The Atomic Weapons Establishment (AWE) defines an abnormal event (AE) as any action or series of actions that causes a deviation from the planned course of events. An AE need not cause physical harm to persons, property or the environment, and AEs may cover quality and security issues as well as safety.
AEs are classified into seven different categories; five of these are considered to be related to safety. The total number of AEs in these five categories was 4,020 in 2008 and 3,911 in 2009. Of these, three in each year were assessed as having the potential, in the absence of remedial action, to challenge a nuclear safety system. None of these six actually did, and none resulted in any harm.
All reported AEs are investigated as appropriate; in particular, the six events referred to above were subject to detailed investigations, and all necessary remedial action has been completed to the satisfaction of the Nuclear Installations Inspectorate.
The aim of the AWE reporting system is to identify all AEs, however minor, and the number of AEs is considered to be a reflection of the robustness of the system and the vigilance of the AWE work force.