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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.
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.
A nuclear reactor, formerly known as an atomic pile, is a device used to initiate and control a self-sustained nuclear chain reaction.
Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion.
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.Control rods are made of neutron poisons and therefore tend to absorb neutrons.When a control rod is inserted deeper into the reactor, it absorbs more neutrons than the material it displaces—often the moderator.The physics of radioactive decay also affects neutron populations in a reactor.One such process is delayed neutron emission by a number of neutron-rich fission isotopes.There is a scale for describing criticality in numerical form, in which bare criticality is known as zero dollars and the prompt critical point is one dollar, and other points in the process interpolated in cents.In some reactors, the coolant also acts as a neutron moderator.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.A nuclear reactor coolant — usually water but sometimes a gas or a liquid metal (like liquid sodium) or molten salt — is circulated past the reactor core to absorb the heat that it generates.The heat is carried away from the reactor and is then used to generate steam.