2.7 Engineered Safety Features

1. Design Policy of Engineered Safety Features

1. Basic safely philosophy

Fundamentals to ensure safety of nuclear reactor facilities are to take all possible measures at each stage of design, manufacturing, construction and operation of the plants throughout their lifetimes so that the plants can be operated, and inspected for maintenance, without damaging the public health and safety and those of the plant personnel.

To this end, nuclear power plants apply the following three steps of measures to prevent any harm to the public by the release of radioactive materials to the environment during normal operations as well as in an accident

1. Making their best efforts to prevent an occurrence of anomalies.

2. Making the earliest detection possible for abnormal conditions with safe shutdown of the reactor before it develops into an accident

3. Managing any accident, if it occurs, by means of various ESFs which include the ECCS and PCV.

Thus, any accident would be prevented from developing further by automatic detection and safety system functions.

In addition, the BWR has unique plant safety considerations as illustrated in Figure 2.7.1. The BWR is designed to satisfactorily cope with the anticipated abnormal occurrences and accidents by these unique safety characteristics.

The first characteristic is an effective self­regulating capability. The existence of a void (steam bubbles) in the reactor core during normal operation due to coolant boiling mitigates an

Figure 2.7.1 Unique safety features of a BWR unintentional power change and stabilizes the power. This is possible, because a power increase, due to several reasons, increases the degree of boiling, i.e. the void fraction, which decreases the thermal neutron flux that maintains fission chain reactions in the reactor.

The second characteristic is powerful core coolability by natural circulation. The BWR has a large potential for natural circulation flow of the reactor coolant This natural circulation can remove up to about 50% of the rated heat generation of the core even when the recirculation pumps are halted.

The third characteristic is stable operation by the automatic reactor water level control. The reactor water level is constantly monitored by level detectors. The reactor water level is automatically controlled, by adjusting feed water flow when the water level changes.

The fourth characteristic is a large capacity of suppression pool as a heat sink. The BWR has a large amount of water in a pool in the suppression

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NSRA, Japan