A criticality accident is an uncontrolled nuclear chain reaction. It is sometimes referred to as a critical excursion or a critical power excursion and represents the unintentional assembly of a critical mass of a given fissile material, such as enriched uranium or plutonium, in an unprotected environment. A critical or supercritical fission reaction (one that is sustained in power or increasing in power) generally only occurs inside reactor cores and occasionally within test environments; a criticality accident occurs when the same reaction is achieved unintentionally and in an unsafe environment. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed is still incapable of producing a nuclear detonation of the type seen in fission bombs, as the reaction lacks the many engineering elements that are necessary to induce explosive supercriticality. The heat released by the nuclear reaction will typically cause the fissile material to expand, so that the nuclear reaction becomes subcritical again within a few seconds. In the history of atomic power development, 60 criticality accidents have occurred, including 22 in collections of fissile materials located in process environments outside of a nuclear reactor or critical experiments assembly. Although process accidents occurring outside of reactors are characterized by a large release of radiation, the release is localized and has caused fatal radiation exposure only to persons very near to the event (less than 1 metre), resulting in 14 fatalities. No criticality accidents have resulted in nuclear explosions.

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