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.
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.
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