All components of the reactor coolant system of a PWR plant are
designed, manufactured and inspected based on the strict quality
control procedures to satisfy the applicable regulations and codes.
Hence, accident occurrence probabilities due to component failures
are very low. However, to assure high safety levels of PWR plants,
they are provided with necessary systems to mitigate the
consequences in case an unlikely accident occurs. If a loss of
coolant accident (LOCA)
occurs, the capability of the reactor coolant to cool the fuel in
the reactor core will be lost, which will trigger the fuel failure
and probable releases of radioactive materials to the environment.
The following systems are installed in PWR plants to prevent such
accidents or to mitigate their consequences.
If a LOCA occurs, and the reactor coolant is lost from the reactor
coolant system, the fuel rods in the core might not be adequately
cooled and their temperature will rise causing fuel cladding
failures and subsequent releases of large amounts of radioactive
materials into the containment vessel. In such a case, the reactor
core is designed to be cooled by an emergency injection of borated
water into the reactor core which prevents significant failures of
the fuel cladding. Also, the emergency injection of water will
suppress oxygen formation by metal-water chemical interactions
between the fuel cladding and the coolant
If a LOCA occurs,
radioactive materials will
be discharged from the reactor coolant system together with the high
temperature and high pressure coolant. The reactor containment
facility withstands the huge amount of energy released in such
accidents, preventing the radioactive materials from being released
to the environment
The containment internal pressure will rise following the release of
energy from the reactor system during the LOCA. The containment
spray
system minimizes radioactive material leakages from the containment
building to the extent possible, by cooling the containment
atmosphere and promptly reducing its temperature and pressure. The
spray water also removes radioactive iodine released to the
containment atmosphere.
The radioactive materials released to the containment during the
LOCA might leak into the
annulus through the containment access air locks, the equipment
hatch and the piping penetrations. An annulus air clean-up system
minimizes the radioactive material leakages to the environment
In the long-term core cooling phase following the LOCA, the coolant
accumulated in the containment sump at the bottom of the containment
vessel will be circulated by the ECCS pumps in the safety component
room outside the containment facility. A safety component room air
clean-up system deals with small leakages of the reactor coolant
from the equipment outside the containment facility.
The ECCS, the reactor containment facility, the containment spray
system, the annulus air cleanup system and the safety component
room air clean-up system are collectively called engineered
safeguard systems and they are essential to ensure the plant safety.
Hence, the engineered safeguard systems are designed as highly
reliable systems in accordance with the safety design review
guidelines. They are designed with redundant and independent
subsystems, so that, even assuming a single component failure
occurring simultaneously with the loss of off-site power,
the systems are able to perform their required functions and
withstand the adverse environmental conditions after an accident.
Also, the systems are designed to allow the performance of necessary
tests and inspections during the normal operation and the planned
inservice inspections of the plant. Furthermore, engineered
safeguard system actuation systems, as part of the plant protection
systems, detect any abnormal situation by processing related process
signals and actuate the engineered safeguard
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3-82Engineered Safety Feature
Systems and Their Functions
Emergency core cooling system (eccs)
Reactor containment facility
Containment spray system
Annulus air clean-up system
Safety component room air clean-up system
General points
Chapter
3 Systems of PWR Nuclear Power Plants
system components. The next subsections offer detailed descriptions
of the functions and the configurations of the engineered safeguard
systems.
The ECCS prevents
significant damage to the fuel cladding in the event of LOCAs, by
injecting borated water from the refueling water storage tank to
cool the reactor core and to give negative reactivity to the core in
order to suppress nuclear chain reactions there. Hie
ECCS also gives the negative reactivity required to shut down the
reactor in the event of a main steam line rupture accident
Furthermore, in the event of an RCC ejection accident, or a steam
generator tube failure accident, the ECCS delivers cooling water to
the reactor core and provides the required negative reactivity to
the core, making it subcritical.
Hie ECCS, as shown in
Figure 3.7.1, consists of a safety injection system (SIS) which
inject high pressure water into the reactor using the safety
injection pumps and accumulators, and a low pressure injection
system which also serves as a residual heat removal system. In some
of the three- loop and four-loop plants, the charging / high-head
injection pumps of the chemical and volume control system (CVCS)
serve as high head injection pumps, as shown in Figure 3.7.2.
In the recent APWR plant design, new type high performance
accumulators replace both the old type accumulators and the low
pressure injection system employed in the conventional plant system
designs. This new design simplifies the configurations of the ECCS
and improves its reliability. In the APWR
plant design, refueling water storage pits inside the
containment vessel are employed instead of refueling water storage
tanks.
The ECCS is designed to satisfy the single
failure criterion, that is, the ECCS consists of
redundant and independent subsystems (in such a way to have two
independent pumps, and duplicate valves and piping), with each
subsystem having sufficient capacities to perform the required
functions even assuming asingle failure of an active
component for a short term after a LOCA, and a single failure of
either an active component or a passive component for a long term
after a LOCA. The phrases "short term after a LOCA"
and 'long term after a LOCA"
refer to the water injection phase and the recirculation phase of
the ECCS operation, respectively. For other systems than the ECCS,
“short term after an accident" and "long term after an
accident" mean about one day and the plant recovery period of
more than one day after an accident, respectively.
Furthermore, the ECCS is designed to perform the required safety
functions assuming a loss of off-site power occurring simultaneously
with the LOCA. The electric power will be supplied by redundant
emergency diesel generators. The
design of the ECCS allows necessary periodical tests and inspections
during the plant power operation to verify the system integrity and
operability to be carried out.
The following subsections are detailed descriptions of the safety
injection system and the low pressure injection system which make up
the ECCS.
i) Safety injection system (SIS)
The SIS consists of safety injection pumps, a boron injection tank,
accumulators, piping, and valves and related instrumentation. The
safety injection pumps,
together with the accumulators and low pressure injection
pumps, have sufficient capacities to satisfy the requirements given
in the “Performance Evaluation Guidelines of the Emergency Core
Cooling System of Light Water Power Reactors" and keep the
maximum fuel cladding temperature under 1200°
C as verified by the safety analyses in the event of a LOCA
of any kind. Moreover, in the event of a main steam line rupture
accident, the SIS injects borated water to rapidly make the reactor
core reactivity negative which suppresses the accident. The
capacities of the accumulators are determined assuming that the flow
from one accumulator becomes unavailable in the event of a medium or
large break of the reactor coolant loop piping; the accumulator in
the affected loop is not assumed to effectively feed its water to
the reactor. The remaining accumulators, together with the other
engineered safeguard system components, will
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Emergency Core Cooling System (eccs)
Functions
System configuration
Chapter
3 Systems of PWR Nuclear Power Plants
satisfy the criteria shown in the above mentioned guidelines.
The SIS has two full-capacity safety injection pumps. These pumps
automatically start upon receiving an ECCS actuation signal. In some
of the three-loop and four-loop plants, where two of the three
charging pumps of the CVCS serve as high head injection pumps after
a LOCA, the suctions of the pumps
are automatically transferred by the signal, from the volume control
tank to the refueling water storage tank, and the pump discharges
are transferred from the CVCS charging line to the ECCS cold leg
injection line. The boron injection tank is on this injection line.
These switching-over of the suctions and the discharges of the high
head injection pumps are accomplished by closing two motor-operated
valves installed in series in each normal CVCS line, and by opening
two parallel motor-operated valves installed in each of the ECCS
water supply lines and the water injection lines.
These duplicate motor-operated valves, installed either in series or
in parallel, are expected to ensure that the valves will certainly
perform the required function, even if a single failure either of
the valves or their power sources is assumed to occur.
Simultaneously with the start-up of the charging / high head
injection pumps, two parallel motor-operated valves at the outlet of
the boron injection tank are opened, and the boric acid solution
(about 12 wt% concentration
(21,000 ppm)), is mixed with the borated water from the refueling
water storage tank and injected into the cold legs of the reactor
coolant loops. The injection of the borated water to the cold legs
and not to the hot legs of the reactor coolant piping is done to
minimize the loss of the injection water through the break involved
in the reactor coolant blow-out flow, effectively delivering it into
the core. When the water level in the refueling water storage tank
reaches the minimum set point, the safety injection mode of the
system operation is changed to the cold leg recirculation mode by
transferring the pump suction from the tank to the containment
recirculation sump. About 24 h after an accident when critical
situations have mostly become settled, the core cooling operation
mode is shifted from the cold leg recirculation
to the hot leg recirculation, where the water is injected to the
reactor coolant piping hot legs to deliver safety injection water
effectively to the core, and to quickly suppress the coolant boiling
in the reactor core. Redundant and independent emergency electric
buses supply power to the pumps. A minimum flow bypass line around
each safety injection or charging / high head injection pump
prevents each of these pumps from being operated without any
discharge flow. The bypass lines are also used for the periodical
tests of these pumps. The safety injection pumps and the charging /
high head injection pumps are horizontal centrifugal pumps.
A boron injection tank,
connected to a cold leg injection line with branch lines
from the delivery header of the charging / high head injection
pumps, holds borated water with sufficient boron concentration to be
injected to the reactor coolant system dining a main steam line
rupture accident. Two redundant sets of electric heaters are
installed on the boron injection tank to maintain the solution
temperature well above the boric acid precipitation limit of the
solution. A sparger is installed in the boron injection tank to
sweep the tank water down into the reactor coolant system by the
injection water flow. The
boric acid in the tank is
continuously circulated by boron injection circulation pumps through
the line with the boron injection circulation tank, assuring that
the boric acid concentration of the tank water is kept at the
required level. The boron injection tank design is not applied to
the latest PWR plants, since the current criteria of the safety
evaluation guidelines are satisfied without the credit of a boron
injection tank.
An accumulator is
connected to each reactor cooling loop. These accumulators inject
borated water into the reactor core in the event of a LOCA to
prevent the fuels and the fuel cladding from being significantly
damaged. The accumulators automatically inject the tank water into
the reactor coolant system, when the system pressure drops below the
operating pressure of the accumulators. The
capacities of the accumulators are determined by assuming that while
the water in one of the accumulators is lost through the break in
the affected loop, the other accumulators
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