Criticality
The criticality is a discipline of engineering Nucléaire aiming to evaluate and prevent the risks of chain reaction nondesired in nuclear installations. It is a under-discipline of the Neutronique. Contrary to the nuclear reactors whose heart must be critical (multiplication factor of the neutrons equal to 1), other nuclear installations (installations of research and the fuel cycle in particular) must be subcritical (multiplication factor of the neutrons strictly lower than 1) in order to prevent an accident of criticality.
The reactivity of an installation depends on various factors:
- quantity and fissile matter nature,
- conditions of moderation,
- conditions of reflection,
- presence of materials neutrophages.
According to the installations, there exist various means of limiting the reactivity, they are the modes of control:
- control by the fissile quantity of material (mass and/or concentration),
- control by the geometry of the process,
- control by neutron poisoning,
- control by the density of surface of the fissile medium,
- control by the moderation of the fissile medium.
The modes of controls are frequently doubled and of the margins are taken compared to the critical conditions: they are the acceptable conditions.
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