Nuclear Reactors (Edexcel IGCSE Physics: Double Science)

• Only one extra neutron is required to induce fission in a uranium-235 nucleus
• 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 emits further neutrons
  • This process can start a chain reaction

• A chain reaction occurs when

A neutron emitted from the splitting of a nucleus causes further nuclei to split and the neutrons emitted from these cause further fission reactions

• Controlling chain reactions is an important part of the fission process in nuclear reactors
• For a chain reaction to be maintained, there must be a minimum amount of fissile material called the critical mass
  • If the mass of fissile material exceeds the critical mass, the rate of reaction accelerates
  • This can cause a huge and uncontrolled release of energy, i.e. a nuclear explosion

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

• In a nuclear reactor, a chain reaction is required to keep the reactor running
• When the reactor is producing energy at the required rate, two factors must be controlled:
  • The number of free neutrons in the reactor
  • The energy of the free neutrons

• The main components of a nuclear reactor are:
  
  • control rods
  • a moderator

Nuclear reactor diagram

The overall purpose of the reactor is to control chain reactions and collect the heat energy produced from nuclear reactions to generate electricity

Control rods

Purpose of control rods: To absorb neutrons

• Control rods are made of a material which absorbs neutrons without becoming dangerously unstable themselves
• The number of neutrons absorbed is controlled by varying the depth of the control rods in the reactor core
  • 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 reactions can take place

Moderator

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
  • These neutrons are called thermal neutrons
  • This ensures neutrons can react efficiently with the uranium fuel

Purpose of shielding: To absorb hazardous radiation

• The entire nuclear reactor is surrounded by shielding materials
• 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 and ensures that the environment around the reactor is safe for workers

Shielding materials around a nuclear reactor are designed to absorb harmful radiation