Chapter
3 Systems of PWR Nuclear Power Plants
The DC power supply systems consist of two safety systems and one or
more non-safety- related systems. The number of non-safety-related
systems is determined based on their loads, which are dependent on
the plant design. All systems are independent and physically
separated from each other. Each system has its its own battery, two
battery chargers, two voltage compensators and a distribution panel.
The two safety DC power systems supply power to redundant plant
equipment including relays, breakers, solenoid valves, and inverters
of safety systems, including the engineered safeguard systems,
essential for the plant safety. The non-safety DC power system(s)
supplies power to the turbine-generator auxiliary system equipment
and to non-safety equipment including relays, breakers, solenoid
valves, and inverters of the systems which are not essential for the
plant safety.
The one-line diagram of one of the two safety DC power systems is
shown in Figure 3.10.6. Hie
non- safety DC power system(s)
has the same structure with the safety DC power systems.
Instrument power systems include four safety systems and a few
non-safety systems. The one-line diagram of two of the four safety
instrument power systems is given in Figure 3.10.7. The number of
non-safety-related systems is determined based on their loads which
are dependent on the plant design.
Each safety system is supplied power by an uninterruptible
power supply unit (inverter). Each non-safety instrument
power system is conventionally fed from an AC
bus either through constant-voltage regulators or a transformer.
However, in the latest plants, in which reactor and turbine control
systems are computerized, each non-safety instrument power system is
supplied power by an uninterruptible power supply and by a backup AC
bus through constant-voltage regulators or a transformer. Power is
independently supplied to each instrument power system from the main
power source (inverter) and the backup power source
(constant-voltage regulator).
Reactor
control center
Safety
protection systems
Important
safely systems
including
engineered safety feature
(Notel
With
mechanical interlock
Reactor
control center
Al Cl
Figure
3.10.7
I
& C power supply system (two of safety systems)
3
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Instrument power systems
Figure 3.10.6 Direct current power supply system (one of safety system)
Board feu | 4 c Icard for j I c
(Note)Wllh mechanical Interlock
Plant Auxiliary
Systems
A water supply and treatment system,
an auxiliary steam system, compressed air systems, heating,
ventilating and air conditioning systems, and a fire protection
system are collectively called plant auxiliary systems. These
systems support the plant operation, as well as provide the plant
with services, such as ensuring the plant habitability, supplying
drinking water, and so on.
Water Supply and
Treatment System
Either filtrated river water or sea water processed in a
desalination unit are used as raw water supply sources for NPPs.
The capacity of the
water supply and treatment system is determined based on a water
supply plan with a scale commensurate to the plant size. In
multi-unit plants, approximately
ton/d ■ MWe
(approximately 1,400 ton/d • unit for 4-loop units) is an
adequate capacity for its water supply and treatment system.
Raw water is stored in raw water storage tanks and sent to
primary and secondary system make-up water storage tanks after
treatment by a water treatment unit. Water treatment units
usually use mixed-bed demineralizers to produce high quality
pure water.
Typical quality standards for make-up water at the outlet of a
water treatment unit are given in Table 3.11.1.
The schematic flow diagram of the water supply and treatment
system for a typical NPP is shown in Figure 3.11.1.
Auxiliary Steam System
As shown in Figure 3.11.2, a typical auxiliary steam system
consists of a steam converter, auxiliary boilers, an auxiliary
steam drain tank, and auxiliary
steam drain pumps. The auxiliary steam
Table
3.11.1 Demineralized water quality specification (Typical at
outlet of demineralizer) |
Threshold (concentrations) |
pH(25°C) |
6-8 |
Conductivity (25 °C) |
1.0 pfl/cm |
Resolved Oxygen (DO2) |
< 0.1 ppm |
Silica (S1O2) |
< 0.02 ppm |
Chlorine (Cl) |
0.1 ppm |
Total Iron(Fe) |
< 0.01 ppm |
system supplies heating steam for components such as a boric acid
evaporator, a liquid waste evaporator, heating, ventilating and air
conditioning system heaters, and freeze-proof tank heaters. It also
supplies steam to the turbine glands for sealing, and to the
deaerator for heating, when steam generators cannot supply steam to
fulfill these purposes during the plant start-up or shutdown
operations.
The steam converter is heated with turbine- extracted steam during
the plant normal operation, and it is heated with main steam from
the steam generators during the plant low power operation. Auxiliary
boilers are used to supply steam when the main steam and the steam
converter steam are not available. The capacities of the steam
converter and the auxiliary boilers are determined based on
requirements to supply the necessary amount of steam for the above
mentioned components during the plant normal, start-up and shutdown
operations.
Based on the requirements, auxiliary boilers and steam converters in
a multi-unit plant usually have capacities of 0.02 ton/h*
MWe and 0.03 ton/h • MWe,
respectively; for a 4-loop unit this means approximately 20
ton/h-unit and 30 ton/h ■
unit, respectively. The auxiliary steam condenses in various
components and flows down to and is stored in an auxiliary steam
condensate tank. The condensate tank water is returned to the steam
converter or the auxiliary boiler by auxiliary steam condensate
pumps. Since such components as the boric acid evaporator and the
liquid waste evaporator, carrying radioactive or potentially
radioactive fluids are included in the load group heated by
auxiliary steam, a radiation monitor is installed on the drain
header from these components to detect any leakage of radioactive
materials into the auxiliary steam system.
The components heated by the auxiliary steam are classified into two
categories:
•Those components including the boric acid evaporator and the
liquid waste evaporator, which carry radioactive or potentially
radioactive fluids and therefore potentially can contaminate the
auxiliary steam system; and
•Those components including the components of the heating,
ventilating and air conditioning systems, which carry
non-radioactive fluids and therefore have no possibility of
contaminating
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