These systems must properly act under any conditions to efficiently prevent reactor core damage. Due to their highly reliable design, they can respond to a single failure in the reactor system. To check their proper function in accident conditions, regular surveillance tests can be conducted during the normal operation and reactor shutdown period. The design of the ECCS consists of several independent subsystems characterized by independency and redundancy which assures their satisfactory function in a LOCA, even in the event of a single failure of an active component for a short term following an accident, or a single failure of an active component or a single failure of a passive component during long term cooling after an accident. Moreover, those components which are installed inside the reactor containment vessel are designed to withstand the adverse environmental conditions following an accident.

The electric power supply system is also important for the plant safety, since the functioning of the engineered safeguard systems and other safety systems depends on reliable supply of electric power. Besides redundant off-site power supply systems, PWR plants have two physically and electrically independent on-site emergency power supply systems which are used when the off-site power is lost. Regular inspections are conducted to check the on-time availability and effective performance of diesel generators and battery banks of these emergency power supply systems.

The reactor containment system is the final barrier which ensures the public safety by preventing FP release to the environment when FP have escaped from the primary coolant system in the event of a failure. The design of the reactor containment system allows regular tests to be conducted which ensure that the design limit for leakage rate of radioactive materials from the containment would not be exceeded. Leakage tests can also be conducted separately on special penetrations such as for piping, electric cords and air-locks, or on a small group of them.

The pipes passing through the reactor containment boundary are isolated by using valves. The valves performances could also be tested regularly. To prevent a brittle rupture of the ferric structure of the containment boundary under any

conditions during its service lifetime, the toughness of the materials being selected is closely considered.

4. Instrumentation and Control System

The instrumentation and control system of the PWR power generating plant consists of the reactor control system and the reactor safety protection system. The safety protection system itself, consists of the emergency shutdown system and the actuation system of the engineered safeguard systems. These systems provide important functions to guarantee reactor safety during its normal operation, and under any abnormal situations including emergency reactor shutdown (caused by malfunction and/or miss operation) as well as countermeasures to prevent accident escalation. Reactor instrumentation and process instrumentation of the instrumentation and control system measure parameters such as neutron distribution, temperature, pressure and radiation which are important in controlling and protecting the reactor system. Each of these parameters is an input signal of the reactor control and reactor safety protection systems.

The aim of the reactor control system is to maintain normal values of operating parameters during normal reactor operation and automatically control the design load changes as well as power disturbances. The design of reactor control system is not specially aimed at protection of the reactor since that is the responsibility of the reactor safety protection system. However, minimizing the actuation of the protection system is a design objective of the reactor control system.

Moreover, the control system is designed to detect any abnormal event at its early stage and it provides an automatically alarm system together with round- the-clock monitoring of the above parameters, in order to take proper measures corresponding to the originating event In addition, several interlock systems are provided to block abnormal situation development due to any malfunction and/or miss operation as well as to prevent unnecessary actuation of the emergency shutdown system.

To maintain sound performance of the reactor core and the reactor coolant pressure boundary, the reactor shutdown system is provided to detect any approach to abnormal conditions and if necessary,

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NSRA, Japan