Defence in Depth (DiD) is widely recognized as a cornerstone concept for providing safety in nuclear installations. It envisages successive multiple independent levels of protection addressing prevention and mitigation of accidents that may cause harm to people and the environment.
On 19 September 2012, the IAEA Department of Nuclear Safety organized a side event during the 56th IAEA General Conference to address the current thinking and applications of the DiD safety approach for nuclear power plants. A panel discussion followed presentations by Mr. Nigel Buttery, Senior Expert from EDF, UK; Mr. Jozef Misak, Senior Safety Assessment Expert from UJV, Slovakia; and Ms. Irina Kuzmina, Safety Officer, IAEA Safety Assessment Section in the Division of Nuclear Installation Safety. Participants reaffirmed DiD's role as a valid and robust approach for providing safety of nuclear installations. Additionally, new thinking and approaches are bound to contribute to a more holistic and consistent application of this safety concept.
Mr. Gustavo Caruso, IAEA Nuclear Safety Action Team Special Coordinator, opened the event and described the DiD concept as one of the most important issues being applied within the Nuclear Safety Action Plan. In his introductory remarks, Mr. James Lyons, Director of the Division of Nuclear Installation Safety, also called DiD as a major component of the nuclear safety regime. In the next talk, Mr. Peter Hughes, Head of the Safety Assessment Section in the Division of Nuclear Installation Safety, emphasized that DiD's relevance is in the practical implementation of the concept, particularly in its use for external events.
During the discussions participants provided several recommendations for future activities. They highlighted the need to improve safety assessment tools and methods to assess compliance with DiD. Furthermore, they stressed the importance of providing a more consistent and uniform terminology, as well as a focused training for the newcomer countries. The IAEA should also consider developing a guidance document on robust application of DiD to further assist in providing safety of present and future nuclear power plants. Finally, nuclear regulatory systems should ensure that regulatory independence is preserved in all circumstances.
The meeting helped to identify challenges to achieve more sufficient DiD provisions for external hazards. Issues and recommendations from the discussions will be reflected in future IAEA's activities.
To achieve optimum safety, nuclear plants operate using a "Defence in Depth" approach, with multiple safety systems supplementing the natural features of the reactor core. Key aspects of the DiD approach are: prevention, monitoring, and action (to mitigate consequences of failures). It provides a hierarchical deployment of different independent levels of equipment and procedures in order to maintain the effectiveness of physical barriers placed between radioactive materials, the workers, public, and the environment, during normal operation and potential accident conditions.
In the IAEA publications the concept was introduced by the International Nuclear Safety Advisory Group (INSAG) in the INSAG-3 Report Basic Safety Principles for NPPs in 1988. Many IAEA Safety Standards and other publications produced later emphasize the importance of the DiD concept and provide high level recommendation on its practical application.