In the operation of a
nuclear reactor
A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat fr ...
, criticality is the state in which a
nuclear chain reaction is self-sustaining—that is, when
reactivity is zero. In supercritical states, reactivity is greater than zero.
Applications
Criticality is the normal operating condition of a nuclear reactor, in which nuclear fuel sustains a
fission chain reaction
Fission, a splitting of something into two or more parts, may refer to:
* Fission (biology), the division of a single entity into two or more parts and the regeneration of those parts into separate entities resembling the original
* Nuclear fissi ...
. A reactor achieves criticality (and is said to be critical) when each fission releases a sufficient number of neutrons to sustain an ongoing series of nuclear reactions.
The
International Atomic Energy Agency defines the ''first criticality date'' as the date when the reactor is made critical for the first time.
This is an important
milestone
A milestone is a numbered marker placed on a route such as a road, railway line, canal or boundary. They can indicate the distance to towns, cities, and other places or landmarks; or they can give their position on the route relative to so ...
in the construction and commissioning of a
nuclear power plant.
See also
*
Criticality accident
A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, or divergent chain reaction. Any such event involves the unintended accumulation ...
*
Critical mass
In nuclear engineering, a critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, its nuclear fi ...
*
Prompt criticality
In nuclear engineering, prompt criticality describes a nuclear fission event in which criticality (the threshold for an exponentially growing nuclear fission chain reaction) is achieved with prompt neutrons alone (neutrons that are released immed ...
References
{{reflist
Nuclear chemistry
Nuclear physics
Nuclear technology
Radioactivity