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Nuclear fission and fusion: GCSE Combined Science Physics practice questions

Foundation and Higher · Grade 6

A second set of questions on nuclear fission and fusion. They are different from the ones on the topic page. Try each, then open the answer and compare your working.

More questions

Question 1 Hard

Explain why the chain reaction in a reactor must be controlled.

Show answer

Answer: The number of fissions would keep growing, releasing too much energy too quickly, so it must be controlled.

  1. The number of fissions would grow very quickly.
  2. An uncontrolled reaction releases huge energy at once and is dangerous.

Question 2 Hard

Explain why fusion needs a very high temperature.

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Answer: The nuclei repel each other, so they must move very fast to get close enough to fuse.

  1. The nuclei are all positive.
  2. They repel, so they must move fast to get close enough to fuse.

Question 3 Hard

Explain why the waste from a fission reactor needs careful storage.

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Answer: It stays radioactive for a very long time, so it must be stored safely.

  1. The waste contains radioactive nuclei.
  2. They emit radiation for a very long time.

Question 4 Hard

Each fission releases 3 neutrons and each of those causes another fission. Starting from 1 neutron, how many neutrons are there after 4 steps?

Show answer

Answer: 81 neutrons

  1. The number triples each step.
  2. 3 × 3 × 3 × 3 = 81

Quick checks

Short questions with options. Choose one before you open the answer.

Check 1

What is nuclear fission?

  1. A large nucleus splits into two smaller nuclei.
  2. Two small nuclei join together.
  3. An atom gains an electron.
Show answer

A large nucleus splits into two smaller nuclei. Fission releases neutrons and energy.

Check 2

What is nuclear fusion?

  1. A large nucleus splits in two.
  2. Two light nuclei join to make a heavier nucleus.
  3. A nucleus gives out an alpha particle.
Show answer

Two light nuclei join to make a heavier nucleus. Fusion happens in the Sun and releases energy.

Check 3

What does a uranium-235 nucleus absorb to start fission?

  1. A proton.
  2. An electron.
  3. A neutron.
Show answer

A neutron. The neutron makes the nucleus unstable, so it splits.

Check 4

Name one fuel used in a fission reactor.

  1. Uranium-235 or plutonium-239.
  2. Hydrogen-1.
  3. Carbon-14.
Show answer

Uranium-235 or plutonium-239. These are large, unstable nuclei that split easily.

Check 5

What is released when a nucleus splits in fission?

  1. Only a lot of electrons.
  2. Two or three neutrons and energy.
  3. Nothing, because energy is used up.
Show answer

Two or three neutrons and energy. The neutrons can go on to cause more fissions.

Check 6

What is a chain reaction?

  1. Released neutrons cause more fissions, which release more neutrons.
  2. One nucleus splits and then nothing else happens.
  3. Nuclei joining one after another.
Show answer

Released neutrons cause more fissions, which release more neutrons. The number of neutrons multiplies at each step.

Check 7

What do the control rods do in a reactor?

  1. They slow the neutrons down.
  2. They carry heat away.
  3. They absorb neutrons to control the rate.
Show answer

They absorb neutrons to control the rate. Lowering the rods slows the reaction down.

Check 8

What does the moderator do in a reactor?

  1. It slows neutrons so they are more likely to be absorbed.
  2. It absorbs the neutrons.
  3. It shields workers from radiation.
Show answer

It slows neutrons so they are more likely to be absorbed. Slow neutrons are easier for uranium-235 to absorb.

Check 9

What does the coolant do in a reactor?

  1. It absorbs neutrons.
  2. It carries heat away so that steam can be made.
  3. It slows the neutrons.
Show answer

It carries heat away so that steam can be made. The steam turns a turbine that drives a generator.

Check 10

Why is a reactor surrounded by thick concrete?

  1. To slow the neutrons down.
  2. To keep the reactor cool.
  3. To absorb the radiation and protect people.
Show answer

To absorb the radiation and protect people. Shielding stops radiation escaping.

Check 11

Where does fusion happen naturally?

  1. In the cores of stars such as the Sun.
  2. In a nuclear power station.
  3. In radioactive rocks.
Show answer

In the cores of stars such as the Sun. Stars have the heat and pressure that fusion needs.

Check 12

Why do nuclei repel each other?

  1. They are all negatively charged.
  2. They are all positively charged.
  3. They have no charge.
Show answer

They are all positively charged. This repulsion is why fusion needs extreme conditions.

Check 13

Why does fusion need a very high temperature and pressure?

  1. To force positive nuclei close enough to join.
  2. To split the nuclei apart.
  3. To stop the neutrons escaping.
Show answer

To force positive nuclei close enough to join. Fast-moving nuclei can overcome the repulsion.

Check 14

What is made when hydrogen nuclei fuse in the Sun?

  1. Uranium.
  2. Oxygen.
  3. Helium.
Show answer

Helium. Helium is the bigger nucleus made when hydrogen joins.

Check 15

Why is waste from fission a problem?

  1. It explodes when stored.
  2. It stays radioactive for a very long time.
  3. It is a gas that warms the planet.
Show answer

It stays radioactive for a very long time. It must be stored carefully and safely.

Check 16

Why is fusion not yet used in power stations?

  1. The extreme conditions are very hard to create and hold.
  2. There is no hydrogen to use.
  3. It releases no energy.
Show answer

The extreme conditions are very hard to create and hold. Scientists are still working on it.

Check 17

Where does the energy in fission and fusion come from?

  1. From burning the fuel in oxygen.
  2. From the electrons in the atoms.
  3. Some of the mass of the nuclei is changed into energy.
Show answer

Some of the mass of the nuclei is changed into energy. A tiny amount of mass changes into a lot of energy.

Check 18

Each fission releases 2 neutrons that each cause another. From 1 neutron, how many after 3 steps?

  1. 8
  2. 6
  3. 4
Show answer

8 2 × 2 × 2 = 8.

Check 19

Each fission releases 3 neutrons that each cause another. From 1 neutron, how many after 2 steps?

  1. 6
  2. 9
  3. 3
Show answer

9 3 × 3 = 9.

Check 20

A student says control rods slow neutrons down. What is wrong with that?

  1. Control rods absorb neutrons. The moderator slows them.
  2. Nothing. Control rods slow neutrons.
  3. Control rods speed the neutrons up.
Show answer

Control rods absorb neutrons. The moderator slows them. Mixing up the two parts is a very common mistake.

Check 21

A student says fusion is used in power stations today. What is wrong with that?

  1. Nothing. Fusion is the main process used today.
  2. Fission has stopped being used.
  3. Fission is used. Fusion is still experimental.
Show answer

Fission is used. Fusion is still experimental. Fusion needs conditions that are very hard to make and hold.

Check 22

Why must the chain reaction in a power station be controlled?

  1. Because the neutrons would run out.
  2. Otherwise the energy is released far too fast.
  3. Because uranium stops being radioactive.
Show answer

Otherwise the energy is released far too fast. An uncontrolled chain reaction is how a bomb works.

Check 23

Why does a nuclear power station make steam?

  1. The heat from fission boils water, and the steam turns a turbine.
  2. The neutrons turn into steam.
  3. The steam absorbs the radiation.
Show answer

The heat from fission boils water, and the steam turns a turbine. The turbine turns a generator, which makes electricity.

Check 24

Which process makes more long-lived radioactive waste, fission or fusion?

  1. Fusion.
  2. They make exactly the same amount.
  3. Fission.
Show answer

Fission. Fission produces radioactive fission products.

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