NUCLEARENGINEERINGTheory and Technology

of Commercial Nuclear Power

RONALD ALLEN KNIEFMechanicsburg, Pennsylvania

American Nuclear Society, Inc.555 North Kensington AvenueLa Grange Park, Illinois 60526 USA

Library of Congress Cataloging-in-Publication Data

Knief, Ronald Allen, 1944–Nuclear engineering : theory and technology of commercial nuclear power 0 Ronald AllenKnief. – 2nd ed.

p. cm.Includes bibliographical references and index.ISBN 0-89448-458-31. Nuclear engineering. 2. Nuclear energy. I. Title.

TK9145.K62 2008621.48–dc22

2008029390

ISBN-10: 0-89448-458-3ISBN-13: 978-0-89448-458-2

Library of Congress Catalogue Card Number: 2008029390ANS Order Number: 350023

© 2008 American Nuclear Society, Inc.555 North Kensington Avenue

La Grange Park, Illinois 60526 USA

All rights reserved. No part of this book may be reproduced in any formwithout the written permission of the publisher.

Printed in the United States of America

CONTENTS

Foreword to the First EditionPreface xvPreface to the First Edition

Overview

1 Introduction 3Nuclear Fuel Cycles 4Nuclear Power Reactors 10Exercises 20Selected Bibliography 22

II Basic Theory

2 Nuclear Physics 27The Nucleus 28Radioactive Decay 31Nuclear Reactions 36Nuclear Fission 41Reaction Rates 46Exercise 63Selected Bibliography 64

xiii

XVll

3 Nuclear Radiation Environment 67Interaction Mechanisms 69Radiation Effects 72Dose Estimates 79

vii

viii Contents

Radiation Standards 87Exercises 94Selected Bibliography 96

4 Reactor Physics 99Infinite Systems 100Finite Systems 108Computational Methods 113Exercises 131Selected Bibliography 133

5 Reactor Kinetics and Control 135Neutron Multiplication 136Feedbacks 145Control Applications 151Exercises 157Selected Bibliography 159

6 Fud Depletion and Related Effects 161Fuel Burnup 162Transmutation 163Fission Products 170Operational Impacts 176Exercises 181Selected Bibliography 182

7 Reactor Energy Removal 185Power Distributions 187Fuel-Pin Heat Transport 191Nuclear Limits 197Exercises 205Selected Bibliography 206

III Nuclear Reactor Systems

8 Power Reactors: Economics and Design Principles 211Economics of Nuclear Power 212Reactor Design Principles 226Reactor Fundamentals 231Exercises 237Selected Bibliography 240

9 Reactor Fuel Design and Utilization 241Fuel-Assembly Design 242Utilization 254Exercises 259Selected Bibliography 260

10 Light-Water ReactorsBoiling-Water ReactorsPressurized-Water ReactorsExercises 282Selected Bibliography

261262

268

284

Contents ix

11 Heavy-Water-Moderated and Graphite-ModeratedReactors 287Heavy-Water-Moderated Reactors 288Graphite-Moderated Reactors 296Exercises 308Selected Bibliography 310

12 Enhanced-Converter and Breeder Reactors 313Spectral-Shift Converter Reactors 315Thermal-Breeder Reactors 317Fast Reactors 321Exercises 332Selected Bibliography 333

IV Reactor Safety

13 Reactor Safety Fundamentals 337Safety Approach 338Energy Sources 340Accident Consequences 343Exercises 355Selected Bibliography 356

14 Reactor Safety Systems and Accident Risk 359Engineered Safety Systems 360Quantitative Risk Assessment 384Advanced Reactors 404Exercises 410Selected Bibliography 413

15 Reactor Operating Events, Accidents, and Their Lessons 417Significant Events 419TMI-2 Accident 423Chernobyl Accident 450Common Accident Lessons 467Exercises 468Selected Bibliography 472

16 Regulation and Administrative Guidelines 475Legislation and Its Implementation 476Reactor Siting 480Reactor Licensing 487

x Contents

Administrative Guidelines 495Exercises 500Selected Bibliography 503

V The Nuclear Fuel Cycle

17 Fuel Cycle, Uranium Processing, and Enrichment 507Nuclear Fuel Cycle 508Uranium 513Exercises 532Selected Bibliography 533

18 Fuel Fabrication and Handling 535Fabrication 536Fuel Recycle 541Spent Fuel 546Exercises 553Selected Bibliography 557

19 Reprocessing and Waste Management 559Reprocessing 560Fuel-Cycle Wastes 566Waste Management 573Exercises 593Selected Bibliography 596

20 Nuclear Material Safeguards 599Special Nuclear Materials 601Domestic Safeguards 604International Safeguards 618Fuel-Cycle Alternatives 625Exercises 628Selected Bibliography 630

VI Nuclear Fusion

21 Controlled Fusion 635Fusion Overview 636Magnetic Confinement 643Inertial Confinement 650Commercial Aspects 655Non-Thermonuclear Fusion 659Exercises 661Selected Bibliography 662

Contents xi

Appendixes

I Nomenclature 667

II Units and Conversion Factors 671

III The Impending Energy Crisis: A Perspective on the Needfor Nuclear Power 677Energy Crisis 678Options 683Proposed Solutions 694Exercises 698Selected Bibliography 702

IV Reference Reactor Characteristics 707

Answers to Selected Exercises 719General Bibliography 721Index 747

OVERVIEW

Goals1. To introduce the basic concepts of both the nuclear fuel cycle and the world'ssix major nuclear power reactor systems

2. To provide a context for a better understanding of the theoretical conceptspresented in Part II

Chapter in Part IIntroduction

1INTRODUCTION

ObjectivesAfter studying this chapter, the reader should be able to:

1. Explain the two advantages and the two disadvantages of fission as an energysource.

2. Arrange in sequence and describe the intent of each process step of the com-mercial nuclear fuel cycle.

3. Explain the concept of and physical basis for recycling of nuclear fuel. Dis-tinguish between open and closed fuel cycles.

4. Describe the role of each of the following support activities in the nuclear fuelcycle: transportation, nuclear safety, and nuclear material safeguards.

5. Explain the following terms as they apply to classification of nuclear reactorsystems: coolant, number of steam-cycle loops, moderator, neutron energy,and fuel production. State the full name and classify in these terms each ofthe six reference reactor types: BWR, PWR, CANDU-PHWR, PTGR, HTGR,and LMFBR.

6. Identify the four major elements of reactor multiple-barrier containment forfission products. Describe the fuel assembly employed by each of the referencereactor types and explain how it provides the first two of the barriers.

7. Perform basic calculations related to fuel-cycle material mass balance andenergy equivalence.

The current basis for commercial application of nuclear energy is the fission process.Figure 1-1 shows a neutron striking an atom of uranium-235 [235U] to produce a

3

4 Overview

b"AH"ONJNEUTRON @-----cJ-----

? H -pNEUTRON 23S U ATOM FISSION FISSION FRAGMENTS

FIGURE 1-1Fission of uranium-235 by a neutron.

fission, or splitting of that atom. From the standpoint of energy production, the reactionhas the major advantage that each such splitting provides nearly one hundred million[100,000,000] times as much energy as the "burning" of one carbon atom in a fossilfuel. The production of more neutrons from fission allows the process to participatein a chain reaction for continuous energy production in a device called a reactor. Amaterial that can produce a self-sustaining chain reaction by itself is said to be fissile.Other fissionable and fertile materials can contribute to a chain reaction without beingable to sustain one by themselves. When the reaction is exactly balanced in a steady-state condition, the system is said to be critical.

One major disadvantage of using the process as an energy source is the generationof radiation at the time of fission. Another problem is the presence of the fissionfragments, which are radioactive and will themselves give off radiation for varyingperiods of time after the fission events.

These characteristics each have major impacts on the design and operation ofnuclear fission systems. The six chapters in the second part of this book treat the basictheories and principles that contribute to the ultimate utilization of fission energy. Theremaining parts then build on this framework to provide descriptions of the designand operation of nuclear fission reactors, administrative aspects of nuclear energy, andnuclear fuel cycle. The final part of the book considers nuclear fusion, which haslong-term prospects as a commercial.energy source.

Since theory and practice interact thoroughly, an overview of the current de-velopment of commercial nuclear power can aid the understanding of the basic un-derlying principles. Thus, the remainder of this chapter provides a brief overview ofthe nuclear fuel cycle and current reactor designs. The reader should note that onlybasic understanding, and not thorough knowledge, is expected at this stage, since eachand every definition and concept is clarified and treated in greater detail in laterchapters.

NUCLEAR FUEL CYCLESThe production of energy from any of the current fuel materials is based on a fuelcycle. Typical cycles, such as those for the fossil fuels, consist of at least the followingcomponents:

• exploration to identify the compositions and amounts of a resource available atvarious locations

Introduction 5

• mining or drilling to bring the resource to the earth's surface in a usable form• processing or refining to convert raw materials into a final product• consumption of the fuel for energy production• disposal of wastes generated in all portions of the cycle• transportation of materials between the various steps of the cycle

The nuclear fuel cycle is substantially more complicated for the followingreasons:

l. 235U, which is the only practical naturally occurring fissile material, is less than Ipercent abundant in uranium deposits (the remaining uranium is mostly non-fissile238U).

2. Two other fissile materials, 233U and 239pu [plutonium-239J, are produced byneutron bombardment of 232Th [thorium-232] and 238U, respectively. (For thisreason, the latter two materials are said to be fertile.)

3. All fuel cycle materials contain small to large amounts of radioactive constituents.4. A neutron chain reaction [criticality] could occur outside a reactor under appropriateconditions.

5. The same chain reaction that can be used for commercial power generation alsohas potential application to a nuclear explosive device.

Each of these five concerns is considered in the following paragraphs and later in thebook as related to the structure of the nuclear fuel cycle.

Uranium Fuel CycleA schematic representation of a generic nuclear fuel cycle is shown in Fig. 1-2. Theuranium fuel cycle described here is used by the light-water reactor [LWR] systemsthat dominate worldwide nuclear power. Variations, including the introduction ofthorium, are considered in the next section.

Transportation between the various steps of the fuel cycle is indicated by thearrows in Fig. 1-2. Waste is necessary in all steps of the cycle, but is shownexplicitly only for the two major contributors-spent fuel and high-level reprocessingwastes.

Nuclear safety, which is charged with protecting operating personnel and thepublic from potentially hazardous materials in the fuel cycle, must be superimposedon appropriate portions of the cycle. Also superimposed are material safeguards toprevent use of fuel cycle materials for nuclear explosives.

The steps preceding reactor use, which generally have little radioactivity, areoften considered to form the front end of the fuel cycle. Those steps that follow reactoruse are characterized by high radiation levels and constitute the back end of the cycle.

ExplorationThe exploration process typically begins with geologic evaluation to identify potentialuranium deposits. Areas that have characteristics similar to those of known contentusually receive first consideration. The actual presence of uranium may be verifiedby chemical and/or radiological testing.

Drilling into the deposit accompanied by detailed analysis ofthe samples providesinformation on uranium ore composition and location. Only after completion of a verydetailed mapping of the ore body will mining operations begin.

6 Overview

FRONT END BACK ENDI

High-LevelWaste

- DISPOSAL

RECYCLE

MINING

j;.Th fhLMILLING

EXPLORATION

ENRICHMENT

UDUFs--PROCESSINGDU30 S

#--?

,,,, A,,,mblv REACTOR INTE§RAGE

"'u D.FUEL FABRICATION II -

DEnr UFsPlutonium

FIGURE 1-2Nuclear fuel-cycle material flow paths.

MiningUranium is mined either by surface [open pit] or underground operations. Majorresources are located in Africa, Australia, Canada, the western United States, and theU.S.S.R.

Economically viable uranium ore deposits assay from 0.20-0.25 wt % [weightpercent] U30 g equivalent to over 12 wt % at one deposit in Canada. Even at the lowestof the assays, however, uranium ore is 30-50 times more efficient than coal on thebasis of energy per ton mined. Because many environmental impacts are proportionalto the amount of ore removed, nuclear energy has advantages over coal in this regard.

MillingThe milling operation removes uranium from the ore by a combination of chemicaland physical operations. One method employs the following steps:

Introduction 7

• crushing and grinding of ore to smaller, relatively uniform size• leaching in acid to dissolve the metals away from predominantly nonmetal ore content• ion-exchange or solvent-extraction operations to separate uranium from other metals• production of U30 S, usually in the form of yellow cake, so named because of itscolor

The major problems associated with milling operations are related to chemical effluentsand natural radioactivity in the ore residues [tailings].

Conversion and EnrichmentNatural uranium is composed of two isotopes-fissile 235U (0.711 wt %) and fissionable238U (99.3 wt %)-which cannot be separated by chemical means. Because manyreactor concepts require that the 235U fraction of the total uranium content be higherthan this, enrichment-separation of the isotopes by physical means-has been im-plemented.

The conversion step begins by purifying the U30 S [yellow cake]. Then, throughchemical reaction with fluorine, uranium hexafluoride [UF6] is produced.

UF6-a gas at temperatures above 56°C [134°F] at atmospheric pressure-isreadily employed in one of several enrichment schemes. The gaseous diffusion methodthat has 'been the world's "workhorse" is based on forcing UF6 against a porousbarrier. The lighter 235UF6 molecules penetrate the barrier more readily than do theheavier 23SUF6 molecules. (According to the kinetic theory of gases, each moleculehas the same average kinetic energy, so that greater speed, and thus barrier penetrationprobability, belongs to the lighter molecule.) By cascading the barrier stages, anydesired enrichment can be obtained. At the present time, slightly enriched uranium at2-4 wt % 235U is produced for LWR use. The uranium left behind in the process iscalled the depleted stream [enrichment tails] and is typically 0.2-0.35 wt % 235U.

The currently popular, and more energy-efficient, gas-centrifuge method alsouses UF6. Here the heavier 23SU isotope is driven to the outside of a rapidly rotatingcylinder while the lighter 235U remains near the axis. Again, cascading individualstages allows production of desired enrichments, including those for LWR use.

Atomic vapor laser isotope separation [AVLlSj relies on atomic or nuclearstructure differences between the isotopes rather than on their tiny mass differential.This and other laser-based methods are subject to significant research and developmentactivity because, among other features, they offer the prospect of very high single-stage separation.

FabricationThe fabrication step of the cycle produces fuel in the final form that is used for powerproduction in the reactor. LWR fabrication begins by converting the slightly enricheduranium hexafluoride to uranium dioxide [U02]-a black ceramic composition. TheU02 powder is then formed into cylindrical pellets roughly the size of a thimble.

The pellets are loaded into long cladding tubes to form individual fuel pins. Thefinal fuel assembly consists of an array of fuel pins plus some other hardware. Fuelassemblies for the light-water and other reactor systems are described in more detaillater in this chapter.

Reactor UseThe completed fuel assemblies are loaded into the reactor core, where the fissionchain reaction is initiated to generate heat energy. As fissions occur, the 235U atoms

8 Overview

are consumed. An amount of 239pU is produced as 238U absorbs some of the extraneutrons. The buildup of fission fragments and their radioactive products tends toproduce a "poisoning" effect by absorbing neutrons that could otherwise participatein the chain reaction. Because the loss of 235U and the poison effect dominate overPu production, the fuel must eventually be replaced as it becomes unable to sustain achain reaction.

Traditional practice has been to replace one-quarter to one-third of the fuelassemblies in the reactor core on a roughly annual cycle. More recently, some reactorshave begun to use 18- to 24-month cycles. By using careful fuel management, fuelassemblies are shuffled to maximize the energy extracted from each during its 3-4years in the reactor.

Interim Spent Fuel StorageSince the fuel assemblies are very highly radioactive when they are discharged fromthe reactor, they are allowed to "cool" for a period of time in a water basin. Spentfuel may be stored at the reactor site or in a special off-site facility for an indefiniteperiod of time (as is currently the situation in the United States). It may also be shippedto a reprocessing facility, usually after at least 90 days of storage.

ReprocessingIn spent fuel processing [reprocessing], the residual uranium and the plutonium areextracted for further use in the fuel cycle. The fission-product and other wastes producedare handled in the waste disposal step.

In the initial steps of the reprocessing operations, the fuel assemblies are me-chanically disassembled (i.e., chopped into small pieces) and dissolved in acid. Theuranium and plutonium are separated from the wastes, then separated from each other.The large amounts of highly radioactive byproducts contained in the spent fuel ne-cessitate very stringent environmental controls for the processing steps and the storageof wastes.

RecycleThe residual uranium and the plutonium extracted from the spent fuel by the repro-cessing operation may be reintroduced into the fuel cycle. Use of these recycledmaterials can reduce uranium resource requirements by up to 25 percent.

The residual uranium is returned to the fuel cycle for reenrichment, as shownin Fig. 1-2. The plutonium is transported to the fabrication operation where it is mixedwith natural or depleted uranium to produce a mixed oxide [PU02 + U02] with afissile content [effective enrichment] roughly comparable to that of slightly enricheduranium. France, for example, is routinely recycling plutonium. Japan and countriesin western Europe also have plans to do so.

Waste DisposalAll steps of the fuel cycle (including the waste disposal step itself) produce someamounts of radioactive waste. Near-surface burial of the "low-level" wastes from thefront end of the fuel cycle is generally appropriate.

Spent fuel assemblies are a waste form at the time of their discharge from areactor. If reprocessing is implemented, the assemblies' waste contents are convertedto "high-level" liquid wastes. These liquids are stored for an interim period (nominally

Imroduction 9

above five years) and then solidified, usually in a vitrified, glass-like form. Finaldisposal of spent fuel or solid high-level waste is most likely to be in a stable geologicformation.

TransportationSince the various fuel cycle operations take place at a number of different locations,transportation is a very important component. Effective transportation systems aredesigned and operated to minimize the risks of:

• release of dangerous chemical or radioactive materials to the environment• accidental nuclear chain reaction outside of a reactor core• damage to expensive components• theft of valuable and potentially dangerous materials

Based on the nature of these risks, specially designed containers and/or vehicles maybe used between various steps of the fuel cycle.

Nuclear SafetyNuclear safety in fuel cycle facilities is usually divided into categories of radiationsafety and nuclear criticality safety. The former includes shielding and containmentof radiation sources plus effluent control to minimize exposures to operating personneland the general public.

Reactors are designed to handle the effects of a fission chain reaction while fuelcycle facilities generally are not. Nuclear criticality safety is charged with preventionof such chain reactions in all environments outside of reactor cores. Because accidentalcriticality is not credible for natural uranium, these safety concerns begin at the en-richment step (Fig. 1-2).

Material SafeguardsAll fissile materials have potential use for nuclear explosives and must, therefore, besafeguarded against theft or diversion. Physical-security and material-accountancysystems are designed to minimize the terrorist threat for theft by a subnational group.International safeguards based on inventory verification have been developed to deterproliferation, i.e., diversion by a nation for the purpose of acquiring nuclear weaponscapability.

Safeguard measures should be commensurate with the risks perceived for givenmaterials. The slightly enriched uranium in the LWR fuel cycle, for example, couldonly be used for a nuclear explosive if it were enriched further. The extreme complexityof the enrichment technology makes implementation of the required clandestine op-erations highly unlikely.

Because spent fuel contains fissile plutonium that can be separated chemically,it is a somewhat more attractive target. Only a national effort, however, would belikely to handle the complexity and hazard (as well as detectability) of reprocessingoperations.

By contrast, recycle with the presence of separated plutonium would offer thebest theft target for the terrorist or other subnational groups. Material safeguard mea-sures, therefore, should be most stringent for this portion of the fuel cycle.

10 Overview

Other Fuel CyclesOther reactor concepts (e.g., as described in the next section) employ fuel cycles thathave many similarities to the LWR cycle just considered. The generic fuel cycle inFig. 1-2 encompasses the options.

The greatest differences occur for systems that use thorium. Because the mainconstituent is 232Th, from which fissile mU is produced, the conversion and enrichmentsteps are not required for thorium. The reprocessing step, of course, must be capableof separating mU from 232Th and the wastes. As described later in this chapter, thereis a unique fuel assembly design for the high-temperature gas-cooled reactor [HTGR],which allows 233U and 235U to be separated mechanically and recycled directly withoutrequiring enrichment technology.

Uranium enrichment requirements vary from use of unenriched, natural uraniumin the pressurized-heavy-water reactor [PHWR] to from 20 to 93 wt % 235U in theHTGR. The liquid-metal fast-breeder reactor [LMFBR] uses depleted uranium-i.e.,enrichment tails-plus plutonium as its fuel material.

"Symbiotic" or "cross-progeny" fuel cycles are based on interchange of variousfuel materials among two or more different reactor types. One such possibility is theexchange of plutonium between LWR and LMFBR systems.

Material safeguards are required for all separated plutonium and for separateduranium whenever it has greater than 20 wt % enrichment in 233U and/or 235U. Thespent fuel usually may be protected at a somewhat lower level because reprocessingwould be required to obtain the fissile content.

NUCLEAR POWER REACTORSAll nuclear reactors are designed and operated to achieve a self-sustained neutron chainreaction in some combination of fissile, fissionable, and other materials. The powerreactors use the fission process for the primary purpose of producing usable energyin the form of electricity.

Common characteristics of power reactors, which are used for classificationpurposes, include:

I. Coolant-primary heat extraction medium, including secondary fluids (if any)2. Steam cycle-the total number of separate coolant "loops," including secondary

heat transfer systems (if any)3. Moderator-material (if any) used specifically to "slow down" the neutrons pro-

duced by fission4. Neutron energy-general energy range for the neutrons that produce most of the

fissions5. Fuel production-system is referred to as a breeder if it produces more fuel (e.g.,

fissile 239pU from fertile 238U) than it consumes; it is a converter otherwise

The first two features relate to the current practice of converting fission energy intoelectrical energy by employing a steam cycle.

Neutrons from fission are emitted at high energies. However, neutrons at verylow energies have a higher likelihood of producing additional fissions. Thus, manysystems employ a moderator material to "slow down" the fission neutrons. Neutrons

Introduction II

with very low energies (roughly in equilibrium with the thermal motion of surroundingmaterials) are called thermal neutrons, with the slowing down process sometimescalled thermalization. Neutrons at or near fission energies are fast neutrons.

Any reactor that contains fertile materials will produce some amount of newfuel. The major distinction between breeder and converter reactors is that the formeris designed to produce more fuel than is used to sustain the fission chain reaction. Bycontrast, the converter replaces only a fraction of its fissile content.

Six reference nuclear power reactor designs are currently employed in the world.Important examples of these are identified in Table I-I and classified on the basis ofthe reactor characteristics noted above. (A substantially expanded version of the tableis contained in App. IV.) The remainder of this chapter considers each of the referencedesigns in some detail.

Steam CyclesMost of the world's electric power is generated via a steam cycle. Water in a boileris heated to produce steam by burning fossil fuel. The steam then turns a turbine-generator set to produce electricity. Condenser cooling water is used to condensesteam in the turbine back to liquid water and, thereby, enhance net conversion effi-ciency.

Nuclear steam cycles have many of the same features as the fossil-fuel case.The major conceptual difference is that the fission-energy heat source is a fixed-geometry fuel core located physically inside the boiler, or reactor pressure vessel. Intwo of the reactor designs, steam is produced directly in the core; in the others, heatis transferred from the core to generate steam in a secondary or tertiary system.

In the single-loop, direct-cycle reactor systems, water coolant flows through thefuel core and acquires an amount of energy sufficient to produce boiling, and thussteam, within the reactor vessel. Both the boiling-water reactor [BWR] and the pres-sure-tube graphite reactor [PTGR] use a steam cycle similar to that shown in Fig.1-3.

TABLE 1-1Basic Features of Six Reference Reactor Types

Pressure- High-Boiling- tube Pressurized- Pressurized- temperature Liquid-metalwater graphite water heavy-water gas-cooled fast-breederreactor reactor reactor reactor reactor reactor

Feature [BWR] [PTGR] [PWR] [pHWRj [HTGR] [LMFBR]

Steam cycle/coolant(s)

Number of loops I I 2 2 2 3Primary coolant Water Water Water Heavy water Helium Liquid sodiumSecondary coolant(s) Water Water Water Liquid sodium/

water

Moderator Water Graphite Water Heavy water GraphiteNeutron energy Thermal Thermal Thermal Thermal Thermal FastFuel production Converter Converter Converter Converter Converter Breeder

/2 Overview

REACTOR STEAM LINE

CORE

PUMP

TURBINE-GENERATOR

CONDENSER COOLINGWATER

FIGURE 1-3Direct, single-loop steam cycle. (Adapted courtesy of U.S. Department of Energy.)

The indirect-cycle reactors maintain high-pressure conditions to prevent boilingin the vessel. Instead, the heat acquired from the core by the coolant is carried to aheat exchanger. Three of the reactor concepts-pressurized-water reactor [PWR],pressurized-heavy-water reactor [PHWR], and high-temperature gas-cooled reactor[HTGR]-employ a two-loop steam cycle, as shown in Fig. 1-4. As the name suggests,the steam generator is a heat exchanger that produces steam for the turbine-generator.It may be noted that steam generators in these systems play the same role of heatsources as does the fossil-fuel boiler or BWR vessel.

As the name implies, the pressurized-water reactor relies on high pressure tomaintain water in a liquid form within its primary loop. Despite the steam cycledifference between the PWR and BWR, they have many design similarities resultingfrom the use of ordinary water as their coolant and moderator. The two are, therefore,grouped together as the light-water reactors [LWR] (whose nuclear fuel cycle wasdiscussed earlier in this chapter).

The PHWR uses heavy water as coolant in a cycle that is otherwise similar tothat of the PWR. The HTGR employs helium gas as its primary coolant.

REACTOR

PRIMARY LOOP

CORE

PUMP

STEAM LINETURBINE-GENERATOR

CONDENSER COOLINGWATER

STEAMGENERATOR

FIGURE 1-4Two-loop steam cycle. (Adapted courtesy of U.S. Department of Energy.)

Introduction /3

The liquid-metal fast-breeder reactor [LMFBR] is based on a three-loop steamcycle, as shown in Fig. 1-5. Primary and secondary liquid-sodium loops are connectedby an intermediate heat exchanger. The secondary sodium loop transfers energy to thesteam generator.

The steam cycles are one aspect of integrated reactor system concepts. Moredetailed information is postponed to Chaps. 8-12.

ModeratorsWith the exception of the LMFBR, the remaining five reactors use moderator materialto reduce fission-neutron energies to the thermal range. Light elements are found tobe the most effective moderators, as described more fully in Chap. 4.

In the two LWR types, the coolant water also serves as the moderator. ThePHWR uses separate supplies of heavy water for the coolant and the moderator. Carbonin the form of graphite serves as the moderator for the PTGR and HTGR.

The LMFBR concept is based on a chain reaction with fast neutrons. Thus, therelatively heavy liquid-sodium coolant was purposely selected to minimize moderationeffects and support breeding, as described in Chap. 6.

Reactor FuelThe designs for the fuel used in the reference reactors are as varied as the steam cyclesand moderators. In one system the fuel and moderator form an integral unit, while inthe others these constituents are separated. The features of fuel assemblies-the unitsultimately loaded into the reactor vessel-for each system are summarized in Table1-2. (Appendix IV contains more detailed information, including representative di-mensions; design, use, and fabrication of the fuel are explained in Chaps. 9 and 18.)

Since the fission process creates radioactive products, reactor systems must bedesigned to minimize the risk of release of these potentially hazardous materials tothe general environment. The philosophy of multiple barrier containment has evolvedfrom this requirement.

As a first barrier, the fuel is formed into particles designed for a high degree offission product retention. The second barrier is typically an encapsulation capable of

TURBINE-GENERATOR

IPUMP

SECONDARY STEAM LINELOOP

PRIMARYLOOP

HEAT EXCHANGER

STEAM GENERATOR

PUMP

CORE

REACTORCONDENSER COOLING

PUMP WATER

CONDENSER

FIGURE 1-5Three-loop steam cycle. (Adapted courtesy of U.S. Department of Energy.)

TABLE 1-2Characteristics of the Fuel Cores of Six Reference Reactor Types t

Pressure-tube Pressurized-heavy- High-temperature Liquid-metalBoiling-water Graphite reactor Pressurized-water water reactor gas-cooled reactor fast-breeder

Component reactor [BWR] [PTGR] reactor [PWR] [PHWR] [HTGR]+ reactor [LMFBR]

Fuel particle(s)Geometry Short, cylindrical Short, cylindrical Short, cylindrical Short, cylindrical Multiply coated Short, cylindrical pellet

pellet pellet pellet pellet microspheresChemical form U02 U02 U02 U02 UC/ThC Mixed oxides U02 and

PU02Fissile 2-4 wt % 235U 1.8-2.4 wt % 235U 2-4 wt % 235U Natural uranium 20-93 wt % 235U 10-20 wt % Pu

microsphereFertile 238U 238U 238U 238U Th microsphere 238U in deplete-i U

Fuel pins Pellet stacks in Pellet stacks in Pellet stacks in Pellet stacks in Microsphere Pellet stacks in medium-long Zr-alloy long Zr-alloy long Zr-alloy short Zr-alloy mixture in short length stainless steelcladding tubes cladding tubes cladding tubes cladding tubes graphite fuel stick cladding tubes

Fuel assembly 8 x 8 square array 18-pin concentric- 16 x 16 or 17 x 37-pin concentric- Hexagonal graphite Hexagonal array of 271of fuel pins circle arrangement 17 square array of circle arrangement block with fuel pins

fuel pins stacked fuel sticksReactor core §Axis Vertical Vertical Vertical Horizontal Vertical VerticalNumber of fuel I 2 1 12 8 Iassemblies along axis

Number of fuel 748 1661 193-241 380 493 364 driver, 233 blanketassemblies in radial array

t More detailed data and references are contained in App. IV.*The HTGR fuel geometry is different from that of the other reactors, leading to some slightly awkward classifications.§All of the cores approximate right circular cylinders. Fuel assemblies are loaded and/or stacked lengthwise parallel to the axis of the cylinder.

Introduction 15

holding those products that do escape from the fuel. The integrity of the reactor vesseland primary-coolant loop form a third barrier. One or more containment structuresform the final line of defense against the release of radioactivity. These last two barrierslocated outside of the reactor core (along with associated safety systems and personnel-oriented administrative practices) are considered in Chaps. 10-14 and 16.

Since uranium dioxide [U02 ] and uranium carbide [UC] are relatively denseceramic materials with good ability to retain fission products, they are favored first-barrier compositions. Encapsulation of the particles in metal tubes or in other coatingsprovides the second barrier. Both features are incorporated in the fuel-assembly designsfor all of the reference reactors.

Light-Water ReactorsThe fuel assemblies for the two types of light-water reactor [LWR] are very similar.Slightly enriched uranium dioxide is fabricated into the form of short, cylindrical fuelpellets. The pellets are then loaded into long zirconium-alloy cladding tubes to producefuel pins or fuel rods. A rectangular array of the pins forms the final fuel assembly,or fuel bundle.

The fuel assembly for the boiling-water reactor [BWR] is shown in Fig. 1-6.The individual fuel pins consist of the clad tube, the fuel pellet stack or "active" fuelregion, a retention spring, and welded end caps.

Upper and lower tie plates plus interim spacers secure the fuel pins into a squarearray with eight pins on a side. The fuel channel encloses the fuel pin array, so thatcoolant entering at the bottom of the assembly will remain within this boundary as itflows upward between the fuel pins, boils, and removes the fission energy.

URANIUMDIOXIDEFUEL PELLET

__/ FASTENERASSEMBLY

I I, \I I

UPPER ...TIE PLATE'

.

h ...",-- JJ-

. -- FUEL CHANNEL

- LOWER TIE PLATE

UPPERTIE PLATE

FUEL RODINTERIMSPACER

BAIL HANDLE

FUEL BUNDLE

FINGER SPRING(TYPICAL of 4)

FIGURE 1-6Fuel assembly for a representative boiling-water reactor. (Adapted courtesy of General ElectricCompany.)

/6 Overview

The fuel assembly for the pressurized-water reactor [PWR] is shown in Fig.1-7. The fuel pins are similar to those for the BWR.

The PWR assembly, typically of 16 x 16 or 17 x 17 pins, is larger than thatof the BWR. The fuel-pin array, and its interspersed non-boiling coolant, is not enclosedby a fuel channel. (Additional detail on LWR fuel is provided is Chaps. 9 and 10.)

Pressure-Tube Graphite ReactorThe fuel assembly shown in Fig. 1-8 is that of the Soviet pressure-tube graphite reactor[PTGR] known as RBMK-1000. The fuel pins consist of slightly enriched uraniumin U02 pellets clad in zirconium alloy quite similar to those in the LWR designs.

The cylindrical fuel assemblies are designed to fit into pressure tubes. Watercoolant is introduced to a fuel assembly as liquid, boils in removing fission energy,and is discharged as steam.

The tubes are distributed throughout a core built up of the graphite blocks thatserve as the reactor's moderator. Coolant feeder pipes, valves, and the pressure tubesare arranged to allow refueling during full-power operation. (Additional detail is pro-vided in Chaps. 9 and II.)

ALIGNMENT POST

UPPEREND FITTING

SGPRAICDER ;aJ. j;W;;; CEA--.I : GUIDE TUBE

11'''"''"'\ '.:.:.: I_ASSEMBLY

_ FUEL RODS116x16 ARRAYI

- . - .. - . -o C:: 0 Ij J G g 'J C C. _ ....

TOP VIEW

BOTTOM VIEW

UPPEREND CAP

SPRING

SPACER

URANIUMDIOXIDEPELLETS

ZIRCALOYFUELCLADDING

SPACER

LOWEREND CAP

I

FIGURE 1-7Fuel assembly for a representative pressurized-water reactor. (Adapted courtesy of CombustionEngineering, Inc.)

Introduction 17

I,.1ill'.... 1

III iIii iIi ill

7, .!.. 20,T

JB

, - Suspension2 - Pin3 - Adapter4 - Shank5 - Fuel Element6 - Carrier Aod7 - SleeveB - End Cap9 - Nuts

FIGURE 1-8Fuel assembly for a pressure-tube graphite reactor. (From NUREG-1250, 1987.)

Pressurized-Heavy-Water ReactorThe fuel assembly shown in Fig. 1-9 is that of the pressurized-heavy-water reactor[PHWR] known as CANDU-PHW [Canada Deuterium Uranium-Pressurized HeavyWater]. The terms PHWR and CANDU are often used interchangeably.

The cylindrical CANDU fuel bundles are designed for insertion into pressuretubes through which the primary coolant flows. These tubes penetrate a large vessel,which contains the separate heavy-water moderator. (The specific design is treated insome detail in Chap. II.)

The fuel pins consist of natural uranium in U02 pellets clad in zirconium alloy.Since these short pins do not have to be free-standing (as is the case for the LWR's),the clad is quite thin. The interelement spacers serve to separate the pins from eachother, while the bearing pads separate the bundle from the pressure tube.

High-Temperature Gas-Cooled ReactorThe conceptual design for a high-temperature gas-cooled reactor [HTGR] describedin Table 1-2 is comparable in thermal output to most of the other reference reactors.It is one representative of a variety of gas-cooled, graphite-moderated reactors around

/8 Overview

END VIEW

, TUBE

FIGURE 1-9Fuel assembly for a representative CANDU pressurized-heavy-water reactor. (Adapted courtesy ofAtomic Energy of Canada Limited.)

the world. Another state-of-the-art system using the same fuel particle form is thethorium high-temperature reactor [THTR], or "pebble-bed" reactor. These reactorsare described in Chap. II, and a related advanced reactor in Chap. 14.

The HTGR has a unique fuel design, as shown in Fig. 1-10. The basic units aretiny particles of uranium carbide or of thorium carbide surrounded by various coatings.Separate fissile and fertile particles, or microspheres, are used to facilitate the repro-cessing separations described in Chap. 19.

As originally conceived, the fissile microsphere contains a small (approximately0.2 mm) kernel of highly enriched 235U in the form of uranium carbide. Employingthree different types of coatings designed for fission produce retention, it carries thename TRISO.

The fertile microsphere has a core of thorium carbide. The version shown inFig. 10 is the double-coated BISO. Other types of microspheres may also be used,especially if fissile 233U, produced from fertile 232Th, becomes available.

The microspheres may be mixed to provide any desired "effective fissile en-richment. " A composition of 5 wt % 235U would be typical for a new HTGR. Agraphite resin binder is used to form the microsphere mixture into small (roughlyfinger-sized) fuel rods.

The fuel rods are loaded into holes in a hexagonal graphite block to form thefuel assembly shown in Fig. 1-10. The block also contains holes for helium coolantflow. In the HTGR design the fuel and moderator are contained in an integral unit.

Introduction 19

235FUEL TRISO ( UIPARTICLE

COATINGSCOATED FUELPARTICLES

FUELPARTICLE

FUELROD

FUELBLOCK

GRAPHITEBLOCK

FUELRODS

COOLANTHOLES

FIGURE 1-10Fuel assembly for a representative high-temperature gas-cooled reactor. (Adapted courtesy of CATechnologies.)

Liquid-Metal Fast-Breeder ReactorA representative fuel assembly for a liquid-metal fast-breeder reactor [LMFBR] isshown in Fig. I-II. Although the fuel pins have the same basic features as those forthe water reactors, they have a much smaller diameter, are clad with thin stainlesssteel, are more closely spaced, and may contain two different fuel pellet types.

The primary fissile composition for LMFBR fuel is plutonium. The fertile com-position is 238U in the form of depleted uranium, the byproduct of the enrichmentprocess. Some pellets contain a PuOz-UO2 combination called mixed oxide. Typicalcompositions have 10-20 wt % Pu with depleted uranium accounting for the remainder.Other pellets contain only depleted uranium.

Basic driver fuel pins contain a central stack of mixed-oxide pellets with stacksof depleted-uranium blanket pellets on both ends. Blanket pins consist entirely of

MIXED·DXIDEFUE L PELLETS

STAINLESS STEELFUEL CLADDING

FLOW CHANNEL

BLANKET

FUELBEARI NGREGION

BLANKET

HANDLI NG SOCKETLOAD PAD

",__-ABOVE CORELOAD PAD

SHIELDIORIFICEREGION

INLETNOZZLE

FUEL ASSEMBLYCRDSS·SECTION

FUEL ASSEMBLY

FIGURE 1-11Fuel assembly for a representative liquid-metal fast-breeder reactor. (Adapted courtesy of U.S.Department of Energy.)

20 Overview

depleted-uranium pellets. In both cases, the blanket material is employed to enhancebreeding by absorbing neutrons that would otherwise escape from the system.

The LMFBR driver assembly shown in Fig. 1-11 has a fuel channel that enclosesthe hexagonal fuel-pin array. Liquid sodium enters at the bottom of the assembly, isdistributed in the orifice region, and then flows through the active fuel region. Thewall prevents the mixing of flow from adjacent assemblies.

Reactor CoresFuel assemblies in the reactor vessel form the core wherein the fission process producesheat energy. Core configurations for each reactor are summarized in Table 1-2. Thefuel is loaded to approximate a right circular cylinder with the fuel assemblies placedparallel to the axis. Coolant flow is axial (i.e., parallel to the axis).

The axis is vertical for five of the reactors and horizontal for the other. For eachsystem a plane perpendicular to the axis defines a radial cross section of the core.(Examination of the fuel-management patterns in Chap. 9 and the core drawings inChaps. 9-12 may aid visualization of the situation.)

In the LWR's, the fuel bundles are held in a vertical position in the reactorvessel. A representative BWR has 748 assemblies, while the PWR has between 193and 241 larger assemblies.

The CANDU has 12 bundles end-to-end in each of the horizontal pressure tubes.An array of 380 such tubes makes up the reactor core.

The PTGR has two fuel assemblies stacked one atop the other in a "stringer."The large PTGR has 1661 such stringers emplaced into pressure tubes in the matrixof graphite moderator blocks.

The HTGR fuel assemblies are stacked vertically. A typical core has 8 fuelblocks along the axis and 493 horizontally.

The LMFBR core contains two basic fuel assembly types. Assemblies of driverfuel pins constitute most of the central region of the core. The depleted uranium attheir top and bottom forms an axial blanket. The assemblies made from blanket fuelpins are loaded around the outside of the central region to form the radial blanket.Through the combination of the axial blankets with the radial blanket, the cylinder ofthe central mixed-oxide core is essentially surrounded by a larger cylinder with depleteduranium for 239pU breeding.

One current LMFBR design calls for 364 driver assemblies and 233 externalblanket assemblies. Although most of the blanket assemblies are placed around thecore periphery, some are interspersed among the driver fuel in the central core region.

EXERCISESQuestions1-1. Explain the two advantages and the two disadvantages of fission as an energy

source.1-2. Sketch the sequence of the process step of an open commercial nuclear fuel

cycle. Describe the purpose and product(s) of each step.1-3. Explain the concept of and physical basis for recycling of nuclear fuel in a

Introduction 21

closed fuel cycle. Expand the sketch in the previous exercise to include theadded steps.

1-4. Describe the roles for transportation, nuclear safety, and nuclear material safe-guards in the nuclear fuel cycle.

1-5. Identify the reference reactor system(s) characterized by fuel cycles with:a. slightly enriched uraniumb. 20-93 wt % 235Uc. required plutonium recycled. no enrichment facilitiese. thorium use

1-6. Identify each of the six reference reactors by full name and acronym. Completea data table that includes for each of the reactors the following:a. number of loopsb. coolantc. moderatord. neutron energye. fuel production

1-7. Explain the concept of multiple barrier fission-product containment and identifythe four basic components. Describe how the fuel assembly in each of the sixreference reactors is designed to provide the first two of the barriers.

Numerical Problemst1-8. Using data from App. IV, make to-scale sketches of the outside cross section

for the fuel assembly from each of the six reference reactors. In a corner ofeach drawing include a circle the size of the outside diameter of a single fuelpin for that reactor.

1-9. Calculate the masses of 235U and 238U per metric ton (t)f of uranium ore,assuming the total uranium is 0.25 wt % of the ore.

1- 10. Coal has a heat content of 19-28 GJ/t of mined material. Uranium as employedin an LWR has a heat content of 460 GJ/kg of natural uranium metal.a. Calculate the heat content of a ton of low-assay uranium ore (see the previousproblem) and its ratio to that of the extreme coal values.

b. Considering that electrical conversion efficiencies [MW(e)/MW(th)] are about32 percent and 38 percent for an LWR and a coal plant, respectively, calculateelectric energy ratios as in (a).

1-11. Repeat the previous exercise for a "giant" Canadian ore deposit with 12 wt %235U.

1-12. The 235U and 238U masses in natural uranium are split between enriched anddepleted streams as a result of an enrichment process. If the input masses andoutput enrichments are specified, mass conservation determines the maximum(i.e., zero-loss) quantity of each isotope in the output streams. Considering a1 kg input of natural uranium, a 3 wt % 235U enriched stream, and a 0.3 wt %235U depleted stream:

t Units and conversion factors are contained in App. II.:j:Metric ton is also sometimes abbreviated as teo

22 Overview

a. Calculate the masses of 235U, and total U in each of the two outputstreams.

b. Calculate the fraction of the initial mU that ends up in each output stream.c. Repeat part (b) for 238U and total U.

SELECTED BIBLIOGRAPHY§General Nuclear Engineering Textbooks

Connolly, 1978Foster & Wright. 1983Glasstone & Sesonske. 1981Lamarsh. 1983Murray, 1988

General ReferencesBabcock & Wilcox. 1980Collier & Hewitt. 1987Etherington, 1958Leclerq, 1986OECD, 1990eRahn, 1984

General Fuel Cycle InformationAPS, 1978Benedict, 1981Cochran & Tsoulfanidis, 1990Marshall, 1983bWASH-1250,1973Wymer & Vondra, 1981

General Reactor Design InformationHogerton, 1970Lish, 1972Marshall, 1983aNero, 1979

Current Information Sources (Newsletters and Abstracts)Atomic Energy Clearing House-reports news on nuclear energy with emphasis on activities relatedto the U.S. Department of Energy (DOE), Nuclear Regulatory Commission, and other governmententities.

Current Abstracts for Nuclear Fuel Cvcle, Nuclear Reactors and Technologv. Nuclear ReactorSafety, and Radioactive Waste Management-published by the U.S. Department of Energy's (DOE)Office of Scientific and Technical Information; each of the four separate issues "announces on amonthly basis the current worldwide information available" on the specific topic area.

Nucleonics Week-published weekly by McGraw-Hili; summarizes current events that involve orrelate to the nuclear industry worldwide.

Current Information Sources (Magazines and Newspapers)Bulletin of the Atomic Scientists [Bulletin I-published monthly (except July and August) by theEducational Foundation for Nuclear Science as "a magazine of science and public affairs"; althougheach cover holds "the Bulletin clock. symbol of the threat of nuclear doomsday hovering overmankind" (in reference mainly to nuclear weapons). it has been a good forum for opinion onradiation effects, safeguards, and commercial nuclear power by some proponents and a few re-sponsible critics; the overall thrust tends to be political rather than technical.

§ Full citations are contained in the General Bibliography at the back of the book.

Introduction 23

EPRI Journal-published monthly by the Electric Power Research Institute; feature articles on thegeneral status of nuclear power and alternative energy sources and on specific research projects;brief abstracts from recently-issued EPRI reports (see "Reports" section below)

IAEA Bulletin-published by the International Atomic Energy Agency in Vienna, Austria; coversworldwide developments in nuclear energy at a level suitable for a general technical audience;includes special attention to IAEA's primary mission, namely international safeguards and non-proliferation and also addresses reactor safety and operating practices; announces new IAEA reports(see "Reports" section below).

IEEE Spectrum-published monthly by the Institute of Electrical and Electronics Engineers [IEEE];excellent feature articles on specific aspects of nuclear and other energy sources (usually several peryear) written for a general technical audience.

New York Times-Tuesday's "Science Section" frequently addresses current issues in nuclear andother energy-related fields in well-illustrated articles aimed at a general audience; focus is generallysociopolitical.

Nuclear Engineering International [Nucl. Eng. Int.]-published monthly in the United Kingdom;excellent coverage of worldwide developments in the nuclear fuel cycle and reactor technology;some issues devoted to a single reactor (often including a colored wall chart of the system), a reactorconcept, a national program, or a fuel cycle step (specific topics of interest are included in theSelected Bibliographies for applicable chapters); "World Nuclear Industry Handbook" publishedannually with the November issue includes reactor and fuel-cycle statistics, station achievementdata, a reactor directory, and a buyers' guide to industry products and services.

Nuclear Industry [Nucl. Ind.]-published monthly by the Committee for Energy Awareness [CEA],a U.S. organization representing electric utilities and other organizations involved in commercialnuclear power; summary coverage of current issues and status, especially concerned with federalregulatory policy and practice.

Nuclear News-published monthly by the American Nuclear Society [ANS], the major professionalorganization for nuclear engineers; summary coverage of current issues and status, plus featurearticles on topics of general interest; updated "World List of Nuclear Power Reactors" appears ineach February and August issue.

Nuclear Safety-published bimonthly by the U.S. Nuclear Regulatory Commission; feature articleson various aspects of reactor and fuel cycle safety and safeguards in the United States and worldwide.

Physics Today-published monthly by the American Institute of Physics [AlP], the major profes-sional organization for physicists; summary coverage of current issues, plus occasional feature articleson nuclear energy and other energy sources.

Power-published monthly as "the magazine of power generation and plant energy systems";frequent overview articles on various aspects of nuclear and other power sources written for a generalengineering audience.

Power Engineering-published monthly as "the engineering magazine of power generation"; sum-mary coverage of current issues in "Nuclear Power Engineering" section plus frequent overviewarticles on nuclear and other power sources written for a general engineering audience (often includesgood color illustrations and pictures).

Science-published weekly by the American Association for the Advancement of Science [AAAS]for the general scientific and engineering community; coverage of world issues and policies in scienceand technology including nuclear power, energy sources, and related cost-risk-benefit evaluation.

Scientific American-published monthly; excellent feature articles on specific aspects of nuclearenergy and other energy sources (usually several per year) written for a general technical audience.

Technology Review-published monthly by the Massachusetts Institute of Technology; good reviewarticles on nuclear and other energy technologies with some emphasis on sociopolitical interactions.

24 Overview

Current Information Sources' (Professional Journals)Nuclear Engineering and Design [Nucl. Eng. & Des.]-published monthly by the North HollandPublishing Company; reports on a range of engineering research related to nuclear energy, empha-sizing structural and thermodynamic analysis and design; occasional issues are devoted to a specifictopic area.

Nuclear Science and Engineering [Nucl. Sci. Eng.] and Nuclear Technology-published monthlyby the American Nuclear Society; the principal technical journals of the nuclear engineering profes-sion for theoretical and applications-oriented topics, respectively.

Transactions of the American Nuclear Society [Trans. Am. Nucl. Soc.]-paper summaries fromANS meetings; up-to-date coverage of current research activities.

Current Information Sources (Reports)Electric Power Research Institute [EPRI]-funded by the U.S. electric utility industry to conductresearch in subject areas of practical interest; reports cover a wide range of topics related to nuclearpower and other energy sources and are usually well written for a general engineering audience.

International Atomic Energy Agency [IAEA]-reports of the Agency's activities; announced in theIAEA Bulletin and other publications.

National Technical Information Service [NTIS]-repository for research reports, including those onnuclear and other energy topics, from U.S. government departments, agencies, and national labo-ratories; inventory also includes selected reports from international sources.

Organization for Economic Co-Operation and Development [OECD]-established to promote co-operation of its membership (including most of western Europe, Canada, Japan, and the UnitedStates); reports from the Nuclear Energy Agency address economic aspects, nuclear safety, regulation,and energy contributions.