It's a cerebral day (easy to happen when you've been waiting two days for the production techs to change a simple needle valve on top of a well head).
Safety mechanisms are often broken down into two categories: active and passive. Active is where an operator or an automatic system must sense and take action to correct for a fault. Passive is where such action will happen without the need for anyone or anything to decide where it happens.
There are obvious examples of active and passive safety. Active safety was a pump must be activated in order to supply emergency cooling water. Passive safety is where a the natural effect of Doppler broadening will cause a pebble bed reactor to become sub-critical if the temperature rises too high, such as in the case of a loss of coolant accident.
But what about the more grey areas? In boiling water reactors, control rods are inserted from below due to the fact that attempting to control the reaction from the steam laden upper portion of the core is ineffective. To aid safety, these control rods are spring loaded. During operation, they are withdrawn against the resistance of the springs trying to push them back in.
Is this passive safety? From one perspective it is. In the event of a total loss of electrical power to the control system, the springs will force the control rods into the core without any outside assistance. However, this is still dependent on the operation of engineered components unlike the PBR system, which depends solely on the immutable Laws of Physics.
The PBR case is an example of inherent safety. The BWR case is an example of a system designed to fail safe. Can these two types of safety be described in the same manner?