114,99 €
Mehr erfahren.
- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Covers all aspects of electrical systems for nuclear power plants written by an authority in the field Based on author Omar Mazzoni's notes for a graduate level course he taught in Electrical Engineering, this book discusses all aspects of electrical systems for nuclear power plants, making reference to IEEE nuclear standards and regulatory documents. It covers such important topics as the requirements for equipment qualification, acceptance testing, periodic surveillance, and operational issues. It also provides excellent guidance for students in understanding the basis of nuclear plant electrical systems, the industry standards that are applicable, and the Nuclear Regulatory Commission's rules for designing and operating nuclear plants. Electrical Systems for Nuclear Power Plants offers in-depth chapters covering: elements of a power system; special regulations and requirements; unique requirements of a Class 1E power system; nuclear plants containment electrical penetration assemblies; on-site emergency AC sources; on-site emergency DC sources; protective relaying; interface of the nuclear plant with the grid; station blackout (SBO) issues and regulations; review of electric power calculations; equipment aging and decommissioning; and electrical and control systems inspections. This valuable resource: * Evaluates industry standards and their relationship to federal regulations * Discusses Class 1E equipment, emergency generation, the single failure criterion, plant life, and plant inspection * Includes exercise problems for each chapter Electrical Systems for Nuclear Power Plants is an ideal text for instructors and students in electrical power courses, as well as for engineers active in operating nuclear power plants.
Sie lesen das E-Book in den Legimi-Apps auf:
Seitenzahl: 400
Veröffentlichungsjahr: 2018
Ähnliche
IEEE Press
445 Hoes Lane
Piscataway, NJ 08854
IEEE Press Editorial Board
Ekram Hossain, Editor in Chief
Giancarlo Fortino
Andreas Molisch
Linda Shafer
David Alan Grier
Saeid Nahavandi
Mohammad Shahidehpour
Donald Heirman
Ray Perez
Sarah Spurgeon
Xiaoou Li
Jeffrey Reed
Ahmet Murat Tekalp
Electrical Systems for Nuclear Power Plants
Omar S. Mazzoni
Published by
Standards Information Network
This edition first published 2019© 2019 by The Institute of Electrical and Electronics Engineers, Inc.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions.
The rights of Omar S. Mazzoni to be identified as the author of this work have been asserted in accordance with law.
Registered OfficeJohn Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA
Editorial Office111 River Street, Hoboken, NJ 07030, USA
For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com.
Wiley also publishes its books in a variety of electronic formats and by print-on-demand. Some content that appears in standard print versions of this book may not be available in other formats.
Limit of Liability/Disclaimer of Warranty While the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages.
Library of Congress Cataloging-in-Publication Data
ISBN: 9781119483601
Cover design: WileyCover image: © MichaelUtech/GettyImages
This book is dedicated to my beloved daughter, Ruth Maria Mazzoni, who is ever present in my mind and whom I am missing greatly since her early departure from this life on January 23, 2012.
CONTENTS
Preface
1 Elements of a Power System
1.1 The Alternating Current One-Line Diagram
1.2 Basis for One-Line Representation
1.3 Main Electrical Components of Power Plants
1.4 Transmission Lines, Switchyards, and Substations
Questions and Problems
References
2 Nuclear Power Plants: General Information
2.1 Introduction
2.2 Environmental Impact
2.3 Nuclear Generation Fuel Cycle
2.4 Evolution of Nuclear Power Generation
2.5 Nuclear Power in the United States
2.6 Plans for New Reactors Worldwide
2.7 Increased Capacity
2.8 Nuclear Plant Construction
2.9 Nuclear Plant Licensing
2.10 Current Commercial Nuclear Plants
2.11 Evolutionary Commercial Nuclear Plants
2.12 Advanced Reactors
2.13 Nuclear Accidents: Three Mile Island, Chernobil, and Fukushima Events
Questions and Problems
References
3 Special Regulations and Requirements
3.1 Regulations
3.2 IEEE Standards
3.3 NRC Regulatory Guides
Questions and Problems
References
4 Unique Requirements: Class 1E Power System
4.1 Class 1E Electrical Systems: General Description
4.2 Specific Requirements for Class 1E ac Power Systems
4.3 Specific Requirements for Class 1E DC Power Systems
4.4 Specific Requirements for Class 1E Instrumentation and Control Systems
4.5 Specific Requirements for Class 1E Containment Electrical Penetrations
4.6 Specific Requirements for Emergency On-Site ac Power Sources
Questions and Problems
References
5 Nuclear Plants Containment Electrical Penetration Assemblies
5.1 Containment Electrical Penetration Assemblies: General (Information on this chapter is based on the requirements of IEEE 317)
5.2 Service Classification
5.3 Electrical Design Requirements (extracted from IEEE 317)
5.4 Mechanical Design Requirements (Extracted from IEEE 317)
5.5 Fire Resistance Requirements (Extracted from IEEE 317)
5.6 Qualified Life
5.7 Qualification Tests
5.8 Design Tests (Extracted from IEEE 317)
5.9 Production Tests
5.10 Monitoring and Testability
Questions and Problems
References
6 On-Site Emergency Alternating Current Source
6.1 General Requirements of the Emergency Alternating Current Source
6.2 General Requirements of Diesel Generators Used as Emergency Alternating Current Source (Information in this chapter is based on the requirements of IEEE 387)
6.3 Specific Design Requirements for Emergency Diesel Generators
6.4 Factory Qualification
6.5 Site Acceptance Testing
6.6 Site Preoperational Testing
6.7 Site Operational Testing
6.8 Site Periodic Testing and Surveillance: Preventive Maintenance Program
Questions and Problems
References
7 On-Site Emergency Direct Current Source
7.1 Energy Storage Systems for Nuclear Generating Stations
7.2 General Requirements of Direct Current Systems
7.3 Design Requirements
7.4 Battery Loads
7.5 Classification of Loads in Terms of Power versus Voltage Characteristics
7.6 Battery Chargers
Questions and Problems
References
8 Protective Relaying
8.1 General
8.2 General Criteria for the Protection System
8.3 Specific Criteria for Protection of Alternating Current Systems
8.4 Degraded Voltage Protection
8.5 Surge Protection
8.6 Protection for Instrumentation and Control Power System
8.7 Protection Aspects for Auxiliary System Automatic Bus Transfer
8.8 Protection for Primary Containment Electrical Penetration Assemblies
8.9 Protection of Valve Actuator Motors (Direct Gear Driven)
8.10 Protection for DC Systems
8.11 Testing and Surveillance of Protective Systems
Questions and Problems
References
Bus Transfer Schemes
Protection of Motor-Operated Valves
9 Interface of the Nuclear Plant with the Grid
9.1 Preferred Power Supply Safety Function
9.2 Interface between the Nuclear Plant and the Grid
9.3 Transmission Line and Switchyard Protective Relaying
9.4 Connections of the PPS to the Class 1E Systems
9.5 Switchyard Grounding
9.6 Switchyard and Transmission Line Surveillance and Testing
9.7 Effect of PPS Voltage Degradation on the Class 1E Bus
9.8 Multiunit Considerations
9.9 Considerations for PPS Reliability in a Deregulated Environment
9.10 Alternate AC Source
9.11 Study of Recent Events
Questions and Problems
References
10 Station Blackout: Issues and Regulations
10.1 Introduction
10.2 Regulations Relating to SBO Requirements
10.3 Specific SBO Requirements
10.4 Alternate Alternating Current Power Sources
10.5 Procedures and Training
10.6 QA and Specifications for Nonsafety-Related Equipment
10.7 Monitoring of the Grid Condition
Questions and Problems
References
11 Review of Electric Power Calculations
11.1 Introduction
11.2 Load and Voltage Calculations
11.3 Motor Starting Calculations
Questions and Problems
References
12 Plant Life: Equipment Aging, Life Extension, and Decommissioning
12.1 Nuclear Plant Licensed Life
12.2 Importance of Maintenance: The Maintenance Rule (Courtesy of the NRC)
12.3 Monitoring Issues Affecting Electrical Equipment, Transformers, Motors, Cable, Control Equipment
12.4 Cable-Monitoring Methods and Techniques (Courtesy of NRC)
12.5 Further Information on Cable Testing
12.6 Switchyard Maintenance Activities
12.7 Emergency Diesel Generators
12.8 Interpretation of “Standby”
12.9 Normally Operating SSCs of Low Safety Significance
12.10 Establishing SSC-Specific Performance Criteria
12.11 Clarification of MPFFs Related to Design Deficiencies
12.12 Scope of the Hazards to be Considered during Power Operations
12.13 Scope of Initiators to be Considered for Shutdown Conditions
12.14 Fire Scenario Success Path(s)
12.15 Establishing Action Thresholds Based on Quantitative Considerations
12.16 SSCs Considered under 10 CFR 50.65(a)(1)
12.17 Inclusion of Electrical Distribution Equipment
12.18 The License Renewal Rule
12.19 Interpretation of Aging
12.20 Effects of Plant Aging
Questions and Problems
References
13 Electrical and Control Systems Inspections
13.1 Purpose of Inspections
13.2 Objectives of Inspections
13.3 Areas of Review
13.4 Typical Approach to the Review
13.5 Acceptance Criteria
Questions and Problems
Reference
Appendix 1 Abbreviations
Appendix 2 Definitions
Index
WILEY END USER LICENSE AGREEMENT
List of Tables
2
Table 2.1
11
Table 11.1
Table Q11.5
List of Illustrations
1
Figure 1.1
2
Figure 2.1
Figure 2.2
Figure 2.3
Figure 2.4
5
Figure 5.1
6
Figure 6.1
7
Figure 7.1
8
Figure 8.1
11
Figure 11.1
Figure 11.2
Pages
iii
iv
v
xiii
xiv
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
35
36
37
38
39
40
41
42
43
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
Preface
This book is the result of class notes for a graduate-level course in electrical engineering, taught by the author at The George Washington University, at several times in the period 1990–2017. The book includes problems and questions which were posed to the students to test their understanding of the material given in the class and to stimulate class discussions. The main reason for the book has been the lack of other suitable instructional material on the subject.
The book is intended for a graduate-level university course in power engineering programs. Also, the book should be useful for training of beginning engineers and as a ready reference for the practicing engineer.
The book summarizes the author's experience in the grass roots design of nuclear generating stations, began at Burns and Roe, Inc. from 1969 to 1979, where he left as Assistant Chief Electrical Engineer. Also important was the experience gained in the modifications to nuclear generating stations, while working as Manager of Electrical and Instrumentation Systems at NUS Corporation, from 1983 to 1989, and the experience gained in performing inspections and plant evaluations for numerous nuclear utility clients, with Systems Research International, Inc. (SRI), of which he is the principal founder-owner. The author has also performed 51 inspections at nuclear plants, as an electrical and instrumentation consultant for the US Nuclear Regulatory Commission over the period 1989–2016.
Throughout his technical career, the author has been very active in the development of standards at the Institute of Electric and Electronics Engineers, Inc. (IEEE), participating in standards working groups. The author remains very active in the balloting of IEEE standards. IEEE standards related to nuclear plants were obviously a fundamental guide for this book.
Numerous sources were consulted in the development of the book material, but the author is very thankful to the following institutions in particular:
US Nuclear Regulatory Commission,
Institute of Electric and Electronics Engineers, Inc.,
Westinghouse Electric Corporation, and
The George Washington University.
Of the many references given in the book, only the edition cited applies for dated references. For undated references, the latest edition of the referenced document (including any amendments and corrigenda) applies.
The author wishes to thank The George Washington University for providing him the opportunity to teach the course. Also my thanks go to the many students who took the course through the years, to the Department of Electrical Engineering and Computer Science, and especially to Professor Robert Harrington, who was a friendly guiding source. The author also has heartfelt gratitude for the encouragement and patience of his wife Maria (Betty) during the writing of the book.
It is hoped that the book may be usable to professionals interested in understanding and teaching the principal criteria for nuclear power plant electrical and instrumentation systems.
Professor Omar S. Mazzoni, DSc, PE
1Elements of a Power System
This chapter discusses the main elements of a power system that are particularly applicable to understanding the basic electrical configuration of a nuclear power plant.
1.1 The Alternating Current One-Line Diagram
A useful diagram to depict the alternating current (ac) system of a nuclear generating station is the one-line diagram (OLD) [6]. The OLD allows for the simplest representation of the main elements of a typical station and the interconnections among them. Figure 1.1 depicts a typical OLD for the high voltage and the medium voltage portions of a nuclear power station. Depending on the plant design and year of start of operations, the OLD may be somewhat different than the one depicted in Figure 1.1. The OLD of Figure 1.1 depicts a preferred approach to nuclear station design, as it offers enhanced independence for the safety-related busses.
Figure 1.1 One-line diagram of nuclear station.
While transformers AT1 and AT2 can be connected to the safety buses and also to the nonsafety buses, they are mainly intended to feed the safety-related busses. The only time that AT1 and AT2 are used to supply the plant nonsafety-related loads is for plant start-up or shutdown.
The plant normal operating conditions are with transformer AT feeding the plant nonsafety-related loads. Transformers AT1 and AT2 and normally energized and feed the safety buses, with both of these transformers being assigned to the two safety-related buses. Each one of the safety-related buses can be fed from an emergency diesel generator (EDG). Under normal plant operation, the EDGs are on standby condition.
After the generator starts up and synchronized to the grid, it is initially loaded with grid load to about 30% normal plant load, at which time the secondary side breakers of transformer AT are closed to feed the plant auxiliary loads.
The preferred mode of starting up the station would be by synchronizing across the generator circuit breaker and feeding the station auxiliaries through transformer AT. An alternate approach, though less preferred, would be through either transformer AT1 or AT2, which would require a bus transfer to eventually feed plant loads through transformer AT.
Automatic bus transfers are necessary to prevent paralleling of two sources, which would impose undue short circuit stresses for the circuit breakers.
Thorough the plant start-up process, the two safety-related buses remain energized and are not subjected to any transfer operations, thereby eliminating the possibility of transients to be induced in the safety system.
The plant design allows for the possibility of testing the EDGs as required by the plant surveillance. The EDGs are tested once a month, and loads are picked up from the system for the period of test duration.
1.2 Basis for One-Line Representation
For a three-phase power system, in order properly represent it in an OLD, the system must be balanced, that is each one of the three phases must carry the same magnitude of current, and the currents must be at 120° from each other. As a result, the balanced three-phase system may be represented by just one of its phases, as they are the same to each other.
Percent and Per Unit Representation
In the definition of electrical power system values, a representation in percent (%) or per unit (pu) is more convenient than actual units such as ohm, volts, etc,
A base value must be introduced to show quantities in percent or per unit. For example, assuming a base voltage of 4.16 kV:
4.0 kV becomes 0.9615 pu, 96.15%
7.2 kV becomes 1.73 pu, 173%
The pu system is preferred because the product of two pu values results in pu, whereas the product of two percent values must be divided by 100 to express the result in percent.
Two base values must be selected to have all other values automatically determined. Generally, kVA and kV are selected. The term MVA refers to MVA for three phase and the term kV to voltage from line to line, then the term current refers to line current. Generally, power is understood to be three-phase power and voltage the line-to-line voltage, unless otherwise specified. A major exception occurs in the method of symmetrical components, where line-to-neutral voltage is used.
Per unit and percent values of transformer impedances are the same on either side of the unit (primary or secondary). Also, the per unit and percent values do not depend on the transformer connections (wye–wye, delta–wye…etc.)
The following relationships apply (see [8]):
Base current,
A
= base kVA/1.732 base kV
Ω = (base voltage, kV
LN
)
2
/base MVA
Base impedance, Ω = (base voltage, kV
LN
)
2
/base MVA
Per unit impedance = actual impedance, Ω/base impedance, Ω
When dealing with unbalanced systems, symmetrical components must be used. In this method, the sequence components are always line to neutral or line to ground, as appropriate.
1.3 Main Electrical Components of Power Plants
Rotating Machines
Generators
Two types of generators are found in nuclear power plants: main generators that convert the mechanical power into electrical power and emergency standby generators utilized to provide electrical power to safety-related equipment when the normal power is not available [5,7]. This course places particular emphasis on the review of requirements for standby generators, as they are intrinsically involved in the performance of safety-related functions.
Motors
Most motors encountered in safety-related systems for nuclear power plants are of the AC induction type, constant speed. Some plants have AC variable speed drives, such as the feed water pumps for boiling water reactor type reactors. Motors for reactor cooling pumps are generally fitted with flywheels, to extend their operation when power is lost. Motors fitted with flywheels present special starting and protection issues.
DC motors are utilized for safety-related motor-operated valves.
Transformers
Distribution transformers are rated from 3 to 500 kVA, and below power transformers are those rated above 500 kVA.
In terms of insulation type, transformers can be oil cooled or dry type. The dry-type transformers include ventilated, cast coil, totally enclosed nonventilated, and sealed gas filled. Oil-insulated transformers are not utilized indoors, due to their intrinsic fire hazard.
Cables
Cables can be classified in accordance with their insulation ratings, which are as follows:
medium voltage, rated 2001–35,000 V,
low voltage, generally rated at 600 V, and
instrumentation and control, rated below 600 V.
Switchgear, Distribution Panelboards, and Motor Control Centers
switchgear, medium voltage, rated above 2000 V,
switchgear, low voltage, rated 600 V, and
panelboards and motor control centers, rated 600 V and below.
Buses
Phase-segregated buses
: Each phase is enclosed in a metallic enclosure, allowing for better isolation. Generally utilized for higher voltage types, such as the generator connection to the step-up transformer (19–24 kV).
Nonsegregated buses
: All phases are in the same metallic enclosure, generally utilized for voltages at 7.2–4.16 kV.
DC Batteries
Generally lead acid are the types of batteries found in nuclear plants. Voltages are between 24 and 250 V.
Battery chargers are provided to keep the batteries fully charged. The chargers provide the DC system load requirements whenever the normal ac input to the charger is available. Whenever ac power is available, the chargers provide the normal DC load and also the “trickle” current necessary to keep the battery fully charged.
Inverters
Inverters provide 120–240 V ac to safety loads requiring power when the main ac is not available, normally provided with autotransfer capabilities between available power sources.
Plant Loads and Their Characteristics
It is important to know how the plant loads will behave under variable voltage conditions. The modeling of plant loads is performed in accordance with the following guidelines:
motors, constant impedance type for starting conditions, constant kVA type for running conditions and
transformers, constant impedance type
lighting, load varies as the square of the impressed voltage,
heaters, load varies as the square of the impressed voltage, and
inverters constant kVA.
1.4 Transmission Lines, Switchyards, and Substations
The switchyard and associated substation are required to connect the power plant to the transmission grid, but are under the control of the power plant operators. The switchyard and its associated breakers, lines, and components are not considered safety related, though they may be considered important to safety.
Strategic arrangements are provided to increase reliability of operation. One common arrangement, which provides very high reliability, consists of three breakers in on lineup with the end breakers connected to a separate bus and two lines between the three breakers (this is the so-called “breaker-and-a-half” scheme).
Operation: Transmission lines are operated by the regional associations of grid operators. Of particular importance to the nuclear plant is the regulation of the supplied voltage, as it will affect the voltage at the plant and the operation of safety-related equipment. Special voltage relays are provided to disconnect the plant from the transmission grid, should the voltage excursion become intolerable for the proper operation of the safety equipment in the plant. (See [5] for extreme external events). The frequency of the transmission grid is also important, particularly when an EDG is paralleled to grid for test purposes.
Transmission Line Protective Relaying
Transmission line protective relaying is discussed under Chapter 9, “Interface of the Nuclear Plant and the Grid.”
Transmission Line Testing and Inspection
Transmission testing and inspection is discussed under Chapter 9, “Interface of the Nuclear Plant and the Grid.”
Questions and Problems
1.1 A prime requirement for the application of the one-line diagram is that the three-phase power system could be represented by just one phase. Could this representation be possible for a distribution system where there are numerous loads between one phase and the neutral? Explain your answer.
1.2 If a power system develops a phase-to-phase fault, is it possible to represent the faulted system with a one-line diagram? Explain your answer
1.3 What analytical technique is available to study unbalanced system conditions in a three-phase power circuit?
1.4 The one-line diagram (OLD) is utilized in the nuclear plant control room by the plant operators to review plant conditions under normal and emergency operation. In addition, a plant simulator is provided. The simulator is mainly used for training purposes. If there is a discrepancy between the simulator and the OLD, which one takes precedence? Explain.
1.5 A main generator circuit breaker is included in modern nuclear plant designs. Other plants have the generator connected directly to the switchyard through the step-up transformer. The main generator circuit breaker allows for isolation of the generator from the switchyard and also allows for availability of the plant station service transformer to feed the plant auxiliaries, without the need for fast bus transfer scheme. Would a generator breaker be considered particularly important to safe shutdown of the plant? Explain.
1.6 Plants that do not have a generator circuit breaker usually incorporate a fast bus transfer scheme to continue powering the station auxiliary buses in case of a plant trip. Why does the bus transfer scheme need to be fast? Provide a brief explanation.
1.7 From description of plant operation, it is not possible to keep the plant buses energized from two sources, as the short circuit capability of the breakers would be exceeded. Is this a normal plant design approach? If so, why?
1.8 If a nuclear plant operating at 100% output gets separated from the grid for a period time, say, for example, 5 min, it will need to trip and shutdown. This is due to the large excess between the reactor energy and the energy represented by the turbine steam bypass plus the plant auxiliary loads (about 6% of full load). The turbine bypass of the reactor energy is typically 25%. This situation is typical of all nuclear plants operating in the United States. What if anything could be done to allow the plant to ride over the transient and keep the generator and reactor running until reconnection to the grid becomes possible?
1.9 Given:
Line to line voltage = 7.2 kV
Line to neutral voltage = 7.2/√3 = 4/16 kV
Three phase power = 21,000 kVA
kVA1Φ = 7000 kVA
Calculate pu values.
1.10 Later designs of nuclear plants have been fitted with a main generator circuit breaker, and some existing plants have been retrofitted to incorporate generator circuit breakers. Comment on the following:
Have generator circuit breakers enhanced the operation of nuclear plants?, if so how?
What safety aspects are enhanced by the generator circuit breaker?
References
Publishing organization websites:
US Nuclear Regulatory Commission: https://www.nrc.gov/
Institute of Electrical and Electronics Engineers: www.ieee.org
International Atomic Energy Agency: www.iaea.org
IEEE 100 Std., “Dictionary of Electrical and Electronic Terms,” Sixth ed., 1996.
US NRC NUREG-0544. “Collection of Abbreviations.”
D. G. Fink and H. Beaty,
Standard Handbook for Electrical Engineers
McGraw-Hill, New York, 2007.
IEEE 141, “Recommended Practice for Electric Power Distribution for Industrial Plants.”
“Assessment of Vulnerabilities of Operating Nuclear Power Plants to Extreme External Events,” IAEA TECDOC No. 1834, 2017.
2Nuclear Power Plants: General Information
2.1 Introduction
This chapter provides a brief review of nuclear power generation, which is necessary to study the electrical systems and their importance to safety. Nuclear plant accidents are discussed with particular emphasis on the accident contribution from electrical and instrumentation safety systems operation.
2.2 Environmental Impact
Fossil fuel generating plants differ from nuclear plants in that the steam supply system consists of boilers, where the heat is supplied by a conventional fossil fuel such as oil, coal, or gas. Burning of fossil fuel creates difficult environmental issues. Nuclear plants, on the other hand, do not create environmental issues, unless safety of the plants is compromised to the point of creating nuclear accidents. Major nuclear accidents have occurred, and some have had a profound impact on the environment, such as Chernobyl and Fukushima.
On the record of the past 50 years, nuclear power has an edge over other forms of providing energy both in terms of limiting day-to-day adverse health and environmental effects, including greenhouse gas emissions and in terms of the frequency and toll of major accidents. Table 2.1 makes this point clear.
Table 2.1Main Sources of Electricity in the World and Their Morbidity and Greenhouse Gas Emissions Per Unit of Electricity Produced
Source (Percentage of world use, 2007)
Deaths per terawatt-hour
Tons of greenhouse gas emissions per gigawatt-hour (life)
Nuclear (14%)
0.04
Small–50
Coal (42%)
161 (US average is 15)
800–1400
Gas (21%)
4
300–500
Hydro (16%)
0.1 (Europe)
Small–100
Wind (< 1%)
0.15
Small–50
Table generated by authors using data from Key World Energy Statistics 2009[4].
The low morbidity is due to several factors, but two stand out:
Most casualties and other health and environmental effects stem from the extractive and transportation industries. Because the same amount of electric power can be obtained from about 200–300 tons of uranium ore as from 3 to 4 million tons of coal or similarly large quantities of gas or oil, these effects are inherently less severe for nuclear power than for the main hydrocarbon sources of electricity.
Safety culture needs to be enforced and strengthened at every nuclear generating plant, so that there will be no more accidents at nuclear power generating facilities.
2.3 Nuclear Generation Fuel Cycle
Mining and Milling
Uranium is usually mined by either surface (open cut) or underground mining techniques, depending on the depth at which the ore body is found. In Australia, the Ranger mine in the Northern Territory is open cut, whereas Olympic Dam in South Australia is an underground mine (which also produces copper, with some gold and silver). The newest Canadian mines are underground.
From these, the mined uranium ore is sent to a mill where the ore is crushed and ground to a fine slurry, which is leached in sulfuric acid to allow the separation of uranium from the waste rock. It is then recovered from solution and precipitated as uranium oxide (U3O8) concentrate (sometimes this is known as “yellowcake,” though it is finally khaki in color).
U3O8 is the uranium product which is sold. About 200 tonnes is required to keep a large (1000 megawatt electric [MWe]) nuclear power reactor generating electricity for 1 year.
Conversion
Because uranium needs to be in the form of a gas before it can be enriched, the U3O8 is converted into the gas uranium hexafluoride (UF6) at a conversion plant.
Enrichment
The vast majority of all nuclear power reactors in operation and under construction require “enriched” uranium fuel in which the proportion of the U-235 isotope has been raised from the natural level of 0.7% to about 3.5–5%. The enrichment process removes about 85% of the U-238 by separating gaseous uranium hexafluoride into two streams: One stream is enriched to the required level and then passes to the next stage of the fuel cycle. The other stream is depleted in U-235 and is called “tails.” It is mostly U-238.
So little U-235 remains in the tails (usually less than 0.25%) that it is of no further use for energy, though such “depleted uranium” is used in metal form in yacht keels, as counterweights, and as radiation shielding, since it is 1.7 times denser than lead.
Fuel Fabrication
Enriched UF6 is transported to a fuel fabrication plant where it is converted to uranium dioxide (UO2) powder and pressed into small pellets. These pellets are inserted into thin tubes, usually of a zirconium alloy (zircalloy) or stainless steel, to form fuel rods. The rods are then sealed and assembled in clusters to form fuel assemblies for use in the core of the nuclear reactor.
Some 27 tonnes of fresh fuel is required each year by a 1000-MWe reactor.
Several hundred fuel assemblies make up the core of a reactor. For a reactor with an output of 1000 MWe, the core would contain about 75 tonnes of low-enriched uranium. In the reactor core, the U-235 isotope fissions or splits, producing heat in a continuous process called a chain reaction. The process depends on the presence of a moderator such as water or graphite and is fully controlled.
Some of the U-238 in the reactor core is turned into plutonium, and about half of this is also fissioned, providing about one third of the reactor's energy output.
As in fossil fuel burning electricity generating plants, the heat is used to produce steam to drive a turbine and an electric generator, in this case producing about 7 billion kilowatt hours of electricity in 1 year.
To maintain efficient reactor performance, about one third of the spent fuel is removed every year or 18 months, to be replaced with fresh fuel.
Used Fuel Storage
Used fuel assemblies taken from the reactor core are highly radioactive and give off a lot of heat. They are therefore stored in special ponds which are usually located at the reactor site, to allow both their heat and radioactivity to decrease. The water in the ponds serves the dual purpose of acting as a barrier against radiation and dispersing the heat from the spent fuel.
Used fuel can be stored safely in these ponds for long periods. It can also be dry stored in engineered facilities, cooled by air. However, both kinds of storage are intended only as an interim step before the used fuel is either reprocessed or sent to final disposal. The longer it is stored, the easier it is to handle, due to decay of radioactivity.
There are two alternatives for used fuel:
reprocessing to recover the usable portion of it and
long-term storage and final disposal without reprocessing.
Reprocessing
Used fuel still contains approximately 96% of its original uranium, of which the fissionable U-235 content has been reduced to less than 1%. About 3% of used fuel comprises waste products, and the remaining 1% is plutonium (Pu) produced while the fuel was in the reactor and not “burned” then.
Reprocessing separates uranium and plutonium from waste products (and from the fuel assembly cladding) by chopping up the fuel rods and dissolving them in acid to separate the various materials. Recovered uranium can be returned to the conversion plant for conversion to uranium hexafluoride and subsequent re-enrichment. The reactor-grade plutonium can be blended with enriched uranium to produce a mixed oxide (MOX) fuel, in a fuel fabrication plant.
MOX fuel fabrication occurs at facilities in Belgium, France, Germany, United Kingdom, Russia, and Japan, with more under construction. There have been 25 years of experience in this, and the first large-scale plant, Melox, commenced operation in France in 1995. Across Europe, about 30 reactors are licensed to load 20–50% of their cores with MOX fuel and Japan plans to have one third of its 54 reactors using MOX by 2010.
The remaining 3% of high level radioactive wastes (some 750 kg per year from a 1000-MWe reactor) can be stored in a liquid form and subsequently solidified.
Reprocessing of used fuel occurs at facilities in Europe and Russia with capacity over 5000 tonnes per year and cumulative civilian experience of 90,000 tonnes over almost 40 years.
Vitrification
After reprocessing, the liquid high level waste can be calcined (heated strongly) to produce a dry powder, which is incorporated into borosilicate (Pyrex) glass to immobilize the waste. The glass is then poured into stainless steel canisters, each holding 400 kg of glass. A year's waste from a 1000-MWe reactor is contained in 5 tonnes of such glass, or about 12 canisters 1.3 m high and 0.4 m in diameter. These can be readily transported and stored with appropriate shielding.
This is as far as the nuclear fuel cycle goes at present. The final disposal of vitrified high-level wastes, or the final disposal of spent fuel, which has not been reprocessed used fuel, has not yet taken place.
Final Disposal
The waste forms envisaged for disposal are vitrified high-level wastes sealed into stainless steel canisters, or used fuel rods encapsulated in corrosion-resistant metals such as copper or stainless steel. All national policies intend either kind of canisters to be buried in stable rock structures deep underground. Many geological formations such as granite, volcanic tuff, salt, or shale are suitable. The first permanent disposal is expected to occur about 2020 (this date may still be questionable).
2.4 Evolution of Nuclear Power Generation
Early reactors – year 1950
Current reactors – year 1970
Evolutionary reactors – year 2000
Advance ecolutionary and passive reactors – year 2020
Other advanced technologies, including hydrogen generation
2.5 Nuclear Power in the United States
As of 2016, nuclear power in the United States is provided by 100 commercial reactors (66 pressurized water reactors (PWR) and 34 boiling water reactors) licensed to operate at 61 nuclear power plants, producing a total of 797.2 terawatt-hours of electricity, which accounted for 19.50% of the nation's total electric energy generation in 2015. As of 2016, there were four new reactors under construction with a gross electrical capacity of 5000 MW. The United States is the world's largest supplier of commercial nuclear power, and in 2013 generated 33% of the world's nuclear electricity.
In recent developments, there has been some revival of interest in nuclear power in the 2000s, with talk of a “nuclear renaissance,” supported particularly by the Nuclear Power 2010 Program. A number of applications was sought, and construction on a handful of new reactors began in the early 2010s, in late 2011 and early 2012, construction of four new nuclear reactor units at two existing plants were approved, the first such in 34 years. However, facing economic challenges, and later in the wake of the 2011 Japanese nuclear accidents, most of these projects have been canceled, and as of 2012, “nuclear industry officials say they expect just five new reactors to enter service by 2020—Southern's two Vogtle reactors, two at Summer in South Carolina and one at Watts Bar in Tennessee”; these are all at existing plants.
2.6 Plans for New Reactors Worldwide
Nuclear power capacity worldwide is increasing steadily, with over 60 reactors under construction in 13 countries.
2.7 Increased Capacity
Increased nuclear capacity in some countries is resulting from the uprating of existing plants. This is a highly cost-effective way of bringing on new capacity.
2.8 Nuclear Plant Construction
In all, about 160 power reactors with a total net capacity of some 177,000 MWe are planned and over 320 more are proposed. Energy security concerns and greenhouse constraints on coal have combined with basic economics to put nuclear power back on the agenda for projected new capacity in many countries.
2.9 Nuclear Plant Licensing
The Nuclear Regulatory Commission (NRC) is responsible for licensing and regulating the operation of commercial nuclear power plants in the United States. Currently operating nuclear power plants have been licensed under a two-step process described in Title 10 of the Code of Federal Regulations (10 CFR) under Part 50. This process requires both a construction permit and an operating license.
In an effort to improve regulatory efficiency and add greater predictability to the process, in 1989 the NRC established alternative licensing processes in 10 CFR Part 52 that included a combined license. This process combines a construction permit and an operating license with conditions for plant operation.
To construct or operate a nuclear power plant, an applicant must submit a safety analysis report. This document contains the design information and criteria for the proposed reactor and comprehensive data on the proposed site. It also discusses various hypothetical accident situations and the safety features of the plant that would prevent accidents or lessen their effects. In addition, the application must contain a comprehensive assessment of the environmental impact of the proposed plant. A prospective licensee also must submit information for antitrust reviews of the proposed plant.
When an application to construct a nuclear plant is received, the NRC staff determines whether it contains sufficient information to satisfy Commission requirements for a detailed review. If the application is accepted, the NRC holds a public meeting near the proposed site to familiarize the public with the safety and environmental aspects of the proposed application, including the planned location and type of plant, the regulatory process, and the terms for public participation in the licensing process. Numerous public meetings of this type are held during the course of the reactor licensing process.
The NRC staff then reviews the application to determine whether the plant design meets all applicable regulations (10 CFR Parts 20, 50, 73, and 100). The review includes, in part:
characteristics of the site, including surrounding population, seismology, meteorology, geology, and hydrology;
design of the nuclear plant;
anticipated response of the plant to hypothetical accidents;
plant operations including the applicant's technical qualifications to operate the plant;
discharges from the plant into the environment (i.e., radiological effluents); and
emergency plans.
When the NRC completes its review, it prepares a safety evaluation report summarizing the anticipated effect of the proposed facility on public health and safety.
A combined license authorizes construction of the facility much like a construction permit would under the two-step process (Part 50). It must contain essentially the same information required in an application for an operating license issued under 10 CFR Part 50 and specify the inspections, tests, and analyses that the applicant must perform. It also specifies acceptance criteria that are necessary to provide reasonable assurance that the facility has been constructed and will be operated in agreement with the license and applicable regulations. If the application does not reference an early site permit and design certification, then the NRC reviews the technical and environmental information as described for the two-step licensing process. There is also a mandatory hearing for a combined license.
2.10 Current Commercial Nuclear Plants
Pressurized Water Reactors
In a typical commercial pressurized light-water reactor, the core (1) inside the reactor vessel creates heat, (2) pressurized water in the primary coolant loop carries the heat to the steam generator, (3) inside the steam generator, heat from the steam, and (4) the steam line directs the steam to the main turbine, causing it to turn the turbine generator, which produces electricity. The unused steam is exhausted in to the condenser. The reactor's core contains fuel assemblies that are cooled by water circulated using electrically powered pumps. These pumps and other operating systems in the plant receive their power from the electrical grid. If off-site power is lost emergency cooling water is supplied by other pumps, which can be powered by on-site diesel generators. Other safety systems, such as the containment cooling system, also need power. PWRs contain between 150 and 200 fuel assemblies.
A typical PWR is shown inFigure 2.1.
Figure 2.1 Typical pressurized water reactor (courtesy of U.S. NRC).
Boiling Water Reactors
In a typical commercial boiling water reactor (see Figure 2.2), the steam–water mixture leaves the top of the core and it is sent to the main turbine. Boiling water reactors contain between 370 and 800 fuel assemblies.
Figure 2.2 Typical boiling water reactor.
The reactor's core contains fuel assemblies that are cooled by water circulated using electrically powered pumps. These pumps and other operating systems in the plant receive their power from the electrical grid. If off-site power is lost, emergency cooling water is supplied by other pumps, which can be powered by on-site diesel generators. Other safety systems, such as the containment cooling system, also need electric power.
2.11 Evolutionary Commercial Nuclear Plants
Westinghouse AP1000
The AP1000 is a two-loop PWR that uses a simplified, innovative approach to safety [5]. Its main characteristics are: a gross power rating of 3415 megawatt thermal (MWt) and a nominal net electrical output of 1117 MWe, the AP1000, with a 157-fuel-assembly core.
The AP1000 received final design approval from the U.S. NRC in September 2004 and design certification in December 2005.
The manufacturer claims simplifications in overall safety systems and increased safety margins.
Safety Features
Rather than relying on active components, such as diesel generators and pumps, the AP1000 relies on natural forces—gravity, natural circulation, and compressed gases—to keep the core and the containment from overheating.
Valves that automatically align and actuate the passive safety systems are utilized. These valves are designed to move to their safeguard positions upon loss of power or upon receipt of a safeguards actuation signal and are powered by Class 1E DC batteries.
The AP1000 passive safety systems include (see Figure 2.3) passive core cooling system (PXS), containment isolation, passive containment cooling system (PCS), and main control room emergency habitability system.
Figure 2.3 Westinghouse AP1000 containment and core cooling passive system (courtesy of Westinghouse).
The AP1000 PXS performs two major functions:
safety injection and reactor coolant makeup, utilizing the following sources:
core makeup tanks accumulators, in-containment refueling water storage tank, and in-containment passive long-term recirculation
passive residual heat removal, utilizing passive residual heat removal heat exchanger.
Safety injection sources are connected directly to two nozzles dedicated for this purpose on the reactor vessel. These connections have been used on two-loop plants.
The PCS consists of the following components: Air inlet and exhaust paths that are incorporated in the shield building structure. An air baffle that is located between the steel containment vessel and the concrete shield building. A passive containment cooling water storage tank that is incorporated in the shield-building structure above the containment. A water distribution system. An ancillary water storage tank and two recirculation pumps for on-site storage of additional PCS cooling water, heating to avoid freezing, and for maintaining proper water chemistry.
Natural Circulation
The PCS is able to effectively cool the containment following an accident such that the design pressure is not exceeded, and the pressure is rapidly reduced. The steel containment vessel itself provides the heat transfer surface that allows heat to be removed from inside the containment and rejected to the atmosphere. Heat is removed from the containment vessel by a natural circulation flow of air through the annulus formed by the outer shield building and the steel containment vessel it houses. Outside air is pulled in through openings near the top of the shield building and pulled down, around the baffle and then flows upward out of the shield building.
The flow of air is driven by the chimney effect of air heated by the containment vessel rising and finally exhausting up through the central opening in the shield-building roof.
Water Evaporation
If needed, the air cooling can be supplemented by water evaporation on the outside of the containment shell. The water is drained by gravity from a tank located on top of the containment shield building. Three normally closed, fail-open valves will open automatically to initiate the water flow if a high containment pressure threshold is reached. The water flows from the top, outside domed surface of the steel containment shell and down the side walls, allowing heat to be transferred and removed from the containment by evaporation. The water tank has sufficient capacity for 3 days of operation, after which time the tank could be refilled, most likely from the ancillary water storage tank. If the water is not replenished after 3 days, the containment pressure will increase, but the peak pressure is calculated to reach only 90% of design pressure. After 3 days, air cooling alone is sufficient to remove decay heat.
The containment vessel is a freestanding steel structure with a wall thickness of 1.75 inches (4.44 cm). The containment is 130 ft (39.6 m) in diameter.
The primary containment prevents the uncontrolled release of radioactivity to the environment. It has a design leakage rate of 0.10 weight percent per day of the containment air during a design basis accident and the resulting containment isolation.
