A criticality accident, sometimes referred to as an excursion or a power excursion, is an accidental increase of nuclear chain reactions in a fissile material, such as enriched uranium or plutonium. This releases a surge of neutron radiation which is highly dangerous to humans and causes induced radioactivity in the surroundings.

Critical or supercritical nuclear fission (one that is sustained in power or increasing in power) generally occurs inside reactor cores and occasionally within test environments. A criticality accident occurs when a critical reaction is achieved unintentionally. Although dangerous, typical criticality accidents cannot reproduce the design conditions of a fission bomb, so nuclear explosions do not occur. 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. However, none have resulted in explosions.