Table of Contents
Nuclear Fission
What is Nuclear Fission?
Nuclear fission is the process of splitting an atomic nucleus into smaller parts. This reaction releases an enormous amount of energy, mostly in the form of heat, light and energy in the ejected particles as kenetic energy. This is following one of the world’s most famous scientists and physicists Albert Einstein’s famous equation, E=mc².
The Process
The Atomic Stage
At the heart of every atom lies a nucleus, a minuscule
powerhouse housing protons and neutrons. It is this nucleus that takes centre stage in the atomic drama of fission.
The Splitting Act
Picture an atomic nucleus ready to reveal its hidden energy.
Next enters the neutron, the trigger for this extraordinary reaction. When the neutron travels past the nucleus, there is a chance it will be absorbed by the nucleus, potentially making the nucleus unstable and triggering a breathtaking transformation called Nuclear Fission. The once stable nucleus is split into two or more smaller nuclei, creating a stunning spectacle of atomic fireworks, this is nuclear fission.
The Energetic Finale
As the nuclei split, they release a torrent of energy – the breathtaking grand finale of the fission process. This energy can take various forms, including fast moving particles, intense heat and blinding light. It's the embodiment of the equation E=mc², where a small amount of matter is converted into an enormous amount of energy.
But here's where the magic happens: it doesn't stop there. The fission also generates additional neutrons. In the case of uranium isotope uranium-235, this is not one, but two or more! These new neutrons start racing off to be absorbed into other U-235 nuclei and therefore creating a domino effect of repeating the above steps (creating a chain reaction).
Control and Caution
Methods of Control
What can undergo Fission?
There are two key terms when talking about materials that can undergo the fission process:
- Fissile materials are capable of sustaining a nuclear chain reaction by undergoing nuclear fission.
- Fissionable (also known as fertile) materials are substances that can undergo nuclear fission upon absorbing a neutron, but they may not necessarily be able to sustain a self-sustaining chain reaction.
Examples of materials:
| Fissile | Fissionable |
|---|---|
|
Uranium-233 |
Thorium-232 |
|
Uranium-235 |
Uranium-238 |
|
Neptunium-239 |
Neptunium-237 |
|
Plutonium-239 |
Plutonium-238 |
An example of the amount of energy released
If 1 atom of uranium-235 underwent nuclear fission by absorbing a neutron, it has the potential to split into the nuclei barium-141, krypton-92 and 3 neutrons, this is not always the case but it is the most common:
Uranium-235
Neutron
Barium-141 (a common
product of nuclear
fission of Uranium-235)
Krypton-92 (a common
product of nuclear
fission of Uranium-235)
3 Neutrons
To use Albert Einstein’s equation E=mc², we need to calculate the amount of mass that has been converted into energy by finding the difference in masses before and after the reaction.
To do that we use a unit called the Atomic Mass Units (AMU), where 1 AMU has a mass of 1.66×10-27kg. The elements within this reaction have the following mass in AMUs:
| Particle | Atomic Mass (AMU) |
|---|---|
| Uranium-235 | 235.0439 |
| Neutron | 1.008665 |
| Barium-141 | 140.9144 |
| Krypton-92 | 91.9262 |
Next we need to work out the mass before the reaction takes place:
Before the reaction there is one atom of uranium-235 and one neutron meaning that the total mass is 236.0525 AMU.
[1.008665 + 235.0439 = 236.0525 AMU]
We also need to find the mass after the reaction to be able to determine the difference:
After the reaction there is one atom of Barium, one atom of Krypton and 3 Neutrons resulting in a mass of 235.8666 AMU.
[140.9144 + 91.9262 + (3 x 1.008665) = 235.8666 AMU]
This means that 0.18597 AMU of mass has been converted into energy as the first law of thermodynamics that energy can not be created or destroyed it can only be transferred into other forms.
[236.0525 – 235.8666 = 0.18597 AMU]
Using Einstein’s equation the total energy release can be calculated:
These tells us that for every atom of uranium-235 that undergoes Fission produces 2.78 x 10-11J of energy.
We can calculate how much energy is released from 1 g of standard nuclear fuel which has contains 5% uranium-235 by multiplying the amount of energy per atom by the number of atoms. First, we need to determine how many moles of uranium are being used where 1 mole will have a mass of 238.02891 g and therefore we are using 0.0002099 moles based on having 1 g of fuel which contains 5% uranium-235 meaning 0.05 g based on the assumption that the fuel is enriched to %5 by weight.
[0.05/238.02891 = 0.0002099 moles]
Avogadro’s Constant is the number of atoms in 1 mole which equals 6.022×1023 Atoms per Mole.
Therefore the total amount of energy released from 1 gram of standard nuclear fuel (Uranium-235 enriched to 5%) undergoing fission is:
How do we use nuclear fission?
The applications of nuclear fission are both awe-inspiring and practical. Here are a few examples:
Matt Moore
World Nuclear Association – Physics of Uranium and Nuclear Energy
IAEA – What is Nuclear Energy? The Science of Nuclear Power
Angelo – Atomic Masses
