The NRC defines defense in depth as creating multiple independent and redundant layers of protection and response to failures, accidents, or fires in power plants.
The idea is that no single layer, no matter how robust, is exclusively relied upon; access controls, physical barriers, redundant and diverse key safety functions, and emergency response measures are used.
[2] On November 19, 1980, the NRC promulgated 10 CFR 50, Appendix R,[3] Fire Protection Program for Nuclear Power Facilities Operating Prior to January 1, 1979, which has a discussion of defense-in-depth.
The defense in depth rule required electric power cables, which control reactor shutdown in an emergency, to have fire insulation that lasts one hour.
[5] However, on appeal, the Federal Circuit Court, determined that the NRC must hold public hearing on any exemption to the defense in depth rule.