For IB Physics SL and HL, control rods regulate the fission rate, moderators slow neutrons, heat exchangers transfer thermal energy between coolant systems, and shielding reduces radiation exposure. Together, these components allow energy from a controlled nuclear chain reaction to be transferred safely.
This is part of E.4 Fission, which is studied by both SL and HL students.
| Component | Function in the nuclear power plant |
|---|---|
| Control rods | Absorb neutrons, reducing the number available to cause further fission. Inserting them farther decreases the reaction rate; withdrawing them increases it. Full insertion can shut down the chain reaction. |
| Moderator | Slows fast neutrons through collisions without absorbing many of them. Slower neutrons are more likely to cause fission in uranium-235 nuclei, helping sustain the chain reaction. Water or graphite may be used. |
| Heat exchanger | Transfers thermal energy from the hot primary coolant to water in a separate secondary system. The secondary water produces steam that drives a turbine and generator. Separation of the systems limits the spread of radioactive contamination. |
| Shielding | Absorbs ionizing radiation, especially penetrating gamma radiation and neutrons, reducing exposure to workers and the surroundings. Thick concrete, steel and water can provide shielding. |
Energy released by nuclear fission increases the internal energy of the reactor coolant. The heat exchanger transfers this energy to the secondary circuit, where steam turns a turbine. The turbine drives a generator, converting mechanical energy into electrical energy.
A common misconception is that the moderator slows down the chain reaction. It actually slows the neutrons, which generally makes further uranium-235 fission more likely. Control rods are the components that reduce the chain-reaction rate by absorbing neutrons.
Exam technique: For an IB question using Describe or Explain, state each component’s action and its consequence. For example: “Control rods absorb neutrons, so fewer neutrons cause further fission and the reaction rate decreases.” Avoid merely stating that a component “makes the reactor safe” without explaining how.
