Just as conventional thermal power stations generate electricity by harnessing the thermal energy released from burning fossil fuels, nuclear reactors convert the energy released by controlled nuclear fission into thermal energy for further conversion to mechanical or electrical forms.When a large fissile atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron, it may undergo nuclear fission.
Some of these methods arise naturally from the physics of radioactive decay and are simply accounted for during the reactor's operation, while others are mechanisms engineered into the reactor design for a distinct purpose.
The fastest method for adjusting levels of fission-inducing neutrons in a reactor is via movement of the control rods.
Heat from nuclear fission is passed to a working fluid (water or gas), which in turn runs through steam turbines.
These either drive a ship's propellers or turn electrical generators' shafts.
Nuclear generated steam in principle can be used for industrial process heat or for district heating.
Some reactors are used to produce isotopes for medical and industrial use, or for production of weapons-grade plutonium.
In other reactors the coolant acts as a poison by absorbing neutrons in the same way that the control rods do.
In these reactors power output can be increased by heating the coolant, which makes it a less dense poison.
Keeping the reactor in the zone of chain reactivity where delayed neutrons are necessary to achieve a critical mass state allows mechanical devices or human operators to control a chain reaction in "real time"; otherwise the time between achievement of criticality and nuclear meltdown as a result of an exponential power surge from the normal nuclear chain reaction, would be too short to allow for intervention.
This last stage, where delayed neutrons are no longer required to maintain criticality, is known as the prompt critical point.