Defence in depth is implemented through design and operation to provide a graded protection against a wide variety of transients, incidents and accidents, including equipment failures and human errors within the plant and events initiated outside the plant.
The defence in depth concept was introduced in the nuclear safety field in the early 1970s. The central tenet of defence in depth is to protect the health and safety of the public and plant workers. Other objectives include protecting the environment and ensuring the operational readiness of the facility.
Successful defence in depth requires creating, maintaining, and updating multiple independent and redundant layers of protection to compensate for potential human and mechanical failures so that no single layer, no matter how robust, is exclusively relied upon.
In nuclear facilities, it is achieved by implementing different layers of protection (different levels of defence). These protections apply to the intrinsic characteristics of the facility, equipment measures and procedures put in place to prevent accidents and, if prevention fails, to limit accident consequences.
Defence in depth is a safety philosophy that guides the design, construction, inspection, operation, and regulation of all nuclear facilities and applies to all stages in the life of a facility, from design to dismantling.
The concept of defence in depth has evolved over time to take into account operational experience from facilities, including incidents and accidents that have occurred, in order to build an ever more effective defence.
The fifth and final level of defence is aimed at mitigation of the radiological consequences of potential releases of radioactive materials that may result from accident conditions. Measures for protecting the public from radioactive releases include evacuation, shelter in hard-wall accommodation, taking of potassium iodide tablets and restrictions on the consumption of foodstuffs. This level includes off-site emergency plans prepared for each site and an emergency control centre. Public authorities implement the off-site emergency plan, which organises emergency operations to limit public exposure to radiation in the event of releases.
Defence in depth can be portrayed as a set of barriers designed to prevent radioactive material from being released into the environment.