have (or at least own up to) having them.
Thorium - What is thorium, why so many people are excited about it, and how it could potentially be used to 'burn' the current nuclear waste stockpile.
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