4.4.4.1Nuclear fission
Physics only — not assessed in Combined Science: Trilogy.
Nuclear fission is the splitting of a large and unstable nucleus, such as uranium or plutonium.
Spontaneous fission is rare. Usually, for fission to occur the unstable nucleus must first absorb a neutron.
The nucleus undergoing fission splits into two smaller nuclei, roughly equal in size, and emits two or three neutrons plus gamma rays. Energy is released by the fission reaction.
All of the fission products have kinetic energy — that kinetic energy is where the released energy actually goes, and in a reactor it is what ends up heating the water.
The neutrons may go on to start a chain reaction.
The chain reaction is what makes fission useful and dangerous at once. Each fission releases two or three neutrons, and if more than one of them goes on to cause another fission, the number of reactions grows with every step.
The chain reaction is controlled in a nuclear reactor to control the energy released. The explosion caused by a nuclear weapon is caused by an uncontrolled chain reaction. In a reactor, control rods absorb some of the neutrons so that on average exactly one neutron from each fission causes the next — which keeps the rate steady. Pushing the rods further in absorbs more neutrons and slows the reaction; withdrawing them speeds it up.
You should be able to draw and interpret diagrams representing nuclear fission and how a chain reaction may occur.
Worked example
Describe, in order, what happens in a single fission event and explain how it can lead to a chain reaction.
- A large unstable nucleus, such as uranium, absorbs a neutron — spontaneous fission is rare, so this first step is usually needed.
- The nucleus becomes even more unstable and splits into two smaller nuclei, roughly equal in size.
- Two or three neutrons are emitted, along with gamma rays, and energy is released.
- All the fission products carry kinetic energy.
- Each emitted neutron can be absorbed by another large nucleus, causing it to undergo fission in turn — so the number of fissions can grow, giving a chain reaction.
Answer: Absorb a neutron, split into two roughly equal nuclei, emit two or three neutrons and gamma rays, release energy as kinetic energy of the products — and those neutrons can each trigger a further fission, which is the chain reaction.
Common mistake
The two smaller nuclei are roughly equal in size — fission is not a nucleus shedding a small particle. That is what distinguishes it from alpha decay.
- Nuclear fission
- the splitting of a large and unstable nucleus, such as uranium or plutonium, into two smaller nuclei roughly equal in size, emitting two or three neutrons plus gamma rays and releasing energy
- Chain reaction
- a reaction in which the neutrons released by fission go on to cause further fission in other nuclei
4.4.4.2Nuclear fusion
Physics only — not assessed in Combined Science: Trilogy.
Nuclear fusion is the joining of two light nuclei to form a heavier nucleus. In this process some of the mass may be converted into the energy of radiation.
That is the whole of the specification's content for fusion, and the two halves are both examined. Light nuclei, not heavy ones — which is the opposite of fission. And the energy comes from mass being converted into radiation, which does not happen in any other process at GCSE.
Common mistake
Fission and fusion are easily confused because the words look alike. Fission splits — think of a fissure, a crack. Fusion joins — think of fusing two things together.
- Nuclear fusion
- the joining of two light nuclei to form a heavier nucleus, in which some of the mass may be converted into the energy of radiation
Before you practise
- This sub-topic is physics only — it is not assessed in Combined Science: Trilogy.
- Nuclear fission is the splitting of a large and unstable nucleus, such as uranium or plutonium.
- Spontaneous fission is rare; usually the nucleus must first absorb a neutron.
- Fission gives two smaller nuclei roughly equal in size, two or three neutrons and gamma rays, and releases energy.
- All of the fission products have kinetic energy.
- The emitted neutrons may go on to start a chain reaction.
- A reactor controls the chain reaction to control the energy released; a nuclear weapon's explosion is an uncontrolled chain reaction.
- Nuclear fusion is the joining of two light nuclei to form a heavier nucleus, and some mass may be converted into the energy of radiation.