Nuclear fission (AQA GCSE Physics): Revision Notes
Nuclear fission
What is nuclear fission?
Nuclear fission happens when a large, unstable atomic nucleus breaks apart into two smaller pieces. This process releases enormous amounts of energy, which we can use to generate electricity in nuclear power stations.
The process of nuclear fission is fundamental to both nuclear power generation and nuclear weapons. Understanding how atoms can be split to release energy helps explain one of the most powerful energy sources available to humanity.
The most common atoms used for fission are uranium-235 and plutonium. These atoms are unstable, which means their nuclei can easily split apart.
How nuclear fission works
When a uranium-235 nucleus absorbs a neutron, it becomes very unstable and splits into two smaller nuclei. This is a precisely defined process that follows specific nuclear physics principles.
The Fission Process:
When a uranium-235 nucleus absorbs a neutron, it becomes very unstable and splits into two smaller nuclei. This process:
- Creates two daughter nuclei (the smaller pieces)
- Releases 2-3 new neutrons
- Produces a large amount of energy
- Makes the daughter nuclei radioactive
Key point: Fission doesn't usually happen by itself. Most of the time, the uranium nucleus needs to absorb a neutron first before it will split.
Chain reactions
The neutrons released from one fission can hit other uranium nuclei nearby. This can cause them to split too, creating even more neutrons. This is called a chain reaction.
If we don't control this process, the chain reaction gets faster and faster. More and more nuclei split, releasing huge amounts of energy very quickly.
Uncontrolled Chain Reactions:
When chain reactions are not controlled, they accelerate exponentially. This rapid, uncontrolled release of energy is exactly what happens in a nuclear explosion. Each fission event triggers multiple new fission events, creating a cascade effect that releases enormous energy in fractions of a second.
Controlled chain reactions
Nuclear power stations use controlled chain reactions. This means they carefully manage how fast the fission happens.
How Nuclear Control Works:
Here's how nuclear power stations control the fission process:
- Special materials absorb some of the neutrons
- This slows down the chain reaction
- Only one neutron from each fission goes on to cause another fission
- The reaction stays steady rather than exploding
Why Control is Critical:
If too many neutrons cause more fissions, the reaction speeds up dangerously and could lead to a meltdown. If too few neutrons cause fissions, the reaction stops completely and the power station shuts down. The balance must be precisely maintained.
Using fission for electricity
Nuclear power stations harness the immense energy released during controlled fission to generate electricity for our homes and industries.
How Nuclear Power Generation Works:
Nuclear power stations use the energy from controlled fission to:
- Heat water to make steam using the energy from fission
- Use the steam to turn turbines connected to generators
- Generate electricity that can power thousands of homes
This process produces large amounts of electricity without burning fossil fuels, making it a clean energy source in terms of carbon emissions.
Remember!
Key Points to Remember:
- Nuclear fission splits large unstable nuclei into smaller pieces, releasing enormous amounts of energy
- Uranium-235 is commonly used because it splits easily when hit by neutrons
- Chain reactions happen when neutrons from one fission cause more fissions to occur
- Controlled chain reactions in power stations are carefully managed to produce steady, usable energy
- Fission produces radioactive daughter nuclei and releases additional neutrons that can continue the process