Nuclear fission — splitting a large nucleus
Fission is the splitting of a large nucleus into two smaller nuclei. A slow neutron is absorbed by a uranium-235 nucleus, which splits into two smaller radioactive daughter nuclei plus a small number of neutrons (usually two or three), releasing energy as the kinetic energy of the fast-moving fission products. A balanced example is U-235 + n → Ba-141 + Kr-92 + 3 neutrons. Fission must be triggered by an incoming neutron — unlike radioactive decay, which is spontaneous.
Chain reaction and reactor control
A chain reaction is set up when neutrons released by one fission strike other U-235 nuclei, causing further fissions that release more neutrons. A reactor keeps the rate steady with three parts: control rods (boron) absorb neutrons so on average only one neutron per fission causes the next; the moderator (graphite or water) slows fast neutrons down so U-235 is more likely to absorb them; and shielding (thick concrete and lead) absorbs the emitted radiation to protect workers.
Nuclear fusion — joining small nuclei
Fusion is the joining of two small nuclei into a larger nucleus. The larger nucleus has slightly less mass than the nuclei that formed it, and this lost mass is released as energy — fusion is the energy source of stars. Fusion is hard to start because every nucleus is positively charged (protons), so two nuclei repel by the electrostatic force. Only at very high temperature and pressure can they move fast enough to overcome this repulsion and fuse.
Drawn from real examiner reports.
Fission ≠ fusion
Swapping these two words is the classic error. Fission = a large nucleus is split into two smaller nuclei when hit by a neutron. Fusion = two small nuclei join to make a larger nucleus. A definition needs both the process (split/join) and the size change (large→small or small→large). "Atoms break apart" or "energy is made" scores nothing. Hook: fuSion joins, fiSSion SSplits.
Chain reaction described too vaguely
The mark scheme wants the full loop: a fission releases neutrons → those neutrons strike other U-235 nuclei → more fissions → still more neutrons. "It keeps going" or "one reaction causes another" does not score. You must name the neutron as the thing absorbed by further U-235 nuclei to carry the reaction on, rather than answering with a vague "it speeds up".
Control rods ≠ moderator
These two reactor parts are often swapped. Control rods (e.g. boron) absorb neutrons to control the rate of the chain reaction — lowering them further absorbs more neutrons and slows it. The moderator (graphite or water) slows fast neutrons down so U-235 is more likely to absorb them. Saying "control rods slow neutrons" or "the moderator absorbs neutrons" loses the mark.
Shielding absorbs radiation
Shielding is the third reactor part and has its own job: thick concrete and lead around the core absorb the dangerous nuclear radiation (and stray neutrons) emitted, protecting the workers and surroundings. Do not confuse it with the moderator (which slows neutrons) or the control rods (which absorb neutrons to control the rate). All three sound similar but do different jobs.
Fusion needs a reason, not just "hot"
Many answer "it needs to be very hot" without the reason. The mark is for the mechanism: nuclei are positively charged (protons), so two approaching nuclei repel by the electrostatic force. Only very high temperature (fast nuclei) and high pressure (frequent collisions) let them get close enough to overcome this repulsion and fuse. Name the electrostatic repulsion to score.
Energy is released, not created
Never write that fission or fusion "makes" or "creates" energy — energy cannot be created. In fusion the energy is released from a loss of mass (the larger nucleus has slightly less mass than the small nuclei that formed it); in fission it appears as the kinetic energy of the fast-moving fission products. Always say the energy is released.
Nuclear ≠ chemical energy
Fission and fusion are nuclear reactions — changes in the nucleus — and release far more energy per event than chemical reactions such as burning a fuel. Nothing is "burning" in a star or reactor. Examiners found candidates confusing chemical and nuclear energy stores when describing how a star forms, so keep the two apart: only nuclear changes release this much energy.
June 2024 Paper 1P Q4(a): candidates confused chemical and nuclear reactions/stores.
Answer "difference" with both sides
For "explain the difference between fission and fusion" (2 marks) give one clear statement of each: fission = a large nucleus splits into smaller ones; fusion = small nuclei join into a larger one. Naming only one side caps the mark — pair the two processes explicitly.
Balance nucleon and proton numbers
For a nuclear equation, use conservation: the nucleon (top) numbers add to the same total on both sides, and so do the proton (bottom) numbers. This lets you find a missing neutron count or an unknown daughter nucleus — 235 + 1 = 141 + 92 + 3 fixes the number of neutrons.
Name the cause in "explain why" answers
For "why does fusion need high temperature and pressure", always name the cause — the electrostatic repulsion between the positive nuclei — then the effect: they need enough energy to get close enough to fuse. A statement with no reason does not score on an "explain" question.
Write a chain reaction as a sequence
Set a chain reaction out as a sequence: fission releases neutrons → neutrons hit other U-235 nuclei → more fissions. Use the marks available as a guide to how many distinct points to make, and use any numbers given rather than answering in words only.
There is no to learn at this level — on 4SD0 the mass loss in fusion and the energy release in fission are treated qualitatively. The quantitative marks come from (1) balancing nuclear equations using conservation of nucleon and proton number, (2) chain-reaction growth, and (3) applying the electricity relationships to a nuclear power station:
| Idea | Rule / formula | Used for |
|---|---|---|
| Nucleon-number conservation | (top numbers) sum of products | find a missing neutron count |
| Proton-number conservation | (bottom numbers) sum of products | find a missing daughter proton number |
| Chain-reaction growth | fissions in generation | how the number of fissions multiplies |
| Electrical energy | energy generated by a reactor of power in time | |
| Efficiency | efficiency of a nuclear power station |
Where = energy (J), = power (W), = time (s), and = number of further fissions caused by each fission. In a balanced nuclear equation the isotope notation is , with = nucleon (mass) number and = proton (atomic) number.
Definitions (mark-scheme form):
Define nuclear fission.
A single fission of uranium-235 is represented by the equation:
Use conservation of nucleon (mass) number to calculate the number of neutrons, , released in this fission.
