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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]
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 criticality accident occurred on December 30, 1958, at Los Alamos National Laboratory (LANL) in Los Alamos, New Mexico, in the United States It is one of 60 known criticality events that have occurred globally outside the controlled conditions of a nuclear reactor or test; though it was the third such event that took place in 1958 after events on June 16 [1] at the Y-12 Plant in Oak Ridge ...
A big reason the lab is working toward producing 30 fresh plutonium cores is some federal scientists and nuclear security officials contend the Cold War pits have grown too old to be reliable ...
A sign on the wall marks the "Toxic Release Assembly Point," another sign with arrows is marked the "Criticality Run." I am at the UK National Nuclear Laboratory in Preston in Lancashire, so you ...
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.
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 ...
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.