The inherent safety features such as the basic fuel design are those used in reactor plant design to limit accident possibilities. In addition, a number of design safety features limit accident probabilities. A number of

these applied safety features are common to most of today’s power reac­tors. By way of example, I shall describe a few.

Monitoring of Reactor Neutron Flux. A prime measurement used to achieve safe reactor operation is the monitoring of reactor neutron flux within the reactor core itself. This is done by a number of independent monitoring systems which measure at various locations in the reactor core exactly what the power level is at any time. These instruments are directly connected to a rapid reactor shutdown system, which operates whenever a predetermined safe upper limit has been detected by the instruments. Therefore, a definite protection system is provided automatically, and secondly, reactor operators have an excellent set of indications of the power level throughout the reactor core.

Reactor Control Systems. The power level of the reactor is controlled and adjusted by means of materials such as boron which are capable of absorbing neutrons. To achieve reactor shutdown such materials are in­troduced into the reactor core. Common methods of introduction include the use of mechanical control rods or of liquid solutions which can enter the reactor water moderator. To optimize the assurance of achieving safe shutdown when required, most of today’s nuclear power reactors have both methods of reactor control available — another example of applied safety systems.

Reactor Safety Circuit Instrumentation. Instruments are provided to monitor all of those plant characteristics where proper performance is im­portant for overall safety, and such systems are connected to the auto­matic rapid reactor shutdown devices. To ensure highly reliable signals in the event of difficulties at any important point, the instrumentation sys­tems include many independent signals so that failure of individual com­ponents or even complete failure of electric power to the whole safety sys­tem will not interfere with rapid reactor shutdown.

Electric Power Requirements. The reactor designer presumes that at some time all of the normal electric power available to the plant will be suddenly cut off. Thus, wherever possible the reactor systems are designed so that they require no electric power to achieve safe reactor shutdown. In those cases where some amount of power is required for safe shutdown, this is achieved by providing emergency backup power sources which normally include diesel-driven generators at the plant and station storage battery systems. These are themselves redundant. More than enough are provided, so if one should fail another would be ready and waiting to per­form the function.

Reactor Process System Integrity. Although improbable, the manner in which coolant could be lost is through a small system leak, which would become progressively worse. Proper material selection of ductile steels eliminates this possibility. In spite of that fact, the safety objectives call for monitoring systems which can detect even minor leakages in the reactor process system; hence, safe shutdown and repairs may be completed be­fore any situation important to safety could develop.

The above five examples indicate the types of applied safety features which have been added to boiling water reactors. Although some aspects of these applied safety features are used for normal operation — that is, the generation of electricity — their primary function is to provide addi­tional safety to the large power reactors. There are many other examples which could have been given, such as the high quality of the design and construction of the primary coolant boundary itself and the inspection procedures used. However, the above five examples are adequate to dem­onstrate that numerous applied safety features are incorporated in the de­sign of today’s nuclear power reactors.