Defence in Depth
Defence in depth is a hierarchical deployment of several levels of equipment and procedures to contain radioactive materials during normal operation and accidents. This is done through proper reactor design and plant operation to protect against both human error, equipment failure and events occurring outside the plant.
The objective of defence in depth:
- Compensate for human and component failures
- Maintain effectiveness of barriers by averting damage to the plant and the barriers themselves
- To protect the public and the environment from harm in the event that these barriers are not fully effective.
| Levels of defence in depth | Objective | Essential means |
| Level 1 | Prevention of abnormal operation and failures | Conservative design and high quality in construction and operation |
| Level 2 | Control of abnormal operation and detection of failures | Control, limiting and protection systems and other surveillance features |
| Level 3 | Control of accidents within the design basis | Engineered safety features and accident procedures |
| Level 4 | Control of severe plant conditions, including prevention of accident progression and mitigation of the consequences of severe accidents | Complementary measures and accident management |
| Level 5 | Mitigation of radiological consequences of significant releases of radioactive materials | Off-site emergency response |
ALARA
What does it stand for?
- As Low As Reasonably Achievable
What does it mean?
- ALARA is the underlying approach to radiation protection where radiation exposure must be kept as low as possible while taking into account financial and social factors.
Redundancy
Redundancy is the use of more than the minimum number of sets of equipment needed to fulfil a safety requirement. This allows for the failure of one or more sets of equipment without the loss of safety systems.
Additional concept to redundancy is diversity. Diversity is the concept of using different systems or components that perform the same task but through another means. This prevents the occurrence of multiple systems failing from the same cause.