over 14 years ago

An accident requires that the event in question was unforeseeable, unlikely or unpredictable.

A criticality accident (also sometimes referred to as an "excursion" or "power excursion") occurs when a nuclear chain reaction is accidentally allowed to occur in fissile material, such as enriched uranium or plutonium. This releases neutron radiation which is highly dangerous to surrounding personnel and which causes induced radioactivity in the surroundings.

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