{{Unreferenced, date=August 2009
Zero power critical is a condition of
nuclear fission reactors 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
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.
Chain reactions are one way that sys ...
with no significant growth or decay in the reaction rate, and at a low enough level that
thermal
A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
considerations are not important to the reaction. For example, a reactor that can produce
gigawatt
The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Wat ...
s of heat might be considered zero-power critical when producing 100
watt
The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James ...
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.
Nuclear reactors