Criticality Accident

A criticality accident, sometimes referred to as an excursion or a power excursion, is the unintentional assembly of a critical mass of a given fissile material, such as enriched uranium or plutonium, in an unprotected environment. The assembly of a critical mass establishes a nuclear chain reaction, causing a sudden surge of neutron radiation (a neutron flux). This flux is extremely dangerous to unprotected humans nearby and causes destructive embrittlement, neutron-induced swelling and induced radioactivity in materials close to the flux point.

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 in collections of fissile materials outside nuclear reactors, and some of these have resulted in death, by radiation exposure, of the nearest person(s) to the event. None have resulted in explosions.