Induced fission (A-level only) Flashcards – AQA A-level Physics (7408)

What is a fission chain reaction?

A reaction in which neutrons released by fission induce further fissions.

How does a fission chain reaction change when each fission leads to fewer than, exactly, or more than one further fission on average?

• Fewer than one: it decreases.
• Exactly one: it is self-sustaining.
• More than one: it grows.

What two things can happen to a neutron that prevent it from causing further fission?

It can escape from the material or be absorbed without causing fission.

What is critical mass?

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

Which factors affect critical mass by influencing neutron escape and absorption?

The material’s shape, density, and surroundings.

How does neutron moderation affect the likelihood of fission in suitable fuel?

It slows fast fission neutrons to thermal energies, making them more likely to induce fission.

What quantities are conserved in an elastic collision between a neutron and a moderator nucleus?

Total momentum and kinetic energy.

In the simple model, how do repeated collisions with moderator nuclei initially at rest slow a neutron?

In each collision, some of the neutron’s kinetic energy is transferred to a nucleus.

Why are light nuclei effective at slowing neutrons in collisions?

A light nucleus can receive a relatively large fraction of a neutron’s energy in a collision.

What two properties make a material suitable as a neutron moderator?

It contains light nuclei and absorbs relatively few neutrons.

What are two examples of neutron moderator materials?

Water and graphite.

How do control rods regulate the fission rate in a thermal reactor?

They absorb neutrons, reducing the number available to cause further fissions.

How does inserting or withdrawing control rods generally affect the fission rate?

Further insertion reduces the fission rate; withdrawal generally increases it.

What two properties are considered when choosing control rod materials?

Strong neutron absorption and suitability for reactor conditions.

What are two neutron-absorbing materials used in control rods?

Boron and cadmium.

What is the role of a reactor coolant?

It transfers thermal energy away from the core, helping prevent overheating and carrying energy for use outside the core.

What three properties should a suitable reactor coolant have, apart from its neutron absorption?

It should transfer heat effectively, remain stable under operating conditions, and be compatible with reactor materials.

Why does a reactor coolant’s tendency to absorb neutrons matter?

Neutron absorption affects the chain reaction.

What are two examples of reactor coolants?

Water and carbon dioxide.