7.21 describe the role played by the control rods and moderator in the fission process

Chain Reactions

Table of Contents Show
• Chain Reactions
• Control Rods and Moderators
• What do moderators and control rods do?
• How do control rods control the rate of fission?
• What is the role of control rods in a nuclear reactor?
• What is the role of control rods in a nuclear reactor why are they made of cadmium?
• 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.

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

Exam Tip

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!

Control Rods and Moderators

• 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

Shielding

• The entire nuclear reactor is surrounded by shielding materials
• The purpose of shielding is to absorb hazardous radiation
• The daughter nuclei formed during fission, and the neutrons emitted, are radioactive
• The reactor is surrounded by a steel and concrete wall that can be nearly 2 metres thick
• This absorbs the emissions from the reactions
  • It ensures that the environment around the reactor is safe

Shielding metals in a nuclear reactor