) ± 10% step load change
) ± 5% /minute ramp load change
iii) 50%
(or 95%) sudden load decrease
The basic concept of the
reactor control system is illustrated in Figured.
1.1. It is composed of several subsystems. The subsystems of the
reactor control system are discussed item by item as follows.
The control rod control system controls the output of the reactor by
automatically adjusting the positions of the control rod clusters in
the reactor core using two primary signals, coolant average
temperature deviation (TAVG-TREF)
and output deviation (QrQN).
The temperature deviation signal represents the difference between
the reactor coolant average temperature CTAVG)
and the programmed reference temperature CCri
i-)
, and is used as the main signal for adjusting the reactor
coolant average temperature to the reference temperature. Hie
output deviation signal represents the difference of the turbine
output (QT) and
the reactor output (QN),
and quickens the responses of the control rod control system during
large load changes.
In cases of step load decreases in excess of 10% or ramp load
decreases in excess of 5%/minute, responses of the reactor are
mitigated by actuating the turbine bypass valves to dump parts of
the main steam directly to the condenser, not through the turbine.
The pressurizer is connected to the hot leg of a primary coolant
loop by a surge line. The temperature of the equilibrated vapor
phase and liquid phase water in the pressurizer is maintained at the
saturation temperature of the reactor coolant system operating
pressure. When the reactor coolant temperature rises or falls due to
plant load changes, the volume of the reactor coolant changes as a
result of thermal expansion or contraction, and the volume changes
cause in-surges to or out- surges from the pressurizer, resulting in
pressure transients in it In cases of pressure decrease, the
pressurizer pressure control system restores the
pressure by increasing the output of the electric heaters. On the
other hand, pressure rises are suppressed by spraying low
temperature water from the cold leg of a reactor coolant loop into
the vapor phase of the pressurizer to condense the steam. In cases
of large surges, pressure rises are limited by releasing the
pressurizer steam to the pressurizer relief tank via the pressurizer
relief valves and/or safety valves.
The pressurizer water level is controlled by regulating the reactor
coolant system inventory. The system inventory is regulated by
adjusting the letdown flow rate to and charging flow rate from the
chemical and volume control system. The pressurizer water level is
programmed to change linearly according to the reactor coolant
temperature which is
programmed in turn to vary linearly according to the reactor power
level, for the purpose of minimizing the burden on the level control
system.
The steam generator (SG) water level control system controls the SG
water level based on three elements, SG water level, steam flow rate
and feed water flow rate.
The feedwater flow rate is regulated using the main feedwater
control valve which maintains the SG water level that is programmed
to change depending on the turbine output During the plant low power
operation (15% or less), SG water levels are controlled by
regulating the main feedwater bypass control valves based on SG
water levels and the coolant temperature difference used as a feed
forward signal.
The above-mentioned programs of the SG water level control system
are designed so that they do not need to be changed throughout the
reactor core life.
There are two different types of startup for nuclear reactors:
startup from cold shutdown
conditions and from hot
shutdown conditions. The cold shutdown conditions are
classified into three sets of conditions: low temperature shutdown
conditions with reactor coolant temperature below
NSRA,
Japan
5~2
Control rod control system
Turbine bypass control system
Pressurizer pressure control system
Pressurizer water level control system
Steam generator water level control system
Plant Startup
I
I I
1
I
1
1
I
I
I
I
1
I
I
I
I
i
1
I
I
I
I
I f
I
1
Other
Loop(s)
Pressurizer
Hater
Level
Control
System
Turbine
Heater
___
reed
Primary
Coolant
Reactor
Control
Rod Control
System
Turbine
.
Bypass
Control.
System
Turbine
Output Signal Qt
(Turbine
1st
Stage Pressure)
,
Main
Water
Pump
Steam
Generator
Water
Levell
Control
System
Average
Temperature TAVG
I
Programed Temperature J j
I IREF
Pressurizer
Pressure
control
System
Spray
Condenser
J
Figure
5.1.1 PWR reactor control system diagram
Generator
Chapter
5 Operation and Maintenance of PWR Plants
93.3°C and reactor
sub-criticality of more than 1% A k/k; refueling shutdown conditions
with reactor coolant temperature below 60 °C
and reactor subcriticality of more than 5% A k/k core; and
refueling operation conditions with the reactor vessel head removed.
The low temperature shutdown conditions are maintained on rare
occasions of unplanned reactor shutdown for long-term repair work.
The refueling operation conditions are for plant refueling
operations. The refueling shutdown conditions are the normal
conditions of a reactor plant before startup operations after
periodical inspections of the plant Thus, startups from refueling
shutdown conditions represent reactor startups from cold shutdown
conditions.
The startups from hot shutdown conditions are reactor startups from
conditions of sub-critical core (1.6 to 1.8 A k/k or more) and no
load coolant temperature of about 286°C
to 292°C . The
coolant temperature is maintained mainly by heat inputs from the
reactor coolant pumps. The hot shutdown mode is an important mode
for the PWR plant operation management, because reactors are safely
shut down with this mode. In most cases, reactors are operated with
this mode and under the above conditions, following emergency
shutdowns and/or unplanned outages.
Since startup operations from the cold shutdown conditions cover
startup operations from the hot shutdown conditions, startup
operations from cold shutdown are discussed below.
Before the reactor startup, plant systems are carefully checked
according to the plant procedures. The system checks include system
alignment checks (systems and components which were isolated for
inspections or repairs are restored and their water filling and
venting operations are completed), the confirmation of states
(positions of valves, power supplies, configurations, standby or
operating conditions, and so on) of components and equipment, and
the verification of various parameters (system main parameters, tank
water levels, radiation levels, and so on). These system checks are
conducted in the main control room and at local sites. Furthermore,
check sheets or other means are used to check that the requirements
for plant startup (boron concentration, water chemistry etc.) are
satisfied, that safety systems
and system components are properly functioning or in standby states,
and that the checks of the plant process instrumentation, power
supply systems, reactor control and protection systems, and control
rod cluster drive mechanisms and their position indication systems
are completed and all equipment is operating normally.
After the completion of the system checks prior to the plant
startup, plant startup procedures are followed. As an operating
practice of the present PWR plants, the reactor is not made critical
until the plant hot shutdown conditions are established. Hence, the
first step of the startup procedures is to establish the conditions.
The reactor coolant system pressure is increased by adjusting
letdown and charging flow rates of the chemical and volume control
system. Power to the pressurizer heaters is turned on. When the
system pressure reaches around 28 kg/cni
, the system pressure is maintained at that level, and reactor
coolant pumps are started. The reactor coolant temperature is
gradually raised by heat inputs from the pumps and the pressurizer
heaters. Along with these processes, the water chemistry conditions
of the reactor coolant, such as oxygen concentration, are adjusted
to allowable levels. After a vapor space is formed in the
pressurizer, the residual heat removal system is isolated from the
reactor coolant system and the temperature and the pressure of the
reactor coolant system are raised according to a specified heating
rate and kept within a range bounded by pressure-temperature
limitation curves for plant heat-up. Meanwhile, water chemistry
conditions in the secondary side of the SGs are adjusted to required
levels by continuously blowing down the secondary side water. After
the plant hot shutdown conditions are established, the procedures to
make the reactor core critical are initiated. At this stage, warming
of the main steam piping is initiated. Feedwater supply to the SGs
is switched over from the auxiliary feedwater pumps to the main
feedwater pumps, and SG water levels are controlled by the main
feedwater bypass control valves. After the reactor became critical,
its output is raised, and steam supply to the secondary system
equipment is initiated. The reactor power is increased by control
rod cluster withdrawal or by boron dilution. Then the turbine speed
is increased
NSRA,
Japan
5
4