Criticality is the state of a nuclear chain reacting medium when the chain reaction is just self-sustaining (or critical), that is, when the reactivity is zero. More loosely, the term is used for states in which the reactivity is greater than zero.
In the context of a nuclear reactor, particularly in a nuclear power plant, criticality refers to the normal operating condition of a reactor, in which nuclear fuel sustains a fission chain reaction. A reactor achieves criticality (and is said to be critical) when each fission event releases a sufficient number of neutrons to sustain an ongoing series of reactions.
The International Atomic Energy Agency also defines the first criticality date as the date when the reactor is made critical for the first time. This is an important milestone in the construction and commissioning of a nuclear power plant. For example, India's Kudunkulam Nuclear Power Plant's second reactor attained criticality on 10th July, 2016 by withdrawing the control rods from the reactor, which led to boron dilution to allow neutron concentration to go up, which eventually led to criticality of the reactor. Qinshan 1, China's first domestically designed and constructed nuclear power plant, achieved first criticality on 31 October 1991.
- "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 Critical Date". Glossary. International Atomic Energy Agency. Retrieved 17 February 2014.
- "QINSHAN-1 Status Page". Power Reactor Information System. International Atomic Energy Agency. Retrieved 18 February 2014.