Nuclear Fission

These note cards will help in particular with those doing the WJEC additional science board. The subject of nuclear fission is included in Physics 3.

Hope these help.


  • Created by: Jay
  • Created on: 22-05-12 11:37

The definition of Nuclear Fission

Nuclear fission is when:


- heavy nuclei are split

- into smaller lighter fragments

- releasing a large amount of energy in this process

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What materials can fission?

- Large unstable atoms

e.g. Uranium- 235

Plutonium- 239

These atoms may fission anyway, unlike induced fission, spontaneously at a low rate.


When split:

- they emit neutrons

- a large amount of energy is released 

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Where can nuclear fission take place?

- In nuclear power stations

Nuclear fission is the process used in nuclear power stations to release large amounts of energy from the nucleus of uranium atoms.

This energy released is used to generate electricity for people to use in everyday life.


- Atom bombs

This process was also used in atom bombs that were dropped at the end of the Second World War on the Japanese cities of Nagasaki and Hiroshima. These bombs had devastating effects hence nuclear bombs being called weapons of mass destruction.

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What is induced fission?

"Induced" means 'made to happen'


This is essentially when is meant by induced fission:

- nuclear fission made to happen

- fission is made to happen for man's benefit (explained later on)

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Why does induced nuclear fission happen?

- Large amounts of electricity is needed to generate electricity


We need:

- fission to take place at a fast enough rate

- to produce these large/useful amounts of energy

- needed to generate electricity

- in a nuclear power station. 

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How do we induce nuclear fission?

Firstly the process of induced fission happens in nuclear fission reactors in nuclear power stations.

What happens?

- Uranium-235 nuclei (large unstable atoms) are bombarded

- with neutrons going at just the right speed

How is the fission triggered?

- the neutrons are absorbed by the Uranium-235 nuclei to form Uranium-236

- Uranium-236 is highly unstable (more so than Uranium-235)

- The U-236 nuclei then fission (split)

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Where does the 'needed' energy come from?


- the energy comes from the mass that goes 'missing' during the reaction



The mass that has gone missing has been converted into energy according to Einstein's equation E = mc²

Energy (E) = Mass (m) x Speed of Light ² (c)

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What form does the released energy take after fiss

The energy takes the form of:

- kinetic energy



some (energy produced from different fission reactions) also take the form of radiation energy such as:

- light

- infra-red

- and other parts of the electromagnetic spectrum

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Why do we have to control the nuclear fission reac

- Chain reactions occur


What happens if we don't control the reactions?

- The neutrons produced from the fissioning of one Uranium nucleus

- will then go on to induce fission in neighbouring U-235 nuclei.


This is a chain reaction.

(when each fission event causes further fission events)

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Why are chain reactions so bad?

What happens due to a chain reaction?

- the number of fissions increases rapidly

- a huge amount of energy is released


Why is this bad?

- Uncontrolled chain reactions are used in nuclear bombs (deadly)

- The energy they can unleash is devastating

- think of the aftermath of the atomic bombs dropped on Nagasaki and Hiroshima

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How can we control nuclear fission?

i.e. stop chain reactions from happening


How can we stop chain reactions from happening?


- Absorb the right amount of neutrons

- so we can keep the reaction going

- at just the rate we want

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What is the right rate?

In a nuclear power station we want to keep the fission reaction going steadily.

i.e. the reaction does not grow


- The rate at which U-235 nuclei are fissioned remains constant

- and we get a steady release of energy that goes on for a long time

- from a small amount of fuel.

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How can we absorb the right amount of neutrons?

- The excess neutrons are absorbed in a nuclear reactor

- by lowering control rods down into the reactor


What are these control rods usually made of?

- Boron

- Cadmium

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What happens if the control rods are lowered compl

If the control rods are lowered completely...

- all the neutrons are absorbed

- and all fission in the reactor stops.

What happens if a fault occurs?

- Gravity can be relied upon

- to pull the control rods down

- and stop the reaction quickly

- even if all other safety systems fail.

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