Edexcel IGCSE Physics: Double Science 复习笔记：7.3.3 Nuclear Reactors

• 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

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

   

• 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

   

• 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