Defence in depth
The concept of defence in depth is fundamental to the safety of nuclear installations. Nuclear safety does not rely on one line of defence but is achieved using a range of complementary means. These factors start with the design and building of a nuclear facility which requires choosing a good design and appropriate site, use of high-quality construction materials and testing before operation. They also cover the whole range of organisational and behavioural issues that are critical to operating a nuclear installation. This comprehensive approach is summarised in the Basic Safety Principles for nuclear power plants prepared by the International Atomic Energy Agency’s International Nuclear Safety Group (INSAG) of experts:
“All safety activities, whether organizational, behavioural or equipment related, are subject to layers of overlapping provisions, so that if a failure should occur it would be compensated for or corrected without causing harm to individuals or the public at large. This idea of multiple levels of protection is the central feature of defence in depth...” (INSAG)
The key objectives of defence in depth are:
- To compensate for potential human and component failures;
- To maintain the effectiveness of the barriers by averting damage to the plant and to the barriers themselves; and
- To protect people, including workers in the nuclear industry, and the environment from harm in the event that these barriers are not fully effective.
To meet these objectives, the strategy for defence in depth is, above all, about preventing accidents. However, if prevention fails, the strategy limits an accident’s potential consequences as much as possible and prevents any escalation to more serious conditions. This means ensuring a low chance of failures in the systems used, combined with redundancy in design, so that if one system fails other independent, diverse lines of defence ensure an accident does not result (defence in depth).
Five levels of defence in depth
Defence in depth is generally structured in five levels. Should one level fail, it will be compensated for, or corrected by, the subsequent level:
- The objective of the first level of protection is the prevention of abnormal operation and system failures in the nuclear reactor. Nuclear reactors are designed to ensure reliable, stable and easily manageable operation. Thus safety-critical components in a reactor are based on very high-quality technologies.
- If the first level fails, abnormal operation is controlled or failures are detected by the second level of protection. This level of defence-in-depth involves the rapid detection of any deviation from normal operation and where possible, its automatic correction by process control and protection systems, without interfering with normal operation.
- Should the second level fail, the third level ensures that safety functions are further performed by activating specific safety systems and other safety features. Nuclear reactors are designed to withstand a set of abnormal occurrences and potential accidents. The third defence in depth level is the control of these accidents / occurrences. This level of defence will aim to automatically place the reactor into a safe condition and contain the radioactive materials.
- Should the third level fail, the fourth level limits accident progression through accident management, so as to prevent or mitigate severe accident conditions with external releases of radioactive materials. All nuclear plants have severe accident guidelines and procedures that operators can implement. These are tested and exercised on a regular basis.
- The last objective (fifth level of protection) is the mitigation of the radiological consequences of significant external releases through the implementation of an off-site emergency plan. The efficacy of the mitigation measures will depend on their overall effectiveness and the speed of their implementation. The response could include recommendations on sheltering, evacuation, food restrictions, stable iodine tablets etc.
group of international nuclear safety experts issues guidance and advice on the whole range of nuclear safety approaches, policies and principles. These are taken up by ENSREG’s members in national strategies for ensuring nuclear safety. One important function of ENSREG is to encourage the application of this guidance across the EU and to act as an EU-level forum for exchanges of experience and best practice in the field.