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Nuclear Energy Fundamentals
Module 3: Nuclear Reactor Types
PREPARED BY
Academic Services
April 2012
© Institute of Applied Technology, 2012
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactors Types
Module 2: Nuclear Reactor Types Module Objectives After the completion of this module, the student will be able to:
Recognize the importance of the nuclear reactor for the operation
of the NPP.
Explain the main difference between NPP and steam power plants.
Explain the simple steam cycle.
Identify the functional requirements of the nuclear reactors.
Classify the nuclear reactors based on the type of the nuclear reaction,
the moderator material and the use.
Explain the basic principle of operation of most common nuclear
reactor types that are in use today.
Module Contents Topic Page No.
1. Introduction 3
2. The Simple Steam Cycle 3
3. Nuclear Reactors Technology 4
4. Classification of Nuclear Reactors 5
5. Typical Reactors and Current Technologies 8
6. Activities 16
7. References 18
ATM 1236 – Nuclear Energy Fundamentals
Module 3: Nuclear Reactors Types
1. Introduction
The nuclear reactor is the heart of any nuclear power plant. It provides the
thermal energy required to produce high pressure superheated steam which
is needed to generate electrical power. In normal steam power plants
thermal energy is released from burning fossil fuels which is used as a heat
source to generate electricity.
2. The Simple Steam Cycle
A schematic diagram of a simple steam power plant is shown in Fig. 3.1.
High pressure superheated steam leaves the boiler which is also referred to
as a steam generator and enters the turbine. The steam expands in the
turbine and in doing so, does work, which enables the turbine to drive the
electric generator. The low pressure steam leaves the turbine and enters
the condenser, where heat is transferred from the steam (causing it to
condense) to the cooling water. Since large quantities of water are
required, power plants are frequently located near rivers or lakes.
Fig. 3.1: Simple steam power plant.
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactors Types
3. Nuclear Reactors Technology
As you studied earlier the nuclear chain reaction occurs when a large fissile
atomic nucleus such as uranium-235 or plutonium-239 absorbs a neutron
and undergoes nuclear fission. The heavy nucleus splits into two or more
lighter nuclei, releasing kinetic energy, gamma radiation and free neutrons.
These products of reaction are known as fission products.
The reaction can be controlled by using neutron poisons, which absorb
excess neutrons, and neutron moderators, which reduce the velocity of fast
neutrons, thereby turning them into thermal neutrons (Fig. 3.3), which are
more likely to be absorbed by other nuclei. Increasing or decreasing the
rate of fission has a corresponding effect on the energy output of the
reactor. In summary the basic functional requirements for a thermal reactor
are:
a fuel such as Uranium -235 or Plutonium-239.
a moderator to change the fast neutrons to thermal ones.
a coolant to remove the heat.
a control system to control the number of neutrons.
a shielding system to protect equipment and people from radiation
(Fig. 3.2).
Control Rod
Reactor Shield
Reactor vessel
Moderator
Fuel Rod
Coolant Pump
Fig. 3.2: Simple schematic for a nuclear reactor.
ATM 1236 – Nuclear Energy Fundamentals
Module 3: Nuclear Reactors Types
4. Classification of Nuclear Reactors
There are several methods to classify nuclear reactors. Some of these are:
Classification by type of nuclear reaction.
Classification by moderator material.
Classification by coolant.
4.1. Classification by type of nuclear reaction
There are two types of reactors based on the nuclear reaction; these are
fission reactors and fusion reactors.
1. Fission reactors: All commercial power reactors are based on nuclear
fission. They generally use uranium and its product plutonium as nuclear
fuel. Fission reactors can be divided into two classes, depending on the
energy of the neutrons that sustain the fission chain reaction, namely
thermal reactors and fast reactors.
Thermal reactors use slowed or thermal neutrons. Almost all
current reactors are of this type. These contain neutron moderator
materials that slow neutrons until their kinetic energy approaches the
average kinetic energy of the surrounding particles (Fig. 3.3). The
probability that thermal neutrons fissioning the fissile nuclei of
uranium-235, plutonium-239, or plutonium-241 is higher compared to
the faster neutrons that originally result from fission. Using a
moderator allows the use of low-enriched uranium or even natural
uranium fuel. The moderator is often also the coolant, usually water
under high pressure to increase the boiling point.
Fast reactors use fast neutrons to cause fission in their fuel. They do
not have a neutron moderator, and use less-moderating coolants.
Maintaining a chain reaction requires the fuel to be more highly
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactors Types
enriched in fissile material (about 20% or more) due to the relatively
lower probability of fission versus capture by U-238. Fast reactors
have the potential to produce less nuclear waste but they are more
difficult to build and more expensive to operate. Overall, fast reactors
are less common than thermal reactors in most applications. Some
early power stations were fast reactors and also some Russian naval
propulsion units.
Fig. 3.3: Thermal and fast neutrons.
2. Fusion reactors as explained earlier in this course fusion power is still
experimental technology, generally with hydrogen as fuel.
4.2 Classification by moderator material
As explained in the previous section, moderators are used by thermal
reactors. There are four main categories based on the moderator
materials:
1. Graphite moderated reactors: These reactors tend to reduce the
kinetic energy of more fast neutrons converting them to thermal
ATM 1236 – Nuclear Energy Fundamentals
Module 3: Nuclear Reactors Types
neutrons than light water reactors. Due to the extra number of
thermal neutrons, these types can use natural uranium or un-
enriched fuel.
2. Water moderated reactors:
Light water reactors (LWRs): These reactors use ordinary water
to moderate and cool the reactors.
Heavy water reactors (HWRs): Use deuterium oxide (D2O)
which tends to increase the number of thermal neutrons and so
these types can use natural uranium or un-enriched fuel.
3. Light element moderated reactors.
Molten salt reactors (MSRs): Use a light elements such as
lithium or beryllium as a moderator.
Liquid metal cooled reactors: Use a coolant that is a mixture of
Lead and Bismuth and may use BeO as a moderator.
4. Organically moderated reactors (OMRs): Use biphenyl and terphenyl
as moderator and coolant.
4.3. Classification by use
The following are the most common uses of nuclear reactors: 1. Electricity
Nuclear power plants
2. Propulsion
Nuclear marine propulsion
Various proposed forms of rocket propulsion
3. Heat
Desalination plants
Heat for domestic and industrial heating
Hydrogen production for use in a hydrogen economy
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactors Types
4. Research reactors: These reactors are used for research and training,
materials testing, or the production of radioisotopes for medicine and
industry. These are much smaller than power reactors or those
propelling ships, and many are on university campuses. Sharjah
University has one of these.
5. Production reactors for transmutation of elements such the breeder
reactors and reactors used to produce weapon grade plutonium.
5. Typical Reactors and Current Technologies
In this section we will discuss some of the typical reactors used around the
globe for electrical power generation.
5.1. Boiling water reactors (BWR)
In this reactor the enriched uranium oxide UO2 is used as the fuel and light
water is used as the coolant and moderator. This is a thermal reactor (Fig.
3.4). The water is circulated by a pump and the water boils in the reactor
vessel itself. The steam produced is fed directly to turbine. In BWR, the
steam is generated in the core itself.
Fig. 3.4: Boiling water reactor (BWR).
The reactor vessel has to be strong and is enclosed in concrete containment
vessel to prevent hazard from the failure of the pressurized circuit.
ATM 1236 – Nuclear Energy Fundamentals
Module 3: Nuclear Reactors Types
The exhaust steam from turbine is condensed and the condensate is sent
back to the reactor core through a feed pump. Another pump is used for re-
circulating the coolant in the reactor vessel before converting to steam.
5.2. Pressurized water reactors (PWR)
PWRs keep water under pressure so that it gets heat, but does not boil (Fig.
3.5). Water from the reactor and water in the steam generator that is
turned into steam never mix. Because of this, most of the radioactivity
stays in the reactor area (primary loop).
Secondary Loop
Primary Loop
Fig. 3.5: Pressurized water reactor (PWR).
A primary characteristic of PWRs is a pressurizer, a specialized pressure
vessel (Fig. 3.6). During normal operation, a pressurizer is partially filled
with water, and a steam bubble is maintained above it by heating the water
with submerged heaters. During normal operation, the pressurizer is
connected to the primary reactor pressure vessel and the pressurizer
"bubble" provides an expansion space for changes in water volume in the
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactors Types
reactor. This arrangement also provides a means of pressure control for the
reactor by increasing or decreasing the steam pressure in the pressurizer
using the pressurizer heaters.
The PWRs are the majority of current reactors, and are generally considered
the safest and most reliable technology currently used in large scale
deployment.
Pressurizer
Steam generator
Fig. 3.6: a) Pressurizer. b) Installing the pressurizer in a nuclear reactor.
5.3. Pressurized heavy water reactors (PHWR)-CANDU
These reactors are pressurized water reactors. These reactors are thermal
reactors and fueled with natural uranium. They use heavy water
(Deuterium; D2O) as a moderator and they also use light water or heavy
water as a coolant.
Instead of using a single large pressure vessel as in a PWR, the fuel is
contained in hundreds of pressure tubes.. PHWRs can be refueled while at
full power, which makes them very efficient in their use of uranium. PHWR
is also known as CANDU (CANada Deuterium-Uranium) because it is a
Canadian design. Fig. 3.7 shows the main components of this type of
reactors. These components are: 1. Fuel bundle 2. Calandria (reactor core)
3. Control rods 4. Pressurizer 5. Steam generator 6. Light water pump
7. Heavy water pump 8. Moderator (D2O) 9. Pressure tubes 10. Steam
11. Cold water from condenser 12. Concrete shielding.
ATM 1236 – Nuclear Energy Fundamentals
Module 3: Nuclear Reactors Types
Fig. 3.7: Pressurized heavy water reactor (CANDU) schematic.
5.4. Advanced power reactors (APR1400).
In early 2010, South Korea won its first export order; four APR-1400
reactors for the United Arab Emirates. The APR1400 is an advanced version
of the OPR1000.
The power generation capacity of the APR1400 is 1400 MW. APR1400 is
PWR and classified as thermal reactor that uses enriched U-235 as a fuel.
The design life of the major components such as the reactor vessel and the
steam generator has been extended from 40 years to 60 years compared to
the earlier version (OPR1000).
The reactor which consists of a vertical cylindrical shell, hemispherical lower
head and hemispherical upper head manufactured by forging stainless steel
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactors Types
lining. Fig. 3.8 shows the main components of the APR1400.
Steam generators
Pressurizer
Cooling pumps
Reactor vessel
Fig. 3.8: Advance Power Reactor (APR 1400) schematic.
5.5. Gas-cooled reactor (GCR)
GCRs are nuclear reactors that use graphite as a neutron moderator and
carbon dioxide or helium as coolant (Fig. 3.9). There are many other types
of reactor cooled by gas, however the term GCRs and to a lesser extent
gas- cooled reactors are particularly used to refer to this type of reactor.
The GCR uses natural uranium as fuel, which enable the countries that
developed them to fabricate their own fuel without relying on other
countries for supplies of enriched uranium. GCRs are designed for on-load
refueling. GCRs are nearly exclusively operating in Great Britain (Magnox
reactors).
ATM 1236 – Nuclear Energy Fundamentals
Module 3: Nuclear Reactors Types
Fig. 3.9: Gas-cooled reactor schematic.
5.6. Advanced Gas-cooled Reactor (AGR)
These are the second generation of British gas-cooled reactors , using
graphite moderator and carbon dioxide as coolant (Fig. 3.10).
Fig. 3.10: Advanced Gas-cooled Reactor (AGR) schematic.
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactors Types
The fuel is uranium oxide pellets, enriched to 2.5-3.5%, in stainless steel
tubes. The carbon dioxide circulates through the core, reaching 650°C and
then past steam generator tubes outside it, but still inside the concrete and
steel pressure vessel. Control rods penetrate the moderator and a
secondary shutdown system involves injecting nitrogen to the coolant.
5.7. Fast breeder reactor (FBR)
FBRs have been designed not only to produce electricity, but also to
generate (breed) fuel, namely plutonium out of uranium. They use a
plutonium (Pu) fuel rather than uranium (Fig. 3.11). The Pu is surrounded
by rods of U-238 which absorb neutrons and are transformed into Pu-239.
The principal reactor cooling system design of a FBR is similar to a PWR.
The major difference is that the coolant in a FBR is liquid sodium instead of
water. There are 2 sodium circuits, where heat transfer is generated by a
heat exchanger. The secondary sodium circuits transfer the heat to a steam
generator. The function of the steam generator is similar to its function in
other types of reactors.
Fig. 3.11: Fast Breeder Reactor (FBR) schematic.
ATM 1236 – Nuclear Energy Fundamentals
Module 3: Nuclear Reactors Types
The FBR fuel is plutonium which needs fast neutrons to generate fission,
therefore water cannot be used as a coolant, because of its moderating
function. The lack of a moderator to slow neutrons gives the word fast to
the name.
Table 1.1 shows the most common reactor types used all over the globe. It
shows also the main countries, the number of reactors and the type of fuel,
coolant and moderator used in each type. Table 4.1 shows that the most
dominant type amongst all types is the pressurized water reactor (PWR).
Table 4.1: Reactor types and numbers.
Reactor Type Main Countries Number Fuel Coolant Moderator
Pressurized Water
Reactor (PWR)
US, France, Japan,
Russia, China 265 Enriched UO2 Water Water
Boiling Water
Reactor (BWR)
US, Japan,
Sweden 94 Enriched UO2 Water Water
Pressurized Heavy
Water Reactor
'CANDU' (PHWR)
Canada 44 Natural UO2 Heavy
water
Heavy
water
Gas-cooled
Reactor
(GCR)/AGR
UK 18 Natural U/
Enriched UO2 CO2 Graphite
Light Water
Graphite Reactor Russia 12 Enriched UO2 Water Graphite
Fast Neutron
Reactor (FBR)
Japan, France,
Russia 4 PuO2 and UO2
Liquid
sodium None
Other types Russia 4 Enriched UO2 Water Graphite
TOTAL 441
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactors Types
6. Activities
6.1. Activity 1
1. Identify the NPP parts by writing the number of the correct power plant
part in the blank.
1. Reactor 5. Steam generator 9. Primary water loop
2. Control rods 6. Turbine–generator 10.Secondary water loop
3. Cooling water 7. Transmission lines 11. Nuclear fuel
4. Containment building 8. Condenser 12. Pressurizer
13. Cooling tower
2. Colour the separate loops using a different colour for each loop. Use the
following symbols to show what is in the loop or part of the loop.
ATM 1236 – Nuclear Energy Fundamentals
Module 3: Nuclear Reactors Types
6.2. Activity 2
1. Identify the NPP parts by writing the number of the correct power plant
part in the blank.
1. Reactor 4. Condenser 7. Nuclear fuel
2. Control rods 5. Turbine–generator 8. Containment building
3. Cooling water loop 6. Transmission lines
2. Colour the separate loops using a different colour for each loop. Use the
following symbols to show what is in the loop or part of the loop.
6.3. Activity 3
Build a 3D physical model of BWR and/or PWR. The material that will be
used to build the model is left to you to decide. Think of your model as a
part that will be integrated in a comprehensive model illustrating a
complete layout of a NPP.
ATM 1236 – Nuclear Energy Fundamentals
Module 4: Nuclear Reactors Types
7. References
1. Why Science Matters, Harnessing the Sun’s Energy, Andrew Solway,
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. www.nukeworker.com
15. http://www.ustudy.in
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