a situation in which an uncontrollable chain reaction arises in a nuclear reactor.
The possibility of an uncontrollable chain reaction occurring throughout the entire core of a nuclear reactor is virtually impossible, since the reactor control and protection system would prevent such a situation from ever arising. However, with the considerable sizes of the modern power reactors, localized brief sources of criticality may arise, which may lead to undesirable consequences, such as damage to the reactor or its core as a result of the rise in heat beyond the permissible level. Such a heat flare-up may be caused, for example, by the shifting of the fuel elements or the erroneous removal of a regulating rod.
A nuclear accident may also be caused by the formation of a critical mass in vessels or devices containing fissionable materials. The possibility that such a nuclear accident will ever arise has been virtually eliminated by the strict observance of safety rules when working with fissionable materials.
IU. I. KORIAKIN