12.7. Defense in depth is the key design principle in nuclear reactor safety. The approach is to provide a series of philosophical and physical layers of protection against the release of radioactivity to the public. In other words, we have one barrier which, if breached, is “backed up” by a second barrier. The second barrier is backed up by a third barrier, and so on.

12.8. The first barrier, or goal, is philosophical, i. e., to prevent accidents from happening. Should an accident happen, the next layer of defense is to provide various countermeasures, in a sequential manner, to control the accident. In our defense, we then provide a series of physical barriers and further countermeasures to confine fission products that might be released. Finally, a containment structure which encloses the nuclear por­tion of the plant is provided “just in case” there is need for further backup. We will examine these principles in the following sections.