What is nuclear fusion?

The joining of two light nuclei to form a heavier nucleus, releasing energy.

What is a thermal neutron?

A slow-moving neutron with kinetic energy similar to that of the surrounding particles; it is readily captured by uranium-235.

How is the energy released in a nuclear reaction calculated?

Mass decrease (reactants minus products) × c², or the mass decrease in u × 931.5 MeV.

What is a chain reaction?

A self-sustaining series of fissions in which neutrons released by one fission cause further fissions.

What is the critical mass?

The minimum mass of fissile material needed to sustain a chain reaction.

Why does a small lump of uranium-235 not sustain a chain reaction?

Too many neutrons escape through the surface before they can cause fission.

What does the moderator do?

It slows fast neutrons to thermal energies by elastic collisions.

What do control rods do?

They absorb neutrons to control the rate of fission.

What does the coolant do?

It carries thermal energy from the core to a heat exchanger, where steam is made.

Why do moderators need low-mass nuclei?

In an elastic collision a neutron transfers the most kinetic energy to a nucleus of similar mass.

Name a suitable moderator, control rod and coolant.

Moderator: graphite or water. Control rod: boron or cadmium. Coolant: water, carbon dioxide or liquid sodium.

What is needed for a steady power output in a reactor?

On average exactly one neutron from each fission goes on to cause another fission.

Nuclear fission and fusionAQA A-Level Physics: Flashcards

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What is nuclear fission?

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What is nuclear fission?: The splitting of a heavy nucleus into two smaller nuclei, releasing energy and neutrons.
What is nuclear fusion?: The joining of two light nuclei to form a heavier nucleus, releasing energy.
What is a thermal neutron?: A slow-moving neutron with kinetic energy similar to that of the surrounding particles; it is readily captured by uranium-235.
How is the energy released in a nuclear reaction calculated?: Mass decrease (reactants minus products) × c², or the mass decrease in u × 931.5 MeV.
What is a chain reaction?: A self-sustaining series of fissions in which neutrons released by one fission cause further fissions.
What is the critical mass?: The minimum mass of fissile material needed to sustain a chain reaction.
Why does a small lump of uranium-235 not sustain a chain reaction?: Too many neutrons escape through the surface before they can cause fission.
What does the moderator do?: It slows fast neutrons to thermal energies by elastic collisions.
What do control rods do?: They absorb neutrons to control the rate of fission.
What does the coolant do?: It carries thermal energy from the core to a heat exchanger, where steam is made.
Why do moderators need low-mass nuclei?: In an elastic collision a neutron transfers the most kinetic energy to a nucleus of similar mass.
Name a suitable moderator, control rod and coolant.: Moderator: graphite or water. Control rod: boron or cadmium. Coolant: water, carbon dioxide or liquid sodium.
What is needed for a steady power output in a reactor?: On average exactly one neutron from each fission goes on to cause another fission.

Exam questions on Nuclear fission and fusion

Uranium-235 nuclei can be split by slow-moving neutrons. One fission reaction is ¹n + ²³⁵U → ¹⁴¹Ba + ⁹²Kr + 3 ¹n, and about 200 MeV of energy is released in each fission.
Explain how a chain reaction can occur in a block of uranium-235.2 marks
A thermal nuclear reactor has fuel rods of uranium-235, a graphite moderator, boron control rods and a carbon dioxide coolant.
Explain why a moderator is needed in a thermal reactor.2 marks
A thermal reactor operates in a steady state at a thermal power of 3.0 GW. Each fission of uranium-235 releases on average 200 MeV of energy and 2.4 neutrons. A uranium-235 atom has a mass of 235 u, where 1 u = 1.661 × 10⁻²⁷ kg; 1 MeV = 1.60 × 10⁻¹³ J.
Explain how the control rods keep the reactor operating in a steady state.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).