The primary means of preventing and mitigating the consequences of acci­dents is the ‘defence in depth’ concept (IAEA, 1996a). Defence in depth is implemented primarily through the combination of a number of consecu­tive and independent levels of protection that would have to fail before harmful effects could be caused to people and to the environment. Defence in depth is provided by an appropriate combination of an effective manage­ment system with a strong management commitment to safety and a strong safety culture; adequate site selection and the incorporation of good design and engineering features providing safety margins, diversity and redun­dancy; and comprehensive operational procedures and practices as well as accident management procedures.

Accordingly to the defence in depth concept, the design, construction and operation of nuclear facilities are conducted under the most stringent quality controls to comply with safety principles, including the development of the necessary provisions to deal with emergency situations in all modes of operation. The owner and the national authorities in charge of nuclear safety perform independent verification programmes to ensure strict com­pliance with safety and quality requirements.

Despite the fact that the nuclear facilities and their safety systems are designed, installed, tested, operated and verified in accordance with the strictest safety and quality standards, the possibility of an accident, a human error or an intentional action that can seriously damage the facility cannot be excluded, although its probability can be considered as extremely low. In very unlikely circumstances, these situations could cause simultaneous failure of operating and safety systems, which could produce radiation exposure of facility workers or uncontrolled discharge of radioactive mate­rial to the environment. Furthermore, in very extreme circumstances, some external phenomena, e. g. earthquake, tsunami or sabotage, could severely damage the plant, its external and internal supplies of electric power or cooling water in such a way that the operator is unable to control the safety systems. This is the situation that occurred in the Fukushima nuclear power plant on 11 March 2011 as a consequence of a big earthquake and a tsunami that partially destroyed the facility.

Both circumstances could result in damage to the health of individuals living or working near the facility as well as to their property and to the environment.