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The World Federation of Science Journalists (WFSJ) is a not-for-profit, non-governmental organisation, representing 55 science journalists’ associations of science and technology journalists from Africa, the Americas, the Asia-Pacific, Europe and the Middle East. The Federation encourages strong, critical coverage of issues in science and technology, environment, health and medicine, agriculture and related fields.

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Defence in depth

//Defence in depth

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.

General presentation of a pressurized water reactor (1,300 MWe or N4) and its main circuits. IRSN.

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.

To prevent the occurrence and limit the consequences of technical, human and organisational failures, defence in depth is deployed on five levels in nuclear installations.

Level 1

The first level of defence is to prevent deviations from normal operation, and to prevent system failures. This requires that the plant is soundly and conservatively designed (with adequate safety margins), constructed (components are manufactured to the highest quality standards), maintained and operated in accordance with appropriate quality levels and engineering practices (redundancy, independence and diversity).

Level 2

The second level of defence aims to detect failure and understand deviations from normal operations. This level includes resources and systems designed to control operating malfunctions, which assumes monitoring that will ensure failures are detected. This includes automatic functions and control systems that can return the facility to its normal operating mode. These systems are designed to correct an abnormal change in facility parameters.

Level 3

The first two levels of defence in depth reduce the risks of failure at the facility. It is nevertheless assumed that accidents can occur during reactor operation. Accidents considered at this level result from a single initiating event (e.g., the failure of a component essential for a basic safety function management of reactivity, cooling of nuclear fuel, or containment of radioactive substances). Resources that limit the consequences of such accidents and ensure basic safety functions are implemented: at this level defence in depth consists of implementing safeguards that ensure the integrity of the core structure and limit releases into the environment in the event of an accident (considered for the design-basis of the facility). This level also includes defining emergency operating procedures.

For the third level of defence, it is assumed that, although very unlikely, escalation of certain anticipated operational occurrences or postulated initiating events (PIEs, that is to say scenarios that might occur) may not be controlled by a preceding level of defence, and a more serious event may develop. These unlikely events are anticipated in the design basis for the plant, and inherent safety features, fail-safe designs, and additional equipment and procedures are provided to control their consequences and to achieve stable and acceptable conditions following such events.

Level 4

This level of defence in depth includes procedures and equipment used to handle situations that are not covered by the first three levels; these are accidents that could result in reactor core melt.

At level 4, the objective is to prevent accidents from resulting in core melt and to limit releases outside the site by ensuring containment (as far as practicable) of radioactive substances in the event core melt nevertheless does occur. The most important objective of this level is the protection of the confinement function.

This level of defence in depth includes emergency procedures and associated equipment resources, specific equipment (e.g., hydrogen recombiners), the severe accident operating guidelines and the facility’s on-site emergency plan.

Level 5

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.