The safety philosophy upon which the Swedish Nuclear Power Inspectorate’s (SKI) supervisory and regulatory activities are based assumes that multiple physical barriers will exist and that a plant-specific defence-in-depth system will be implemented at each plant. The physical barriers are situated between the radioactive material and the plant personnel and surroundings. In the case of nuclear reactors in operation, the barriers comprise the fuel itself, the fuel cladding, the reactor pressure-bearing primary system and the containment. Defence-in-depth entails applying several levels of different technical systems and operational measures as well as administrative routines in order to protect the barriers and maintain their effectiveness during normal operation and during anticipated events and accidents. If this fails, a system for emergency preparedness should be in place in order to limit and mitigate the consequences of a severe accident. An effective defence-in-depth system is based upon sound management and control of safety, an organization with adequate financial and human resources and personnel with the necessary competence working under suitable conditions. This is the basis of a good safety culture.
When a facility is in operation, all barriers should be intact. This means, for example, that a containment leak should normally result in the shutdown of a reactor, even if all other barriers are intact and safety is thereby not jeopardized. Defence-in-depth systems are designed so that they can withstand deficiencies during a limited period of time required for corrective action. For example, a competence analysis or parts of a safety assessment may be lacking for a certain period of time without SKI requiring the facility to be shut down. When such deficiencies occur, SKI talks about reduced safety margins.