Zero power critical is a condition of nuclear fission

Nuclear fission

In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...

 reactor

Nuclear reactor

A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

s, that is useful for characterizing the reactor core. A reactor is in the zero power critical state if it is sustaining a stable fission chain reaction

Chain reaction

A chain reaction is a sequence of reactions where a reactive product or by-product causes additional reactions to take place. In a chain reaction, positive feedback leads to a self-amplifying chain of events....

 with no significant growth or decay in the reaction rate, and at a low enough level that thermal

Thermal

A thermal column is a column of rising air in the lower altitudes of the Earth's atmosphere. Thermals are created by the uneven heating of the Earth's surface from solar radiation, and are an example of convection. The sun warms the ground, which in turn warms the air directly above it...

 considerations are not important to the reaction. For example, a reactor that can produce gigawatts of heat might be considered zero-power critical when producing 100 watt

Watt

The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...

s of heat through a fission chain reaction.

Most nuclear reactors are held at a zero-power critical condition as part of the start-up sequence, to assess the condition of the reactor itself.