Nuclear Reactors (Edexcel GCSE Physics)
Revision Note
Chain Reactions
Only one extra neutron is required to induce a uranium-235 nucleus to split by fission
During the fission, it produces two or three neutrons which move away at high speed
Each of these new neutrons can start another fission reaction, which again creates further excess neutrons
This process is called a chain reaction
The neutrons released by each fission reaction can go on to create further fissions, like a chain that is linked several times – from each chain comes two more
Worked Example
The diagram shows the nuclear fission process for an atom of uranium-235.
Complete the diagram to show how the fission process starts a chain reaction.
Answer:
Step 1: Draw the neutrons to show that they hit other U-235 nuclei
It is the neutrons hitting the uranium-235 nuclei which causes the fission reactions
The daughter nuclei do not need to be shown, only the neutrons and uranium-235 nuclei
Step 2: Draw the splitting of the U-235 nuclei to show they produce two or more neutrons
The number of neutrons increases with each fission reaction
Each reaction requires one neutron but releases two
More reactions happen as the number of neutrons increases
Examiner Tips and Tricks
You need to be able to draw and interpret different diagrams of nuclear fission and chain reactions. Generally, things move to the right as time goes on in these diagrams, but it is important to read all the information carefully on questions like this. If you have to draw a diagram in an exam remember that the clarity of the information is important, not how pretty it looks!
Nuclear Reactors
In a nuclear reactor, a chain reaction is required to keep the reactor running
When the reactor is producing energy at the correct rate, two factors must be controlled:
The number of free neutrons in the reactor
The energy of the free neutrons
To do this, nuclear reactors contain control rods and moderators
Diagram of a Nuclear Reactor. The overall purpose of the reactor is to collect the heat energy produced from nuclear reactions
Control Rods
Purpose of a control rod: To absorb neutrons
Control rods are made of a material which absorb neutrons without becoming dangerously unstable themselves
The number of neutrons absorbed is controlled by varying the depth of the control rods in the fuel rods
Lowering the rods further decreases the rate of fission, as more neutrons are absorbed
Raising the rods increases the rate of fission, as fewer neutrons are absorbed
This is adjusted automatically so that exactly one fission neutron produced by each fission event goes on to cause another fission
In the event the nuclear reactor needs to shut down, the control rods can be lowered all the way so no reaction can take place
Moderator
The purpose of a moderator: To slow down neutrons
The moderator is a material that surrounds the fuel rods and control rods inside the reactor core
The fast-moving neutrons produced by the fission reactions slow down by colliding with the molecules of the moderator, causing them to lose some momentum
The neutrons are slowed down so that they are in thermal equilibrium with the moderator, hence the term ‘thermal neutron’
This ensures neutrons can react efficiently with the uranium fuel
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