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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 mere fact that an assembly is supercritical does not guarantee that it contains any free neutrons at all. At least one neutron is required to "strike" a chain reaction, and if the spontaneous fission rate is sufficiently low it may take a long time (in 235 U reactors, as long as many minutes) before a chance neutron encounter starts a chain reaction even if the reactor is supercritical.
The rapid uncontrollable increase in reactor power in prompt-critical conditions is likely to irreparably damage the reactor and in extreme cases, may breach the containment of the reactor. Nuclear reactors' safety systems are designed to prevent prompt criticality and, for defense in depth, reactor structures also provide multiple layers of ...
Each nuclear fission produces several neutrons that can be absorbed, escape from the reactor, or go on to cause more fissions in a nuclear chain reaction. When an average of one neutron from each fission goes on to cause another fission, the reactor is "critical", and the chain reaction proceeds at a constant power level. Adding reactivity at ...
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 fission cross-section ), density, shape, enrichment , purity, temperature, and surroundings.
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 with no significant growth or decay in the reaction rate, and at a low enough level that thermal considerations are not important to the reaction.
Criticality accidents are divided into one of two categories: Process accidents, where controls in place to prevent any criticality are breached;; Reactor accidents, which occur due to operator errors or other unintended events (e.g., during maintenance or fuel loading) in locations intended to achieve or approach criticality, such as nuclear power plants, nuclear reactors, and nuclear ...
A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction. They are used for commercial electricity , marine propulsion , weapons production and research . Fissile nuclei (primarily uranium-235 or plutonium-239 ) absorb single neutrons and split, releasing energy and multiple neutrons, which can induce further ...