The core organising principle of nuclear safety: successive, independent levels of protection, each able to act if the one before fails. The usual scheme has five levels, from preventing abnormal operation, through controlling faults and design basis accidents, to mitigating severe accidents and finally protecting the public with off-site emergency response. It applies to physical barriers, systems, procedures and people alike.
Losing one train is fine, that's defence in depth, but two trains down means we're one failure from nothing.
Veterans use the phrase to challenge shortcuts: any change that quietly removes one layer, however minor, is treated as a safety issue in its own right.
Also heard as
- defense in depth
- DiD
Related terms
Fission product barrier
One of the successive physical barriers that keep radioactive fission products away from people and the environment.
Redundancy
Providing more components or trains than needed, so a safety function survives if one fails.
Diversity
Using different designs, technologies or physical principles for the same safety function to guard against common-cause failure.
Single failure criterion
The design rule that a safety system must still do its job despite any one component failing.
Emergency classification
The graded levels used to declare a nuclear plant emergency, each triggering a defined response.