Module 3: Nuclear Reactor Types
PREPARED BY
Samir Hamasha
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
1. Introduction
2. The Simple Steam Cycle
3. Nuclear Reactors Technology
4. Classification of Nuclear Reactors
5. Typical Reactors and Current Technologies
6. Activities
7. References
 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.

A schematic diagram of a simple steam
power plant is shown in Fig. 3.1.
How does it work?

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.
The nuclear chain reaction Review
 It 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(neutron absorbers),such
as : Xenon-135 or samarium-149, which absorb
excess neutrons, and
 Using 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.

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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:
1. a fuel such as Uranium -235 or
Plutonium-239.
2.
3.
4.
5.
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
There are several methods to classify nuclear
reactors. Some of these are:
• Classification by type of nuclear reaction.
• Classification by moderator/ coolant
material.
• Classification by use.
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
 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. 4.3).

Thermal reactors

The probability that thermal neutrons (speed 2200
meters per second) 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
enriched in fissile material (about 20% or more ).

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.
2. Fusion reactors as explained earlier
in this course fusion power is still
experimental technology, generally with
hydrogen as fuel.
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 neutrons than light water reactors.
Due to the extra number of thermal neutrons,
these types can use natural uranium or unenriched 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 unenriched fuel.

3. Light element moderated reactors.
Molten salt reactors (MSRs): Use a light elements
such as lithium or beryllium as a moderator.
2. Liquid metal cooled reactors: Use a coolant that is
a mixture of Lead and Bismuth and may use BeO as
a moderator.
1.
4. Organically moderated reactors (OMRs):
Use biphenyl and terphenyl as moderator and coolant.
The following are the most common uses of nuclear reactors:
1. Electricity

Nuclear power plants
2. Propulsion

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Nuclear marine propulsion
Various proposed forms of rocket propulsion
3. Heat


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Desalination plants
Heat for domestic and industrial heating
Hydrogen production for use in a hydrogen economy
*(The hydrogen economy is a proposed system of delivering energy using hydrogen. The
term hydrogen economy was coined by John Bockris during a talk he gave in 1970 at
General Motors (GM) Technical Center. Hydrogen is therefore an energy carrier (like
electricity), not a primary energy source (like coal)
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(conversion) of
elements such as the breeder reactors and reactors
used to produce weapon grade plutonium.
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)
5.2. Pressurized water reactors (PWR)
5.3. Pressurized heavy water reactors (PHWR)-CANDU .
5.4. Advanced power reactors (APR1400).
5.5. Gas-cooled reactor (GCR)
5.6. Advanced Gas-cooled Reactor (AGR)
5.7. Fast breeder reactor (FBR)
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. 4.4).
How does it work?
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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.
The reactor vessel has to be strong and is enclosed in
concrete containment vessel to prevent hazard from the
failure of the pressurized circuit.
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.

PWRs keep water under pressure so that it gets
heat, but does not boil (Fig. 4.5).

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).

A primary characteristic of PWRs is a pressurizer, a specialized pressure
vessel (Fig. 4.6).
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 reactor.


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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.

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. 4.7 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.

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

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GCRs are nuclear reactors that use graphite as a
neutron moderator and carbon dioxide or helium as
coolant (Fig. 4.9).
There are many other types of reactor cooled by gas,
however the term GCRs and to a lesser extent gascooled 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).

These are the second generation of British gas-cooled reactors ,
using graphite moderator and carbon dioxide as coolant (Fig.
4.10).



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.
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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. 4.11).
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. 4.11: Fast Breeder Reactor (FBR) schematic.


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