A nuclear reactor uses a moderator, control rods and coolant

Nuclear Energy & Binding Energy — AQA A-Level Physics

Moderator: slows neutrons down

Surrounds the fuel rods inside the reactor core.
Made from materials that are poor neutron absorbers: water or graphite.
Fast fission neutrons collide with moderator molecules and lose kinetic energyThe capacity to do work. Measured in joules (J).The energyThe capacity to do work. Measured in joules (J). an object possesses due to its motion. through elastic collisions.
After ~50 collisions, neutrons reach thermal equilibriumAn object is in equilibrium when the resultant force on it is zero. The object is either stationary or moving at constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹..The state in which two objects in thermal contact have no net heat transfer between them because they are at the same temperature. with the moderator (~2700 m s⁻¹).
First few collisions may excite moderator nuclei (energyThe capacity to do work. Measured in joules (J). released as gamma when they de-excite). Subsequent collisions are elastic.

Control rods: absorb excess neutrons

Made of boron or cadmium — materials that absorb neutrons without becoming dangerously unstable.
Lowering the rods further into the core absorbs more neutrons, decreasing the fission rate.
Raising the rods absorbs fewer neutrons, increasing the fission rate.
In an emergency shutdown, all control rods are fully lowered to absorb all free neutrons and stop the reaction.

Coolant: transfers thermal energy

Usually water, pumped through the reactor at a cold temperature to extract heat from fission.
Transfers heat via a heat exchanger to a separate water circuit that produces steam.
Steam drives turbines connected to generators.
Water is often used as both coolant and moderator (high specific heat capacityThe energy required to raise the temperature of 1 kg of a substance by 1 K (or 1 °C).).
Other coolants: molten salt, helium gas.