The purpose of the first level of defence is to prevent deviations from normal operation and the failure of items important to safety. This leads to requirements that the facility be soundly and conservatively sited, designed, constructed, maintained, operated and modified in accordance with a management system and appropriate and proven engineering practices. To meet these requirements, careful attention is paid to the selection of appropriate design codes and materials, and to the quality control of the manufacture of components and construction of the facility, as well as to its commissioning. Design options, including the selection of processes, that reduce the potential for internal hazards also contribute to the prevention of accidents at this level of defence. Attention is also paid to the processes and procedures involved in design, manufacture, construction and in-service inspection, maintenance and testing, to the ease of access for these activities, and to the way the facility is operated and to how operating experience is utilized. This process is supported by a detailed analysis that determines the requirements for operation and maintenance of the facility and the requirements for quality management for operational and maintenance practices.

The purpose of the second level of defence is to detect and control deviations from operational states in order to prevent anticipated operational occurrences at the facility from escalating to accident conditions⁵. This is in recognition of the fact that postulated initiating events are likely to occur over the operating lifetime of a nuclear fuel cycle facility, despite the care taken to prevent them. This second level of defence necessitates the provision of specific systems and features in the design, the confirmation of their effectiveness through safety analysis, and the establishing of operating procedures to prevent such initiating events, or else to minimize their consequences, and to return the facility to a safe state.

For the third level of defence, it is assumed that, although very unlikely, the escalation of certain anticipated operational occurrences or postulated initiating events might not be controlled at a preceding level and that an accident could occur. In the design of the facility, such are postulated to occur. This leads to the requirement that inherent and/or engineered safety features, fail-safe design and procedures be provided to control the consequences of such . Such engineered safety features will be capable of preventing extensive damage to the facility or significant off-site releases and returning the facility to a safe state, and maintaining at least one physical barrier for the confinement of radioactive material. This barrier may be provided by a combination of a ‘static’ barrier with a complementary ‘dynamic’ barrier (e.g. a ventilation system), which together provide effective confinement of radioactive material. The most important objective for this level is to prevent releases of radioactive material and associated hazardous material or radiation levels that require off-site protective actions.

The purpose of the fourth level of defence is to mitigate the consequences of accidents that result from failure of the third level of defence in depth. The most important objective for this level is to ensure that the confinement function is maintained, thus ensuring that radioactive releases are kept as low as reasonably achievable.

The purpose of the fifth level of defence is to mitigate the radiological consequences and associated chemical consequences of releases or radiation levels that could potentially result from . This requires the provision of adequately equipped emergency response facilities and emergency plans and emergency procedures for on-site and off-site emergency response.