Understanding Nuclear Fission Revision Notes | GCSE AQA Higher Physics

1. What Makes an Atom Split?

Picture the nucleus of a uranium atom - it's like an overstuffed suitcase, packed with 92 protons and 143 neutrons (235 nucleons total) all crammed together. The electromagnetic force is trying to push all those positively charged protons apart, while the strong nuclear force is desperately trying to hold everything together.

For most atoms, this balance works fine. But uranium-235 and plutonium-239 are different - they're so large that they're right on the edge of instability. One tiny push, and the whole nucleus can split apart violently.

Here's the crucial point: spontaneous fission is extremely rare. For fission to happen reliably, the unstable nucleus must first absorb a neutron. This extra neutron is like the final straw that breaks the camel's back - it provides just enough extra energy to overcome the forces holding the nucleus together.

2. The Fission Process - What Actually Happens

When a uranium-235 nucleus absorbs a neutron, it becomes uranium-236, rendering it highly unstable. Within a fraction of a second, the nucleus splits into two smaller nuclei, releasing 2-3 neutrons and gamma rays, along with approximately 200 MeV of energy.

But that's not all that comes out. The fission also releases 2-3 neutrons plus gamma rays. The energy released is enormous - about 200 million electron volts per fission event. To put this in perspective, burning one carbon atom releases only about 4 electron volts.

Worked example: A Typical Uranium-235 Fission Reaction

Show what happens when uranium-235 absorbs a neutron and undergoes fission.

1. $^{235}U + ^1n \rightarrow ^{236}U$ — The uranium-235 nucleus captures a slow-moving neutron
2. $^{236}U$ becomes highly unstable — The extra neutron makes the nucleus too heavy to stay together
3. $^{236}U \rightarrow ^{144}Ba + ^{89}Kr + 3^1n$ — The nucleus splits into barium-144, krypton-89, and 3 neutrons
4. Energy released ≈ 200 MeV — Plus gamma rays are emitted

Answer: One uranium-235 fission produces two smaller nuclei, 2-3 neutrons, gamma rays, and 200 MeV of energy

**Trap**: Many students think the fission fragments are stable. Actually, they're highly radioactive because they have too many neutrons for their size. They undergo further radioactive decay over time.

3. Chain Reactions - The Domino Effect

Here's where nuclear fission becomes really powerful. Remember that each fission event releases 2-3 neutrons. If even one of these neutrons hits another uranium-235 nucleus, it can cause that nucleus to split too, releasing more neutrons.

This creates a chain reaction - like dominoes falling over and knocking down more dominoes. In theory, one neutron could trigger an avalanche of fission events, each one releasing enormous energy.

But there's a catch. For a chain reaction to be sustained, you need enough fissile material in the right shape. Too little material, and most neutrons escape without hitting another nucleus.

In nuclear reactors, the chain reaction is carefully controlled using control rods that absorb neutrons. In nuclear weapons, the chain reaction is allowed to proceed uncontrolled, releasing all the energy in a fraction of a second.

4. Drawing Fission Diagrams

Exam questions often ask you to draw or interpret diagrams of nuclear fission. Here's what you need to show clearly in your diagrams.

Worked example: Drawing a Nuclear Fission Diagram

Draw a diagram showing nuclear fission of uranium-235 and the start of a chain reaction.

1. Draw an arrow pointing at a large circle (U-235 nucleus) — Label the arrow as 'neutron' and the circle as 'U-235'
2. Draw the large circle splitting into two smaller circles — Label these as 'fission fragments' - they should be roughly equal size
3. Draw 2-3 arrows coming out of the split — Label these as 'neutrons released'
4. Draw these neutrons hitting other U-235 nuclei — Show at least one neutron causing another fission event

Answer: Your diagram should clearly show: incoming neutron → fission → fragments + neutrons → further fission events

**Key diagram features examiners look for**: Clear labels, arrows showing neutron paths, roughly equal-sized fission fragments, and evidence of the chain reaction continuing.

5. Exam Technique and Common Mistakes

Nuclear fission questions test your understanding of the process and your ability to interpret diagrams. Here's how to maximise your marks.

**Common mistake**: Writing that fission 'uses up' neutrons. Actually, fission consumes one neutron but produces 2-3 neutrons, giving a net gain. This is what makes chain reactions possible.

**Mark-winning language**: Use precise terms like 'fission fragments', 'enough fissile material', and 'self-sustaining chain reaction'. Avoid vague phrases like 'atoms break apart' - use 'nuclei undergo fission'.