Radioactivity Study Guide

In a nuclear reactor, a chain reaction is maintained by the fission of uranium-235 or plutonium-239 atoms, which release neutrons that then go on to cause further fissions. To generate power safely and efficiently, this chain reaction must be carefully controlled. This is achieved primarily through the use of moderators and control rods.

Moderators, typically made of graphite or heavy water, serve to slow down the high-energy neutrons produced during fission. Fast neutrons are not effective at causing further fission in uranium-235, but thermal (slow) neutrons are. By moderating the neutrons, the probability of them being absorbed by other fissile nuclei and maintaining the chain reaction increases.

Control rods, usually made of materials like boron or cadmium, are highly effective at absorbing neutrons. These rods can be inserted into or withdrawn from the reactor core. By adjusting their position, the number of neutrons available to cause fission can be precisely managed. Inserting the rods further into the core absorbs more neutrons, slowing down the reaction rate. Withdrawing them allows more neutrons to perpetuate the chain reaction, increasing the rate. This mechanism ensures that the energy output remains stable and prevents the reaction from becoming uncontrolled, which could lead to a meltdown.

In a nuclear power station, the controlled nuclear chain reaction within the reactor core releases a significant amount of thermal energy. This energy heats a coolant fluid, typically water, which then flows through a heat exchanger. In the heat exchanger, the hot coolant transfers its thermal energy to a separate water supply, transforming it into high-pressure steam. This steam is directed to turn a turbine, which is mechanically linked to an electrical generator. As the turbine rotates, it spins the generator's coils within a magnetic field, inducing an electric current. After passing through the turbine, the steam is cooled and condensed back into liquid water, often using a separate cooling water supply (from a river or sea), and then pumped back to the heat exchanger to be reheated, completing a closed loop. This continuous cycle efficiently converts thermal energy into electrical energy.