Because of the need for protective action to be available at all times when the reactor is operating and the need for continued cooling and monitoring when the reactor is shut down, systems must be provided to assure high availability of electric power.

Primary Coolant Circulators
The largest single plant load is the drives for primary coolant circulation. Since it is important to maintain coolant circulation and since these drives are generally too large to be supplied by engine driven sources, provisions should be made to supply the coolant circulator drives from two or more sources.

Frequently, arrangements are made for the main generator to supply two or more power lines. Provisions in the switchyard enable the plant distribution system to be supplied from the plant generator or from one or more of the outside lines.

In spite of possible connection of plant loads to multiple external power sources, it is possible to lose all external lines, for instance, by a tornado. In this event, a local source of power to supply critical ac loads is required.

For these purposes, engine (diesel)-driven generators are usually used. Credit can sometimes be taken for local hydro generators or gas-turbine generators if these sources can meet the requirements. These power systems must be designed so that they provide power to the station following a design-basis event.

An ac power system (generation and distribution), a dc power system, and a vital instrumentation and control power system are provided. An example of a safety grade power system is shown in Fig. 5-21.

In the ac system, each of the redundant load groups must have access to both a preferred and a standby power supply. The units of the standby supply must have sufficient independence from the preferred supply and from one another to preclude a common failure mode.

Load assignment must be such that the safety actions of each group are redundant and independent. Protective devices must be provided to limit the degradation of the system and maintain the power quality (voltage and frequency) within acceptable limits.

Following a demand for the standby power supply, it must be available within a time consistent with the requirements of the engineered safeguards features and the shutdown systems. In the dc system, batteries, distribution equipment, and load groups are arranged to supply critical dc loads and switching and control power.

Redundant load groups, and corresponding battery sections, must be sufficiently independent to preclude common failure modes. Each of the redundant load groups must have access to one or more battery chargers; the batteries are to be kept charged.

The battery supplies must be sized to be able to start and operate their assigned loads in the expected loading sequence for a length of time commensurate with the protection provided.

Battery chargers supplying the redundant load groups must have sufficient capacity to restore the battery from its design minimum charge to its fully charged state while supplying normal and post accident loads.

Each charger supply must have a disconnecting device in its ac feeder and one in its dc output line. The dc system must be equipped with surveillance equipment to monitor its status and to indicate actions.

The vital instrument system is provided to power the instrumentation needed for reactor protection and engineered safety features. Since there may be considerable variation in the instrumentation in various plants, the vital system may be required to supply ac or dc or both.

To preserve freedom from common-mode failure, the vital supply must be divided into redundant and independent systems with adequate status indication. Provisions for testing, adjustment, and repair should be included in the parts of the emergency power systems to improve reliability and availability.

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