What is nuclear fission?

Nuclear fission is the splitting of a heavy nucleus into two lighter nuclei of roughly comparable size, together with several neutrons, releasing energy.

Describe how fission is induced in a uranium-235 nucleus.

A slow-moving neutron is absorbed by the nucleus, which becomes unstable, deforms and splits into two fragments while emitting two or three further neutrons.

What is a chain reaction?

A chain reaction occurs when the neutrons released by one fission go on to cause further fissions, so that the reaction sustains itself once it has started.

What is meant by the critical mass of a fissile material?

The critical mass is the smallest mass in which enough of the neutrons released are absorbed rather than escaping through the surface, so that the chain reaction is just self-sustaining.

OCR A-Level Physics B H557

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Ionising Radiation and Risk

Fission, fusion and nuclear power

3 things to know
Components 01, 02 and 03

Overview

Recall questions on nuclear fission and inducing it in uranium-235, the 200 MeV released and its mass equivalent, chain reactions and critical mass, moderators, control rods and coolants, nuclear fusion and why it needs high temperatures, deuterium–tritium fusion, why fusion releases more energy per kilogram, and the benefits and concerns of fission power.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

State the approximate energy released in a single fission of a uranium-235 nucleus.

Numbers

Calculate the mass converted to energy in a fission that releases 200 MeV.

Key term

What is nuclear fusion?

Questions from this subtopic

Each one with the model answer underneath.

1
What is nuclear fission?
Model answer
Nuclear fission is the splitting of a heavy nucleus into two lighter nuclei of roughly comparable size, together with several neutrons, releasing energy.
2
Describe how fission is induced in a uranium-235 nucleus.
Model answer
A slow-moving neutron is absorbed by the nucleus, which becomes unstable, deforms and splits into two fragments while emitting two or three further neutrons.
3
State the approximate energy released in a single fission of a uranium-235 nucleus.
Model answer
About 200 MeV is released in each fission.
4
Calculate the mass converted to energy in a fission that releases 200 MeV.
Model answer
The energy is 200 × 10⁶ × 1.60 × 10⁻¹⁹ = 3.2 × 10⁻¹¹ J, so m = E/c² = 3.2 × 10⁻¹¹ ÷ 9.00 × 10¹⁶ = 3.6 × 10⁻²⁸ kg.
5
What is a chain reaction?
Model answer
A chain reaction occurs when the neutrons released by one fission go on to cause further fissions, so that the reaction sustains itself once it has started.
6
What is meant by the critical mass of a fissile material?
Model answer
The critical mass is the smallest mass in which enough of the neutrons released are absorbed rather than escaping through the surface, so that the chain reaction is just self-sustaining.

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