PhysicsICSE Class 10 · Nuclear Fission and Fusion

Nuclear Fusion: Thermonuclear Reactions, Fusion Equations and the Energy of the Sun (ICSE Class 10)

Updated 16 October 2026 · Fact-checked

Nuclear fusion is the joining of two light nuclei, such as isotopes of hydrogen, to form a heavier nucleus, with the release of a large amount of energy. It needs temperatures of millions of kelvin so that the positive nuclei can overcome their repulsion, which is why it is called a thermonuclear reaction. Fusion powers the Sun and stars.

Two light nuclei need millions of kelvin to fuse; the product has slightly less mass, and that lost mass is released as energy (E = mc²).

Illustration drawn for QuestionBuddy.

Understand Nuclear Fusion and the Energy of the Sun

What fusion is. Nuclear fusion is the process in which two light nuclei combine to form a heavier nucleus, releasing a large amount of energy.

Examples (each balanced in mass number and atomic number):

$_{1}^{2} H +_{1}^{3} H \to_{2}^{4} He +_{0}^{1} n + energy$ (deuterium + tritium)
$_{1}^{2} H +_{2}^{3} He \to_{2}^{4} He +_{1}^{1} H + energy$
$_{1}^{1} H +_{1}^{2} H \to_{2}^{3} He + γ$ (a step in the Sun)

The hydrogen isotopes are protium $_{1}^{1} H$ , deuterium $_{1}^{2} H$ (D) and tritium $_{1}^{3} H$ (T).

Why it needs very high temperatures. All nuclei are positively charged and repel each other. They can fuse only if they come extremely close, so they must be moving very fast. This needs temperatures of millions of kelvin (and high pressure, to keep the nuclei crowded together). Because heat is needed to start it, fusion is called a thermonuclear reaction. Light nuclei such as hydrogen are used because they carry the smallest charge, so the repulsion is least.

Where the energy comes from. The mass of the product nucleus (and particles) is less than the total mass of the nuclei that fused. The lost mass $m$ becomes energy $E = m c^{2}$ . When hydrogen fuses into helium, about 0.7% of the mass is converted, compared with about 0.1% in the fission of uranium, so fusion releases more energy per kilogram of fuel.

Energy of the Sun and stars. In the Sun's core the temperature is about 15 million kelvin. Hydrogen nuclei fuse in several steps; overall, four hydrogen nuclei become one helium nucleus, and the lost mass is radiated as light and heat. The Sun loses over four million tonnes of mass every second in this way, yet it has enough hydrogen to shine for billions of years.

Uncontrolled and controlled fusion.

The hydrogen bomb is an uncontrolled fusion reaction. The very high temperature needed is produced by first exploding a fission bomb inside it.
Controlled fusion for electricity is still being developed: creating and holding a gas at millions of kelvin is very difficult, since no material container can touch it.

Fission and fusion compared.

	Fission	Fusion
Process	a heavy nucleus splits	light nuclei join
Started by	a slow neutron	very high temperature and pressure
Fuel	U-235, Pu-239	hydrogen isotopes (deuterium from sea water)
Products	radioactive fragments	mainly helium (not radioactive)
Energy per kg of fuel	large	larger (about 7 times)
Use today	nuclear power stations	the Sun and stars; hydrogen bomb

Advantages of fusion as a future energy source: fuel (deuterium) is plentiful in sea water; far less long-lived radioactive waste; no runaway chain reaction, since the reaction stops if the temperature falls.

Nuclear Fusion and the Energy of the Sun: formulas and key facts

A fusion reaction: $_{1}^{2} H +_{1}^{3} H \to_{2}^{4} He +_{0}^{1} n + energy$; Deuterium + tritium.
Energy released: $E = m c^{2}, c = 3 \times 1 0^{8} m/s$; m = mass lost in kg.
Mass converted: $Fusion of hydrogen: about 0.7% ∣ Fission of uranium: about 0.1%$; Fusion gives more energy per kg.
Conditions: $Temperature: millions of kelvin ∣ High pressure ∣ Light nuclei$; Hence 'thermonuclear'.

How to solve Nuclear Fusion and the Energy of the Sun questions

Fusion questions ask you to complete an equation, explain the conditions, or compare fusion with fission.

1Add the mass numbers on the left and the atomic numbers on the left.
2Subtract what is known on the right to find the missing particle; A = 1, Z = 0 is a neutron; A = 1, Z = 1 is a proton; A = 4, Z = 2 is an α particle (helium nucleus).
3If a coefficient is unknown, divide the missing total by the mass (or atomic) number of one particle.
4Decide fission or fusion: light nuclei joining is fusion; a heavy nucleus split by a neutron is fission.
5For energy, find the mass lost in kg and use $E = m c^{2}$ .
6For 'why high temperature?', mention positive charges, repulsion and the speed needed to overcome it.

Nuclear Fusion and the Energy of the Sun in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Why a hydrogen cylinder is not a bomb

Suppose a friend worries that the hydrogen gas cylinder in the school laboratory could one day 'turn into a hydrogen bomb'.

Explain why fusion cannot start in the cylinder.

Try it first, then show the solutionSolution

Fusion needs nuclei to come extremely close, but all nuclei are positively charged and repel each other.
Only at millions of kelvin and very high pressure do they move fast enough to overcome this repulsion.
At room temperature the hydrogen nuclei are far too slow; if the gas burns, that is a chemical reaction involving only electrons, not fusion.

Answer: Fusion needs millions of kelvin; at room temperature the nuclei cannot overcome their repulsion.

Memory hook: Cold hydrogen never fuses.

Fuel from the sea

Imagine a news report says that one day power stations could get their fuel from sea water.

Which fuel is meant, and why would such stations be safer than fission stations?

Try it first, then show the solutionSolution

Sea water contains deuterium ( $_{1}^{2} H$ ), heavy hydrogen, which can be fused.
The main product, helium, is not radioactive, so far less long-lived radioactive waste is produced.
Fusion stops if the temperature drops, so there is no runaway chain reaction.

Answer: Deuterium; little long-lived waste and no runaway chain reaction.

Memory hook: Heavy water from the sea, helium to the air.

Tricks to remember Nuclear Fusion and the Energy of the Sun

Short memory aids. Say each one aloud once, then use it on a question the same day.

Fusion fuses, fission fissures
Fusion joins (fuses) light nuclei; fission splits (cracks) heavy ones.
Use it when: Naming the process.
Thermo = heat
Thermonuclear: heat (millions of kelvin) is needed to push the positive nuclei together.
Use it when: Why fusion needs high temperature.
0.7 beats 0.1
Fusion converts about 0.7% of the mass, fission about 0.1%: fusion gives about 7 times more energy per kg.
Use it when: Comparing fission and fusion.

Nuclear Fusion and the Energy of the Sun: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Pick the fusion reaction · 1 mark · MCQBoard level

Which of the following is a fusion reaction?
(a) $_{92}^{235} U +_{0}^{1} n \to_{56}^{139} Ba +_{36}^{94} Kr + 3_{0}^{1} n$
(b) $_{1}^{2} H +_{2}^{3} He \to_{2}^{4} He +_{1}^{1} H$
(c) $_{11}^{24} Na \to_{12}^{24} Mg +_{- 1}^{0} e$
(d) $_{86}^{220} Rn \to_{84}^{216} Po +_{2}^{4} He$

Try it first, then show the solutionSolution

Two light nuclei join only in (b). (a) is fission; (c) is β decay; (d) is α decay.

Answer: (b)

Example 2: Two tritium nuclei · 2 marks · Short answerBoard level

Complete and name the reaction: $_{1}^{3} H +_{1}^{3} H \to_{2}^{4} He + x_{0}^{1} n + energy$

Try it first, then show the solutionSolution

Mass: $3 + 3 = 4 + x$ , so $x = 2$ ; atomic: $1 + 1 = 2 + 0$ ✓. (1)
Light nuclei join to form a heavier one: nuclear fusion. (1)

Answer: x = 2; nuclear fusion

Example 3: Lithium and deuterium · 3 marks · Short answerHarder board level

In one fusion reaction, $_{3}^{6} Li +_{1}^{2} H \to 2_{Z}^{A} X + energy$ .
(a) Find A and Z and name X.
(b) Why is the total mass of the two X nuclei less than the mass of Li-6 plus H-2?

Try it first, then show the solutionSolution

(a) Mass: $6 + 2 = 2 A$ , so $A = 4$ . Atomic: $3 + 1 = 2 Z$ , so $Z = 2$ . (1)
X is $_{2}^{4} He$ , a helium nucleus (α particle). (1)
(b) Some mass is converted into the energy released, $E = m c^{2}$ . (1)

Answer: (a) ⁴₂He (b) The lost mass becomes energy.

Example 4: Core and surface of the Sun · 2 marks · Short answerHarder board level

The Sun's core is at about 15 million kelvin, but its surface is only about 5800 K. Explain why fusion takes place in the core but not at the surface.

Try it first, then show the solutionSolution

Fusion needs nuclei moving fast enough to overcome the repulsion between their positive charges; this needs millions of kelvin and high pressure. (1)
Only the core is hot and dense enough; at 5800 K the nuclei are far too slow to fuse. (1)

Answer: Only the core is hot and dense enough for nuclei to overcome their repulsion.

Example 5: Mass lost by the Sun · 2 marks · Short answerHarder board level

The Sun radiates about $3.8 \times 1 0^{26}$ J of energy every second. Find the mass it converts into energy each second ( $c = 3 \times 1 0^{8}$ m/s).

Try it first, then show the solutionSolution

$m = \frac{E}{c ^{2}} = \frac{3.8 \times 1 0 ^{26}}{9 \times 1 0 ^{16}}$ . (1)
$m \approx 4.2 \times 1 0^{9}$ kg per second. (1)

Answer: About 4.2 × 10⁹ kg every second

Example 6: Fusion or fission per kilogram? · 3 marks · Short answerHOTS / competency

Suppose fusion of hydrogen converts 0.7% of the fuel's mass into energy and fission of uranium converts 0.1%.
(a) Find the energy released by fusing 1 kg of hydrogen.
(b) Find the energy from the fission of 1 kg of uranium.
(c) How many times more energy per kilogram does fusion give?

Try it first, then show the solutionSolution

(a) $m = 0.007$ kg; $E = 0.007 \times 9 \times 1 0^{16} = 6.3 \times 1 0^{14}$ J. (1)
(b) $m = 0.001$ kg; $E = 0.001 \times 9 \times 1 0^{16} = 9 \times 1 0^{13}$ J. (1)
(c) $\frac{6.3 \times 1 0 ^{14}}{9 \times 1 0 ^{13}} = 7$ times. (1)

Answer: (a) 6.3 × 10¹⁴ J (b) 9 × 10¹³ J (c) 7 times

Example 7: Case study: bombs and power stations · 4 marks · Case-basedHOTS / competency

A hydrogen bomb releases fusion energy in an instant, but no fusion power station supplies electricity yet.
(a) Why does a hydrogen bomb contain a fission bomb?
(b) Why does a hydrogen bomb release far more energy than the fission bomb inside it?
(c) Why could a fusion reactor not 'run away' like an uncontrolled fission chain reaction?
(d) Give one advantage of fusion fuel over fission fuel.

Try it first, then show the solutionSolution

(a) Fusion needs millions of kelvin; the fission explosion provides this temperature. (1)
(b) Fusion converts a larger fraction of its fuel's mass into energy (about 0.7% against about 0.1% for fission), and a large mass of fusion fuel can be used. (1)
(c) Fusion is not a neutron chain reaction; if the temperature or pressure falls, the reaction simply stops. (1)
(d) Deuterium is plentiful in sea water (or: the product, helium, is not radioactive). (1)

Answer: (a) To reach the temperature (b) Fusion converts a larger fraction of mass into energy (c) It stops when it cools (d) Plentiful fuel / little radioactive waste

Where students lose marks in Nuclear Fusion and the Energy of the Sun

The slips examiners see most often, and what to write instead.

Mistake: Mixing up fission and fusion
CISCE examiners' comments, 2025 Q8(ii): shown a neutron splitting uranium-235, candidates named the process nuclear fusion.
Wrong working
'U-235 split by a neutron: nuclear fusion.'
Right working
'Heavy nucleus split: fission. Light nuclei joined: fusion.'
Do this: Look at the nuclei: heavy and splitting, or light and joining?
Usually costs: 1 mark.
Mistake: Forgetting the coefficient of a product
With '2 X' or '3 ⁴₂He' on the right, the mass and atomic totals are multiplied by the coefficient.
Wrong working
⁶₃Li + ²₁H → 2X, so X has A = 8.
Right working
2A = 8, so A = 4; 2Z = 4, so Z = 2.
Do this: Divide the total by the coefficient.
Usually costs: 1 mark.
Mistake: Saying fusion needs 'heat to burn'
Fusion is not burning. The high temperature gives nuclei the speed to overcome electrostatic repulsion.
Wrong working
'Hydrogen burns in the Sun to make helium.'
Right working
'At millions of kelvin, hydrogen nuclei move fast enough to overcome repulsion and fuse.'
Do this: Mention positive charges and repulsion.
Usually costs: 1 mark.

Exam tips

Write 'nuclear fusion (thermonuclear reaction)' when naming the process.
Show the mass and atomic totals on both sides when completing a fusion equation.
Explain the high temperature with 'positive nuclei repel', not just 'it needs heat'.
In comparisons, give matching points for both: process, condition, fuel, products.

Practice questions on Nuclear Fusion and the Energy of the Sun

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which of these is NOT true of nuclear fusion?
(a) light nuclei join to form a heavier nucleus
(b) a large amount of energy is released
(c) it is started by a slow neutron
(d) a little mass is converted into energy
Hint
What starts fission, and what starts fusion?
Answer
(c) it is started by a slow neutron
Q2 · 1 mark · Very short answerBoard level
Name the two isotopes of hydrogen most used in fusion research and write their symbols.
Hint
Heavy and super-heavy hydrogen.
Answer
Deuterium $_{1}^{2} H$ and tritium $_{1}^{3} H$ .
Q3 · 1 mark · Very short answerBoard level
Complete: $_{1}^{1} H +_{1}^{2} H \to_{?}^{?} He + γ$
Hint
γ has A = 0 and Z = 0.
Answer
$_{2}^{3} He$
Q4 · 1 mark · Very short answerBoard level
State one similarity between nuclear fission and nuclear fusion.
Hint
Where does the energy come from in both?
Answer
In both, the products have slightly less mass than the reactants and the lost mass is released as energy ( $E = m c^{2}$ ).
Q5 · 2 marks · Short answerHarder board level
Identify X and name it: $_{1}^{2} H +_{2}^{3} He \to_{2}^{4} He + X$
Hint
Left totals: 5 and 3.
Answer
A = 5 − 4 = 1, Z = 3 − 2 = 1: X is $_{1}^{1} H$ , a proton.
Q6 · 2 marks · Short answerHarder board level
In the reaction $_{5}^{11} B +_{1}^{1} H \to n_{2}^{4} He$ , find n and check the atomic numbers.
Hint
Divide the mass total by 4.
Answer
$11 + 1 = 4 n$ , so $n = 3$ . Atomic: $5 + 1 = 6 = 3 \times 2$ ✓.
Q7 · 2 marks · Short answerHarder board level
Why is fusion often called a 'cleaner' source of nuclear energy than fission?
Hint
Compare the products.
Answer
Fusion of hydrogen gives mainly helium, which is not radioactive, so far less long-lived radioactive waste is produced. Fission gives strongly radioactive fragments that must be stored for thousands of years.
Q8 · 2 marks · Short answerHarder board level
Suppose 0.7% of the mass of hydrogen is converted into energy when it fuses. Find the energy released when 2 kg of hydrogen fuses in the Sun.
Hint
m = 0.007 × 2 kg.
Answer
$m = 0.014$ kg; $E = 0.014 \times 9 \times 1 0^{16} = 1.26 \times 1 0^{15}$ J.
Q9 · 2 marks · Short answerHOTS / competency
Fusion uses hydrogen nuclei rather than heavier nuclei such as carbon or oxygen. Give a reason linked to electric charge.
Hint
Z = 1 versus Z = 6 or 8.
Answer
Hydrogen nuclei carry the smallest positive charge (Z = 1), so the repulsion between them is least and they can fuse at a lower (though still very high) temperature. Heavier nuclei repel far more strongly.
Q10 · 2 marks · Short answerHOTS / competency
The Sun converts about $4.2 \times 1 0^{9}$ kg of mass into energy every second. (a) How much is this per day (1 day = 86 400 s)? (b) Why does the Sun not run out of mass quickly?
Hint
Multiply, then compare with the Sun's huge mass.
Answer
(a) $4.2 \times 1 0^{9} \times 86400 \approx 3.6 \times 1 0^{14}$ kg per day. (b) The Sun's mass (about $2 \times 1 0^{30}$ kg) is so enormous that even this loss is a tiny fraction; it can shine for billions of years.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Overall change in the Sun's core?Show answer
Four hydrogen nuclei become one helium nucleus.
2.Approximate surface temperature of the Sun?Show answer
About 5800 K.
3.Temperature of the Sun's core?Show answer
About 15 million kelvin.
4.Fusion fuel found in sea water?Show answer
Deuterium (heavy hydrogen).
5.Why does a hydrogen bomb need a fission bomb?Show answer
To produce the very high temperature needed for fusion.

Nuclear Fusion and the Energy of the Sun: frequently asked questions

Is fusion completely free of radioactivity?

No. Tritium is radioactive, and the neutrons released in deuterium–tritium fusion can make the walls of a reactor radioactive. But the amount and lifetime of this waste are far smaller than for the fragments from fission, and the main product, helium, is not radioactive.

Is the fusion in the Sun the same as deuterium–tritium fusion?

Not exactly. In the Sun, ordinary hydrogen nuclei fuse in several steps, overall turning four hydrogen nuclei into one helium nucleus. Deuterium–tritium fusion is the reaction scientists find easiest to use on Earth.

How much fusion is in the ICSE 2027 syllabus?

A basic introduction with equations: what fusion is, why it needs very high temperatures, that it powers the Sun, simple balanced fusion equations, and how it differs from fission.