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How does a nuclear chain reaction continue inside a reactor?

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Labeled diagram showing the process of a nuclear power plant with a pressurized-water reactor. Water circulates through the reactor core, heats to produce steam in a steam generator, which drives a turbine connected to an electric generator. Steam is then cooled in a condenser using water from a cooling tower, which releases nonradioactive water vapor. Control rods, pumps, and transmission lines are also indicated. — nuclear power Schematic diagram of a nuclear power plant using a pressurized-water reactor.

A nuclear chain reaction continues inside a reactor through the ongoing process of nuclear fission. When a heavy atomic nucleus splits, it releases energy and emits several neutrons. These neutrons can then strike nearby fissile material, inducing further fission events and releasing more neutrons to continue the cycle.123

To maintain a steady-state reaction, the reactor uses a multiplication factor, k, which represents the ratio of fissions in one generation to the next. For a controlled, constant power level, k is kept equal to unity. Key components facilitate this process:1

Moderators: Materials like water, heavy water, or graphite slow down fast-moving neutrons to thermal energies, making them more effective at inducing subsequent fission events.14
Control Rods: Made of neutron-absorbing materials such as boron, cadmium, or hafnium, these are moved in or out of the core to regulate the number of available neutrons, thereby controlling the rate of the reaction.15
Coolants: Substances like water or liquid metals remove the heat generated by the fission process to prevent overheating and transport energy for power production.14