Defence in depth is the use of multiple layers of protection in nuclear safety that complement each other, rather than relying on a single piece of equipment, operator or interceptor. The aim is first to prevent deviations from normal operation, then to control the events that may occur and still limit the consequences of further situations.
This approach doesn't just consist of physical walls. Facility design, quality production, maintenance, measurement systems, operating procedures, training, safety culture and emergency preparedness are parts of the same whole.
Why is multiple layers of protection needed?
In complex engineering systems, equipment failures, human errors, and external events cannot be completely ignored. Defense depth aims to prevent failure in one layer from turning into a directly harmful outcome.
It is important that the layers are as independent as possible. If the same cause of failure can affect all backup systems, simply increasing the number of systems does not provide sufficient protection. Therefore, diversity, physical separation, and common cause failure analysis are part of the design.
Five-level approach
In IAEA safety documents, defence in depth is generally described in five objective levels:
- Avoid deviations and malfunctions: Robust design, quality production, correct operation and maintenance.
- Detecting and controlling abnormal operating conditions: Monitoring, control and protection systems.
- Controlling accidents covered within the scope of design: Shutting down the reactor, removing residual heat and keeping radioactive materials within limits.
- Limiting the consequences of more severe cases: Additional systems and accident management measures.
- Reducing on- and off-site consequences: Emergency plans, measurement, communication and protective actions.
These levels are enforced at a given facility through numerous systems and administrative measures.
What are physical barriers?
When describing light water reactors, barriers such as fuel matrix, fuel cladding, reactor cooling system boundary and containment building are mentioned. The goal is to control the movement of fission products across multiple physical boundaries.
Although physical barriers are important, defence in depth is not just the number of barriers. Systems that monitor the status of barriers, safety functions that protect them, and personnel who implement decisions are also included in the evaluation.
Are safety, security and safeguards the same thing?
Although the terms are related, they describe different fields. Nuclear safety concerns preventing accidents and limiting radiological consequences; nuclear security concerns protection against malicious acts; and nuclear safeguards verify that nuclear material remains in peaceful use.
Resources
- IAEA, Concept of Defense in Depth: https://nucleus.iaea.org/sites/nss-oui/Published%20Chunks/m_6ed2bf4f-b00e-4436-bde0-abf0d5a94e9d/c_acc399ce-18eb-4953-9e0d-a609fbe33dbc__1_0.Html
- IAEA, Safety Assessment for Facilities and Activities: https://nucleus.iaea.org/sites/nss-oui/Published%20Collections/m_0d27280c-1967-4e9c-aa22-4394ea3acb5b/m_0d27280c-1967-4e9c-aa22-4394ea3acb5b__31_0.Html
- U.S. NRC, Reactor Concepts Training Course: https://www.nrc.gov/sites/default/files/doc_library/cdn/legacy/reading-rm/training/reactor-concepts-training-course.pdf