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Nuclear Energy Fundamentals
Module 4: Nuclear Reactor Design
PREPARED BY
Academic Services
April 2012
© Institute of Applied Technology, 2012
ATM 1236 – Nuclear Energy Fundamentals
2 Module 4: Nuclear Reactor Design
Module 4: Nuclear Reactor Design Module Objectives After the completion of this module, the student will be able to:
Explain the fuel assembly geometry, specifications and material.
Describe the fuel assembly forms.
Explain the refueling types, frequency.
Explain the active dimensions of the nuclear reactor.
Explain the function of the moderator and the material used for
moderating nuclear reactors.
Explain the function of the reflector and the material used as
reflectors in nuclear reactors.
Describe the control system of some common types of nuclear
reactors.
Explain the principle of operation of the cooling system of some
common types of nuclear reactors.
Explain the function of the RCP.
Identify the main components of the RCP.
Identify the different types of radioactive barriers used in nuclear
reactors.
Describe the construction of the pressure vessel of BWR.
Describe the construction of the pressure vessel and the
containment building for some reactor types.
Describe the construction and the types of the steam generators.
Explain the function of the steam generator.
Describe the construction the pressurizer.
Explain the function of the pressurizer.
Explain the principle of operation of the different safety systems and
sub-systems used in NPPs including (RPS, ESWS. ECCS, Emergency
electrical systems and ventilation and radiation protection).
Explain the way to initiate the nuclear reaction in the reactor.
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactor Design 3
Module 5: Nuclear Reactor Design Module Contents Topic Page No.
1. Introduction 4
2. Fuel Assembly 4
3. Moderator 8
4. Reflector 9
5. The Chain Reaction Control 9
6. The Cooling System 12
7. Protective Shield 16
8. Steam Generator 22
9. Pressurizer 24
10. Nuclear Reactor Safety Systems 25
11. Activities 31
12. References 32
ATM 1236 – Nuclear Energy Fundamentals
4 Module 4: Nuclear Reactor Design
1. Introduction
In this module we will focus on the primary systems and the basic
functional requirements of nuclear reactors. These include the reactor core,
reactor vessel, reactivity control, reactor coolant system, steam generators
(SG), pressurizer, and the reactor safety and protection systems.
2. Fuel Assembly
The reactor core is the main part containing the nuclear fuel. The solid fuel
material is fabricated into various small shapes called, pellets which are
usually put together and called as assemblies or bundles.
A reactor core may contain from tens to hundreds of these fuels
assemblies, held in a fixed geometrical pattern.
2.1. Fuel assembly geometry
The individual fuel rods are arranged in assemblies where there are three
basic types of fuel geometry. These are square which is used in most
reactors, hexagonal is used in VVER (a PWR developed in 1970 in the
Soviet Union) and cylindrical that is used in CANDU.
(a) (b)
(c) (d)
Fig. 5.1: Fuel assembly geometry: a) square b) cylindrical c) hexagonal d) spherical.
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactor Design 5
Some high temperature reactors such as pebble bed reactor which is one of
the promising nuclear reactor technologies known today use spherical fuel
geometry (Fig. 5.1).
In PWR for example, the fuel assembly consists of a square array of 179 to
264 fuel rods which is a long, slender tube in which nuclear fuel is
surrounded by cladding material and inserted into a reactor. There are
three types of cladding material, namely, zirconium alloy (Zircaloy),
stainless steel and magnesium alloy (Magnox). The fuel rods are assembled
into bundles called fuel elements or fuel assemblies, which are loaded
individually into the reactor core and 121 to 193 fuel assemblies are loaded
into an individual reactor.
Fig. 5.2: The PWR fuel assembly specifications.
ATM 1236 – Nuclear Energy Fundamentals
6 Module 4: Nuclear Reactor Design
Fig. 5.2 shows some size specifications of the PWR fuel assembly. The
cladding tube contains around 350 to 400 pellets with both ends plugged.
Those pellets are fixed with a spring.
The table below shows the basic specifications of PWR fuel assembly. As
indicated in Fig. 5.2 and table 5.1, the cross section size is about 21 cm and
the fuel assembly length is around 4 m.
Table 5.1: PWR fuel assembly specifications.
Type 14×14 15×15 17×17
10ft 12ft 12ft 12ft
Cross-section size (mm2) 197 214 214
Fuel assembly length (mm) 3,473 4,057 4,057 4,058
Fig. 5.3: The PWR fuel assembly specifications.
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactor Design 7
A modern BWR fuel assembly comprises 74 to 100 fuel rods, and there are
up to approximately 800 assemblies in a reactor core (Fig. 5.3). The
number of fuel assemblies in a specific reactor is based on considerations of
desired reactor power output, reactor core size and reactor power density.
2.2. Fuel material
The fuel material is the material from which the fuel elements are made.
Typical fuel materials are uranium metal (U metal) and uranium dioxide
(UO2); however, the material could be also a mixture of uranium dioxide
and plutonium dioxide (PuO2) and thorium dioxide (ThO2).
2.3. Fuel form
Nuclear fuel can be in many forms. It can be in the form of a chemical
compound, ceramic, or metal alloy. It can be pellets, rods, or even
dissolved in liquid salt or liquid lead.
2.4. Refueling type
Refueling is the method by which the
used fuel assemblies in the core are
replaced by fresh ones. Basically, the
fuel may be replaced either individually
or in small groups during reactor
operation on-load (at rated reduced
power), or with significant portion while
the reactor is off-load (during refueling
outages). On-load refueling is typical for
the CANDU or GCR (Magnox) reactors.
AGR may be refueled either at reduced
load (still on load) or off load. Other
reactors such as PWR, BWR and FBR are
refueled off-load (Fig. 5.4).
Fig. 5.4: A nuclear fuel assembly
being loaded into the
reactor core.
ATM 1236 – Nuclear Energy Fundamentals
8 Module 4: Nuclear Reactor Design
2.5. Refueling frequency
Refueling frequency or sometimes called the fuel cycle is defined as the
time period in which a significant part of the core is refueled. This
characteristic applies only to reactors with off-load refueling. The fuel cycle
length is the average time period in months from the end of one refueling
to the end of the next one. Most common refueling frequencies are 12, 18
or 24 months when a quarter to a third of the fuel assemblies is replaced
with fresh ones.
2.6. Active core diameter and height/length
The active core diameter is the diameter of the circle encompassing the
active fuel assemblies in the core. Excluding the reflector or reactor vessel
shielding. Usually the reactor vessel varies from 2 to 10 meters but it can
be more than 10 meters in some reactors such as GCR (Magnox).
The active core height is the active part
of the core excluding structural
components and support. The name
height or length is based on the actual
fuel orientation (vertical or horizontal)
in the reactor core. Usually, the core
height varies from 2 to 5 meters, but it
may be also between 5 and 7 meters.
Some GCR (AGR) reactors have core up
to 8 meters height.
Fuel
Fig. 5.5: cross-sectional view of a
nuclear reactor.
3. Moderator
A moderator or neutron moderator is a material that reduces the speed of
fast neutrons, thereby turning them into thermal neutrons capable of
sustaining a nuclear chain reaction involving uranium-235. The moderator
is used in thermal reactor and the materials used as moderators include
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactor Design 9
ordinary water, heavy water, graphite, beryllium and certain organic
compounds.
The moderator should be well distributed within the fuel zone or core. In
some reactors the fuel materials and moderator materials are mixed
together.
4. Reflector
The reflector reduces the leakage of neutrons by reflecting back the
neutrons escaping from the core. The same material used for moderator
can be used for the reflectors in the case of thermal reactors. The light
water, heavy water and carbon are mostly used as reflectors.
The use of a proper reflector helps to reduce the size of the reactor core for
a given power output since the number of neutrons leaking are lesser and
help to propagate the fission process instead. It also reduces the
consumption of the fissile material.
In the fast reactors where fast neutrons are utilized for fission, nickel,
molybdenum and stainless steel reflectors are used.
5. The Chain Reaction Control
Control rods are used to control the nuclear reactor reactivity and power.
These are made from neutron-absorbing material such as silver, indium
hafnium and cadmium. Other elements that can be used include boron,
cobalt, dysprosium, gadolinium, samarium, erbium, and europium, or their
alloys and compounds. Control rods are inserted or withdrawn from the
core to control the rate of reaction, or to stop it (Fig. 5.6).
In some PWR reactors, special control rods are used to enable the core to
sustain a low level of power efficiently. Secondary shutdown systems
involve adding other neutron absorbers, usually boric acid as a fluid, to the
system.
ATM 1236 – Nuclear Energy Fundamentals
10 Module 4: Nuclear Reactor Design
Fig. 5.6: The mechanism of using control rods to control the reaction.
Besides the control rods, this system includes a number of devices, sensing
elements that measure the number of neutrons in the reactor, and other
devices to regulate the position of the control rods. The control rods when
lowered into the reactor absorb the neutrons to reduce the neutron
population and when raised allow the rise in number of neutrons. In some
reactors the reaction is controlled by varying the level of moderator. In the
heavy water moderated reactors like CANDU, a combination of moderator
level control and neutron absorber rods are used.
Control rods are usually combined into fuel rod assemblies (Fig. 5.7). In
commercial PWR for example, 20 control rods are used per assembly. These
are inserted into guide tubes within the fuel element. A control rod is
removed from or inserted into the central core of a nuclear reactor in order
to control the neutron flux. This in turn affects the thermal power of the
reactor, the amount of steam produced, and hence the electricity
generated. Control rods often oriented vertically within the core. In PWRs,
the control rod drive mechanisms are mounted on the reactor pressure
vessel head and the rods are inserted from above (Fig. 5.8).
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactor Design 11
Control rod guide thimble
Instrumentation guide thimble
Grid
Fuel rod
Bottom nozzle
Top nozzle
Control rods
Fig. 5.7: The control rod combined into PWR fuel rod assemblies.
While in BWRs the control rods are inserted from underneath the core due
to the necessity of having a steam dryer above the core (Fig. 5.9).
ATM 1236 – Nuclear Energy Fundamentals
12 Module 4: Nuclear Reactor Design
Fig. 5.8: In PWR the control rods are inserted from above.
Fig. 5.9: In BWR the control rods are inserted from the bottom.
6. The Cooling System
The rods must be surrounded by coolant (liquid or gas); otherwise
temperatures can rise to levels hot enough to melt metallic components
over a prolonged period. This opens the possibility of a serious meltdown, in
which molten, highly radioactive material from the reactor core falls
through the floor of the containment vessel and into the ground below.
This system removes the heat released from the reactor core. It consists of
pipes through which the coolant is pumped. When passing through the
reactor cores, the coolant picks up the heat, transfers the heat to another
working medium through a heat exchanger and then returns to the reactor.
Gases, heavy and light water, and liquid metals such as sodium, lithium,
potassium, can serve as coolants. In a reactor, we must be able to control
the amount of heat produced. The heat produced depends upon the
number of fissions taking place per second in the reactor, which in turn
depends upon the number of neutrons present in the reactor.
In a PWR for example (Fig. 5.10), water in the reactor core (primary loop)
reaches about 325 °C; hence it must be kept under pressure of about 16
MPa to prevent it from boiling.
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactor Design 13
The secondary loop is under less pressure (6 MPa) and the water here boils
in the steam generators. The temperature of the steam leaving the steam
generator is 280 °C. The steam drives the turbine to produce electricity,
and is then condensed and returned to steam generator in contact with the
primary loop (circuit).
The water entering the condenser from the cooling tower or sometimes
from a large body of water (tertiary loop) draws heat from the steam that
leaves the turbine and its temperature rises from 25 °C to reach 35 °C
when returning back to cooling tower or the water body.
Primary loop 330 ºC, 16 MPa
Reactor pressure vessel
Steam 280 °C, 6 MPa
Steam generator
Condenser
Reactor Coolant pump
Water pump
25 ºC
Generator35 ºC
Turbine
Secondary loop
Tertiary loop
Fig. 5.10: Cooling system in PWR.
Compared to the PWR the BWR (Fig. 5.11), has only a single circuit in
which the water is at lower pressure (about 8 MPa) so that it boils in the
ATM 1236 – Nuclear Energy Fundamentals
14 Module 4: Nuclear Reactor Design
core at about 285 °C. The reactor is designed to operate with 12-15% of
the water in the top part of the core as steam.
The steam passes through the steam separator to the a steam drier plates
above the core and then directly to the turbines, which are thus part of the
reactor circuit. Since the water around the core of a reactor is always
contaminated with traces of radionuclides, it means that the turbine must
be shielded and radiological protection provided during maintenance. The
cost of shielding the turbine tends to balance the savings due to the simpler
design. Most of the radioactivity in the water is very short-lived, so the
turbine hall can be entered soon after the reactor is shut down.
Feedwater in from the condenser
Steam out to the turbine
Reactor coolant pump
Reactor coolant pump
Reactor core
Feedwater inlet
Steam drier
Steam separator
Steam outlet
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactor Design 15
Fig. 5.11: Cooling system in BWR, 3D and 2D illustration.
6.1. Reactor coolant pump (RCP)
The nuclear reactor coolant pump is one of the main components of the
nuclear reactor cooling system. It provides the circulating force of reactor
coolant to help in transferring heat energy through the different cooling
system parts. It also provides the driving force of spray water inside of the
pressurizer. Fig. 5.12a shows the main parts of the reactor coolant pump
and Fig, 5.12b shows a real picture of the pump.
Flywheel
Reactor coolant section nozzle
CasingImpeller
Motor rotor Motor stator
Reacor coolant dischare nozzle
Motor
(a)
ATM 1236 – Nuclear Energy Fundamentals
16 Module 4: Nuclear Reactor Design
(b)
Fig. 5.12: a) RCP main components. b) Real picture of the RCP.
7. Protective Shield
The fission reaction is accompanied by emission of radiation like , and .
Exposure to these radiations is dangerous. In order to protect the persons
working near the reactor from these harmful radiations the reactor is
enclosed in steel and concrete which are capable of stopping these
radiations. This arrangement of protection is called Radiation shielding.
In nuclear reactors, the following components or systems are playing a role
as barriers to radioactive release:
The fuel ceramic.
The metal fuel cladding tubes.
The coolant system.
The reactor vessel.
The containment building.
Where the containment is the final barrier to the radioactive release. In the
following sections we will discuss the later two.
7.1. Reactor pressure vessel
Usually a strong steel vessel containing the reactor core, moderator and
coolant (Fig. 5.13), but it may be a series of tubes holding the fuel and
conveying the coolant through the moderator. In a typical PWR, the reactor
pressure vessel is about 13.5 m high and about 4.4 m inside diameter, and
has wall thickness exceeding 22 cm. The active length of the fuel
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactor Design 17
assemblies may be in the range of 4 m.
7.2. Containment building
The containment building is a steel or reinforced concrete structure
enclosing a nuclear reactor. It is designed, in any emergency, to contain the
escape of radiation. The containment is the final barrier to radioactive
release.
A typical containment building is a steel structure enclosing the reactor
ATM 1236 – Nuclear Energy Fundamentals
18 Module 4: Nuclear Reactor Design
Reactor pressure vessel
Pressure vessel head
Fig. 5.13: BWR pressure vessel and internal components.
vessel and normally sealed off from the outside atmosphere. The
containment is designed to contain or condense steam to a maximum
pressure in the range of 410 to 1400 kPa. This done as a short term
solution but for large break accidents other systems are used for long term
heat removal.
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactor Design 19
7.2.1. Types
Since the most common nuclear power plants are the PWRs and the BWRs
we will focus on the structure of these two types containment building.
Polar crane
Steel containment liner
Reinforced concrete containment
U‐tube steam generator
Main steam line
Main coolant pump
Upper internal
Reactor core
Control rod drive mechanisms
Fuel transfercanal
ATM 1236 – Nuclear Energy Fundamentals
20 Module 4: Nuclear Reactor Design
Fig. 5.14: PWR reactor containment building.
7.2.1.1. Pressurized water reactors containment
For a pressurized water reactor, the containment also encloses the steam
generators and the pressurizer, and is the entire reactor building (Fig.
5.14). PWR containments are typically 10 times larger than a BWR and in
most PWR designs. The spent fuel pool is located outside of the
containment building.
Modern designs have also shifted more towards using steel containment
structures. In some cases steel is used to line the inside of the concrete,
which contributes strength from both materials. In case of accidents the
containment becomes highly pressurized, yet other newer designs call for
both a steel and concrete containment.
The containment building can be classified based on the shape, size,
generation, etc. Fig. 5.16 shows three different containments designs based
on their shapes. These are the can design, the spherical design and the
combined design.
(a) (b) (c)
Fig. 5.15: Three different shapes of the PWR containment building; a) Can
design b) Spherical design d) Combined design.
7.2.1.2. Boiling water reactors
In BWR's (Fig. 5.16) the containment consists of a drywell where the
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactor Design 21
reactor and cooling equipment is located and a wetwell that is also known
as a torus or suppression pool. During accidents, the reactor coolant
converts to steam in the drywell and the pressure builds up quickly. Vent
pipes or tubes from the drywell direct the steam below the water level in
the wetwell. This condenses the steam and reduces the pressure. Both the
drywell and the wetwell are enclosed by a secondary containment building
(Fig. 5.17) which is maintained at a slight negative pressure during normal
operation and refueling operations.
Wetwell
Drywell
Reactor pressure vessel
Spent fuel pool
Secondary concrete shield wall
Fig. 5.16: Cross-section sketch of a typical BWR containment.
In some BWR designs the spent fuel pool is inside the containment but in
most of them it is located outside of the containment building.
ATM 1236 – Nuclear Energy Fundamentals
22 Module 4: Nuclear Reactor Design
The BWR containment shape looks like a cuboid which is very different from
PWR shape (Figs. 5.17 & 5.18). Since the steam going through the turbines
is coming directly from the reactor it is still slightly radioactive the turbine
building has to be considerably shielded as well. This leads to two buildings
of similar construction with the taller one housing the reactor and the short
long one housing the turbine hall and supporting structures.
Secondary containment
Primary containment
Fig. 5.17: Primary and secondary containment buildings in BWRs.
Fig. 5.18: The BWR containment building design for the reactor and turbine.
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactor Design 23
8. Steam Generator (SG)
Steam generators are heat exchangers used to convert water into steam
from heat produced in a nuclear reactor core. They are used in pressurized
water reactors between the primary and secondary coolant loops and they
are considered as a part of the cooling system.
In commercial power plants steam generators height can be up to 21 m
and weigh as much as 800 tons. Each steam generator can contain
between 3,000 to 16,000 tubes, each about 2 cm in diameter (Fig. 5.19).
The coolant, which is maintained at high pressure to prevent boiling, is
pumped through the nuclear reactor core. Heat transfer takes place
between the reactor core and the circulating water and the coolant is then
pumped through the primary tube side of the steam generator by coolant
pumps before returning to the reactor core (primary loop). That water
flowing through the steam generator boils water on the shell side to
produce steam in the secondary loop that is delivered to the turbines to
make electricity. These loops also have an important safety role because
they are primary barriers between the radioactive and non-radioactive
sides of the plant as the primary coolant becomes radioactive from its
exposure to the core. For this reason, the integrity of the tubing is
essential in minimizing the leakage of water between the two sides of the
plant. There is the potential that, if a tube bursts while a plant is operating,
contaminated steam could escape directly to the secondary cooling loop.
Thus during scheduled maintenance outages or shutdowns, some or all of
the steam generator tubes are inspected.
In other types of reactors, such as the pressurized heavy water reactors of
the CANDU design and liquid metal cooled reactors, heat exchangers are
used between primary coolant (metal or heavy water) and the secondary
water coolant.
ATM 1236 – Nuclear Energy Fundamentals
24 Module 4: Nuclear Reactor Design
Tube bundle
Tube support plate (7 total)
Secondary moisture separator
Primary moisture separator
Fig. 5.19: Vertical u-tube steam generator.
8.1. Steam generator types
There are three types of steam generators; vertical u-tubes with inverted
tubes for the primary water as in PWR (Fig. 5.19). The Russian VVER
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactor Design 25
reactor designs use horizontal steam generators, which have the tubes
mounted horizontally (Fig 5.20). The third type is the Once-through steam
generators that convert water to steam in a single pass (where water
enters at one end and steam exits at the other end (Fig. 5.21).
Fig. 5.20: Horizontal steam generator.
8.2. Steam generator tube material
Many types of materials are used to
manufacture the SG tubes. These can
be high-performance alloys and super
alloys such as type 316 stainless steel,
Alloy 400 and Alloy 600MA (mill
annealed).
SG
Fig. 5.21: Once-through steam
generator.
9. Pressurizer
The pressurizer (Fig. 5.22) is used to control the pressure in the reactor
cooling system so that boiling does not occur in the reactor. The
pressurizer also is used to act as a surge tank for the system taking up the
level variations in the system. Heaters are installed at the bottom of the
pressurizer for heating the water inside the pressurizer to about 345 ºC to
ATM 1236 – Nuclear Energy Fundamentals
26 Module 4: Nuclear Reactor Design
produce a bubble of steam. The steam bubble is used to maintain the
pressure in the sealed primary system at around 16 MPa.
Manhole
Spray hole
Nozzle for safety valves
Nozzle for control valves
Spray protection
Nozzle for volume compensation pipe valves
Heating rod Drain nozzle
Fig. 5.22: The pressurizer.
Automated pressure control valves (called power operated relief valves) and
safety valves, connected to the top of the pressurizer, can open to control
and maintain pressure. As explained in module 4, the pressurizer is part of
the PWRs. The pressurizer is about 13 meters tall and weighs 80 tones.
10. Nuclear Reactor Safety Systems
Safety is one of the most important design aspects of nuclear reactors.
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactor Design 27
Various systems are included in the different designs of NPPs to ensure
maximum safety measures.
The following safety systems are used to ensure maximum safety:
Reactor protection system (RPS) Essential service water system (ESWS) Emergency core cooling system (ECCS) Emergency electrical systems Ventilation and radiation protection
10.1. Reactor protection system (RPS)
A reactor protection system is composed of systems that are designed to
immediately terminate the nuclear reaction. All plants have some form of
the following reactor protection systems:
1. Control rods
Control rods can be quickly inserted into the core to absorb neutrons and
rapidly terminate the nuclear reaction.
2. Safety injection
In this system the nuclear reaction can be stopped by injecting a liquid that
absorbs neutrons directly into the core. In BWRs for example, the liquid
usually consists of a solution containing boron which can be injected to
displace the water in the core.
10.2. Essential service water system (ESWS)
The essential service water system (ESWS) circulates the water that cools
the plant’s heat exchangers and other components before dissipating the
heat into the environment. Because this includes cooling the systems that
remove heat from both the primary system and the spent fuel rod cooling
ponds, the ESWS is a safety-critical system.
Since the water is frequently drawn from an adjacent river, the sea, or
other large body of water (Fig. 5.23), the system can be endangered by
large volumes of seaweed, marine organisms, oil pollution, ice and debris.
ATM 1236 – Nuclear Energy Fundamentals
28 Module 4: Nuclear Reactor Design
In locations without a large body of water in which to dissipate the heat,
water is recirculated via a cooling tower (Fig. 5.24).
Fig. 5.23: The cooling water is drawn from large body of water.
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactor Design 29
Fig. 5.24: A cooling tower is used to reduce the temperature of the cooling water.
10.3. Emergency core cooling system (ECCS)
An emergency core cooling system comprises many systems that are
included to safely shut down a nuclear reactor during accident conditions.
The condenser is not used during an accident, so other methods of cooling
are required to prevent fuel rods melt down.
In most plants, ECCS is composed of the following systems:
1. High pressure coolant injection system (HPCI)
This system consists of a pump or pumps that have sufficient pressure to
inject coolant into the reactor vessel while it is pressurized. This system is
normally the first line of defense for a reactor since it can be used while the
reactor vessel is still highly pressurized.
2. Depressurization system (ADS)
The function of this system is to depressurize the reactor vessel and allows
lower pressure coolant injection systems to function, which have very large
capacities in comparison to high pressure systems. It consists of a series of
valves which open to vent steam under the surface of water in the wetwell
incase of BWRs or directly into the primary containment structure, in other
types of containments. Some depressurization systems are automatic in
function but can be inhibited, some are manual.
3. Low pressure coolant injection system (LPCI)
This system consists of a pump or pumps which inject additional coolant
into the reactor vessel once it has been depressurized.
4. Core spray system
This system reduces the generation of steam by using special spray nozzles
called “spargers” to spray water directly onto the fuel rods. Reactor designs
ATM 1236 – Nuclear Energy Fundamentals
30 Module 4: Nuclear Reactor Design
can include core spray in high-pressure and low-pressure modes.
5. Containment spray system
This system consists of a series of pumps and spargers which spray coolant
into the primary containment. It is designed to condense the steam into
liquid water within the primary containment structure to prevent
overpressure.
6. Isolation cooling system
This system is often driven by a steam turbine, and is used to provide
enough water to safely cool the reactor if the reactor building is isolated
from the control and turbine buildings. As it does not require large amounts
of electricity to run, and runs off the plant batteries, rather than the diesel
generators, it is a defensive system against the total loss of ac power (i.e.
failure of both offsite and onsite ac power sources). This condition is known
as station blackout (SBO).
10.4. Emergency electrical systems
These electrical systems usually consist of diesel generators and batteries.
These are not needed during normal operating conditions because nuclear
power plants receive electrical power to power their systems from off-site.
However, during an accident a plant may lose access to this power supply
and thus may be required to generate its own power to supply its
emergency systems.
1. Diesel generators
Diesel generators are used to power the NPP during emergency situations.
They are usually designed so that one can provide all the required power
for a facility to shutdown during an emergency situation. A facility can have
multiple generators as spares. Additionally, systems which are not required
to shutdown the reactor have separate electrical sources (often their own
generators) so that they do not affect shutdown capability.
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactor Design 31
2. Motor generator flywheels
Loss of electrical power can occur suddenly, and it can damage or
undermine equipment. To prevent damage, motor-generators can be tied to
flywheels which can provide uninterrupted electrical power to equipment for
a brief period of time. Often they are used to provide electrical power until
the plant electrical supply can be switched to the batteries and/or diesel
generators. The flywheel used in the RCP in Fig. 5.12a is an example.
3. Batteries
Batteries are often used as the final backup electrical system and are also
capable of providing sufficient electrical power to shutdown a plant.
Electrical inverter is needed to convert the DC power generated by batteries
to AC power to run AC devices such as motors.
10.5. Ventilation and radiation protection
In case of a radioactive release, most plants have a system designed to
remove radiation from the air to reduce the effects of the radiation release
on the employees and public. This system usually consists of the following:
1. Containment ventilation
This system is designed to remove radiation and steam from primary
containment in the event that the depressurization system was used to vent
steam into primary containment.
2. Control room ventilation
This system is designed to ensure that the operators who are required to
operate the plant are protected in the event of a radioactive release. This
system often consists of activated charcoal filters which remove radioactive
isotopes from the air.
ATM 1236 – Nuclear Energy Fundamentals
32 Module 4: Nuclear Reactor Design
4. Activities
The class will be divided into four groups and each group will do one of the
following activities and then groups will discuss and share findings.
Activity 1
Conduct a research on the recent accident in Fukushima nuclear power
reactors in Japan due to tsunami. Present your work in form of a power
point presentation and share your findings with your classmates.
Activity 2
Conduct a research on the accident in Three Mile Island nuclear power
reactors in the USA. Present your work in form of a power point
presentation and share your findings with your classmates.
Activity 3
Conduct a research on the accident in Chernobyl nuclear power reactors in
the former Soviet Union. Present your work in form of a power point
presentation and share your findings with your classmates.
Activity 4
Conduct a research to answer the following question: What initiates the
nuclear reaction when starting the nuclear reactor? Discuss your findings
with your classmates.
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactor Design 33
6. References
1. Why Science Matters, Using Nuclear Energy, John Townsend,
Heinemann Library.
2. Nuclear Energy (Fuelling the Future), Chris Oxlade and Elizabeth
Raum (Heinmann Library, 2008).
3. Future Energy, Improved, Sustainable and Clean Option For Our
Planet, Edited by Trevor M. Letcher, Elsevier.
4. Nuclear Safety, Gianni Petrangeli, Elsevier.
5. http://www.bwr-pr.com/
6. http://en.wikipedia.org/wiki/Boiling_water_reactor
7. http://www.nuclearfaq.ca/cnf_sectionA.htm
8. http://commons.wikimedia.org/wiki/Category:Schemata_of_pressuriz
ed_water_reactor
9. http://www.nuc.umr.edu/~ans/poor.html
10. http://nuceng.mcmaster.ca/refer/facts.htm
11. http://www.apr1400.com/
12. http://www.solcomhouse.com/nuclearpowerplants.htm
13. http://www.ecology.at/nni/index.php?p=type&t=gcr
14. http://www.mnf.co.jp/pages2/pwr2.htm
15. http://www.ask.com/wiki/Nuclear_fuel
16. http://wiki.answers.com
17. http://nuke1010.blogspot.com
18. http://www.brighthub.com/engineering/mechanical
19. http://www.animatedsoftware.com/hotwords/nuclear_reactor/nuclear
ATM 1236 – Nuclear Energy Fundamentals
34 Module 4: Nuclear Reactor Design
_reactor.htm
20. http://accessscience.com/content/Nuclear-Fuel-Cycle/459600
21. http://news.isc.vn/en/politics/nuclear-illinois-helped-shape-obama-
view-on-energy-in-dealings-with-exelon.html
22. http://maecourses.ucsd.edu/mae198/content/turbine.shtml
23. http://www.eskom.co.za/nuclear_energy/
24. http://cr4.globalspec.com/
25. http://wn.com/BWR
26. http://www.ask.com/wiki/Containment_building