According to the IAEA “Code on the Safety of Nuclear Power Plants: Design” [6] safety systems are “systems important to safety to assure the safe shutdown of reactor or residual heat removal from the core, or to limit the consequences of anticipated operational occurrences or accident conditions". Anticipated operational occurrences are events that are expected during the operating life of nuclear reactor, as opposed to the accident conditions which are events that occur unexpectedly and used as design basis for engineered safety futures to limit the release of radiation. For the RBMK-1500 reactor, the above mentioned main safety functions are assured by the following safety systems: Control and Protection System, Emergency Core Cooling System, Emergency Feedwater System, Accident Confinement System, Pressure Relief System and Reactor Cavity Overpressure Protection System.
3.1 Control and Protection System
The reactor protection system consists of several subsystems, which are designed to run back the reactor power or to scram the reactor in response to a variety of trip signals. A power runback to 60% of full power occurs in response to AZ-4 signal. A power runback to 50% of full power occurs in response to AZ-3 signal. The power runback is accomplished by inserting control rods until the desired power level is achieved. A reactor scram can be either a fast-acting scram (FAS signal) or the normal scram (AZ-l signal), which is slower. A fast-acting scram is intended to quickly insert control rods for conditions which could lead to the rapid generation of positive reactivity. The slower AZ-l rod insertion is effected on response to transients which would not produce a rapid positive reactivity insertion.