Induced fission (AQA A-Level Physics): Revision Notes

8.1.7 Induced fission

Key Concepts in Induced Fission:

1. Thermal Neutrons:

• Thermal neutrons have relatively low kinetic energy, making them more likely to be absorbed by uranium-235 nuclei. High-energy neutrons tend to bounce off rather than be absorbed, hence thermal neutrons are critical in maintaining the fission process.
• Once absorbed, the uranium-235 nucleus splits, producing daughter nuclei, free neutrons, and releasing energy.

2. Chain Reaction:

• Each fission event releases neutrons, which can go on to induce further fissions in nearby uranium-235 nuclei. For the reaction to sustain itself, at least one of the neutrons released in each fission must cause another fission event.
• Critical Mass: This is the minimum amount of fissile material needed to maintain a self-sustaining chain reaction. If the mass of the uranium is below this threshold, neutrons escape without causing further fission, and the reaction eventually stops.

Nuclear Reactor Components:

Nuclear reactors harness induced fission to generate electricity safely and efficiently. Several key components ensure the process remains controlled:

• Moderator:
  • The moderator slows down the neutrons produced by fission to thermal speeds, making them more likely to be absorbed by uranium-235 nuclei.
  • This is typically achieved through elastic collisions between neutrons and the nuclei of the moderator material.
  • Water and graphite are commonly used as moderators because they effectively slow down neutrons, are inexpensive, and do not react chemically within the reactor.
• Control Rods:
  • Control rods absorb excess neutrons, thereby controlling the rate of the chain reaction and ensuring it doesn't accelerate uncontrollably.
  • The height of the control rods within the reactor core can be adjusted to control how many neutrons are absorbed and therefore, regulate the energy output.
  • Materials like boron and cadmium are used in control rods due to their strong neutron-absorbing properties.
• Coolant:
  • The coolant transfers the heat generated by fission to a secondary system where it is converted into steam to drive electricity-generating turbines.
  • Common coolants include water (often serving a dual purpose as a moderator) and other substances like helium and molten salts.
  • Coolants must have high specific heat capacity to absorb large amounts of thermal energy without quickly heating up themselves.

Diagram of a Nuclear Reactor

The typical layout of a nuclear reactor includes these components arranged within a containment structure, as shown in the diagram. The process involves control rods inserted into a reactor core containing uranium fuel rods. Heat generated by fission is transferred via the coolant to produce steam in an external loop, driving turbines to generate electricity.