Criticality accident

A criticality accident, or simply a criticality, is an accidental nuclear fission chain reaction that goes out of control. These accidents are sometimes called critical excursions, critical power excursions, or divergent chain reactions. These reactions are the result of accidents involving fissile material (material that can undergo nuclear fission), and they can have very harmful results on the environment, as they let off large amounts of radiation. During normal conditions, a critical reaction (a reaction that can keep itself going) only happens in safely guarded location. During a criticality, these reactions happen outside of these environments, or in a way that isn't safe. Criticality can be reached by solid materials (for example, metallic uranium), liquid materials, or by powder slurries.