AQA GCSE Physics Foundation (8463), Foundation tier · Atomic structure › Nuclear fission and fusion
Practise Nuclear fission. 8 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Nuclear fission is the splitting of a large, unstable nucleus such as uranium-235 or plutonium-239. You need to describe what happens in fission, explain how a chain reaction happens and how it is controlled in a nuclear reactor, and draw or interpret fission diagrams. GCSE Physics only, not in Combined Science.
Key facts
Fission: a large, unstable nucleus splits into two smaller nuclei
Fuels: uranium-235 and plutonium-239
Usually starts when the nucleus absorbs a neutron; spontaneous fission is rare
Products: 2 smaller nuclei + 2 or 3 neutrons + gamma rays, all with kinetic energy
Chain reaction: the released neutrons cause further fissions
Nuclear fission is the splitting of a large and unstable nucleus, such as uranium or plutonium.
Spontaneous fission (splitting without a trigger) is rare. Usually the nucleus must first absorb a neutron.
The nucleus then splits into two smaller nuclei, roughly equal in size, and emits two or three neutrons and gamma rays.
Fission of uranium-235. All the products move off fast: they have kinetic energy.
Energy is released. All of the fission products have kinetic energy.
Chain reactions
diagram
The neutrons released can be absorbed by other uranium or plutonium nuclei, which then split too. This is a chain reaction.
Each fission releases 2 or 3 neutrons, so without control the number of fissions grows very quickly.
In a diagram, show a neutron being absorbed by a large nucleus, the nucleus splitting into two smaller nuclei, and 2 or 3 neutrons coming out, each going on to a new large nucleus.
A chain reaction: each fission releases neutrons that cause more fissions.
Controlled and uncontrolled chain reactions
In a nuclear reactor the chain reaction is controlled, to control the energy released.
Control rods absorb some of the neutrons. Lowering them further into the reactor slows the reaction; raising them speeds it up.
For a steady rate, on average one neutron from each fission goes on to cause another fission.
The energy released is used to heat water to make steam, which turns turbines to generate electricity.
The explosion of a nuclear weapon is caused by an uncontrolled chain reaction.
How to answer each type of question
Describe the process of fission
3 to 4 marksGrade 5
Start with the nucleus absorbing a neutron.
Say what it splits into.
Say what else is emitted and that energy is released.
Example. Describe what happens when a uranium-235 nucleus undergoes fission.
Show the model answerHide the model answer
The nucleus absorbs a neutron (1). It splits into two smaller nuclei of roughly equal size (1). Two or three neutrons are released, along with gamma rays (1). Energy is released / the products have kinetic energy (1).
Don’t lose marks
Saying fission starts with the nucleus absorbing a proton or an alpha particle. It absorbs a neutron.
Saying the nucleus splits into lots of pieces, or into protons and neutrons. It splits into two smaller nuclei, plus 2 or 3 neutrons.
Saying control rods absorb 'energy' or 'radiation'. They absorb neutrons.
Drawing a chain reaction where each fission releases only one neutron, or where the neutrons do not go on to other nuclei.
More tips
Memory tricks
Fission = splitting (a fissure is a split or crack).
Fission recipe: 1 neutron in → 2 smaller nuclei + 2 or 3 neutrons + gamma rays + energy out.
Control rods catch neutrons: more rod in → fewer neutrons → slower reaction.
Exam technique
In chain reaction diagrams, label the neutrons, the large nuclei and the smaller nuclei produced.
Write 'the nucleus absorbs a neutron' rather than just 'is hit by a neutron'.
When comparing a reactor and a weapon, the key words are 'controlled' and 'uncontrolled'.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 4
Define nuclear fissionThe splitting of a large, unstable nucleus into two smaller nuclei.
Grade 4
Name nuclear fuels used for fissionUranium (e.g. uranium-235) and plutonium (e.g. plutonium-239).
Grade 5
Describe the products of fissionTwo smaller nuclei of similar size, two or three neutrons and gamma rays, all with kinetic energy.
Grade 5
Compare a reactor with a nuclear weaponIn a reactor the chain reaction is controlled; in a weapon it is uncontrolled, causing an explosion.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Name one element used as a fuel in nuclear reactors.
Uranium
Name the other element that is commonly used as a fuel for nuclear fission.
Plutonium
State one thing, other than energy and two smaller nuclei, that is released when a large nucleus undergoes fission.
Two or three neutrons (or gamma rays).
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
(a) What is nuclear fission? Tick (✓) one box.[1]
The splitting of a large, unstable nucleus into two smaller nuclei
The joining of two light nuclei to form a heavier nucleus
The emission of an alpha particle from a nucleus
The absorption of an electron by a nucleus
(b) Name one element used as a fuel in nuclear reactors.[1]
(c) What must usually be absorbed by a uranium-235 nucleus before it undergoes fission?[1]
(d) As well as two smaller nuclei, what else is released when a nucleus undergoes fission? Tick (✓) two boxes.[2]
Two or three neutrons
Gamma rays
Alpha particles
Electrons from the atom
Show the answer and mark scheme
(a)Answer: The splitting of a large, unstable nucleus into two smaller nuclei
(b)Answer: Uranium
uranium / plutonium
(c)Answer: A neutron
a neutron
(d)Answer: Two or three neutrons; Gamma rays
Question 2Medium6 marks
Uranium-235 is used as the fuel in many nuclear power stations.
(a) Describe what happens when a uranium-235 nucleus undergoes fission.[4]
(b) Name the other element that is commonly used as a fuel for nuclear fission.[1]
(c) Spontaneous fission is rare. What does this tell you about how fission usually happens in a reactor?[1]
Show the answer and mark scheme
(a)Answer: The nucleus absorbs a neutron and splits into two smaller nuclei of roughly equal size, releasing two or three neutrons, gamma rays and energy; the products have kinetic energy.
the nucleus absorbs a neutron
it splits into two smaller nuclei (of roughly equal size)
two or three neutrons are released
gamma rays are emitted / energy is released
the fission products have kinetic energy
(b)Answer: Plutonium
plutonium
(c)Answer: Nuclei usually split only after absorbing a neutron.
fission usually happens only after the nucleus has absorbed a neutron (so neutrons are needed to start / keep the reaction going)