Nuclear fission and fusion (physics only) – GCSE Physics exam questions with mark scheme

1Grade 6–7Triple only5 marks

Nuclear power stations use nuclear fission of uranium-235.

(a) Explain how a chain reaction occurs in a nuclear reactor. [4]

(b) Give one difference between nuclear fission and nuclear fusion. [1]

(a) a (slow-moving) neutron is absorbed by a uranium-235 nucleus(1)
(a) the nucleus becomes unstable and splits into two smaller nuclei (daughter nuclei), releasing energy(1)
(a) two or three neutrons are also released (ignore mention of gamma rays)(1)
(a) these neutrons are absorbed by other uranium-235 nuclei, causing further fission(1)
(b) fission splits a large / heavy nucleus (into two smaller nuclei) whereas fusion joins two light nuclei to form a heavier nucleus allow fission splits nuclei, fusion joins nuclei allow fusion needs very high temperature (and pressure) / happens in stars, fission does not(1)
Answer: (a) Neutron absorbed by U-235, nucleus splits into two daughter nuclei + 2–3 neutrons + energy; those neutrons cause further fissions. (b) Fission splits a heavy nucleus; fusion joins two light nuclei.
Examiner tip: The chain reaction needs the released NEUTRONS to go on to hit further nuclei — say this explicitly.

2Grade 4–5Triple only4 marks

The equation shows one possible fission reaction of uranium-235.

$$^{235}_{92}\text{U} + {}^{1}_{0}\text{n} \rightarrow {}^{94}_{38}\text{Sr} + {}^{A}_{Z}\text{Xe} + 2\,{}^{1}_{0}\text{n}$$

(a) Determine the values of A and Z. [3]

(b) Give one way in which this reaction is different from spontaneous fission. [1]

(a) total mass number on the left = 236 (allow 235 + 1 seen, or 236 − 94 − 2 seen)(1)
(a) A = 140(1)
(a) Z = 54 (allow 92 − 38) if A and Z are swapped, max 2(1)
(b) the uranium nucleus must absorb a neutron first / the fission is caused (induced) by a neutron, whereas spontaneous fission happens without absorbing a neutron (allow spontaneous fission happens on its own / without a trigger; ignore ‘spontaneous fission is rare’ alone)(1)
Answer: (a) A = 140, Z = 54; (b) here a neutron must be absorbed first.
Examiner tip: Don’t forget the 2 in front of the neutrons — it counts as a total mass number of 2, so A = 236 − 94 − 2.

3Grade 4–5Triple only6 marks

The diagram shows the start of a chain reaction in uranium-235. The daughter nuclei and gamma rays are not shown.

(a) How many neutrons are released by each fission shown in the diagram? [1]

(b) How many fissions take place in the 3rd generation? [1]

Determine the number of fissions in the 6th generation. [1]

(d) In a nuclear reactor the chain reaction must be controlled.

Explain how the chain reaction is controlled and why this is necessary. [3]

(a) 2 / two(1)
(b) 4 / four(1)
(c) 32 (allow 2⁵)(1)
(d) control rods (are lowered into the reactor core) (allow boron / cadmium rods)(1)
(d) the rods absorb neutrons, so fewer neutrons go on to cause further fission (allow so on average only one neutron from each fission causes another fission)(1)
(d) so energy is released at a steady / safe rate (allow to stop the reactor overheating / to prevent an explosion; allow an uncontrolled chain reaction is what happens in a nuclear bomb) ignore ‘to make it safe’ unqualified(1)
Answer: (a) 2; (b) 4; (c) 32; (d) control rods absorb neutrons so fewer fissions occur, keeping the energy release steady and preventing an uncontrolled (bomb-like) reaction.
Examiner tip: Say what the control rods absorb — ‘they slow the reaction’ on its own does not score.

4Grade 6–7HigherTriple only5 marks

Each fission of a uranium-235 nucleus releases 3.2 × 10⁻¹¹ J of energy.

The mass of one uranium-235 nucleus is 3.9 × 10⁻²⁵ kg.

(a) Calculate the energy released when 1.0 g of uranium-235 undergoes fission. Give your answer in standard form. [3]

(b) Burning 1 kg of coal releases 3.0 × 10⁷ J.

Calculate the mass of coal that would release the same energy as your answer to part (a). [2]

(a) number of nuclei = 1.0 × 10⁻³ ÷ 3.9 × 10⁻²⁵ (allow 1.0 ÷ 3.9 × 10⁻²⁵ for this mark only if 1.0 g not converted is the only error — then max 2)(1)
(a) = 2.56 × 10²¹ (nuclei) (allow 2.6 × 10²¹)(1)
(a) energy = 8.2 × 10¹⁰ J (allow 8.21 × 10¹⁰ J; allow 8.3 × 10¹⁰ J from rounded 2.6 × 10²¹) an answer of 8.2 × 10¹³ J scores 2(1)
(b) mass = 8.2 × 10¹⁰ ÷ 3.0 × 10⁷ (allow ecf from (a))(1)
(b) = 2700 kg (allow 2730 / 2735 / 2.7 × 10³ kg; allow 2.7 tonnes)(1)
Answer: (a) 8.2 × 10¹⁰ J; (b) ≈ 2700 kg of coal.
Examiner tip: Convert 1.0 g to 1.0 × 10⁻³ kg before dividing — leaving it in grams gives an answer 1000 times too big.

5Grade 4–5Triple only5 marks

In one fusion reaction, a hydrogen-2 nucleus and a hydrogen-3 nucleus join to form a helium-4 nucleus and particle X.

$$^{2}_{1}\text{H} + {}^{3}_{1}\text{H} \rightarrow {}^{4}_{2}\text{He} + \text{X}$$

(a) Identify particle X. Give a reason for your answer. [2]

(b) The table gives the masses of the particles.

Particle	Mass in kg
hydrogen-2 nucleus	3.3436 × 10⁻²⁷
hydrogen-3 nucleus	5.0074 × 10⁻²⁷
helium-4 nucleus	6.6447 × 10⁻²⁷
particle X	1.6749 × 10⁻²⁷

Calculate the decrease in mass during the reaction. [2]

(a) neutron(1)
(a) mass number of X = (2 + 3) − 4 = 1 and atomic number = (1 + 1) − 2 = 0 (allow X has no charge and mass 1; allow it is ¹₀n) reason only scores if linked to the numbers(1)
(b) (3.3436 + 5.0074) × 10⁻²⁷ − (6.6447 + 1.6749) × 10⁻²⁷ (allow 8.3510 × 10⁻²⁷ and 8.3196 × 10⁻²⁷ both seen)(1)
(b) 3.14 × 10⁻²⁹ kg (allow 0.0314 × 10⁻²⁷ kg)(1)
(c) it is converted to energy / released as (radiation) energy (allow transferred to the kinetic energy (store) of the products; ignore ‘destroyed’; do not accept ‘it is turned into neutrons’)(1)
Answer: (a) a neutron (mass number 1, atomic number 0); (b) 3.14 × 10⁻²⁹ kg; (c) it is converted into energy (radiation / kinetic energy of the products).
Examiner tip: Mass is not ‘destroyed’ in fusion — say it is converted into energy.

6Grade 6–7Triple only5 marks

Scientists are developing power stations that use nuclear fusion of hydrogen isotopes.

(a) Fusion only happens at extremely high temperatures, such as in the core of a star.

Explain why. [2]

(b) There are no fusion power stations generating electricity commercially yet.

Suggest two reasons why. [2]

(a) (hydrogen) nuclei are (both) positively charged so they repel each other (allow electrostatic repulsion between nuclei; do not accept ‘atoms / electrons repel’ unqualified)(1)
(a) at high temperature the nuclei move fast enough / have enough kinetic energy to get close enough to join (overcome the repulsion)(1)
(b) any one from: very high temperatures / pressures are difficult (and expensive) to produce or maintain; it is hard to contain the very hot fuel / no material can withstand the temperature (allow plasma is held by magnetic fields, which is difficult)(1)
(b) any second from the list above, or: so far more energy has to be put in than is (usefully) produced; the technology is still being developed / is very expensive(1)
(c) any one from: fuel (hydrogen isotopes) is plentiful / can be obtained from seawater; produces little or no long-lived radioactive waste (allow less radioactive waste); there is no (risk of an uncontrolled) chain reaction / meltdown; releases more energy per kilogram of fuel ignore ‘no carbon dioxide’ (true of both); ignore ‘renewable’(1)
Answer: (a) Nuclei are positive and repel; only at very high temperature do they move fast enough to get close and fuse. (b) e.g. extreme temperature/pressure hard to achieve and contain; more energy input than output so far. (c) e.g. abundant fuel / less long-lived radioactive waste.
Examiner tip: In (a) say it is the nuclei that repel because they are positive — not the atoms or the electrons.

7Grade 6–7Triple only6 marks

Both a nuclear power station and a nuclear bomb release energy from a chain reaction of uranium-235 fission.

Explain how the fission chain reaction releases energy and how the chain reaction in a power station differs from the chain reaction in a nuclear bomb.

In your answer you should refer to what happens to the energy released. [6]

Fission: a uranium-235 nucleus absorbs a (slow) neutron; becomes unstable and splits into two smaller (daughter) nuclei of roughly equal size; two or three neutrons and gamma rays are also released; energy is released(1)
Energy: the daughter nuclei and neutrons have (a lot of) kinetic energy; they collide with surrounding atoms so energy is transferred to the thermal energy store of the fuel / reactor core; gamma rays also transfer energy; (in a power station) the coolant carries this energy to heat water to make steam which turns a turbine and generator(1)
Chain reaction: released neutrons are absorbed by other U-235 nuclei which also undergo fission; each fission can cause more fissions so the number of fissions can increase rapidly(1)
Power station (controlled): control rods absorb some of the neutrons; on average only one neutron from each fission causes a further fission; the rate of fission stays constant so energy is released at a steady rate; rods can be lowered to reduce the rate / shut the reactor down(1)
Bomb (uncontrolled): neutrons are not absorbed, so most go on to cause fission; the number of fissions increases very rapidly (eg doubles or trebles each generation); a huge amount of energy is released in a very short time, causing an explosion(1)

Level	Marks	What it looks like
Level 3	5–6	A detailed, logically ordered explanation of fission and the chain reaction that links the products’ kinetic energy to heating, and a clear comparison explaining how neutron absorption keeps the reactor’s rate steady while the bomb’s rate grows rapidly.
Level 2	3–4	A description of fission and the chain reaction with some reference to energy, plus a valid difference between the reactor and the bomb; the mechanism of control or the energy transfer may be incomplete.
Level 1	1–2	Simple, separate statements about fission, neutrons or the difference between controlled and uncontrolled reactions, with little linking.
No relevant content	0	Nothing worthy of credit.

Answer: Neutron absorbed by U-235 → splits into two daughter nuclei + 2–3 neutrons + gamma; products have kinetic energy transferred to the thermal store of surroundings. Released neutrons cause further fissions. In a reactor control rods absorb neutrons so about one per fission continues and energy is released steadily; in a bomb the chain reaction is uncontrolled, fissions multiply each generation and the energy is released almost instantly as an explosion.
Examiner tip: Top-level answers say how control works (rods absorb neutrons so about one per fission continues) — not just ‘the reaction is controlled’.

8Grade 8–9HigherTriple only7 marks

The graph shows how the power output of a nuclear reactor changed over 90 minutes.

At one time during the 90 minutes, the operators lowered the control rods further into the reactor core.

(a) Give the time at which the control rods were lowered further. [1]

(b) Between 20 minutes and 50 minutes the power output was constant.

Explain, in terms of neutrons, why the power output was constant. [2]

While the power output was constant, how many neutrons per fission, on average, did not cause a further fission? [1]

(d) Each fission releases 3.2 × 10⁻¹¹ J.

Use the graph to calculate the number of fissions happening each second at 80 minutes. [3]

(a) 50 minutes (allow 49–51 min)(1)
(b) the number of fissions (per second) / rate of fission stays the same(1)
(b) (because) on average one neutron from each fission goes on to cause another fission (allow the rest are absorbed by control rods / escape, so the number of neutrons stays constant)(1)
(c) 1.4(1)
(d) power read as 400 MW and converted to 4.0 × 10⁸ W / J/s (allow 4.0 × 10⁸ J released each second)(1)
(d) number = 4.0 × 10⁸ ÷ 3.2 × 10⁻¹¹(1)
(d) = 1.25 × 10¹⁹ (per second) (allow 1.3 × 10¹⁹) an answer of 1.25 × 10¹³ (400 MW not converted to W) scores 2 marks(1)
Answer: (a) 50 min; (b) rate of fission constant — on average one neutron per fission causes another fission; (c) 1.4; (d) 1.25 × 10¹⁹ fissions per second.
Examiner tip: ‘Constant power’ means each fission triggers exactly one more on average — say ‘one neutron per fission’, not just ‘the reaction is controlled’.

9Grade 1–3FoundationTriple only6 marks

Nuclear power stations release energy from nuclear fission.

(a) Name two elements whose nuclei are used as fuel in nuclear reactors. [2]

(b) Complete the sentence.

Before a large nucleus can split, it usually has to absorb a ____________ . [1]

How often does spontaneous fission happen? Tick (✓) one box. [1]

It happens very rarely.	☐
It happens in about half of all fission reactions.	☐
It happens in most fission reactions.	☐

(d) Complete the sentence about nuclear fusion. Use words from the box. [2]

heavier

heavy

light

neutral

radioactive

In nuclear fusion, two ____________ nuclei join together to form a ____________ nucleus.

(a) uranium (allow U; allow uranium-235 / U-235 / uranium-238)(1)
(a) plutonium (allow Pu; allow plutonium-239) do not accept a second isotope of the same element for the second mark; ignore thorium(1)
(b) neutron (do not accept proton / electron)(1)
(c) ‘It happens very rarely’ ticked only (more than one box ticked = 0)(1)
(d) light (first gap) — do not accept heavy(1)
(d) heavier (second gap) — do not accept heavy(1)
Answer: (a) uranium and plutonium; (b) neutron; (c) very rarely; (d) light … heavier.
Examiner tip: Fusion joins two light nuclei to make a heavier one — don’t mix it up with fission, where a heavy nucleus splits.

10Grade 1–3FoundationTriple only6 marks

The diagram shows what happens during one nuclear fission reaction.

(a) Name particle A. [1]

(b) The neutrons released by this fission can be absorbed by other uranium-235 nuclei and cause more fissions.

What name is given to this process? [1]

(d) How many neutrons are released by the fission shown? [1]

(e) What does the wavy line labelled D represent? [1]

(f) After the fission, nuclei B and C and the neutrons all move apart very quickly.

Which energy store do B, C and the neutrons all have? [1]

(a) neutron (do not accept proton)(1)
(b) chain reaction (allow nuclear chain reaction; do not accept ‘fission’ / ‘nuclear reaction’ alone)(1)
(c) (roughly) the same size / about equal (allow B is slightly larger than C; do not accept ‘C is much smaller’)(1)
(d) 3 / three(1)
(e) gamma rays / gamma radiation (allow γ; allow electromagnetic radiation; do not accept alpha or beta)(1)
(f) kinetic (energy store) (do not accept thermal / heat alone)(1)
Answer: (a) neutron; (b) chain reaction; (c) roughly equal; (d) 3; (e) gamma radiation; (f) kinetic.
Examiner tip: The fission products are roughly equal in size — a common wrong idea is that the nucleus just chips off a small piece.

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