Fission: nucleus absorbs neutron, splits
Nuclear fission: a large, unstable nucleus (e.g. uranium-235, ) absorbs a neutron and then splits into two smaller nuclei, releasing 2–3 fast neutrons and a large amount of energy. Both elements are needed for full marks: the split into two smaller nuclei AND the release of neutrons and energy. Nucleon number () and proton number () are conserved in the equation.
Fusion: light nuclei join, mass deficit
Nuclear fusion: two light nuclei (e.g. hydrogen isotopes, and ) join to form one heavier nucleus. Energy is released because the product's mass is less than the combined mass of the reactants (the mass deficit is converted to energy). Fusion needs very high temperature ( K, to overcome the electrostatic repulsion between the positive nuclei) and high pressure (to raise the collision rate).
Chain reaction and reactor energy chain
A chain reaction occurs when the 2–3 neutrons from one fission are absorbed by other U-235 nuclei, causing further fissions; if each fission triggers at least one more, it is self-sustaining. This needs the fuel at or above its critical mass. Reactor energy chain: fission in the fuel rods heats a coolant → steam → the steam turns a turbine → the turbine drives a generator. Control rods and the moderator regulate the rate.
Drawn from real examiner reports.
Fission splits, fusion joins
The most-cited error: candidates reverse fission and fusion. Fission = split (one large nucleus splits into two smaller ones); fusion = join (two small nuclei join into one larger). Writing "fission is joining" or "fusion is splitting" scores zero. Mnemonic: fission → fissure (a crack); fusion → fuse together.
June 2023 Paper 21 Cambridge 0625 examiner report: fission and fusion were listed as "not well understood". 4PH1 November 2023 Paper 2P examiner report (proxy): "many candidates confused fission and fusion" — the highest-frequency error for nuclear energy content.
Energy from mass deficit, not bonds
Energy is NOT released "because bonds are broken" — that is chemistry thinking and scores zero for a nuclear reaction. The mark-scheme mechanism has two elements: (1) the total mass of the products is less than that of the reactants (mass deficit); (2) the mass difference is converted to energy (, qualitative at IGCSE). Never write "bonds broken".
November 2023 Paper 42 Cambridge 0625 mark scheme Q8(b)(ii): the two-element mark scheme for energy release requires (1) smaller total mass of products and (2) mass difference converted to energy — so the bond-breaking answer scores zero.
Control rods absorb; moderator slows
Don't swap the reactor parts. Control rods (boron/cadmium) absorb neutrons to reduce the reaction rate. The moderator (water/graphite) slows fast neutrons to make fission more likely. "Control rods slow neutrons" is wrong (that is the moderator); "moderator absorbs neutrons" is wrong (it slows them). Control rods need the word absorb.
November 2023 Paper 42 mark scheme Q8(c): control rods confirmed as "absorb neutrons to regulate reaction rate". The absorb vs slow distinction is a classic definition-precision trap on Cambridge P4 Extended questions.
Fission is triggered by neutron absorption
A fission does not happen "spontaneously" — it is triggered when the nucleus absorbs a neutron. Only stronger candidates state this; many write "the nucleus splits by itself". The absorbed neutron makes the large nucleus unstable so it splits, releasing 2–3 neutrons that can trigger further fissions. Always begin a fission description with neutron absorption.
November 2023 Paper 21 MCQ Q37: "Only stronger candidates were aware that the absorption of a neutron triggers fission." — the highest-frequency error on fission MCQ questions.
Fusion releases energy too
Both fission and fusion release energy — some think only fission does. In each case the total mass of the products is less than the reactants, and the mass deficit is released as energy. The difference is only the process: fission splits a heavy nucleus; fusion joins light nuclei. So "fusion absorbs energy" is wrong — the fusion of light nuclei releases it.
Fusion needs high temperature and pressure
Fusion needs both conditions, not just one: very high temperature ( K) AND high pressure. The high temperature gives the nuclei enough kinetic energy to overcome the electrostatic repulsion between their positive charges; the high pressure raises the collision rate. Writing only "high temperature", or saying the heat "melts" the nuclei, loses marks.
Energy-release: mass deficit in two steps
For "explain why fission/fusion releases energy" give two elements: (1) the total mass of the products is less than that of the reactants (mass deficit); (2) the mass difference is converted to / released as energy. Both are needed; never write "bonds are broken".
Match the command word
Match the command word: State the process in one phrase (fission = splitting; fusion = joining). Explain energy release with the two mass-deficit elements. Describe a chain reaction as a sequence. State the function of… each reactor component in one sentence.
Describe a chain reaction as a sequence
To describe a chain reaction, write the sequence: a U-235 nucleus absorbs a neutron → splits (fission) into two smaller nuclei → releases 2–3 neutrons and energy → each neutron is absorbed by another nucleus → causing further fissions. Show at least two generations.
Words to use and words to avoid
Use the precise words: nuclei (not atoms), fission/fusion (not just "nuclear reaction"), mass deficit (not "mass is lost") and neutrons (not particles). Avoid "bonds broken", "explosion" (a reactor is controlled) and "voltage".
Extended syllabus only — applies to Papers 2 (MCQ Extended) and 4 (Extended Theory). Core candidates (Papers 1, 3) do not study §5.3.
There are no numerical formulas required for Cambridge IGCSE §5.3. The key relationships are qualitative:
| Concept | Relationship | Notes |
|---|---|---|
| Energy release | Mass of products < mass of reactants → energy released | (qualitative only — do not apply numerically at IGCSE) |
| Chain reaction threshold | Each fission triggers ≥ 1 further fission → self-sustaining | Requires mass of fuel ≥ critical mass |
| Fusion conditions | Very high temperature AND high pressure | Temperature needed to overcome electrostatic repulsion |
| Reactor power output | Controlled by depth of control rods in core | Deeper rods → more neutrons absorbed → less fission → less heat |
Key vocabulary: Always use "nuclei" (not "atoms"), "fission" (splitting), "fusion" (joining), "neutrons" (not "particles"), "mass deficit" (not "bonds broken").
(E) What is a chain reaction in the context of nuclear fission?
(E) Describe how a chain reaction is produced in a sample of uranium-235 fuel.
[4 marks]
