descriptions of selected accidents that have occurred at nuclear reactor potx

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ORNLNSIC176 [Information is our business

DESCRIPTIONS OF SELECTED ACCIDENTS THAT HAVE OCCURRED AT NUCLEAR REACTOR

FACILITIES

‘OAK RIDGE NATIONAL LAB., TN ‘APR 1980

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CN eames J [moecia hn mit reno

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iii _

Contract No H-7405-eng-26 Engineering Technology Division

DESCRIPTIONS OF SELECTED ACCIDENTS THAT Have occURRED AT NUCLEAR REACTOR FACILITIES How, Beretnt

and

Members of the Staff of the Nuclear Safety Teforeation Center

Date Published: April 1980

Prepared by che

(aK RIDGE NATIONAL LABORATORY Oak Ridge, Tennessee 37830 operated by

UNION CARBIDE CORPORATION for the DEPARTMENT OF EXERGY

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[Preceding page blank, contexts FREPEE ii: c0 ai Fuspanewrans 2.1 Basie Theor!

2G Ligit-Water Reactors for the Production of Flectririey SEARO 19

1.1 fuel Melting Incident at the Fermi äzactor (1266) 37 5:2 Eincteseal Cable Fires at San Onofre 1 (1908) a TU Fuel veledasss at Se Laurent (1959) pata a 3k Uncovering of tie Core at Le Crosxe (0/0) 200 2

‘at Robinson 2 (970) 2 .2 sĩc, 2 2 3.6 _Dinchacge ef Frimary Syaten iat Dowell at Dresden 7

5.9—Waive Separations at Turkey Point 3 (9713 8 : 2 Sil furbine Basement Flooded at dua Citses (9I2) ese 3 ‘LL Steam Generator Dazaged in Hot Tests

3.13 Seawater Intrusion into Primary tu nem,

31s’ Fracture of Shate of Wain Reactor Coslant “Pum at Sueey 1 (975) 3.15 _inauvereent Griticality During Wefueliag ‘at Vermont Yankee (1977)

3.16 Operator Sucked shrough Hanhoie into” Container at Surry 2 (973)

= or sẽ

3-16 TƯeetrieal Gable Tire at Brome Ferry 2 913) Zils Sens Failure in Hain Goolant Pape u AC Rob MNnOn 2097S) x.xàscccrvax.c,ct2x.ecaxixixvc£LL— 64 1.20 Hydeogen Euplonion at Cooper Iniures Two (1973) rns S4

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Page 3.21 Explosion Destroys Off-Gas Building st Cooper (1976) : or 3.22 Unplanned Cririesiity buries Refueling ae Millstone 1 (1976) 69 3.23 Short Causes Instrument Failures at Rancho Seco 1 (1977) 69 3.24 Fire in 0ff-Gas Syacer at Brovns Ferry 3 (1977) a 3125 Stuck Pressurizer Relief Valve at Davis-Beese 1 (1977) ee 1 3.26 One Inlured tn Hydrogee Explesion ae Millstone 1 (1977) - n 3.27, Disasseabiy of Burnable-Poison-Rod Acsonbiy ae Cryseal River 3 (1978) 28 3.28 The Accidene at Three Mile Island 2 (1979) 80 3129 Less of Coolant Inventory at Oyster Creek (1979) sls 86 4 PRODUCTION REACTORS 5 92 4.1 Blockage of Coolant Tube in the Hanford KW Reactor (1955) eons 92 4.2 Fuel Fire at Mindscale (2957) v 13 Failure of Prinary Seras System in the Reactor 93

at Hanfora (1970) os ấu 95

5 EXPERINENTAL AND RESEARCH REACTORS 99 5-1 Core Danage in the NAX Reactor at chalk River (1952) n 99 5.2 Operator Error Causes Fuel Melring ín EBRC1 (1555) , 101 5.3 Ruptured Fuel Elenent Couses Extensive Contamination of Reactor Building at AU (1938) 103 5.4 Improper Instrumentation Results in Fuel Melting im NERE-3 (1958) 105 5.5 Leakage of Organie Seal Coolant Causes Fuel Damage in SRE (1959) 3 106 5.6 Fuel Flenent MelCing fa KTR (1960) 108 5.7 Theee Fatalities ip Accidenc at Sici (1961) „ 108 518 Pressurizer Failure in SPERT-3 (1961) vị HỆ 519 Mydrogen Fire ae PHC3A (1962) fore 13 5:10 Fort Flenent Melting at Oak Ridge Research Reactor (1963) us 5.11 Rupture-Loop Faiiure in PAIR’ (1963) 120 5112 Loss of Coolant Damages Core at Lucens nà 18

DISCUSSION AND CONCLUSION 134

REPERECES aa?

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FOREWORD

‘This report vas prepared at the request of the President's Commission fon the Accident at three Mile Leland in order to provide the members of the Commission with sone’ Insight into the nature and significance of accidents in nuclear facilities However, the report thus conceived was recopiitzed to be of interest to a wider audience; therefore, ve are pleased to give it the broad distribution afforded by thie Oak Ridge National Laboratory-tuclear Safety Information Center report

In selecting the accidents that are included in this compilation, ve screened al thode ava1labie in the computerized files of the Nuclear

Safety Infornation Center While we can state with gone certainty that this file includes all accidents that have occurred at comercial nuclear facilities in the United States, ue can also state with equal certainty that there must have been accidents in foreign nuclear pouer plants of Which we have no knowledge In fact, several of the foreign accidents of which ve have heard (e.g., the sodius-vater explosion in the Russian

fase breeder reactor Beloyarsk 3 in 1975 and the release of CƠ; from the Caechoslovakian gas-cooled hesvy-vater-noderated reactor Bohunice 1A in 1976) are known only through sketchy informal accounts Such accidents cannot be included here because #0 few details are know to us On the other hand, this report does include six foreign accidenta where the information was documented

Although H.W Bertini is principally responsible for the preparation of this document, he was assisted by several members of the staff of che Hacleer Safety Information Center, including J R Buchanan, W R Casto, lin, B, Cottrell, R B Gallaher, and R L Scott, who partietpated in the development of the selection eriteria (for the accidents reported), prepared the draft on a fav of che accidents, and reviewed the resulting document Chapters 1 and 6 were written by Us B Cottrell

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vi

regard to this last category, it is noted that che Nuclear Safety Infor- mation Genter annually publishes # compilation of all Licensee Event Reports submitted to the Nuclear Regulatory Commission by U.S comercial

nuclear pover plants (seo the Bibliography)

‘The presentation of che material in this docuzent is ained prinartly at the educated layperson The use of acronyms is avoided vhere practical and, when used, they are spelled out the first time they appear Follow ing the Introduetion is a brief discussion of the fundanencel principles of nuclear reactors and a description of some of the reactor systeas chat are used {n the production of electricity in the United States In this brief presentation we did not attenpt to describe all che different types of reactors ~ much less the special features of each Although the {nformation on the accidents included herein cones from a varicty of sources, ve have endeavored co standardize che presentations and to include identification of the facility involved, date of the accident, 2 brief deseripeion of the accident (including shy unigue circumstances), and discussion of the accident consequences, In all cases the docu- mentation pertaining to each accident is cited so that interested persons may go to more detailed source material for additional information

Wa B Cottrell, Director

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vit

PREFACE

‘The Nuclear Safety Infornation Center (NSIC), which was established tn March 1963 at Oak Ridge National Laboratory, is principally supported by the U.S Nuclear Regulatory Commission's Office of Nuclear Regulatory Research Support is also provided by the Division of Reactor Research ‘and Technology of the Deparraent of Energy NSIC is a focal point for the collection, storege, evaluation, and dissenination of safety inforna~ tiên to aid those concerned with the analysis, design, and operation of nuclear facilities Although the sost widely known product of NSIC is the technical progress review luslean Safety, the Center prepares reports and bibliographies as listed on the inside covers of this docurent The Genter has also developed a systen of keywords to index the information which ie catalogs The title, author, installation, abstract, and key- words for each document reviewed are recorded at the central computing

facility in Oak Ridge The references are cataloged according to the following categories

1 General Safety Criteria 2 Siting of Nuclear Facilities

3 Transportation and Handling of Radtosctive Materials 4, Aerospace Safety (inactive \1970)

5, Heat Transfer and Thermal Hydraulics

6, Reactor Transients, Kinetics, and Stability 7, Fission Product Release, Transport, and Removal 8 Sources of Energy Release under Accident Conditions 9 Nuclear Instrumentation, Control, ané Safety Systens 10 Elecerscat Fouer Systene

11, Containnent of Nuclear Facilities 12, Plant Safety Features — Reactor 13 Plant Safecy Features ~ Nonreactor

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vận

17 Operational Safety and Experience 18 Design, Construction and Licensing

19 Internal Exposure Effects on llumans Due to Radioactivity in the Eevironnent (Inactive Septenber 1973) 20, Effects of Thermal Nodifications on Ecological Systeme (nactive Septesber 1973)

21 Radiation Effects on Ecological Systems (inactive Septesber 1973) 22, Safeguards of Nuclear Matersals

Computer programs have been developed that enable NSIC to (1) operate a program of selective dissemination of information (S01) to individuals according to thetx particular profile of interest, (2) make retrospective searches of the stored references, and (3) produce topical indexed bibli~ opraphies, In addition, the Center Staff is available for consultation, and the document literature at USIC offices is available for exasinatton NSIC reports (i.e., those with the ORNL/NSIC and ORSL/NUREG/NSTC nunbere) may be purchased from the National Technical Information Service (see tn side front cover) ALL of the above services are free to NRC and DOE personnel se well se thetr dtrect contractors They are avaflable to ali others at a nominal cost ae detersined by the DO Coat Aecovery Policy Persons interested in any of the services offered by NSIC should address Anquirtes to:

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3 TNTROPUCTION

This report vas prepared at the request of the President's Conaission fon the Accident at Three Nile Island to provide the members of the

Commission with some insight into the nature and signifieance of accidents ‘that have occurred at nuclear reactor facilities én the past Toward that end, this report presents a brief description of 44 accidents which have occurred throughout the world and which meet at least one of the severity ericeria khách we establ ished

‘The accidents selected for inclusion fulfill at least one of the

following conditions: (1) eavsed deach or significant injury; (2) released a significant asount of radioactivity offsite (e.g, sany tines the

naxinun perniseible concentration for extended periods of tine);

{9) resulted in core danage (nelting and/or disruption), or core danage vas suspected although it did not actually occur; (4) resulted in severe damage to eajor equipeent; (5) caused inadvertent eriticality; (6) uae a precursor to a potentially serious accident; or (7) resulted in signifi- cant recovery cost (e.g, greater than half a a{llion dollare)

These criteria are expected to encompass all significant accidents Ae the sane time ít should be noted that they also encompass some

accidents hich are not uaigue to nuclear reactor facilities However, for the sake of consistency, all those which meet the established eriteria fare included Similarly, ‘there 4s sone subjective judgment involved in evaluating the severity of many accidents When in doube, ve have chosen to inelude the aceigent in the compilation,

‘As noted above, the accidents selected for inclusion here occurred throughout the world He believe that our knowledge of U.S reactor experience is euffictencly comprehensive to ensure that all relevant accidents that have occurred in this countey have been considered However, we are well evare that our knowledge of reactor experience in

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đeeunent — first, because the U-S experience (with pover reactors at least) constitutes approximately half the vorld experience and, secondly, because the experience outside the United States 4s derived prinarily from other reactor types Furthermore, these foreign pressurized-vater reactors and botling-uater reactors are not built co U.S criteria and safety standarés

This report encompasses all cypes of reactor facilictes, except critical facilities Thus, the accidents included in this report involve (1) contrat station pover plants, (2) production reactors, and (3) expsri~ ‘mental and research reactors, and they are grouped accordingly while

the principal concern of this document is vith accidents that occurred at central station pover plant rectors, the experience with other types of reactors is also relevant, although primarily in a generic sense

Hovever, because of the tremendous differences from one type of reactor to another (and sonetines even within a given reactor type), it is genarally not possible to extrapolate the accident sequence (in detail) from one reactor type to another Thus, the experience vith erieical facilities (the simplest reactor form) {s so far renoved from what could happen at @ central station power plant reactor as to be completely

Snvelevant Furthermore, good reviews of accidents in critical facilities already exist?

In this report we identify the reactor involved (by type, designer, ‘operator, location, ané pover level)? and then present a brief description of the accident itself, including @ brief comentary on the causes and

consequences — vhere such information was available Ve have undertaken no investigative work om, nor analytical evaluations of, accident causes of consequences; we sinply describe the events that took place and

report the conclusions that vere reached in che sources chat are cited In reading the accident deseriprions, the reader should note that the word Voperator” 12 used rather loosely and may refer to any of the

operating personnel at the facility, including, in some instances,

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2 MUCLEAR REACTORS: FUNDAMENTALS 2.1 Baste Theory

2.1.1 Atens and nucles

‘An stom of any elenent consists of a very small, heavy nucleus surrounded by a cloud of electrons, ubich are very 1ight negatively charged particles The dimensions of the electron cloud are much larger than those of che nucleus If one were to scale a fluorine atom (nine electrons and a nucleus) to dimensions roughly equivalent to those of the solar aystem (nine planets and a sun), one vould reduce the mass of the sun about ten times, reduce ite size (ékaneter) by about one-nalé, land make the distance between the planets and sun about fifteen tines greater, The resections of concern in a nuclear reactor are only chose involved with the nuclei of atons

[A simple concept of the nucleus is thet ir Le s cightly bound cluster of bite of matter called neutrons and protons They are about the sane size, but the proton has a single charge of positive electricity whereas the neutron has none If a proton is added to a nucleus, the atom becones © different chemical element with different chenfcal prop- erties If a neutron te added to a nucleus, the atos becones a different Leotope (Lies, an atom of slightly đ$£ferent weight or atomte mass) ané acquires different nuclear properties, but the element, and hence its chemical properties, remains unchanged For exanple, the isotope of Uranium whose sas nunber” is 235 (295u) 19 needed to make a suclear reactor function, but the szotepe of uranium ghose mass umber te 238 (238y) cannot be used for this purpose because of its different nuclear properties However, both isotopes have the cane chemical properties The sane is true of plutonium: che plutonium isotope 73°Pu 4s a nuclear “fuel” whereas ?* Pu ie not

The sinplest nucleus is that of hydrogen (iH), for tt consists of & single procon By adding a neutron, one gots a different isotope (7H),

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hydrogen Unlike any other element, the isotopes of hydrogen have

different nanes This one, 2E, is called deuterium (D) Since deuterium combines with other elenencs in the sre manner as ordinary hydrogen

CN), ác can combine with oxygen to form water In addition, since its mass is twice that of hydrogen, the water formed by deuterium (00) ts called "heavy" water It has different molean properties than thạc of ordinary, or "Light," water

2.2.2 Fission and the auclear chain resection

‘The energy that becomes available in a nuclear reactor is explained by Einstein's fanous formula, F = ne?, where £ {6 the total energy of the matter, is its mass (or veighe), and ¢ ts the velocity of Light ‘The interpretation is chat matter and energy are equivalent; i.e., if a certain amount of matter ie made to disappear, an equivalent amount of energy will appear The reverse is also true: if energy is made to đđágappear, chen matter will appear

The fission process that takes place in nuclear reactors is based

fon this principle In this process a neutron ts "captured" by the nucleus of 2 2? atoa; that is, 2 neutron strikes and penetrates the nucleus, thus forming 7#8U, Hovever, this new nueleus, vhen fered in this way, {6 highly unstable; it breaks apart (fissions) almost instan- taneously nto tuo fraguents plus few free nevteons If one were co determine the weight of the debris (che tye fragnents plus the free neutrons) after the fiseion and compare this weight with that of the 226y atom before the fission, one would find that matter had disappeare

that ia, the debris would weigh Less than the original atom of 236u,

Since matter has disappeared, then, according to Einstein's equation, an n created

equivelent amount of energy must have bạ

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atone that have been struck, either by the Fission fragneRnts or by the neutrons, vecoil and vibrate and thus the mediun is heated tence,

energy in the fore of heat se created by the disappearance of matter, which occurrad in the fission process

ALL Elssiong are nor exactly alike; that is, th fragnents forced in one fission are sonevhat different from those formed in another, and the nunber of neurrons emitted in one fission may be different from the ‘number emitted in another

Recall that the fission deseribed above was caused by one neutron and that a feu neutrons ware emitted when che 235U atom fissioned If enough 225 is present, and if other material with the necessary prop erties is also present, then at least one neutron released by one fission will cause another fission, and the process ie repeated continually The reaction is thus self-sustaining because each neutron chat is

"captured" in causing a fission is replaced by other neutrons, sone of tuất

ton generates hi

which cause other fissions, ete Since each £4

appears that a continuing source of heat has been devised This would bbe true, except that it takes a certain anount of #25 "fuel" to support a self-sustaining chain reaction, ané some of the fuel ie destroyed, or consuned, in the fission process The control of the process is described below

The extra neutrons that are released by each fission (1.0., those evtrone chat do not cause additional fiseions) either escape fron the reactor or are captured by nonfissionable nuclei, It is important that neutrons be husbanded so that there will be a sufficient number to sustain the chain reaction

2.4.3 Gritleality tn a nuclear reactor

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conditions vhích congticure a crisis will also be explained More

detailed background information must be presented before these explanations can be made

Mot al the neutrons that are released in & £{ssion reaction are omitted instantanoously A small percentage (approxinately 0.73 in 235y f4geson) are enitced Later — about 0-1 sec later on che average The neutrons emitted instantaneously aru called "prompt" neutrons, and those enftted a short tine later are called "delayed" neutrons Both the prompt and delayed neutrons help to initiate and sustain the chai:

reaction occurring withia the reactor

‘The term “suberitical” is used to describe the reactor configuration vhen ít is less than self-sustaining The term “supercritical” is used

to describe the configuration hen the nuaber of fiselons is increasing over a period of tine rather than remaining constant over time, as vhen che vesctor is simply critical This occurs vhen more than one of the neutrons that are exitted in each fission cause nore fiseions For

example, vhen the reactor ie sinply eritieal, a single neutron out of che two or three that are released in a single fission causes another fasion, and a short tine inter one of the neutrons released tn that fission causes another Fioston, and @ short tine after that one of the nevly released neutrons causes still another fission, ete thus, the

fission rate is constant over tine and so is the neutron population in the reactor When the reactor is supercritical and, say, tvo of the neutrons from each fission cause tvo other fiesions, then che fiver

fiseion would be folloved by tuo fissions, and a short time later by

four fLesions, then eight fissions, ete.; this, the rate at vhich fissione are taking place, as vell as the neutron population, vould be increasing with time

Ht should be pointed out that the reactor can be critical with any murder of neutrons present so long as the nunber causing fission

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the reactor renains critical fn enormous number of neutrons partieipate

in the fissioning process, even at low power For example, if only

10 million neutrons were causing fissions in a resctor ubich vas critical, the heat generated vould be go enall thar t would be difficult if not impossibie to measure, even vith the most sensitive instrunents When @ reactor is operating at full power, there are about 1017 (one hundred billion million) neutrons in the core at any inetanc

one more term must be defined before proceeding to ehe subsequent chapters, and this is "reactivity." The numerical value associated with the reactivity is a measure of the eriticality The eriticality is a Loose tern which broadly defines che general nuclear condition of the reactor The reactivity 1s a more precise measure of the criticality

For example, che reactivity is taken to be zero when the reactor is critical, If the reactivity ¢s +0.00001,

critical; ££ ft 4¢ 40.001, the reactor 4s more supercritical If it is -0.00001, the reactor is barely suberitical; if the reactivity 1s “0.1, the reactor is highly subcritical

TE the reactivity {# greater than +0.0073, the reactor 1¢ said to be prompt crirical, and the rate of fissions u{ll increase at a very

he reactor is barely super~

rapid rate Under these conditions, the chain reaction is more than self-sustaining by the pronpr neutrons alone, without the need for the delayed neutrons This situation in a auclear reactor would probably Lead to a damaged core because the power would increase 90 fast that ft would be difficult to control Ths is a crisis situation

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time the pover increases ro the desired level The reactivity ie then brought ro zero again (critical configuration), and the reactor renains at the higher pover The reactivity is made negative (suberitical configuration) to decrease the pover, at one would raise the gas pedal to reduce the speed of a car

‘Table 2.1 aumarizes the above comments, Note that reactivity ‘equal to zero is equivalent co the gas pedal being held at = constant

position

2.2 The Componente of a Nuclear Zeactor

The nạn component of a nuclear resetor is the "sore." Ir is sur~ rounded by a thick (B+ to 10-in.) steel vessel called the pressure vesset,

whose thickness {s determined by the operating pressures of the aysten The core of a resctor 4s the region where nuclear Fission cakes place and consequently where the heat is generated It consiats of three major components: the ful, the coolant, and the moderator A Fourth component, the reflector or blanket, is sometines used A reactor can be generally characterized by specifying these components

‘The fuel used in pover reactors in che United States 12 an oxide of

‘uranium, U0;, which 9 4 cough ceramte that aelts at a very high tenpers ture (2865°C (5189°F)} The fuel is “enriched ~ that is, the amount of the fisesonable isotope 235y in the uranium 45 increased over that which is nomally present in uraniom ore, The percentage of 2950 in urantum as i 4s found in nature (natural wrandun) te 0.721, whereas the uranium used in pover reactors 4e enriched te 2 to 32, (Note that this enrich- ment is considerably lese than that required in the uranium used in an atomic bomb.) The U0 fuel is surrounded by 2 thin netal shearh called the cladding The purpose of the cladding is to proteet che Uz and co prevent the escape of radioactive Fission products (to be described below) The cladding is made of an alloy composed msinly of zirconvm, Other metal, such as stainless steel or aluninun, has been used sn

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10

‘The coolant reeovee the heat that is generated in the core In power reactors in the United States, light water is used as the coolant Gases such ax helium (He) or carbon dioxide (C0:) are used in other reactors, and Liquid sodiun is used in fast breeder reactors

The function of the moderator 14 to reduce the speed of the neutrons that are released in the Fission process Uraniun-235 has a greater propensity to capture neutrons, and consequently to fission, chen the speed of the neutrons is reduced, When they are released, the neutrons ove at very high speed Teir speed is reduced to the nost efficient

levels for fissioning when they scatter off the nucle of the moderator and slow devn In power reactors in the United States light vater serves the dual function of coolant and moderator Other types of

reactors use heavy water or graphite as the noderator

The material chat surrounds che core is called the blanket or reflector When this material is used as a reflector, its nain purpose de to scatter the neutrons that might otherwise eseape, deflecting then back into the core Berylliun hae frequently been used as a reflector

material in experinental reactors, but not in pover reactors When the material surrounding the core ¢ used as a blasket, the prine purpose is to permit the Cransmutation of che blanket material co a fisstonable isotope, For example, natural uranium, which {5 composed almost entirely

(99.272) of the ssotope "3U, ís betng used ae 2 blanket in some U.S

power reactors When *34 captures a neutron, the fissionable isotope 235pu 4a formed Thus, the blanket not only helps to prevent the escape of neutrons, but also serves as a “breeding ground” for the £issionable Aeotope #Ư#Pg, which contributes to the pover of the reactor vhen it, Én turn, fissions by neutron capture

Control of criticality or reactivity 4e achieved by long rods made of material chat readily absorbs neutrons These rods are called control rods (or poison rods), and they are aads of beroa carbide power or

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nh

coolant to enhance the control because boron is good neutron absorber ‘When the control rods are withdrawn from the coré, the reactivity

sncreases; thie movenent of the rods is referred co ax the insertion of reactivity

When the control rode are fully inserted, the reactor is substantially suberitteal (reactivity less chan zero) As che control rods are with-

drawn, reactivity increases until the reactor is critical (reactivity = 0); this is the critical "configuration" referred to above For reasons of efficsency, pover reactors ave designed co be eritical at full pouer when all the control rods are almost completely withdramm or conpletely withdram, depending on the reactor type, In these positions, further withdravai will add Lfttle or ne reactivity The imediate and complete insertion of all the rods is sonstines calleé a "scram," and sonetines a

reactor "trip," vhich, in the pariance of power plant engineers and operators, means to svitch something off A scram can be initiated ‘autonatically or ie can be initiated by the resctor operator by pushing

the scram button on the control console

‘The mechanisms that move the costrol reds, called the control rod el in U.S

Grives, are located on top of or underneath the pressure ve

power reactors The tern "reactor" refers to the pressure vessel, the control rod drives, and the core, vhích includes the control rods

2.3 Radiosertvity

The source of radioactivity is an unstable mvcleus Such a nucleus will tend tovard stability by transmutation (radioactive decay) to a

nucleus that is stable Many tranemcations in succession may be required before a stable nucleus is obtained, and this entire sequence of decays

During each transautation, or radioactive decay, a particle 1s given off from the nucleus, and a gamma ray usually

ie called a decay chain,

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1

St can be stopped by the skin; the electrons are more penetrating, fol- owed by che neutrons and ganna rays, The gana rays are high-energy x rays; both are electromsgnetic radiation, as are radi waves The nucleus contains no electrons; chose that are emitted in radioactive Gecay cone from the transnucation of a neutron to a proton within che nucleus

À measare of the rate of decay of the radioactive muciei is the “nalf-Life.” This is the time it takes for half of all of che nuclei

life ia short, the Level of radioactivity will be reduced quickly because most of the

that are present at any instant to decay If the hal

radioactive nuclei will be transforned to ucles of other elements in a short time, If it is long, the radioactivity will remain for œ correspond~ ingly longer time

The fragments produced by the fissioning of a fuel nucleus (called either ELasion fragoents or fiselon products) consist of clusters of neutrons and protons aad ara, in fact, che nuclei of other elements ‘They are usually highly unstable when they are created by the fission process and hence are radioactive A large variety of fission fragnents

fe, and each has a ifferent half-

are formed during the fiesion proce Aste

‘The energy carried by the enitted particles and the gamma rays during the radioactive decay of the fission products constitutes about 6 1/2% of all the energy generated from each fission These enitted

particles are almost entirely absorbed within the core of the reactor, ang hence their energy {8 transferred to the core vhere it contributes

to the total heat that is generated by the reactor

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1

the fission products after the reactor 1s shut down ie called “decay heat." Ie is substantial, and steps must alvays be taken to ensure that this hest is renoved after the reactor is shot don

OF the particles that ave given off during the radioactive decay of the fissfon products (alpha and beta particles, neutrons, and gamma rays), only the neutrons will cause other nuclei to becone radioactive ‘The half-1ife of those fission products chat do lead to che enission of neutrons is so short that they are reduced to nogligibie anounts in a few minutes; hence, they are of Little concern One can then say, sn general, that the radioaceévity of any substance thet might cone fron a reactor will not cause nearby materials to become radioactive thenselves.” Radioactive eubdstances can “contaninate" other materials by clinging toi than (e.g, a3 a deposit of dust, a vater layer, otc.), but 2 nonradio- active waterial will remain nonradioactive even if it is inmersed in radioactive material

However, most of the material in the core of a reactor will become radioactive because Che core contains an enorsous nunber of neutrons ghen the reactor 1s operating at pover The cooling vater that passes through the core becomes radioactive Radioactive eritivm is formed by neutron absorption tn the szall amounte of deuterium in the water, and radioactive nitrogen-16 (188) 1s formed by neutron absorption in oxygen Algo, corrosion products of the metal piping, which are produced in anall quantities, becone radioactive as they are transported through the core by the water, In addition, traces of fvel parcicles (Uy), called "eeamp" uranium, sxe found on the outside of the cladding These traces cone from the manufacturing process Most of the {seston products that

"there is a minor exception to this statement, and st applics only when hydrogen er conpounds of hydrogen are exaoeed to radiation Sone of the ganha rays enitted from fission products have sufficient energy to jar a neutron loose from the deuterium found in natural hydrogen These eutrons, vhen absorbed, can cause a evbstance to becone radioactive, But because the Fraction of desterium in natural hydrogen {5 so small

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1

ave forned ia thts tranp fuel ordinarily remain imbedded in the fuel, but chose that escape go directly into the water, since there is aothing

to prevent thes from doing so The vast majority of fission products are formed in the fuel that 1s ineide the cladding, and most are pYe~ vented From ontering the water by che cladding However, some of che fission products migrate through the cladding, and some of then escape ehrough enall defects in che cladding (A certain number of defects are allowed by the Nuclear Regulatory Coemission.) When all of these con~

tributions to the radioactivity of the water are summed, the toral level of radioactivity ie stil much lees chan chat of the fuel, although it

is sufficiently high to be of concern

In the event of a severe breach or melting of the cladding, sone of the radioactive fission products can escape into the cooling water The vater will then becone highly contaminated, Many of che technical

apecifieations chat set Limits on reactor operation are formilated to prevent the cladding and the fuel from melting Molten fuel can slump against the cladding and interact with it, causing a breach It vas the escape of fission products from welted fuel into the cooling water that was the source of the high levels of radioactivity in the vater in the accidents involving melted fuel that are described in the following chapter

This section le concluded with a few definitions thet are pertinent to the measuresent of radioactivity

Curie (Ci): The curie is the unit used in measuring the “activity” of a radioactive source, i.e., the nusber of afsintegrations (or radio~ active decays) oceuering per second, where 1 ci = 3.7 * 10!° dis/sec Te approximately represents the number of disintegration per second in 1 gram of radium A millicurie (eCi) is one-chousandth of a curie

Roentgen (B): The roentgen (R) is che unit used in messuring the fontzation” capability in air (or, equivalently, the potential for

depositing energy in air) of x rays or gamma rays, Biological damage in “in this context, tonization is the process of knocking off one or ore electrons from atoas or molecules, thereby creating fons High teaperatures, electrical discharges, of nuclear radiation can cause

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1

tissue fs related to the degree to which x rays or gemma rays will Jonize air, or deposie energy in alr, Measuring instruments deteraine the radiation level in roentgens per hour (R/hr) or milliroencgens per hour (@R/ir) The dose of radiation that one can expect 4s deternined by mulesplying the radiation level in an area by the time apent in that area For example, if a person spends one-quarter of an hour in an area where the radiation level is & m8/hr, his/her total dose vould be

A/M he x 4 aR/he = 1 aR

Rad (radiation absorbed dose): The rad is che unit used in measuring the degree to which energy from any kind of radioactive source is absorbed in any material, This unit is not associated with the roentgen For a given level of radfation consisting of x rays and ganna rays, the dose seasured in rads or in roentgens is about che sane

Res (roentgen equivalent san): The rex i¢ che unit used in neasuring doth the energy deposited in any material and the potential for biological damage The rea takes account of the fact that the various kinds of

radiation, (1.e., x rays, gama rays, beta particles, alpha particles, and neutrons) dasage tiseue and body organs in eifferent ways, For x rays ond ganna rays, the dose received by soft tissue will be about the sane if the level of radiation given in roentgens {s the same ae that given in ren

The average person £8 exposed to about 200 nrens over @ period of a year from cossfe rays, medical x rays, x rays fron television, ete A dose of 600 rens vill kill most people

If 4 person spent 10 hr in an area vhere the radiation level vas 20 mrens/hr, he/she would receive the same dose in that 10 hr that the average person receives in 1 year Thus, a person can spend a short

time Jn an area vhere the radiation level <s high ond still get only @ vẻ in the

anal) dose This should help explain the urgency expres

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16

2.4 Elecerse Power Plants

an electric pouer plant utilizes sone form of fuel to generate heat and subsequently converts the heat fo electricity The electricity is distributed via power transmission Lines end sold to custoners

A simplified schenatic diagram of a typical electric pover plant is shown in Fig 2.1, In a conventional plant, coal or ofl is burned ané the heat generated turns the water in the boiler to stean The steam passes through pipes to the curbine A large shaft connects the turbine ta the generator The stean causes the turbine and the shaft to epin, ‘and the spinning shaft in conjunction with the other coaponents of the generator results in the prodvetion, or generation, of electricity In other words, the heat energy of che steam 1s converted to mechanical energy in the turbine, and che generator then converts the mechanical energy into electrical energy, or electricity

The teas passes from the turbine to a condenser where the stean is condensed to water, which is then punped back to the boiler The con- donser extracts heat from the steam by passing cool vater through pipes lover wich the steam flows and condenses The cool condenser vater is

thus heated This heat is renoved by passing the heated water chrough Large cooling tovers or by transporting it to holding ponds shere it is air cooled es exec Ce a laaessel steam WATER

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„

In a nuclear pover plane, the muclear reactor system simply replaces the boiler show Ja Fig 2-1 A vendor of reactors supplies all the ‘equipment necessary to produce the stean that goes to the turbine This ‘equipment is called the nuclear steam supply systes The electric

Utility that oune and manages the pover plaat purchases the other equip~ reat (i.e., the turbines, generators, condensers, etc.) from other

sources The present cost of the nuclear stean supply syste is about 5100 milion out of a total plant cost of $1200 to $1400 million for œ Large [1000-Mu(@)] plane

The size of che plant is gauged by the electric pover 4t produces, which {2 measured 1a megawates (Mi) One megawatt equals one million watte, The electric pover produced by the plant is measured in nega~ watts of electric pover [¥(e)], whereas the heat, or thermal porery

that 4s generated by the source of heat within the plant to produce the electricity is measured in megawatts of thermal pewer [MM(t)] Onty about one-third of the thermal power that is produced by the heat source can be converted to electric pover; thus, the megauatts of thermal pover produced by a plant is about three times the megawatts of electrical over In other words, power plants have an efficiency of about 30%

‘A 2000-HH(e) plant 1s considered Larg:

needs for a city with a population of sheot 600,000,

it will supply the electrical 2.5 Classification of Reactors

Reactors are broadly classified according to the purpose for which they were built Hovever, various types of reactors can be used to satisfy the sane purpose Descriptions of the types of reactors that

are fneluded in this report are given below

2.5.1 Reactors for central station electric pover plants

‘A reactor that is used for the production of electricity falls into this classification Light-water reactors, heavy-uater reactors, Liquid~ wnetal fast breeder reactors, and gas-cooled reactors are ali being used

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1s

Light-vater reactors (1.e., reactors thạc are both cooled and oderated by Light witer) are the primary source of nuctear electricity

Án che United States and in many other countries of the vorld,? including Austria (1),” Belgium (7), Brazil (3), Bulgaria (4), Ceechosiovakta (4), Finland (4), France (40), German Democratic Republic (7), Federal

Republic of Germany (26), Hungary (4), Tran (4), Tealy (7), Japan (25), Korea (4), tuxeabourg (2), Mexico (2), Netherlands (2), Phiilipines (2), Poland (1), South Africa (2), Spain (16), Sweden (12), Switzerland (7), Taiwan (6), and Yogoslavia (2) The United States has 7) tight-water reactors in operation and 124 in various phases of construction

Alehough the U.S.5.2 has 12 Light-vater reactors, Light

cooled graphite-noderated reactors, of which it has Z1, are the main source of nuclear electricity in that country

Heavy-water reactors are the primary source of nuclear electricity An Argentina (2), Canada (23), India (6), and Pakistan (1)

‘The United Kingdon has 37 gas-cooled reactors, which is the type they find most Favorable

Several countries find the Liquid-netal fase breeder reactor (LMPBR) sufficiently promising to pursue on a large scale France, the Federal Republic of Germany, Japan, the United Kingdom, and the U.S.S.R have LMFERs in operation or at various stages of construction

Since the main enphasis of this report is on the reactors used in central station electric pover pleats in the United States, a nore

detailed description of these reactors, namely the light-vater reactors, Se given in Sect, 2.6, Descriptions of the other types of reacgors used in central station pover plants can be found in the literature listed in tthe bibliography

2.5.2 Production reactors

Production reactors are used to produce the fiesionable isotope 2#ƯBu IE ts produced hy the absorption of a neutron in the nucleus of

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as

can aton of 2#8y, There are a variety of these reactors They are noderated by graphite or heavy vater and cooled by gas or Light water The fuel used is usually natural uraniom, Detailed descriptions of

these reactors are not generally available because they are classified 2.5.2 Experimental and research reactors

Experimental and research reactors are grouped together in this report because they are snall [less than about 30 18¥(:)] and experi~ mmencal in nature However, they are different fron each other

A research reactor is designed for the purpose of conducting scientifie research, mainly that Snvolving the interaction of neutrons with the nuelei of matter They are algo used at universities ac an ‘experinental tool for inetruction in nuclear engineering They are generally cooled and moderated by Light vater, and the fuel cladding is vsually aluminus

An experinental reactor, sometines called a proof-of-principle reactor, ia the first step in the development of a full-scale central station electric pouer reactor of # particular concept It ie buslt

primarily to determine whether the concept actually works or not, and if

At does, to determine some of ite characteristics Since a variety of reactor concepts have been formulated, there ere various kinds of experi~ mental reactors

2.6 Light-Mater Reactors for the Production of Electricity There are tuo types of reactors chat ave used for the central

station generation of electricity in the United States Both are Light~ water reactors; one is the pressurized-vater reactor (PUR), and the other 4s the boiling-uater reactor (SHR) Both are described in more detail below

2.6.1 Pressurized-vater reactors (Pais),

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20

2250 Ab/tn.? or about 150 times the somal atmospheric pressure) A Simplified schenatic diagram of a PWR in conjunction vith an electric pover plant is shom in Fig 2.2, The pipes and other equipment chat handle the vater that flows through the reactor constitute the prisary system The pipes and other equipnent that handle the steam that goes ke the turbine and alse the condensed water that returns constitute the secondary systen

In a PWR, the prinary systen Mater passes chrough the core of he Feactor where it is heated; chen it As pumped through che steam generator and returned to the core The heat that is picked up by the water of

the prinary system hile {t is in the core ts transferred to the vater of the secondary systen in the steam generator This transfer of heat

conDeNseR ΠSSS wo mang

COOLANT Tu OMe — tung win =

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2

turns the secondary systen water into ste: ‘and cools the primary eysten Figure 2.3 is an illustration of @ typical pressurized-vater

reactor Primary vater enters at the side of the reactor pressure vessel chrough the inlet nozzle and flows downward near the inside sur~ face of the pressure vessel to the bottom of the vessel, Then it turns upward, passes through the core where it picks up heat, exit the vessel through the outlet nozzle at the other side, and flovs to the steam

generator (piping co steam generator not shown)

Figure 2.4 is an illustration of a typical steam generator The hot water coning from the xesetor enters at the botton of Che «team generator, passes through thousands of small tubes, exits from the bottom ae cooler water, and <s punped back to the rescter The tubes

keop the water fron the primary system separate from chat of the sec ondary system ater of the secondary systen, vhích comes from the condenser of rhe curbine-generator, passes into the steam generator

Chrough the feedvater inlet and flove around the hot tubes where it Se turned tate steam The steam flovs out of the top of the steas generator and goes to the turbine-generator

Figure 2.5 ie an illustration of the nuclear steam supply systen of a PUR The system de very large — the main coolant pimps, for example, are about three to four stories high

‘The function of the pressurizer is to nantatn che pressure tn the Prinary systen The pressurizer 1s connected directly to the prinary system by a pipe Figure 2.6 i an illustration of a pressurizer The botton half of the pressurizer is F£lled wich water and the top wich stear, which le under pressure and acts as 2 cushion for minor vater oF pressure surges, The pressure of the steam is transmitted to che water at the bottom of the pressurizer and, in turn, co che ater of the primary systes via the connecting pipe If the pressure of the systen gets too low, heaters at the bottom of the pressurizer turn on and boil sove of the water; the steam generated is added to that at the top, which increases the pressure, If the pressure gets too high, cool water

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2 conTRoL, oD ORIVE MEGHANISM -THERMOEOUPLE INSTRUMENTATION supponT veoce 4

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pressurized-2

Fig 2.4, Typical stean generator in systen of a prescurized-vater reactor

aay

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2% |

Fig 2.5 Schenatie arrangenent of the nuclear stesm supply system of @ pressurized-vater reactor

from the outer steel shell) extend out at various points from the pressurizer, They are used for attaching additional piping called Lines Safety Lines and pressure-relief lines are connected to the nozzles at the top Safety valves and pressure-relief valves are

Anstalled in these Lines, and their function is to relieve the pressure AE te gets coo high