Criticality (status)
In the operation of a nuclear reactor, criticality or critical state is the state in which a nuclear chain reaction is self-sustaining but not growing. Subcriticality or subcritical state is the state in which a nuclear chain reaction is not self-sustaining. Supercriticality or supercritical state is the state in which a nuclear chain reaction is self-sustaining and growing. Sometimes, less preferably, criticality takes a wider definition, and refers to the any state in which a nuclear chain reaction is self-sustaining, no matter growing or not (encompassing criticality in strict definition and supercriticality).[1]
In terms of reactivity, reactivity is 0 in criticality, less than 0 in subcriticality, greater than 0 in supercriticality.[1] In terms of effective neutron multiplication factor (Keff), Keff is 1 in criticality, less than 1 in subcriticality, greater than 1 in supercriticality.
Applications
[edit]Criticality is the normal operating condition of a nuclear reactor, in which nuclear fuel sustains a fission chain reaction. 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.[2]
The International Atomic Energy Agency defines the first criticality date as the date when the reactor is made critical for the first time.[3] This is an important milestone in the construction and commissioning of a nuclear power plant.
Prompt criticality
[edit]The event of fission must release, on the average, more than one free neutron of the desired energy level in order to sustain a chain reaction, and each must find other nuclei and cause them to fission. Most of the neutrons released from a fission event come immediately from that event, but a fraction of them come later, when the fission products decay, which may be on the average from microseconds to minutes later. This is fortunate for atomic power generation, for without this delay "going critical" would be an immediately catastrophic event, as it is in a nuclear bomb where upwards of 80 generations of chain reaction occur in less than a microsecond, far too fast for a human, or even a machine, to react. Physicists recognize two points in the gradual increase of neutron flux which are significant: critical, where the chain reaction becomes self-sustaining thanks to the contributions of both kinds of neutron generation,[4] and prompt critical, where the immediate "prompt" neutrons alone will sustain the reaction without need for the decay neutrons. Nuclear power plants operate between these two points of reactivity, while above the prompt critical point is the domain of nuclear weapons, pulsed reactors designs such as TRIGA research reactors and the pulsed nuclear thermal rocket, and some nuclear power accidents, such as the 1961 US SL-1 accident and 1986 Soviet Chernobyl disaster.
See also
[edit]References
[edit]- ^ a b "Criticality" (PDF). IAEA Safety Glossary. International Atomic Energy Agency. 2007. p. 46. Retrieved 17 February 2014.
- ^ "Criticality". Glossary. US Nuclear Regulatory Commission. 11 December 2013. Retrieved 17 February 2014.
- ^ "First Criticality Date". Glossary. International Atomic Energy Agency. Retrieved 17 February 2014.
- ^ Rhodes, Richard (1 August 1995). Dark Sun: The Making of the Hydrogen Bomb. Simon & Schuster. ISBN 978-0-68-480400-2. LCCN 95011070. OCLC 456652278. OL 7720934M. Wikidata Q105755363 – via Internet Archive.In the description of the Soviet equivalent of the CP1 startup at the University of Chicago in 1942, the long waits for those tardy neutrons is described in detail
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