Atomic Structure - 4.4.1 Nuclear Fission (GCSE Physics AQA)

Nuclear Fission

Splitting Nuclei

• We can split a nucleus. Through the process of nuclear fission, we can split a large, unstable nucleus into 2 smaller particles. This is made possible through the use of a nuclear reaction.
• Nuclei undergo induced fission. Nuclei don’t often split up on their own; instead, they have to undergo induced fission. During this process, they absorb a neutron, which leads to fission.
• Spontaneous fission is rare. Sometimes, a nucleus doesn’t have to absorb a neutron to undergo fission. This is called spontaneous fission, but this is very rare.

Process of Nuclear Fission

• The nucleus absorbs a neutron. In most fission reactions, the unstable nucleus will absorb a neutron as the first step.
• The nucleus splits into 2. Once the neutron has been absorbed, the nucleus will split into 2 smaller nuclei. Energy is released through the fission reaction.
• Neutrons and gamma rays are emitted. The nucleus will also emit 2 or 3 high energy neutron when the fission reaction is happening. As well as 2 or 3 neutrons, gamma rays are also emitted.

Products of Nuclear Fission

• Fission products have KE. The neutrons and 2 smaller nuclei released in a fission reaction will have kinetic energy. The KE allows the fission products to move away from the original reaction site.
• Gamma rays carry excess energy. Any energy that is not absorbed by the products of the reaction is given to the gamma rays. This means that the gamma rays will move away from the reaction site as well, often at very high speeds.
• Neutrons can start a chain reaction. The neutrons released from a fission reaction will have kinetic energy. Therefore, the neutrons can move around and become absorbed by another large, unstable nucleus. This will lead to a second fission reaction, resulting in a chain reaction.

Controlling the Chain Reaction

• Chain reactions can be dangerous. It is very difficult to stop a chain reaction, meaning it can become dangerous very quickly. Many neutrons are produced by each fission reaction, leading to multiple fission reactions.
• Nuclear reactors use control rods. Inside a nuclear reactor, there are a controlled series of fission reactions occurring. We do this by using control rods inside the reactor.
• Control rods absorb neutrons. By placing control rods into the reactor, we can absorb excess neutrons. We know that neutrons induce fission reactions, so by absorbing some of them, we are limiting the number of fission reactions that occur.
• Nuclear weapons have uncontrolled reactions. Inside a nuclear weapon, there are uncontrolled nuclear fission reactions happening. This means that there is an uncontrolled chain reaction, which ultimately leads to an explosion.

Representing Nuclear Fission

Below, we can see a representation of nuclear fission. You may be asked to draw a diagram like this in an exam, so here’s how to do it:

1. Draw a neutron. This is essentially a small circle. You can label it ’n’.
2. Draw a large nucleus. As you can see in the diagram below, the nucleus must be made up of smaller particles.
3. Draw 2 more nuclei. Since this is a fission reaction, we need to draw the 2 neutrons that are produced.
4. Draw 2 or 3 neutrons. Fission reactions produce more neutrons, which we need to represent on our diagram as small circles again.
5. Draw more fission reactions. From one of the neutrons, draw an arrow to another nuclei. This is the continuation of the chain reaction.

The end product should look similar to the diagram below: