TABLE OF CONTENTS














Introduction




1.0
Overall Unit
1-2
1.1 Energy
Conversion ……………………………………... 1-4
1.2 Water Moderated Reactors ……………………………... 1-8
1.3 Reactor Safety ………………………………………….. 1-8
1.4 Defense in Depth ……………………………………….. 1-11
1.5 Reactor Safety Fundamentals …………………………... 1-13
1.6 CANDU Station Systems ………………………………. 1-15
1.7 CANDU 9 Operating Characteristics …………………...



2.0
Reactor and Moderator

2.1 Reactor Structure Assembly ……………………………. 2-2
2.2 Fuel ……………………………………………………... 2-10
2.3 Moderator Systems ……………………………………... 2-12



3.0
Reactor Control

3.1 Reactor Control Requirements …………………………. 3-2
3.2 Reactor Instrumentation ……………………………...… 3-6
3.3 Reactivity Control Devices ……………………………..
3-11
3.4 Reactor Regulating System Programs ………………….. 3-20



4.0
Heat Transport

4.1 Main Heat Transport ……………………………………. 4-2
4.2 Pressure and Inventory Control ………………………… 4-8
4.3 Shutdown Cooling ……………………………………… 4-13
4.4 Heat Transport Auxiliaries ……………………………... 4-15
4.5 Heat Transport System Operation ……………………… 4-17



5.0
Steam, Turbine And Feedwater

5.1 Steam Generator (Boiler) ……………………...……….. 5-3
5.2 Steam System ………………………………………...… 5-5
5.3 Turbine ………………………………………………..... 5-9
5.4 Condenser

………………………………………………. 5-11
5.5 Feedwater System ………………………………………. 5-12
5.6 Generator ……………………………………………….. 5-16
5.7 Conventional Plant Services ……………………………. 5-18



6.0
Special Safety Systems

6.1 Shutdown System Requirements ……………………….. 6-2
6.2 Shutdown System Number 1 …………………………… 6-7
6.3 Shutdown System Number 2 …………………………… 6-15
6.4 Emergency Core Cooling Systems ……………………... 6-26
6.5 Containment System ……………………………………. 6-32

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Copyright © 2005 Dr. George T. Bereznai

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INTRODUCTION



COURSE OBJECTIVES
At the successful completion of this course the participants will be able to:
Describe the following features of a CANDU Generating unit:
the principles of overall unit operation and control
the functions, equipment and operation of the main process systems
how each major system is controlled
how reactor safety and the protection of the public is achieved;
Conduct normal and abnormal operations on a simulated CANDU-9
Generating unit, including:
power maneuvers
poison override operation
recovery from a reactor trip
recovery from a turbine trip
responses to reactor, heat transport, steam and feedwater system
malfunctions.




This text has been prepared to support the Nuclear Power Plant Systems and Operation
course, which has the following main components:

• modules in science fundamentals, equipment and systems principles relevant to CANDU
reactors;
• modules in CANDU reactor power plant systems and their operation;
• self-study of this text to support the above modules;
• problem solving assignments to reinforce the understanding and application of the course
material;
• operation of a CANDU-9 power plant simulator;
• reviews in a workshop or tutorial format to answer questions and exchange information
on topics that are of interest to the majority of the course participants.

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The traditional approach to teaching nuclear power plant design and operation has been to
begin with the scientific theory and mathematical representation of the fundamental processes
that take place in a nuclear power plant, studying simplified models, individual pieces of
equipment, eventually combining these into systems and finally synthesizing a complete
generating unit. This approach may be called ‘bottom-up’, since each building block must be
understood before subsystems can be formed into systems and eventually into a working
whole. Although this approach has been used successfully for many generations of students,
it is not considered appropriate for a class of adult learners with varied experience in the
nuclear power field. Such individuals will typically be experts in one or more areas relevant
to nuclear power plants, but few if any will have a good understanding and experience with
the overall operation of specific power plant types.

The approach followed in this text and in the course it supports is called ‘top-down’. It is
built on the assumptions that the participants want to achieve an overall understanding of
how a nuclear power plant operates, that each of them are already familiar with many of the
underlying science fundamentals, equipment and system principles of nuclear electric
generation, and that each participant will want to study different aspects of nuclear power
plants to different degrees. As such, while the lectures will treat topics that are necessary for
everyone to achieve the desired level of common understanding, it is left to the self-study
sessions for each individual to pursue various topics to different depths. The Simulation and
Problem Solving sessions are designed to ensure that the desired level of understanding is
achieved by every participant. Any shortcomings identified during these sessions will be
addressed during the review period, and if necessary will result in changes to the content of
the lectures and/or the conduct of the self-study and simulation sessions.

The sequence and content of the lectures and the assignments are designed to achieve the
terminal course objectives in the most direct way, subject to the knowledge and skill level of
the participants. As much as practicable, the final learning outcome of each session is to be
presented first, followed by covering as much detail as is necessary for the participants to
gain the desired level of knowledge. For example, if the participants are familiar with reactor
theory and with light water reactors but not with the specific features of heavy water reactors,
the lecturer should begin with the latter, and only cover the other topics if they are needed to
understand the operation of the heavy water reactor. The subsequent assignments would
include questions and problems that were designed to verify the assumed level of theoretical
and light water reactor specific knowledge, and if significant shortfalls were discovered,
these would be taken up in the subsequent review session and/or lecture.

ACKNOWLEDGEMENT
The material for this text is based principally on the CANDU 9 480/NU Technical
Description, AECL document 69-01371-TED-001 Rev. 1, published in January 1995.
Diagrams and text have also been based on various AECL and Ontario Hydro training
manuals.

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Nuclear Power Plant Systems and Operation
Chapter 1: Overall Unit
Dr. George Bereznai
page 1 - 1



CHAPTER 1

OVERALL UNIT



CHAPTER OBJECTIVES:
At the end of this chapter, you will be able to describe the following for
a CANDU nuclear generating station:

1.1
Energy conversions from fission to electricity;
1.2
The main functions and components of each major system;
1.3
How an energy balance is maintained between the reactor and
the conventional side of the station;
1.4
How the unit as a whole is controlled;
1.5
The fundamentals of reactor safety;
1.6
The main systems and operating characteristics of a CANDU
generating unit.



Nuclear generating stations exist for the purpose of converting the energy obtained from the
fission of certain nuclei to electricity. This energy conversion takes place via a number of
intermediate stages that require many pieces of equipment organized into several systems
under the control and protection of both manual and automatic operations. This chapter
presents the main features of a nuclear power plant, so that as each system is studied in
greater detail in subsequent chapters and in other courses, the reader should always be able to
place such detail into the overall context of an operating station.

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Nuclear Power Plant Systems and Operation
Chapter 1: Overall Unit
Dr. George Bereznai
page 1 - 2


1.1 ENERGY CONVERSION

The basic nuclear generating station energy cycle is shown in Figure 1.1. Fuel containing
fissile material (Uranium) is fed to the reactor where fission takes place. The energy liberated
appears in the form of heat, which is used to boil water. The steam produced from the boiling
water spins a turbine-generator set, where the heat is converted first to kinetic energy in the
turbine and to electricity by the generator; the electricity produced (denoted as megawatts) is
supplied to the electric power system.


h e a t t o t h e a t m o s p h e r e
STEAM
TURBINE-
REACTOR
fuel
PRODUCTION
GENERATOR
MW
Fission to Heat
Heat to Heat
Heat to Electric
h e a t t o c o o l i n g w a t e r


Figure 1.1. Basic flow of energy in a nuclear generating station.


It is important to recognize that while the transport of heat from the reactor to the turbine
takes place in one or two closed loop systems that are highly efficient, the transformation of
the heat energy of the steam to the kinetic energy of the turbine is accompanied by a large
loss of energy as the steam is condensed to water prior to recirculating it back to the steam
production system. Approximately 60% of the heat energy removed from the fuel is rejected
to the condenser cooling water. As we will see, several other systems are also cooled by
water. Under normal operations only a few % of the energy is lost directly to the atmosphere.

As indicated in Figure 1.1 spent fuel is periodically removed from the reactor. On the
generator end the flow of electrical energy is shown to be in two directions to indicate the
electrical energy consumption of the station itself.

This very much oversimplified representation of a nuclear generating station will become
increasingly more complex as we study the details of the many systems involved directly or
indirectly in the energy conversion processes, and in ensuring that these processes are always
under control and are operated in a safe manner.

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Nuclear Power Plant Systems and Operation
Chapter 1: Overall Unit
Dr. George Bereznai
page 1 - 3


Energy Balance

Nuclear generating stations are designed to operate for extended periods at a constant power
level, requiring that a steady state balance is maintained between the rate of energy released
from the fuel in the reactor and the electrical output of the generator. This must be achieved
despite inherent variations in the burn-up of fuel in the reactor, disturbances in the energy
conversion processes, in the demands of the electrical power system and in the energy
exchanges between the environment and the station.

As a minimum the plant control system must be able to adjust reactor power to produce the
desired amount of electricity. Since under normal operating conditions the generator is
synchronized to the electric power grid, the electrical energy produced by the generator is
determined by the energy of the steam admitted to the turbine. A mis-match between the
energy produced by the reactor and the steam energy required to produce the desired
electrical output will result in a change of steam temperature and pressure. Because steam
pressure measurements respond more quickly than temperature measurements, it is steam
pressure that is used to indicate an imbalance of energy between reactor and generator, and is
therefore the parameter chosen as an input to the control system to maintain the required
balance.

A very much simplified plant control system is shown in Figure 1.2. The key inputs to the
control system are:
• reactor power
• steam pressure
• generator output (MW).

The station control system is designed to keep steam pressure constant while matching the
stations output to the desired setpoint. If the setpoint is the desired level of megawatts, then
the control system adjusts reactor power by changing the position of the reactivity control
devices, and the control system is said to be in ‘reactor lagging’ mode. If the setpoint is the
desired reactor power output, then the control system adjusts the steam flow to the turbine by
changing the opening of the governor valve, thereby altering the generator’s output, and the
control system is said to be in ‘reactor leading’ mode. The choice of which type of setpoint to
specify depends on the operating status of the generating station and the requirements of the
electrical power grid, and input to the control system by the authorized station operator.

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Nuclear Power Plant Systems and Operation
Chapter 1: Overall Unit
Dr. George Bereznai
page 1 - 4


setpoint
reactor
power
Station Control
System
reactivity
steam
control
pressure
STEAM
governor
TURBINE-
REACTOR
valve
PRODUCTION
GENERATOR
MW
Figure 1.2. Simplified Nuclear Generating Station Control System.


1.2 WATER MODERATED REACTORS

Most of the nuclear power plants in operation around the globe use reactors that are both
moderated and cooled by water. Reactors that use enriched uranium use ordinary (or light)
water as both moderator and coolant. The reactor core is contained in a pressure vessel with
no separation between moderator and coolant. Two main types of light water reactors have
been developed, the Pressurized Water Reactor (PWR) and the Boiling Water Reactor
(BWR). In the former the reactor coolant forms a closed primary loop in which it is not
permitted to boil under normal operating conditions, and the steam is produced in a
secondary loop. In a BWR the coolant is allowed to boil and the steam is fed directly to the
turbine. The main characteristics of PWR and BWR reactors are shown in Figure 1.3 and
Figure 1.4. The next two sections outline some of the main design and operating features of
these types of reactors.

Reactors fueled with natural uranium must use heavy water instead of light water as the
moderator, and in order to achieve maximum neutron economy, many heavy water moderated
reactors also use heavy water as the coolant. The currently used designs are of the
pressurized primary loop type similar to PWRs, but instead of a pressure vessel, pressure
tubes contain the coolant and the fuel, while the moderator is in a low pressure, low
temperature calandria vessel. Since this text deals extensively with the CANDU (CANadian
Deuterium Uranium) type of pressurized heavy water reactors, the illustration of a CANDU
in Figure 1.5 is provided only as a means of easy comparison with the PWR and BWR
reactor types.

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Nuclear Power Plant Systems and Operation
Chapter 1: Overall Unit
Dr. George Bereznai
page 1 - 5








Moderator



H2O at 15 MPa
Coolant H2O at 15 MPa
Fuel
U-235,
enriched
to
3-5%



Moderator and coolant are combined.


Refueled off load every 12-18 months.


Light water coolant transfers heat to boiler.





Figure 1.3. Pressurized Water Reactor.

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Nuclear Power Plant Systems and Operation
Chapter 1: Overall Unit
Dr. George Bereznai
page 1 - 6









Moderator
H2O 6-7 MPa
Coolant H2O, Enriched to 2-3%





Moderator and coolant are combined.


Refueled off load every 12-18 months.


Steam flows directly to the turbine.



Figure 1.4. Boiling Water Reactor.

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Document Outline

• TABLE OF CONTENTS
• INTRODUCTION
• CHAPTER 1: OVERALL UNIT
  • 1.1 ENERGY CONVERSION
  • 1.2 WATER MODERATED REACTORS
  • 1.3 REACTOR SAFETY
  • 1.4 DEFENSE IN DEPTH
  • 1.5 REACTOR SAFETY FUNDAMENTALS
  • 1.6 CANDU STATION SYSTEMS
  • 1.7 CANDU 9 OPERATING CHARACTERISTICS
• CHAPTER 2: REACTOR AND MODERATOR
  • 2.1 REACTOR STRUCTURE ASSEMBLY
  • 2.2 FUEL
  • 2.3 MODERATOR SYSTEMS
• CHAPTER 3: REACTOR CONTROL
  • 3.1 REACTOR CONTROL REQUIREMENTS
  • 3.2 REACTOR INSTRUMENTATION
  • 3.3 REACTIVITY CONTROL DEVICES
  • 3.4 REACTOR REGULATING SYSTEM PROGRAMS
• CHAPTER 4: HEAT TRANSPORT
  • 4.1 MAIN HEAT TRANSPORT SYSTEM
  • 4.2 HEAT TRANSPORT PRESSURE AND INVENTORY CONTROL SYSTEM
  • 4.3 SHUTDOWN COOLING SYSTEM
  • 4.4 HEAT TRANSPORT AUXILIARY SYSTEMS
  • 4.5 HEAT TRANSPORT SYSTEM OPERATION
• CHAPTER 5: STEAM, TURBINE AND FEEDWATER
  • 5.1 STEAM GENERATOR (BOILER)
  • 5.2 STEAM SYSTEM
  • 5.3 TURBINE
  • 5.4 CONDENSER
  • 5.5 FEEDWATER SYSTEM
  • 5.6 GENERATOR
• CHAPTER 6: SPECIAL SAFETY SYSTEMS
  • 6.1 SHUTDOWN SYSTEM REQUIREMENTS
  • 6.2 SHUTDOWN SYSTEM NUMBER 1
  • 6.3 SHUTDOWN SYSTEM NUMBER 2
  • 6.4 EMERGENCY CORE COOLING SYSTEM
  • 6.5 CONTAINMENT SYSTEM

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