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A reactor consists of an assembly of nuclear fuel (a reactor core), usually surrounded by a neutron moderator such as regular water, heavy water, graphite, or zirconium hydride, and fitted with mechanisms such as control rods which control the rate of the reaction. The physics of nuclear fission has several quirks that affect the design and ...
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 ...
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.
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 ...
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.
If k = 1, the chain reaction is critical and the neutron population will remain constant. In an infinite medium, neutrons cannot leak out of the system and the multiplication factor becomes the infinite multiplication factor, k = k ∞ {\displaystyle k=k_{\infty }} , which is approximated by the four-factor formula.
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 ...
A new reactor is assembled with its control rods fully inserted. Control rods are partially removed from the core to allow the nuclear chain reaction to start up and increase to the desired power level. Neutron flux can be measured, and is roughly proportional to reaction rate and power level. To increase power output, some control rods are ...