Report on the AMcclint' at the'

Chernobyl Nuclear PoWer. Station

0UE-1250

(TU.S.) Nuclear Regulatory .Comission

Washington, DC

Jan 87

.7 - 7.

I -

~~lUUT~

.1mi •

k ago"* * touinin
BIBL00RPHI DAASHEE NIJREG- 125-0

I i 1. TITL. ANO S"TITLE

Report on' th~e Acc'ident at the

Chernobyl Nuclear Power Statior

I

I

3 LIAVI OL8%k

.......................................... ~

______4 -.

January 198 a Dore ",P1UImO I

I AUT"0• IS

Multiple Authors

9

MONT" i Januar

1.ya"

1987

Department 

Electric 

of Energy -Inst., of Nuclea wr--,-------- ....---- .------ Power Research Institute V .. M% Oft W-Pasi .ow Environmental Protection Aoency Federal Emergency Managemet, Agency. . - i--- Nuclear Regulatory Commission ._"_._ _ __•

U. S. Nuclear,, RegulatoryE c ot ssnion d.ed r i ilty A cy WashingtonD. C.ý "20555- ~ ~ 'i'.

oSWO"P"A"N~T&AR NOTES

A5YRACI - .,.ae

This report presebts the CoiAM~e .,finfogatias obtained by various organ- izations regarding the accideot (MWd -the comsq8*eces 04: the accident) that occurred 

April 

ý.t uikt 4 of the-nuclear power station at Cberasbyl in the USSR on 26. 1986. Each organistation baa indepeodently accepted responsibility for one or more chapters. thre various authors are identified in a footnote to each chapter. Chapter provides a8 overvieW of the rteprt. Very briefly the other chapters cover: desi f. teei l nuCptrc2,te clear station Uiit 4; Chapter 3, safety analyses for E3t.1 ; .Chapter apts the accident scenario; Chapter 5, the role of tbhe operator; Chapter 6, a- assesneat of the rad.oactive release, 

muergency 

dispersion, an tsanport;-Chaterd 7L, 2e a tiLes. associated with actions; emid-Chte6-ifatiem t-e health and environmental consequences from the accident. ltes8e subject cover the major aspects of the a.ccident that have the pote• •il to'pcOet n;w sjnfe*Mation and less•os for the nuclearlindustry in seomral.et;Z

The task Of evaluatingft''_M '4g~~ iouta6 kaaiiw tedte -'s

pursue according to the relevace ofmthe suJect to thei o ,. To..s findings will be Loased" qrtlb h emiatog~''~ The basic purpose of this report s -to pr•vide -the; afomettif ýo wb.kcn such 1ssfsnments

can be ,.ade.

k

3⁄4

K

1.

Chernobyl Accident, Russian Reactor, Severe Accident, Core Melt., Radioactive Release, Reactivity Accident, Graphite Fire, & Pressure Tube Reactor

OW044sw e"Aft"D, te~i •am$t*l

Unlimited

UnZcl assi fled

Unclassified

Z0)9 A
-. : ; " :aII-- c u ~

.,,n7-'Cernbvi uclar

dv t o

a9

Report on,& t Accid-ent _-th Chev~iNucdear~

PowerStation

SS

U.S. NUCLEAR ..REGULATORY

,COMMLSION

-',Prepared by:

ent ofý Energy

I'.. -Electric Power Research Institute

' :Environmental Protection Agency

Federol Emergency Management Agency

Institute of Nuclear Power Operations

Nuclear Regulatory Commission
-ABSTRACT

_This report presents the compilation of informatix-,* obtained by' various organ- izations regarding the accident (and the consequences of the accident) that

occurred at Unit 4 of the nuclear power station at CbernobyiL in the USSR on

April 26, 1986. Each organization Ls independentl7 accepted responsibility for one or more chapters. -The various authors are identified in a footnote to

each chapter. Chapter 1 provides an overview of the report. Very briefly the

other chapters cover: Chapter 2, the design of the Chernobyl nuclear station

Unit,4; Chapter 3, safety analyses for Unit 4; Chapter 4, the accident scenario; Chapter 5, the role of the operator;Chap-ter6,.an assessment of the radioactive

release, dispersion, sandtruasport;-.Chapter 7, the activities associated with .emergency actions; and Chapter, infoat:ionm on the health and environmental

consequences from the accident ,•,••These subjects cover the major aspects of the

accident that have pthepont, -pew. information and lessons for the

nuclear industry ,in-.general'.

The task of evaluating .the iufomtion i-obtained in these various areas and the

assessment of the potential Aiplications bas been left to each organization to

pursue according to the relevance.of the subject to their organization. Those

findings will be issued separaLely by the cognizant organizations. The basic

purpose of this report is to provide the information upon which such assessments

can be made.

ttI
Pagte

Abstfract ............................................................ .... iii

Acknowledgments ........................................................ ix

ýýater T:-;

I Overview .......................................................... 1-1

2 Plant Design ......................... . . . . . 2-

2.1 Reactor, -Fuel,- and Fueling z.-achinei i'.... ................... 4 --

2.2 Fluid and Heat Transport ,Systems............................. 2-18

2.3 Reactor Physics ................................................. 2.24

2.4 Instrumentation ands.Control. ........................... 2-27

2.5 ElectricalPower System................................... .2-36

2.6 Safety Systems .................... ........................... 2-37

2.7 Reactor Operations..................................*.. ..... :2-49

2.8 References ......... .................................. 2-51

3 Safety Analysis .......................... .......................... 3-1

3.1 Introduction.............................. * .................. 3-1

3.2 Soviet Safety Analysis of the Chernobyl Unit 4 F.,.;actor ....... 3-3

3.3 Independent Safety. Review .. . 3-27

.3.4 References..................................................... 3-53

4 Accident Scenario... ....... ................ ........................ 4-1

4.1 0,verview... ; --. ;; . .. ..... . ... -,.. . .. . . . . . . . . .

4.2 -Event.x1ed tb ih4eiAc•cdent.. . I ........ 4-2

4. 3 EZvents:Vurift.-sa*d Atr( thel-c~dde. .................. ........ 49;

4.4 References .......... .. . . . ........................ 4-12

5 Role of Operating Personnel ........ .................... 5-1

5.1 Operator Actions and Plant Activities Before the Accident... 5-2

5.2 Immediate and Short-Tern Operator Actions ................... 5-3

5.3 Sumary of Key Operational Events and Errors ................ 5-6

5.4 Operator Actioms Following tha Accident ..................... 5-9

5.5 References ................................. 5-10

6 Radionuclide Release and Ataospheric Dispersion arid Transport 6-1

6.1 Radionuclide Release ........................................ 6-1

6.2 Atmospheric Dispersion and Transport ............ ............. 6-8
CO:TENTS (Continued)

Chapter f Page

6.3 Consistency of Soviet Estimates of Rad'.onuclide, Release

With Observed Data From Other Countries ............... ...... . ,10 References............6-12

7 Emergency Preparedness and Response...... ........................ 7-1

7.1 Emergency Plans. .................................... :7-1

7.2 Emergency Organization and Facilities ....................... 7-6

7.3 Alert and Notification System .............................. .7-7

7.4 Protective Actions Taken........ ............................ 7-8

7.5. Radiological Monitoringand Exposure Control,......... ....... 7-10

-7. 6 -.'ed ical Treatment. ................. 7-14

7.7. Soviet .,Guidance _,on.'Acep ble•-i vels. of Public

Radiation Eposure.. 4 .. v :. ............................... 7-16

7.8 Decontandtio.............................7-18

7.9 Site :Recovery. ..... . ............................ 7.10 Relocation-and Xeienr (Off Site) .......................... '7-21

7.11 Public.-EducAtionaadImoxmtion Prog•rams.....:. ". .....- §7-21

7.12 Training. rram......................................................... 7-22

7.13 Summary. . .. -. .................................. 724

7.l4Refrences. .............................................. 72 7.14 References ..... 7-27

8 Health and Environmental Consequences ...................... ....... 81

8.1 Pathways of hNmaaExposure .................................. 8-1

8.2 Health Effects ........................................ 84

8.3 Radiological Effectsa on the Soviet Union .................... 8-6

8. 4 Radiological.Effects on turoFe Outside the Soviet Union ..... 8-11

8.5 Radiological Effects on the Umited Etates ................... 8-14

8.6 Global Effects on Agricultwre and Food ...................... 8-24. 8.7 .-.kological•Iffects,•, ,:........... ......... ....................... 816

8.8 References. ..... .. -.... *...* 8416

Figure Pg

2.1 Cross-sectional vievofR -000 ................................ 2-9

2.2 Fuel channel 2...................

2.3 Zirconiui-to-stainless 'steel transition Joint. ................... 210

2.4 Assembly of graphite riags on pressure °ube and graphite block

cooling ......................................... ..............-....... 211

2.5 Cross-sectional view of reactor cavity........................... 2-12

2.6 Cross-sectional view of reactor building elevation, Chernobyl Units 3 and 4 ................................ 2-14

2.7 Layout of main building of Chernobyl Units 3 and 4 ..................... 2-15

2.8 Scbematic drawing of the 36-rod fuel element ..................... 2-17

2.9 Cross-sectional view of the fueling machine ...................... 2-19

2.10 Grab book of refueling machine ................................... 2-20

vi
'A_,iý,COrflNTS. (Continued)

Figure Page

2.11 Normal and .emergency cooling system of the Chernobyl Unit 4

Leactor. .... .........I ............................. ... .......... ... 2-21

2.12 Schematic drawing of control rod cooling system ................. 2-232

2.13 Gas circuit system .............................................. 2-24

2.14 Effect of reactor Operation on the coolant void, fuel

temperature, and moderator temperature reactivity coefficients... 2-27

2.15 Control rod design ............... ........... i ............... 2-31

-2.16 Schematic drawing of fully withdrawn and fully inserted

control rods......... ................................ 2-32

12.17 Functional diagram of a*.control rod irlyc.. 2-35

of-the reactor:_.ergency. +cooling system...... .... 2-38

2.19 Schematic drawing of themsystem for discharging steam from

the main safety -valves inteo'tepreslure suppression pool of

the accident locAllitlastis .,ta , ...................... 2-41

2.20 Systep .o protect the eactor vault from excess pressure ......... 2-43

2.21 Schematic diagram of the accident localization system ........... 2-45

2.22 Evolution of reactorparsmeters during startup ................... 2-50

3.1 Nuclear safety regulatory bodies and docume-ts in the-USSR ........ 3-6 I

4.1 Chronology of the accident at the Chernobyl Nuclear Power

Station .......................................................... 4-3

4.2 Key reactor parameters for the last five.minutes before the

accident ...................................................... 4-6

4.3 Chernobyl data evaluation of power vs. time during core

destruction phase ............................................. . 4-. 10

4.4 Photographs of the residues from model fuel pins (SPXN rods)

nfter tests in the +CDC.,(capsule driver core) siz/.Iting power .excursions fnr m reacuivity laseitio c.id......... .... 4-1

5.1 Schema tic diar O0.................... 5-2

6.1 Daily radionuclide release In the ataoephfte from the - -

damaged unit ....... 6-3...................... 

Table Z +e

2.1 Development of Soviet-graphbltmodarated, -water-cooled

reactors ............ ............................. ..... 2-2

2.2 Chernobyl Unit 4 design parameters ......... ...................... 2

2,3 Fuel assembly desigp parsmeters forCharnobyl Unit 4 ............. 2-18

2.4 Calculated reactivity coefficieuts for R ...................... 2-26 I

2.5 Types of :ontrol rods ..... ... * ......... . . .......................... 2-29

2.6 Control rod specifications ....................................... 2-32

4.1 Chronology of tbh accident at the Chernobyl Nuclear Power

Sta o ........ ................................................. 4-13

, •. .. •• ll l ~ eo le l le ~ll ll 1. ,. l ~ ~ l lll.l4 
CONTENTS (Co tined)

Table mage

5.1 Operator violations of ýprocedures .............................. 5-8

6.1 Core inventories and total releases, at he time of the

Chernobyl accident ................................................ .6-2

6.2 Daily release of radioactive substances into the atmosphere

from the damaged unit........... ,;•...................................6.

6.3 Radionuclide composition of release from the damaged unit of

Chernobyl Nuclear Power Station .................................. 6-6

6.4 Estimates of percent of core inventory released based on

measurements outside the Scviet Union... 6_12

7.1 griultralprouct in-_c'tepermitted: radioactive_

Contamination was-found-to beexceeded.............................. 7-13

•7.2 Levels of 1-131 in milk, Ny 196. .............................. 7-13

7.3 Criteria for makig erProttaibn of thepopulation.. 7-17

:11 

f.

8.1 Radionuclide contributions- .to external dose based on

spectrometric measuraemntse lin southern-Finland on

May 6 and 7, 1986. ...... ........................................

8.2 Radionuclide deposition in two U.S. areas ........................

8.3 Maximum radiation levels found in Europe following the

accident, by country ...........................................

8.4 Haximum individual doses tosgroups in the United States

due to exposures or-,intakes.,•n the first.year after the

accident ......................................................

8-2

8-3

8-12

8-15

- 1
The following individuals have :contributed substantially-in the production

of this report: From the U.S,. Department of Energy - S. Rosen, E. Purvis,

D.. -HcPherson, 'F., -Tooper and their contractrs-,, otably PacifiNoirthwest . Laboratories (J. McNeece and L. Dodd); from the. Environmental,.,Protection

Agency - S. Myers, D. Janes, J'. Puskin, C. Nelson, and*N. Nelson; from the Electric Power Research Institute - G. Vine; from the Federal Emergency Hanagement Agency - .- Sanders and V. Adler; from the Institute of Nuclear Power Generation - W. Conway and S. Visner, and from the Nuclear Regulatory Commission -

Sheron, C. Allen, F. Congel, and S. Archarya...

the Nuclear 'Regulatory Commission provided a cordinti 'function, wih regard

to the production, of ,the repirt.. C. 'Allen'fromthe IR¢acted as ther project "..manager for coordinating the production oftthe report.-'The Nuclear Regulatory Comission also provided substantial editorial (1. -arwell and staff), graphics (J. Thot-Thombson-u •i astaff)' :.:typig ý(L. ffcKenzie and word-processing staff), and printing.(N. Dsgttdand staff) -services supporting the publication of this

report. . .

,Ix
CHAPTER 1

OVERVIEW

in response to the Chernobyl Nuclear Power Station acciden°. in April 1986, a

group comprised of representatives from the Federal Government and the nuclear

power industry met to compile factual data and information relevant to understanding that accident. Specific organizations, as noted below, prepared descriptions of the accident. The, individual inputs are herein compiled and

represent, therefore, the views of the responsible organization.

iThe effort drew heavily on th:ze sources during the preparation.ofits report.

The first: source is a report prepa'ed in the Soviet Union (USSR' 11986); that was

presented to the Internatioral . ic Energy Agency (IAEA) at a meeting held

August 25-29, 1986, in Vienna, Austria (IALA Experts' Meeting). The second

mzjor source of information came from discussions with Soviet representatives

attending the IAEA Experts' Meeting in August 1986. The third major source is

a report prepared by the International Nuclear Safety Advisory Group (INSAG,

1986) for the Director General of IAEA (Post-Accident Review Meeting,'

August 30-September 5, 1986).

The focus of this report is limited to the factors bearing directly on the

accident at Chernobyl. It does not extend to all aspects of the design and

operation of the Chernobyl plant. As such, the report includes information

on the relevant areas of plant design, plant safety analysis, the accident

scenario, the role of operating personnel, radioactive releases, emergency

response, and health and environmental consequences..

Chapter 2 was prepared by the U.S. Department of Energy (DOE). It describes

the unique design of the Soviet high-power, graphite-moderated boiling-watercooled reactor at the Chernobyl Nuclear Power Station. This uniquely Soviet

design evolved "rom early demonstration and plutonium production reactors.

General characteristics of the RM1 and its predecessors include the use of

graphite as a neutron moderator and light wate-r as the coolant. Pressure

tubes, contained in vertical channels in the graphite, either contain lowenriched uranium oxide fuel or are used as locations for control rods and

instrumentation.

The use of boiling wser as a coolant ia a pressure-tube, graphite-moderated

reactor distinguishes the RBIK desiar, from any other reactor design. Other

distinguishing features of the RBHI design include:

• on-line refueling

* single uranium enrichment level

* separation of core cooling into independen. halves

* use of computerized control systems

• separate flow control for eac' pressure tube

* positive void reactivity coefficients under most operating conditions

slow scram system

1-1
steam suppression ssystet .

programmed power .setbacks (rather than scrams) for various abnormal ccjxai t oions .'"•*++• '• :-.i.•". lo• coolant--fuei.ratio. -.

accidenL localization systems

%.hapter 3,-prepaied by the Blertric Power Research Institute (EPRI), is directed

at a safety ana•lysis .of...Chernobyl- _Unit4 one of 14 operating. RBMK-.1000 reactorplants. Significant differences exist in RBMK-1000 designs, as they h:ve

evolved from the early Leningrad design (first-generat4in RBtK, eight tctal

units) to the more 2odern Smolensk design (second-generation RHBK, six total

units, including Chernobyl Units 3 and:4). '-This evolution of the RBHK design

is often difficult to-discern in Soviet literature, and details of the plantspecific differences among the 14 plants are not available. However, descrip- .. tive.material of second-generation RBK-lO00 reactors is more: complete:

especiaAlyass a .result -of information in-the Soviet :report... on .the-, act.ident,

(US,18),The *Afety analy*sis, in this- chapter ýsometimoes .%present 0cm s * r. i.., enri aalsi f ý_second-generation -PSNK-460 r'easc'tors.--here -known

differences exist betveen first- .and s&cond-generation reactors, a brief dis

cassion is included of teeff+ctsof thosedifferences o.2 the RNK safety

analysis, but ean. a"nlysis-oft,,e olde+- :desin is not included in this report.

Since many of the- design feitures'unique to the second generation do not appear

to Lave been backfitted into .the,-first generation, the reader is:cautioned

against assuming that•safety capabilities discussed here apply to the eight.

older RBmK-1000 reactors.

Chapter 4 was prepared .by :the r.S..Nuclear Regulatory Commission (NRC). It

presents the. events leading to the accident at Chernobyl Unit 4 on April 26,

1956. The events are detailel-in narrative form and are summarized in

Table 4.1. The.accident-.chronology.includes relevant information on several

aspects of tae plant design characteristics and operation and includes the

operator and procedural errorsethat contributed to the accident. These factors

were important in the.sequence of-events that. ultimately resulted in an uncontroll( d power excursion that destroyed the reactor and breached the integrity

of the reactor buildine. -he-focus- in the chapter is on the response of the

syslte to the ai'rJus mats. zts Upotln used in reconstructing the sequence of events was 9 Of mry reports cn the Chernobyl accident

prepsreA by the WI 3`ttitta OR the U1tilization of Atomic Energy (USSR,

1986) and the Intermatio"mLl,_1earSafety Advisory Group (INSAG, 1986).

Chapter 5, prepared by ..tbe_.titute for Nuclear Power Operations (INPO), explores the role of operatitS.personnel at Chernobyl Unit 4. During the performance of a turbineVgnerator test eo April 26, 1986, Chernobyl 'iit 4

experienced a core-damagin&gaccident._- A severe excursion was accompanied by a

pressure surge and fire that..destroyed the reactor and breached the surrounding

building. The test procedures had not bean adequately reviewed from a safety

stanepoint. Nanagenrat control of the performance of the test was not maintained; thebtest.procedure was not followed;-safety systems were bypassed; and

control rods were misoperated. Operators lost control of the reactor during

the performance of the test. Chapter 5 focuses on the operator actions during

the event and on the breakdowns to management/administrst.ve controls.

I:

[

.4

~-C-,

Chapter 6 wps prepared by the U.S. Nuclear Regulatory Commission (NC). It has

as Its firet topic the magnitudes and timing characteristics of release of radlonuclides iron the Chernobyl Unit 4 plant. Its second topic is the atmospheric

1-2
. dispersion and transport, of th r-released radionucl ides-.reult.inginenviron mental contaminatin within nd- Outside- of the Soviet geographic bundary,

-Radionuclide release -ana.aitmospheric -dispersion adtriansport from "'•C•eribl as

described in Chapter tare derived from the i&formation contained primarily :in:

the two reports cited.(INSAG -1986; SSR, 1986). ---The last section:of Charter 6_ offers a short discussion on ctousistency of the estimates of the radionuclide

release providedin- the Soviet-, xr t-,with.the observeddats from-:-

side the Soviet boundary. osvd. forin o.t_

Chapter 7, prepared by the Federal Emergency Management Agency (FEHA), documents the av.ilable offuite- and 0nsite emergency plans and preparednessmeasures that were in place for the Chercobyl-nuclear facility. also 0It escribes

the Soviet response to the accident, and relates it, where feasible, to the

-preaccdent-emergency planning and preparedness activities. Where: known, .eme~rgency .response .orga'iizations re identified and their. .. roles, are' described.

._The a~lert and. notif icatilon _4yste sd by theSvesi aaned The po ýýtective actions-take byl thAve r ls ecied, incuing'evacUation,-

sheltering, use of radioprotective drugs., and medical arrangements.. -PYinally, Soviet information -pertinent •todecontmntion, relocation, .and re-entry is

documented.

Chapter 8 was prepared: bytheEnvir'omntal Protection Agency `(EPA). It.examines the radiological, health and environmental consequence" associated with the

Chernobyl 

effects are 

accident. 

discussed 

"adiation 

for populations 

doses and 

at 

their 

the site, 

reported 

within 

or 

30 

calculated 

km (18.6 

health 

mi) of

.7

the site, in the balance of the European Soviet Union, in other European countries, ano in the United States. Because of limitations in the exposure data, however, most of these estimates must be regirded as tentative.

Data for the Soviet Union were -drawn chiefly from the Soviet report to the IA-.

(USSR, 1986). Estimates for other. European countries were based largely on

information reported by the World -Health Organization (WHO, 1986a and 1986b)

and individual European governmental agencies. For the United States, measuremerts made by or reported to EPA were ..employed.

References

INSAG, 1986 International Nuclear Safety Advisory Group, "Summary Report on

:the Post-Accident •Review. Meeting on the Cherrobyl Accident,"

August 30-September 5,. 1956, GLC(SPL.I)/3, IAEA, Vienna,

September 24, 1986.

USSR, 1986 USSR State-Committee on the Utilization of Atomic Energy, "The

Accident at tte Chernobyl .Nuclear Power Plant and Its Consequences," Information compiled for the lAEA Experts' Meeting,

August 25-29, 1286, Vienna;' 1986.

WHO, 1986a World Health Organizetionj Regional Office for Europe, "Summary

Review of Measurement Results Relevant for Dose Assessment,"

Update Revision No. 7, Copenhagen, June 12, 1986.

1-3
WHO, 1986b World .H~ealth. Organizatin 1eio OfficE frErpe Smr

Report of Workin.g Groupo !"6iAssessment of Radiati6n- Dose Cmi't-.

mentei Europe 7 D9,.; ote ahern, by Accident, Bilth."ven,

June:!ý 25-2, 1986, Copenhagen, Septemiber. 8, 1986.

1 5
SCHIAP:E' 2R

-'?LPANT DESIGN

The Soviet high-power, -esuei-ib etor (Soviet designation: • )BIK) is a

graphite-moderated, ýboiling-water-cooled ..reactor. -,This unique design, which

has been constructed only:in' the ,tSoviet Union, evolved from early demonstration

and plutonium production reactors General characteristics of the RBIK and its

predecessors include the use of' granphite as a ne•. -rir uioderator and light water

as the coolant. Pressure tubes, contained in vertical channels in the graphite, either contain low-.enriched uranium oxide fuel or are used-as locations for

control rods ani iastkrentation.

•The.: use.! of boili$ngwater• •as a coolant i•n iapressurerttube- giraphit&m.derated

*"reactor distinguishes i~the design from any other reactor design. Other distinguishing featiures-.ofthef ý R deajwg include

* on-line refuel.ing.

* single uranium enrich tlevel

• separation Of corecoling into in endent halves

* use of computeri-ed control ..systems

separate flow control-for each pressure tube

positive void reactivity coefficients under most operating conditions

* slow scram system

* steam suppression. system

* programied power setbacks,(rather thaný,acrams) for various abnormal

conditions

low coolant-to-fuel ratio

accident lccalizatioa .syste"s

These features are described An detail-1ater in this chapter.

The Soviet ,nuclear;;og m arch ',deelc sad ment .onse0eral

types of reactors 1Wd thecostruction4ad• operaition of

various prototypes.i the id;l96Oa, - --the Soviets decided to develop two types

uf power reactors: thbe VVXl.:(pressurized-water reactors) and the IBlKa

(boilins-water reactors).

The evolution of the general design parameters related to Soviet graphitemoderated, water-cooled -reactors s-ýpresented in Table 2.1 (SMenov, 1983;

Rlimov, 1975). The units at thetiberim Atomic Power Station were built as

dual-purpose reactors .:(J•iv, 1975).to produce both plutonium and electricity. The Beloyarsk reactors are demonstration plants and are unique because they superheat the steam in the reactor-. core.__,

S. Rosen, 1. Purvis, D. McPherson, and F. Tooper of the U.S. Department of

Energy (DOE) and DOE contractors, notably Pacific Northwest Laborstorief

(J. ;Neecre and L. Dodd) cqpiled this chapter.

2-1
Table 2. 1 Development of Soviet graphite-moderated, wuter-cowoled reactors

Item

Year of operation

•lectrical capacity (mle)

Thermal capacity ("WtO

Nmber of fuel channels

(equilibrium loading)

Ymber of stem superheat chaunels

Steam prelsure at tumbise , (at.)

Stem teortUre (0C)

Total uranium loading

(tonne)

Average' enrichment (%)

Burnup ("WARg)

Specific power (kl/kg)

Fuel cladding material

Obainsk

1954

5

3.0

Ii.

54.6

Stainle~ss

s't•.e

SAPS*

1958

>100

Peloyarsk-l

1964

100

286

0

998

268

87

500

Beloyarsk-2

1967

200

'530

998

.266

87

560

48

3.0

14'.6

16.7 •

Stainlesse

steel

Chernobyl-4

1983

1000

3140

1661

0.i"..

BMKs

Ignalina-1

1984

1500

0

4.3

Aluminum

67

1.8

4.0

11.3

Stainless

steel

65

280

189

2.0

22.3

25.4

Zirconium

1% niobium

65,

280"

19

2.0.

21.6

Zirconium

1% niobium

*Siberian Atomic Power

generators.

Station - SZx identical units using double coolait circuits with steam

I
The first RBHK was a lOOO-HWe plant.brougjht on. line- in. 1973 at, the, Leigrad Atcjic Power Station • ".-th Chernebyl Unit 4 reactor ii considered a sec•ondgeneration plant because, ethe design includes a number of safety features- not

present in Leningrad •Unit. -

At the time of the accident,:-•1 RBMK- 000- ractors were In operation ai.. ddition to a 1500-NWe REII. plantoperatingat. Ignalins (Table 2.1). The M " 1500

design differs little trau •M-1000-design. te The, cores are, :essentially

identical. ',ýP]ahs:e:La exis-t: s to !butilXd eeven larger plants vith electrical capacities

as large as 2400 lIe.

The Soviets had seve reasons -for pursuing the RBWK design. Theýe reasons

included .(Semenov '1983)

an extensive engineering experience base with graphite-moderated, boiling- X

water-.cooled-reactors .

* ,existing, madufatua g lants' coud'.fabricate -major.cI co nets

the reactor sizeoii liktedibi considerations • related to fabrication,

transportati, or.insa•tion, of. co• nents

a serious loss-of-coolant accident larger than that considered as design

basis thought tobe .virtully.ibpossible because of the use of numerous

pressure tubes .rather than a single pressure vessel

• very efficient fuel use

• use of online refueling could achieve a very high plant capacity factor

The Soviets considered-the MW to ýbe their "national" reactor and showed con.

siderable pride in thedevelopment of the design. A number of design issues

were identified by the Soviets and. addressed in newer designs. Economies of

scale, control, and safety vre ,three such issues:

• Economies of cae..• ,he .economica were -recognized to improve substantially

by going to -ger desips. Asae t, one 1500-1BWe RBM iswcurrently

operating aa-.ye-mg 4W in construction.-"",Plans -exist for "

plants as large as -2400 MIe.

Control: The 83IS-1000 -was recognized to have stability problems and was

difficult to-control, particularly at low power levels. The approach to

resolving these problems was to place increased rel.iance on automatic control systeas and adopt a-slightly higher fuel enrichment and slightly

lower graphite moderator density in order to decrease the positive void

coefficient..

Safety: The Soviets. re-evaluated the safety systems of their reactors.

- As a result, later -3J -desigpns, incliding Chernobyl Unit 4, incorporated

improvements in emergency core cooling systems and steam suppression pools.

A summary of the key design p4rameters of the Chernobyl Unit 4 reactor is liven

in Ta,'1r 7.2 (USSR, 1986).

2-3
Table. 22 hernobyl -- 7t design parameters

I tern -D ecript Ion

General Design Characteristics

Reactor type ' Vertical pressure tube, boiling water,

moderated

Refueling .. On-line

Design power generation. .W 3200 

Total reactor coolant flowrate 37,600 tonnes/hr (23,026 lbm/sec)

Core Description

Core- dimensions (active ."zone).

Height 1 0 A (23. 0 ft)

Diameter .- 11.8. "(38.7 ft)

Volume 7ý5•0:4 O (20,655 ft3)

Total number of fuel channels 1661

Lattice spacing 25 cm z 25 cm (9.8 in. x 9.8 in.)

Moderator material Graphite

Maximum allowable measured -7500C (1382°V)

temperature

Material density 2.6ii- /c•s (103 lb/ft3)

Reflector.ý dOmentona i -.

Top and bottom .84 a (1 -,64 ft)

Sides 0-O.88 A (2.89 ft)

Graphite core weight 1700 tonnes (3.74 x 106 lb)

"-6s

J-a

Q4

g-A

Fuel Description

Design

Uranium aterial

Cladding material

Enrichment

-Two 18-rod elements connected in series

U 2

Zr-li Wb

2.0 vt% V-235

2-4
Table 2.21 (Continued)

Description eItem 

Fuel Description (Conti.ý.d-)

Fuel A 8ssembly" eion Selt ent -',6.9 * (22.61 ft0

Maximum cladding temperature 3230C (613 0 F).

Maximum fuel. temperature -2100*C (3812*F)

Total uranium weight 140 tonnes -(418,500 lb.)

-`-Maximm fuel exposure 2. iDk

Wate , R ecirculartion System,

System material Abstenitic stainless steel

Independent flow loops 2

Steam drums -. , total, 2.per loop

Pumps 8 total, 6 normally operating

Pump dynamic head 1.96 HPa (284 psi)

Net positive suction head - .6 lia (87 psi)

Main pump suction anddischarse -90 co (35.4 in.)

header diameters

Main pump capacity 5500 to 12,000.1 3 /hr

(24,200to 52,100 Via)

Dirensions of indivi&dsua-l pres sure 5.7 z 0.35 cm (2.2 x 0.14 in.) tube inlet piping (010:z vall)

Dimensions of individual pressure 7.6 x 0.4 cm (3.0 x 0.16 in.)

tube outlet piping (OD z wall)

Fuel Channel

Number . 1661

Pressure tube diameter (0D) 1.8 ecm (3.46 in.)

Pressure tube wall thickness 0.4 cm (0.158 in.)

Materidl Zr-2.5% Kb

Connection Diffusion welded Zr to stainless steel

joint in core sone

T,4'~

* .- AR

M,

* ~ 4 'g

2-5
-1Ta -hannel le 2. (Continued

• I em-,.,.,• ,.,:.,•.-,_.•;:• ..... ,• .... D]escription.. .

Fuel Channels (Continued)

Individual channeljflov - Manually-adjusted #ntrol regulatng"valve,

Inlet temperature -270°C?(518°F)

Ou-;ýet temperature (avg. 28140C f(5430F3

Operating pressure .- 6.8 HPa (986 psig)

-Quality , - . 4. ,(average steam), 20. mxmm

Average tube power-- 1890 kWt

Axial peak/avg.. powerratio1 40

Radial peak/avg. power -ratio .48

Steam Secondary System,

Steam collector yPdmsr system steam drum separators

Number of collectors 4 total, 2 per loop

Collector (ID x lengt.),. .. 2.6 30.984 m (8.5 x 101.7 ft)

Steam flow rate (total) _5800 MT/hr (3552 lbe/sec)

Power generation 1000 Ee (two 500-EWe turbine

g eritors)

Beat rejection. ,wit o atsrý eservotr :(condenser)

generators . .

Feedwater inlet temperatmre toi_-steas '•1-656C A3290F)

separators •"

Control Shutdown and .$fety q hutdown

syste'

-Type 3 4 eets *Acssed is alumintm,

Number of control shutdown assesblite

Neutron absorption materiel

Control rid spacing

ýIoweroid• and retrieved from ab~ove Dy a

belt .6able and motorized drum

211

34 C

500 m x 500 m (19.7 in. x 19.7 in.)

2-6
STable 2.2, (Continued)

Item Description

0ontrol Shutdown` an4Sfety- Shutdown

S- s em (Continued) .,-

Control rod tiravel, 6.425a mf(20.5

rod-4.5m

ro-0Om

4..

-~

'~- 55 4.-

* 452 -~ S -s>-' 55.S~

4

(14.8 ft) and axial control

(23f)

L0011U• 0etnoa 5_ernarate.e

do~wnvarf

Conro trd u~ltrertimm time 20seicond,

9O2t reactivity. Insertion timle , 0secod

Owe Epre ssure Control. &ate

Type Partial 4sti

ster-cooling syistem with

low 'in iAndividual chnnlso

a Ahr (15Yt ~ tlr

Z

san suppfression of releases

Enclosure

Function

Design press

Operationandthe reactrs cavity, ilet.pipin

-Reactor core inlet and piping system

-C-ten steamtr from piping break or

steam separator relief valves

ure rUclosure areasdesigned for either

0.-4S MPa (65 psig) or 0.18 HPa

(26ipsig) beko. ta ........ •:......•,•.,•; • • • ter- from pip• resko~tn••..... ....

740 epastor relief '-vale irectdo - - ,t&6ding water in bubbler pond below

" .a•Csctor. Water spray above bubbler

Pond helps condensation process.

2.1 Reactor, fuel., a uua chine

2.1.1 Iighlights

Chernobyl Unit 4 is-!& 1000-,e, vertical pressure tube, boilini-water reactor

that uses online refueling. Tbe core and reflector are in a cylindrical

graphite stack witb a diameter of 13.56 m and a height of 8 a. The reactor is,

penetrated by about 2000 channels that provide locations for fuel, control rods,

and instrumentation. The fuel is 2% enriched uranium oxide clad with zirconium

containing 17 niobium (Zr-lI b). Fuel elements are constructed in 18-element

clusters connected to a ceatral support tube. There are two subassembly

clusters approximately 3.5 w (11.5 it) long in each fueled pressure tube. The

2-7
fueling machine is. a massive piece of e u "ipment that operates over the reactor

operating• 'floor and' is'Ides-igned tfolad ful while the reactor is at full poer.- -

2.1-.2 Reactor (Dollethal 1980b usS 1986)

Chernobyl Unit 4 is 'aI'1000-We, vverticl•. pressure-tube, boiling-water reactor that uses online reiueling. -The plant contains two independent primary.,, recir- culation coolnt loops-r•{ tt. ere separae h&lves of the reactoar. Figure 2.1

shows a schematic cross-section of the Leningrad first-generation RBIMK-1000, which is representative: f -Chernobyl Unit 4. Each loop has four primary recirculation pumps (with three -functioning under normal operating conditions) and two steam separators..

The primary coolant from these pumps discharges to a common header to.-which:. 22

group distributi1on`,ihe aders are'" c"onnected., Supply lines for individuAl.ýprei6sure -tubes originate a tonaeneheduply line. containsa• manuallyoperat

flow-regulatings" valve andilov melter. The 'coo-lant is..dirce upte16 ue!' channhels pa st -the "fuelasmle (see FIigure' 2.2). T he JinlIe't water reaches the saturation temperatureit-a-bout one-third-of the-length of the fuel.element. Nucleate boiling occurs-- ove zrte idif -the fuel length.

The pressure tubes in the ýcore sare made of z=irconium containing 2.5% niobium. The Zr-2.5% Nb is diffusion -weldedto.stainless steel piping by heating it -to 600°C under a vacum ý(see Figure 2.3) .-The joints are constructed- separately and joined to the tube- 8ass8 ly before installation. The top and bottom tran- sition joints are located immediately above and below the graphite reflector. A permissible rate of heating and cooling of 10 to 15°C per hour has been established on the basis of thermal and strength tests.

.Chernobyl Unit 4 has 211 cont.rol dishutdown rods. The rods are functionslly " divided tion or scram 

into manual rods control sho~ rads,"automatic bsorbi control rods, emergency power reduc- g rods, and compensating rods.

The 1700-tonne graphite moderator-is stacked in the shape of a vertical cylinder 11.8 m (38.7 ft). in diameter and 77" (23 ft) high. -Each column is composed of 25 cm z 25 tm (9.inz9. laa.) apbitebloc Teuiblcks are60

(23.*6 in)high, -anid b,iki 4Ued b1osat. Latalled An the-SO-cu (19.7-in. top -and- bottom reflectors a toa rai 1for teskheightof80 0i(26 a -2 ft). " The outer side reflector 088 (.9 1t) .1. thick, making a total stack diameter of 13.6 m (44.5 ft). The :aide reflector graphite columns are pinneo with cooled steel tubes to enhance rigidity and provide reflector cooling. The -moderator and reflector -columns are-capped• n both top and bottom with a thermal shield. The top caps are steel-blocks 250 ' (9.84 in.) thick and the bottom caps are also steel but 200 '-m (7-87 4a.) thick.

A gas mixture, nominally 80% helium and 201 nitrogen, is fed into a chamber below the reactor where it is distributed across the bottom face of the reactor. The gas mixture flows between the graphite columns, providing a heat-conducting medium for transmitting the graphite heat to the coolant channels. The space between the tubes in the channels is fitted with graphite rings, which are fitted alternately to the tube and graphite chanoel opening (see Figure 2.4). During reactor startup operations a pure nitrogen cover it used. The graphite temperature can reach 7500C (13820F) under these conditions. The gas mixture is monitored for moisture to detect leaka&e from the tubes.

2-8
I

Figare, 2.1 Cross-Aectional iie'w of RIM7~p- AtA ~
11

I

* 4h

~'1

C4

ap

lN

I -

P:

V

2-10
I

Centerline

)662*Fl

)I f6OSIF)

,o(590*F)

284*C

1543*Fl

'Pressute Tube

0 4 6m0 S n.)

cm (3 46 re

n13 68efin

V. .. ... ...... 

• i~• 

Figure 2.4 Assembly of graphite

rings on pressure tube and

graphite block cooling

2.1.2.1 Reactor Core, Reactor Cavity and Vessel (Dollezhal, 1980a, b;

Dubrovsky, 1981; Dollezbal, 1q77; .USSR-, 1986)

A-cross-section of the reactor ccre,

The reactor ic located in a cavity 2K

(7! ft x 71 ft x 84 ft). The reactoc

cylindrical vessel forr.ed by a 14.:-,:.

steel shell. This shell is bou"dcl

biological shields. The shell, Lop',

shields, forms the closed reactor si,

:vy.-nd vessel is shown in Figure 2.5.

x 21.6 m long x 25.5 a deep

ILe core is located in a sealed

(47.6-ft) x 9.75-m high (32-ft)

and bottom by upper and lower

the top and bottom biological

The 16-mr (0.63-in.) thick reactor v(. servee sairly as a gas barrier and

structural restraint for the graphite. reactor vessel contains the circulating helium-nitrogeu atmosphere for L'. graphite moderator at a pressure of

about 0.0015 KPa (0.22 ps•g). The spat outside the reactor vessel is filled

with nitrogen at a pressure of'0.0017 MI'a (0.25 psig), which is greater than

the oressure in tCe reactor vessel.
'e

U

Wtided

Tube Ducts

UP.per B iological .. ... < :, ':. ..

Rollersf

Graphite

Reactor Vessel

Nitrogen

Annular

Wiwe Tank .

-Lower

J Shwid

-or Support

~8l1

t) in dia """

ided to a

Irical tube

grate. The

er1 to

ical sbioedrive

-. 79

7 Pre. su. Boun.ar,.!.

Figue 2.5iCt~as~ct!fioi_*tl i.,<.f -reactor cavity

2.1.2.2 Upper and Lover.Shield vand eactor Suppott (Dubrovsky, HS

Dollezhal, 1977; Dollehal, l:980b• USSR, 1986)

The upper biological shield is a.cylindrical shell about 17 a (56 1

meter and 3 a (10 it) thick. -It consists of two circular plates ie

cylindrical outer shell. :Additional strength Is provided by vertic

ing ribF. 0peninsP for Lhe pressure tubes consist of welded cylind

ducts. The space between the ducts is filled with serpentine sggre

entire assembly, whichiweijhi '1000 tonii(2.24O05 lb), retts onto

accommodate thermal expansion. Xs addition to providing for bioloi ins, it also supports the weight of the fuel channels, control rod

channels, the upper reactor outlet piping, and, tioe removable fl.vr covering.

The lower biolotiral shield is 14.5 m (48 it) to dism'ter sod 2 m (6.5 ft)

thirk. It is similar tn construction to the upprr binollfcal thield. This

2-12
shield- transmits' theis.d'; the, graphite Anil" lower pipin$- toý the main reacto'r

su~pport immediately,, under it.

The main reactor support 4s made ot two steel platsi stiffening ri o.3 "

Theflorse~s S •0•h•s,81dto6g~b ''l~d plate wi sthe f enriegr. 's- 5.3 '

(17 ft) high placedperpendicular to each other (cruciform, shape). This sWupport transduits the, weight of the lover shield and the ri eol r to thelding

foundation.

2.1.2 3 Uipper Flo Sa (urovaky,18;Dleza,18 USR 1986)

The floor of the reactor haila-cotrteofremovable eCti'ons tf.e r allow,

access to the fuelnchanes;t io-!r6--m-iit .iieoieds, and controdsl r drives.

The floor set..ves as she AboiologilJ and ( in.a barrierk. 1The re.-

movable sections are made of steel, structnres filled with iron-bariumt-serpfentine

concrete and rest onpthe -et % s-nel du biologicald shield. r' is

extracted from th 6ý reacto§ th5nhgp q~ ntefoo opoiefrc ing

inle theh firoe r,. t. prvd

'end to preventsthTpossbnterintiv ste

building;iym nein h eco

2.1.2.4 Side tio icat d--198; SolleShal, 9 90,b USSR

1986)

'double-walled vessel, (D 66 m( )nside d4iaimeter (l ) -zh 19.0, (62ik t

outside diameter (OD) su obste reactor' edvessel inside the reactor'cavity.

The vessel consists of-16 wa ter-fied compartments and prov des shielding in

the lateral direction. I-c e Vessel , Wlls ,are 30m (2 0 in.) thick. eThe space

between the water-filled-shield and the walls of the reactor cavitytis fill8d

with sand. The space between the water-filled shield and the reactor vessel is

filled with nitrogen.

2.1.2.5 Reactor Cavt li-s (brovaky, 191 USSR 198)

The reactor cavity walls are msde oft feforce concrete 2 a (6.5 ft) thick.

2.1.3 Reactor Nall .(Dubrovid 11 'hielifa- 1981; Dollezhal,.. .198c,: eý.; Usike

1984; USSR-,,18)>K~;i

'A cross-section of b .ild 8pet fuel d4to Lhe s shoonl ns Figure 2.6. A

plan view of-Units 3 and 4 asonA iue2.7.

26 hall (the the reldtng Adreactor -aeabov cover, of the reactor) is a

lUrge open workspace containing the refueling machine and an upper, higbh-ay

area with a 50-tonne-capacity overead travelint crane. The refuelin& machine.

which weighs about 350 -tounmesq,1,_ mone F1a on a traveling bridge. The inside

dimensions of the reactor -ball ýare about 24 a -wtde x 80 a lons a 35 a high

(79 ft x 262 ft- z 125 ft). -Th1e lever bay As- constricted of reinforced concrete

&and has, walls about 1.3 a (5 ft) thick. T1he massive walls and columns support

the fueling machine and provide shielding- -for the ate&* separators- located

adjacent to the reactor hall. A-- spent fuel storage pool, 1,9 located in each

reactor ball. The bigh-bay portion of the reactor hall is of steel frame

construction using precaat concrete panel sheathing for the walls. Tbe reactor

hall roof, atop the high bay, is supported by steel trusses about 6 m (20 ft)

deep. The mass of a preassembled roof block is 50 tonnes. The reactor has

four roof block&, and each block is 20 a x 24 • x 6 a (66 ft x 79 ft x 20 ft).

2-13
as

tg-vmfFmuw 2. 12'20t ins auln aw. 3 l metaw wU-S3Wu-wtn.ICnmwO-Adma mmsus6W4 o~ 1 ~ 'west. S. 0.u~ 5 60 104 -mi-edti-m-lfig etane: 10 -Main citculatng-YMn II Elgetrc teo mkin cwlculoftlpump.; 12, OniinSepaf sta. 3W. *at e.s pomw ulsd 8si9wedts=s:qwija 14 m-hnim IS 5tfen SMKtOOecs 100M-, S. 6-AcII cottdmi~nmet avsO9. 17- Sut1s pnD.A 0" %e ýs* Mg weir Cellii oia 6 1w sw 2 om;2beard w.h~o oalns 2Eaas ~mato latlcaens 3 Pau

Figure 2.6: Ckroe-sectional view of

,,Chernobyl Units 3'and 4.

ieaftorý buildingelevation, " .4
2~

- ~ I ~4WA Ii ,

51 1 .~1wA

hit taxis W 

Figure 2.7 Layout fi nbil OfCherobyl Units 3 and 4

.So•~c •~Dubrovsky, 11981, p. 95.

21.4 -Reactor- Building and Turbine Generator aill (Dubrovsky, 1981; Konviz,

1981; USSR, 1986)

rk~e• overal-l-dimensi~ons"-of be-ctoruild~~tins, not including the turbine

generator hall and conneLting mounting fframe, are about 72 m wvde x 160 m long

x 50 a high (236 ft x 525 -ft., ••164 ft) (see. Figure 2.6). The distance fromground elevation to the top oftei ghb ay-is 71 a (233 ft). The reactors are

Separated by a wall and shared ventilation .systems. A ventilation stack is

mounted between the- two units drectly above the ý-enera1 --ventilation equipment.

Mke control rooms of'ýC& 1ro~ 3nt r eaaeylctdiSdg@ 41d e 

Large room.

rhe 'reactor building is enelly -constracted. of veintorced co:•crete, most of

ihich is precast, but thick walls o10aer: 70 cm (2.3 ft)] are bi t by the

3recast cast in situ" -metbod usiagprefabricated reinforred forr. panels.

Aore than 200,000 m2 (2 million ft') of building surface on each power unit

As a special protective coveritg stated• to be polyethylene, presumably for

!ase of surface decontamination. *

turbine generator hall, about Wec wide x 400 a long x 30 a high (167 ft x

312 ft x 98 ft), adjoins the reactor building. The saace between the turbine

Square brackets denote information believed to be ty.je but not found in Soviet

2-15
generator hall and'the reactor buatiding is oiccupied . an intermedie bul ing.

floor .are 

Theuperflor occupie ae 

by ccpieda ',entral by d&-arator and f pipe aisleý, and- ihe lower

switchgear, stor age 

control. board, 

an 

unit 

f.shelve., control ba-ds h se.. t elect rical e+uii.ment . -

21.5 Fuel Assembly oesign (Dollezhal', USR W981; 96

The 

+ . l 

fuel 

r r o d .

assembly 

• E a c h 

onits 

, 4 

of, 

": 

two circular.lb-rod clusters, connected by at cen- -i-ý+" " • • , .. .. .d t•f- + . + - :•• . . . - . . . . - .. . . . .: .. ..+ . l r ft) E c n adConsists ring of 6 rods and of an inner. an-outer rino~f12 rods,held by: stainless steel spacer grids and 2 end plates. Th ulrd r opsed ofcldigtbs(rl ) containing sintered aium ozd pellets. 'The central rod is made of Zr-2.5•, Nb. A . schematic dawn oth asebysshowninFgr2..Dtlso the design are given in Tablea, 2.3........

21.6 Fueling 

.achine (Dollethal, c, Sb, 1986)

The refueling aystem-includes.. a-W @'tlune cran t at spans .the re ctor area; a carriage tht opeaes along the crane;-ils; and the refueling mchine, which Is held by the carriaie-.-V 

is ~ ~ ~ i-•.•••...::..- :-. _i c by ~ hel 1 th cariag (Fg r . ) h hole assembly weighs about 35ý0 tonne (770,00 _ . Tbe 

fuel channels and over,: th positioned over any of the 1661

to refuel five fuel channels: dui 

. t .a 

g aa. T .n .machineisdsned .24-o i o l at l ... refulin .... 

The is,ý&ao-ed' ....... atprmgi~- d-l -. .. refueli system is designed-toefuel period while -t least 10 channels every 24-hour

sen of defetiv 

the reactor 

fuel mel, iabtdown. 

t s and- mo Aefueling ruh•wtoutr at full powrprisrpae Pnermits relae generationf 

..n 

d 

n . ef 

Ti .

e fueling m 

m 

e andnral refueling without interrupting power blies from storage to 

n .caUbe used to move -irradiated fuel assem- the rasctor or from ome reactor position to another.

Wh ile centered over a fuel channelthe ' refueling machine l a cylinder tha Cotan a qelýkc ....... 6q&•u machine. lowers a cylinder tha cotais asea, wichfit vr-the. outside of the fuel channel nozzle. Th 

The cyiner 

cylinder, whic 

whc :L*,+.osrt ' +i

can on-board ve- --

h ti 

ank-. 

par. 

h 

of te preessuVe vessel, is filled with water from

cap isaready for remval. 

st- - is , .Pressurired, at which time the nozzle A grbab hook,o loated inside the pressurized cylinder

grab hook are rmntely-ci•eed ao4 +he -mla- ,+- nova ug Ozten of thesios : actuating device, " lotIM-Ahe outside of the lug is t h e n r o t at e d . T hk i"+ . . .+ - P , I " then otatd. Tis rtatin ineals the nMOzzl -plug gasket and releases the b l g . e the: t me :. plu in place. The pressure pressurized refuelng in the hi e cylader i9 h e t Internal, low;.. prf ~ •..hs n to hilber than the pressure tn the iN, the shield thus, pl; - ' t ozzle Plus frCi being ejected. The nozzle

into pl~a, sheldplu~ te th suPeaO"10 r~od and the fuel assembly are lifted

cartridge the Pressurized bolder. The cylinder cartrid.geis of.,the rotated refuelig tpemtisronfa machine and reandwtia gauge (used 

attached 

to check 

nozzle 

the 

and 

f"el 

shield 

ch"-i . er. The fresh fuel assembly, with plug, 12 tm lowered Into the fuel channel.

)uring-.this 

rro. the pre1surizd 

entire refueling 

cylinder operation wster is pumped at a crntr-lied rate inO the flchannel tlenrnts. After the fresh to cool the discharged fuel

'Sal engaged, 

fuel is in place., the nozzle plus locking device is

Irpressurixed and 

sealing 

the cylinder 

the plus 

is 

gasket. 

retracted. 

The refueling machine seals are then A biological shield plug is moved

2-16
'4

-J-6

ote- that the fuel le*Sth i, eci+basembIy- .I .- 3.43a (11.2 wth a a-t), 20-rm

(0.79-in.) Sap between the subassemblies. T•be upper and lover assemblies have

their rod plenums at the upper and lower ends, respectively.

Figure 2.8 Schematic dravi*g of tbe 36-rod fuel element (18 rods ica ech

of two subassemblles)

Source: Dollezhal, 1981.

.2-17
Fuel ii'se.b•y• design pa.am ek tiit" for ,. C henin Init 4 - ,."• Table 2.'3:

Parameter . Value

Subassembl ies per assembly 2

Number of ro-ds-:pe--r 46 asikblo~~.~-

Assembly out.er diiamete'r 9 mm (3.1 in.)

Length of assebly fe reion 6.9 at) * (23.0 

Length of active, fue per rod 3' 43 . (11.25 ft)

Plenum length 17 5 cm (6.9 in.)

Cladding tube outer diimeter 13.6 - (0.5 in.)

Cladding radial vall.thickness 0.9 -m (0.035 in.)

Cladding material

Fuel material '0

Fuel enrichment - 2 0 wt 7. U--35 r--23-' 

.Fuel pellet.diamete'11.5 - (0.45 in.)-

Fuel pellet length -`.15. 0 (0.59 in.)

Minimum pelle 46e8sit -&104/,cc (0f.37.6 lb/in.)

Pellet end Dshed -

Fuel cladding gap --0. 18 to 0.38 m-

(0. 007 to 0.015 in.)

Fill gas compositi'on". Be

Fill gas pressure 0.1 P&P (14.7 psi)

Water-to-fuel volume .ratito 1.23

-V.-- ~

9-V<* ,$

IA,' .9

9A9~

* 7.

into, position belov the .refuting -machine, and the discharged fuel, element. is

transported to the spent fue•l -.storage pool.

-2 fluid and deat lTrawort. Sysltes

'.2.1 Highlights - -

Three principal f udn'saw .- aerytm are usedinteCroylUt4

reactor: (1) Vthep i 7 i g ytem, ehhcoolsthe core- andproduce - ..

-power; (2) the. rod cooling system, wontlhich provides cooling to the control rods and the reflectors; ,a (3) the .reactor gas circuit, which enhances

heat transfer from the :graphite.moderator. -Ito the preasnre tubes.

2.2.2 Primary Cooling. System (Sedov, 1979; Novosel'skii, 1984; Dubrovsky, 1981;

Dollezhal, 1951;-loromn, 1980; Kulikov, 1984; tVSR, 1986)

'The Chernobyl Unit & reactor contains two-1ndependent primary coolant loops,

each of which cools half of the reactor. .-A schematic drawing of the cpoling

system. is shown in Figure. 2.11.. Ech loop._ .as four _primary coolantpump s, three

of which are normally i tn-t the fourth acts as a backup. Each pump has a

capacity of 5500 Lo 12,000 us/hr (about 24,200 to 52,800 gm) and a dynamic

head of 1.96 MPs (284 psL). -The discharge line from each pump also has a check

valve, to prevent backflow should the pump fail, and a flow-regulating valve.

The pup.ps are fitted with heavy flywheels to provide a 120-second rundown time

in cant of a loss of electrical power to the pump, and to provide interim cooling until nA'tural circulation can be established IFtural circulation is expectrd to be established 30 to 35 seconds after &in pumps are deenersized.
77

LZGEND: 2-Cartridge; -2fSupport *Itoipm t; 3ý-Upper Pressure Vessel Seonion; ,4-Grab Drive; 5-Grab Actuating Chains; 6-Framevork; ?-Chain Winding Necbanimo; S-Cartridge Rotation Drive; 9-1,dd. e-rssure Vessel Section; 10-Shutoff Device;. li-Planual ?ube Location Device; :12-2elevision Tube Location Device; 13-lovable Lower Shielding; 14-Lower Pressure Vessel Section; 35-Special Book; 16-Se.lng• Drive;. 17-Shieldi..; 18-Cram-; 19-Carria.e.

Figure 2.9 Crouu-soctional view of the furlifig "Cahinie

2-19
.. .. .. .. , ,R e.. 

'•it•t:•Zt. .

•" d, .' .. ,

RM

2ach of these Fall~~

permit natural- _cirutio hugte stalled pumps.

-Plilions

Gooa, RaC*

........................ ,•...........•••..,•, .... .. ,.!:.

~Jews

,ueling machine.

to theW.. ý,Two46W mmly ope ft and tbe outlet of the pump after shutdown 5f the four in-

:4

zeato

The coolant from the 'P s flows toA cmon.'header and then co twenty-two 32.5-cu (12.8-in.) diaemeter datrai' or hders on each half of the reactor. The individual supply pipes of 5.7-m (2.in.) diameter, and 0.35-cm (0.14-in.) wall thickness to- the presWre tubes are connected to these dis- tributor headers. Lack supply pipe contains a manually operated flow- regulating valve and £f1 i•meter. .. Pressure tube coolant flow, and thereby steam quality, is set by adjusting these flow-control valves on the basis of calcu- lated channel power, calculated power distribution, and measured inlet temper- ature. The coolant is directed frm the supply pipes up through the fuel channels. The full core coolant flow at 200% power is 37,600 tonnes per hour. The inlet water, initially at 2700C (51307), is Leated to the average saturs- tion temperature, 2864C (S43el).

2-20
trator

'~2. 

",

W* CbW'44.

Figure 2.-11. Normal and emergency cooling systems of the

Cthernobyl -Unit 4..reactor

At approximately 2.3-a(755 i.tc).ito-the active core, bulk nucleate boiling'

-ýoccurs, and this. pces..continuaes alongthe remainder of the ch*nnel. The

average exit steam qnulityot the core is 14.5%, and the maximum exit steam

quality Is 20.1%. ~~

'The steam-vatar- 9*W~Aftuz~sfee rsue ue r ~dvdal .carried by :pipes .... , d ter and-0.4-cu (0.16-u.) wall thickess to four trontd.--,se arators, 2.6 m (8.53 ft) in internal

diameter and 31.0 -mi. (10.1. f)A" A& ength. Two separators serve each loop.

Steam exits from the tp-:.f ea.chseprator into two 426-rn (16.77-in.) diameter

steam headers. BDeteean .theseparator.*outleta and the main turbine or steam

dump inlets, these two h*eaders7jijon U .fom a single 630-m (24.8-in.) dimetar header, which passe . 1frm th e tor building into the turbine gallery.

There are four 630-m (24.8-ih.)••headers. T-:hese headers are cross-connected

to headers that can f :eedeitrh steam duo or one or both of the 500-eWe

turbine generator sets. .

The pipe section locatsd :before the-turbine main steam valves contait- various

steam discharge devices: eight mim safety valves with a throughput of 725

tonnes (1.5 million Ibm)-of stem per hour, four tutbine condenser fast-acting

steam dumv stations with a cavacitv of 725 tonnes (1.5 million lb.) of steam

per hour (two per turbine plant), and six service-load fast-acting steam dump

stations.

2-21
Steam. at 6. 46 Mls(97pia,20C(3F) n 0i61.11 or ois relative humsidity

Sfed: from. the; iin sea.,e'.-aer into the first. . ta ofhe four-, stg•.. higk.

pressure tubine. :bSom 1ighI+]ess e • m ,:fi, bled•I- off Vstjream of te turbine

inliet' v'alvet, as well: ~asm f r66m inar-stage #9"i t~hg-pisr turine,- and

sent to the heating-side of the reheater/sueraters, .g the jet J pumps for te '

main condenser air ejectors, a h e m'ainnd tu.rbieu.if. e- a, -tem.

After exiting the high-pr~essure e .turbine, the stea*ass . g e s t hog • d h. oeo o f t h e•oe 

two separator/reheaters and.ia dried and sup`4.oer•h d to 0..40 VOW (58s!i a).and 2630C (4730F) b+•i tes•8 iif... th f r::

260 (7 0F eore entern one ofteor 4-staige low-pressure- tunrbiui.

Inter-stage steam is ed fro 6ariou *tap in the o turbines.t

service condensate .reheaiid:•itersaduiiay the oads. :.:-•.d:;;.-+:.li ..... 'r+": "

After leaving the low1-pressure turbine, the steam e. nters one of the four sections of the main conde'nsr where it ondensesa. 04 Ea (5.8 psia),-From the:

condensers, the watr s'umpdoac .t o -- _-manztamsear-os(;in-fu

ýelectric bigh-pres'sure -feeps a dsi po isher(for pu ificto

and water them"iistrIro,' _nd*• '-rator wiha

attached explosive gas recombiner The feediater entest sfparntors,

at 6.964 HPa, (1010 paia) a"nd.656C • .(-3290F).-.Iti's4 mixed- with 2oC (U. 3F) ..

saturated water to-povdreciltion water 270oC (518).).

Twelve downcomer- pip Area-itta-ched to"t-hebottom of :cac seam seprator. ..

These .pipes connect .wit~h <ac o:headerthat feeds the. auction of the primary.

coolant pumps. headr",.and thpmpdischarge Whis header described earlier, YI

are 90 €a (35.4 in.) in internal.diameter.

Under certain (unspecified, but r-esumably low-power) conditions, steam from the

reactor can bypass the main: turbines and be discharged to the main condensers

via a steam dumping sys6tem•.•i-a-78e consists of a series .of reducers,

which pass the hish-pressure.steim into- oneof two bubble. tanks where. it is.

cooled before being sentý,to..ý-the , ma:incoandenser.

2.2.3 Control Rod,.Cooli• n (•0ys:9 S, . 198.6; Dollezhal, 1981)

A sytemsepaatefromtherius~aoig systam is- .provided to- coiol thkecn

trol rods o h .2.T ytmas r--

video cooling-to - of ,core.

Approximately 1100 u• /rcooingm).f mo~an water from a supply reservoir

.(known as the -emerglpacysat norage tank) at 40_ C (1040) flows under gravity to

the control rod cooling cOhan-s ;(and reflector cooling passages) at the top of

the reactor. The coolant i he co••trol rod channels flows downward through the core. The flovrate i ea'choit.es channel is-s approximately 4 m8/hr

(18 Vpm), and orificeat the bottam prevent the rapid loss of water even if

the supply is terminated. -The .4,vo'lue of 1the supply reservoir is governed by

the condition that it'/should supply the rated flow for 6 minutes after the

supply water from the lower circulation tank is interrupted. After flowing -...

through the control ;*od pressur tubes, .the water (650C, 1490F) is cooled and

returned to the circulation tank. , Water -is pumped from the circulation tank

back up to the supply reservoir. Part of the water in the reservoir is sent

tbrough a purification system consisting of mechanical filters P-.d ion exchange

beda.

2-22
• " "s- - •.~A • ,

Controi or - o lo s

.. e o n a 1) Cle.nine i!6 

Fedtro ~.pr~To etetiof 81lowdown

System

MAW

Figure 2.12 ,.Schemkatic draving of control rod cooling System

2.2.4 Reactor Gas Circuit (UMS- 1986;,,Voron, 1980)

The reactor gas circuit:' (see Fi•Ure 2.713) circulates. a nominal mixtiureof 80%

helium and 20% nitrogen:gas thrtoug spaces in:the graphite moderstor at a rate

of about 200 to 41)0 normal m:lhr •(762 to 14,12.5 normal ft 8 /hr).. This action

* improves the heat-traffrm;b its'

preventsoxidaticmof th spbteý

p permits 1JM- s"ese1-

during operation

The gas mixture is fed into0 , cannl.t below the reactor and. distributed across

the bottom face of the reactor. -ý,.The mixture then flows between the graphite

columns, 1.roviding a beat conduction. medium for transmittiag the heat generated

in the graphite to the.process cannls.•Monitors are provided at the top of

each chani~el to sense the re~ltive1h0Midity and• -temperature of the exiting gas.

.These data are used to detect 'ay leak ,that "y be present in the various

pressure tubes.

The gas scrubbing system consists of ,set of contact catalyzers, scrubbing and

dewatering units, and cryogenic cooling -system units'. In the contact catalyzer,

hydrogenation with M2 takes place at & temperature of *-160C, with the formation of water vapor and combustion of CO to CO2 and the release of heat. The

reaction takes place in an oxygen atmosphere in the presence of a platinum

2-23
Active,

N2

Supply of Nit I T Metal Struct,

H

eleae Thoug ::. 

~~~~i6 Ativity Reduction Systei -

-HeN N2

• :: " :; :; Nitrogen-. """"" :" •

Helium

ScS(rubbing,-

System

Na

orum ,

Nitrogen.,

~Toth-".Pressurew Supplyýý'

;• 6$upession

-Active

Ventilation

Active Drainage. pi, e•

rogen to the

ures of the Reactor

eN2

It'

yerthe "as"pue hog

O crubbing n mseigfilters. Adsorption takes

re scrubbed from the bellimling unit. Amy impurities

cold trap at a temperature

be graphite temperature It

of 750eC (13820F) during

Figure 2--13 Gas circulit sy

cetalyzer~I~tarbe0010 th 4 tecontact -Castl

bait, 01Ah and mechanical.

place adC 3 8 C, dwatear vapor impurities a:

nitrogen gss,04*8h -hmpasst* the Cryogenic co*

regasining- in 49 as are' Frne in this unit by a

of -1850C.

During startapi, pure itrogen cover gas is used. T

allowed to. inrAselp to a ~mezimm measured value

this tim..

2.3 Reactorhyics

2.3.1 Viahlights

The unique design features of tht reactor core, from a reactor physics perspective, are its graphite. moderator, large size, and large core load of enriched

2-24

1,.-'.-
uranium fuel. The graphite moderator plays a, signifcant role in defining the

characteristics of1 the reactivity feedback coefficients. The large, core, size

causes it to be loosely coupled, and the -large fuel load causes it to contain

many critical masses. These special design features produce unique neutronics

characteristics and complex reactivity control requirements.

2 '3.2AReactivity Coefficients (Do1lezha1,, 1980a;. USSR;" ;19867;- Romanenko,....._:

1982; Virgil'ev, 1979)

Reactivity feedback coefficients are associated with the temperatures and densities of the reactor core materials. The five primary coefficients that determine the neutronics behavior of the reactor during both normal operation and

accident conditions are coolant density, graphite temperature, coolant tempera- ture, fuel density, and fuel temperature-. The magnitude and sign -f these

t*e ff icients=-are dependent on4 ,thee-corekloadingvof .neutron absorbers(control.

Srods iisupp emental absorber aduued ue. ., e water-filled chnnels).and the. • 

isotopic content of the fuel. Becaue the loadiegof absorbers in the core and

the isotopic content of the fuel change with tize--, the reactivity coefficients

change with time.

Reported values for the coefficients as a function of core configuration for

1.87. U-235 fuel and for the 27 U-235 fuel for Chernobyl Unit 4 are given in

Table 2.4. Of the five, the effects of fuel density and coolant temperature

are minor because the ranges of possible density and temperature changes are

small. The remaining three coefficients, however, significantly affect the

reactivity state of the core. Each of these three is discussed: in detail.- .

2.3.2.1 Coolant Void Coefficient..

The coolant void coefficient is positive under most operating conditions. This

is due to the large graphite-to-fuel ratio, which produces a yell thermalized

neutron spectrum with no water in the fuel channel. The magnitude and sign of

this coefficient are strong functions of void fraction, control rod positions, . --fuel enrichment,. fuel- exposure,, and supplemental absorber .loading. -Since these

factors vary considerably over the reactor volume, there is a large variation

in void coefficient. As shown in Table 2.4, the coolant void reactivity coefficient is positive in most operating conditions., and it becomes more positive

as the rea-tor continues to operate. Figure 2.14 shows that the void coefficient becomes constant at approximately 1000 effective-full-power days.

2.3.2.2 Graphite Temperature Coefficient

The graphite temperature coefficient is positive. Increasing the graphite

moderator temperkture hardens the energy spectrum of the thermalized neutrons.

The net reactivity effect is a combination of decreased neutron absorption in

the water coolant (positive), increased neutron absorption by U-238 (negative),

and increased fission reactions- in the-plutonium- isotopes (positive). -..The......----.

latter effect continues to increase as the fuel undergoes burnup; thus, the

reactivity effect associated with increasing traphite temperalure becomes more

positive as the reactor continues to operate (Table 2.4). Figure 2.14 shows

that the graphite temperature coefficient becomes constant at approximately

1000 effective-full-power days.
Table 2.4 -t-Caýlculated reactivity coefficients fir' RMI-.

,-_.,.i: . S.tate of core

Item Y. 1% U-235 .0 -235ý

Exposure (MWD/kg-) 05100.

Re'activity per W.(~ 60065 000 0.0042 0.:0048

Number of equivalent -

rod worths 3020 20 30

Number of supplemental 236 n18 0 1

absorbers

!Co•olant oid (f)* .i uO 2 40. 92zlz-O 1. 0 2x10- .

0 4 3 0 52 4 .M_ 466iGraphite temperature 0 . 3.2zI0 . 5.4x10- -6.xO- •-

Water temperature 45x10-dgd (p/0 C) -'••"÷i

Fuel temperature '1 0 a . OxlO-5 -1.AxIO-5 -l .2x0-5 " (ploc) ,.. .: ......... ~

Fuel density " .'Z1I0- .- 0.22x10- 2 -1.3x10-2

(p/gm/cms)

I( void.

2.3.2.3 Fuel Temperaur Cefcin

The -fuel. temperatureý coefficient 1. tý,ý,he only coefficient that is negative. As

h nu .... pera ient less .becomes ngative AS

th reato oprates,4 •-s satat t app-,roxi -ty •70 e•ffecive -,:/•

f ull-pover days.

2.3.3 Reactivity CitrolIsqir I .st

The reactivity control sy•ytem is designed to compensate for any reactivity

changes. The ntIber and pacingr of the control rods are used to control spatial

variations in the power. Te total reactivity worth is sufficient to bold the

reactor aubcritical inder all cond'itions.

During startup of the Leninlgrad lmit IT M reactor, local power oscillations.,

occurred with a frequency-of approexsitely 24. hours. -.Ia. an effort -to. reduce .......... .

the teudency toward local power variations, the later RaN reactors were

designed with a fuel enrichment of 2% U-235 and a+ reduced graphite denwity. An

increased reliance on automated control v&s also initiated to assist in reducing power oscillations.

2-26

ý44
- 4

21- If-2 "

-2 .

-0.5

l-t

XiiJure' 214 E'ffect .,of reactor. opeato ,A'A , .on, the coolant void, ;fuel temperature, and

0 1000 :2000 moderator tamperature reactivity

t. ff.pi 4mw ffc~icients

The large enriched uranium fuel load creates m.any. critical wsses in the core.

The reactivity control system -is- dsiged to 'hold the core subcritical under

all conditions. The controlrod system-alone is not sufficient to hold the

core subcritical for the initial fuel loading. During the initial loading, one

supplemental absorber rod is loaded for every six urarium-fueled channels. As

the reactor operates and the initial reactivity is burned out, the supplemental

-absorbers are replaced with uraiuni -fuel. The.-positive reactivity coefficients

add to the difficulty of maintaining the reactor subcritical during accident

conditions.

As discussed earlier in this•section, the-values of the reactivity feedback

coefficients are dependent. on.;theore4 •roading .of absorbers and the isotopic

ccaposition of the fueL.it o s nmeof& the coefficients-varies

considerably. --Fr•om • Fur :14, the coolant void, giraphit

temperature, and fuel. t: eratuisoef ic .u are at their maxim values at a",

reactor average fuel exposureo"f iUAD fk4..(The average fuel exposure at

Chernooyl Unit 4 at the time• f -ýthe accident -was approximately 10.3 MiD/kg.)

Because of the control rod configuration-at the time of the accident (virtually,

all rods fully withdrawn), the void coefficient vas 1.5 times its normal value.

The delayed neutron fraction and tbe -pronptlneutron lifetime determine the

dynamic behavior of the reactor-in response to changes in reactivity. For an

RBW( lattice with an exposure of 10.3 WdD/k, these vnlues are 0.0048 and 0.77

msec, respectively.

2.4 Instrumentation and Control

2.4.1 Highlights

The reacthr, iL highly instrumented and relies upon extensive computerized

c'ontrol for optration. Control rods are grouped as follosv: manual, automatic

2-27
regulation, scram, and,: shortabsorbing rods. In addition, auxiliary absorbing

rods are used during a• mult-•yeau -eiod while the-rector is achieving an

equilibriumu expsr level-.

2. 42 Core Instrumentation' ndControl Rod yAstems (SSR', 1986)

Reactor instrumentation collects.& &n poessdtneed -io' 1oto reactor

power. At least six sensor.sys*tes are used in the Chferobyýl Unit 4 reactor:

Beta-emission sensors . .re."I ocated inA- 12, fuel channels: in the central 'part

of the core at seven diferent heihts itýoeasure the axial distribution

of neutron flux.

Additional beta-emission- sensors are installed in 130 fuel channels to

.measure the radial flux distribution.

:,Fission, chambers, sdtomau neutron i~hi h~atrssatd

- are- arranged-:in.fu .hnea c$at s~rialarbubzd the cor tin

.-the radial reflector.

Thermocouples aeinst•a•l•d 3dif fe rnt heights in 17 .,ýertical chanes.

to monitor grapht epItue

Gamea-spectrome uer.piobes that- easuretihe activity of th. steam/water

mixture in the drain pipes at the seprator inlet (near drum separator in

Figure 2.11) are used ,to6m.nim tor:Aeaks i. fuel-element cladding.

The relative-humidity andftimperature o6f a helium-nitrogen mixture, which

is pumped through the gap betweeL the tube and the graphite, is used to.

monitor leaks in pressure tubes.

2.4.3 The Monitoring a.d Control .System (Dollezhal, 1980d; USSR, 1986)

Vrm- monitoring and control 'ste poi"ý.• edof two basic subsystems=: the

cottrol and protection system, and the reactor process monitoring system. The

la..ter contains: the- centrall -i:zedSýss~ (Soviet deintion: Skala),.

2.4.3.1 -The. Costral eetLse

The control and protectionsytem (SovieAbt(deignation: SUZ) regulates both the

power and the power distribution in .tbe reactor. It also provides automatic

dwerseucy protection if -the-.-power •evel-eeefs aset limits (see Section 2.7).

The control and protection system provides

•*. control of the power .•level d,-(b mthe. eutron-f. .) of the reactor and

its period under all operitinStregimes from SxlO-it to 1.2 times full

power

e startup of the reactor from the shutdown state to the required power level

automatic regulating of the reactor power at the required level and

.changes in that level

2-28
b mnual (from the opr os ý'control desk regulatng; of the power dens ity

distributio~n-.* thogou , 'e core aid- kegulating 'of tcreactiVity to- COM pensate for bur ref.ection, and othr effectsA ."

stailzaio i powe dest idstirbtc

automatic otabilizatifbhof the radial-aitmuthal pover Aensity distriution.

in the reactor

' preventive protection, i.e., rapid control led reduction o If the reactor

power to: safe l s r c level i..of wr, prOtection

level 2 is #ra of full pOwer).

•ergeucy e w protection, when the' -ara meters of the reactor or: generating Unit

change as a result of.an.a•c•dent (protetic-n level 5)

.Overall lpower contrel can be divided. into .ree groIups: manual,, .automatic, and

emergen.cy. Local power conitol Aen-be divided intotwo groups,utomatic and

-emergency. Eahgopi ecie rel.Oealpvrcn~oi rvded

~by, 211 contrtol rods (earier~i "mctbro M l) 16Te66d ar e,1 funinal

divided into manual- odet 4iigpaton: 7) -ietwos!it of auto- matic control rods (S e on:• andiom: LOWa), emergency power reduction

or scra- rods (SoXi e.g), ad ASrtened absorber.- (So viet

designation: lSP). dti.,mUMem suiplemental absorber rods in the

core are used to bold dwn• h Initial 'etcets. reactivity. These additional

absorber rods are grdull replaced with fuel during burnup. The fuiier and

function Of these variows, tYe8 r are listed in Table 2.5.

.. ITable2.5 , Types o control rods

Name ; SYmbol-- :ftmbek- .,unct ion

locl6 automatic

Automatic power

Scram

Short absorbing

...........~. i:s used to shape power and a

:_portion is reserved.

reglaie-~-JA >12 Minanpower - amape bw;using -. ; igal fi fm lteral -

reglatem R42 -b aintain total reactor power.

AZ

if?

-Three sets of four ganged rods.

24 -Scra rods -normally withdrawn

from core.

24 Used to control axial power

shape - manually controlled,

and enter from- .ottois of

reactor.

Auzi)lary absorbers VP 240 Replaced by fuel during burnup.

Compound of noron steel (22

boron).

2-29
The bsorin t f h i fabricate in a eeve,

dsigsn (-see Figure 2-.5 d ý,Table 2.6) T ron ca rbide i-s enclosedý fiin a-.

sealed annular element:JoMe fromin• 1lmin, alloy. The , ,AR and AZ rods w ,*--:• 

are assembled from:.aaio US-'n siections. -;the USP rods r a4ssembled from ,three absorbing sections:. Alrs elordinotecre: from the top

except the USP rods•, •wh•ic re raised rm th o m.

With the exception of the. R rods, all the rods. havesections, tvdisplacewster anid, thus, nice theeffectivenessm ofthe control The hods. displacer

sections are cylindrical andare fomdd of aliminum alloy with sealed end Caps.

The five displacer sectio are,,filled with sleeves and cylindrical graphite

blocks. When a control .od is fulywitdraw,.the displacer in located

symmetrically with respect.to cbere so that the 1-a rod channel sections.

on either end are filled, with water (see Figure 2.16). .,

These 1o-. (3. 2-ft) water-fil-led-ectt46ni cre, strongnurnasobr. Drn

th initl inis'ertion .0f fu1y4,14,, cs (,scram), the Va8ter- ini

th otom- section is replaced by ewal -asorbin gahtdisplaicer. -

"The result- is a loca-l, positive ?ctivty 'Increase in the bottom meter of the

core. The magnitude f tVity iAs is-dependent -on thenu• ber of

rods fully withdrawn.....44 .-

Manual control is pro e ym iatrol (RR) rods. Theme rods are divided

into four groups. One.gro Is.s located-in the periphery of the core, and the

remaining three are located in the 'etrally core. -The central rods are divided

into three regular, inteAriiix4e latticesi. C:__..ontrol of excess reactivity is accom-i

plished by the BR rods.i.n one -of ýthese central groups and by the peripheral rods,

which are moved up or down to equalize the current in the peripheral ionization

chambers. The rods of each central group are moved sequentially to maintain the

position within ft. 5 w, (20.: L ofneachother. .- ,,,The rods of the two other

central groups are at teeteeupror.lo r positions depending upon the

reactivity reserve. - . -- 

The overall power control'systeim 6nist1 oif three identical sets of automatic

regulators. Each set consists of -for • Loa tion chambers placed around the

..reactor. 4nomto AAe ue i I-synchronizing ný the-,mveen

of the four.automatic mimf. .4iti- P. . -chme•---: 

different iti1ity' ""sv*to ui in either the low-power. range,

from O.5% to l1( ofw flr powrz Tawri p r range, from 5% to 100 of

full power. In the 1ev-po -agth _.: is,: -..automatic regulator (3AR); in

the working-power range h.ere ai two. (IM ad 2-R). One of the working-range

regulators is switched on; the secondis in "hot" standby. The second

gegulator is automatically switched. on If the first regulator malfunctions.

An emergency signal ts .imrated fh t limit of a chamber is exceeded and

tVhe signal is recorded on at.at, t-o masurn g channels of di-fferent groups.

If an emergency signal is rated, the emergency control rods are lowered.

This action protects the reactor as a Whole from power excursions, and it also

protects the reactor from peripheral local power excursions.

The power density distribution in the reactor ti stabilized'by the I, a1l

automatic regulating system and by the local emergency protection sfy' ,em. The

former It designed on the principle of independent power regulation in 12 local

2-30
* z

4.... .....

RR. -R&AZ Rods

Void br Coolant Passage

OR. AZ and USP* Rods

SAR .,Rod has no

I

"PS? rods have their displacers on the top avd the 34C absorber on the tottom.

Yfgure 2.15 Control rod design

2-31
Tale 2'.6 1oto riod seiain

Compositi nnoo,

Control ma-terial

Cladding material

.control length/se

Total length/sect

Displacer lengthj

Outer B4 C diamete

Inner B4C diamete

--. -rcn arbide (B4 C)

- Aluminum (Al) alloy.

ctioi 908.4 cm(87in.)

.ion_124~(03i.

iection .00 Cm (39.4- in.)

6.5 c• (2.6 in.) . 5r '.75, cm (2.3 i.)

(ha"Jter cl -add ing diam'"Jeter L 7. -J cm (2 .8ý in) Outer clasd dingthfciine ss 2c 008 ' i. inner claddin iaetr 4.0 cm (2.0 in.) e dg. thickness .2. cm (0.08 in.)

. .:•i• .. • • •. .. .; }•• •:• .• . . . .. . . : ? ., :-. , . •. ! ;•.•:.• , . •,• • :,,: • -• • "•:.- • , -;• .. :.•:%,..: .. . ..- . . .. ... . . .

-VA66oy

Figure 2.16 Sctematic drawing of

control rods fully vithdravn and fully inserted

2-32
.zones, of the: reactor by.me'assof 12 regulatng rods. The local, automatic regu- et i ng system rods are c'ontrolled by two det•etors• ....si.tione. in the core aroun".

.the local automtic regulating rods at a distance oft OW63am. from the rods.

The local automatic 7'regulatiug system is:- switched inothe automatic mode in the

power range after the rqui redpoweJr, density distribution,,has been achieved. J ' transitional reLetelocal thas,6 -automatic regultin system, considerable advantages, because Vt not only measures 4"d reuatsthe overall power, hri

reultsrntI

t:_ailso.,smoothesý ýout povrditort-ion-s aued- by local petubaionsý _`4th equipment.

The local automatic regulaitig sy.ste is the primary system used to-automatically regulate overall poeri the po"er- range f rom. 10I to 100 -f full power. The automatic regul-atinj, system forL4overall, p er is uswed for standby and is automatically sWitchedon hen the.. local Automatic regu atings .malf unctions.

Th ner~tib /vithdrawl speed of the automatic control ' rods is. limited to

0 a ntase 4 so that use mt does not:eceed' the limits, established•y theNuclear

Safety Regulations for the rate of addition of positive reactiviywe 12 rods

ofthe 'local systo m:,tA*- i _•• 10 •,.. 7-0d vio: wh~ •-

of thegulocalsystem ovrn-unwed at th; time (0.7 10 /stc). _ia built.! li.i-r. 

tation prevents the coutinu.uwithrawal of the automatic regulatory rods for

more than 8 seconds.

When a power-overshobtL sl' O sigalappears inf one, of th canl ftelocal

emergency protection one, ,heithdra l of the local automatic regulating rods is automatically blocked. When emergency power overshoot signals appear i.n both channels of the local emergency protection zone, two local emergency protection rods are lowered into this zone of the core until at least one of the emergency signals disappesrs•. In this case. the overall power of the X

reactor is reduced by au atically- lowring the power transducer settings at

their operational rate change.

The withdrawal of more thn 1S to 16 of th manual regulating and emergency pro- tection system or -shortened absorberxad upon,-any malfunction is prevented by .a "power -blocking" circuit. 1his fccit ,•utmatically. detercito s the number

of rods tht aybe'w-hauumbe- r Is grveater 1than,1 to M10 the .circuit. ts autONStialpdas i hK ftum seI driepwe suppl source and no additional rods :cý e ithdram•p nfre, tbe core. Three power blocking channels process th e~ signals1 by a v-mut-ofithree lgc

2.4.3.2 Reactor Process . ssitoritg System

The Chernobyl Unit 4 type Rl :reactor process montoring system provides the

operator with informstionia ývisusl and documentary form on the values of the

parameters that define the• raactor.$ operatig regime and the conditon of its

structural elements (e.8., pce chuanels, control channels, reflector

cooling, graphite stack., and meta l :structure). ...

The following systems relate-to -the process monltotins system:

2-33
channel-by-channel coolant flo*rateonitoring in theprocess ad control ....

channelsJ

• ,onitorigf io.temperature the graphite sta.ckand te metal aructure.,

* channel irtegrity: mnitor:in frm the. temperatiure and. humidity of the - surrounding gas

phsi power density monitoring system

• fuel cladding failure detection

* Skala central monitr System

-,Information from the reactor process- monitoring system is collected and pro- cessed by.- the Skala cen mo ori system and by individual instruments or

&independentý, Systems. (chan fail.. e detction,0, phYOical poerm onitoringsy.-

"te-fe cladding fi .' e t ec"i" "mote t h io n paramet.ers.

The Chernobyl Unit I. type IbM reactors have

points:

the following numbers-, of monitoring

fuel channel flowrate mt :. 1661 points

control channel flo t measum t - 227 points

temperature measurem 40i, oftheme1talstructure and. biological

381 points

measurement of the graphite stack-and plates - 46 points radial and vertical power measurement - 214 points

gas temperature measurement-2044po ts

measuremeat-of coolanta.ctivityt .- ý41661 points

ihielding -

The results of computer calculations _are given in the form of -eartograms of .th•e

reactor. A cartogram is a-comutr printout organized to. be geometrically similar to the layout of chanlasiAn the reactor..The cartogram lists the parameters for each channel (e.g., ,the type of.vcll -charge, the rod position) and also

identifies the bottest egioms.-

2.4.4 Description .1thet il cbaniiy 83)-Q

The rod drive mechanism is usedut raiseo, -lower, and monitor the position of

the control rods (Figure 2.17). Thew•echanim has a dc motor with a built-in

elec'romagnetic brake that stops 4rotatton of the shaft when vcltage is applied.

Th motor transmits rotation through a-searid transmission link to a drum. A

belt-cable wound around the d.supports the control rod.

-Rotation is

rod moves.

has reached

monitored bya ,elsyn senso,.,Cams driven by a screw mov vben the

Limit witches activated by the cams indicate when the control rod

its extr•m• upper or lower position.

In the absence of motion command, -the circuits of the armature and the excitstion winding of the electric motor are de-energised; voltage is applied to the

electromagnetic brake; and the drum, which holds the belt-cable and rod,

remains motionless. .hen a command to extract the rod is transmitted, voltage

2-34
2S

2 . -i... 

-,- ~ - ~ ,. ---. 3⁄4w t. ,-

Figure 2.C.7,-'- ctl diagrm f a-control rod ddfve mihanism

is removed from the brake,-iedrin-is released, and the electric motor-raises

the rod. Mfotion-continues intil-either -a4_ stop signal is given or the upper limit switch is activated.

Rods are inserted&into the-core' tof om, hree ways:

.() When_ a signal t•.o loverhe- -..is ,iireceived, the electromagnetic coupling is de-energzed .ad,*becueof, teweigiht of the rod, the drive initiates

a lovering movement, ring ia .elf-ezciting dynamic braking mode.

(2)- The drives anzisi ni•tiatl movement mode. when-.,ol"tageis • applied to .'--ittt.~wAA4~~b~k is4!e-eegieeab

cause of t-e eigtf of the ,th dr1i* initiates a lowering movement in

dynamic braking, mode ith a, wek-Current.

(3) It is also postible toivi Ae. te otrt. initiatk the lowering of the rod, thus reducing the transition time. "In this case, full voltage is applied to the armature circuit and.-tothe iscitation winding, and power is cut off from the Olectroma tiUcibrake. T-hbe drive initiates a lowering movesent in the motor md.-Thea power is cut off from the armature winding but not from the zcitim wi ing. The rod continues to fall, but its

motion is slowed by the presence of electrical -current in the excitation

winding of the motor.

The safety system bas five different levels of response to reduce the power level. These levels of reaponse are discussed in Section 2.6.

2-35
2.5*' ElectricA!.l. Power System

2.5.1 Highlights-. -i

The electrical sys... "te used .. t Chernobyl Unit 4 included normal working

.. .....-- h• •!~ ~i-bl --- -dat In t.i even46ppe '

andresrvesuplie fr.power. ýIT~(Pes o mrec power sources (bitteriesn

and automatic eble iiediately in the event- the

primary, sources failed:..5> ? -,.. -

The equipment i.. grouped: into oie of thiree categorries. depending on allowable

power-interruptica times: .fractions of a :second, fractions of a minute, and

extended. • -

2.5.2 Categories of Electrical: Equipment (Plyutinskiy, 1983;,USSR, .1986)

Man mehaism ad, devices within the reactor '0pertn syte reui eteetric

power ornomal r p io .. These.inciud. e tee porsof`electi.c 'drve, ~ Cou~ ctrol -avsadmeoKoioi~adcontr

systems'.

All electrically. driviein eii tithin the plant is categorized into one of

three 'dpnaiitýt t

2.5.2.1 Category 1.

ýEquipment in this g roup cannot tlerate- aa- interruption, in power supply or can

tolerate only very brief -interruptions of: between fractions of a second and

se7eral seconds. A power supply-is Absolutely essential for this group after a

scram. The power users in this group and in the Category 2 group are.sub- -

divided into "safety-related-process systems users" and "whole-unit users" for

which a power supply. i. .absolut-elyessenttial,. .even when the plant's in-house

power supply has been toaly-sht f -ff.

Catego-ry1 safety system users Include theisolating mechanism for the accident

localization (containment). sys:tem: anm ydroen removal system, the fast-acting valves and gate valves .c. rgeucy core-cooling system lines and mmitoring,

protection and automaticcmao 0 evices -of afety system.. tmoule-unit users include the -Skal.ý na.t control-and pr.tectiom.• sste,-

-the dosimetric ~~tdgyt~ ftereactet, the twsz kim anid g~aieratr,

and the fast-actifg jpzeMsu V mechanism.. Th ame rgency power for these

systems com. from str batter0 ith static transformers to provide 0.4-ky.

2.5.2.2 Category 2

Equipment in this category- can-tolierte Asterruptioma In the power supply from

tens of seconds to teos of.m6iutes.•£ ,,_Pqe supply for this equipment is absolutely e~sential after ascrem.-Safty-systm that use Category 2 equipment

include mechanisms of the mergecy-core cooling system and the accident localization (contsanle*t) system. bole-unit users are mechanisms of the auxiliary

turbine generator systems, certaim auxiliary reactor systems (intermediate circuit, cooling systems of th fuel cooling pond, blovdown &04 cooling system,

etc.). The backup power source for this category of equipment is provided by a

diesel Senerator.

i

2-36
' 2.5s at o 3 "a Z .... 

Al euimet ohe ha~sao CteorCategory atgy2 it. consiee Category. I3 .. . .. .............. . c... p u mp s f i ed , .'0 

use of flyiwheel 'w on w"min " eci latia. pus a-lrlow s these pumps to be c assified

power supply ... ':'•'•""• specale. :int:ernal . .system.; power generators.treb h main .enteg ors, ,tore. turnecd b." the .eow T,

supply system When isps occur;diesel generators, which...ar startedup auto- ...

matically and areI €capable of roviding powr within.15 seconds oishap of a 

occurringpin the powr-suppy system; storage batteries; and medium power generators at nearbyhydroec'torc pyoer.plants' (or thermalelectric power plyants)

that are not connected to. the power system but are operating only to supply the

.internal networks of the nclear power plant. The followingpower.supply ainen "

networks" sart used220q,' o bi inera -ot . eltil 1lant ton..... loads: .suppl p

• a6-3SO/ 2

20-h,5network supply;:ingmi n ida ulatingwe .umppl andetherk .large.;

elcrc mots psti 6/0c.4-ky 4istep-dowgneransormewic rs .satdýu uo

* a 380/220-V, 50-Hz Category 1 network providing power to the control"

computer system

, 2.5.3 Tne Diesel Generao s tarrtio 'ttes,, .. "1986)" .. .'....

Three diesel generators provide backup poer-to Chernobyl 'Jait 4. These genera- •'

tors, each with a capacity r o f5500 ,k, were usec as an inependent power supply

fort tre nokt onemergency power sup '~o y setions. ATe steartptime ,of the disupy tel

-generator nast 15 .seconSd.Tn dinesear eerator start. .a.utmeatically. p but . .w 

nelecric .o... ielgnrtors were used to supply power to the

-- re mot mprtn euimetduin te nir tmeo '

a coplte oltage losarom .up 

all source ecept networkg battering.,i rcatnpm

2. aety Sys~tems(USr, an 986)04k tpdonta f

The Chernobyl Unit I. xtype ks reactor safetl system s rovide for 20k

wemeriency core sooling

S main coolant loop oIerpressure protectpon. supply'-net

rea0to2 a space overpressure protection p spyeo

* m.tgation of radioactive releases

T steam pressure suppression C.

•Seersr wasgen gas reoval•

2-a3k' up th
A.

57

In addition, protection against abnormal operating- conditions in provided forr

by automatic power reductions and full reactor shutdown by insertion of control

rods.

2:.6.2, Reactor Emergency Core Cool ing& System

The emergency core cooling system (ECCS), -shown in Figur 2.18, is designed to,

pro•ide.-cooling of the reactor in- the event. -of accidents;._resulting in damage: to

the Core inlet cooling system.

The ECCS in brought into operat ion by the opening of a fastactin electric

gate valve. Power is iuppied by. storage batteries.

The nitrogen: from the, CCS ta•nks is prevented from reaching the reactor through

the- automatic :closing of two gate valves...

1

The ECCS was designed to- satisfy the following main requirements:

It-must supply water to the damaged and undamaged halves of the reactor in

quantities that will prevent melting, m.ssive .overheating, and-cladding 44-ilure of the fuel elements.

V.,

(2) The.,S must operate autmaitically on rceapt of the-,- smaeiindsig-bs

accident signal" (abr:eak in the main coolant pum discharge piping). The

Fe"d pump

Figure 2.18 Schematic drawing of the reactor emergency cooling system

2-38
basis for distinguishing between the damaged and undamaged halves of the

cooling system are

(a) an increase ir pressure in compartments containing primary coolant

piping (indication of pipeline rupture)

(b), coincidence with either of the following two signals (showing selec-1.,

tion of the dAamaged half):

drop in water level in the steam separators of the damaged half

of the reactor

* decrease in the pressure differenti-I -ttween the main circulat.ion pump pressure header and the steam :Vparstors of the damaged

half of the reactor

(3) The speed of operation of: the CICCS must ensure that water is supplied to.

the damaiged'half of the reactor within 3.5 seconds.

(4) There must not be an unacceptable reduction in water supply to the reactor

channels as a result of a pipeline rupture.

(5) The system must perform its safety functions in the event of any failure

independent of the source event, in any active or passive element having

moving mechanical parts.

(6) The system must comprise a number of independent channels (subsystems) anA

must function with the req,.i::ed effectiveness in the event a failure occurs

independently of the source event, in any one channel (subsystem) of this

system.

(7) In the event of drainage of the ECCS vessels, nitrogen from the vessels

must not be allowed to reach the reactor.

(8) The ICCS must operate as intended in the event of a maximum design-basis

accident coinciding with a loss of internal power from the power unit.

In order to comply with the above essential requirements, the ECCS comprises

three independent chbnnels (subsystems), each of which ensures not less than

501 of the required outjut. Racb channel (subsystem) includes a fast-acting

section and a section providing prulonged afterheat removal. The fast-acting

section supplies water to the damaged half of the reactor during the initial

stage of the accident. The afterheat removal section comes into operation

after the fast-acting section has ceased to operate. The fast-acting section

of two ECCS channels consists of a system of tanks filled with water and nitrogen at a pressure of 10.0 MPa (1450 psi), connected by pipelines and headers

to the distributing group headers of the primary coolant system.

U. ch of the two fast-acting sections consists of six tanks of 2j m3 (880 ft3)

volume each. The total initial volume of water is approximatelv 80 m3 (2800 ftW),

and of nitrogen, approximately 70 m3 (2500 ft 3 ). Each section supplies not

less than 50% of the required quantity of water to the damaged half of the

reactor over a period of not less than 100 seconds. The per.•od of operation

depends on the magnitude of the coolant lcak.

2-39
the f

t f .e s-actin .se t n of t e t i d C S channel ~u pies i ater from th-e'

.,t 9: sec••-tiq a"4f:,ti,-: tirt ECCS APPAI,

electric feedpm, wich-~. ~a'sures aum ipp1y; of not *esa tha 0 o h rqie

design-basis. 

amsount. of water to the danaged half of theý rat r. Intk he event amaim accident coin with a- seitn p the

water from the electric'- p.mp i s rapid 45 to 50 seconds

while the pump uz . i'tcdmf with uxe Aeur genserator.

The,.,. o:t o.mo'•lsecion providet coreacto bot the eomanged " andsundamaged parts of ]tor. thed rIt comes ope l

moment ut which the- fast-.a• r section fo :of the aCCS ceases to operate.

The long-term after t'. r-emoval: 4eco of each of the tbtee ECCS hi& is:

consists of cool:ing'p s.o. te dimagedhalfof the reactor, and the ccoling pimps of the und tamage halfof•tthe reactor.

Therpm for the damagss eS -he•lf of tke reactor- o each of;est tethe fro arne'a consurstsoi to p-.mp c..onncted in parallelp wjivt ta rat e of aprimtl 5O04tnues/bhr (300 ur/sc) tW1ýs! not

esah than 50 of tthe requiredate for -*he' dam agedhalf in the event of a ma8 x mm resign-bas.rd a a • The.lters ,idrawn by the punps froms thedpre.sure suppression pooL.otri4em~eai~t~ ytm isf.cooled- by the. service

water in the heat exhngrmoneo e intake line of o the two pumpso and reaches thelCChadst ughte .ichre le. TO -flow restrictors are

installdconthre dischge les tepumps and are designsd to ensure the steady functioning 'of the -u-.66 eergency situations characterized by a

sharp dropein pressure-of oresulting .the`.actor.' coolant circuit from a ru- tured pipe.

Each of the .CCS chan nes contains om.•ae p and supplies water at a rate of approximately 250 hr (L~O ihes) ths , n•t no t less than 50o f ithe flow required fornthets m hlf In a maximum design-basis accident. Vater

is drawssfreo the -tanksc tining clsea •cotdensate and flows to the-headers of the Cylinder section valve. The flow restrictors

in the discharge inets f pm perfom the soam functions acjo those in

the damaged Ldf of tihe eatr.

Thi• System is desinedt.emia6 that t e sible ure level is not exceeded. This is dome ath fypoidn r steamt into *the pressure sup- press ion pocl. The 4yte iMcludes-,relief valves aNd a system of pi.pes and

headers that conduct the -steam dAto. the pressure suppression pooal of the acci- dent localization aystem.

The system was designed wW~vthe 4*J&Ctive of satisfying the following main

requirements:

* pressure in the main cooliag system not to bee w ded by move thazi 15%

of the working pressure

* be operational when the-pressure is the coolant circuit reaches the

mlnimms operatfng value

* to close tb' matz %afety valves

2-40
to. w~ork- wnder, conditions of cyclic dynamic load' upon, operation of t e miainh safetyý valives:

to speeds introduce, s team into the water of the. pres sure suppression Poo at. . that re closet•Io, tho••t oiin.d, even `44'h ei oh safety valve

is in oper~ation- (this- is unecessary foqr- shok-re ateam, Ddnain

A schematic drawingI of the system for discharging. stea fro the miain safety valves ,n-ito.the:pressuresuopressionpool of the acident i o sstem is shown in Figure 2.19.

The system consists of eighat mainý safety -val~ves with.a total output of 5800 tonnes/hr (3500 underaical 1imc), circuit pressure, i:e., , -an output which is equal to the nominal-i steam. output of the reactor. installation. Con- ..trol of each main safety- valvei r(wiot an -output of 725 tonnes/hr.(MO.lbm/sec)) is bY, .a di-ect y acting use valve ý(lever-gravity type), equ.ipped iha

tubmiersible-'nozzles,,,

mately 1200 nozzles I,.

I U-, A I I+ ,

F mW Saw to

Tewboqeesators

Prev mmpe"o - pk&4 MaA

Figure 2.19 Scbemstic dravinS of the system for discharging steam frc the main safety valves into the pressure suppression pool of the accident localization system

2-41
When an overpressure- ,4d0iti Is etected-, the64 s stems are.. intendedt

in the following se qence:

76 kgf/c1 (1081, p(Si -. main safetoperate 7 kf/i 05 si) 2 main 6afety valves: operat

78 kgf/c 2 (1109 psi) I min safety valve operates

81 kgf/cM2 (1152" psi)l 4 main safety valves operate

Staff working in the tuni control :room and- i-n the reactor control room can have

the capability to manua-lly -oen:theomin safety valves.. ;

2.6.4 Reactor Vault 00erpressire -Protection

This system ensures that tAe permissible pressure in the reactor vault is not

elceeded.in .an acc..dent involving: the rupture of a single.- fuel channel.-,ý (The

,system is not- igned to' handle muliple ruptures.). :Protectioný is 'achieved by drawi~ithe siteiA And ga -mixture'' freteratraaeit es mAnd -gas

ds"c:har'ge`rv," 9omartmnt -oft pesr suppression-pool and susqunl 8 ito -

the pressure i'uppression lihtself (see.Figure 2.-0). ..

The system is designed"o.sat if ,o6o __geqr ts.

• prevents the excess.pressure-in, the -eactor vault-•from exceeding 1.8 kgf/cm. (abs) (25.6 psia) ii- the event of •a- oule-endec break of one fuel channel

(e.g., failure of oe: transition joint)

* prevents water from the..steam. adgas discharge compartaent of the pressure suppression pool from entering the reactor vault in the event of a

design-basis accident .Žo.

ensures that the ftecto- vau fire • iably isolated from the atmosphere

The reactor vault is4 cnetdoth: dste..und gas discharge compartment of the 4-1

pressure suppression pool by t"set of pipes. (This is a special compartment of

the pressure suppression pool-systems :having a water iepth approx 4 aately 1 m

(3.25 ft) greater than t mstof h ool sets of four -... 0-sm (118-i.)exi pies(fen a d 2 *~ "d fou --atthe ibottom ý.Lthe reactor

,space) connect ~ * )Pipes 'oto4O- that, go to ,the,' steam ýand gas, discharge compartment. be-, 4 o&'•th 600-m pipes are 2 m (6.5 ft) below

the surface of the water. . •That•isfm• • normal operating conditions, the

reactor space 13 separated-, -fr-m te- tsphere by a 2-a seal.

Id the event of a rise in pressure. n the reactor vault to 1.2 kgf/cmt (abs)

(17.6 psia), the seal pens.:and thestem -and gas mixture enters the pressure suppression pool through t he- sm discharge pipes. When the pressure in the

above-water part of the cortae•t -eaches .1••1gf/cm2 (abs) (15.6 psia), the

check valves open and the steam and Sao mixture enters the stem distribution

corridor. The stem and as mixture then enters the water of theopressure suppression pool by means of the stem discharge pipes. The gas from the

reactor :space, bubbling through the layer of water in the compartment/pressure suppression pool, is cooled and maintained in the compartments of the accident

localization zone. After a necessary holding and cleaning period, the gas is

discharged into the atmosphere by the hydrogen disposal system.

2-42
50 mPipe

J- • 

3W mm.... Pipe

PSFPiPessurep

Suppression z c0f Poa

Comnpartment

Enclosing Main 16

Steam Dischargea Vilve (x2) - Semteam Distr'buta

p s tue fLt .-Stem striution o

Uppel. e fld 

pper shield,

. 7 27t•" • n ... . - .--. cmm -. - d -l t l]

pressure -tube ":•utFe:tloss -J]rn•' will cause overpressurizstion of the reactor space...If 

shield will 

the pressure- e ,ce -0. 3 O~a (44 psi), the upper biological lift up. .,ince-:,the foOL-c.dnnels are welded to the upper shield,

control 

its upward 

rod 

movement 

channels 

vili 

are also 

lead..to massive tube .filures. Furthermore, since the coanectied-to will be lifted out of the the upper shield, the control rods core'.

2.6.5, Accident Localizatios System

The accident localization system is. designed. to mitigate radioactive during accidents releases

system. Piping within 

involving 

localization 

failure of certain piping of the reactor cooling zones includes

primary pump suction headers primary pump outlet pressure headers

2-43

e -
* the group distribution headers

* the coolant supply pipes between the group distribution headers and the fuel channel Inlets

Piping not within localization tones includes

* fuel channels (they are enclosed in the sealed reactor vault)

" sections of the foe] channels above the upper biological shield not enclosed vithin the sealed reactor space

" steam-water crossover.pipes from the fuel channel.outlets to the steam

separators

.. steam separators .

* steasilines from 'the steam'.sepatators to the iuwbiines

* dowincome'rs" from' the ite"a, separators to tie pump iWet heade~rs'

The accident localizatioa~yatem tonsists of a set of sealed compartments and

rooms interconnected by valves end piping. :The main system compcaents are

* two compartments *ith a designppressure of 0.25 sPa (36 psig) each enclos- ing four main cooling pUmp inlet and outlet headers

" th. steam distribution corridor, with a design pressure of 0.25 HlPa (36 psig)

" the pressure suppression pools, with a design pressure of 0.25 HPa (36 pasg)

• the portio', of the luilding with A design pressure of 0.08 fPa. (12 pais) enclosing the group distributionheaders and the fuel channel inlet piping

A schematic diagram of the. eccident Iocalluation system is shown in Figure 2.21.

h various co a M tams tm accident locali.ation system are co- aected by thret types of.valves- .

* check valves (Figure 2.21, item 9), installed in the openuigs of the cover separating the inlet plping "ad the steam distribution corridor

* release valves (Figure 2.21, item 10), installed in the openings of the

cover separating the air space above the pressure suppression pool and ~ the two primary pug compartments

panels of check valves (Figure 2.21, item 11). installed in the partitions separating the steam distribution corridor and the two primary pump conpartments

The two primary pump compartments and the steam distribution corridor are con- nected to the pressure suppression pool by steam outlet channels (Figure 2.21, item 17).

2-a
2- 1. 17 12:

11 9- 16 5

/

17

13 t1 i4 8 7

Figure 2.21 . Schematic diagram of the accident localization system

.,In emergency situations the system fctioua in the following manner ' If 8a

,failure occurs. in the _pziaaq puiinet or ioutlet header, the resulting steam

formation leads to a pressiure•rse i ;te•faffected compartment. The check

valves between thecoamp th eamdistribution -. corridor (Figure 2.21,

item 11). open at a.OpreVceeds.2 kPa (t.29 psi).-e the

-Pressure -reach~s :te a lqud clu`r rm

siteam outlet chann-l", th si*Wre 'begins to flow into both elevations of the essbbbi throug the ater

the steam condenses amdthair-sol ected-in the space above thewater.

'When .the pressure in the-air-spac e ds 5-kPa (0.73 psi), the release valves

between the air spa-.e and the other prisary pump cow.artment open and part of

the air flows into that C r. et. Thu, _its.volume is used to reduce the

.pressure in the compartment.,susta.ing the pipe break. During the course of

this accident, the c€.eck ,vlves•rFi•ure 2.21, item 9) remain closed.

If a failure occure is the -roup distribution headers or in the supply pipes between the. roup Oistr-.bution headers MWd the fuel channel inlets, the resulting

pressure rise opens tht check valve•s leading into the steam distribution corridor. From the corr.dor, via' the steam discharte channels, the steam-air mixture

goes into the'water volume of the pressure suppression pool's central region. When the pressure in the air space above the water exceeds 5 kPs (0.73 psi) the

release valves connecting the air space with the two primary pump compartments

2-45
open. 

to. reduce In 

pressure 

this iit.. inihe on, the volumes of bot primary i pum portments are u4ed oom cdntainingther rd. pip ng . . .

Toý prevent the ipa o f radioactiv4e material lotaide Athe reion . he

localizat ion syst, lls lorad e'ingsv -h ar eqUipppedwt spca1'' seal penetrations at liifiesp ihre they are.t-irersedi by pipes oreecetrial cable. In addition,. cautoff .n 5ealing7 *vve syi . ensures-- isolationa of tihe localization zones by cutti ng o-f th comiunication• Ines btween the sealed and non-sealed locat4ns

2.6.6 Pressure SuppressionftSstem

The purpose of this- sa steins-to..icodense steam ford

* during an accident involving failure of some sections of the primary coolant t.sytstem

##dring theý aCtuation of th.Ae. "mi safet IValves

* during leaks throu .the main safety valves under normal operating

The system :conditions. is a dual-•eioneinfored concrete tank vith a metal lining (see Figures 2.19, 2.20,11 and2.21). ,hebspace i each elevation is divided by longitudinal rtition traverse partitions into three sections: to ary pumpcompartments) and one cen- tral (under the steam distribution coidor). The longitudinal and transverse walls have tpenings for water ind air. The lover elevation is filled with water. The depth of the water laver Is 1.2 m (3.9 ft). The total volume of water in the is 3700 two 

S 

elevations (131,000 is 

ft3

3200 m _(113,q000 ftS), and the volume of the air space .• ,

SPteam goes. into. the_ water., volinwotr~ough the stesm discharge channels.- Th-e nump- ber, diameter, and spcn fteSe itiuinpipes and their ,depth

- ~~~~~~I- 1 , ÷ .;', - -•. , , _T• ' •. ,.1 . .! i. ' e "i 

under water are determined fro tests m a:large scale model. These pipes ensure full condensatlono4f ite sthe.• L e ater volum.

'The accident localists ....................--.-...-.. frbe zSmi1.an a ,system Heat for: hydrogen &6ovP' 1 : -' " . . .: .. .. .' " o ~~~ h ......... f e ld :.Lc ~•': '•....... .. -. • • ::"....... .. •

Beat from the sealed locatin oft4he cidet --localization system 1i removed _by a sprinkler Cooling syystem, and by. us ,type condensers located in the

steam distribution corridor.

2.6.7 Nydrogen Removal system

The hydrogen removal system .creates , megative pressure in the accident locali- zation zones, then measures the concentration of hydrogen and removes the hydro- gen upon its occurrence. The hydroge remval system consists of an electric beater, a contactor, a-condenmer, a moisture separater, and a gas blower.

Under normal operating conditions the gas-air mixture passes through the elec- tric beater, contactor (in the presence of hydrogen), condenser and moisture i

2-46
separator, and-, by Means of the gas blower, through

is discharged into the atmiosphere.

Z.6.& EmergencyShtos

the filtration Plant, and

The reactor is-. proteicte igainst emergepnies`by the automatic insertion, coreP*

all absorber rods (eept o enid rods• -. t4fr th short os

Twenty-four SUZ rods uiformly distributed throughout the reactor are. selected .

for the emergency protcton- noe the total number of manual regulating

and emergency protectio rods. .-ien the- reactor is startedup, the 24 emergency

protection rods are the•fir•statoeraised to the per -limit switches. The

withdrawal of any other r•odsi- tamiitcally prevented until the emergency

tystem rods have been raised,.

The xteliab ility o h egnyi protection,, system nw h eibe-ucinn

of th ~cntrosya manul? s achevedboy having sixzitdpnetr~ f3

to-36 o~t~l~r~be eaWdistiute uniformly over the reactor.- aIrdi

moved by its own servo drive under the control of its individual power and

logic block. The fiilume f oe or even several servo drives, ork control blocks

is not serious, ui~ hre "are 187 r~ods.. Since eac U s o marrouddi

the reactor by rods .fdifferenit groups, th failed rod-is always surrounded by'

neighboring rods in *orking order.

The design of the SUZ.drive mechsia .. m ensures automatic insertion of ill SUZ

rods (except the shortene&d.rods) ito the core in a power failure. The reliability of the protection system is ensured-by functional redundancy (redundant

monitoring channels) for each parameter and equipment redundancy (redundant

channels for logical processing of, the s gnls). ..

In view of the large contribution of unclear power plants with RBMK reactors to

the general power grid, it.i•s"-ecessary to-reduce to a minibmm the outages of

such plents A differ ialapproch t.r emrgency situations in the reactor

and generating unit has,-. Itherefore, been adopted in organizing the emergency

protection system. 1, -eenin" the ture-oftthe emergency, there are a.

number of difrent--~l7 OW"ris1zgm) h gnypoeto.- ..... emere protection wit cqilete sa.oo of ithereactor-~-emrec n+ - -" 

emergency protection ct• •uni -eemergency situation has passed

(partial AZ-5)

preventive controlled reductioan f.reactor power at an increased speed

to safe levels (AZ-i,- AZI, and ,;AZ-3)

The safe power levels for vTarIOUS emergency situations anid the speed of preventive power reduction are determined. by calculation aid confirmed experimentally.

The highest level of emergenct protection is AZ-S, which is achieved by. inserting all the SUZ rods (except the shortened absorber rods) into the core up to

the lower cut-off switches.. This regime is entered in the following situations:,

a power overshoot of 10% of full power

a reduction in the period to 10 seconds

2-47
* adrop or excess ini theiý -1'iril. U"in he s~rto

* a rop in: the f eedvater _,throihougput

a.a pressure excess in. . p:rii .eparators. * a xc cpressure aztion- coprtments, drug. • -. separators, or. lover ater lines.

a& pressure:ecssi tae reactor cavity * a fall in the level'.i in heý $1 coolant 'tank

Ac,a reduction- in ,vater flow through the- SUZ, channels -" -' trip of: two turbineg o or of the onlyý operating turbine generator

• trip of three oftefoAr• - operati'A in circulation pumps- in u•mp

room

voltage 

one of the loss 

protec 

in W4 the tion ..pl anta p r supply system or indication of leiel :rei: es (C-3, AZ-2, or AZ-I) without its

* being carried out, or order.from the c-oMand units (AZ-5 button, dclutch- . .•ing:-key) attheControl eskds and . ,&a numberi of other locati.ons.iný the

I~itbeevnj~ofan emergency power overshoapriJP- sodrd h resulting rod inse;tion stops when the original cause of the emergency has dis- appeared (when the power has- bhien-re ced to he: appropriate level). .Tis • '. makes it possible to• • e .p t p r regime if the power overshoot signals have been-caused b•ypower di.tor and the emergency -situation

can be 

e 

removed by rapid pari"-l' -•reuti:o of the reactor power. The same is ta perating e a in the case of significant local true in transitioba-l0. r"time .... . cse"

perturbations. t .prtial AZ-•Srgime; -can only operate. for a shirt -time, for g if the SUZ rods are lowered* to. a. sgnificant extent into the core during a partial AZ-5 event, the reactor will be completely shut down just as in an AZ-5 regime.

The 2-3regme redctin 4,to 20 offul owr) is ordered" e there is an

emergency load rejection

* by two t i es or

y theonly operatig .mne

The AZ22 ie (re 06te45%1ifr) its ordered -whew, there: i •

* an outag otftý,.of. ''~ mrtro

" an emergency loadf rejec i m ofto turbine generators

The AZ-! regime (reduction to.601 offul power).is. ordered when

- One of the three operatin maim circulation pumps in any pum room is switched nff. ,L

IlTe water f low inL thek -primar7ycircuit falls.

* The feedwater flow falls.

* The water level in the drm saprators falls.

* The group closure key for the throttle regulating valves is actuated.

In AZ-I, A2-2, and AZ-Siregimes the reactor power is automatically reduced. at

a rate of 21 of full power per second to levels of 601, 507., and 201 of full

power, respectively, by 'the online automatic power regulating system. The

emergency rate (speed) of power reduction and reactor operation stabilization

2-43
ev5a,'~~t reduction are- obtained` 'by automatic switching c ~ji~a~ng ireg of the, 40p~mnt~ SUZ rod~k,~

i • i::<•i" ~~~------------------------ •,•'•++ at ai, safe power I

into. te automati

2.7 Reactor Oper

2.7.1 Highlights

-v-..4 .

-'--4-

at ;ibi

Normal 

Startup'.and"shutdown "oPlerting mdrdes 

m 

cotisft, Of startup and sh~utdown and full-power op'eration. mustse 'astbe performedin " .rescribed sequence t imit ihermal pefome in a presc to

stresses in the metal oaenta of the reactor. uig full-power operation, plant operating candit•os are-m• ntained within: specified boundaries; to ensure

reactor safety..

2.7.2, Reactor Startupa;nd Shutdown

RBHK retactors- ae startd upfh the main circlto 1,s'dig pressure nop-e -' r'a t!on,,a~t a

vitijin- agjiven 

and, at',a steam sprtt~e ~e eetdb-teomao range. :?be required cavitation Margin of-the min circulation pump is ensured by reducing the p outpu ottl-re latin valves install" thp~i-te. tt ~e g • 

av ite.4schoirge. 

fuel The .. coolin'.ater feowrate in'all the channels of the core l-a mSiotored ctitin of the reactor is carried out atA !sliding" pressure in the tem ase.pa ra torsi. 1 e , pressure isthe ot cOstat but :increasesas the temperature rises.

During 

by-the emergency 

staru adniil 

fqed atiag ofOthe reactor, the main -coolant loop -is fed p. s-Reactor poWer during startup and initial heating is maintained at an averiae level of 2- to 3 of nominal capacity. The thermal power of individual fuelchannei s (uArng this.process can be as much as.6% of nominal because of theAon-uniformit-y of. power density distribution -inthe core.

N

with 

Reactor_ 

oetoor power-ascension 

hreo'h and, iitialeatin 

ma 

o the co...oling loop can* tak e place -

the reactor. At Circuatio Pumps operating on each side of ityonitor the water forate -through each. f~uel chami an atb ahe r time, enaure an. adequate vup:ý-cavita- tionmargin.Ata ac ~ 3 fo nlhecliglp-i ,heated toUteau~ 32?.~hrs~tessi -structures of h ea ;the.at- rte P ---to ---0C (1Y) per hour.

At a pressure of 2 to 4kjf/cm (28 to 56 psi) the dU-aerators begin to beat

at 

up. A vacuum begins• •tbul dýaedenserv.s. ft the of the turbine being started a separator pressureof a.bout 15sh0f/c .(213 psi). Once the vacuum has been created, the 

generator to build up speed. The turbine is normll-y y r• i c-onected to the grid vhen the pressure in the separators is about i~g .m16 (011 .'psi). Further increase in the parameters, up to rated values', t ,e place 1a parallel with the buildup of electric load.

Figure 2.22 gives from the time the

until the turbine

an enaple of the evolution of the: main reactor parameters reactor -reachea theminim.. power level that can be monitored

generator is aSnchconized and connected to the grid.

The cooling main circulation pumps retais In operation during scheduled shutdown anJ of the reactor. before the onset of shutdown cooling, the reactor

2-49
2-

i,. • • 

20 100 150

40 20 i ly, 20 

S r. . . ... 7

_____ 0 10_ _____ _____"___

Tim.. h 12 14 18- 20

LE.GEND: 1-.water temerature (T) in reactorirculati pressure in separators; 3 - ermal power (i) of reactor.

-4igure 2.22:, 4Ewlati Vaaetr durngstatu

.disconnected 

power is reduced to the after-heat level end ;the unit turbine generators are from the grid san-shut off. Wu reactor power"is.reduced to the 20% level, the capacity of the, min circuation pumps in service is reduced to "6000 to 7000 rn8

1hr (27,000•t. 31,000 u'.). he coolant loop is cooled down to a temperature of 120 to :30oC W(2 8to 2667)by 8radually lawering coolant loop .pressure by discharging steam ina cotrolled manner from the steam separators Uo the turbine codensers or to the proces codnser. To achieve a grnater degree of cooling, a pca htoncooln system composed of pimps and beatJ

exchangers is used.

Thermal stresses in the metal structures 'of the reactor limit the cooling and heating rates. During shutdown cooling, the rate of tempt-rature reduction in' the coolant loop is determined principally by the rate of controlled steam dis- charge from the separators. Therefore, It Is not difficult to keep the cooling rate at the prescribed level under these conditions.

A

2-50

- I • . •4
2.7.3 Operation tI Power.

to a power level of 500 NRt, coolant is circlated. through the reactor by -

the main circulation pmps operating at 600, to 7000 m,3/hr (27,000 to 31,000

gpm). At a power of 500 HWt, the throttle-regulating valye is opened and the

capacity of the main circulation pmincreases to 8000 mA/hr (35,000 gpm). At

power levels above 500 MWt, the reactor operates at a constant main circulatnio

pump, capacity. .:ýWhen the~ power level exceeds 607.of rated power, no fewer than

three main circulation pumps should be operating on each side of the reactor.

The hydraulic distribution. of •n W& reactor core is such that, when rated

capacity is reached, the throttle-regulating valves are fully open and the

total flow through the reactor is. 48,000 m8 /hr (212,000 gpm).

2.8 References

Cherkashov, 1981 Cherkashov, Y. X., ýet al1.,, "Mathemaitical and Experimental

Study of-limergency Cooling ofRBMK-1500 Reactor in Case of

Maximum Credible Accident, " At2!ay e hlektricheskie Stantsii,

Vol. 4, July 1981.

Cherkashov, 1984 Cberkashov,,Y. Hf., "Safety Design of the REHK-1000 Reactor,"

IAEA-SM,268/84, Operational Safety of Nuclear Power Plants,

Proceedings, of an IAA symposium in Marseilles, May 2-6,

Dollezhal, 1977 ollezhaL, N. A., et al., "Design of the RBHK-1000 Reactor,"

Paper presented at Risley under the SCUAE/UKAEA Agreement

for Exchanges of Information on Pressure Tube Reactors,

Atomnaya Eiergiya, February 1977.

Dollezhal, 1980a Dollezhal, N. A., and I. Y. Yemel'yanov, "Physical

Chracteristics-of the Core," Chapter 2 of Chinnel-Type

Nuclear Energy Reactor, Atomizdat, Moscow, 'arch 1980.

Dollezhal, 1980b ZDollezhal, XN.A., and I.!. Teuel'yanov, "Design of a

ReactorPlant.,",Chapter 3-_of Chane-Tpe, uc Uar -Energ

Dollezhol, 1980c Dol-ieshal,_N. A., andI., I. Yemel'yanov, "The Recharging

ý,Machine," Chapter 5 Of Channel-Type Nuclear Energy Reactor,

Atomizdat,1-MNoscoV, March 1980.

Dollezhal, 1980d Dollebahl, ]. ýA., and I. T. Yemel'yanov, "Control

Devices T"_,Chapter 6 of Chsanel-Type Nuclear Energy Reactor,

pp. 119-123, 131-138, Atomizdat, oscow, March 1980.

Dollezhal, 1980e Dollezhal, N. A., and 1. T. Yesel'yanov, "The Recharging

Machine," Chapter 10 of Channel-Type Nuclear Energy Reactor,

Atomizdat, Moscow, March 1980.

I'.

"I 

t

Dollezhal, 1981 Dollezhal, N. A., "Graphite-Water Steam-Generating Reactor

in the USSR," Nuclear Energy 20(5):385-390, 1981.

2-51
Duroayt 1981 7Nbrovsky,-V.(d. , ontuinoIulerlor Pants,

Mi Pfsiihets', Moscow, 1ý i...

Egiazarov, i977 ;E .n•a A. .e Kuz•'mK i "Stiktup and Familiarization With an1 RBHK-1.000 Reactor Power Plant (Experimental . invetiigation. of Core Phys cs and Physcal. Startu of an

131KRector)." Paper rsetdasmiar at Risley, widr t9 SUAE Cj. Agreement for Exchanges of nforma-

• -t:rion Presau•re Tube Reactorst. t a ner i. ],

Febr-uary -1977 (EPRI Ref. No. 77"021 ).

Isayev, 1984 .:IsiN'ayev,. -V., et al., "Unloading Aditional Absorbers

F•ro the• 0311-1600 Core," At.m*nr'a Enerkiyp 56(5):280-282',

1984.

Kluo,197,5 fli iav,, Al Phy*c~a sics: anhd Nuclear: eactors, Mir

Konviz, 1981 lonviz p V-.S., "Work Continues: on Construction ..Chernobyl '4May AES," Moscow 'Inerseticheskoye Stroitel 'sno

~4'2 6. 91

Konviz, 1984

Kulikov, 1984

Levin, i983

ýonviz, V,.8.., and L. V. Golubkov, "Chernobyl'skaya AES

T-hird-Phase Construction Begins," Moscow Energeticheskoye

Stroitel 'sao, 1984.

iEnlikov, 1. V., "State of the Art and Oevelopment Prospects for.Nuclear Power Stations Containing RBMK Reactors,"

,Atoamnaya Knerxiya 56(6):359-365, 1984.

Lavin, T. N., and -remen', M. Q.,, "Particulars of Startup, Adjustment on Third Generating Unit of Chernobyl AES,"

lrassian6:4450-54, 1983.

V •

Markov, 1094 r-h',v, Y-. '., ,t at al , " rgency Localiation System at

.T...dlk.Vit ,;44- rnobyl AhS," ,Mocot Enerseticheskole ..... .. • 0• ige• m 1:61-3, •Jamury 19.4.

MCCI, 1966~& IterC-4"1.- Consulting Enineers, "Accident at the

-: -e bl ,Plant•. Unpublished analysis,.Columbus, Ohio,

Novosel'skii, 19 Novosel'skil, 0. Y., V. B. Rarasev, et al., "Tests on I•,"erovd Steam Separators in the Third Unit at the Chernobyl Nuclear Power Station," Atomways EnertiYa 57(12):382-385,

March 1984'.

Plyutinaskly, 1983 .Plyuttnskiy, V. I., and v. 1. Posorelov, "Automatic Control

and Safety of the Thermal Power Generation Components of

-uclear Power Plants," Inerpoatamizdai, 1983.

Pushkarev, 1979 Pushkarev, V. I., et

cients of Reactivity

"6(6):386-389, 1979.

&l., "Ways of Altering the Coeffiin RL31 Reactors," Atomnays Fnerjaa

2-52
Romanenko, .1982 Romanehko, V:. Sl., and A. V. Krayushkin, "Physical Character. istics of-an RBMJ Reactor in the Transitional Period,

Atomnaya Ene tziyai 53(6):636-373, 1982..

Sedov, V. N., et al., "Corrosion Products in Main: Technological. Systems of Atomic Power Plants With an Rk-1000

Reactor During Operations," Atomnaya Energiya 46(1):23-28,

Sedov, 1979ý

Semenov, 196j

Usik, 1984

USSR,:ý; 1986:

Virgil'ev, 1979

Voronin, 1980

Yemel 'yanov,. 1981

Yesel'yanov, 11984

Semenov, B. A., "Nuclear Power in the Soviet Union," IAEA

Bulletin 25:(2), June 1983.

Usik, A. M., et al., "Progress Report: Chernobyl'skaya

AES' 4th Power:Block,," Moscow Energeticheskoye Stroitel'sno

1 2:8-11. 11984

USSR State ,Coumittee- on' the Utilization''bf Ato~mic Energyý,,.

"The Accident- at., -Chernobyl Nucl~ear Power,'Plant :and-fit~s'"`Con-ý,

sequences," Informaticn compiled for the IAEA Experts' Meeting, August 25-29, 1986, Vienna, 1986.

Virgil'ev, Y. S., "Thez-mal Expansion of Structural Graphite

and the Effect of Neutron Irradiation," Atomnaya Energiya

47(5):305-308, 1979.

Vorozin, L.. N., "Nuclear Electric Power Plants," Atomnaya

Energiya, July 1980.

Yemel'y*~ov, I. Y., "Design Measures To Ensure Operability

of Nuclear Electric Plants With RBiK Reactors Under Emergency Conditions," Atomnaya Znergiya 50(4):251-254, 1981.

Yenel'yaDov, I. Y., etal., "Study of Operating Conditions

of Power-Generating Unit With RBMK Reactor With False

Turning or- of-Reactor's Emergency. Cooling System," Soviet

Powee Engineering 3: 10-13, March 1984.

"a'

Vt',)

"'-'-Sr

I,

-~1

"-'S-p-S

2-53
M>-cAPTER 3

-SAMETY -NALYSIS

This chapter presents a simary of Sviet safety analysis of the RBN-1O000

reactor, as well as an independent review of RBMK-1000 safety. The base line

for\ this chapter is the hernobyl Unit 4 generation oR RBMK. reactors. Section 3.1 summarizes the Jescripti4ve mteri-al contained in Soviet literature.

Section 3.2 explains. ho6w 'i* - reactor responds to a variety of credible

is challenges, available.z- to As' the degree Adequte detailed technical: information on, the plant such the independent revIew section should be viewed as•`s'% anb

extensionof Chapter 2 ("Plant 1ign") explaining:, how, the RBMK-lO00 reactor' -. responds as asystetm.to tranaients and accidents. May-ntri rporstsb Western. countries~ since, thbe: avc'i6Mt' have .attempted to6 analyz 63( RM-reac~tor.

safety Such• Western reports are referenced, when used:, but Soviet, source documents are used wheneveripos.ible.

The Soviet report on the accident at Chernobyl (USSR, 1986), and earlier Soviet

literature, contain extensive.Ainforation about 31 reactors. Most of that

information is descriptive.,0i not analyti.c. Hence, there exists a rather

complete body of knowledge f whichito.assesble Chapter 2 ("Plant Design"); there is less informtio a0o • Soviet: safety analysis. The available informtion on Soviet safety analysis is very seneral in.nature, is-nit plant specific or site specific, and presents the qualitative results.of generic analysis, usually not quantitative details.

Chernobyl Unit 4 was one Of.14 op-erat'ing 311-1000 reactor plants. Significant ,differences " ..... I .. exist (fira in 31-1000I i ,-deig s, As, -they have .evolved fro. the early Lenin!-!

4g.rad -design- (f irst-generaion, RUEt 8: toltvl un-nitol to -the more modern Smolensk design (second-geveration, -6: total-mAits, .-including Chernobyl Units 3 and 4). his evolution ofthe di is ofte difficult to discern in Soviet liter- ature, and say deutale _-f the, pln-pei A ifferences -mong th 4 ats.

are not cleari..21 Th dscpt Weulrial of he #M1-10.00 secon g1e10 lmI mach more exesv ta aomtimm0 about the current status of -first-generation. AMKUO eatr.Svetitrtr does' ntL. iscss wehr the dei fa- f 

tares unique to the a.odenrt~ aeben- WU backf itted into the first generation.. Thereforeo, safty caabilities: discussed here may or may met apply to the 8 older RD-1 -1000 reactors. Also, since the single operational R1SM-1500

unit (Ignalinik Unit 1) operates with less safety margin to "boiling crisis"

,to [critical assunin heat this flux discussi limital O• thn31-1000 iactraa similar caution applies c•1be-appliedto the 33M1115$00 reactor.

3.1 Introduction

3.1.1 Purpose of This Chapter

Chapter 2 described the Cbersobyl Unit 4 reactor plait design, and Chapter 4

explains what happened at Chernobyl in April and May 1986. This cha.ter will

G. Vine of the Electric Power Research Institute (E1PI) compiled this chapter.

~3-1

2•

Ua.

A
help explain why it .hapend bygvn b givin the tlý;o seadier anwe fqýupes* adin n of- -plant

response to firqen1t c response to frquently,'occurring transients, cc: erdb~i•dn a. the transients discussed and credibl e- aci"n14Hnyo

zctident. Reiin the 

in 

individually 

this chapter actually did occur durin rnobyl helps provideý a bt tifuýert ins of what happened inthe c.mlex Chernobyl accident.

In .di in . l,•.• • i~!s.lo•o•'l ̧ :i!i~ • , . i'no• U s .• rn .e :•• r

In addition, some Westeri: countries may choose to develop Chernobyl" for their o countries and own 

'lessons learned from

review of Soviet reactors nuclear power industry. k safety.. :is, zan impotan erft' ofti process. For exampl]e, an accident such as thie oeie,,t -Chernobyl could raise questions. about the riginal safety analysis and the design basislof the plant. Even if the accident was due 

the 

partly 

plant safety 

to operator 

analysiserror 

to 

or 

assure 

manement 

ourselves 

breakdown, it is, prudent to review

sight does not exist that a contributing design overA broad investigation 

that 

of Chernobyl 

could hea paIrallel in our own safety analysis. reactor safety is necessary for 'this :1purpose. Finally, 

thnenby 

13 'RBNK 

Uit4had. 

reactors. restain- in operation, many with, fewer safety features

aationa border~ ~'A'~acti'lreview:i ý,'owa-qfthes reatr r ie close to inter-. the safety ofte-H eincan hel answer questions about Soviet corrective actions.

rhis chapter does not pi*asjdgment-o1 the design- or operation of the Chernobyl

wn 

eactor. 

published 

This 

information 

chapter is.iLteided 

from 

to-be factual and constructive, and:is based the Soviet-Unibn wherever possible. A major iource ^f information is the•-oviet r ep-oro the. accident at Chernobyl (USSR, :986)., An independent safety review:•oUChernobyl .s based on Western approaches. included to help us understand, erspective. -A comparison of 

the design performance from a more familiar

s not the objective of 

the relative- merits of U.S. and Soviet reactors this report.

.1.2 Summary of the Safety Review ::,

iernobyl 

Ltted from 

Unit 

the 

4 

evolutionary 

is. one of "the, newer Ram-,000 reactors, and as such has beneriginal Leningrad design. 

-" i 

As a 

roveomts RB- reactor safety since the seco"d-generation RBKM reactor, tit 4 had an accident .Chernobyl localisatiom system ('AL) underneath ,gned to condense sta aPrevost the reactor, de t0e release of ratdiociiyL. ,pe breaks in lover ýr~ic'tor,_we-9C ag O-UGAsspiug., Sock brreaks rioniee the Soviets to-bet .ozfet....... c• •6Went-,"- The reactor protetion Snd ergencycore: cooliagsystem include features to shut down the actor, cool the core, sad` p1n u damae in such an event.

other "maximum credible accidents" Are:defined in Soviet literature, but a aber ,f transients are studied, ýsuch as less of feedwater, turbine trip, and :Iirculation 

luced power 

pump 

during 

trip. 

many 

The. 

such 

Soviets 

transients. a*ttowptto keep the reactor critical at ,The reactor protection system pro- lea automatic power reductions to 60%, 501. and 201 power for selected tran- .nts in order to avoid full plant shutdowns.

ice the emergency core cooling system and accident localization system are igned for a 8at e maximm credible accident, other credible accidents are discussed 

be of sufficiently 

in Soviet 

low 

safety 

probability 

analysis, presumably because they are considered to Justify disregarding thea in the

3-:'
design basis. Exasples of credb aci ts" with, p.ottiallyserious consequences not 

piping,~~~~~ 

.discused 

semppnadmi mai 

ot in e: rt s ofeacor ekit ed epping.; rapid reactivity excur.- sin; and omeacdnsiitaigi h core rgo-isefsuch as a

discussion). 

blcked f low- chfanne117 -or mutipl w-can"il r~uptures (seie S&Vion 3.ý3 f orfute

analysis 

Many of thee accident sequences not. addressed by Soviet safety were part of CtheyChrnbyl. accident. A• important result of the deci- sion 

is 

to consider only p e bpebreaks below th reactor as: credible is that there no containment saukrrdnoing .:react uteotl:•pipi Vi'te . Figure~~~~~~~~ udn 2.) eco~te iigaove tereactor (see

3.2 Soviet Safety Analysis of, the, Chernobyl. Unit 4 Reactor

This sectian vill sumari ý te• - tu•loWOrmation available on the Soviets' ow assessment of their R-BH reactorsw.i.th emphasis on-ý the second-generation RK design and' Chernobyl Unit 4.-T ources of -iSoviet safety analyses.are-...- •riua rily te Soviet reporton the ac.4dent at the Charnobyl nuclear power: plnt(SSR, 19~) Soiet tchnica )aers-on- ter RU-l000 recor, and Sfewviev tetbooe aov ae-ilable in••• West.

k exist review between of these Soviet. Soviet and ~~ ......-

docents o4 cates that 'oame. fundamental -differences s-e-to- sapfety analysis. Some of these lifferences include:

:1) 

criteria 

The Soviets place hevielianceýOn:- systey testing to verify that safety are met. Fobr. e iae, ite- Svets have conducted extensive in- .plant experimentation, • have •recetly developed one or more scaled test -facilities 

clear 

that can duplicate 20-lO0 functions without the use of nu- fuel. The Soviets state the?'r analytic capabilities are good, but

Western. 

their capacity for computer-a.sisted analysis may be more limited than in

d-ictive ..

modeling 

countries. , ciet Ltechnica literature contains eass xpretest pre- ad, lessXpost-estci:oIe validation than is in U.S. tech- , lite . pproach to safety analysis may be adequate for studying routin opeatWis a••nd the ability to cope with :routine 

lim.-i;ations 

transients. 't i7 likely, however, Ihat their approach has placed on their iabliy to predictplanptperformance. in abnormal reie beyond 'these MM&imh - t4 treapod. Lion*~ -of, test -results,.

2) equremnts- fr cmplte e -mm JAtt of all safety: anal ysis calcuations-- tyi U9nion. cal of Also, Ithe Soviett.safety Wetrnlcean -proces -r not as extensive in the Soiet crite i nrally eaphasise overall safety objectives,. without seifyin.-g tVedetA l ed: criteria or methods to be used.

t) 'Available Soviet safety analysMisf the VYD (P]l) design, is more complete than available RBl analysis:..Soie V.4t safety requirements are ge8er- ally more stringent than RE.safetyrquire•e•nts.% The Wt is the Soviet export design, w•ereas the R -Is not e•prted outside the USSR. Hence VVWR designs tend to be sore cvmpatible with Western safety criteria; and

Western 

Soviet safety analyses of the Wit often rely on Western studies. This influence on 'the VVIR has led to -som recent Soviet applications of VM safety approaches to .t0e •=.

3-3
(4) Soviet safety analysis. tedst place greatepr emphasit: on, prevention. and

ea.Irly mitigation of eetd dein-a isacidents thaiý it- doe~s o the

cosqece n itigation: of severe accidents. beyon neig thbasis of,-

the plant. As- A, r"e~su-lt--j,-c.a -itl ~ analysis pears i.n-,. ovi'et literature on isue ý suc h rvn io is ad miia~ti of rativity

excursions, or coping wihtesoe hmclenergy i, the RBNk (graphi te,

,zirconium, H12 generatioa ian cidtetc.)

3.2.1 Soviet Design 'Phi los ophy ,

(This section is based on, inormatio-n. in 1981-1983 Soviet literature.)

The following requirements form the basis of safety for Soviet nuclear installations (Cherkashov, fI98):

a. th. plant~ -mus t `be:, eigned. bIL in such a way that the

probability of accidnsi pt to a minimum; y

b. atvany t ime-, even',iu ~cidnno; -. radioactive. ubstan 9-1

or radiation from them should enter the serviced areas of the

nuclear power station or the; surrounding environment;

c. the personnel servicing the nuclear plant must possess adequate.,

knowledge sad experience of-opera'tion. /

The overall safety of nuclear poe4rtplants in the Soviet Union ncludes a wide spectrum-of-measureks, h most important of w, h

are (Cherkashov, 1984; Sidorenko, 1981): 6

a. securing high quality manufacture and installation Of components;

b. checking of components at all,_ stages;

c. development a izatio i effective technical safety measures to prevent :accidents, -to Comensate for possible malfunctions, and to decrease the comsequeaces of possible ,accidents;

d.. d.velopmen niit • so•t• -............ activiy

releasedtin caio" Ueri Onali i;e s

e. realization of technical sad -moidrgatizaational measures to ensure

safety at all -stages of ,coinstruction. and operation of nuclear

power plants;

f. tegulation of technical and.,organizational aspects in securing

safety; and

S. introduction of s System of state safety control and regulation.

3.2.2 Soviet Nu-lear Safety Regulation

(This section is based on information in Soviet literature published in 1983

(Semenov, 1983).

I

3-4
The regulation! of saifetayibty, off icial documents is- one of the main tools for ensurighe safbtfey.,f. nutclear .power plants in the USSR'. The 

structured 

state sueriso 

prior ucerl ý: lant 'safety [as; twas to theaccident•]ýwas ccomplished by:d

The State Coittiee on "up eivision of Safe Operations in Indus- try and Mining, Under supervision, of the Council of-. Ministers of the USSR (sG or4.e' kndioriWfrt& MUSSR), whiceh supervised co - pliance with Regulations and standards of engineering safety in design, construction, and operation of nuclear power plants;

The 

which 

State Nuclear.' Safety Inspection (Gosatomnadzor of the USSR),

safety supervised: compliance with rules and standards cf nuclear in design,,,contruction, and operation of nuclear power plants; and 00 -ionst . f err

O 

Th- tate'Sinta, lbspecton of 'theUSSR, under, t]e inistry of ublicHeailthý., which sup'erv .ised compliance with rules and stan- dards of radiation safety in design,consto4*on,,, andperat o of nuclear powepans

The established systemof, three . upervisory bodies largely determined the structure of the whole' co.mpleof regulatory documents on nuclear power plant safety.

A regulatory document on nuclear power.plant safety in the USSR, "General Reguand 

lations 

Operation," 

To Ensure 

was 

the Safety of Nuclearf -Power Plants in Design, Construction

were 

introduced in 1973. i.n 1982, the "General. Regulations"

clear" 

revised. 

Power Plants 

The 

During.Design-,. 

new document-is ,tit~led, General Safety Regulations of NuThis ntruction, and Operation" (GSR, 1983).

struction 

document covers all types ..of-co" rcial reactors, operating. or under conteneral way, 

in 

without 

the USSR. 4n this", p•prach requirements are presented in a concrete details.-In most cases the General Regulations

lone); 

snly prescribe tasks which have.to beslvedto ensure safety (what must be they do not determine the solutions (how it should be done).

Other regulatory dcoosi44 ýS uidss rules, procedures) 'de elo . further lishing and. specify .-more'oncretely the 8-1:eal Reglaitions," e-stab- the basis for actiVities -fd igners and corresponding supervisory bodies. .One-of the.main-documents in the field of .engi- neering 

ponents 

safety is "Regulations forDign and Safe Operation of Com- for Nuclear Power .Plants, Test'4and Research Reactors, and Installations."

The basic document in- G•satx6adzor's.a-activity, "Nuclear Safety Regu-. lotions for Nuclear Power Plants," was introduced in 1975. It regu- lates nuclear isfety, -governing -ot only criticality problems in reactor operation, but also refueling, transportation and storage of fuel assemblies. It contains the mwin technical and organizational requirements to ensure nuclear safety in the design, construction, and operation of nuclear power plants, and the training requirements for personnel associated with reactor operation.

3-5
In the f ield of. radiation safety, the. basic docum'ent byr which theý health. and4;. inupeciito.n protection bodies are. gi`ded in "Radiatio In Safety St jandrd a s These -76) stand•a rds were wored 4 ot on O the basis of iecenain ofa time era iona -- Comision o' hdilo

ic.al .Principles Protection and...b.s.... ..... stm f do.' ..e-limit s and of thir appliCation' TeHalth. Regulations for Design

And OperatiOn f Nsuclr Pow• • Pl a •ts" issued in- 1978, further de- velop and, spe y e••asic, S-7 document to. include siting, moni- toring, and insec ionpol. 1⁄2

The sytmof g iatry documents on nuclear power plant safety is complemented by tie sysftem oif stte.tandards devel Ioped and estab- lished by the Stte Committee onSi ards (dosstandart of the USSR). The system of s ards extends •tm te of regulatory documents by ensuring nuclear plant safety through establishing requirements for miany component', wnterials, processes, etc..

Figu .3.1 is taken. from , esenow's study (193).,

Figure-3.1 Nuclear safety

it the USSR

regulatory bodies and documents

Source: Semenov, 1983

3-6
3. 2.3, RBH-1000 Comiiimice! With, G6eneral` :Safety Regulations

.The 1982 "General, Sfty eulton" (0,' 41983)i ,1dicus'sed. in- the previous, subsection, establish a4.b'road set o'f oulory regulations for all nuclear

power plants;. The regulatirns aremore•,: comprehensive than those established

in.1973. The provisions fo'rbrin ic existin-g' nuclear power- plants into con- formity with the revised reuwltions vre'- to be established in each specific_, case by the Soviet 'egii' -- iFsIti nots- kowwether Chernobyl Unit 4 had been brought intor conformity wth.th. revised safety regulations before the accident.

The regulations (GSR, 1983) discis som design and regulatory documents that should exist for Chernobyl:

The o ea v .eig itshould contain. a special volumi" .

.."Technical Substantiat.ioi of struc tio ad O . rating:Safet of '-F . Vuclia 0_4e -~ !{I,~~ldb h ee~1cnrctr -IX chief designer, and.... . .. tor, accoring . t the "stindard

content of the TOB."

The main document that .sures the operating safety of the nuclear

power plant is the technological 'egulations containing the rules and main procedures for safe operation.: of tie plant, the general proce- dure: for performing erai :rlated to- nuclear power plant safety, and also the limits n.dconditions of safe operation.

T-he regulations are worked out by the board of directors of the nu- clear power plant with participation of the scientific director, chief designer and general• designer, and is confirmed by the operat- ing - organization."- Thi sugts "local regulations with no higher, approval. 1

lone cf the above documents b eeni mde aailable to the International Atomic

Energy Agency (IAEA) to assist.A • i ng the design and operation of the hernobyl Unit /. reactor".. - . -

~i~hrepcttocoiiam~it ~ 3 "~tic"s -it -Is wrky difficult to h onomty •it those reguletions. :wavailable - loviet literature on.the -1W • -e4igsdes mot provide a complete under- itanding of how some of th"e: rlre.uirmeats Are met. Documents that might lemonstrate that Chernobyl NiftL-4 cWpWi"ed: With the general regulations would )e 'of 8z-at value to safety-enginers .i.-.reachine--r. understanding of the iccident. •EA. To date, the SovieVt-Unio ', .. • has :: mot pro ". A these documents to the

%be evidence from the accident suggests that Cernobyl Unit A wis not in com- iliance with the following specific reqUm its n the 1982 regulations:

2.1.4. The systems and devices of nuclear power plants important t* safety should be designed, manufactured, and installed with regard to possible mechanical, thermal, chemical and *iscellaaneous effects

that arise as a result of olanned accidents.

3-7
•2. !1. 9.

safety:

service

tenance

,T~h'e systems *ad devices, Of nuclear power plants ii

sh-ouldb ,Diiý.ubject to- a.-periodic check throuhu

life of t6,.,uclGar.,power plant and after -e"pair

anid chec:ashould no t lead to a reduction of 'the.

2.2.2X. The fst oierI coefficient, of. reactivityhol usual not.

be positive in' iy operting modes, of he nuclear power plnt d Ind, i,

a s.•:tates of he tem for~~ssptio of. heat f6ro te •coolant of•

the first circu6itk. ~ih.fs Odwer coeffilcient of reactivity in. aft.

operating modes" is8 psitive- the %reactor af ety in stekdy transient,

and emergency modes- hold' be ensured• and substantiated in the design.

2.3.1. At least io,:inbdeendent reactivity control systems (two

independent members or.tvo independent .groups of members),:,. prefeiril

based.o-, feet~ epovdd

a:.3.2. At least ý_twie. oflk tM 1 rovie independent r-eactivi contl' "#t 

state to th nbrtcal state independently of each other and should

maintain thsat.,s *~e~ tift temperature- of. the coolant and*

moderator.

Conversion to, the subcritical state- should occur rapidly enough to

prevent damnae of ts above the permissible limits at

any considered initial, event

2.3.3. At least one of the provided independent reactivity control

systems should ensure conversion from any normal operating state to

the subcritical state undersany temperature conditions and during,

transit-ionprocesges of-tecosid ewed initial-- events..

Conversion to th•e ubcritical state. should octo

prevent damal e of the fuel ts above the permissible limits at

-any considered initiral event-- aoding to the principle of single

-failure in a givenfn•ly tt during failure of the most

.effective resactivit gepond. .

'The toa ag fV- e. ivity -in the--indicated-ý trnIit ......r e 'Wniei6W' isto several temperatu and mode

ranges, using part- ahieA:mdited y•sot-em6: for each range (part of

the members and soms)-,vith application of the prin-d

ciple of unit failure for each part of the system.

2.3.4. At least one-o, the • Provided independent reactivity control

systems should ensure, conversion, from any normal operating state to

a subcritical state ,ad shouldmaislntain this state with regard to

possible release of ieactivityduring prolonged shutdown cooling

under any normal conditions and those that take into account initial

events according to, principle of unit failure in the given system

and with failure ofthe ,most effective reactivity control member to

respond.

2.3.5. The reactivity control system, together with the core characteristics, should ensure the absence or rapid suppression of such

3-8

I
power fluctuations 'd energy release. distribution,, ias a, result of which the fuel elements may bdamagd above the, limits,,:for, norml operation during the peratAn -period of-the core.,

2.3.6. The reactivity control system" if there is a single disturbaace in the monitoring and control system, should ensure suppression of positive reactivity related to the coLtrol mimbers being brought to reactivity (within the design'rates) without damage to the fuel elements above the limits cr normal operation.

2.3.7. The maximum efficiency of the reactivity control members and the maximum possible rate! of- crease of reactivity in the case of erroneous sctionsb of- prsonnei' or of single disturbance of any device of the nucle6ar. po0erpl•anit.hould be limited, so that the effect from, -a- subsequentjincreaseý of power does not lead':

to excess mimmpmiibepressure in the first circuit;

ýto impe rmissible deterioration of the efficiency of heat dis- sipation. or iwltdovu J* the fuela emnts.

2.8.1. Localizing systems should je provided to confine radioactive materials that have escaped from the reactor installation during an accident withinthe bounds-provided by the design.

2.8.2.. The first circuit lihold either be located entirely in con- tainment buildings or, as in the. case of planned accidents. localiza- tion of released radioactive isterials. within the boundaries of containment buildings should be ensured. Directed discharge of- radioactive materials, into tbe environment is permissible in indivi- dual cases if it is substantiated ,in the design that nuclear power plant safety is, ,enuredwith-this ,discharge..

2.8.4. Localizing syst hould1 perform their functions during$ accidental leaks of coolant of the first circuit with, regrd to the ,possible mechan.ical, thermal.d ical effects.

3.3.). The foll11 il • s hu b carried out before peim- nt:•-operac.onforMIty Of the: Icleari pGr -plant structures to the design should be checked;

startinS-adjusting 

tests of individul operations should be completed (including systl MW

complex .estjag of the muclesr power plant should be Mleted (including physical and power startups of the reactor).

The procedure for putting the nuclear power plant into operation is carried out in the order established by existing regulations fer the corresponding enterprises and accordInS to these "General Regulations."

3-9
contain 

Sthat regulate startng-adjusting per on should

the Viewpoint 

a- list of the ope ,'.tib that are.00tentially. hazardous from of sa,_fet for cx e o era-ions wait: ma inncontr

measures c,onvert that• prevent e core te to a 

occurrence 61percritical of accidents. state) and a list of the

3.2.4 REMK-1000 Design-Rasis Tranxients.

The 

for normal 

1982 requirements (GSR, 1983) establish the. maximum-, dimage to fuel elemeonts

and O.1% of 

operation at 1% of fuel elements with defects of the gas leak type,

fuel occurs. 

fuel elements foruwich direct contact of the coolant and nuclear The requirements then specify that

An excess of these limits is, not permissibleupon a single one of thee fol owing violations of normal operation:.'

a. malfunctions of the reactor control andmoniltoring system; -b. 

C. 

loss of power supply to the main circulating water pumps; switching off of turbogenerrtors; d. co"lete loss -. xter.al. power.supply sources; e. leaks of the first circuit, N filled by standard makeup systems.

The 

for 

RB31000-OO 

some but not 

designall 

provides 

thet 

for automatic power reduction or thtatdown (scram)

schem for 

aboveviolations (,tee Section 2.6.8). The protection

,discussed below.

each of the above desin-baesis violations of normal operation is

One 1983 Soviet report (Cherkashov,-. 1984): states that

The following are considered the Most likely transient conditions resulting from failure of 'equipoment:

eo emergency shutdown of power stationturboenertors 3 marlfunction of Al coolant circulationPiets *failures Aa the LgbsdwpW;svMy -system T total os yi or matlstO i d d iower ... plant

The 

emergency 

criterion of nuclear power statiom safety assumed forthe above condition tis the absence of 6ryoui t on the fuel pin surface..

which 

meats. 

correlates to. the -fuel element defect criteria of the general require- rach of the above "most below. conditions" olkkelyeetran to hient discussed

3.2.4.1 Protection for, Design-4asts 'TransiLents

The 

reduction, 

Soviet protection 

but 

system for most transients is designed to initiate a power

ý(Voronin, 1981) 

act to shut down (or scram) the reactor. One 1981 Soviet textbook differentiates between transients and accidents as follows:

Under trnin odtos h rmaysa st e h oe ualt 

all 

in o eration at the admissible power-level with observation of requirements with respect to reliability of heat transfer from

3--10
the. reactor •ore nd•. safety~~ of the nuclear power plant. Unde e - itency, condiftios~ prmayolisielshtwnfteracr

and- the power brdr nit 6-d1excld"e to- d•magi •to the. n'Ucler fuelin

the ractorand bsic quipment ad h p.ipliea of the. nuce

steam generating, plant..

(1) Malfunctions ot i1*?R6eadtor Contro•l••And Monitoring System

One of the worst recttor rSystem trt diitoring maif unctions .oul a,.e 

continuous rod vithdrawal accident. Such an accident would increase reactor

power to an unsafe level.A high reactor power scram opweoerhtof10

of nominal power in. -listed- inthe. Soviet report on the accident at Chera-byl (USSR, 1986). However, the.abilityof .t high power scram to respondvith

adequate speed to a rod, withdrawal .accident and to, ?revent fuel damage is not

clear. A .1981 :Soviet-.-textbookýl.Dollezhal, ,1980) indicates that the number and&,,

eff icie'ncy of ascram: iods 'are b:ýaft sed oni "thbe. max imum, pssible rapid vaiai on of

reactivity. Two- coadnýtioss were considered: drouoftcuecanlsia

coli eacorlate, incore life, and olasoftemina th ore and cooin

Of fut"el -elements- early ind core life when th ivid" coffici it 1reactivity isa

negative. However, the ratech the reactivity variations would occur is less severe than.,the~~ rt cd ,the 'Cherbyl a*ccidtnt. Also, the

list of most likely' i : o itia_6doets not include reactor control and

monitoring system ialfunctions.-

(2) Loss of Power$ Sup- o h lWCirculati. 'Walter: Pumps

Section 2.6.8 lists the: follo Automatic actions in relation to this

transient:

AZ-5: Emergency shutdown o "the. reactor... -(scram) :on ,shutdown of three or more

main circulating pumps: in one loop..

AZ-2::. Reduction t501 SOof ratedpowe'r-because of loss of onk f two turbine.

generators and its. associated mainWcirculating pumps (MCPs).

AZ-i: Rdcint 0L frt poefbththe- reactor and gnrao bec--ause of shutd ofa melitireemaiir•lting piins in- 6ms loop v, whe.n

-the water thogpti ed u lkki te~ 1rmar .circuit. Th acrprote tion system alo niiaesa auoaic rpOf' one r#Cifcu1ation POW In thO

opposite loop, so flow- throug the, core *ill be. balanzed. This prevents power oscillations and thermal -transientes. 41983 Soviet report (Cherkashov, 1984.

discussed investisations inwhich theb output of the remainig8 pumps increased

from 8000 mb/hr to 11,i000 ./hr.-on LhaAown, of one pump and reduction in steam

quality.

Shutdown of a coolant circulating Pump 1islisted as one of the most frequent

transient conditions.

Note that toss of two out-of three main circulating pumps per loop is not

covered by the protection logie. lBased on protection logic described in the

Soviet study on the Chernobyl accident (USSR, 1986), loss of two out of three

ICPs in either loop will not r-educe power below 50%. However, "Operation

and Maintenance of Nuclear Power Plants" (Voronin, 1981) indicates the plant

would trip on lose of two out of three pumps per loop.

3-11
0.) Switching, Of f Thrb~ine Geator

Section 

transient:

.6.8 li11ts, tie fol3oub auomati'c act~ions in relation to this

AZ-5: Emergency shutdown (scram) on loss of both turbine generators: (including loss of on-site Ac power..-

AZ-3: Reduction to 20. power,.on o:ad rejection by both turbine generators.

AZ-2: Reduction to 5Ofaepow er on loss of one turbine generator.

Emergency shutdown, of power sitation turbine generators frequent transient is listed as a quite

from fullpow-er by' both, 

condition. In the case of' asimultaneous load ctiouý turbne generators,, power is reduced Jn-h ouseý ....... -oad s,- and iVWtw l~ikel that one or mote scarto22 

(4)" C lolete 'Loss; of External Power 8ppli Sources Section 2.6.8 1istsir-iutmst€ tisi: relation to this -transient. It - does list "loss o -ras 

emergency shutdown a condition resulting in an

power" indicates (scraim) .. ',This se f.the term "loss of in-house electric

generator 

that -l..l lant. ac' loads -are: powered normally from turbine output. (One Soviet report-iists The .this transient as an "accident.")

in a 

following 

1984 Soviet description 

report 

of a "loss .Of power to. internal equipment" appeared (Smolin,: '1984).

When the internal equipment current, 

in a nuclear power station is deprived of the main ci-rculation-pumps while stop along with the feed pumps, the emergency•bhutddiovnequip-ment operates and the automatic

pressure 

shutoff valves ahead of Ute turbines are closed, which causes the- to increse and he afety.valves to open. Then the presAfter 

sure in the lpt fall, tbegins -andthe , safety valves should close.

been found 

abou ! mi,-.the emergency feed_ pnmps are switched on. It has

stable 

with a ul sm a hecked on the reactor that cniiar bI etinp-isle zloop by natural ,circulation,. -and coolin the or -4s tas z aww&y, 04.plications.

(5) Leaks of the First Ciarcuit., Filled,.by-Standard Makeup Systems

Section 2.6.8 lists- three conItions relating to recirculation system (Soviet designation! first circuit) leaks which ir

(scram): cause an automatic emergency shutdown

• uncompensated coolant leak greater than 55 kg/sec

• 

• decrease 

high pressure 

in steam separator water level outside set limits in either half in reactorrpiping spaces (leaktight compartments)

Each 

on transients 

of these three 

is 

conditions is indicative of a large leak. The regulation

capability 

directed at smaller leaks that are within standard makeup (no need for emergency cooling actuation). tion 2.6.8 It appears from Sec-

(Voronin, 1982) 

and from "Operation and Maintenance of Nuclear Power Plants"

unless the 

that smaller leaks do not require an immediate plant shutdown leak is in the graphite region, and therefore leaks within makeup

3-12
option 

system 

of 

tpsity 

reducing 

'Wvill not cause an automatf, scram. T•e operators he te

rep~air.`,o fipoabe,. power, isolating the leak. iff possible, and- prsu- ingoine

or st v r u tli.ew.•ould 

IThe rup-ture, of ai single- channel lower (Inlt ieý

lists five 8.speciic 

not cause an automatic scram. Section 2-.6.8

tor vault: (e!g.,-oisture lcoitions, 

in graphite, 

which are indicative of ac lek' inside the reacwhich excess pressure in graphite), all of

observe. 

sthould. 

d-,

be, monitored by the opert n would requireamaul ca if"-

(6) Partil. or Total Loss of; Feedwater

Althoudhedot 

water pupply aystem 

liste above 

are 

as a sign-basis transient, failures in the feed- listed as a likely transient condition. Other Soviet documents, list loss of fteedwater as~a desigr!-basis- transiet, possibly it is 6eci s

egenerat-ors~r.- 

ýani, outcome:2of 

ThnoralowrupytotemaIin 

either-a loass:of offsite poiwe'r or- at loss, of, both turbine-,

i.g#enera.tr-• - eedater pup sturbin ou • All -- . -additito6nlV 

f I feedwatr pumps are ac motor driven..In

as well as 

"in feedsiater"'Pumps can be powered"'by external ac power sources,

Chernobl M, 

tu.erntie generator output. The Soviet report on

runback to 60%-(AZ4 

states thuaja drop:in feedvater flow causes both a power s n emergency shutdown,.without setpoirytt *'for -specifying a feedwater

feedwitetr owýk-auses 

eiiibithr'-action., 

t"eirgency 

_-Section 2.6.0% states that a-' loss of M0 or msore of'~ shutdown.,

with 

One 1983 

a losso.,fot 

Soviet, 

feedwter.flowto 

report (berkashov, 1984) discusses the complications associated the steam separators.

In this. situation 

pletely. stopping the.emergency safety system is triggered, corn- the fission chain reaction, and the reactor power . ..... ..reducedtothe-dcJay hat removal level. The turbogenerators

-signsl 

Iturbine, generators) are unloaded ou receipt of a pressure reduction-

.c.¢onpditio 

iem,• thseparators, with the rate of unloading under these

•regulation.on;ditions 

-1bei. -gireaterhan with normal operation under pressure

bringing-t~~heeac~~ 

ortwith the triggering of protection devices

- to-A. "wer-power level. The increased rate of "rati.•z revents anyig ficat--'re4ducion of

... is a ..posslibiit ~presuea~t emt, 

sPlying a 

water 

th -dwaer 

to uppl 7-is5 cut of f thre the•.separatOr. b means of " emerg•.•esency 8 edat 

These-pups 

with .. a total output of about 101 nominal.

start of the 

are swt c o automtically about 10 seconds after the

water 

lo.ss- ofnormal- feevater. [If the loss of main feedto 3,'minutes. 

sis• "used by..ioss of in-house electrical power, then about 2

,the e r 

a.re •to quired Set emergency feedwater p~rps loaded on 7'eýny diese generators.)

A featureWofithetwra 

switched off after 

ient, ts that the coolant circulation pumps are

feature permits 

triggeri•nS of the emergency safety system. This

prevents 

a*reduction o* water level in the separators and

steam 

stem from being trapped in the downcomer system. Trapped

to a deterioration 

could lead to cavitation of tLe coolant circulation pumps and in the condi•ions for convective circulation the circuit. After in

the reactor by convective 

shutdown 

circulation 

of the pumps, decay heat is removed from of the coolant.

j-ia
3.2.4.2 Analysis of Design-Basis Transients

As stated above, most of the design-basis trausients are provided for by automatic or manual scrams or power reductions. However, n .uantitative analysib

could be found in Soviet literature that documents how a- atic actions will

keep fuel element temperature within limits. Specific examples of transients

that appear to be within the RBM--1000 design basis, but do not. appear to bC

dnalyzed for safetyjin the Soviet-litera are;e•-

(1)

(2)

The leak or rupture of a single fuel pressure tube in the graphite region

is the limiti.ng design-basis event for the RBHK-1O00 reactor vault. Detection and manual reactor shutdown are prescribed-in procedures, but effects

of tube rupture on surrounding graphite and the gas pressure boundary are

uncertain. The reactor vault pressure boundary might be breached in such.

an event if o6perator dtetetion ,-and-man'ual .ac~tion are delayed.

The. b4J-lO jeco~liu~bpictected. againist' the leak or tuptu4re- i-p

a single -pressure tub.ue'-stem! outlet ripe, 16cated below the refueling

floor but above the upper biologicai Ahield and gas boundary. Although

makeup capacity is adequate to handle this event, the escaping steam and

water are not contained. Such a rupture has not been analyzed in the

available literature for its effects on adjacent outlet pipes, on control rod drive mechanisms subjected to high-pressure steam, or on nearby

refueling operations ii' progress.

Y3) With a positive void coefficient, and complex systems and procedures for

maintaining adequate heat transfer margin to critical heat flux (CHF)

limits in each individual tube, the plant operators face demanding responsibilities. IL is not clear what measures exist to assure they arc

capable of detecting and selecting the proper course of action for each

of the large spectrum of credible malfu3ctions in the reactor control and

monitoring system.

3.2.5 RFBMK-1OO0 Design-Basis Accident

The Soviets employ the concept of "moximium credible accident" (NCA) or maximum

permissible accident (HPA) in their approach to designing the RDK-IOO0 safety

.systems. The MFA is defined as the largest credible pipe break in the primary

circuit. The size of that largest "crecible" break has evolved over the years

ta the size of the main circulating pump inlet and outlet piping (900-mm

diameter). Early RBDW-1O00 designs (first generation) did not consider large

pipe breaks as credible accidents.

The existence oflan emergency core cooling system to cope with pipe breaks was

mentioned in 1975 (Konstantinov), and general descriptions of this system

appeared in 1977 (Yemel'yanov). However, the emergency core cooling of this

period consisted of the high-pressure tanks anJ pumps only, without a tie-in to

a bubbler pond (or steam suppression pool), which did not yet exist as a source

of emergency nakeup water or as a heat sink for pipe breaks (see )Agure 2.1).

A 1979 Scvlct textbook (Dubrovsky) discussed a 300-mm (12-in.) pipe breaK as

the maximum credible ac:ident, although other references shortly thereafter,

discussed a 900-mm (36-in.) break. Soviet documents in that same time frame

(e.g., Margulova, 1)97#) discussed "bubblers" and "technological condensers"

that were installed i *h- turbine building to condense main steam safety valve

3-14
di-scha~rges ol (not steas blowdown f rom pipe: breaks). h f :irst oiet do61cii- Dents to de-scribe the accident I.ocalization s yte (ALS, t bubeonid # ss tesr installed beneath the reactor builing)- and its design basis (her 1900-.

break), 

RBHK-looo appeared in 1979 (Dub'vvsky), four to five years before the irst - reactor went into o;erationwith the AU. D"•s lied' (see F 2.6. from 

represented 

Dubrovsky, 

a "semidesi 

0979). -Therefore, for neal dcd~ 900-w1 iera b accident" fcr oper.ting" .-.. R... 0 reactors: It vea the bi 0r ity to protei- thoat fu fr overheating, t-t it was .. not bsi •he . for. ay containment function, 4hich did not yet exist.

Soviet literature 4[urbu.-g the late 1970. and &arly 1§80. concerning emergency core cooling and accident localization is much more afbiguous about first- generation than second-generation RW rkeactors. -.:•Soviet literature reveals the following about: . .. ... .ystems:

(1) igh-pressuejcto t j44o. Ib oe ntecae ppebreaks is, provided. Soviet r•fern •'clearly tt second-generatiOxe." RBIK - 1-000 emergency. core co••oln st:-aredesigned to handle the largest recirculation-. i diameter), and suggest that first-generatio- Injetiol ysem arenm equivalent.

(2)1br ,reitlledto.quench essl "safety valve discharges. These . bubblers" were; s6ma i arison -to the bubbler ponds to be installed later 

hadon tecond-en.ajo RR-10 reactors. The-older RDNK-1000,plants two bubblers:,- each- a,;;vessel aot10 ft in diameter and 70' ft long. They did not have thecapocity or the physical connections to condense

the steam from large pipe breaks..

(3) Originally, 

boxes" 

first-generaton, edrulation piping was installed in "strong designed to-..-thoti" UP. to about 4 atmospheres pressure (about 60. psi). But theseWstrong hoes.or-_aults hadno means of relieving steam pressure. froma.pipe brnk to condensing system (first-generation RBMK-1000 

Pnly 

reactors do -ot have a bubbler pond beneath the reactor). Bence, smallcold MC Wcotained:.ever, 1the liters- Lure and a1 -cma revie se meeja )'eaca ~ ~a~L~t 4a nVienna during th psaccident"

a ional rnP bubbler 

tha tqnuls fomb these ~5 ~ ~ 2K

Soviet refe•re• ksbew oby hae e providd. o cý .old eo .dthat st~tes whether or not-such tunnels

locali~.~ton 

are backfitte 

s 

on lftist-generation ROM reactor, or whether such te . r : -cpabl*,iof handling a 900-rn break.- Therefore, older (first-generation) RM••'O00 reactors may still have a limited 4, ability to cope w-th t htJ e l-arg-,st break size: ICCSs apparently have been upgrad-ed to provide. a&q ta Vore cooling -but-breaks my not be contained and 

never 

could vent to the atmosphere. uble ponds beneath the reactor could be backfitt*dl4 older-•senerati.• RU-1000 reactors.

(4) Extensive Soviet literature appe.rJ in 1979 (Dubrovsky) that described •wz/.r~'ncy 

capability 

core Cooling systems, bubbler pond design and testing, and the of the R]U-1000 design to handle a 900-m (36-in.) pipe break.' BHowever, the literature did not differentiate between first- and second- .Renerat ion RBy-1000 systems, and could be eiiinterp,'eted a& imlying that a,. FMK svyte.a were designed to lotalize largp pipr brreak% "

3-15
he lead plant for the new accident localization system (ALS) design underneath he ppeared reactor in was 1979, Smolenak (Dbrovsky), Uin• nit: 1. 

ut Descriptions Smolensk, of the AU featuring Smolensk did'ot go into commercial operation ntil 1983. In fact, the first RBMK-1000 plant to go into operation with an LS 

molensk was Chernobyl 

Unit 1, the Unit 

etes first 3 •ince CiernobylUflit 3 was ready for startup before program vas conducted 'at Chernobyl Unit 3', nd was reported in 1984 (Markov). Chernobyl Unit 4 was built with these in- roved large-break loss-of-coolant-accidet .(large-break LOCA) capabilities, ad therefore was designed to, andle a 900-rn break.

E 

1984 Soviet paper (Cherkashov) :descrbed, the following testing and analysis the 9O0-m pipe break event:

The design of the safety system A-isbased.on !the.premise that the-most serious emergency situations may with- . facture' of the large

i6 d~ ii ................. • nf• t design,' l rg pipework of the priarycirc 'teRIK er unit de"ign ............. provides :,technical 

the 

means to- ,prevent the' 4ýMbrg 'of a ta-a iture into service areas nd priclrly beyond 'he power"itation boundary.

The fracture of a large- ipe A••• yunlikely., Experiments on full-scale models haveshownb t :ata.-.-t.apressure of 8.5-9.0 MPa the fracture of a pipe with a diameter -f about 800 m is possible if the depth of fatigue cracks i _approxinately 0.75 of the wall thickness and the crack length exceedsi470 -S. Operational monitoring of the state of the metal ensuresý .the exclusion of the sudden fr'acture of pipework since the critical.,dehfct..ize is large and is reliably revealed during planned shutdowns of the unit. During inspections, the metal is examined and inspected using special methods (ultrasonic defecLoscopy, acoustic emission). Despite tUis, the nuclear power

of station the instantaneous design provides ieasures utonensure 'its safety in the event transverse rupture of the largest pipe.

The leak rate is initially ab•ot-6 7m0/sec in the event of the com- plete instantaneous rupture of ,a900-i-dindemter pipe, and 40. m3

with /sec the same fracture of - rndiameter pipe.. As a result .of the analysis of emergency .4itmts, A'ep 10n Signal h ,ave been chosen for the ,acsbco ge coolinsystem: an increase in pressure..seart .ts contining circuit pipework, and a reduction in level .i-a separator down to a value exceeding the departure from the 6iWAl Tal uefor transient conditions.

The 

main 

most dangerous pipevork fracture is in the discharge line of the coolanZ circulation pap,-since this imstantly cuts off the coolant delivery to the channels in the half of the reactor in which the emergency has occurred. It .s this hypothetical accident which determines the characteristics of the reactor emergency cooling sys- tem, including its rapid action and maximum output (about 1.1 m3 /sec).

Water from the emergency cooling system enters each group dirtribu- tion header. Non-return valves ore provided in the pressivre header at the inlet to the group distribution header to prevent the useless discharge of water through the fracture. The reactor emergency coolwater 

ina system consists of two subsystems: (1) the basic subsystem with tank unitt; (2) decay beat remowvl subsysteme with special pmaps

3-16
and water reserves in tanks. The cooling water is fed from pressure

tanks (and After their discharge, with the aid of pumps) to te emer- :.

enAcy cool-ing system header of each half of the reactor and from

there through pipework to each group distribution header. High-speed

valves on the water supply lines to the, headers open on receipt of

the signal that the emergency cooliti system has been- switched on.

The procedure for switching-o the b.sic4ubisys teaf:the emergency,

cooling system guarantees the decay hkoat removal from the core in the

event of a complete. or partial large diameter pipe. break and precludes false trips inthe eventofp, emrgencies not related to a coolant circuit rupture.

In the initial period of an emrgencywith a full pipe break in one

pressure header there iýso ooolant -flow through. one-half of the

core, the residue of, water- .Por~tes and is- discharged through the

ietd- -r tof the fuel ca Sviets estimate COmPlt dryou

in: one second' . 'Mir.is ±klbed ,by a rapid beatup of fuel pin cladding. At the msom t of restoration of coolant flow from the emerge~ncy cooling _system, the cladding will ae etd pt 65070C

Further increase to teperature of the cladding is slowed, down and

is then completely otopped by the transfer of heat to the steam-water

mixture and steam underv aon-44ilibrium conditions.

Maximum cladding terperat.re -Is very sensitive to the interruption time of

cooling water. A Soviet calculation, (abinov, 1983) of peak cladding temperature with a 5-second interruption was over lIO0°C.- With only a 3-second

interruption, peak temperatures can :e 2000C lower. (The Soviet criteria for

fuel performance following an accident are given in the safety regulations

(GSR,. 1983) as: ...

* maximum fuel rod temperature,,< 12006C

* maximum zircaloy-water reaCtion in the core, < 1%

" maxim= depletion of fuel claddiag thickness, < 18%)

The investigation ofti msecsituation rqie .iso

experiments to atmily'lbs -fidi amsfer in the fuil chnn ls nder

condi ~ ~ ~ xrciom i iinifwte e epae supply of, coolant. --The

-esults-: of thoe. e3e4"04 were the.m used to calculate the thermal

conditions of the f1l0-pins.6-%t s, sown that even in such a hypothetical situation thereI was 1O? 1 netration of water into -the fuel

-pins.

All equipment and pipevork of the êulation loop of the reactor

is located in closely sealed c•4partments preventing the 4ischarge

of a steam-gas mixture from the maclear power station into the atmosphere in the event of pipe work ruptures, since the steam-Sas mixture

is removed via special tunnels into a localization unit where the

steam is condensed. 1he coapartmsents are designed to withstand An

overpressure of 0.4 NPs, which is not exceeded even with a full

instantaneous rupture of the largest pipevork.

3.2.5.1 Other Design-basis Accidents

Soviet literature discusses other design considerations and less severe events

including transients, various equipment failures, and human errors. A few large

3-17
pipe breaks in locations other than the main circulation pump discharge are discussed briefly and not analyzed because they are judged less severe (e.g., distribution headers, MCP auction pipes). The.only other pipe break discussed in detail in the available Soviet literature is a break in a 53-in pressure tube inlet line. this Small pipe rupture is treated uniquely in the design, because the smaller size of these lines permits a lower pressure confinement area beneath the core Jaat in not subjected to the energy from large breaks in adjacent-confinement areas."

No other design-basis or beyond-design-basis Accidents are discussed in the Soviet literature that 6as 9ben:reviewed. Nor example, control rod malfunc- tions are discussed, but reactivity insertion accidents are not defined or analyzed.

,Main steamline breaks and main feedlirt breas" -a.reximples of important pipe -_breaks that do not appear to be ana lyed as desig accidents. The de- scription in Chapter .2 of the ounda F of the primary'piping confinement areas shows that the steam separators and 2ei•t inlet and outlet piping are not part of the confinement/bubbler pond system. The main steaslines and main feedlines do not appear to be equipped with main stem isolation valves or main feed

indicate 

isolation valves at the-4st4i separator ro'o boundary. These factors seem to that the Soviets do'not consider main steamline and main feedline - breaks in their desigu basis. The anailysis of these accidents is important for a complete understanding of If-1000O safety for the followirg reasons:

a They represent credible pipe breaks of large size.

They would discharge steam outside a confinement area and outside the bubbler pond designed to condense steam-from pipe breaks. They would be unisolable Lreaks allowing radioactive steam to reach spaces that could contain vital equipment.

would 

They would cause a rapid-steam •demndand steam pressure decrease that in turn create rapid and severe voiding in pressure tubes. Not only would this create OF concerns for fuel naseibly heat transfer, but it could initiate a 'severe powes Lzaasiat because of the positive void coefficient.

Since 

culation emerSency shutdown $Otpoiits are established on the basis of recir- pipe breaks inistead of fedline orsitemiline pipe breaks, it is not clear which setpoint will i4nitiate a shutdown or power reduction, or how long it will take. -Since the void coefficient power excursion would probably react Inch more rapidly than protective action, the tran- sient could permit an excessive amount of reactivity insertion before power could be reduced.

Finallyy, maim steamline and main feedline breaks in the steam separator room would not initiate .merae=y core cooling systems, since initiation criteria would not be set.

Note that Section 3.3, this chapter, discusses these accident sequences in reatrr detail.)

.- .- -•.

1- 18
3.2.6 RBHK-1000 Changes With Potential Impact on Safety

and 

Soviet 

operation 

literature contains extensive discussin of changes made in the design

in 1973 at Leningrad... 

ofthe RBMiK-1000 reactor. since- the first unit went into operation: from the first to the seaond The major eneration, changes were but made as the design transitioned other evolutionary changes hae also been made over the last decade.

Not all of these changes were made for Safety reasons. For example, more

operation 

changes were made to simplify construction or improve the economics of mReK-l00 and maintenance. Some of these changes were made t.o upgrade the safety of the RBin design. A raeviewof these safety-related additional changes provides

and 

insight into s MA- ie00o safety problems, Soviet analysis techniques, the degree to which these problems have been corrected. (More recent Soviet

problems 

literature 

have 

before the Che'nobyl Unit 4 accident stated that all the RBHK-1000 been resolved.) '

3.2.6.1 Reactor Control Proble and Instabilities

Soviets 

Th' following 

before 

discussion 

thecaccidentait 

of RBM-l100 problem and solutions identified by the

impact on safety. Xostof 

Ch•r-obyl is limited to areas with probable the ioSti reports (Dollezhal, s from 1983 and 1984 Soviet 1983).

Deep deplecion of the nuclear fuel vith a low initial enrichment characteristic is

nuous fuel 

of :RB•-type reactors, which is provided for by conticapacity recharging at the operating facility. Fuel recharging at

plants with 

is constantly accomplished at all RB-K-1O00 nuclear power

centrationýdecreases 

the help of an unloading loading machine. The U-235 conamount of 

from 18-20 to 'k'3.7 kg/ton of Lvanium, and the

such 

fissionable: plutonium reaches 162.8 kS/ton of uranium. With

physical 

a change in. t1e isotopic composition of the fuel-, the neutron- characteristics--of ,the cell are significantly altered. If in the steady-state 

teristics regime of fuel recharging only the local characcharacteristics 

(e.g., the poer) fIsthe hanels are altered but the of. =t ctors as ahle ,remain practically con- s tant, then Ithe most, In~ sits- -:ms ýpysical-.chiaracteri-. stics, in particular reat-iy coefficients the graphite, (stea•m, thermal ofperiod 

theml from he •tVgup) occur during the initial of operation, etareatr lo with fresh fuel and addion 

tional 

the 

absorbers. 

isotopic 

1the values of t.ea coefficients depend not only

absorbers composition of the fuel, but also on the presence of in the active zone.

Experience 

theoretical 

with the operation of the i1M-I000 has confirmed the

are withdrawn, 

conclusions that as the fuel is depleted and the absorbers

of the 

the reactivity coefficients Increase and the stability energy distribution decreases. A radial-azimuthil tribution, enerSy dis- for which the form of the monsteady deformations mined is deterstable. 

by 

Measures 

several of the lowest harmonics, turned out to be the least

have 

related to stabilisation of the energy distribution been carried out in two directions:

3-19
an increase in the automation by virtue of! the creation of

a branched system for regulation of the reactor; and

a purposeful change in the composition of the nuclear fuel.

(1) Increased Automation

Improved automation and control can be grouped into four areas. First,

A qualitatively new.systemof local automatic regulation of the

energy distribution (, ) ad local emrgency protection (LEP) which

operates from intrazonal detectors has been created and introduced

into upera-tional practice. The LAR system fulfills the function

of automatic stabilization of the lowest harmonics of the radialazimuthal energy distribution.iaintainuag a specified capacity of

the reactor, this system canOby virtueof auxiliar lements operati.ng.:in the individual mode, at~to*mtically regulate th caactyý in

individual regions of the active zone. The LEP system accomplishes

emergency power reduction in the case of local bursts of power, which

can arise due to the:failureof LAR elesents or for other reasons. A

structural peculiarity I of the LAl and LA consists of the use, for

regulation of the capacity and-proLection of the reactor, of groups

of (from 7 to 12) slave mechanisms with a regulating rod uniformly

positioned in the active zone and surrounded by two LEP detectors

and four IAR detectors. The average- correction signal of the LAR

detectors is used to control the rods. Triaxial chambers located in

the central hermetic sleeves of the HGA serve as the detectors of the

IAR-LEP system.

The Soviets claim "the 'LAR-P: system has exhibited high reliability and effectiveness, based on operating experience."

Second, nuclear monitoring of radial power distribution, which works on the

power level of 130 fuel assemblies uniformly distributed over the core, using

in-core detectors. ?be vertica 66"torifr system measures the neutron density

at seven points. along; the, e fch of 12 fuel assemblies..., '. detector

signals are passed to a vompnute, Ahe contrio com lex.

Third, a data-processing program whic :calculates the power of all fuel assemblies from the detector signals and fron calculated reactor physics data, the

safety margins to maximum allowable power for the particular flow through each

channel, the maximum permissible levels of the detector signals, the void fraction, the power generation of each channel, etc.

Fourth, a computer at a center outside the rescLor installation, which periodically carrie, out nuclear end optimization calculations.

(2) Increased Fuel Enrichment

Computational investigations have shown that when the inits- , riýT'•tsent of the fuel in U-235 is increased, not only do, thr vf.,-i, I

prrt*ies 0 the reactor improve, but its econoh-i( it•Iit-L I;

freuse due to an iucrease in the depletion depth and, a dr( r,

the spccific consumption of nuclear fuel. An inmprtant d,'.

3-20
of the: v~ar ia tioni o f the. time constatit of the ffirst azimuthal harmfonifcý Vf off the thleomton-11 diort hi, e ne rgyý diitribution(b )'n the tea reactivity toefi n h14.e sabihd' h smale thIau

of the posiftive Tsg*team reac ,tivity- coeffciet ttige thestb- ity of theý energy d1stribution: `nd the si ler te mnitorXng-of the

reactor. The most rational 'mthod f"r decreasing the steam coeffi- cient is an. i2raeo h".ai f'tecnetaino -235 nce

and the moderator nuclii.n the active zone. A-decrease in the ateam coefficient due, to a change toa "fuel :o~f-2. enihmn is esiae

to be approximately Iis 1.3 Pi ,here the effective fraction of delayed basis for neutronsý. the .adoption Thes*eý of [Sovieti- the coaclusions' have served as te solution of increasing the enrichment

of the RBMK-1000 fuel to 2 ...

(3), Results . .. ,.. . . .. , . . .. .

The Soviets believe :that'.-

The 8-year .operation-obfifsystemswhch provide for the control and .regulation. o eft merg-..:sj-y tibutonin RBK-l1000 has confirmed the correctness of the .engineering solutions which have been taken as the basis for their development. ', The combined and consistent function- ing of the three-syst ks--the*. monitoring and protection system [HPSJ, which operates off lateral ionization chambers; the system for physical control of -the: enekgy distribution (SPCED) with respect to radius and height of the active zone, which uses O-emission neutron detectors of the cable type; and the Skala system for centralized control (SCC) -has facilitated the reliable control and regulation of the•- energy distribution in ,allý. .operating modes of the reactor. The accumulated experience ýof :the assimilation and subsequent opera- tion of the. monitoring and control .systems has permitted developing and incorporatinges•ures di-rectedi..at a further increase in the -reliability of their toperation. Mong-these measures one can count the conversion Of thelogicpo.rtion .ofthe MiPS to more reliable inte- &rated circuits ihittd-appreciably developing its

.functional *osib/i *i ci .-by several.times -n-the -dimensions of' th elcocq t,. the replacement. of ..the- cable- link in the slave mechnims of t- •'Sby a belt link to increase . their operational-reserve,.wo and introduction of noncontac - thyristor circuits •fors•'otrongcontrol of the MPS servomechanisms. The service term of the -detectors for control of the energy distri- bution with respect to the radiusof 4 the active zone exceeds the operating time ofthe.BGA k in-which they are mounted. In order to increase the reliability.-of operation of the detectors, soldered connections have been replaced 4y welded ones. The detector assem- blies for control of the.energy distribution with respect to the. height of the active zone preserve their effectiveness for 4 years.

(4) Testing and Analisis

A great deal of'attention has been devoted to the perfection of ther- mal automation and emergency protection systems in the interests of increasing the reliability and safety of the operation of RBMK-O000 nuclear power plants. The equations of kinetics, hydrodynamics, and

3-21
heat 

systemis 

transfer,. and aIgorithms of the .. erationý of the equipmnt and.

plant are 

fo~rl 

used 

automatic, regulatibloTi thepramtr iif' of a" nlerpoe in 4a•tiod wh1ic•"h sbeen develo•0d for

Soviet the investigat references- irn discu f.ansition 

ams e-.wUoter Aind emergency conditions. Some programs- it•,wodmnsional channel-by-channel deling capabilities. Upon coarison-of te0 results of calculaations"with the data of the dami ,processes in -RPRK-;lOOO 

satisfactorily 

oprain 1 nts iW4 es'n. 64abligshed thtte model,

- gency 

describes thedynamics. of.the power unit. Some Amer- conditions, associated1 aainly with. the transition to natural

In 

circulation of the coolant hasve tit studied on, special test stands.

under 

order 

conditions 

to justify the rel1ibili'ty of he cooling of the active zone of natural: circulation, three series of experiments

Le6ningrad 

have been performed 

units and 

under 

th seodKus 

natural conditions on the first and third niti steady- state and rn sitional regimes;~

.3.2.6.2 -Problems With; Emergency Core Cooling. for' lecirculation.Pipe Breaks

As previously discusseitai•ecti"Ont 

largest re o the recirculationpi"pi 

accident for .. dimeter"). is considered -the maximum credible the RBMK-1000 reactor,. ,,The second generation of RBK-,1000 plants has been designed to handle this event. -The Soviets state they have done such

and 

testing 

bubbler 

and analysis to verify 'that their.improved emergency cooling injection pond pressure 'supression will perform as intended.

-Improvements 

improvements are 

in the ECCS :design;have been made since the initial design. These smmarized below.

( Th) e capacity of-,theECCS.inwjection studied. Apparently, 

and pressure suppression systems was

ponds 

.the limited steam 'condensing capacity of the bubbler

ing 

led to the addition of ::a-surface condenser using a freon-type coolpool 

system, directly undr the, core. This surfac:- condenser and adde!

pools. 

spray 

The 

systems augmted the., thermal capacity of the water in the abiltytotcoI water with a system of heat exchange-rs apparto,16 U -rtof the initial buI . oods -although ol .1ta'ýp - ha Wve increased.

(2) The 

locations response 

was 

of 

modeled 

the bber po.'.P to' sirnilated pipe break events in various Ua i-,s _ itest faci11-ity and reported in 1984 (Turetskiy). The 

results 

response 

were 

to recirculati06pipe breaks was considered adequate. Test

(USSR, 

also presented in the. Soviet report on the Chernobyl accident 1986).

(3) Soviet 

main steam 

documents indicate that-the ability -to discharge steam from the safety valves to the external bubbler vessels and sub-reactor

generation 

bubbler ponds was !art of,-the origJa•l design for first- and secondimprovements 

RBMK reactora, respectively. NBovever, recent reports indicate

safety valves. 

have been made in the -amber and modes of operation of these

bubbler 

Such improvements nay have created the need for increased pond thermal capacity.

:.i?.++

Also, 

simulate 

a 1984 

a stuck-open 

Soviet report (Smolin) discussed a test that was conducted to safety valve following a simulated loss of ac power.

3-22
water Thscm~xtest aimulatedI 9, loss of. rec rculAtion flowV loss: of feed flowa tural circulat-ion6 c-old 4no ,± te ctth tor. These tests

lead 

donstrated 

to boilint 

that 

in, 

'depressuri 

theation* c iuidby acua-jn I5et-vrv iU,

drops below loop uddisrupion of e en pressure

feedwater 

550-650 psi;.. --Auxiliary feedwater must be initiated or: man rest-4 e for a••dequte core cooling.

-(4) rOne. of the. most di-ff•iclttt-'. 

the power osciOllti 6661esas tie challedge of how to control"

into one-half 

that result from inadvertent actuation of the ECCS of the reactor. -.'A. i984 Soviet report--(Yemel'yanov) a -series of calculations detailed

right-half tiat studied' the power flash-ups and left-half/

by the positive 

power 

void 

Imbalances that follow inadvertent ECCS actuation caused coefficient. 

operating philosophy It is important to recognize that Soviet emphasizesi. maintaining power operation throughout these transients.. •.. ,--

Power excursionsl in exego~ -b~' rblwthe inta 01power,, condition 'ere crlculons W.ere ',' calculated for void coefficients typica-lof. initialioadiig. (new fuel) and fuel at _steady- state overload (Mxl -b" o)'ý oia iety .of1.co•tmbination ofautomtic

presented 

control rod re-cponoses vmed iiacerpts from that report are below:

There is finite-,probability 

cooling of false response of the emergency system: (SAR), with, imalfunctions with erroneous• 

in automatic devices or tctions of the•operator. Bere the most probable. ... case-is 

reactor. 

the feeding of water from the SAOR into one-half of the

in boiling 

False response oi the SAOR results in a sudden change conditions in the reactor,. which via the reactivity

The 

s..teamn 

nature 

effect can caus -a sudden disturbance of neutron power. andamountoof 

mined the.reactivity disturbance are deter- by the. sinajpndmAgnitude. of the steam reactivity coefficient, ***

The action ,of th eul n rtction systems(Ss)pasn

--

important 

the 

role 4 h P ss- Ahssituatio. Ractors of in aoisu ~ ~ t uoai owrrglto A_) sst perato, h& esignals from lateral ionization chaber (Ils) Th oratift of the AR is aimed at maintain- ing the resulta-nt .•siSnalý of f -- iietrically placed chambers

moving 

equal to a specific value and isi-mplemented by the synchronous

reach end 

of a set of -foutr rods. .en the rods of the working AR cutoff witches, h _:automatic changeover 'standby to the AR takes place. Disbelance, provided interlocking of the Ala is

drawal 

for the case_,of failure LI.,the AR (spontaneous withtoe synchrorizatioe 

of the get.of.four rods of the AR because of a failure in

balance systie; falns appearance. of negative dispurpose 

because 

of 

of a break in tihbcamber's circuit). For the suppressift power flash-ups caused by sudden changes in reactivity with the compensation of which the AR cannot cope, the A•fK is furnished with an emergency power protection system. Upon (AWM)

reactor 

a signal from the AZH, all rods present in the

the reactor. 

(exc-_pt the shortened absorber rods) are entered into The operation of the AZH ceases when a signal

I -
a Ve', indicating that, esergencyas er t.' 't 

exceeded.-. ,,,ier. .o :; ,e,

.In dditio toý Au,• t Raf rt""ct•.o•ri rei aeso furnished With a .s stern of lc90l auto:matic, (lAs ich krols simultaneoousl

control the-power *nd puprs the mot raid distorioni of. the

form of radiarl-.azimuthal' energy. release. 'A 12-channell LAR AYP.-

:-,,tern has been provideid eu.iliid inii de iign o f foiU' 

.phase 2 (lseonod-generation reactors.* T-his LAR. version wasas

modeled. Di sbalance in'teroc inges ben imiplemeted in eac

LAR channel.,in all LAR versions.: A, local e*mergency protection.

(IAZ) system has been ipl•m•ntedý iithin the fra'ework of the

LARs in addition to ian A •..

fl

False turning on the SAOR as' the result of ,a- change in "the ra te

.,of flow, And enthalpy, of: the bleat. tiiansfert me'dium init ially" re-L

suits .in-s~uddez drpiU Lne ancont in thecoe Ten ,.

followin a icrease in -thi o wpl ater 'rom the SO

vessels (for, 4 to- 5 .eonsmaximtm), "deexcitation" of this

ssem. begis th pyovter frn the SAOR is reduced, And.

in 32seoditicmptey tq e.

'Cases of the feedoingof iwter from." the SAOR into one-half of the

reactor were considered.

.-Maintenance. of power.is accomlished either by the AR system or

the LARLAZ system, and the emergency power protection system

takes part in suppressing power flash-ups.

Because of a.dela the irculati loop for the first .30 to

35 sec, the enthalpy in the inlet does not depend on the behavior

of the pressure and the 'opration of the regulator for the water

level in separatodrum.sA.-.

With a negative rctivi effectwh regard to, steam content,

,turning on40he8Bi acmane b theý additionopf po-si-,

-tive reatvt am Aiob" ls-s 1 4oe 

-on the other.i~ hadthe, ý5firbst reaction ýto turing4 0-f-the SAOR-

'is a power dptý. stm t powe:r regulation system tries to

compensate the kreactivity d iistrace• and to maintain the power

at a specif ic O-ooing the.flash-up, zlevel.. the drop in the

rate of flow from the SAR..requires from 'the power regulation

system a sharp response,.in th posite dixection.

Thus, with u-a 0 the leadi edge of the SAOR discharge pulse

is potentialty dangerous, and vwth aU 0 ý.'the discharge drop

following the first, fi.as.-upleads ta apower surge. It was

established in the process of .calculations that potentially

dangerous situations 'arise in the triggering of the SACR. This

situation is caused 'ly the response of the AR system, and

negative reactivity disturbances rvtich are asymmetric with

respect to the halves of the rear'.,r.

The Soviet !nalysis was conducted for the following conditions:

3-24
(1) AR system only, without tiking into account tUe operation of the AZH and disbalance ir-terlockin.

42). LA1R-LAZ systm "o'nly., Without. taking Into account the 'operation of the AZ?! and disbalance interlocking (3) AR system with the assistance of _tA.f.(scrahusystem),-bu ou

•--b 

disbalance 

: . ...... • : • :-• •" " • ' _ý '" _ý .... _ 9 sy t n b t~w ith ou t.." interlockin? . . ..... ... .. - - (4) XAR-LAZ system with the assistance of the AZH (acres system), but without disbalance interiocking

In the first two conditions, power flash-ups were as high as l40-150% power from an initial 100% power condition.. In .the. second tvo -conditions, the scram system helped suppress the triggeri.ng setpoint. 

the. flash-up,; 

b)ther 

butut 

oIds(AR 

o -yoperated until power was restored.below ow"LAR-IAZ) owre adjusted to maint"ain. full-power condit ons hile•W.a ,AZM.system was operating.

Other tests wete run which -included disbalance interlockirg. This Interlocking caused a full reactor ..•b t6-os•w4 :04 seconds because of the power $kbalaace. The interlock prevents, R =nd &A-MZ rods from being pulled out to compensate for the partial insertion of acrem rods.

The study concluded that the transients were acceptable. The option of main- taining power (primarily with the UAR-LAZ system) was judged the beat option for --maintaining full-power operation during inadvertent ECCS initiat.ions. 3.2.6.3 Materials Problems

Early problems were reported with the transition weld between zirconium pressure -tubes and the stainless steel inlet and outlet piping. Welding technology was improved, and those transition welds are noa designed to vithstand limited temperature transients (up to 150C per hour).

The reliability of, fuel-oselybly s-.satricton has been increased. Basedeon the results of the etartu* nsdJu 4FtL.qxperimental Lnvesti4tious, sad operating experience, 769m.' Ante ecestruction of the individual reactor subassemblies emd thee•tz-tof tbecirculation loop have been introduced. A 1981 Soviet report (81doreako I1981) discussed improvements in quality control:

At the precent time engineering staAards requirements worked out for all pieces of are being equipment which are import.At for safety. A component of this problem ts the development of scienti- fically substantiated intervals between Inspections for each class of equipment. Another prOblem is that of developing methods of conti- nuous or quasi-continuous monitoring of equipment neutron noise, stress waves, (acoustic and etc.). The development of such methods to the and introduction full extent will make it possible to go over to a qualitatively new level of monitoring during operation and may possibly lead to a review of equipment failure taken into account in atomic power plant projects today.

'I

-a>

-4

a

-A

3-25 *1
3;2. 6.4 Steam Separator'Probliems

A1984 Soviet, reprt o'oe'kV isusdtecage in; steam sepa raorP

diameter fromt 2.-3 . o26 .ft, to C~ i) rhicanepoddadti6nal operating margin b en igh levels (tat could result i4n moisture

carryover to the turbines) a'd.lo'wlevels l (tit could allow cavitation of re

circulation pumps on loss of f e64ater on pipe breaks). Increasing the steam

s,,eparator, diamete prmittedmnoe -t~im~ to rso4t-tasetcniin and

.proveinded a n in.eto during -yaious .ac.i.entsi traIsieats and ..

Also, the pipelines of -the ,steam-awter com-pications were being redesigned, the steam pipes in the space of the searator rooms were being. rearranged, asind

optimal shimming of h 1team-discharge fittings of the separators had been

introduced for equalization of the steam loads. and elimination oftmisalignment .

of the levels lengthwise.an& bet e:' - , -, ""Centi5e:ara-!rS. 

y, improents we me automati co o s ta .

steam separatorepressure and levela. -The tructureoft-he regulation systemn as

well as hardware was improved.

3.2.7 Soviet Use of o istic• A1naysi•s• • chniques

The following informatiofn is takes from. a 1983.Soviet report (Sidorenko):

In the early pe.-iod -f the. 'development of the Soviet atomic power

industry, the formulati on of safety; requirements was characterized

by purely intuitive and engineering approaches. At the present time

the quantitative-probabilistic approach is increasingly becoming the

basis. The studies being developed and expanded in the Soviet Union

on quantitative-nrobahililtic,,.analysi5 are directed primarily toward

these goals. The -elaborationr of' additional safety requirements

for atomic heating plwants has been based in great measure on the

quantitative-pr.babitliatic approach'.

For reliable app analysis of

safety in the Ig'a it it I eceai :i -,,to have the pertinent

statistical dti a"b i: .' 'ad icrfor

most natura phena ýýn_ if Ltisl , da t t re

bility of specific• q edin• _ e t6 ic industry are limited

at this time. This e , .isp ible i for the deterministic approach in the •,aisi/cigq tfItioU Peration stage. Certain

elements of the qusntitative#probabillstic approach, however, do

exist here and they *re laid outin the standards-technical

documents.

As a rule, the parameters of -the Attural phenomena taken into account

in the design are chosen on the basis of a quantitative-probabililstic

anklysis. For example, the design for the construction of an atomic

power plant makes provision for .n earthquake with an average

recurren':e period of up to 100 years, and the maximum design earthquake is assumed to bave parameters which, according to the calculations, have a probability of 10-4 yr- 1 . The choice of the design

values for the wind, snow, and other loads when takini, the meteorology into account is also based on statistical data. (Some reports

3-26
a 

tqua"ke•o 

.er 

Wft I 

',ott against natura phenomena such: as ea i A PR )I design than to the....e- design.)

'There 

probabilistic are direct 

apprach- indications for the use of the quantitative- during designing of pover' lant equipment and

analysis 

systems. Thuswt•e bGeeral Regulations" envisage a quantitative of the ve frthe 9. Nt -Melabl libil.tyo he..sy.xst ihich-. leadsto a-o search 9cm.-, -quanti tate analyioftepo- bility 

situations. 

ok damage to heequipment, and: realization of various failure considered 

been developed 

in the design -stage. Special procedures have

failures,i 

for these purposes. In addition to the postulated

ures of 

te at oic power plant design may not take account of fail- systems (elements) whose reliability to estimates. is fairly high according

,approved., 

As; statistical 

the, domain:of•applIcati8nof 

data are a&ccumu ate-d eMthe-pertivent methods are '-

t quantitative-probabilistic Sapproach 

of 

inIJI the j0rces "f PP designing and monitoring on the part the s•upeisory -organs will ,grow.

3.3 Indended ,aeyReview

* .--. ~ 1

I

-4"

'--'I

This 

RMI-1O00 

section 

reactor: 

presents 

plant. 

an independent safety review of various transients on the

tiators, utilizing a consistent 

_It reviews a broad range of credible accident iniaccident initiators 

forma'. The organization of the transients and considered 

Western approaches.

in this section is patterned generally after

Computer 

been performed 

models 

for 

of, 

the 

the R!MK-lOO0 have bee'n developed. Quantitative analysis has

.he use of analytic modeling sequences of .greatest relevance to the accident. Through

in& the specific techniques, valuable insight was developed concernsequence, and about 

behavior of RBMK-1000 reactors duriag a Chernobyl-type accident

yses was performed 

:the characteristics of RMH-type reactors. One set of analDOE and nC rese•archpr6otr 

.using.-comuter codes and models that have been developed under

analsispacage -for-analysis of LS. and fast -,reactors. Safety

.relponse were used, iaagrtia Ometronics with,*ki thraIyruic and 'st~ructural by..l14 %4"M frm'gNe ational -Laoratory, tional Laboratory, Oak, Rdge Nati -Brookhaven Nz - l Laboratory, and Pacific Northwest Labc-ra- tory. These .aalya.ss u eddn modeling the behavior of the accidpl. Th'

of 

results 

Energy's 

are 

TeamAnalyses 

or s ein -NE-0076, "Report. of tbe U-5: I..- f the Chernobyre -4 AFS A cidý-.. -set of Chernobyl analyses is in. progress at EPIL.

Most of the information contained in thi" secti(r:- comre ,ýports or can be i#ferr~ed from information providn.-r, by tPzý :an RNM transient that involves an 1n•r

power increase by virtue of the posit t-.v.. avoided when the outcome of a particular principles, or the conclusion is specul', presented because models, design detail]... insufficient to permit quantitative a, criteria also are qualitative: Sequ+rf- lcant fuel damage, and sequences with tH

cause significant damage to the partial systems are considered tnacceptable.

I , ý, -

3-27
Most sequences are considered from an initial normal full-power (100%) condition. In addition, any sequence for which the outcome might be significantly

different or potentially more severe in- , low-power condition is considered at

both full power (100%) and low power (typically 1.0-20%). The. degree of:average

fuel burnup or "core life" assumed for most of these analyses is equivalent to

that for Chernobyl.Unit 4 at the time of the accident (about two effective full-

. ~pwer years or about 10.. GwD,/burnip. . ....

Transient and accident sequences are ý,categorized by their initiating cause via

the. process variable whose change may-.have a deleterious effect on the nuclear

fuel. Each postulated initiating incident is assigned to one of the following.

.seven categories:

(1) Transients involv increasesnL'heatrey. y of over-

!_r cooling events includes, amongothet ingStransientsý. Increased: steaim flow causes" 'icreaed voiding'ýin the czhanne-ls ............. thus increa "and es react vi ypoihich could threaten .. fuel

cladding from overhe.aing. '.

(2) Transients involving decreases in heat removal. This cat.2ýgory of undercooling events is primarily composed of loss-of-b-,t-sink events. These

events lead to increased temperatures and pressures, and typically reduce

channel voids, thus adding negative reactivity. However, these events

present a threat to fuel integ-ity if heat removal cannot be restored.

(3) Transients involving increases in reactor flow rate. This category includes transients involving increases in recirculation flow rate, including

credible reactor inlet temperature changes (and resulting reactivity

changes) as a result of the increased recirculation flow. These events

typically involve an initial improvement in power-to-flow ratio, and thus

less voiding.

(4) Transients invol--ingdecreases in reactor flow rate. This category includes transients involving decreases in recirculation flow rate, primarily . due to losses of main circulating pumps (NCPs). These events typically involve an initial degradation in power-to-flow ratio and thus fuel element

heatup and increased voiding.

(5) Transients involving reactivity aud power distribution anomalies. This

category of events includes a variety cf control rod withdrawal events,

control failures, reactivity imbalances, etc. This category is primarily

a result of errors in th!, positioning of control rods or fuel, and

includer, errors in o:n- lint refueling.

(6) Transients involving, increases in corant inventory. This category includes rvent!' th.' ir.ight increase total coolant ivventcory to the point

that tC vc stV.cSa•, separator water levels occurred,. which could threaten

the- tut;(--, genecrators with turbine blade damage from water entrainment in

th,' stcal s:. Since the RBK is a boiling water reactor, these events

will gejoiralvy riot reuult in increased system pressure.

(7) _ .i ' ___iLdecreases in RCS inventory. This category includes f

1.' events that decrease coolant inventoryU.e., loss of steam Peparator

Irv•-!), other than excessive steam demand zvents. This category primarily

3 -28
consists of e rane• fredible los-of-coola small t

large breaks.

he .asienats cover ie• ti section 'are s" 1e l rupedic•

with- the bove ca~ttipn sche :

(I) Increases in. Reat.Rem oval

e

* I~i stan in rek (frow, ful -p:over and~o pow-er)*k

* Stuck-open. safety reie vae (-for multiple ..uck open SRVs**) Ex.essive steam',demnds fi. full and low. power (steam, pressure regulmtor. failedopen .or rapid_ tuine generator loading; turbine

bypass f-aie op-en.**

* Loss of feedae.r heaters or other reductions in ftefta'. te.- nrature

S I.. adv.ertent initiation .f decay heat removal,

* Single tubinie psenrator trip; partial. load, rejections'

* Simultaneous trip of both turbine- generators S Turbe g ra trip(s) . out bypass

* Loss of feedatier

I oss of ffs'ite paver

6 tationf blackot (ltoss -of all ofisite, and onsite ac power)**

* oss of deca he MatMoval

(3) Increases in Reactor' lowrate

Startup of an idle main circulating pump (MCP)

MCP startup. with -idle coolant:-puiop ý branches. ast abnormal- -temperature

(4a) Decreases in Retct-or Florate

-single nC, trip fauflpwe - Loss of all force VP c) flowfrm W-lll power, low power

* MC throttleý valv flowcontrol failare,(failed shut)

* ls hannel blockage, inlet -(flow isolation

valve,:shutoff)

(5) Reactivity and Power• uP iazIbbti Asolies

* Continuous rod.withdra lident Z•-Gsingle rod (full power and low

power)

*These transients are saot discussed" n the Soviet literature and are judged to

be beyond the design capabilities of the plant or beyond the design capabilities of the plant or beyond the Aeility of operators to control.'

**These transients appear to present a difficult challenge to the plant. Proo0t

operator action (within a few minutes) Is necessary.

3-29
' p.'wor)*: Continuous ir6'withdrawal aiccdent - rod' banks (full power and, low

".ispcellaneous c~ntrol rod withdrawal kerrors: and rnis.operaotion .

* Refueling err nldigipoe fue plcmn eg, roperenrichment)**

Rod drop out bottom of reactor (short absorber rods only)* or.

Loss of inventory in-control rod cooling system

(6) Increases in RCS t. . ..

* Inadvertent ]ccs •ctuation*•

* Excessive fedatr flow

(7) Decreases in RCS Inventory

* are-rekLOUA ofecirculation pipe MHCP outlet)

* laarge'-b reak LOCA of group distribution hiea der

7' Large-break JOCA of *team separator downcome-r, or IICP auction header

" Main feedvater, pipe break**

" Small break-in,.channel inlet line(*ý or • for cercain break

size) -

* Small break inn cha 1'eoutlet line o-r refueling connection* or

* Pressure tube ruptu L.msinsidereactor vault (graphite region)* or

3.3.1 Other RbM-100 ,SafetyReview

This particular approach to..afetyýreview is not the only acceptable approach.

Other organizations in the United States and overseas have used different forsmts for RBMK-1000 safety .review. The U.S. Department of Energy's team analysis

of the accident-sequenceprkovidedithat team with the opportunity to develop an understanding of RK safety-characteristics. "The U..S. nuclear industry has

developed a position paper:o-, th.be l accident that summarizes some of the more important desin hratristicsofthe RM reactor. The IAEA and

INA also have conducted' safety •rev• e of the -accident and reported their

results.

3.3.2 lRecurreing 9I-amt. W1p!j7 SaittyMa s

A number of reactor coreh•eamL potenti. -re modes, and other elements

of RBMK-1000 safety.;,ana.lyst .. "h individual transient analyses. These elements a.etsmarised in.this sei.tion to Avoid repetition in individual transient analysisirsetion.

3.3.2.1 Graphite

In the RBM-1000 despign, sthegaphite-moderator heat is removed by conduction

heat transfer to the fuel-channls during normal operation. The heat generated

*These transients are not discussed in-the Soviet literature and are judged to

be beyond the design capabilities of the plant or beyond the design capabilities of the plant or beyond the ability of operators to control.

**These transients appear to present a difficult challenge to the plant. Prompt

operator action (within a few minutes) is necessary.

3-30
in. the graphite during nomal .full-powejr operationw due -to e~utron- joeation Lis _g . p _ _P• • - Mg.::.../ .• .....

approximately 5.-o a oepoweo 7 Iit h graphit nomlY OPe

750aes 0C. a t aý tempertr i hra e f 600-T650 C, with, a% maximum ie, rtueo Thus, folldvin a .ccident . --inddiio d heat, there. is sig . .. ....... . t O: t ye, a

nificant quantity of ?to eat in the graphite. which Must be removed. Even . hen. anevent commencesf: low power, r tored heat in the graphiteis still'

high, despite the iowr neutron flux z :ithe graphite. At low power, primary flow- is reducedp to *a intain•poper hteme al. from.graphite . .• .

tc coolant in the pressur. ti" is re., ucO d accordingly. Also, at 160 -power the helium-nitrogen cover is'can, to a nitrogen-only cov-r gasi vith much

poorer heat transfer properties.

The graphite moder tar k=acio- and•. +. L.•.•. s the ote rgraphite-ta reacto nd

graphite-air (oxidation) reactions. 'The, graphite-steam reaction is a, high- -.,- t em rature -endothermic reaction tbat. producees'- hydrogeniand carbon.mnoxide

("cal as"or watr gu")..the high-tetmperatr hgr*pte-airx oxiaktion

r Ie Iaction is ih guy &tijemic, but requires'er hijli rtmmpertas Aii large SAWon a,.of xyge ogtu~td Provisions irpade i h 11Kdsg for.

exc usion of air from•` thsraphite -mans- of thehelium--nitrogen cover gas, in the reactor vault re•. hsecov as- is monitored- for water and steam

during normaoeain~misuei h oe gas* during Pormal operation can14ut - i"-n- d lti6n of the. graphite beyond the design basis, thus icreasigtet-h. mal :conductivity:jgp resistance for heat

transfer from the Sraphite .to t•e• fuel cthanl coolant. No-provisions are

evident in '-he design ofiithe -l00reactor -6K. to mitigate the potential con- ...

sequences of flammable gas.prdution asa ,. result of ani accident involving graphite reactions.

The dimensional stability of the graphite in the presence of high neutron A

f1uences is an important :desi -isue.A5 ritishreview of-theR---- reactor .. .: ........ (NNC,,--1976) reported.tht:daim i - on the o rdrof .2. m y occur

"during reactor operation th-highý- lit ýXgraphite. Lower quality graphite, - .11.0

..... oprati.n y ~. phit &:~eaue'dplto•o 

would-be expected to e .eri e ate im allr dChan es. Tbe- imensional

:changes are important. to sfty. ' bcause teability to transfer the heat

geeroaed indth graphite-tot~~Ie channel coolant met not be degraded by

genrae d inose-papckedI •sm:•• :W Itbe P- id epe r-etnis bliy :!•

alig nthe problems. Vault~mm•?mmges i he ss-nouus of the grap oie .presult in

r creased ps a thr graptte preaeol a barrier -toe cto can- . ductive hbeat -itirians Thtsbd .ritrwouldtty. Therefore, infcreass tahit

rtelerasure which may led ..-oreato r t e 8raf dhepleti of thdgraerphteduri, .Ang normal operation; thisinudre lt is gwýreater conductive-beat transfer

resistances. .eehgr 8 r atue s c ontribute torete rates of

hydrogen and carbon momoxid ction folicing an accident which admitsyi steam to the graphite to 'the r'"eat-or vault.

The close-packed desil oft6.*e grapite pieand the pressure-retaining'ability oftereco yutcretet-. probim in heevnt of one or more pressure tube

ruptures. The reactor vault ;has very -little free volume, a small-capacity gas

treatment syflem and limited relief" capacity. Therefore, in accidents which

release large voiuemseof gasvor stem into the graphite region under pressure,

there is a potential for -graphite bl1ocks to be damaged, for the. graphite pile ~ to be "blown &part," and -for the pressure boundary to fail.

3-31
athg .aetemp~eratures, uw.dergo an~ exo....hermic reaction, and produce° hydrogen : and metal' oxides:. Thi .ecto..raeis.nepoenil.untono temera- :•I

t ure,, and begins- at.... :approximtel-y 1,150.0C. ....Following a• losf-or-coln titedthat zi-rcalloy-wa-ter reactions would originate3 in the :r"l.Rito

zIrcae. lloy 00 rodý ei claddingg fuel a h becuse. etpo the. henncladdingbyte deca

pressure boundarynculdn. oue 86, V . Zircalloy-water reactions in th fuel channel- ten a of the heatup of the fuel channel

material r by radiatio t eat transerg from the fuel rods (5 . away), or by ydIhav- ing fuel rods lu Th agacint te pressure tube. The zxrctalloy-water .reaction a ... would be limitedn by te availabimlityof both water and a zirconium. l eo The cl water inventory in the core can be depleted before all the zirconium wThi reacts. i the s 

z1=rca-.onyu• fn 2bel-ýrlq iýcls'ddkng of~i tcause hatuloss-of-cthe, colading jW`tevent, tie ay .. preatiore thle imit pein tokta hrogepod`ct eaion by eda tring the rsetieon-ofl.a.r

mayehave bocurredtion th aceidennt.el the'herniup th foe 

mtemprialur bo. 600ia6 0°*fi•. &dtransferditin fulrd from the e . n eawa rOtby fuela :!

zirconiue rcsosnium-iobeis Te- tubbe tepethe e. The pas eofser-t:on

Tnheapessue throuhe cabepretsedr beultoareal thsre prconsrcted ofd Thveis

zircJ it iu. an I hydre prentube v oaine ss-of-core, c on ent te

pressure o tb ,tre uld b Teginto inrease t dieol from the stam

water saturation t at , Oa imately 285iC to the graphite operating temperature of 600-6,50PC. --,,'--Addit'ionAl rait ion heat~ from the nearby fuel

rods as they also begin abetatuprtransient would raise the temperaturelof the zirconium pressure tubes to even higher temperatures. They passage. of super- -heated steam through the pressure tube durinag this time period would have'a

slight cooling effect-on the pressure tube inner surface.

.Zirconeim-niobium .... haeltl Ai stre•nth above .750C. the -i. event of a . los s of cool ing, -it aepcted týhat the Pressure tubes would fail. within the

grphite vault ara .becausea. f t-h c. intion of normal pressures (70 bars) and T Ion ead of ,coolag-toecoseveral pressure tubes would

result in the rapid over pressurization.oif t he reactor vault shice the vault

rupture discs are, desv .2,$to cemoda.e the ru of Only d one .pressure tube

in the reactori It aessur1*ittob *Ad eupture of the lt dueel sm

addition woul resttod if lm f "oI atto the rphait of t!ebeled yp;s -are sufficiently high, -*aga~it neoh i oxidatio reaction' cvuld

result.

-Thus loss of cooin tosvrlflcrn~coud result in asevere accident

condition if core cooling-is not..re-established within-a very short time period. As mentioned in Section 3.2.6.3,vaWr~ious ;Soviet reports have discuss"d the dif- ficulty in creating. a. strong transition weld between stainless steel and sirc- alloy. A transition diffusion. weild- is ousiedat the top and bottom of the fueled, portion of each pressure tube inside the reactor vault. The weld is designed to handle temperature changes up to 150C per hour. Many metallurgical experts consider this weld to be the weakest point in the RBM primary system. The

Soviets discutsed problems with this weld in the 1970s, but later reported the

problem solved.

3-32
.3:.31 Transients. Involving, Increase's in- Beat Removal

This category of, overio. •eventsiacludes, among other things,- increased steam

flow transients. incressedtaie fl-w causes increased voiding in the caannels

and thus increases rkeactivity and powr, which could threaten fuel -ladding because of overheating.

3.3.3.1 4-qilain Steamline Bretak

This accident is not discussed in &uy Soviet literature. The rupture of a main

steamline in an RBR-1000 reactor ý*ould be an unisolable loss-of-coolant accident that bypasses the--suppession'pool. The rapid, steam, demand would result

in voiing in one-half of the core. This Voiding would cause a, power surge and

fuel heatup transient in the affected half of the reactor, driven by. the posi- ,tive .v6id: -coef-ficient-,. _-.Theý pressure drop would cause bulk bi-ling i thee 

4prlaary neesi --,op' atn ther lsoforeciuat"o.Žh rsuedo

probleu~se T Uresi 1984)

Since the steam separators are.t. one of the accident localization system (ALS) strong boxes, teew ould5b -AL& pressurization signal to actuate the ICC systems (the RCC logic uat see simultaneous loss of steam separator level

and ALS pressurizationin -orider, t ntheCC' injection valves). The failure

to inject colder ICi waterat this point would allow the channel voiding to continue and the neutroui power-.to tinue ýýto rise from positive void reactivity. Without protective action, this uncontrolled reactivity excursion would continue

until very high fuel teerstures added sufficient negative reactivity from the

fuel reactivity coefficient to stop the power rise. Extensive COF violations

would be likely. It is pOssible that manual or automatic start of emergency .. eedwater would occur,-b-t; thisprovids 1 02 of full feed flow,ý not enough to

control the power excursion or hbandle :the invcntory makeup demands of a main

steamline break..

The steam released frm a.min sramine break in either steam separator room woud fll hespace ao e uperbilogical. shield, and below, the .shield

-blocks coveringth pressm U" exevli ..connections. Steamwudesaet

'the refuelin fJ*r t-I t~ be 14,~e blocks. Thllk stm woul

impinge oIa control 4`04 drivm a , d damage- thecontrol rddrive

motrs uffcietlyto zevmtopeatin. Ifstem bo towni rapid, some

shield blocks could be;A-a e d ff thef4loor.

Since emergency shutdown ae IViats established on the basis- of recirculation pipe breaks instea,,dof.ýt*mlie .pipe breaks, it is not clear which setpoint will initiate -areactorshutdown or power reduction, or how long it will take. Since the loss of coolant.,lain the form of steam instead of water, the

rate of water level drop: in the afected steam separator(s) would be slower

than in the case of a rcirculation pipe break. Without detailed information

on scram setpoints, it is Uifficult to predict the mesu, of scram. It appears that if the event starts &rM high power, the scram would occur on high neetron

flux. However, if the event started at low power, the low steam separator water

level setpoint might be reached before the high neutron flux setpoint.

A main steamline break from low power appears to be worse than an equivalent, accident at high power for four reasons. First, the magnitude of the positive

3-33

I
void coeff icient ist much larger at low power,, wbhic leads to- a, more: severe.

period reactivity, of time excursion.", biefoe aii Second, toai sic low. power, the transiien t proceeds for alne ic protective actin od begsin to; reverse: ,the

off positive selected reactivity- lCPs and excursi trottling on..1id eicltonfo .rdcdb trin HCP flow to maintain the desired channel exit

stear. quality. The REdK &sign- does- at appear to be equipped with the caps- bility to increase recirc'tlstion flow quickly in response to rspid steam demand

or power increases. Steamline IrTeaks at ow power .ould create#a•e-sere- •perI-P to-flow mismatch hat would permit 'xtet -e fuel overheating before autobtic protective action. At high powe, aUtot~c scrauwould be iitated' before

severe powerto-flow mismatch would*occur. Fourth, nuclear instrumentation has

limitations in accuracy and response time- t low power, again.permitting the

event to progress rapidly bfore itru tion detectsa prroblem.

A review of -the, Soviet •literature indicates that the RBHMK-1OOO. plant is -not

iqu6ped wit main steam isolation valves or min steam check valves. This

meas tat tealin -~min brak ~ unislabe;and' tht ~Ause' of -cross,-.

connectionx,- eWee 11steinm sepeartors and apparent stemink 'cross ncoaiiettions. "to :bt turbine gseneratorsa mingle main temmainer break wvil probably lead to

the blowdown of all four steam ieparators.. .I•;4t is possible that: some steamline

break locations (e.g;., • in the. trbinie baill), are. isolable,. or at. least can be partially isolated so that alol four stems rators do not blow down. However,

blowdown of only the steam sgparators-on oie side of the reactor, with no automatic protective action, wldo lead :t6o svre power excursions and left.-half/

right-half power oscillations. 41he vawilble Soviet reports provide very limited information on the -R3. maintii system design, valve .placement, and

normal lineups.

Indications available in the control room include a loss of steam separator level, an initial decrease in steam...pressure aud outlet feeder tube t.mperature, and an increase in Steam quality sd" neutron "flux. Immediate- actions would be

required by the operator, w.,ho must mually scram the reactor if ;an automatic

scram does not occur, ad ini-tite ,11W and IC injection. This -ccident could

result in a rapid power encursioa rapid fuel beatup before a scram would

occur.

,Since the break flow an.1 f~~l b ietdt he steim." sprtor Ise the turbine ball, o, o"em f.e., .. . of wh ich are crectad to t he ..,

suppression pool, .long-term;1.crcýdationcooling of the -core wi spprsion-, pool water would not -be po asibleo. •is rs .t be ansafetyissue not

addressed by the desip of the plat.

3.3.3.2 Stuck-Open Sfety.Rellef• Valve

A stuck-open safety relief valwe (SlY. is4an.unisolable steam discharge similar

in sow. respects to a small .steamlie break. -Tbe plant response would be similar to that discussed above for a stemlias break, with a few important eoxcep- tions. First, severity would probably be less because the size of the safety valve opening is less than the size of a min steamline break. Second, SRVs

discharge to the suppression pool, so the questions of pressure suppression,

offstte release, and availability of water for lont-term cooling are not'issues.

Third, most stuck-opin SRV sequences would be initiated by other events (e.g., load rejection) that cause a reactor scram. Thus, the transient resulting from

3-34
i tuckv-open; SRV its -"u"dd by a, masin steamlinie b reask Lransleint, and Prompt

protective action: is, nmo likely

.....-9 e as ad , . ety!-va iti,-.n • ,e ,•-;-cb ..... t-, w••.. :,..

c en:. st f vl•ve s that- ld-, occur after tur iine

trip or load rejection e -vents fr..: high: power. Thes.e events would reqtir`e steam pressure relief via iurbbine b'ypssor SRVa. This SRV filure.is anlyed

and presen ted in Soviet literit ure (Turetskiy, 1984) .and is seen to be a potential. problem are., ac i to,-he Soviets.

A stuck-open relief valve. w4ould result in loss of inventory, increased channel

voiding and reactivity, ad eventuail depressurization of the main circulating

system. The event •y•ayot resu•ltAn .the Actuation of the kCCS (tdiscarge rAt•e

may be insufficient to actuate both low. stem separator level and containment

pressurization setpoints simultaneously). The likely initiating event.- (load, rejeceo tripping of he' feed er pumps -and' the: ese c r ... ....... (MFV~sS, an..h ectr uigthe earily phase of,#f3-nnuteiera

upl vmam'i su l power tot. eegec eda~e~np, a core i*

cooingis si~tedby ~,coastdoWuo the E.nd nýabtural 6r iation. h :efectiveness of c•olinb• - atura•l• c• ation is a'ffected by rate of

.depressurization ftecruaiglo.TeSveshv investigated ti

on a test loop an foui.the • , .:• -.. fo .,Z- .tlie "rates, , , So greater ie s hav 'than 0'.-2 Namin, effec- . -

tive cooling cannot'b suedvt~u retrn feedwater flow.

'This event-requires ra idop rator. 'sponIe -- but should be a benign -event for

:the case of one stuck-en tre-li.ifvalve ,if feedwsater is not lost; more than one stuck-open SRV could :result inndequate core cooling (see Turetskiy, 1984).

3.3.3.3 Excessive Steam Demands From Full Power and Low Power

.is :event is very s imilar toth.aist ineý break -accident .and .-could be. .initiated if thelsteam pressure regulator .fal le open, if a turbine gSeneraqor .loaded rapidly, or if...the-. turb~ine bypass -.valve failed open. --Rapid. depressuri-_

-zation and channel oidin 4iwold reut1, c,.zausing a, power excursion due to the ...'positive void coefficient. sponse as severe as the main stesmline

break would occur if the, t blowdown is large .enough. -Nowever,

som o thtblodu pa-.~b slte,1o thi ramsient-might be . t of etrlier ttt tibis

eve rai•ses.the..m C.eemms fo -Ti --failu to6achieve timel y.utomat ic

:protective action that wore disusdm" forthbe iuin steamline -break. -xtensive

1CII violations would-be likely becaueofprttive action delays, power-toflow mismatch, and voiding.. Te-,potential :exists for a sisnificant power

transient with so ±ineiate • scrm...escalating into a-,serious transient that .could result in significant fuel-overheating, : Prompt operator action is essential in terminating ths.evet,bystOppig the .,steam blowdown if possible and

b* initiating a scram, 3FW,•a• C:injection.". It is possible that even with

prompt operator action, core dame could occur in this sequence.

An In :he case of the main stemlinu break, -this event would have more severe

consequences if lattiateO from conditions of low power or low power-to-f.cw

ratio.

3-35
f ~

33 3.4, Loss of Feedwater Heatez.. or Other Reductions in Feedwater T erature

'This overcooling transient isI much less, severe than,. those, discussed"' Aboe6

Reduced subcooli g MCofP in"'letsay necessitate a power reduction, but fuel,

damage from this sequence is ve;ry unlikely.

3.3.3.5 Inadvertent Initiation of Decay Heat Removal (DfR)

This overcooling event ais one that might occur if the RBk-10O0 design is Vulnerable to the inadvertent initi4tion of the shutdown decay heat removal system

while the plant is operating atpiwer. 'L Not _enoush .is known about DHR system,

design and opera•tions to addressthisip•oential vulnerability.

3.3.4 - Transients Involving Decreases in Heat Removal

This categ ory, 6fdercooline.. .eveits cos• •i I.- of loss.-o -eat- .pr 

sink eventso. Ths ovnt 'lad ihcreased ~'emperatUies and pressures, andi

ý;t ypi ca~ll-y: will re'duce, channel voids, tsad ignegative reactivity. Rov~e.'*:,

these events pxresent a threatt1 r.:fuelJterity if heat removal cannot be

restored. . -

3.3.4.1 Single -Turbin Generator Trip,. -Partial Load Rejections

The trip of one -turbinegene•rator, result,-in, the following automatic actions by

the plant:

• decrease in plant paoer level to 50U at a rate of 47 per second

• opening of steam du4p or.turbine -bypass valves to relieve excess' team due

to. slow rod.>. in&etions dec7 heat '

continued feedvater : •o a , min_ýiirculating pump flow to reactor

Since onsite l .c normally .adsare powered by turbine generator output, it is, likely that a loss . ae w "turbi•e :seitr,' ould cause a loss..of ýabout: one-half

of all onsiteo - : .ý_ *urbine, ,t-rip msy ..not case the loss -

ý,of Isof iinrebtM mo'ip)2or one~ hf-s (los zo 5%

or more of feedviter t csji*s ;lan) mre. cy" -shu . down. This indicates-.that

.some MCPs and IFW/ s t'ýOw: WW :• CP per loop)l ighta•tn poweredeundfrom off site. "per loop) i

A normal trip of one turbine generator would. require the r_-gulation of the

remaining feedwater. flwa..:d mainirculation pump flow in order to reestablish

the proper power-to-flove-rsatilo,- -an•dthus maintain proper steam quality in the

-reactor outlet Vpiping a-id... ute steam flow to the other turbine.. Since the

four steam separators -appear.:to ,tý.dboth turbines, the -irculotiiu flow to the

reactor must be decreased quickly to a level supporting a 507. power level. It

is not apparent from the Soviet literature bow this is accomplished. One ICP

supports 407 power, and two NCPs support .0% power.: Therefore, if the turbine

trip caua..d the loss of one WCP in each loop, flow would decrease to- roughly

80%. The remaining MCP flow from two MCPs in each loop would have to be throttled rapidly to 507 flow to match the new 50% power level. Without this decrease in recirculation flow rate, voids would collapse, and steam quality

would fall, resultint in a decrease in steam production. This condition would

3-36
adld 60egativ A iv:ra,, rectivity and further dieras voids. Thus te tripp ing of one

turbine could reslt. in:' a. tripf. - he secontd, ecause .:of low sge- flow.

This event is- a design-basis transient for the RIK. reactor, and;• should not b:e•

a: safety problem if systes respond as designed.

~'3;3.4. 11 Siitaueii~Ti of 6h Trie 10knefrtors, ulLa

Load rejection is defind as the dropping of the plant external load b*- the unit

turbine generators.

Tripping both turbine generators results in ihe- folIowing automatic ations by

the plant: -

reactor -trip Punr most', conditions)

Z start6feegwy diesel geear pra le)

As discussed abovei.n iSections 3..3..,onsite ac loads are .normally powered by

turbine generator- • -. herefore,-a-trip of both turbine generators -would ,

cause a tempora~ry lo~ss•-of ll.onsite4--ac -loads, .and should initiate the start of

all emergency diese geniIerators, so that emergency ac loads, such as emergency

feedwater pumps, can-6 e operated(after 3 .minutes).. It is likely that a trip

of both turbine generators wud.cause a loss of all HCPs and a loss, of all

feedwater.

The AZ-3 automatic powerred`ution setpoint discussed in Section 2.6.8 (reduction to 20% cl rated power instead of emergency shutdown - "scram") ozcurs when

two turbine generators drop off -the grid and internal power is left on, i.e.,

load rejection. " '.-

The loss of both tnriline generators will result in a large reduction-.in_-stea-m

flow, a steam pres-sur*eicrease, -and, steam void collapse. Steam dumps,.turbine I

bypass valves, and safety relief valves (SRVs) will open to relieve pressure. •

TUrbine bypaLs my oftd beavailable in thas event, since it is likely that .the

min! -condens er oli b lost'tiol6oiU*g os falosiepwr TeEwold relHive tOte pess-l t .wr ihrate, 'Ti~eaiebby ~ i

.,-,wu rbIe b-p m I 

definestheali p t though not :the-Ut ,ator-my 

have a large turbineyiiipss capacity, theSvet .. .report on the Chernobyl.aci-

.dent (USSR, 1986) -states the R -1000.reactor is provided with adequate SRV

capacity to handle af8ull -load rejection. .It is also likely that this event

was considered in the desigz- of. suppression -pool: cooling systems.

If all MCPs were lost ,-aturalicirculation •would -be established during the NCP

coastdown period -ino abt-30, ecoi ds. -Sufficient cooling of the core can be

accomplished by natural circulation and inventory addition by emergency feedwate'r actuation.

System pressure would remain at or slightly above normal operating conditions

throughout the initial portion of the transient (until emergency feedwater flow

is established) but stema separator levels would fall durinR this time perod

as inventory loss through the steam dump and MeVs would not be replaced. The

level in the steam separators probably would not be depleted in the first

3 uinutes .before emersency fedwstater injection.

3-37
This. event is a t.din-basis transient.fort.he Rht reactor,.i and

a safety problem -4i0 • syst~ .Rms n a• s n. .......

shoqid not be

331.4.3 Turbine G6enerator Trip(s)V Without Bypass

This event is similar, toth esingle or simiultaneous turbine trips disused

ct re f valve capacity" petarbe to bes lges

enough to handle a full load rejecion if- turbine bypass" fails., -to operate.

•3.3.4 4 Loss of .Fedwater

The loss-of-feedwater -event is. a design-basis transient in which either a

.partial loss of normal feedwater -or i complete loss of.normal feedwaterispostulat Forthe tcase -of-. a,: loss of one .out our, mai PUM-.m. .of 

(?fFWsy cu oan•fisi c:ingFoo•. m....e.ists iz. . oviet.lite.ature o teexis tece........im

,of auU4tomtc.-prot-ct-io~n. Same Sovit, retferncs -- gget that follown the

ilos~s of o+e - P, the decreatsed fo- othe., a stam separators7is sensed, the

turbine power outp"n reactor poe, is automatically decreased to-either 502

or 02ofnoma fllpowr.Ifths utmaic power reduction is not provided,

then the loss ofta sing1e WWA.,(if ý amoticed) probably would lead to an automatic shutdown (scram) on low iiter level in one or more Steam separators.

For the case of a lossof SOf -A••eof.main orý mor feedwater flaw,-the main circul-ating pumps (MCPs) areutomatical:ly. ipped in the affected loop. This trip is

required in order to avoid cavitation of the kCP auction due to loss of subcooled feedwater. Pump cavitation could damage the MCPs, and mil.t interfere

with the establishment of natural circulation for long-term cooling .of the

reactor, if ,.1CP discharge loop., seals fill with steam or gases out-of solution.

The loss of 501 .f1dvtr0 f•ow.alsoresuits in a reactor -scram signal and the

initiation of the .. mrgencyfeedvater system to provide feedwater flow to the

affected steam separators. Long-tem-cooling of the reactor is via natural circulation. Natura~i~l'cicutiosJ•s-ais ded by the bypass line around the main

circulating pumps.

'For the case-o -f6j--m,& m ie.ls fedvate f _ow terautma plant

response is etdl

.initiation .of cLi • - I• I:.-I U• t . II -dte-startup .the 'rgec y .

.feedwater pip - is pedozued iittilly. llowing the scram, sPteam prersure will be relieved"is safety valvsto t , suppression pool. feedwater

must be supplied to provide• a a•te cre Cooliag. If a safety valve sticks

open, the depressurization can interrupt natural circulation cooling of the

core.. 3

This event is listed .inany: ; :Veporkt.eas ua," a design-basis transient.. The

event appears to be capable of :beitg aMigated by the autotatic actuation of

emergency systems, but would require immdiate operator action, if a safety

valve stuck open or auxiliary feedwat.r did not start.

3.3.4.5 Loss of Offsllt 'Iftwer

This event Is very similar it' a aimualtana'ous turbine trin or full-load-retection

event. Onaite ac power loads are normally powered from the turbine generator

output, instead of from a traraformer powered from the grid. T-e loss of offsite power would not normally cause the loss of onsite loads directly, but

3-38
could initiate a loadýrejectitOfb ich. could in turn drop all onsite loads as ariesult, o the' t le t~rips

If turbine 16ertr an hiandle a loat' rejection transient without dropping

onsite ac loads, then reactor power is reduced automatically to 20. If onsite

p rwe is lost (turbine,-,:generators trip),, the rea ctor would, scram ,automati~call~y...,.

Al. ai cs..ain p~umps, all eedw4.ter pumps, and condenser Vacuum woul be

lost. The emergency d•ieselgnerators will start and supply power to the

emergency feeiater -p.ps within about 3 minutes. Natural circulation will be

established, following thýe P goastdown period of about 30 seconds. System

pressure willf 'increase-because of the decreased heat removal, causing turbine

bypass actuation. Safety relief-.valves (SRVs) will actuate if initial power

level .exceedsbypass capacity. SRVs relieve steam, to the.. suppression pool,

wheet ile-onnsdadrmi available for continued cooing. CogOer 

As- pre uou .1 -,discussed inScios33 4 an '313.4.2. it spible that

some .nsite..t c•L•ads8 may be powered from off site via an auxiliary transformer.

If s, at 'the tkel lods would bon Cfrmech loop, -and at least one

MW? hissiuaian im -r ight define "loss of offsite pOwer"- (10SF) differently, oiderin LOP, to be only the loss of offsite power.to Onsite loads

(via an xIliary -transformer). With that definition, a LOSP would not cause a T

load rejection •or. .turbe trip, but instead would cause the loss of the suggested

one NP..per. lp, a .0ne ý two tor -.HFWPs. In this case, the plant response would

be similar tos inetuArbine trip, with a power reduction to 60% or as low as

507 power.

The event .appears to-be addressed by the automatic actuation of emeýrgency sysdoes operator action. P.

31.3.4.6 S.tation-.,lackout*

The loss all• stits taf •ac power results in immediate plant behavior similar

.... tothat ýor telssowever ofoffsite ac power. -in this event, the electri-

.cllyim 4111 pp and CC pumps are not a I vailblebecuse

-*f Aothe7 -i .. ratorunits ... i-th a em water to. the

syst m, e nztory, wil s i l . a ur l c r u a i n w l tel on- ...... time-at eisntitePitry -circuit reaches bulk boiling. Adequate core

cooling .ia degraded--ati rl circulation may continue- as long- s there is ade-

-quote water-iý t.e Ste , separators. Tie time t tzh- at the steam separator

water inventory is a.-function of initial power level, and assumed values of

IM decay beat,,andgr•aphite enslible heat. After steam separators are empty,

natural circlatio. Vcooling of the core ti loL. Rovever, assiuming electrical

power still s not .been,-resored, decay beat ramoval can continue for another

short period 'of time- a*,& percolating" mode. boiling continues as long an

water exists in the core region. Water in the aeam separator downcomers at an

equivalent elevation will flow into the core region via recirculation piping 1i3

a "manometer mode". "ben -ystem water inventory is depleted to a level nez:

the midpoint of the core, steam cooling of the upper core region will no lorger

be adequate, and the reactor fuel elements would begin to dry out and undtrgo

beatup. Tuel temperatures exceedinS the fuel design temperature under af-cidrnt

coaditions of 1200*C can be experted.

*Lose of all offaite and onsitt sc p'-er.
An important safety concern is- the possibility of a return to&: criticality because of core boiloff and the positive moderator c'iaracteristics of the reactor.

One Soviet textbook (Dollezhal, 1980) indicates ir general terms that adequate

control rod shutdown margin exists in a' completel' voided core.

The RBiK plant :is "not equippe•d>iAh any steam-dri:en feedwater pumps or injection pumps to prevent this sequence from leading to fuel damage when water

inventory is depleted.

This event is capable of causing core damage if ac power cannot be restored

from either onsite or offsite sources.

'"3-3,47 -Loss ý; iDecay Hveat Remova" -(• - -

This e ,ent opl cau& fue daaei ogtrm shtown deay heat removk-V

,-.;sys~tems a~re.lost', fr a sufficient period of tine. Few det ails are known bout

RBHK long.-term DHR syste.m, and this postulated event represents different

initial plant conditions than '.hose in effect at the time of the accident.

3.3.5 Transients Involving Increases in Reactor Flow Rate

This category includes transients involving increases in recirculati'in flow

rate, including credible reactor inlet temperature changes (andresulting reactivity changes) as a result of the increased recirculation flow. These events

typically involve an initial improvement in power-to-flow ratio, and thus less

voiding.

3.3.5.1 Startup of an Idle Main Circulating Pump (MCP)

This event will increase total core flow and reduce channel voids, adding some

negative reactivity. An idle pump would typically oe started up to support

higher power demands. The pump start would then be followed by increased demand

fox .stesm flow, which would restore normal channel voids ned exit steam quality

to its previous value.

Controlling large swings in void fraction and exit steam quality fol-loving WCP

startup appears to be done by throttling MCP flow to maintain a consistent

.power-to-flow ratio. For example, operators might have procedures (or even pump

starting interlocks) that require the discharge throttle valve on each NCP to

be shut before pump start. The throttle valve could iLeha be opened at a sufficiently slow rate to balance the flow increase with increased steam demand,

thus avoiding owings in reactivity. From the Soviet literature, we have no

indication that this procedi-re is followed, and no indication that tLe MCP

throttle valve is designed for this use. On the other hand, we have no indication that MCP startup has created any operational problems.

This transient is part of the normal anticipated opersting requirements of the.

plant on ascension .to full power, and shtuld not be a safety problem if systems

respond as designed.

3.3.5.2 MCP Startup Witi Idle Coolant Pump Branches at Abnormal Temperature

Even if an idle pimp branch were allowed to cool down slightly, it is unliAely

that any core reactivity problems would result from pump start brcause the RBI
reactor does not hae a large negative-temperatureý coefficenet' that, co6uld'

crea Le a rectivitexcursion-.from iectin codriater into thecoe In

jecting hotiter water iuto: ther co-re, -ýTu increas voids ad d reativilty bu. this situationf is consider.d 4i very probable, especially since it is un i kely •

chat large te•teratue increasesw could. originate outside the core of a boiling

water reactor.

It appears the aIr1kgest temersature etc 'rsion fr4o idle coolant could occur if

an individual pumpbranch couldabe isolated and Looled down for maintenance

while the reactor remains in operation. If that cold pump branch were unisolatec and restarted-suddenly 4ithout'trewia-ring, a sudden slug of cold water

would be injected into the reactor. This event-would cause a l3rge negative

reactivity insertion, and:. could initiate power oscillations similar to an

inadvertent; s'afetyinjecti-on, ta described in Section -3.2.6.. • It alsoMwould- v.

_Cause!asvrehra -tasei othe5 .. temperatuareý-sestv tasit.-iojn,-ed at' the bo tto of---the -zirc i sre tubes.

' i.3.6 Transients Involving Decreases in: Reactor- Flow Rate

This category includest"ranstienit inolving decrease3 in recirculation flow

rate, primarily due to., osmes.of wmin circulating p a (CPs). These events

typically involve an,- initial -degradation -in power-to-flow ratio and thus fuel .element heatup and. increas" voiding.

-3.3.6.1 Siggle NCP Trip From- Full :Power

This event ij a design-basis transient for the RBRK-1000 reactor and initiates

an automatic power rnback to 602 and a trip of a sLngle pump in the opposite

loop. rhis event i, handled bthe automatic actuation of -safety systes, and

:does not appear -to require any.- mediate oaoperator action.

3,3.6.2 Loss :ofAll -orced C i.Fr Full Power, Low.Power

This -event wouldprogress, -1b he. same asa-&turbine trip, or loss of- offsite -

• powerA f w would, occur eason other

than lossoofffaýita omrJrjeto , or tr..+A Aturbine total-loss 4of -<:

fo~rced Nfo ii"44' ol t -iiaemrgency- shutdown: scram),adti fbt

-turbine generators. -Cre--,oolig wold+i -be provided initially by tlow coastdovn

-driven by the NCl?. +flywhee Mter about •}-30 seconds, natural circulation would ....tbko over as -the -,primazy -means-of=decay beat removal. Turbine bypass valves

would open to relieve- ,excess pressure created by *e loss of beat sink. - At high

power, safety relief •vaves I(/s) wul dopen to relieve this pressure to the

souppression pool. . . . • .: , '..el

Failure to scrsm :ontotil loss of flow would create an Immediate boiling criais

because of the saeriou violation of balanced power-to-flow operation. It is

likely that CRY violations would occur In all fuel channels, followed by overheating, rapid voiding, further reactivity Increases, and damage to fuel.

Since the$s effects'would occur within seconds on loss of flow, the Soviets -

have focused much attention &n this area. Very large MCP flywheels provide

good coastdown characteristics. Also, there is a quick-response reactir scram

signal based on senatag the loss of two or more circulating pumps in one loop

(instead of requiring the detection of a loss of all lCPs or instead of drpend-I

Ing ov other signals such as turbine trip, to initiate the acram).
Since the automatic protection system willi iniatascm and turbinetrip

:on loss. of flow,, this even can be considered'to. be a desi'gn-basis; transient' be--

c eof its close similarity tothe simultaneous turbine trip. or los6s-ofoffsite-power transient. -Therefore, this. transient appears t.: be. mitigated by

the automatic actuation saiýafty systemss, without imediate operator. action.

3.3.6.3 NCP. Throttle ValV1e•-Flow Ctroailure (F1i!led, Shut)

The inadvertent closure,-o-fa oine C dischar-ge throttle valve would create a

2artial loss-of-flow event simfilr o:: apump trip. It has the potential to be

worse than a pump trip for two-reasons-.- -irst, if- the throttle valves are

quick acting, their. closure would haltý flow rapidly:without the advantage of

the MCP coastdown. .Second, if the•e• loss-of-flow logic to the protection, system..

only: senses H&P ope.ratonb .a ralve closure would go unrecogni.zed , -

.creatin.an adve-repower.-to--ow.sma~tch.in-- one-half of .the ereactor. Exl-cess -

voids-an higher stea qua1ty would be-. cre ated-,in, that -half, add'ingposit i've-.

reactivity..-Flow- rom thetrm•i•ni pps would increase somewhat. Increased

neutron flux would- probably-e.:detected, and control rods would be inserted to

prevent CHF violationsc ad fu6eloverheti. Operators would detect the valve

closure by valve. positio iid.tiui,: flow oro discharge pressure indication, or excess reactor peror steams quality :indications.

The automatic protection :,sy.,.te -oralars ', "an indications to the operator,

would result in a powerirduct ?ion t• 6•%. ecause of the similarity of this

transient to a single NCP trip event, itis limkely that this event would not

present-a safety problem.

3.3.6.4 NCP Seizure

This event is similar pto.pM trip, except that flow coastdown would not occur..

'Also, the RBMK loss-of-flow logic.maynot monitor actual flow rate, so-automatic

actions are uncirtain...:2I MT-hef fects o hI s.event, -if undetected, are discussed .-bove in Section 3.3.6.3 -'(pump discha-rge -throttle valve failure). Power should

-be reduced to 60% in:. :his -event . :If •detected and responded to .rapidly,.-,it :;ould

not present a safety prlm U1fmdaetected, game fuel, d.amage_ mi*$t occur. -

3.3.6.3 MCP 8haft: Ireak"A

This event is aimilar ,,to thbe, NC? eizureAdiscussed above, The same comments

about flow coastdown and-:vcertainty -about .detection and automatic response -'

aloapl t hi vet Pwr' shudbe reduced to 60% in this event. If

-detected and responded torpil,-i oudnt present a safety problem. If

undetected, som fueld occur.

3.3.6.6 Copee LessofFw n neCnel

The complete loss of coeling flow it one pressure tube could result from the

closing of the manually operated inlet pressure -tube -sgulating valve (which

can be fully closed). -

*Fiow blockage, inlet isolatliq valve shutoff.

3-42
iou t ~rapi dryout. and& etpoe fetdfe

.The immediate impact "4du dbe -d'h~'-p' f. th J

chnnl nda oclpowe :ncr~se casd6yth nrai oi inj hia power

increase .coud~ ,propaga ieher fuel,, channels iheiinediatemtrx,

alto, not to the 6ere as in the aiffOc ted ch ai innel.

Steam binding did counter.-cu:.ent. flow litations would prevent cooling of the

affected channel from the steam separator. Soviet literature indicates that

cooling in thie "r•versi (bu.l•, •e (planned, shutoff of •A pressure, t einlet on a shutdownr. ector)"" uiu maintenanceis only: allowed after, 72. hours

of shutdown. Before.ths,,decay hat levels are"too high to permit effective

cooling.

The incore instrumentation and automatic controlý systems for control rod Posi-.

tioning- may respond.to this, event by drivieagrods into -the core, in. the vicinty

-.of- the: -affect•ed nel, iii.otder to maintain then preset power- evel. -

The- evyetbt should -bi. detected by th cladding, leak detectioný system.- h oea'

tors uhol&. tiniite a manulM acras bsed on thse indications

Based on an adiabatic b up t conditions, the time interval between terminati of f• Id" cladding damage is measured i msecomds ?he

fuel would reach meltin:gtem eratures within 1 minute. This time frame I s very

short compared to t"e timehat.w`od be required for detection and possible

manual actions tor6e ;4,•PIemth ave.•• ;(Een- if the valve were reopened, severe

fuel damage is likely to occur because of the sudden quenching.) Also, the

severity of this evnt apears -to -be beyond the capability of locpi. -reactivity

control systems to -mitigate the local power excursion.

It is probable that this event would result in fuel.damage in the affected

---channel. EAtensive fl' -meltin ouldw-probably cause a -rupture-of, the akffected

Spressure tube, and asin•gle ]pressu-tube IOCA into the graphite region. Adja-

.cent tubes couldbe. adrsly eaffe ,;-by ,the heat.up and local: reactivity excursion. Iscaping atmb •-salhydrogen from a v•ptured tube could damage adjacent

,graphite. Additionsi pressure-tuibe.. ruptures are possible as these effects propagate. Multiple tAb. rqktm AIM beyond-the. designcapability of the reactor

Vaultan umUo~ ev6iftaly. caný -tb vault-pesr b:a ofail..

This- event is -beyo -the design basis' of the .plant'and odlklyas.evr

damage to the reactok."-

3.3.7 Transients I•nvo•vingeactivLty.and Power Distribution Anomalies

Thscaeor f vnt ucnsavr~iretr %of control rod withdrawal events,

.control fallures, reactivity I*.la ces, etc. This category is primarily

dedicated to errorstALta;e .positioInin-of-control rods or fuel, and includes

errors in on-line reMueling.

3.3.7.1 Continuous Rod Withdrawal Accident - Single Rod*

This event is not anlIysed in the available Soviet literature end is not indicated to be a dealsn-basis event. The uncontrolled vithdraval of a single control rod would result in a local power increase in the reactor core. This power

*Full power and low power.

3-43
increase would lead to increased vo6"id fracfion and steam quality in adjacent.

fuael channels Since the core has ,aly 21 ull-lengt ontrol rod for 1661 fuel channels, the withdrawal of ofe control rod ight affect 8, or more fuel

channels. Because of the positive.void coefficient, the control rod withdrawal

.would result in further ocal Power, increases in the local core region, 'hich,' probably would result in violating: the CRF limit in those affected fuel

channels:.

Because of the positive. void, coefficient, this event-is not self-terminating. The RBMI( reactor must rely the ar-esnse-of slow control rodo, and the fuel

Doppler reactivity coefficient to *j;'i L. rod withdrawal Accidents. The Uncon- trolled withdrawal of one control rd would likely result in the insertion of .adjacent control rods by. the automatic power distribution control.system.

couping th fue f ~bnnel is wffcie t to analyze tis event. ti

pos gt ulated; that, te rmial ti"on f this event.vul requir~e the colete shutdown of the reactor by either the-automatic:-systs (e.g., :110 nominal core power reactor trip) or opertor actio&; Aomw fnelhannl coldexperien-cedriyout

and heatup of the fuel'durinthis izevent, Ami , with the potential -for sm fuel: ele,- sent damage. Because of the core's.:*'- loose neutronic coupling, it. is not clear that insertion of adjacent contrlrods, even if done promptly, can avoid a dangerous situation in-the locl egion.surrounding the withdrawn rod. .

If the automatic control s estm aýr-capable of detecting and mitigating the

continuous withdrawal of one control rod, then it is likely that the limiting (worst case) sequences would be ones t.at inL. iate from low power. The amount

of positive reactivity inserted and degree .if voiding would be greater by the time the local high-pvwerconditio wsetect ecause of pr itrument

response at low power.

3.3.7.2 Continuous Ro ibran., cidn od3ns

This event is not analyzed ImGavaiLable Soiet iterature And -is :not indicated- ..to be a. desisn-basid event. t-e sweat L& difficult todef inebc e 'appears that lArsk .er iictu osar toerated n-agroup-or bn modi. - The lare-n er.o m ct and -USP) appear to be moved

individually and seuetilly, so a lare roup withdrawal of RR or USP rods

.appears unlikely. A withdrawal .frthe scr-- rods (AZ) is an unlikely accident, because the scram rods aOre-maintainedA -. 8 a fully withdraw position during critical operations. Te. group .withdraval of the 12 or more automatic regulat- ing rods (ARs) appears the most likely, yet these rods may .be moved individually instead of In bank dur:ingnormalopIeration..:It Is also possible that multiple withdrawal of AR rods..ismo-borelikely i ro ps of 4 rods .because of the way the

control system is designed. -Ane'Soviet report mentioned a possible synchroniz- in& error that could withdraw four'rods.

The effects of a continuous rod withdrawal accident of a bank of rods would be

very similar but more severe than the single rod withdrawal accident discussed

above (Section 3.3.7.1). The group withdrawal accident would be worse for.two

reasons. First, such more reactivity would be inserted; and second, it is unlikely that automatic control actions would be effective, since the automatic

*Full power *nd low power.

3-"44
regulating thatculco rods nr sigleod td a ,li kely the rods beingwithdrawn ia g-up rin; wfihdra•wa.

Since automatic compensationist"14. "nlikely, this eVent would proceed rapidly until terminated by an automatic scram on high power, or by a manual scram. It is likely that .CHEviolatio f _ueL cladd ingoverheating, and fuel damage culd occur before the power excurifon could:be terminated. Again, the positive void

discussed 

coefficient would multiply the effectsof the reactivity, excursion. 'Also, as above in the _sinie .rod ithdrawal. event, this accident would pro- bably be worse if initiate at lo power.. -

This event is very difficult.to ianslyze becauseof its complexity and lack of detailed core information. O•On the basis of a preliminary review. and a Soviet beyond the itera;,turei•,,.• capabsii i.t 4p'ears-. pratoruto.control.- to•be. 

-of b-6yond-the design basis ..of thei "plt, ad -

3.3.7.3 Miscellaneous Rod ihrawal. Error and Nisoperation

During normal reactor .operatic t. h o6olant •oid fraction must. be maintained at a specified level by orpertor ' on. TJ. e. operator does this by adjusting the control rod positions -ad/or regulating the coolant flow to individual fuel channels. The instrumentation .system. measures the flow rate at the inlet of each pressure tube, _'i ihe individual'•assembly. power and-steam quality at the outlet of each pressure tube. ý.,From the literature, this measurement and calcu- lation cycle occurs on a- continual basis with a time interval of 5 to 10 minutes.

Failure of the operator to recognize a OF violation, or failure of the sea- suring instrumentation :to iden. tify :iapproach ,to: critical heat flux boiling in the reactor, can lead to a :local.power transient. The event results in in- creased 

that voiding in. the affected. fuel channel -with a resultant power increase in channel. Because.of -the Wek,1metrosica coupling of the fuel channels, the increased power level in the affected fuel. channel would result in an in- crease in power level i4n adjacenIt chnnl -4b.adjacent..channels would then - '-experience ýthe adjacent.hae. increased,, Thu mi ' furt l a increase the level I:wr of prpa :te to* t•he ,;!e tilr• -

the nuclear instru aion" m d.ii-damceptable power.-increase in -a .region of: th reactor and generatd oig lforcontrol rod insertion. Proper con- trol rod positioning (either. m l or.atomtic) is very Important to these power excursions, improper rod motion I the most likely initiator of local .pover transients; and proper control rod positioning is the oa.!,! practical means of stopping them. --(AdJustiLg. individual. channel flow is a slow, compli- cated operation.)

!'nother important control rod .isoperation that can have severe safety conse- quences is the excessive manual withdrawal of control rods. As evidenced by the Chernobyl accident, excessive rod withdrawal can degrade safety for two reasons: First, rods pulled to full height have little initial effect during a scram because they move into a region of little rod worth. Second, rods pulled to full height actually insert positive reactivity during the first 3 seconds because of the displacement of water out the bottom of the control rod channel by traphite rod followers.

3-45
I , . . . •• -'•I-•

:Failure o the nuclear ii1trumentation system.to detect power surges, or failure. ojf' the, opera"t or' to. responid. ith the Appropriate- control, rod Adjustments,,.oud result in- overheatinft of- t4it",re &Md possibleý fueldamagei.

3.3.7.4 Refueling Errors Iuicluding, jiproper Fuel Placement*

This categoryOf eVentludes++ar~userrors in loading fuel or absorber+ t-ce ..... rror in di ... i- l so Ab or ,u .... ro'lds. Possible errors inc lude: fu++el or mbnv r oto

* feorabsorbers loaded into Improper. locations

• loading fuel of r t(either -ih to low Itoo late in the fuel

cycle, or too eigaly in the cycle).

indquate .cooling of, f uel rods_ du;ring- fueling oeainrsligi ;Zdowa o ng mnd ating .1 -operatirnsien a is .......... .. ll-' insert.ed r

rapid insertion of new fuel .f high enrichment without compensating con-.

trol rod insertion (Inserting highly enriched fuel with the refueling machine could-have theeffect of tinuertini a "booster rod.")

These errors cannot:beanamlyzedwiath avaiilable information.

3.3.7.5 Rod Drop Out o ktA of eactiorA*

This event is A special CA•-tgory of rd withdrawal accidents, involving the downward withdrawal of short absorber rods (USP rods) out of the bottom of the

reactor. The slow withdrawalio •these rods by control rod drive mechanisms has beendiscussed in Sections- 3,.1. -and.- 337.2. This speci..al categor i for the unique possibilit y for vey rapid' reactivity insertions by 'a rod drop acci- dent involving USP rods. Since these. -rods..are pulled into the reactor from the bott.m• of the core by rod, dri"' n A..mounted above .the core,, the..possi- bility exists for rapid, gravty-assisteda,+rectivity insertions as a result of control rod drive failurde so WismdisefaSiement, cable break, etc.

'Secaub. -this "sequemee -woul 1s tsCUVityt a ater rate than*ohrrd ..withdrawal JOUbeng .seer tra -Ans ent. .. 

3.3.7.6 Loss of Iaventr ... contr.o.oi System

The cooling system for the coMtrol rois (C'S( systea) is a major contributor to the overall effectiveness!of the control rod channeli (rods plus cooling water) as neutron Absorber, beaet oi, "water is itself a significant absorber

of neutrons. aowever, +the oo wter also helps moderate neutrons and thus tends to increase the te l neutron flux.

Because of the dominant absorbing property of the cooling water, a loss-of- coolant accident in the asepaatei, '"lo-pressure, lc-temperature control rod cooling system would add a significant amount of positive reactivity to the

core. One very preliminary estiaste is that a total loss of control rod cool- ing would be equivalent to the complete withdrawal of about 10 to 15 control

*EZ.., improper enrichment.

**Short absorber rods only. i'

3-46
rods. SUch an accident would not only *addt alargeat-.ms. ount activ Voiver 

ity .but it wiould: rin1g into question thebiity th eco af cievea

shutdown of the neutron chain reaction after a scram.

The Chernobyl Unit 4 riector h0s redundant control rod cooling systems with backup makeup,- wate*r-available(-aeiUSSR, 1986). 'Also, automatic reactorýscraw .- are initiated by both .a drop of ievi in the CpS coolant expansion tank and areduction in flow through the CIPS channels. Therefore, it appears this accident sequence is haidled adequately. by automatic protective systems.

3.3.8 Transients Involving Increases- in CS Inventory

Thi cegryinclUdesi events that might in o primary, circuit ki e .

tory tthpi taecsiv e st eam-sear torwters vl c~rd h i- .. uld thr+eat -.t•~teur~bin geneatos with•. turbine;blade damaeý fro v."er

ain nth-eam e,. ystem -Since- IN teactor.* sa -b~i+i ..- reactor, these events. will. enerlly not result in increased primary preisuw-e.

3.3.8.1 Inadvertent CS A+tivat.ion -

This transient was discussed in detai-l•/n Section 3.2.6.2. It creates power oscillations because:.oft theeactivity differences-between the affected and

unaffected halves of the reaitor.#;Si- cethe volume of-high-pressure injection .4

water is limited, it;1is niot-ant.icipated that steam separator overfill would

occur. During the tests discussed in the referenced section, the high-lIvel

scram setpoint in the steam separators. apparently was not exceeded.

3;3.8L.2'. Excessive J.eedwate rFlowA. tfre • .feedwater malfunction .coat occurred rol:, mlfuanction -in ::ou~ld.dz>eslut. orfdl_..:. Ai *:reactor.. I .. ... ;..++ only ione-half of the reactor (most probable case),

power oscillations, such as..in- the inadvertent RCCS actuation discussed above, might result. The total volie and flow rat. could be. greater than an ECCS

.injection, but t .the.. .A.... e -zcur-im l d be be less severe. 

If_. _this. transi o ov a suta l time.. l period",ehl , .. steam . separator scram setpoint wouald,-b reached.ý-Ahrsinably, high-level setpoit the Soviets. selected a that o~ld~ro~t~t their turbine generators from moisture

carryover.

3.3.9 Transients Involving Decrea-s in 1M1 Inventory

This category includes a llt..tatdeceae primary cisrcuit inventory (i.e.,

loss of steam separator Ueve1) other than excessive steam demand events. This

category consists primarily of a -ranp of credible loss-of-coolant accidents

from small to large breaks.

3.3.9.1 Large-Break LOCA of Recirculation Pipe (tCP Outlet or Combined

Discharge Header),

The rupture of one main coolant pump discharle line would result in the imaedI-

;.r Increase in volding in one-half of the reactor, and the discharge of steam

and water to the high-preslUts containment area. This stesm would discharge to

the suppression pool. The check valves on the group distribution headers would

3-47
clos from e,, thus, prevetiiig break flow back- f row the reactor -.7 4hve rae w'ater flo, the steam .e-p to),u ,,-,o.v"iediat; cooling of he, c-ore. The

increAse in pressure in the containment area along with' the rapidly decreasing level in t*.e steam separator(s) woulderesult in the gener. o n s a

and a. reaLor emergency shutdow (scram) signal within few seconds after the

event initiatio ie C is. s ed ioi ense the affected; loop- and initiate flow in ";e 3.5 secod. 'CChi This extremely b short response. time is required. to ntrol '6:0indications, ofthe -,event .. i,: avoid violating C : limits. Th:e probAble control OOm i of h .ve would include a decrease in the te••erature- measured in the outlet feeder tube due to reverse filo firm•. eaffected .. sitea separator(s) of.saturated or light- ly subcooled water, nInreas ,se- i. ..pressure-in the a ssociatedi cotainment cow-.

partment, and a decrease: vitestea separator level. It appears that the ECC

signalalso.initiates iertgency feewater and trips. main circulating pumps Snetebreak 'NFs.loainisprtd from the ICC Jeade'r-,nd,,grop ~:c~lctos~yad~c vlwe te iiiiatonofthe ICC should. pr ude -suffi-L letiifb ai chec affcte rftiuatr hal ,e' T" 

brea wfrom tdpipe ddi.be directed.. to the suppression pool,

which also serves as A backup water source for the ECCS.

Details of the piping restraints for the MCP discharge pipes are not available.

A potential vulnerability in- the rupture.of one discharge pipe is the possibil- ity that it could propagate to otrdischarge pipes because of pipe whip and/or jet impingement., This propagation. of ruptures. could result in an accident ,':beyond the: capability ofthe•e•me.irecy core ,ooling systems.

This event is considered the maximum credible accident for the RBHK-1O00 plant, and appears to be capable of being mitigated by the automatic actuation of the

emergency sys tems. Although, this event.does ot .appear to require.- imediate

operator action, there.i s-sme doubt Among reactor safety experts that the coincident "loop-selecti on"'logc -can, sense- the affected. loop and.- initiate adequate flow in a-short 46 g tie period -(3.5.iseconds) to prevent fuel,damage. in all situations.

3.1.9'.1 'Large..-Iresk )iC f Groimp* Distribut'ia leader

Thgutr farO-~p~ i~b~fmhie woul result.,in the- InmediAte Increase in .voiding I.•....ts. a. .ci4ted 4" fulchannels, and the discharge of

which steam and would water to t'h, lowpressureqcoatimeRt rt area underneath the reactor, vent to- the swpressia onol.:Reerse water flow from the steam

•separator(s) would p- vide •"mediate cooling of the core. The increase in

pressure in the low-pressure .&co:tainent as i. along with the rapidly decreasing level in the affectewd.t-se rator(s), would result in the generation of

:an ICC signal and a reactor .nergemcy4shutdown .(scram) signal at some short

time after the event Initiation. 6iprobsble e control roam indications of the event would include a decrease. i the temperature measured in 40 outlet feeder

tubes due 't reverse flow from the steam separators of saturated or slightly subcooled water, an increase in.pressure in the associated low-pressure con- tainment compartment, and a decrease in steam separator level. The initiation

of ICC would provide sufficient water flow to the steam separator(s) for long-" term coolinS of the fuel assemblies in the affected channels. The break flow from the group distribution header would be directed to the suppression pool, .which ser,. as an alternate water source for the ECCS.

3-4P
jn~

Details of Xihe pp••i t;r-aaf•h gup collectors are not available.q-. ,

e group r o `k k.... ..... ... .. .. -e•i~ ii t h - upu potential vulneaiiyi k utr of: 6n group dstrkbtin header, is the.

possibility that it could propagate, to other group collectors-because of pipe- whip: and/or jet Impingement. Thi effect could- result in fan accident beyn d

the capability of. the emergency core cooling systems.

This event appears,to be handled y•the automatic actuation of the emergency

systems without requiring ijm•edia.te operator . n..a.c.tio 

3... Ii m, ýeparator Downcomer/NCP Suction Header -Inside . the RecirýlaijiPipe Room" 4

-These zýtwor-evenits ol exhibit very- sim~ir behavior and'thus -ar,ýe osded

together Th2beki~ihrgn ouoe ipe .or teMP~uto hae

;would--esl in.thii probable falr of theimain cruaingunsi h. ffecte ~f the".eactor. and-thedraini:ngof the stea m separator(s) -in the

affected half. Since td sultin the rapid increase of lower

piping room pressure .ecr .s.-in level in thesteam -seartOir(s),

and emergebe .CC ener d- CC flow, Would

be directed to the fuel.:.channls, s long -ter, cooling would be- assured due to

the break flow being 41irected to te suppression pool.

If the rec.rculatloonoimcmerrupta e. is outside the suppression pool pro- tected recirculationmpipe room, then it would be similar to a-mainfeedwater

pipe break, which in discussed-.below .... -This event appears to be handled by the

automatic actuation of:the: rgencyý.osyems without requiring imdiate operator action.

3.3.1-4- Main- Feedvater P*iPe Break,

-A break in the feedwaterlie f" aerr pumps and the st.a...ear.-

tor-is not.& destg~ný-baosis~acc ntaccordinugto the Soviet literature. For

... various hebreak .feedwateris.break wills.-det.e location theplant..

exone-ote c-eenthe main fe-4iater 1=0 .--

a4d th steam

-loved by a .comnnctiim, for thbeý OnRDIcY £e ter system. .-,or "brea locat~ions

between the tant transient would look much.

dowstlike a pam raflt 'te r eak 1hck'°a- 3.3.4.4). -1or break :.lcations

downstream of -the cbedk lvew 461 -result a in the loss of feedwater.

fow to the stem separators m eha f 4wreactor and the blowdown of the

water inventory-in -rainrt t Aheafffecid s4team through the break.

Nor these latter brek a , e r feedvater system could not be used to provide feedister .to the steam sparstors. The loss of feedwater flow M

to the main circulatigpw-.ould result.: in the cavitation of the ma ci- liculating pumps, and possible damage. .heWA-•s probably would not trip on a

loss-of-feedvater-flow signsl, -`beca=e this signal is measured by feed flow

instrumentation which.probably would mot sense breaks close to the steam separstor. The accident would result in the smtying of the steam separators in the

affected half of the reactor and the possible loss of inventory from the steam

separators in the unaffected half of the reactor due to the connection of the

steam separators via main stemaline cross-connections (see Section 3.3.3.1 on

main steemline breaks). This accideret would -ot result in the automatic V_

3-49
t

iLAitiation, of the emrecy core. copling systemt. ECinttonrequires .

.simtaneous- sgnals fromý low: stam separ;trrevel sdfo' ihcnaimn ' pressure. This high-pressure signal would ot occur, since. t:.steam: separ- .

ators and main feedwater piping are not l Iocated in sApaces tat bow doi, to t4e

suppression pool. The.reactor eergenc ,utdo Pstem (scram) would initiate because of the loss of. team seppra•or. iventory. ,If the break- i inside

eihe sea sparator ioom, the' cocrsfrdMae caused bp se scpng through the gaps- in the. rfueling- floor aso apply here.

Operator action to. initiate the ergency core cooling system, initiate emrgency reactor shutdown -(eram),-.nd 'to close themain steam cross-connection ... would be required to mitigate th' .aiccident. in addition, there would be a continual, loss- of reactor. coolant inventory out of the break location. This -accident raises a:concerni for the availability of..the m•kteu c - -pi . :ater

The rupture of one inet feeder tube would :result in an ismediate increase in. :: voiding in the. associa~ted. fuel, charne, an!d the discharge of steam, and water to. the low-pressure iS vault nqre• •rth.the.reactor. -The escapingsteam would'

vent to .the suppression pool.-rverse water flw 1from thei seam separ"tor would provide se ilmediate cling of ,the affected channel. This reverse

flow normally vould be as great or *•reater than normal c3hannel flow,-and prob- ably would not increase voids noreadd positive reactivity.n The bove descrip- Tionhapplies to . cuete rupture of oee inlet feeder i tube.W m Obviously, an

diminish mall leak coolant in- an aaffected inlet flow.in -feedertube the ould channel. nt cause. Therefore,f flow reersal , -but: would - :.. /: • there eaists er.. t

unique -inlet li .nesbreaksio e (m.: aller. than- . ete rupturef, larger than small

leak), that wouid stagenate 6f•lo in 'gof the affected channel. break -Thissize is ab. worst-case inlet-schavnol neobreakd , itand could damage fuel in th affected.

salnnelk. Thisi pl-ausilfe.sta tow-scario" uldo-be 'sm-lar tro a ct•"le- . Losinsh ofolow- ems fn I tbml(e.-Liected-Cannel. Teefr te).itsa

uhn.inceu e ine prnessu ie inthe- lo-rsue ti vaul alo gwihter sol

aecreasing level"in.th~e:s•_teamn e• a r associatedame th the ruptured channel ..

luea) proabl would 'result in '4't~he generation fa ICh igaanndl :Ti rak sie Iso

ShUt owa sixwl t ;so me-undefined' i-e afer. ::/intiation of the event. .Dependtlde ecreassin 

a decrease rater level. *:The Control-r.-zm indications of the event would in- in the"-te- erture ameasured i* the outlet feeder tube due to

reverse 

ater, n 

flow from the -steam •separator(s) -of-esaturated or slightly subcooled - increase in pressurn in tetessociatod ouw-pressure cowiarthent and a

•ecrease in steam separator level. The operators should initiate a manual scram

a ccordnce with procedures. -if they fail to do so, an automatic scram andne CCS actuation will probably result from continuous loss of coolant. r Tba milation of the eCCS .ould provide sufficient water flow to the steam meprsator -.

zor maintaining iosg-tetu Cooling of the fuel assembly in the ruptured channel. The break flow from the .ruptured channel would be directed to the suppression pool, which serves as an alternate water source for the CCS.

3-50
De0tails- of the p Ii#1.ng estraints for the, iLilet .feeder tubes are not available.

A p vl ra rupture of. one feeder tube it

that it could propagate t the r feeder tubes because of- pipe whip. and/or jet:

iingement.ý Tisstuation••o could eventually result in _-i accident beyond the

capability of the emergencykcooling systems.

In general, the riu-re of an Lilet-.eeder line appears to be capable ot beinmitigated by the automatic acttuaion of the-emergency systems and does not require immediate operator action.It ias considered by the Soviets in the design

basis of the RBI•kI'O reactor . "Iowever as discussed above, it appears that

a certain medium-siz.e- inl-et. line break could lead to flow stagnation and fuel

damage. Immediate operator-action (scram, ECCS actuation) would be required.

3.3..6 Sallrea-in hannl Otlet Lineý or RueigConvection

The ruttur .f i outlet.feeder tube i..d.. esultin e imeiate incease 1.

voiding in the associated: fuel channel and the discharge of steam and water to

the area above -the reactor, vault- -Ti.s region is not within any pressure suppression system bouidaries._-t•• ie wter flow from the main :circulating.

pumps initially would provide aeqae cooling of the affected-channel. however, because of the positive voidi cbefficient, the increased voiding would

cause an immediate- power increase in.the -affected fuel channel. This power

increase: is likely to. cause .overheating and propagate to adjacent fuel channels

because of the increased& neutron fluence. The level in the affected steam separator(s) would decrease-slowly because of blowdown through the ruptured fuel

channel.

The local power contrl: *1 t • dhould adjuat .:rods in.an.attempt to control the

local power increase.. :-2The imediate-reaction of the plant operators should be

.a manual scram or at leastam l isetion of the control rods in the vicisity of thew break- i order.to : ontrol"tbe pwer distribution. The probable

indications of this br•eakwouldic~ludea slow lcvel decrease in one or more of

the steam separators, and pressure in one or both

detectors, they p d- Ao*ldb turated or- slightly subcooled water fl.owing from the steam- -separator. ForZ bresk locatn downstream of the detectors,

a superheated stemi.,sdicationr Vrobablymidd -be registered by the detectors.

.Because of the break locatimonitis•accident would not automatically initiate

the RCCS since no lovervault pssure increase would be detected. Also, an

automatic scram of the.reactor1would not be expected since it is proable that

-no scram setpoint wOuld.be 1 eceeded:by this accident. Normal circulating water

flow would continue and accident tertinotion would require operator action to

scram the reactor and begin a cooldown process. The break location cannot be

isolated since there is no alve- between the pressure tube and the steam separator. (If valves existed to pemit isolation, it is highly unlikely, that plant

procedures would permit Isolation in this situation, since fuel melting and

extensive zircalloy-water reactions in the affected tube would result.) Once

reactor cooldown is achieved, the affected pressure tube would have to be

defueled. Then, closing the feeder tube inlet valve and installing a freeze

plug in the outlet feeder tubesdownstream of the break would isolate the break

lo•'ation.

3-5
A,~

Since the break flow: is to ihe a&real above the reactor vault, it is likely that

the coolant eventually would-be dischargedýo outside of the plant. The dscharge

would not be returned to.'he isuppression pool. This raises the question of the

adequacy of water isppies for log-t.erm cooling of the core.

Details of th•e-pin" restraints forthe inlet feeder tubers are:znot available.

A potential vulnerability in, the pture -of one feeder tube is the possibility

that it could propagate to other feeder tubes as a result of pipe whip and/or

jet impingement. This situation .tcould eventually result in an accident beyond the capability of the ergency cooing systems.

Another concern is the likelihood that high-temperature, high-velocity steam

will impinge on control r rod drive motors, drums, and associated power and contocabig.. Sucih d'amaget:coul -,pre-clude. rod motion andprevent manualL and _ -

-a._utoma tic, 4crams-. -As-fcner stedmage that colild -,occur-toý uu~air '

instruetto an otol rocoln systems.

This event has the .potential•for icalating-Into a very serious event which

could result in szigificant-core daage-i•prompt operator action is not taken

to scram the reactor and begin the cooldown process.

3.3.9.7 Pressure Tube Rptureu sidei-the-Reactor Vault (Graphite Region)

A rupture of a pressure tube inside ,the reactor vault is considered by the

Soviet designers to be beyond the design basis of the plant, based on expectation of "leak before break" and the monitoring ior pressure tube leakage. However, the pressure-relief capability of the reactor vault via rupture discs

or relief valves isrbaed om'tbe •-ea 4vter flow from the rupture of .-one pressure tube.

The rupt,-re uf one pressure tub inside -tbe reactr vault would result in the

immediate increase in.voidingin; t ,associated iuel channel, and the admission

of significant quantitwiesof-stemad ter,.to. the graphite cover gas. Because of the positive dratc'mfic .n th.e water. in: the fuel,.channel,

'!an immediate powerl-ý.. lase4 bt fn uem lwould be-expe_-riencied, -which -is

postulated to prop> tadaet fuel-channelii s because of h n~ae

neutron fluence. e ahe., ra h-,e-l•p 6ricrease might be less near the

break because of a -decreise i.a/n-igraphite tempersture in the immediate vicinity

of the break as the stem 9mtd'water cools:,-the aphite slightly. The level in

the affected steam separator(s) would-decrease slowly as a result of blowdown

through the ruptured fuel,-channel--I,,ýovever, :te feedwater control system would

probably compensate and mantainz adequate level.

The rupture of more than one pressure tube is'beyond the design basis of 'the

RBMX-1O00 reactor. Such an event would exceed the stated relief capacity of

the reactor vault and could overpressure it. :xcess pressure might deform or

rupture t•x vault, or it might lift the upper biological shield enough to

relieve pressure to the upper core exit piping region.

The immediate reaction of the plant operators should be a manual scram or at

least a manual insertion of the control rodp in the vicinity of the break in

order to control the power distribution. The probable control tom indications

would include an increase in the temperature asO Jumidity of the c'rculating

cover gas, and a slow level decrease in one or more of the steam srparatorr.

Y-Ž~ ~

it

!,~ ~

'3-'

~'-~'~& A

'S' '3
pecause6 of the break location, this accident 'would. not. autouatically. initiate the: em~ergency cre, cooling se sineo lower vaul t 'pesure increae0 -would beý since detected it is probable Alo an- that automatic. 

no scra. 

scram of the reactor would not be expect-ed, signal.would be generated by this accident.

Normal circulatingwater-flow would continue and accident termination would require operator action toý. scram the reactor and, begin a cooldown process.. The. :break loc.tion cannttb"e inoated, since there no valve between the pressure tube and theý steam separator. ý(If valves existedý" to permit. isolation, it is highly unlikely plant Uth procedures would permit isolation in this situation,

would since fuelm4..ltinýg result,. i-n-e•e" : t e•tensive. Actor had- zircalloy-water. reactions in the Affected tube cooled down, te d ged t-ube wuld he to

be defueled. en., closing the feeder tube inlet:valve and. installing a freeze plugin the outlet .eeder tube would isolate the break location for repair-I.

.The,. iactof stem cut tin _o• adjacent tubes in' the.e -reactor ivault must 4so 4 e.

* firetlctia sh anaysis. own infossi I- eating uni(ts tat a steam jet. from the: b k i:ect-Tf`Serfrea locat ion can erode local-. teriials, su-chas graphite, and accelerate this. ma0cerial which can then cut adacenit tubes.

This event-has the'po•tential-for escalating into a very serious event which could result: IinsignificAnt core damage if prompt operator action is not taken .to scram tW reactor .,.dbegin the cooldown process.

3.4 Refvrences

Cherkashov, 1984 Cherkashov,.Y. N., "Safety Design of the RKBK-1000 Reactor,"

... AE.,.-SM-268/84, 

rc e edngs 2epratinnal Safety of Nuclear Power Plants," ofAar. LA A susium in .arseilles, Nay 246,

--1983, .AA,.Vienna, 1984.

DOE, 1986 UJ.-•-epartmen• of Energy, "Report on the U.S,. Department .. f - nergy aI eAm Analyses of the Chernobyl-4 Atomic Energy St.t on AccidentSequence,'! .E/NE-V076, November 1986.

Dellzhal~19S f•tllesihal, N~. nd 1. I .m'ao'

JIv er, anerrvRactor, Atomizdat-9 Ioscow, 71980.

Dollezhal'-1983 N. ADolezl A., I. Y. Yenel'.yanov, et al., "Some Charac- .. ristics ýof and. Experience With the Operation of Nuclear

• PowerPlants With RMNK-1000 High Powered Water-Cooled Chana~l~ectors(331)," Atomnsys Enerziya 54(4), April 1983.

Dubrovsky, .urovsky 1979 V. 5., "Construction of Nucear Power Plants," Afir Publishers, Moscow, 1979.

GSR, 1983 "General Safety Regslations of Nuclear Power Plants During .-Desigp, Construction, and Operation i(OPB-82)," AtoEmaya mA'enusi 54:(2), 1983.

Kabanov, 1983 3abenov, L. P., "Effectiveness of Emergency Cooling of Water- Cooled Power Reactors," Teploenerattika, No. 2, pp. 4-10, February 1983.

041

3-Aý53
Ions-tant inov,

Margulova, 197

Markov, 1984

NNC, 1976

Novosel' ki

Semenov, 1983

Sidorenko, 198

Sidorenko, 198

Smo;in, 1984

Turetskiy, 1989

USSR, 1?86

V(Wronin, 1981

Yemel'yanov, 19

1-975 Konstantinov, L., "Reactor Desigp Safety Anaiysis in Reg#rdto, Internal Hazards. for Pressure-Tube Reactors (USSR),"

Repct No. 0 AE-CONF-75-769-067, 1975- meeting on. nuclea r

power plant planning, IALA, Vienna, 1975.

78 Margulova, T., Nuclear Power Stations, Izdatel'stvo Vysshaya

•Shkola-,Moscow, June .1978 (EPRI Ref. No. 78-004).

Markov, Y. P., et al., "Emergency Localization System at

Third Unit of Chernobyl AES,""Moscow Energeticheskoye

St.-oitel'sno 1:61-63, January-_1984.

National Nuclear Corporation, "Ltd., "The RussiLn GraphiteModerated Channel Tube Reactor," NPC(R)1275, United

KingdOm,March '1976.

.19814 'N.ovose!1 skii, 0. Y., V. B... Karasev,,, et al., "'.'Tests on':

:Improved Steam Separators in the Third Unit at the.Chernobyl Nuclear Power Station," Atomnaya Energiya 57(12):

382-385, March 1984.

Semenov, B. A., "'Nuclear Power in the Soviet Union," IAEA

Bulletin 25(2), June 1983.

Sidorenko, V. A., and 0. .. Kovalevich, "Safety Control of

..:Atomic Energy Sources," Atomniya Energiya 50, February 1981.

3 Sidorenko, V. A., 0. N. Kovalevich, et al., "Standards for

Safety of Atomic Power Plants; in the USSRr,'" Atomnaya

Energiya. 54(4), April 1983.

Snolin, ,J. N., V.. I. Esikov, et al.., "An Experimental

Study of-Emergency Cooling Conditions in RBMK Reactors on

Load Disconnenction," Soviet Atomic Energy 57(2):507-512,

August 1984.

4 .Turetskiy, L. .., et al., "Accident Localization System -of

Energy Units With RBMK-1000 Reactors," February 1984 (EPRI

Ref. No. 84-0061''

* i * V

L~

I

I

I

ii

I

* I

'77

USSR State C'o-"

"The Accident

Consequences.

Meeting, Au,:.

Voronir, L. '

Nuclear Powi-

..n the Utilization of Atomic Energy:

.':,bhernobyl Nuclear Pcwer Plazt and Its

tion compiled for the IAEA Experts'

, 1986, Vienna, 1986.

ter 1 of Operation and Maintenance of

.ts, Energuazdat, Moscow, 1981.

Yemel'vanov, I. :., V. P. Vasilevskii, et al., "Safety

Problems of Nu;'ear Power Plants With Boiling Water-Cooled

Graphitt, Channel Reactors," Atomic Energy (USSR [Soviet

ALOmic Energy] 43(6):458-464, December 1977.

3-54
Yesel'ynov. , 1984 Yai yano,, '1.," I., Y., et " oOeangC .it on d 

ofPower GeneratingUiWihRIKacoWthFle7- 2~ fog v,.98• ' on o ojsea ctor!,s aEmrgec. olngSstm"Jso

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3-55
CHAPTER*4

'ACCIDENT SCENARIO

This chapter presents the factual •nformatiOn availabt.; to the United States on

th- causes and in-plant consequences 'ofthe accident at Chernobyl. A chronological listing of the eventI 1.provided In Table 4.1 .t the end-of this chapter; all aspects of the acci'd•et th io e the reactor was mved from

normal ope -';.-on until several days after the accident occurred are identified

and discus,.,.. there in chronologic; rde.-

4.1 Overview

The accident occurred duri6g a test of the turoine generator system. This test was designed to demonstrate that _follow•i a reactor trip, with the resulting.

loss of offsitte power dis a ti- .f the sts supply-to the.tarbije, -th otat-"

ing inertia of the t :rine gene'ratot.r wouii.esufficient to generate enough eiec-.

trical power to energize certainsafetysystems until the diesel generator system could be started and accept the electrical loads. This test had been performed

earli.er at similar plants...,hes.,pecifict.purpose of this test was to determine if a new generator magnetic field reSulator would maintain the voltage output

.from the generator for : longer"period.

In the process of establishing the teast conditions for the reactor, the operators brought .the-plant. toan unstable operstIn- condition..-How,.ver,. for a nm-. ber of reasons, the operators .hose toru, the test fro this ustzble- condition.

To prevent the reactor from automaticallyrshutting down, the operators purposely V

.bypassed several systemsAwitqi tant to safety. The role of the operator in this

accident is discussed. in -_ap; r5.,0_

with the- safety s -ystems pase• t• psawasin an unstable and vulnerable

condition. -iThstoo.u rm~mtLra& table, onditi s wa tke

positive void reactiii"y oficient.allowed t .e .eff.ct.. •eacivity

to Increase as thetvol -Of . team increased in the core. Other significant -•

parameters included the low Ini~ti~al "PmI leel low subcooling, Low initial

steam void fraction in the coret fuel burnup condition, and control system

characteristics. The design "chracteristics.f the Chernobyl plant are detailed

in Chapter 2. All

The initiation of the test.caused * teste•m volume in the core to increase. Under the unique test conditions '(for-hi-Ichl te plant was not designed), and with

the safety systems bypassed, a significant insertion of reactivity resulted. The

resulting power increase produced additiotal steam voids which added reactivity

S. Sheron and C. L. Allen of the U.S. Nuclear Regulatory Commission (NRC) compiled this chapter.

4-1
and further increaedr the po9wer-. vluatio tod in

brugt og to, a p -propt cri tical condition. Asseassme6n"`t of- oviet (nd, other ana lyse0s also iDiteth depsto heneg the i6n fue Ia ufficitent

ýto melt* so me nlss to dat -ffe.-h suggest tefloigpossible sequence ruptured of the events. fuel c-ladd4 The r apid expansion associ*a tied.. with: melting, quickly nd. injecte.. f nd and m n fe ito the

coolant channel. The inerac-tion of the coolant with te. hot fuel fragments produced steam veryraiidply'-_;. The hi eýeratures and rapid. production of

steam quickly ove•pretss ure ttabes in the core 'e'ion..The

.pressure tubes then failed and overpressurized the cavity regio.n around the.

.,graphite blocks. Sufficient force •was generated to lift the top plate off the reactor This postulated and possibly sequenceo-.f• to fail. evts t iactoir building and eject .core material.* .. .. •can.. asuociated: with the first "explosion"

heard by operators'at teplnt. A second "explosion. was. reported to have .'occurred approximately 3 seconds after. the initial one.

Various specul-at-ioson t-.'ource •• o • sf noise include a second criticality, a hydrogn:- dtnto, rvbi;&b;rrereration.

In sumary, the event was caused bya :. 'ce atiion oI of procedurladmaaeet-..t' deficiencies, humiun:errorsjW -vstdeii h ractetr14tics.

4.2 Events Leading to tmeAccident

-The events leading to :,,th acc4identat the fourth unit of the Chernobyl Nuclear -Power Station on April: 6,-1986 .re discussed in this section. The events are detailed in narrative form and are a marized- at the end of this chapter in .-Table 4.1. The accident chronology identifies the violations of operating procedures and principles that .placed tk.e reactor in succeedingly unstable. configurations. Information used .i.'reconstructing the sequence of events was obtained from review: of- r0e6 rt ite h yl accident prepared by the USSR State Comoittee aNuclear on-the..Utilizatio Safet of Atomic Energy (USSR, 1986) and the Advis report on the Cheruobyl

.accident (INSAG, 1986). 1igure .1 illutrates.the chronology of events41 leading to the accident.and-1s provided:-to supplesient the detailed scenario.

-plant Unit 4 

was- 

-of :to the: be Chernobyla to tatitms Ist"it prto nDeebr1S~' e ip rom pril I,- 1986. .AAt that '

ýtime the active core cotait"ed 1659 ue asebies (TVs) with an average burn

up of 10.3 IND/Ik. Mobst-4f:it lh f ul-semblies ` !(about 75%) were assemblies from the first loading .ith a8-burnup o121 AlN-./kg (USSR, 1986).

*Precise details of the "vents during this time my never be known with certainty. Further information could contribute to our understanding. Such information as

geometry, composition, and distribution of materials would help us analyze the nature of the "explosion." Information on the distribution and conditions (strains, fractures, and distortion) of structures and structural materials would help assess the magnitude of. the forces involved.

4-2
SiON 

........... . O RA TORS L RO

BY. LOAD D:ATCHER 

RACH 2RE EREAU MM

. 119 OPERATORS. BLOCK

RACTOR TRIP SIGNAL S

Wo~ 

MSTEAM SEPARATOR

g 

~LEVEL AND PRESSURE

w 13-00TG 17

SISCONNECTED

OPRACTOR TI

-0 ZiPWR-C 

T S:RI

DESCENT RESUMED

. . . I / , _ , _ . . . .

-12:31 OPERATORS

SWITCH OFF LOCAL 

IOPERATORS 

START 2

30AUTOMAA TC COORTRM 

AMTON41JC COOL.ANJT

kw*PS. FLOW L1ANTS

Figure 4.1- Chropolog fth aciettte Chernobyl- Nufclear

10we r 044Ato (not to0 -scalte)

Tb. events leading to tAhe acA started a.'tý,O1:00* em Apriil,ý125 when sztation:-

persmelstarted-rdca 

ecq e rdn ,ts r~~e.D

1335 pwerha ractr bes rducd tin3100 Wt :to about 1,11600 Wt. ubn

.enerator No. 7," .oe of t1e i•.s to -aiin týUurbine generators, v then

.removed from service. 

iree fectrica 

f,7sU receiving er from the discon-

•nected generator, incluimps 

aton.f 

fe.r.motor-driven 

feedvatar pips and fotir

of eight main circulating , .- wre switched to the busbar of turbine generator No. 8, the genieator tIo• b -ete test. To. ensure the reactor core

,received adequate oling.d ingte -- ei t, thelremaining feedwater and

main circulating pumps 

'ere 

aligned. tote.staions 

service transformer (offsite power).

*R eferences to time will use a hybrid military time designation. For example,

0100 becomes 01:00. The purpose is to provide a framework for more detailed

time reference (e.g., in seconds such *a 01:00:30) where such ioformation ts

available and relevant.

4-3
At 14:00, te emergency cooling sysem As "CC isconnected toprvn

gec.core a, reve_

inadvertent ac tuaion duigtets te ECCS is', des i-ghe to, :ensure the -core

.,ý,remains cooled& during ptu atidkloss of-colnt icc1:t-_idns.A ttistmeth

test wias deilayed ~tthe reqeto h laddsacea te.itojin

was left in service for an. additional nine hours. During this time the ECCS

was left isolated in*iolatibhn ofoperating procedures. Although the. Soviet and

INSAG reports state that the "CCS could possibly have limited the consequences

of the accident, m . engineer•belee tt this violation did not have .

any direct effect on course eof the' accident.

After the load- demand vas.lifted at 23:10, power reduction was resumed in: prep- .

aration for the test (t•esi specficatioýnisequired the experiment to be performed at a reactor power ltween 700 and O000 Mt). In-keeping with low-

,Vpower 

.which monitors:,ad 

operating procedures, 

a.iiintainiin 

the 

local 

local 

power 

automatic 

in i12 core 

control 

zones, 

rod 

was 

positioning switched off. 

system, 

ApV

parntythebakiautmati cotrlrod positioning, system, whichi operatev ~ný

,conjtunto wihteoapwerctolmaintains ,control o.f avrg ore;,. ,.

power, d not been- sete •totheproper :ctor

power subsequently dro 1pd 4-blow 30. , Ht

The operators wre ableto tilu:ecr' owrat 200 N•tby 01:00 on :

April 26. However, as a. result--of enon buildup in the fuel, Ohich is a natural

occurrence that introduces, large amounts-of negative reactivity during prolonged

low-power operation af-tr.high-polweroperation, the operators had to manually

withdraw the control Sods beyoind-safe operating limits to increase' power. The •

unit's effective shutdown :,margin was reduced to 6 to 6 control rods, well below

the minimum reserve margin of 16 :equivalent control rods required by the plant

operating procedures for safe operation of this. class of reactors. Reactor power

still could not be raised :to ,the level required for the test.

Despite the serious rductioniný the excess 4sbutdown reactivity margin and the

inabi lity..to meet established•. test p.zl r .conditions, the decision was made to.

ýýproceed with the t .est. At01:-03 and 010 tw4tandby main circulating pimps,-N

one per recirculation loop, vere 'starte-d'amdJoined with the six pumps already

running. According•t•o th•eA• t pla,.for othe eight pumps, two on each

reiruation loop vldb voved -ine tbe ~turbine coastdown test a"dth

,-remfiaining. four: ps vewlw1 'i§~oeainto, provide core coolig.A

o arlie, ed the'oriia•,tes t••o e lled for thper be . t.. lst

700 Wt. Mader t c t f pul provide the necessary cooliug. ?However, at -s-powur' l"eve pfu f20 pumps were not necessary,.and:

operating at .this power ,l l wi#b eight ps running violated maximum flow

limits. With eijht p rsuaniftg, ot. flow was greater than desired, rising,

-to 56,000 to 58,000o,,/hr.,-t same4pw the flow was measured to:be 8000

$/hr, well in excess-wof t0 oml.1000Ou 8 fhr.

Because the operators were:uable toi'increase reactor Power beyond 200 MW (as

a rtsult of xenon accnumlation), they had a very low power-to-flow ratio and

low core void fraction.. As a result, tthe core hydraulic resistance was subt,,antially lover than WOuld-be expected in the test. Continued operation

beyond this point violated operating procedures because of the risk of pump

cavitation, vibration, and possible breakdown under these conditions. In . k!

addition, the increased flow through the core caused A reduction in steam

generation and a drop in steoa pressure and liquid level in the steam drum
u,... re ... : , - - w ., the ke Z r s or p. a , r, as . I e e...

separators FigUre t for the

,.:last 5ý minutes beorC

At 01:19, to prevent An automatic shutdown of the reactor under these, varying

level and pressure conditions, the operators; blocked' the, rieactor;'protecti"on

(scram system) signals related to the pressuret ad liquid level in the s team 4.a

drum separators. '16n dditi, -at 'this time, the feedwater flow to .the. st"

drum separators Wa4 increased (by• • •s s•4uch fou times nominal) to. restore the"

depressed water level. This action lowered. the core- inlet temperature and further reduced steam p•rdu•ction.. Aany power level, but particularly r t low

power, a reduction in c.ore voids:(reduced steam generation) produces & negative reactivity insertion :inplatis usin•gthe RM: -1000 design; that is, it

causes a reduction in reactivityithi n 30 seconds, .the autoimtic control

rods had fully withdrawn from the core 't compensate for the reduction in

reactivity.~ Theoperators then assisted the automatic control rod. system.by

withdrawing the mtanual-control rods. Th-actions 'ýof,.the, operators oecopn sated.for ithe-_reactvyre io'nd the -'-"t-omat`ibc--

into the cr"r... rods bea mon bak

By 01:22, reactorparametrs ere _rslattvely, a Uble And; -the dec'isio~ný.- wa made

to proceed with the .test.teci signals associated with the

turbine stop valves on both turbines were blocked to prevent the automatic shutdown of the reator when • thes ale -:e: re closed. It was believed that: if.. the

reactor was kept in operation, -the testCOuld be repeated- quickly should the

first attempt prove unsuccessful' - Ie. thetst procedures did not provide for

disabling the automatic shutdown logic associated with the isolation of both

turbine generators. Feedwater flow was reduced Just before the initiation of

the test.

At 01:22:30 a computer.printout,.from.the f:fastrcti'ity ,prgram -evaluation 

,showed that the available-excess reactivity.iargin had dropped to a level requi ring the reactor -to -be sHhut.w.iiediately. -. owever , this requirement was

- ignored so that the test icoulddbe c.qlet. .:The. axial neutron flux distribution was distorted by this :time .a"nd.the majority of the control- rods were

probably rendered ineffev'tiv'e for powe cot rl

A0:23:04,, the,-stop valves~oý LOSmri bmrotor- iNo-. wer cl~osed to begjin

th .e test :and -the four min c~1irculatin is on 0--the generat~ot "busbar, begana to

:coast down. The reactor continned to operate ,;t 20(, IIt since automatic reactor

.shutdown had been disabledA fol|ig- teisolation of both turbines.

At the start of the test, ýthe bulk -of the reactor coolant system was near.saturation temperature as . resIutof ,the.reduced feedwater flow and the excessive

circulation flow produced by-all.eit p ir nn during the lov-powr

condition. In additios, ,the core -was 'at; very.low void fraction. -A core

flow decreased (reflecting the tosstdow• of Uhw four main circulating pumps)

and inlet temperature increased ao, a result of the earlier reduction in

feedwater flow and with power production still At 200 t•t, steam production in

*Note that much of the information is Figure 4.2 is based on t Soviets' computer simulation of the accident, particularly of the last minULr Xiefore the

.power excursion.

'-5
;;rI ;;j: :;

3;;

a;;

a

a.

Ik 1;; :32 222 5~

* .. *0* .* a .* =

i

4 4.

-A

-3

-c

-6

- II

U~itI~A %- 51 La Flcv, f~eedwter kg/s ) 0 

Pe~s.~ bw steamdram S& 0 --- h Fw. steasmAO/s)0 60

edy A-IEatu setd 0. .2--0 Sta asqaiy(Exit, of core. %

Oed gra AR-2 (*pdace finsertedl 0 1' -- PSteaim v& q-ptyIorveae void fraction) 0

Red ruwe AR-I Pheatien inserted) 0 1.2 --- S Level (seam drum. mm) -20 0:

J,

1 e '4.* key reactor parameters for.the•,last

before the accident, 01:19-01:21

nutet
'gel. --1,1*-*

a..I:?::jjj::i

IJ

-,,

* S6

1!!! U

A

-s

-6

-M

Untmu Power IW

Rfetiv*,. sm M% -

Pressure. steao drum (W.)

Red yuse Aft-2It(raction inserted)

Rid V'W AR-3 (fractiOn miseted)

&0S000 " --... N

1.2 ---

1.2' -- S

RPo. Feedvator (kg/s) 0

FlOW. steam (hg/s) 0

Fuel te". MC)2

Stem -asn quality (Exit of -core. %) 0

Stem vat. quaity voidrtio) 0tCerage. 

Level Isteam drum, mm) -10

60

tt0

Figure 4. e reactor parameters for the last ff.e m..inutes

before the ac~cident, 01:21-01:23:30
¶1i.;II:~;

ii;~*i

†

St

.. S *.3 4

if.

Ir

so

KEY TO THE CUR4V.ES ON. FIG. 4.26

t ~AX

-- A

-- -. 140

-6

Reativity, Spa 1%)

Rod grow AR-I ffracta,mbrtd

Rad jrw AR-2 (froctiosa~ittd

Mod V"w A-3 (frcti@A kingted)

0

-t

54

0

0

0

0

120

.5

90

46000

1.2

1.2

1.2

L

-- p

-- s

qwo, tcPt/52

Flow. Feedwatei i s0

Flow, stem (hg/a) 0

Fuel temp (*C) •.,

stem mss q.uagtlio.yEtof core. %) 0

Stem vol. "ia lt(or-avea void frac :,o

Level (steam u m) -1200

S

600.

.600

2000

t2~

0 w

4ii4

2c Key reactor parameters for the l•st fiveminutes:

"ý' ,before the accident, 01:23:30-01:24:4 . • ' ' +;: +•] .. .- + , ''

+++++++ .. .... + . V++ j... + ,+ +•+K:;. + +++ +.

.•+ + .+ . , + .
the. codrie began' to Dncrease. ~The RMfl-1000 design responds t'o:the formatiion of steam 

void~~ cefcnt.* 

in the ~~ core region Vihou-t'.pro'pe~r 

with an increase in power (because o h.e aIsitive ... control, ac~ti on. this rieprsn' ~ ef propagating conditio. ... t .wa.it this-point that the poer excursion began.

Shortly after the sta of the p.ower increase (-At 01:23:40, 36 seconds. after the 

scram 

turbine stop valves had clo"ed), te uit shift manager gave ihe order to- the reco en Z~,wich1 is -t~he4 highet level of eegny.ud function availiable. .hisn woul allownsertion- of al, control and shutdown rods into the core., __euse ofte redecedue shutdown margin and the distorted* axial neutron f-lux distribution, the rods would. have had. to travel well. into the core before encounte -sufficient "eutron flux to-be effective. -Further, because of the use of. foliowers in the. cotrol rod design, it :is possible. that initial, discussion control of control; rod motion,,could rod• design introduce reactivity. (See Chapter 2 for-a. "an d::nctions.) After the scram :was ini- :

tiated,~~~~~~~ fsvr hck eerprel et Inme in the control 'ioom'Jn

control• 

an oeratr ob _ ~hat~e cotro rods had failed, '. )fully isr. h rodAdrives werei the. dee•ergized iný the hope #;at the rods would fall Into the core undertheir 'weight.

4.3- Events Duringand After the -Accident

Analysis indicates tha y 2 lar positive reactivity insertion had occurred and considerable-energy v•wasproduced in the fuel.. At some point the: fuel melted, expanded, PA failed the cladding. According to the Soviets' evaluation of their'. c .mter.analysis.of the event, fragsented fuel was in-. jected into the coolant chaneI and interacted with the surrounding ,steam-water mixture.** This resulted in a :high-pressure failure that ruptured fuel chan- nels, sheared the connecting.piping to the reactor, and breached the roof of. -the reactor building.-' Thispasof ithe cldent is generally associated with the first "explosion" th•t, wis reprtd, e A i'second "explosion" was reported to have occurred approximately 2 to 3.'o¢d after the first. The high-pressuret failure destroyed .i ch: oftereactr-, -ruptured the water-filled biological

.*Under the Unique. coftties~Ih ttAbe voidra cvtce iin ws

**Soviet experts estimate•h Athees"ery •d•position in the fuel-exceeded

300 cal/Sm over the .course.:of th.:-accident, and fragmented the fuel and dis- persed it. Figure 4.3 (Sberon, 1986) shows an integral of the Soviet calcu- lation of reactor power verus timedring the power bursts. --y dividing this integral over .the.j irst peA*k4by"t mass of fuel in the core, an average energy deposition of 300Scal/gm is Som wobtaie fuel will have a larger specific energy.

A number of experiments on fuel performance versus energy deposition have

,.in been the performed SPERT CDC in 

of 

the 

model 

United States, Figure 4.4 shows the results from tests fuel pins as a function of energy deposition (MacDonald, 1980). Consideriig such results, the Soviet description of the event seems reasonable. Given the uncertainties in the calculations (particu- larly in the power distribution and the power time history), a more detailed description would be difficult to develop and justify withojt considerably more information.

4-9
'A ~-, 4

10

0

C

* j.~. ~ ~

- '~A<7 AD~i~P IIIWI

0~

CL

A~A

0

S

U

S

S

*

0

1.

0

S

* A

C..>..'

Al

"1

The

Ly 30

4 W

Time - eonds 0123:42 01:23:48

Figure 4.3: Cheruobyl-data_ valuation of power va. time during --tore destruction phse (Sheron, 1986)

;Source: Soviet. analysis provided in Figure 4 of

shield- tanks, and jected:,gra I, from the top of thebuilding

hot fuel and reactoz•mtriu wererted to have started approxiaate)

.fires on the aur..undi, structurs.

"Firef ighti ng .- it fra ?rlya [ pmiyarve at0:S.AT@ eventth fire 'from, pra. ihra ca mtrstadtheir 0f9 tsi O ...... the--turbine :buildin, hicb tunito.L -- Dy 03:.-4 the firesIn .Atbe

turbine buildi w•ere . erl control,. Dy 5:00 all fires (other than: those in

'the core) were ezianibed• Mind •3 1"t i lshut down.

•.ter building by iJectieg wateir (200 t o 300 tonueS/hr) with the .auxiliary fedwater pmps at the-stem dim-qratora :a the main circulation pp suction

headers. frmstor tank wdditionally,waeor ws injected into the intact

portions of the reactor via the 6'ow reconnected ECCS.

4-10 .

4

II.

-I

.

7
radial aveage.

fuel enthapy,

ca/g U0 2 •.

U

h ~

* .,

~2O5

- -~

lea sule Figure 4.4

Photograph of the residues: fri model' ful" pins (S'P* rods), after teuts. i the .' C

driver core) simulating power excursions from reactivity inserition accidents

Source: Hacdonald, 1980, p . 585.

D'.' ~ ic~ '*
Units- 11 and .2 not Iwere shut down until 01: 13on"46 -AP i ril1 27.

Betwei.en-,April -27 and May 10, 5000ý tonnes of tboron coponds, dolomite, sandclay, and lad were d~rop~ed :,y he icopter onto the. daaged reactor to, engure that the fuel rlibe mie suibcritical- And, to redue the. dishageordactive material . f m the site. most of the material was dropped between

April 28 and:y 2.

14I

[

r

:2~2

By lay 6, the discharge 6f r"adioactive material had dropped to. several h-ndred

curieb per houtr. Nftitrogen •gas 'was, pumped into, a space under the reactor building to cool the ifik remainig, in: the reactor.

4.4 References

INSAG ~ 1986 lnte~atio AlSfety Advioy GroupG !SumarY" Repotnth , ,d t i t,, I ,-:• v i.e w : '. . . ... . . .

ot-Aien Revie qeting on the herbyAciet, ~Y~ugus t3Oj Septeue 5,96 SPLl)3 A ,Vina

September 24, 1986.

MacDolnald-ld90l•acoad 'et al., tAssesIsment of -Light-Water-Reactor

- .ige During" a, 'Reactivity" .. Init ia ted Accident," -Nuclear 

.--S -, Vol. 21, No. 5, September-October 1980.

Sro, 19.86..-, erO~n.,,.3., "The Accident at Chernobyl,. Presentation to

CS.NI paiicipants, Paris, September 1986. (Available in

S.;he -RC Public .Document Room, 1717 H St4 NW, Washington, D C.)-

USSR, 1986 .... USSR- State.C.Ciittee on the Utilization of Atomic Energy., "TheA;4Cciden•tat. -,the Chernobyl Nuclear Power Plant and Its Consequences,,:"-Information compiled, for the IAEA*Experts.'. Meeting,

_.. u~st 25.29;,.1986, Vienna, 1986.

>,--~ 2.

2A~

•K

4-12
Table 4.1 Chronology of the accident at the Chernobyl Nuclear Power Station*

Ddteit ije Powner (NWt)

April 25

Event Comment

01:00.00 3200

13:05:00 1600

14:00:00 1600

Began power dercent to 700-1000 KWt as required for test.

Turbine generator No. 7 disconnected. Powe

for auxiliaries (4 main cooling pumps,

2 electrical feedwater pumps, etc.) transferred to bus bars of turbine generator

No. 8.

Emergency core cooling system disconnected

as required by test procedure.

A slowreduction in power was plagned

to help reduce the effects of xenon

buildup• The range for the test power

level (170 to 1000 MWt) was chcsen by

the test designers.

er At this time four pumps were drawing

power frot turbine generator No. 8,

two from the. ýgrid and two .on standby

but connected to the grid.' As part

of test'• t•eequipment connected to

turbinegnerator No. 8 will run down

with it when s 'team to it i shut off.

This was a: 'violation of safety principle; ho!eVer, the test required dis,-

connect•ing the ECCS to avoid possible

ECCS actuation during testý (not actuated bYLDCA).

Continued power reduction delayed for 9 hours

on orders of the power grid dispatcher to

meet demand.

The hold in power would reduce the

rate of xenon buildup at the test

power lev el.

23:10:00 1600 Power reduction resumed.

*As proviA'd in the Soviet report (USSR, 1986) and the INSAG Summarv Report (INSAG 1986).

'0 1 '4ýA
IJ

Table 4.1 (Continued)

Date/time Power (Mt) Ivent Commnt

26

C

4-

)0:28:00 Variable Iwitebedhoff local automatic control aE .er'- mitted at low power operation. Because bulk

power setj.int was inadvertently left preset to: a low pwer level, t. ; •actor power was driven Aiiw tob below 30 ?:L

51:00:00 

Vartiblei 

200 'Power was stabilised at tl .s lievel. Attei~ts to :Iinreose ýpower to- Ab esired 700--1000 Mflt • le"vele+e, "uccessful- because of the 1m; smust;_@t o -"perating reactivity 6irga (IU.

(~~1:03:@0 200 At ~01-0:0S0, one additoa .. coig~~

01:03:0 

to waesplaced isto sertice, and at 01:03107 an- otherr 6nco•• pm Olig was placed into .:ser- vice T.is. iesulted. ina. total of 8 pumps is se0 .4e4as specified for toe test progam The totalifow increased in excess of the mantism flow allowed given the (low power) opetating conditions. The coolant tempera- ture approached saturation. Low power-to- flow ratio possibly reduced core void to 10%

or less. This resulted in a dtcresae in

steam pressure and water level in steam

separators.

bOperating reactivity mseAn is a ealcuhtd reactivity margin based p 

sltuations, 

"titiais. It is given in temn of umber of control rods where 30 this may be lowered to ian sbolute miniama of 16 (UMR,

Abrupt power reduction would result, in a negative reactivity insertion from,

xenon buildup requi.ring removal of control rcds to maintain critiali•ty,.

Xeon~pisnig uit p" gre ter than~

.tbýr anticipated for, test; s a result

te OR1 ýwas 6-8 rodsi, substantially

low the 16-30 rods reqvired by pei fi'cation.

4ased on thei pl4an4; test w-e (300-1000 MWt), only 4 main qcooIn&

pumps were required, or safe operati ".l Thsinthis coufi1$,`ra~tion., .4 pmp would coast down at tetniition'

andjd•,4 pumps would remain running. The oviieta stated that this .mode af opra- i 1t 0(i.e., low core' .oid, very low 0W,1 i

power measurement response, and.-umps

near cavitation) is veryunstable. The.- Soviets were concerned that the pums . may have been operating close to point of cavitation.•

Further control rod& vithdrawal., occrred because of reduced void content. Thsis reduced the O111 evenuatre.

on,8 power . and controlroddistriution and .is, the minimums allowable. In ,certain 1,986; :pp. 8, 28).

.- ''A "A"J'" 'A AjN A A i, j'. 4 0 Q AA ~~A 4~,
Date/time Power (Ngt) Event Comment

FC~on t~iued)

Before

01:19:00

200

01:19:00 200

The operators blocked the emergency protection signals related to. low steam pressure

and low water level.

OrmOar1nual replenishmeIat of, feed

water to st-- separator, As thW colder

water: resichbt the, react or core. thre was-a

sharp drop is the aom fraction of the coolant Mi. correp ding reduction in reactivity an ~k;rdecrease.ý

SAlk •(teti pwerregulation) r rpodsd . movd pu~dto C qeaite for reduction: in,

power. some sanual rode were removed

etrely1N

Theý tý'fo t6'ac team separator

had increased by a factor :of three over the

balanced flow for this power level.

01:19:30 200 S

By blocking these protection sign:als,

operator was able to continue operat, t-

,ing plant and avoid scram.

jiiparently anxiou's t stl. .... . war level well withino the no6rmal

rane,-.the operator sharply increased

t fc eendwater floweatov its, nsinal

Tbot Soviet r0eort implied t.hat this

ýmaneuver was not pe'ridtted,.

htifedvatier flos tthe' stea

separator at the nozzles of the. downcomers, leading to tepr ..ary: coolant.

pumps.- Hence, the cre iltfo

temperature responds.'rapidly to4an

.change in feedwater. flow (approziMately 20-30 second transmit, time).'

*Trom this point on, much of the information is based on calculations

in their report (USSR, 1986).

per~formed by Soviet analy'sts. decibed & 
Table 4.1 (Continued)

Date/time Power (Wt) Event Co.ment

Continued)

01:19:58 200

01:21:50 200

01:22:10 200

01:22:30 200

Turbine sti" bypass valve

closed.

Opertor riced fee ae

f rom A valow of four ,timesl

Core outi stean-quality i

of two-thi" -of balanced f

Op0erator n6t" from Skala 'P

MM was about 6 to 8 rods,

value where Immediate react

required.

A: pritnout ,•Ot:¢h actual co

outputs andthe position ol

ing rods was obtained .at ti

Skala system.**

(steam dump) Thiswas done to raise the steam pres- sure which was too low. However, it

continued to drop slowly-until the"

startpof thetest (01:23:0'). The ", valve was probably only partial op ....

since,' ,reactor steam was being; use to

p erte trbn 'generator, fow rasapy Thisý 6Wuse inc~rease ofin et t erathe balanced flow ture and compounded the, ev 1 mmutte later

ncreau4 ..•*eTre •ore v"i'd content increases.

easing at 'a valu

low.

rintout that the No actbio was taken.

far below the

or shutdown is

ire flux monitor The flux- was "practically arched" i.. .all the regulatw the,.radial direction and double peake:'d is t•me from the axialiy with the higher peak in'the

top. section of the core. At this

highly unusual power shape, theerror,,tr, in estimating the control rod, worhs

was extremely large, hence the error

in estimating the ORN was also .arge.

ftesulting from approximately 20,second transient time from stem g rum to:,core inlet. **Skala system is the program that evaluates (among other things) the opt"ing reactivity margin, (USSR, 198).
Table 4.1 (Continued) "

Date/time Power (lVt) Event Comnetn

April 26

(Continued)

01:22:45 200

01:23:04 200

a

-J

leedwateii tfow rate stabilized. Pressure

began to -*ie

rigalto reactor shutdown on closure

of bth tmtiae ýgenerator steam, valves was

gwitced disablid.,:1 Turbine generator No. 7 had-e :f -ch erlier. •.

tobege tsA8 .est -the; turbe generator It

turbinest v w lo d and threturb

a.evve"remained closed.

Tiow rstý bega 6 to fll as` t four- masin, coolng powredby turbine *gene'ratof' 11o. t began to run dw.Stesm pressures,

ao begaim to'increase. duei~ to removal of t

blikp goenertor steam load sod the reductic

(by operator action) of the feedwater rate

about 1 minute before.

Pressure Increased, some. AR rods :began wit

draying becsusc of collapsing voids.

Thivas in vioja4tion of'ý' thke test

-program and' nomal a erating 'proce- S res,. By aIvoidiin reactor shutdown

",. it would be possible ' to r4peatthe

test..using r Ieactor-ýge'nerated siteamý.ý

Thise' factor's cius~ed an, increase iwA

hcoolantvoid fraction (positive:

re"ativity insertion)-, and, a resultiv rz.

in-rpase increase.

veiuse reactfitycoer ccnditions a.-t

the -time .of -the-test,4 the void. fractio 'Is belie4• ed to he increased

.10-yt •simes 'more sharply than it would

hav cone at normal r This sha increase in void fraction result Ai~

a hrp, increas in ractivity.

(Sqoviets have stated' that. the'f ormalvoid reactivity coefficient of,

2. OxIO-4/% steam volume in~cresdt

3.OXlO-4/Va team volume at t~est

conditions.)

01:23:10 200
Table 4.1 (Continued)

Datotime Power (MWt) Event Comment

Aoril 26 (C-ontinued)

01:23:21 200

01:23:31 Inc'

01:23:40 ai

J•|

w

el

As four wps coasted down, the flow dropped,

void and power increased. To compensate,

seme AR; 0ro began to drive in.

eaigiatv~ n oweri inc.rease Id ,furtIhler be-i,

cause-of pumpcoastdown and increasing init

Wmpers ter buttn..

Th& -uap period estimated to be much,.

less that," 2 scods The Doppler efect

teiý t ie eet and rod insertion. Coolant flow continued to decrease

(due to rtundown of 4 wsin cooling pumps).an

the p oer, continued to increase.

Operotorerd'baning nbise. and, saw rodi

were stopping before they reached bottom and

disengaged the servo drive couplings to allow

rods to fall into core.

Control cods vere unable•* tW balancei'' '

reativity additions ..

Soviet clculaton idicawtes power. -

at2td tine to be 53 Wt: and rising.-

CSIbidiat'ed fuel tempratures increased

P"'e surge • esiiiite"&'.to ilceide, d1

times full power.

torted the core so that the- control-,-",

rods could not be bottomed.. The baging nies may have beeninitial rupýtures of pressure tubes or burskting of

the reactor cylinder.

O1:23:U* -

ftstimoted from Soviet analysis:(WR, 1986).

n"II

'II~ Q[
Table 4.1 (Continued)

Date/time Power (PIft) fvent 

Coent

Aporinl26

01:23:45* -

01:23:46* -

01:23:47* -

0.1 : 23:48•.

Pover tiueihItkly led to fuel framentatiho, causin a Iarxe steam spike (rapid pressure,. increase) which reversed flow to close maifi:n reactor, pp dchk valveis as indicated by_

reco-ded btai r l o •,t

'Stea Arm t pressure.:ecedd "accident

eel !!•s mpro.el. scod

aLte, ft t- in coolant flow as channels ruptured because of pressure spike.

Nigh-pressur~e failure blew top off reactor. and destreyed :reactor.

Second" low ad o 'oIse approximately 3 seconds, after first, - hot fragments and sp&.rks emitted 

fragments 

fri6 top of reactor building. Not caused about 30 fires.

Pressure rise rate ýcalculatedW bi.. Soviett was ý64 VA.' .s> .

IU 4nitial rupturesoccurreed , (01:23:441, this flow- increase ould.K he!.:consistent wihth pper biologi- ci 

Sla...in 

thield bein# ife by the pen t-up the reactor cylyinderan the' resultant severing o':l pressure", tue and control rod ',--channels'.

A . . . ',. A

'A A~ 01:24:00 'A

Fr~oi. this point on,, tereisnme

observation in•Ortion from the control room, and proceeded from outsdete

reactor building.

ftstimated from Soviet analysis' (SS, 1986).

1~

,AAA~ A S~A A

A ~A.A rA.

A 

A.. A., A Ar.. I

- 11 1 . - . I I.IL; I . ` ýI ,11 

AAA 

A I A IA ell A. >A AAA IA'.,.¶AAA< 1. . . ... AAA : A I P. ! A4A r A, , ,,,- ' tý: ' tL, ""

A. .. ~ A.AA A~r.AAAAA'A¶IAA A.<.¶AA.A.`A42.<...,'..A,,. A.AA1 .. ,.AA V A~.> ~ ?! r A A ' A2~rrA .. A ArAI A A AP

.. 'A ,.X¶..A . ~ ~ ~'A AAA'AA~tA,~A.AAA
4.'

I* . 4, Table 4.1 (Continued)

Date/time Power (Hir) Event Coiment

Wril 26 (ContLinued)

01:2&:00

(Continued)

02:54:00 -

Shortiy aftifthe accident, an attempt was -his was :unsuccessful.

mode: to reduce the temperature in the.1reactor gsscnannlylrge

cavity and eveat, combustion f t.- gahite Mixture •• of". -

uigemetgeacy ond auxiliary feewaer. and carbon monoxide capabl~e .of.,theu

pu s. ;.Deciimio-wams later aade to fill the • explsbion:.if mixed. w-i.th owxy e, ere,

reactocvt•y t ' Vith "h.eat discharing and: C> creatediin.core region..

filte'rin materials."

f unts t from Pip t add CMe bl Three,,i# aryi ftir sites were:

friive turbine robm above tur ine

gene-rator xo~. 7

~(2) react~or: room

(3), partiall1y destr~oyed co partment's, adjaceent, to reactor room.

Fieigtnguits focused. on fightg

fire, ini turbine6 room to prevent spread

a

0

Nand i extinguishers and

fire-cranes" were used in the compartments.to fight fires03:34:00

03 :54:00

05:00:00

Host of the.: fires on turbine room roof were

out.

Fires on reiactr building roof were out.

All fires (othr than those in the core) were

out. Unit 3 shut down.
Table 4.1 (Continued)

Date/time Power (lit) Event Comment

April 27 ,i

01:13:00 - Units I and 2 shut dow.

April 27

to

0 Decisia " 4as later made to fill the reactor cavity vith, "eat discharging and filterln$g m aterials.ni ,,: . ,, , • ,,,

e r) 5000 tonnei of baron,', coujounds, dolomite,. sad, clay, end lead Onto dmafg.ieeT's

-Dischatge ot radio0activity dropped to several hmdied cifrie per hour.

Witrogen 

reactor building. 

to its pumped 

Temperatures 

into.. space under the Profiles of reducing fuel heatup was rose, stoppe,._sol.. ,,.

As inumrance! atuinst "extremely failure of. improbable" Completed by end of June. the lover tier of structures, an artificial "beat discharge horizon" was con- structed. under the core.ý

a.

04

14 sow"
'~5.

-~-~ .- ~ I

CIAPTIR 5

ii

.ROIE OF. OERATING-PKSIMNEL

This chapter focuses onhe.: oper.atoractions and the breakdown in managemLnt/

administrative' controls tatcontributd in a major aiy to the accident. Some

descriptive materia*•i, •epeate tfrom ChapOer 4, -.& appropriate to provide

perspective on operator actions.

Since the accident'-st; Cei ibl, .muc.h ha-s, itbemeknown :about the sequenceof

events before, aring, "an- afer thacde. ovrvery little has bteen;

prvie re gar in~ ~ratr 7epdnctauig uties, -and respon-:--

sibilities.

.At the time oft .eist46hitpeon wreosi;attefuoea&ting units. It -,k •-o• cla ow du•tiesan responsibilities were divided

among these personnel.

-Information avial idctsthat Chernobyl Unit 4 wtas one of the best of

the 14. operating Ma-lOGO units.-. The "training and experience of the operating

crew may have focused'maily:..o.stdy-ste operation since the reactor operated

continually as a base-loaded with wit on-line refueling. Evidently very little,

If any, training had been conducted on a plant simulator. Only one simulator

at another site has been .mentioned As pssibly. ervi the. .training needs of

opera tors Of all PAK:uit.

The Soviets believe. that th .jzrkoious maicsilent performance created an attitude

in plant personnel thati c -lo adherence to procedures.,was unnecessary.

'The -Soviet,% fetAhat it spzeics go"be- Tem operationldt dmntn

ovrotideace. . •-, 

4 ~ .

Th MM -II units-! hu ~ec~yaso ip~ci~tdmr peration.

,Chernobyl Unit 4:-bad batiion etwo Toars. It is0.ot known whatefentas

had occurred at RIWD •unitsthiat mWa ha•e.e•e precursors-to the April 25, 1986

accident or what corre-tive-actioas had be-e taken in.the areas of design,-

'operations, or training.

The plant operating Procedures --are sot avaiable. 3w~ethe Soviets have

- described three operating restrictio.ai•.n.articular that bear on the accident.

(1) Procedures prohibited -steady-state operation below 700 Wt 22- of full

power. The basis for this. restriction was UtAe dominating positive steam

void reactivity coefficient and vstable operstion at low power. The

S. Visner and W. Conway of the Institute of Nuclear Power Operations (ISPO)

compiled this chapter.

3-1
overall positive~ reactivity coefficient below hi&"s power level would

exacerbate.a core -transient.

(2) The "equivalent of SO- control rdeds" wats always to be maintained as. excess reactivitymargin. Ztits the understanding of U.S. engineers that this

requirement was a means of specifying an overall rod configuration that ensured Presence a of cert~ain neut mizimam initial. negativereactivity rate during a.• scram. I ron na 6t•h e t magnitude 4he 

of the positive *tea" voidtreactivity coefficient.

(3) The discnarge flow of anyiof&th main circulation pumps was not to exceed

a specified imt-it t avid cavitation.

Figure 5.1 is aschemati~cdrawing._Of the plant, intended solely for use- in

following the operationala cs ofthacient.,

5.1 pertorActonsand lan~ Mlviieslefre the-"Accident 7

On April 2.5, 1916, the plant staff "was to.conduct a special test on Uni t4 just before it was abut d f ýOr imemai eane. The test wvas beingconducted .to demonstrate that-theattrbu I peratora could continue to power important loads during a station bllac u until ,the .diesel generators took over. ;-In the

test, the steam supply _tome -,ofUth turbine generators was to be cut off. The test would determineb m the geerator ,wculd continue to supply power near rated voltage as it coasted• • n aincoolant circulation and feedwater pumps were to provide the main :electrical load. This test had been performed at least twice before, but ithe: enerator output voltage had decreased faster than desired. Changes therefore were made in generator field control, and the test

was to be repeated. The Interntionl ,Nuclear Safety Advisory Group M(SAG)

OMM DA f

9-09 c C C•shg r •o 1 &avrDoaheRec0

Figure 5.1 Schematic diagram of the. PJM-100OO

5-2
siumnary report. toe' that "The initiative o hýts-d i 1iii,6r

thee test and the provission p ofh •i ?!• 

(INSAG,~q 1986) s~j~ihee otechnica -rather thn. nucle'air exprs ONSsA6, 1986).

The test called for the fol'ig* Actlotio n.. ;

(1) reducing reactor'.'po,-we'r •tob een700 ad 1000 ?1Wt -,Or between 22 id 31%

of full power

(2) blocking the emerency .corerooli• systen (ECCS) to prevent inadvertent

actuation during the test (see Figure 5.1) (Presumably, the-ECCS would

remain blocked only for the expected short duration of the test.)

()realigning the mi'sin circulation pumpsP. AP f our wer conced to the. Vtub6e generator o-, wuhich thae test would be-,co~niduc'tedi -and lthe remaining

*' zour. • main.cii•uilation pumps were : onnecte.d to',.the station grid. (the.test .:-.,,

would start with all eigkt pumps operating while in normal operation only

six pumps operate,; two p s act as italed slpsares)

(4) shutting off teamto 'the t.St turbine ...geertot ini.tthe coastdown

It is not, known if the test procedure mentioned additional test conditions

(e.g., power history) ,:oray special precautiona to be taken or how to proceed

if the plant did not respond-as. e•xpected.

It is reported that the test, viewed as a simple turbine test, received only

perfunctor' review. The station's technica!l safety group apparently did not

review the procedure. The IUNAG report -noted that, "the. procedures were, poorly

prepared in.respect to safety, •(IA, 1986).

-The test was directedby- n engine• r •:.bo hdexpertise only in the -turbine

gxenerator/electrical rea. 11* briefeito the operatio staff included directions to carry out the '.st proce , -following station procedures. as necessay-<twould apertMat tie t"a engner essumed repnibilUt and

authority.A duigtestfmam.if.,-,<V

-The operators feltra, sense .qf urgency to- coqlete th ts~.Te test would

,have been delayed for ome' ý.tIil iit ere sot t,,year pefre at the scheduled time.

The reasons f or, the ,sVensef UrgenUcy or. it#s rigin were not-satisfactorily

,explained in the Sovietreporft.

5.2 Immediate and Short-Term ftgastor_ -Action

-On April 25,t 2986, at 024009', the o*eratore&* re reducing power (began power

descent) in preparation for the coastdown test A'S turbine generator go. S8

(For an exact chronology, see Chapter 4.)

Twelve hours later, the reactor power reached SO%, and turbine gen~erator No. 7

was shut down. The reason for the power decrease being so slow way have been

.to allow the xenon poisoning. to equilibrate with th~e loweri~ng power levels.

The operators then blocked i~ht emergency core cooling 'system In oci-ord.&oce v'it'e

the test. procedure. Shortly thereafter, h power reductz-.'itis &S tt"!

5-3
nine hours because the load dispatcher requested's continuing supply of power. The emergency core cooling syatem remained blocked for this nine-hou period,' in violation of operating procedures. -The ease of access to controls for

blocking and unblocking this system is not apparent.

The power reduction resumed at 23:10. However, because of an operator error

in the process of switching the autosatic.control systems from spatial power

control to global power level control, the power dropped quickly to 30 MWt.

The automatic global power control system had not been properly set. T' compensate for the loss In reactivity from the collapse of steam bubbles during the power decrease, the operator withdrew control rods. .

The power level was stabilized at 200 iWt at 01:00 on April 26. Xenon poisoning continued to increase.. _,To .copenasate, the operators withdrew additional

rods. Thus sore rods. were :w.thdr;wn than the operating procedures allowed;

procedures required mainta a -reactivity margin "equivalent of S0. control . rods." The excessiVe-41t, "l.of iods placed the emergency protection system•:_-

in a configuration that reduced the initial shutdown reactivity rate when scram

was required. Also, thewithbdrawal-of rods "ade the void reactivity coefficient more positive.

The operators were unable to-increase power to 700-1000 Wt, as called for in

the test procedure, because of the small excess reactivity available. Oper- ating at a power level as low as 200 MHt was a violation of plant procedures, which prohibited continuous operation at power levels below 700 W•t. In this

power range, small changes In power produced relatively large changes in stem

volume and reactivity, making it very difficult for the operators to control

power level and steam separator water level.

Two additional main circulation pmsps were started in the next 7 minutes, one

in each loop; thus all eight pump;s were running. This action was in accordance

with the test procedure. Bowever, the test~p#cedure called for a much higher

power level. At 200 M•t -,6 of full power -• very little steam was being pro- duced within the core, so the resistance to.flow was low. The coolant flow

rate now exceeded •th allowed-Umits .which ,re set to prevent cavitation. The

additional flow th e-h ta. t of the coolant 4n the fuel

channels, resulting Wip~ber s'team pzsre$ d owr water level.1u the steam

•drumi. Nost importantly, -tbes circuNtac W; lso brought the core inlet teaperature of the coolant very close to saturation.

The operators experienced difficulty In conttvlling steam drum pressure and

water level because at low power the controls~were too coarse. (Also, changes

in feedvater flow to control water level changed steam voids and reactivity in

the core.) To avoid -n automatic shutdown, itO1:19 the operators blocked the

emergency protection signals for reactor secram that related to steam drum pressure and water level.

During this period, reactivity continued to -drop because of xenon buildup and

decreased voiding, requirin8 further control rod withdrawal to maintain power

at 200 Milt. The operators supplemented the automatically actuated control rods

by withdravint manual rods. It is likely that the operators were also busy

adjusting the local power distribution in the core.

S-4
At 2:•3pr0 hV: ,x. r .noted, from t-he% cputer printout that hei 'vi-able reacivit magnrlted. to6 the ujbe 6f os and their position -in the,ý

reactor, 

cor, eddrope i-.e.,•six 

wA 

to.eght 

l beOWthlel requiring imediate shudow o the

required "equivalent" rods versus the 30 "equivalent" -rods by op-rertin procedures. Because-a scram in this situation could' initially 

followed 

addi reictivlty, special or emergency procedures would have to be to shut •e~~~~~~~~~ down te reactor. Nevertheless, the operators opertt. continued'with 'On•i•:::•/•;: ...= .... the tes t.

At 01:23:04, the operators blocked the reactor scram that would be automati- cal'ly aOctivated:-by the`hutdown- of the second turbine generator, No. ;8. (The. first turbine generator, No. 7,-had been shut down earlier). The test proce- dure did not call for blocking this scram logic. The scram logic was.blocked.

so the test coul.be. repeated:f1 sa r

At t Sam t d! the-peratoes- closed' the - t u neg a

established for a iseveretransient, as ollo

(1 eMreactor asiý critcl a t at a very low owierllevei .here it waso un- andItto& control.

(2) Steamvoidi-percentage in-the core was small,.b;t the water :temperature at the core inlet, was near saturation, giving the potential ior rapid voiding over. a. substantial..ýregion, in the core.

(3) The overall coefficient of reactivity was potitive, with the steam (void) coefficient predominant.

()Control rods wire nar the top of the reactor in a region of low reac tivity differentialv•orth (low "bite"). It would take several seconds for "::th•e reciable negative reactivity. (Additionally, the rods. appal.rently:would insert positive reactivity initially as the graphite r .. f•oll ers -displaced.w. , ter from the lower.,region ofrthe :core.)

Thbe four. izcli-~. poerdfrmturbinle 4mmerator No. 8ýbeiaat coast donderesig thveerfow tfom theWcea .igreactivity to the fuel-channels. . This-allowed more -steam and initiatinS a power rise.

*- power 

The rising .'powarAcreased the steam voids which in turn further increased due Uo the overall.positive reactivity coefficient. Thirty-six seconds

visor. 

into the test, a anul scram was initiated on an order from the shift supercounter 

,,kecaus. the actrol rods were near the top of the core, they could not the, inreasing reactivity. A very severe power excursion took place.

A 

the 

loud 

reactor 

noise 

was 

(also 

heard, 

translated 

-and 

as "loud report," "shock," and "banging") from

an operator noted that the control rods had not

would 

fully inserted. -NAe then de-energized the control rod drives hoping the rods drop under their ova weight. Two to three seconds later, the operators heard -second loud noise as the reactor was destroyed.

The core reactivity exceeded prompt critical, and the power, by Soviet calcu- lation, reached 100 times rated full power. The energy release- lifted the

-- U! U

- I

5-5
1000-ton reactor cover plate, severing all the fuel channels;. the refueling

machine. and its crane collapsed onto the reactor. Hot Segments from the core

were ejected from the reactor, and approximately, 30 localized fires started,

involving roofing materials and other combustibles. The disintegration of the

fuel stopped the chain reaction.

.Steam and water from the reactor and water from the 'ruptured shielding tank,

were released into the reactor hall and compartment below the reactor core.

The graphite .in the reactor was ignited, and a severe fire resulted. Hydrogen

and carbon monoxide were produced, but the role they played in the accident is

not clear.

The rapid destruction of the. react

>..-;, .•e high radiation levels, and high -.- temperatures probably, preclu~ded ret.ndformation from instrumentation on

conditions in the core.

While radiation and temperature levels in the reactor hall became excessively

high, the control room remained habitable, at least temporarily. Some of the

operators left the control room to investigate what had happened and to assist

in controlling fires; they were among the earliest casualties.,

Using the auxiliary feedwater pumps, the operators injected water into the

reactor at the rate of 200-300 tons per hour (about 1000 gallons per. minute) at

the steam separators and at the manifold between the separators and the main

circulation pumps. This water came from an intact emergency core cooling system

tank. The valves Lnd pumps used remained functional, and the nectssary controls

may have been in -he control room.

.5.3 Summary of Key Operational Events and Errors

The design of the plant placed a heavy dependence on adherence to administrative controls and procedures for safe operation. The following major operational events or errors and administrative or management control breakdowns led

to the accident:

(I) Overall management control of the test and its integration with plant

operations-were not clearly established. The. test was diiected by an

engineer with expertise in the turbine generator/electrical area only.

Plant managemeat did not ensure that normal restrictions on plant operations were observed.

(2) The test procedure did not receive an adequate safety review. It is

reported that there was a possibility that the accident might have been

less severe If the ECCS had not been blocked. In any case, it my reflect

the attitude of the station staff towards violations of operating procedures. Also, necessary safety precautions and instructions in the procedure were evidently not adequate; this situation was not corrected in the

review of the test procedure.

(3) The operators felt a sense of urgency tc complete the test. The test would

have been delayed for a year had it not been performed at that time. The

reasonx for the enser ot' urgency were not well explained in the Soviet

report but may have .been caused by outuide or management pressures. Since

the evolution occurred over a 24-hoi r period, more than one operating shift
aSoio ihvolVet na io hiday These ucted early in. the morning and. just before y.facors may have influ-encd. prformanc".

(4) The power ion for the-test Ie•' was, interrupted for nine hoursi at the load disp :..+ . quest.... This delay changed the initial, . core. conditions from conditicns c, .te plated in the test procedures:. The delay in' starting the test may aIas' 4ave i.ncz'aed the pressure on the operating staff to. make up fto-r the 'time

(5) Due to an operator erzror in improperly setting the control point on the automatic global p6o r controller, the power level dropped rapidly to 30

1Wt when the operators switched the automatic control system from local to global. The reactivity loss from the resulting collapse in steam bubbles and increasing xenon poisoning prevented the return to the 700-1000 KWt

power level specified in the test procedure.. .

_(6) The prtr did1not, fol low: the:tiest pr'ocedure.:

The procedure testwas and started stat.on~ at operating ai'lOor level-that Violated both the test + instructions. --(The test was started

at -6%pwer-instead_ 2•2ofs specified _in the .test .procedure.-),.. At .-lowb poevk ttleing generated, -k0o the eight circulation pus p;roduced a-flow rate above allowable limitL. With 'the high flow•1rate ad6 low power level, the water inlet temperature .to the core was- very close to saturation. Under these cond h.ions, an increasein•-4powr caused asmuch greater inc-ease in steam voids and

reactivity-than normal. +

* The reactor scram signal for the trip of the second tUrbine generator

was blocked,:--;vhich violated station safety procedures and was not. 'called -for by.' <•M te procedure.-

(7) Other safety, -systems were -ailao. defeated:V

B locking t-he-.•.stam.. artor.pressure -and water level scrams allowed 7 .oreacto o aioi, delp i table conditions.

-Control 

plant 

negative 

proc"Uefe.. 

rods 

reactivityr 

were ruthdra 

this 

Ulting 

ve•wsdone 

+ we+ll 

from 

beyond 

to 

void 

compensate 

safety.-lmits 

suppression 

for zenonu+buildup 

in 

specizied 

the core.

by 

and

V

This errorurendere d-the Vmrg cy protective (scram) system ineffective.

The emergency core. coolig systes was deactivated for more than nine

Iours while ;the plant ,awes operating, rr.cerary to normal procedures. Mad this vyeta bees available, accorcq. S to the INSAG report and the Soviet report, the accident may h. '" been less severe.

(8) The plant operators and station management did not demonstrate an adequate uwderstending of the safety implications of their actions. Their willing- Dess to conduct t13 test at a very low power level, with abnormal and unauthorired control rod confisuration and core conditions, and uith ..afety features bypassed indicates an insufficient understanding of the

reartor and its potentia! behavior.

3-7
T.,ble 5-.1, r diptedIn pitrt'-from the Sovit reot, ausmriies' the: major oernational, violations'.

er taile0 inf oration, in the areasi sted below. would- be.fo, coiderable -

help min oreý fully- understanding some oprtoa -1 pt oteacciet

(1) The ,exact duties•-epnib-ilities, a id auithority of tesaion staff

members.

(2) The qualificaions4, training, and experience of the people on shift. The

kind of plant simulator training they had. The frequency With which they

were retrained.. :o6w knowledgeable they were about reactor behavior, including reactivity eOects ani traient and accident analysis.

Table 5.1- ions of p procedures

.... s=:•t s ! Consequences

(1) Power level below that ,, :Large :positive void reactivity

saecifiedd by<.. t~oficient

Reatordifficult%.to -control

',O~er&l1 -power coeff icientu

pOositive

(2) Control rods mispositioned -Unauthorized (and probably

umanalyzed) configuration-,

Emiergency protective ,sys tem'

.ineffective

(3). ýOperated 4IVei-0t-" main , Reduced voiding .but coolant'

circulation- ý ith- tmperature near saturation

'coolant-f ,. escedýýig , .

Authori~d levels

'(4) Bllocked te-Actor Scram Lost-automatic, scram, protection

sDignl item Ua o. bo0th atstart of test

turbine senerstOrs

(5) Blocked reactor -scram o'n- Lost r-eactor protection system

water level dsta bsdO thermal parameters

pressure I:rtve.dr.i.-

:separator

(6) Turned off the emergency lost possibility of redu~cn8

core coolint system , severity of accident

5-S V
(3) The rigor and -onsistency oif staff dherence to, safety requirementsin procedures and opeting rules, and o ese wee enforce•d.

(4) The. administrative contos obypass ing -or blocking saety ytes NOW frequently this was done. How easy or difficult it was, from a an operational standpoint to bypass or block safety systems (i.e., accessibility of these controls).

17,) Thetexct contentotetrbe generator test procedure_ i anoot 'yan other

procedures to which- it may :haveerred.

(6) Why it was so important to perform, t4e test at the time it was attempted; why it couldn't have been postponed.

j7). A detailed explanation-of what-is meant by "operatinqg, reactivity margin," ' orUw it -relates to thn number of control'.6ds ert sider thi e htthe, -n basis: for, tiilsjrýrequlireimenatýý,was." The rel-ationshi p btween ;--.".,,ýopera.ingS reactivit ma4rgin rement of 12-uinserti on --of oto rods.

()Tbe procedures .-operatoars wieire requiý, red .to follow in sbutting down the

reactor when conto rdihr~wa l ad--inadvertently decreased the "operating reactivity: margin" - below the level that required the reactor

,to be shut down.

(9) Events that had occure d in any ofh3-,8t111he plants with lessons that had been included instaff training and operating practices at Chernobyl Unit 4.

(10) The operating shiftceules for Chernobyl Unit "4 in the month of April ý1986.

5.4 .Oerator Actions ?ollowig. the wAccident

Within one-half hour, firefightbro from-the local area arrived to suppleent zthe. plant's firefighting•teams. ea ig t -the emergency. lbThe. mJor objective -Vas to keep the fe• • mn r g i-t . Special oat,•ntiOe ýwasiven, to prtectingý cableý` r ams ma i ahtrg reas.

At 02:25, plant personnel iafoZma vjovetovr'nt officials in Moscow of the event at Unit 4, with the prel-miary assnt that it was controllable with local

-resources. Also, at this time, t U*oparators started to inject water into the

r6actor using the auxiliary feedvater-system. The operators recognized that

-discharging the pressurizedi, ccimlators in the ECC3 would be inadequate because of broken pipes ,in theprimary system.

An assessment of the damage indicated that the pemanently installed radiation- monitoring equipment was inoperable or off scale. The tame wan true of the

power, flow, and temperature -Instrumentation. Information is not available on

any measurements of radiation levels where personnel were fighting the fire. PotassiuP iodide (KI) was distributed at 03:00 to personnel at the site.

5-9
An offsite response 6' eaded by A. Aiag1yan,.director o t h u .... a- Power

Plant Institute, arrived: at 05:- . 0 a ti' he Soviet- gvern- ent Centra Committee arri0 c rentral ekrgenc et rn

up,: with, comple~te -authority to deal with-the accident. Inifoirmation is6 not available on! the. transi~tion of control and- authority Irom the local-:plant imanagement.

By 05: 00, the f iveis co hi -turbine building. roof and. near. the re'actor had -be'Len

extinguished. Also at this ti6e, Unit-- 3 wias shut down. We don't kow why _,,Unit,3 was kept .run g untilth-e and woywit. as.shut down at that time.

Later in the day, the injection- Of the auxiliary feedwater was stopped. It was

judged ineffective Ic.cau e Of broken -pipes, inthe reactor system. There was

also concern about flooding an ontaiatingS Units 1, 2, and 3.

Units I and 2 were shut down the followinga morning at"Ol: 13. These units had b~come contaminated internally becausetheir.vntilation ,systems had-, remained

n iou the z•ad-oa.tive th damaged Unift 4 betgan. No, tzplanation Iis given ~on whyý the ~vefitilitio systms s-7tayed onl

or why.;the units 'continued to ~operate for ýalmkost -24 hours. after the Accident -in Unit 4.

5,.5 References

INSAG, 1986

USSR, 1986

I

InternatiohalN-uclear Safety Alvisory Group, "Summary Report on

the. Post-Accident Review teeting on the Chernobyl Accident," August 30-Sep br 5, 1986, IA'A, GC(SPL.I)/3, Vienna, Septem- ber 24, 1986.

USSR State Committee on the Utilization of Atomic Energy, "The

Accident at the Chernobyl Nuclear Power :Plant and Its Conse- queces" Ifoastes omile foir the IAEA Eprts -eting,

August 25•-9, 11986, Vienna 198.

~< 4 A.. • A -" . "'%.

/

5-10

I-
-,. il - - - - ..na . . n iiu 

RADIONUCLIDE3- RLAS AD *DAA1SPNERIC DISPERSION AND TRANSPORT

The 

istics 

first 

of 

topic of this-capter deals vith the magnitudes and timing character- release ofreadiniuclidei 

topic is the 

from the Chernobyl Unit 4 plant.I ts second atmospheric.t.dispersionh and transport of the released radionuclides

geographic 

resulting in environmental ctmination within and. outside of the Soviet boundary.

-'The 

-Agency Soviet 

(IAEA) report Experts' (USSR, 

heeting-.in 1986) prepared for, the International Atomic Energy Vienna,-Augut 25-29, 1•986,••contains - , ody.of information a large on0t& e sujcsfi s. chapter.4 4 -pared -for :the IAEA by t Further, the report pre- terns iey: Adisory Grou p(IA at 

(INSAG, 

its Post-Accident 

1986), 

Review~ -Mting in Viena, August 30-September 5, 1986

contained provides _,revi .annotati os, -and implications of the,.iafoiatibn

by the Soviet 

in the Soviet _rt, ad•• • It a1-iformation-and insight provided

INSAG report 

experts in theAugust 25-29 Vienna meeting. Preparation of the

uical experts,, 

included 

vel. 

particiption of and-inputs from a large number of tech- know :their in s.pective.4fields, ..including -from various countries the Lnited States.

The 

scribed 

Chernobyl radionuclide release and atmospheric dispersion and transport deprimarily 

in 

in 

the folloving two sections, are derived from the information contained the two reports, just cited and partly in, the U.S. interagency draft

S-.... 

report 

a fort 

that 

discssion 

was prepared before the Vienna meetins. The last-,section contains on consistecyof .the estimates of the radionuclide release

'Soviet 

provided 

boundary.--

in the Soviet report with teobserved data from regions outside the

6.1 Radionuclide Release

"When the. Chernobyl ~matrhtd~~iCoro 

was a large -structure *ere destroy4 "there reles 

with the Chernobyl afr~mU rm the Plant. 'Theý ph meaassociaited

materials 

accdet ereatly ianJ.aced by deslin features and

Sfrom 

unique to t OW..SE-IOOO reactorfiubch differ in many basic respects

clide 

those of U.S. com• rcLal -powerzractors. The Chernobyl data on radionureactors 

release 

because 

are not directly releant to the predicted releases from the U.S. of fundamental.differences 

to in release mechanisms and barriers the release to the atmosphere.'

On the basis of radiation.measurests*and-various 

.of environmental media within technical analyses of samples

experts 

a )304m some around the Chernobyl plant, Soviet estimated that a total ýof about. SOMCi of noble gases (approximately 1001 of the core inventory) and a total of about SO MCi of other radionuclides

F. 

piled 

Congel and S. Acharys of the U.S. Nuclear Regulatory Commission (NBC) com- this chapter.

6-1
(approxmaely 3-47 of;the core i'nventory e e irelese to the envi-ronment

over a, period of 10 dys (f rom April .26 to f. y 6). Throughout this period, and particularly on the-first day, the. release was -accompaied bylargeamounts f I..

eeyhicOh .elevated-the cdio ide plumto-great heights (see Section 6.2).

Abouti 20. NCi of non-no66ble Sam release. occzurred. on the first day of the Accident (April 26). The o•tal monabsegs release. as boposedof about 120.of

the cesium, i Odine, tellurium invetories and about 3-67 of the inventories

of other radionuclides in the reactor core at the time of the accident.

The core inventory of-principal radionuclides at the time of the accident,

decay-corrected to, ay 46 1986, and the percentage released are shown in

Table II of the D ISAG repot and inable 6.1. The. estimates in these

tables are generaliy consistent. ith those made by the experts from the United

States and United Kingdbm before ite Vienna meetings.

.Table 6.1 Core inten••oe.' and totalo releaces ~ tthtiiEAii oftecernob1y. accild et.....

_`_ý hialf-life 'tor + iPrcentage .

A(days) (Ci) released

K.:-r-85 39,30 089 100

Xe-i33 5.27 46 I00

4-131 8.05 35 20

Te-132 3.25 8.6 15

•Cs-134 .750 .5.1 10

Cs-137 -. 1x10 4 7.8 13

+- 2.8 -.- +-130, 2.3

Zr695 65.5 1i' 3.2

tRu-103 39.5 11 2i.9

-106.- .368 .54 2.19

Ua-2140- 12.8 78 5.6

.Ce-141, - 32.5 ,.. 119 2.3

Ce1~ 284 86 . sr-29 4.0 ..

UNp-239 W2,35 3;4 3 - 238 + 0.027 '+3.i5,i@ 3

-Pu239 C- 4810 -0.023 3 --240 1 . 410 0J032 301

-?u-241 Ca-Z4•2 4800 164•+ ++•+• +,••+"+ %.+ 4.6 0 ;-+• 7 3

3

e~eaycorete t My6, 1986, and calculate

a.s prescribed by the Soviet experts.

Sourcef: •I O, 1986, Table Xl.

*The Soviet estimates of all releases and release rates except for the noble

gasee have an uncertainty ran#e of Wt%7.

6-2
.. .... . .... ..... ..................... .. ...& t c u t e .,

The release of, radionuclides from Cbernobyl.did oo as a single acute event. ather ee Only &abouth- t Zj5,'. of releaseu i y . rook •' place durig the" firs- tdayer. of the. acident; th ~totetl ecurte s prtracted rcesoe 1-day period-. 'Tmerauurhoeitmes of airhend groun S n__ theUnion were btained and meanurements of the radioactive plume were made dependent from aircrafts.. Fruom thesýge d4ma a,i the Soviet e1xpertws constructed a time . release rate -chrt ho in Figure 6.1 (see also Table 6.2). (Note:

Figure -6..1 was provided by.ltheý8o•viet•e•perts::durin the.August .. .25-29,. 1986, meeting. Tble eVienna a6.2.i o-b-d on Table 13 of -.the Soviet report. 'Quanti- ties of radionuclides show• in•n Figure, 6.1 Ind Table 6.2 are decay-corrected to Hay 6, 1986; for example, 12.-HUi shown for April 26 when back-tracked from Nay.6 to April 26 would result-i--about-20 -Ci for the actual non-noble gas release on April 26.) The release sPon in Figure 6.1 can be categorized by four stages:

12.0

d_

ý8.0

7.0-

364

.A8 

1

4.0

0 1

DAYS

mssd.gsd .me ~y S. im.

Figure 6.1 Daily radionuclide release iatoethe atmosphere from the damaged

unit (not includI4ng oble, Saseso)

Source: Soviet experts at the Vienna meeting (USSR, 1986)

6-3
Table 6-.2 Daily release of radioactiveikubstan~ea intoteaopbee fom the 4agdui

including nble gases)*.

Days af ter ;2?uaSftity.r~eleasedýý

D]at* ac~cident (Wfi)ý

/,26 402

4/27 1 4.0

4/28; 2 3.4.

-4/29. 3 .6

4/30 A ~ . S 5 2-.0 

5/2 J, 4 4.0

5/3 7, 5'.0

5/5 9 >8 0

5/6 -10 0'

/9 1.4 ý0.01

45/23 22oexlO-r r

*The error,,0in tth release e evaluation

i;s ±50. Cotiuin oi are the

"Aosiuietric. Lipme-t error, ,the error

in measuring the"4istopic cositon

of air and soil, sapls,, AMd the error

-in aging _are area.

*the values are,ýadjusted to May 6,1986,

wit allowance . . rg dioative decay

(the Vrelaeadise on April2v6-,1986, amounted:

blyioll-toabut- MCiat the tine of release)f

So '-: USSR, 1986,, Table 4.13.

(1) The firrsptod stateisp t WUi. • lease /tiorst o.n the first day (A#ril 26) of the accident. (:hDayian0_ige--LOfre,6. 1) wfilwi.h occurred without warning., In

this stages very efergetic m cranical disOharge of dispersed radioactive

fuel took place so a sloa z ,pls ion, and fuel fragmentation in the

reactor. e ope t de vaporized f rom overheated and prob- .ably molten fuel raphosition Of iradionucl Ides in this stage of-release

corresponds o ztastet the C e dposition o fissionproduts of iO the 

fuel but enriched -in n2uClidso oaieegmt iodine, telluriumn, and

cesium.

(2) In the second stage, from:-Apr-! 2A to ay2, the release rate decreased to

a minimum value of tme-osixth! the average release rate for the first day. Soviet experts crqdi-t this -decrease-to measures 'undertaken to terminate

the burning of graphitetand filtration of radionuclides emerging from the

core. One of the measures taken wee aerial deposition of about 5000 tonnes

6-4ý

4..'
of a varietyof -e800b materials (boron carbide - 40, tondes, &doromite -t tonnes,••

c lay/s and -10 ton~nes, a~n& lead -- 406O to neS-j ,b'ee April 27; and Kay

.10 motlybt efr-il -28 aidiA ay' 2.- this- phase-, thje compoition of

the released' radioauclides was approximatgly the same as their"composItion

ik-n the fuel. : e

(3)In the third ,stage, thbe release was, charac-terized:b ai incraei

the .releas e rteý retahing adiyalue of aot0~ oa th is ay's,

release. In the initial Patof tis stage, the release was prkimarily of

vola t ile 1c4 ,monents,'-.tnd -a sequen ty the 01impostifion 0_6'fr, raiouclides in

the release aga••irsembled their comosition in the fuel (on .ay 6).

Soviet expertspas iated ths inrease in the release;ato (a) heatup of

the fuel-by residual- -,decay heat ;wich they estimate raised the fuel temperature to above , PCC and (b) possible .carbidization of uranium dioxide

(lo,), making it easier for, fission products to escape. (It is not clear

from the Soviet or -INSAG reports as to whether the graphite burning: was

still continui"n into this phase.) j -

(4),. tThe fourth (:final) stage, starting on May 6,, is. characterjized by asde * ' dereseinthe release rte to suu01 f h initial rate, and con-.

. tinuing to decrease-thereatr "'Soviet experts attribute this to special

.,.measures take roducti- of cold nitrogen on. ay 4 or:.5.: into the

reactor vaultt and ih, formation-oftmore refractory compounds of fission

products as a- "sult of their interaction with the material deposited.

Variation in the daily r*elese rate anmd coup6ositiln of :the, radionuclides released from the' damaged r.zactor are showni -Tables 6.2 and 6.3, respectively.

(Table 6.3 is based: on ,)Tabble4:-:,.14 ..of the Soviet report.)*

The distribution of fue•ldepited around -Chernobyl was4 as follows:

onsite - -•O.3.0.-5 of the core

0-20 km -.. ,21 of the! core

. beyond- 20. km '-1l-of, thi core--,

Samples of IJO2 were- found to have' been ox'idized to i30. :It is not clear., however, hetherthis conrsio ccured with lantor afterrelease t te

-Chemical.form it.helesed rof radionuclideC:Wre said to be quite variable. Ates re ini the .range of less than

1 micrometer to te•as ef 6•4'€ eers. Air and fallout amples showed the presence of "hot" particles As-iched,..prmari-y,..ainradionuclides- of one element -

such as nothing but ,cerlim or-cesium.,-Spherically-shaped hot particles consistIn& of only rut mhe e tcte itsdthe Soviet Union. Further

-characterizations of.te #physical -s&Ad c ica natureof the radionuclide release and determinations, .of-particle size-disribution of aerosols are being

und~ertaken by the. Soviet ,w*rts. 4

The magnitude,, timing, duration, and energy ýoo the radionuclide release, peculiarities in the variations in the rate of re~lease throughout the release period,

the sudden drop in release rate at the end-o *Lthe prolonged release period, and

the formation of hot particles consisting of 4ingle elements were unusual. The

6-5
Table 6.3 Itadionuclide composition of release from the damaged unit of Chernobyl Nuclear Power Station*

Activity of release (MCI)

Core activity release Nuclide** 4/26/86 :-.5/6tS 6*** up to 5/6/86 (•)

Xe-133 ;5 .S .Possibly, up to 100

Kr-85m 0.15 Possibly up to 100

Kr-85 - 0.5 Possibly up to 100

1-131 4.5 7.3 20

Te-132 4 -1.3  15

Cs-134 0.15 0.5 10

Cs-137 0.3 1 13

Ho-99 0.45 3 2.3

Zr-95 0.45 38' . 3.2

Ru-103 0.6 .2 2.9

Ru-106 0.2 " 6 2.9

Ba-140 0.5 4.3 5.6

Ce-141 0.4 -- rZ~j. 2.3

Ce-l" 0.45ý 7-2.4 2.8

Sr-89 0.25 2.2 4.0

Sr-90 0.015 0.22 4.0

Np-239 2.7 ::1.2"** * 3.2

Pu-238 e.1xlO- 1 08x10-8 3.0

Pu-239 0.1xlO-8 .+0.7x10-s 3.0

Pu-240 0.2x2O-$ lz1O-3 3.0

Pu-241 0.02 0.14 3.0

Pu-242 0.3x1O- 2z10-6 3.0

Co-242 "Z10-8 2. z1O-2 3.0

ci

*Error of estimate: *:50"; explanation Table 6.2

in footnotes to

**The data presented relate -to the -•activity of the main radionuclides -measured -.on =diometsi*, ' lyses.

***Total discharge up to Nay'6, 10 sic) -after April 26, 1986

(see the footnote belov).

****Data are presented as provided by khe Soviets. The Integral release must,o• eer, be a monoto•zically increasing function.

Source: USSR, 1986, Table 4.14.

isotopic content and character of aat~rial released are heavily skewed toward the nonvolatile radionuclides and octinides, due to fuel fragmutation as the result of the power excursion or othir mechanical release mechanisms.

Some characteristics (reactivity excursion) and conditions (presence of graphite and air& of the Chernobyl accident are not prototypical of severe accidents in

6-6
light water reactors. ietees, somez-aspects of- t6% radionuclide releas

frmCenblcan,6 heinile'rist-oo w ithn' the- context of."the existing dataý base.

and tertcl onierisfor raincie' re eae as descrie below:

(1) Erichment of the release by- volatile, c~om ent (iodine,, tellurium,

cesium):

.At elevated temperatures ,yradionuclides ocanuderýgo a condensed-to-vapor

phase change'and t vaporss are swept away from the fuel. Vaporization

release me chaisms•-ire the predominant feature of modern tools for predicting radionuclide- release. Vaporization releas, rates for radionuclides

are largely deterained by their rlative volatilities. Aerosols generated

by vaporization ate enriched, relative to the fuel, by the volatile

species such as iodine, tellurium, and cesium.

(2) Composition of -ndo Ylatile and7 actinideradionuldes in the :release.-,

-similar to tainthe fuel:

This is probably the result of some mechanical release mechanism in vhich

particles of fuel werel, eroeolized,'carrym with them the associated

Inventory, ofOridionlides- rfltheory is that fuel particles were formed

as the resulto:ff--f'xragmentation during the p,.ver excursion and that these

,were siebsequentl' entrained in air flowing through the core. -Another

is .theory thetfuel aerosols were produced as a result of oxidation of

W02 to U309~.

(3) Enhanced release rate beginning about 6 days after the accident:

Although no definitive explanation for this has been offered,.some possible expl-antiomeitherinividually or in combination are as wfIIooiw:

(a) ateral nce epostionwas toped (about May 3),,the meltigo

deposited lead and the py••riysisl of dolomite came to an end, s6 beat

losses fromthe :debris.dropped, the temperature of the debris rose,

and va.o..rizatiof- re ea:-*se icreased. •,again 

-- wtril •inabvaoiatoIo enhanced . . thei . cheical -reactions :'o in thedebris

(c) Oxidatiobincreasw from aom uni•dentified mechanism.

(4) Sudden drop in the .release rate fte o i Nay 6:

-No defini tive.explanation: for this k. been offered. Novever, three pos-

:sible -hypotheses are be -follows: .

(a) •itroSen gas injected under pressure beneath the core succeeded in

cooling the cter-and preventingýfurther oxidation reaction.

(b) During the third phase of the release, parts of the core debris reheated because of residual decay heat and may have liquefied because

of reduced heat loss through the. molten cover provided by the deposited material. The !iquefied debris relocated, eventually faling

6-7
nt pipe ver runs where it froze C cas -Into the pipe runis -may Ihave prevented- Ah quncedeisfo y f Ar 6 teiehdd",dersf o n urther release.

(c) Thbe principal Soviet explanation is: that the materials. dropped on, the ceinterc ith~ te: radionuclides to prodýuicei on- ile chlemi- cal forms.-

From theý preceding'_ dec iptos l tfpar htth rnblrles a strongly influenced byteique iemical conditions of the Chernobyl accident associated with air ingesi to the core •, graphite-fuel interactions, gvaphite interactions with the radionuclides, and accident management strategies followed by the Soviet offiials after destruction of the reactor building and the ini.' tial release in order to, :control further radionuclide release and to cool the reactor.

6.2;1 Atmospherric Dispersion and, TranO'ort

Radionuclide releasefrom Chkrobylý cluded, bes ides noble gases and volatile species, ejection ofa large quaitity'. of fuel material in the forms of fragments

mostly 

and particles Ain an assortmentof s aiizes. The- fuel fragments fell on the ground near the aste. Wt"itheu'iaelapaticles (sizes varying from less than one to tens of micrometer) li-O" fell on the :ground by gravitational settling and dry deposition processes While -being, dispersed in the air according to the prevailing meteorologica oi tions and carried by the wind heavier ' particles falling in larger percentages icloser-to-.6the site and lighter particiesltrans- porting farther out from.ihe s•te.- -,Because.of preferential depletion of heavier particulate material from thelplume, only extremely small-size particles of fuel and volatile fission products, and fission products in gaseous or vapor

tion forms 

on transported the plume over duringtkransport large to very large distances from the site. Precipita- _would haVeý-caused further depletion of par- ticulate or soluble materiaslt-.from .the plume.. .(There is. not much information on- precipitation according to the during INSAG the report,-ithere 10O-day.release 

was period in the Soviet report. However, no ihavy rainfall at the reactor site

or in Kiev over the period.April I26-!N.y 30 because, rainclouds moving toward the area were dispersed W" silver iodi4e w'*as.pr;ayed -on them from aircraft..) ,various deoiiPpoess(rvtaimlAtlndyadvt 

:resulted dqpostos in theod L in contamination. Us04t~if h eeso i

Over the 10-day _period: o'f radinomuclide .relea(e from Chernobyl the mieteorologi- cal conditions in the -.resionsIsurroundingt04 -plant were quite complex.- Varying rate and composition of -thexradionuclido release, large amounts of energy accompanying the release, and complex meteorological conditions led to very complex patterns of air .'d.4gron:dcoatamination, both within the Soviet Union and in other countries. Bowver, of cthepatter contamination were determined very quickly by means ofW evironmentalmonitoring..

The cloud which formed at the time of the accident produced- radioactive trail. on the ground in a westerly and northerly direction.depending on the meteoro- logical conditions 8overming the transport of air masses. Subsequently, for a considerable time, a stream of gaseous, volatile, and aerosol products continued to flow from the accident zone. The most intense stream was observed during

6-8
'the f-irs 2- to. 3~ day.- a fte thcz accident inD the noritherly direction vher radix-

: atifn evels' reached 17000 mR/hr' on, April, 27 arnd, 500 mR/hr on kpril 28 at diStances' 5 to 10,km f romA the reactor, it_ an altitudeý of ~200 m. The height of the

st~ream 6n April 27 ceeded`,ý_.?9-20 m in thenorthwesterl 43direction at -aot30k

from te: reactor site; the radiation -evelve at tt height was about 1, m./br.."

Diri4.g the Ifol,,owinq Aof dayýs, the height th est-rea did, Dot:exce'ed 200 to 400'm

The followingJ is a& brief- suziizary.,of-.meOteor-ologica-1 injformatio pro,._vided -in the,

Soviereprt, ichcb hovever, is. not dequate fo. dtile analyia vla

tion of p.lumedispersion and transport over large-distances to regions outside

the SoitUni on

April26

In ithe area around Chernoby.. -. -=

-- , Ground. leiel.wind wva'ivariable and-lijht2 R

At -5 ki s -'

".. .At altitudes of-700 to 1,57 k,: vand-as toward the northvest at a

speed,.of 5 to 10. st/sec.

Wind- leaving, the.-thernbl area

* Long-distance transport of air masses. in the ground layer was to

-esterly ,and" ortherly directions - radionuclides reached areas on

the frontier with Poland on April 26-27.

-At altitudes of 700 a to 1.5 ki-, wind vas to a northwesterly direction and subsequently,. turned to the north.,

Aoril 27-29:

Radionuclide transport was inthe ground layer aofair at .a eight o2.00.

•:inanrtherly .an~~dmorthioesterly direction from the station.

eteooogia codtim riag 'April26 toý 29 establishtd. the tasic Vlosecausi, fallout inbs that direction.

'Tbepeeigmtoooia cnro as described in the Soviet reporet are

consistent wth. those-in then .S. tnter-agency draft report prepared Wefore the

.,Soviet w t:became•. available-. The follo*wig information related to long•-

range trasport -of radionuclides 1.. based on theU,:.S. inter-agency draft report.

The Cherbobyl accident aMphsiues the importance of large-scale atmospheric

transport and diffusion .or major releases of radioactive- materials at various

elevations e.t"ndinsgup to I ka or higher. lteleases of material into the A

atmosphera well above the surface are generally subjLe~t to considerably different transport and diffusion conditions than release near the ground.

From available radiological monitoring information, including sampling by aircraft, after the accdent a number of relatively distinct "debris clouds"

were identified at various heigbts and locations in the atmosphere. To reach

6-9
t h•e west coast of the United, States by May 5, 1986, som fraction of the ini- tial release apparently was. injected relatively high o atmsphere, 'othe at.,

6-k klevelsmabout (or. even: higher) above the surface. The "debris clouds"

which meandered over Europe apparently were tranaported at or below about

1.5 km.

The principa. ,atmospheric transport and' diffusion-model- availablein the United

-Sta •• •to estimate regional and global dispersion following the accident was

the Atmospheric Release Advisory Capability (ARAC) model developed and used by the Lawrence Livermore Laboratory. This model was used to examine vario)us

release scenarios regarding the vertical distribution of radioactive taterial

released into the atmosphere.

The model uses the particle-i~n-cell ,.concet' ,foatwospheric-.-di~ff"ion,- ý-is aa

:!•::,•!processes: were. modeled, for. Cher~nobyl!•: Miteorological :•data :available- from

.. .selected cities throughout the world typically include hourly or 3-hourly sur- face observations (e.g., wind speed, wind direction, cloud cover, temperature,

dewpoint temperature, visibility, and precipitation). At a fewer number of

cities, upper air data whih are obtained less frequently from soundings

(radiosondes) provide d(..a on wind speed, wind direction, temperature, and

dewpoint temperature for selected elevations (e.g., constant pressure surfaces

such as 850, 700, and 500 millibars which correspond to elevations from near

the surface to over 10 ki aloit). The model considers primarily wind speed and

wind direction profiles which shcp, both horizontal and vertical tempor3l and

spatial variati 'ns with atmospheric. stability inferred from other measurements

such as vertical temperature gradient, c.oud cover, and wind speed. Analyses- - of long-range transport and diffusion are somewhat limited because of the large

distances between weather 2tations (which affe':ts spatial variations) and the

relative infrequency of upper air soundings (whi-ch affects temporal variations).

However, the analyses by the ARAC comuter code showed reasonable agreement

with known, if very limited radiological monitoring information from outside

the Soviet Union, considering the recopmized uncertainties in both the calculations and tLU measurements.

-The wet deposition -processex, vaghout and rainout, are extremely important in

examining -the consequences of the Chernobyl accident. ,Hoever, because of the

complexity of modeling these processes and the relative uncertainties associated

with modeling and very limited precipitation data, .-the effects of wet deposition can only be considered subjectively. Precipitati*n appeared to have been

light and widely scattered during the initial releases and plume traniport.

However, subsequent precipitation throughout Europt- and Asia caused widespread

ground contamination and plume depletion.

1 6.3 Consistency of Soviet Estimates of RadJonuclide Release With Observed

Data From Other Countries

It is obvious from the Soviet report that large aw ants of radioactive Luterials

in the reactor core were released into the surr..jdint environment. Much of

the radioactive materials released was carried sway in the form ot asses and

aerosols by normal air Icurrents. Radioactive materiabr were wideth' dispersed

in this manner, alth)ugh moc remained inside tL- .oviet Unioll.

6-10
In the, weeks following the Chernobyl accident,. e'ev~ted levels, of rdioactivity were deteated in air •inwetier, and food- and on th". groud in; mny European '

ýcoutntries. On tebailis of'-measured data oný radionuclide- concenitrationsi

v r.ou environmenta imedia And outdoor gas ine-si-y leesi hse cou"-

ries, any expertrOUpse to assess the magnitude of0 theChernobyl "radionuclide release efore Soviet 'the report was available. -owever, these

were very difficult attem.i., bicause information in. several areas needed for

extra-long-range plume transport ,calculations was lacking. Comprehensive

meerooial dat for, A; very. large, region boh in s~ide-,, andiouts ide ,,..the-i Soviet

Union were not available.ý,Radioniuclides .only in gaseous or fine par iculate forms were transported to ••agedistances (hundreds to thousands of kilometers),

while the larg.rr particles most- likely fell out in the closer-in regions within4

the Soviet territory..Lack of datan: mthe particle size distribution and physical and chemical properties of the released material with which to model the , fallout processes during transport added to difficulties in.evaluating the

• !: irele~ase.magni~tudes. to. fit the environmentalim.'asurementSa.tlagei:stances. . -The ificultiewee further be - .o. ... t.

ý'of te var.o.szoqput~er oe sd$fife cutrefr (iirakngo Jrthe ,changing pl ume trajectory du '~hangn eerlgclcniin pr ticularly the wind direction and vertical movement.of air mass) and (2) analyziugS the characteristicso..f.sigleDcmtiple, and vertically or-horizontally splitplume(s) developed1 frm negtiEM rel-ease-of radionuclides which-was vertically distributed at the source .up.to:large heights and lasted as, long as 10 days with widely varying release rates. -ecaue.of the difficulties posed, each

expert group used its bo.specntnr f simplifying assumptions specifically, suited to its computer code for _etimatin, the Chernobyl release magnitudes .%hich would reproduce t:iemeasukrd envio. etal data outside the Soviet Union.

Some of these estimates (which, hoveer, have large uncertainties) are shown in

Table 6.4.

Considering the large ncertai•tie•s iarth&e,:,eestimates, .:S•• errors in the Soviet

estimates shown in Table 6.3, and uncertainties in environmental measurements

: other countries, .it i.s.reasonable :t. ,conclude that estimates. provided by t.he-

:Soviet experts are consistent with-the-estimates of experts.of other countries

at least for th ore-.ltle raiomaclides. -It shonld be. noted that the less-volatile radiouclýide r pa tly ot transported beyond .the boundaries of the Soviet dti;A t-thi m 0064dere.. the volatile radiomulides. This

behavior is cowsstn with 6.tmqttte esi-volatile radiomiclides

.were transportted as ::•fafl dheswhc ws of comparatively lage . size. These larger a•rslse aparestly-depleted from the plum more*

rapidly than the aerosols form-ed-,bycodensation of volatile radionuclides.

"easurements in Sweden. indiate that most of the iodine in the- pl was in

vapor or desobable particulate -form. -.It is not clear, however, whether the

iodine was released from t4he reactor -building .1n these forms *-- whe'her it ý- s

converted to these forms ldurifn transport. ,.Although there has been no report- ing on detection of the nobleolss Lin theV envIronment associated with the

Chernobyl release, it has been generally assumed that 1002 of these inert gases

were released quite early In the accident.

6-I 1
Table 6.4 Estimates .of- percent of' core.. inventory released

base'd on measutreaentsý outside-,theo So~vtiet Union.

Radionuclide

group British*, Fre 6ncth* Can'adian*i LN(U.S)* WO(US, *

Noble gases Large - - a 100 -

Iodine 15-20 - 75 b 20(9)

Cesium 15-20 20 16 c 20(12)

Tellurium Small 7.- - 3(1)

Barium Small 0.7(0.4)

Ruthenium .. 1* 1-2 .- ... ".. -- .(07. 3)

0.01-00 Ce "006(0.62) Zr

Neptunium Small 0-.02-0,:04 - . :0.04(0.1)

0.t04(0. 1) Ce

Strontium small -16 d

*Reported in OECD/CSNI/IGECA. meeting oi June '12, 1986, Paris. :(Reference available in NRC's Public:Doctment -Room, 1717 R St., 1W, Washington, DC.)

100-200 MCi, b = 10-50 MCi 1-131, c = 1-6 HCi Cs-137, d 0.001-0.07

.j:, MCi Sr-90. Reported in L NL, 1986, Table 2.

***Maximum values for the fi-rst day1,s,.:release to fit the observations in Sweden.

Figures vithin parentheses are.• up;Azdates-reported. in OECD/CSNI/GRECA meeting, January 14-15, 1987, Par!is.(Reference.available, in MRC's Public -Document

Room, 1717 1 St., 0, Washington, D.)

S a A. .. 3 

INSAG, 1986

LLNL, 1986

USSR, 1986

International NucieaiSafety Advisory Group, "Sumary Report

on the Post-Accident-,ReUvi Meeting on the Chernobyl Accident," August 30-September,'5, 1986, GC(SPL.I)/3, IAEA, Vienna, September 24, .1986.

Lawrence LivermoreNational Lasboratory, "ARAC Response to the

Chernobyl Reactor tAccident," UCID-20834, July 1986.

USSR State Cemmittee on the Utilization of Atomic Energy, "The Accident at tbe Chernobyl Nuclear Power Plant and Its

Consequences," Information compiled for the IAEA Experts' tieeting, August 25-29, 1916, Vienna, 1986.

6-12
1CHAPTER 7:

EMERGENCY PREPAREDNESS AND RESPONSE

In this chapter is gathered and sum•arized all available information on the.

Soviet Union's emergency preparedness for potential accidents at .the Chernobyl

Nuclear Power Station; 'the Soviet response actions to the actual emergency are

reported.

.The fact-finding program in emergency preparedness and response includes:

.7 -() d..ocimenting: -both- of fsite and onste eegency, planning, andpreparedness

iýmesue that- were in pla~ceo the- Chernobyl nucleari facil-ity-and (2) g ther

whee fasileto he reacidnt'mergency-planni ng-and -preparedness acivities. Emergency response, organizations are identified and their roles de-

-scribed, where known. tiit 1he A notification system used by the Soviets

-is exained. Sovieteperien it khýe-range of protective, actions taken is

also studied, including evacuation, qheltering, use of radioprotective drugs,

and planned medical arrangements andtheir implementation during the accident.

Finally, Soviet information perti.nto.decontamination, relocation, and re.--

entry is documented, includin descriptions of the radiological monitoring

program(s) being used for -containated, -decontakinated,-and disposal areas.

The information which forms the basis for this chapter comes from the meeting

held under the auspices of the Internationai Atomic Energy. Agency (IAEA) in

-Vienna, Austria, 1from August 25-29% ,1986,f 0and .a report from the International

Safety Advisory Group (INSAG,1i986) which 'met from August 30-September 5, 1986,

also sponsored by the -IAEA.ý -.-The-'chapter is also based Qn -information from

-press clippings, monitored _$adi..brdcasts-, Soviet press..ccounts, etc. '

..Specific sources are refe••en •dr, the reliability of. some of this

-information, particuladrl a.0outs cannot be firmly established at this

time. ', , ' ~A

7.4 Emersency -Plans ~ ? *

,'Considering the informatiom-availiIble-, knowledge on the status of Soviet radio-

-logical emergency planning for.the Chernobyl power station is limited. Known

information includes "

-a Russian paper presented at anIAJZA workshop in 1980 (Dekrestnov, 1981).

reporting a general planning--framework that describes a plant location

strategy, !accident classifications, public safety measures, and an accident management organization structure for nuclear power plants

M. Sanders and V. Adler of the Federal Emergency Hanagtment Agency (FEMA)

compiled this chapter.

7-1
- the massive mobilization of resources 'd" ri the So.e'' r.spnse along wlith tht. ]BekreutnTv paper mind Russian descriptions of response; activiti~es, indicating prior planning Activities. ,ns~~ian thy it d 'te

Russian admissions tat response plans 'is they e~xi ste.d we re of limited, value to response teams arriving at Chernobyl, and substantial'ad-hoc

planning. ensued,

... . Russanregulation.for nuclear power plants currently bein translated

and reviewed. to determi the extentL to which they co-,tain eergency planning materials

In the paper presented at .ih IAEA workshop in 1980 and coauthored by an offi- cial of the USSR Ministry.of Fnergy (Bekrestnov, 1981), a strategy of locating nuclear power plants 25 to 40 km (16 to 25. pi)l from. cities is .identified. Also, accidebntr atý these:plants are. separated into.three i categoies based upon degree o;evrt adplnigof safeaty m-ea sures is ba'sed, upoqn t he mo st severe Saccideniticategories, andteripct.pSftpectisfr the.. public are itakenbased upon- xpected bo dyoses- (see -iet~iona7.77)-and incliude6the :6"follow- ing measures described in detail by rusaneports and officials in Vienna (Bekrestnov, 1981,-pp. 148 and 149): .

* temporary shelter

limited stay in thle, open a.i -decontamination vof- skin and clothing. * limited consumptiOno-.i.f contminated food

iodine prophylaxis-(KiI)

In addition, the Bekrestnov paper describes the hierarchy of an accident manage- ment organization and.information needs and responsibilities and/or authorities by. managerial level. the .At top ,.oft. the .management organization is a "coordina- tion center" involving both Sovernment authorities and plant personnel, divided into five sections and attending to.. one vof the -followinS problem areas (Bekrestnov, 1981, p. 1:,50)-::

' Constant surv -ineof the' odtos Ofwh ,prtn power plant

* .,radiatin "- %o

dosimetric inspection of theterritpry.arond .the plant And the environmental protection xone

* protection uf the population and provisional evacuation, if necessary

• modical aid for the :•population and plant ,personnel, Including iodine prophylaxis (KI).

The coordination center -appeArs to be very similar in function to the "special commission" described by the Russian deleSation in Vienna as having ordered and coordinated all protective measures and deployments of resources and personnel (INSAG, 1986, p. 79). Indeed, the delegstion emphasized the importance of a "centralized coordination center" (INSAG, 2986, p. 80).

7-2 • :4•
The remainder of this chapterin.4cludes detailied descriptions of- a massive, ,MoLilizat ion of- personnel ndeimntt- ccmpis medica transportation,ý medical

treatment, remedial action., evacuationtransportation, 'adiologic•a onitoring , .

decontamination, pubIc alering, acce ýs- control, secur relocationite

tunnelig dkcosrcin dwater ue4ll drilling". At h eylat

this would have requiredthe exis-ten.ce 6f, standard procedures, -source listings, and general plans foXr response tcivil emergencies. The Soviet IAEA

paper, for example, lists one emergency-response action as, "put plan for popu-

-lt~protection int opration (if. necessary)"I -(Bekrest n ov, ` 98 0)

In addition, it was learned4at -Viena that the Soviets have a. planned 3-.km

(1.9-mi) safety zone around' eachnuclea r power plant, and they restrict building of factories within a raius of 36to 10 km (1.9 to 6.2 mi) once a- nuclear

power plant is built. -The 30-i. (18.6n-mi) zone within which evacuation took

:place-, was. an "ad hoc" measure reksultin from the severity of the accident

(Warman',1986bp 1). a. ~ ~ ~ ~ B',- ) - I'iat ~

'twas aloidcted at ,Vienna thit. saincer 199~te vietainhsada

se -fprotective: action guide whi'-or t Asis " fo po etveactions : f ..... : : et• "•ro ec~ve" cti n ~u des. _4k ii form.;' -..e .s~s--•1or ::'•:: -protiective :•' " , • ' ,•..• t ost.•:':" , <:: ":-

including sheltering, .e.acuation, andprotective action decisionmaking.*

Similarly, the Russian deieiat'ioun alsostated art the meeting that when the response team from Moscow and other Iocations arrived at Chernobyl, it found the

response plans had only :imited valu •that- and the teamhad to resortto "ad

hoc" planning (Sandersi • .. m198). assive "scale of the accident probably was

a major factor in forcing ad- oc planning, as it was noted to have overwhelmed

local resources. This was because the release of several million- curies initially with similar though smaller releases daily was not included in Soviet preplanning (INSAG, 1986, p.-79.;.Warmn,# 1986a, p. 3). For example, a major

difficulty was that, because of the "actual situation... not all existing

ýarrangements could be appii (INSAG, 1986, p.7).

-The extent of ad hoc .emergencyplannikg for evacuation transportation during

the emergency is illust-rated n Sie news report by an interview with

Gennadiy Vasilyevich .. edov, mienlitia jor general and Ukrainian deputy minister of internal af fai-rs; be -stated:

,Wben the problemi opf UV1%Acmt g rsegahred '-Vmm - -

all. divisional Iinsp~uor adtld hi: lt Vs have all da tas

,on how many buildings'-!ud g.t .ia tere] in your respective

division. Weis obtidhsdta mmd -determined the necessary .number of motor buses,. a• wll-as , worked -out a: plan for the

evacuation. In this xrespect,.- everything must be clear and well

organized. In suck •cases.chaos ... i ermi.ssible.(Zhukovskiy,

1986].

Another Soviet news report g.azve imilar normation as follows:

The main burden of all :the difficulties connected with evacuation work was borne by.precinct inspectors in the city of

Pripyat. The speed and precision of the evacuation itself

*fMarshall Sanders, FWflA, personal coinunication, October 1986; also see Section 7.7.

7-3
depended rargely on thm .Li"tsi were- rawn up through the..................'-'

night andý for, half' othnxtdysaff ers were ass igned

dJuties, depoending. on the nu~mber of homes* and doorways, trainsorainneeds, 'were calculaited' -.:3usas were. alloc~ated: to each

sector''and weree; iven pr•edc routes, [Illesh, 1986].

The Russian delegation indicated- that". thi, -ad hoc panning• .enabled the •vacua- .

tion of 45.000 people._ from Pripyat to be6 executed vi•hin .ess..w tharee hours.,

- -but.-thtth'. ad hoc eaction of-90,00 peopl'e- fro' the remainder of a 30 km

zone around the plant ,as. much more difficult. They identified the refusal of

most of the rur.. populationto leave tir Animals behind as a maojr difficulty (Warman, 1986b, p. .2)-.,-An earlier. Soviet news report had: also identified

this difficulty, indicating that at the initial stage,, there was considerable confusion" (Gubarev, i986). "The problem was remedied through the use of military trucks (probably also ad hoc) for.+evacuating ,aboutvK 19,000 cattle-

"C (Saniders, 198,6,' pp. 2 and 4).-

Another ver.i U, a taken b eoviets, b:,iefore evacuangs. Pripya ,

may have- eithe been covered by n psiby for .n--ýplace civi defens.e) or

was an ad hoc action. This 'Ws the covering. (with: aolymer.substa•nce).,of l-and

areas along roads. to be Used _A a cuatioa routes (Sanders, 1 p. 4'). Thisni

-action was required beiause nlike many' evacuations in nuclear p*owr plant

drills and actual hazardousmterials ncidents, the Chernobyl evacuation had been preceded by a severe irelease of &hazardous substance over evacuation

routes. This route•prparati msure-demonstrated effective foresight, was unique, and was apparently- uite-successfu. Information provided at Vienna

indicated that the 5,.000 Pripyatieiacuees received an average body dose of

3.3 rem, far below Soviet., Usndards for levels. of exposure (Warman, 1986b,

p. 3).

These evacuations andtheXr prepatiions, •are described in, more detail in Sec- tion 7.4. Another important: -protective-a.action, potassium iodide (KI) distribution, is also mentioned here because Sovet officials indicated that virtually

llpeasants enthusiasticlt ly -took alt (rm n, 9a, p. 6). ýThis Im-.

plies Soviet plans were, .I-a plc o a lability and distribution.of the

TeeAre seerlindicatimn thst-,the Soi ts intend to mrove emergenc-y p

,paorednessii Yevgeni Veike6 , -"-te v•ce president of the Soviet Academy of

+Sciences, was quoted Oasaybn •ther.obyl event wvil Influence and

-affect our decision in future' t4ehnica la d administrative policies" (Post,

1996d). A top-ranking Soviet ýnclear power official, Gennadi. Veretennikov,

also was quoted as saying that ,te. vernment has issued new "operating instructions" fo:- all8its -nuAclear .4tstations a isoon as the Chernobyl accident .51

occurre,' and that the directivesc'overed unspecified "organizational measures" '

(KY Times, 1986c).

The Soviet Union also b69 programs, regulatory bodies, and regulations for nucleer safety. The supervisio,,of nul*ear power plant safety is established by I

the following regulatory agencies, vhich oversee compliance with regulations

and standards In the design, onstruction, and operation of nuclear power plants

for the functions indicated (Semenov, 1983):

7.'4
State Comittee on; ý,upervis ion- 6f Safe Operations £nk, Industry adlmn

under' the ipervisiion of•h e Council.of Minister's df UheRU••SSR ieen 

safety, ern

State Nuclear, Safety, Inspection nuclear aty

State Sanitary Inspection f the USR under the Ministry of Public

']Ialt -ra~iatio safety -i*

The primary regulator,. docmet - on:' nlesr pow er plant safety in the USSR was

issued in 1973 and is e•titld. "General Regulations To Ensure the Safety of

iNuclear Power Plants in• a•gn, iConstruction, and Operation." This document

prescribes tasks requiredi to6 ; e ensure- safety (Semenov, 1983).. Other regulatory documents are .,Pegulations for D..esign. and: Safe O o mnents for

Nclear-Po -rPwnd erPlants Test a esac Installations - and

-Nucleuai afety Regulations for Nuclear Pover Plan ht+s. ".... T ha 4 rd-Tst, q"'

toJensure nuclear sfety l phe A truc io,•dand opera ton: o nuclear'

power plants, and ti a g requir t fotr personnel involved with reactor

-operation (Seeo.:A9

The primary document for radiation safety is "Radiation Safety Standards"

(RSS-76)•. This document refleMCts ecommendations of-the International Commission on Radiolosical; Protection ( P)ad stablishesa system of dose limits

(Semenov, 1983). ',A- separate.ý cumetJk"The ealth Regulations for Design and

Operation of Nuclear.Power Plantsi in 1978, extends the application of

the basic RSS-76 document •o6msitin,.montoring, and inspection (Semenov, 1983).

.. Copies of the documentsm•etioned above-were not available for revie;..therefore,

it is not known if any-eL them tin integrated plans :for radiolooical 'eerSency response. Dose, limiti. s.= traii .-requirements -for reactor operating _personnel, which +appare tlaricluded therein, are relevant.to radiolooical

,:emergency planning anrspns sadmy hae provided guidance during the

Chernobyl accidekat. -nadtoOtebasis of a suggestion at the Vienna

metntheUlnite States haii, ebtia bcopies of the,.following Soviet- regulaoy_ documents, UbeteM:~ '

S "Wis AiU 56MI- Citrioes. an. Operation if.N1clar U Por vr + :

Plants" (SP,ý-NP ý-79;epteebr 1981L)

* Geerl orCetrl)3u-1ISue ;Safety of Nuclear power Plant Design, Constructin adOeras rt ops'-Ol-52 1972)

Th~ese d0cuments are entasae n wural ill1 be reviewed to deter'

vine the extent of- emergency-,inng co 'wthich based upon the document

titles, to expected to, be of a generic maturie.,

Out knowledgeable source, *a-nuclear engineer formerly associated with a nacl*4r

power program to a tuaropeaSim caniat country,has stated that radiological -emergency planning at commnist nuclear power plants is extensive. He olso

stated that be is convinced that planning Is; tifed in very closely with their

civil defense system and Is highly centrali.?'

*Aladar Stolmar, personal communication, July, 29,.-.1986.

7-5
However, desp ite al often foregoi ng- informationit is-ilnot deiiey

,.ýknown whether a, isite-specific overal eme rgen cy plaý n or, perhapisidvda

plans dealing with functiona suchas:'evacution and - distribution were in

place at the time of.the accident. •It was noed in thes IAEA report on the

Vienna meeting (INSAG, 1986) that the short time a vailable during the meeting prevented a detailed discussionfpreplaing cmonents. 'POMP It was suggested in. 

the report that this, tpic dicussed sbe at a future meeting, particularly with

reg-r4,d to•t•iechnica L.-aspects ndcriteria, ecountered, &problem ,and essons to,

be learned (INSAG, i986, p. 81)..One"particular p0lanning omponent of special

interest, whether it. was preplaned 4or planned and, executed on an ad hoc basis, .

is the covering of evacuation route land areas with a polymer substance. Con- .

ventional thinking about evacuations 'as precautionary measures before evacuation routes were contaminated by major releases of hazardous substances may

benefit greatly from. increased information on how-Chernobyl emergency rzsponse

'-planners and opcrational personneldeial with this problem.

7.2 m~renc Organizaition .and. facilities> r,-~'

The available information clearly indicat•es that many organizations and itunctional groups took part in the response.to-t.e emergenc a4t the Chernobyl power.... station. 'It also indica8 t•Ut pifcoiission"' ordereda and0, coordi.nate '

emergency response activities ia(INSAG, 11986, 'p. 79). Whether the coimission was

configured like the prototype' "coordination center" described in Section 7.1 ts

having five sections dealiig with .-,various .comlponents of emergency response is

not known. Also not 'known, islwhether th• commission was comprised of the apecialist team dispatched. im iatewly from 1oscow, plus the local authorities and,

plant officials they were sent to assisti (INSAG, 1986, p. 77). However, it is

known that Soviet officials-at.Vienna emphasized the importance of a centralized

"emergency I coordination centre with all the authority ::and powers to direct the

response organization. Also, .te.seneric.-functional responsibilities described

for the Chernobyl coordination center .•are..essentially the same as those delineated in the 1980 Soviet paper (see :Section .7.1) presented at an IAEA workshop

.(INEAG, 1986, p.8 -O ersnw 19914, p!.150).

-Despite the special.; c issifmt,1 Sovieti•• formation provided at Vienna and earlier Soviet tatht, ergeu-q .as response h r b

.of .adequate tqimn NWd £AciiisIma ) der~tato oftesveiyi

iof the accident byplaatýpersonnel .d Ioa ~ifficiais,. ._A Soviet- officia n

dicated that personnel deaingwAh -,theac 'mi.t did not have all the equipment. .4

they needed. Deputy-Premier- Ivaila/evs aid that "better facilities" were

needed. After the accident-....e iIcated.that,.,we have invited our designers

and machine builders here. MWe .are ,showin them what is required in such circumstances, what facilities .here .ought to be...the things that we lacked"

(Chicago Tribune, 19S6). :=Ane.amle -of the lack of special equipment cited ..

in Vienna was the absence, of hydraulic.ifterstto place firefi~hters on the

burning roofs at the plant site (INSAG,.1986, p..63).

Another factor hindering emergency.response was identified by the Soviets at

the Vienna meeting in August when they reported initial problems in accurately

reportinS the severity of the accide:.A:situation at the plant and off site

(INSAG, 1986, p. 77). The director and chief engineer of the Chernobyl power

station were both subsequently dismissed for mishandling the disaster at the,

plant. Pravda reported that they failed "to insure correct and firm leadership

in the difficult conditions of the accident and displayed irresponsibility and

7-6
inabiity toý organize. They were unablei :to gi~ve. an' semeuht of what had

happened and t tk cardinal asretorgnz efective- work of- all the'

d 'partments in 'the iiquidation oif the consequences of te iccident" (NY Times,

186g). Similarily, the chairnant,'of tli~e Soviet news agency, Vile t'in anin, has

stated;. that "he' firt reports of the pbowr planit vifer incomplete and turned

out to be incorrect (UI,. 1986;1 aso see Section 7.11). The protote emergeny 'organization set fr .in..the 1:986, Soviet paper at an lA Vorkshop (k tnov, 1981, p. 150) lists the-foll1wing ffr responsibilities hnucear power plant

offcial s:

. Cow"•are accident ittheoretical accident categories.

• Form preliminary conclusion about the category. of failure.

* Form conclusionlbDout consequences.

* Form conclusion about radiological situation in the region.

Inform authoritiesg. Pu.t. pl an for poplto poection into prto eesr)

Releva nt to-the-sate i hrbe~ sn ~p Iti ic ta ts,_ementI. .-M ay 44, SZoviet:

leader.Hikhail .Gorbachev Inca-id that in the future, greater attention will be paid to the reliability,4of eaq ip t and-".questions of discipline, ordr and

organization" aot.e•nucle.pwr-plt.(Post,--1986a). -'

The Soviets indicated that the initial protective actions were coordinated from

the Communist Party;headquarters"i:.tý "early morning hours of April 26 (Warman, 1986a, p. 5). It is not kn6wunwhetbr -the arrival of the specialist team from

Moscow resulted ina-wove to-an•ther-location.

Given the major roles played' by the specialist team sent from Moscow and the

special commission which, once established, directed the emergency .response, -dissemination of organizational and operational "lessons learned" information

by thm could prove quite, usefulfor sultch. rganizations-in other nation.

7.3 Alert and Notificatio ':syste"

Although details arelcking, 'iat•eis possible to characterize generally the alert .and notification systemt-a.t •u used,.to..espond to the Chernobylraccident., Since the -accidentv o h Lncre ih nApi6anth 'uijteil 

initial protecti. c-..e _p' cehe rg t

public, vietoffic esident -of the f feted area

until 08:00 (Varisan, -986a-,p. Noti.fi•ation was carried out by the tystem of Soviets in each ar.touse a bloik*, who also distributed potassium iodide (KI) , andl 111whowere assisated -by. youngtomunist Party members. w(Warman, 1986a, pp. 4-5; INSAG,.1986,p.77). inother source indicates that the evacuation of Pripyat was announced at 12:00., wirth buses arriving from Kiev at 14:00

to evacuate those without '-eLICleo (Was., 1936a, p. 6). In addition, it is

apparent 'hat no siren system.was used, and that .telephones could.not be relied

on because most Russians do,-not have them (iWarman, 1986a, p. 4). One Soviet

official stated that "In principle, warning Ibthods in my opinion require some

thorough study and discussion" (Varman, 1986a, p. 5).

A nuclear engineer experienced in planning at nu•1ear power plants in communist

countries stated that p-blic alerting ts primarily accomplished by a wired radio

system installed in each house or apartment near the power plant. According to

7-7
hithe door-toý-door notificationa i probablyabckpaastenuetate

respideints have takrecomened actions .*

The foregoing information- c ,scibs the alerting anden-tification of

Pripyat but does`ntidescribe when and how the other residents in the 30-km

-one were alerted' to, theý accident and 6notified that they were going to- be- evac-"

Auted. Particularly iwn, Vi of the large4size of this zone, dissemination of

this. information could prove useful to eme

planning for rural.- 1ounities. rgency planners, particuarly those

7.4 Protective Actions Taken

The available inform-ation.indicates i•t.h.Sviet protective actions during the

Chernobyl emergency consisted ofl heltering, administration of KI, evacuation, -.,;.- decontamination, and measures to prevent :adiation exosurein,. the ingestion

phy sen 7,1, cti"on: all the protective 4measures

ment-. ofiha resources anbd p n-elt weo- re ordered labA cO di and dep loywere used in main r deiin o acin - (Snes 196

pp. 2 and 4). .ý

Sheterngof the- geea-publc i Prpyt ncluding the closing o col

an inegatns aste che rotcive acinfrom the time of the acci-

.dent early on temrigfApi26hruhnoon on April 27 (SA,1'986,

ppi~. 77-78). Concurrent-ly, potassil.ioi•de (K!) tablets .were distribute'd door !•

todoor and KI was ultimtaely. consumed-?as wYell by the 45,O000residents of '~

Pripyat and some 90,O000peasants in 71 villages within 30 km (18.6 mi) of the nuclear power plant (earman, 1986, P. 6).

decision to shelter the residents OfPripyat rather than iThe to evacuate them

.on the day of the a•cident vas based -n the permissible levels of* radiation wmeasured in Pripyat, while at he s time hgh levels awere measured along9

potential evacuation routes.2 Therl-and wind4conditions associated with the

.initial releases carried •most :of !:the radioactive materials above and around :! Pripyat (Sanders, :1986 p. .3)~~.~~.~h gPriphaccident occurred at-1:24 on0

April 26, the offi thertlo o iicai uin-to shelter and stay•id rst

• occurred at night, RussinXutboritiesrei'r!• d that it Would be./mpractiea1 . to wake people nU to toell:them tosta in aiben The time fm from 02.00 to 08:00a

dwas spent in emergency.ngplanin ad btaiiand di-stributing K! tablets for1986

• issuance to individuals !at 08:00 ;(Warma, r1986a, p. 5). •

T•he Russians were-apparently veli•prepar .i fori.:large-scale distribution of lK/i!

tablet to . the general evidencedkby ipublic,•as tbe distributions described above.o

The XI was distributed to :prevent.. the accumulation of radioactive iodine in the iii

thyroid glands of members of ,the .general pUbtlic. -Thousands of measurements of

1-131 activity in the thyroids of .the exposed-population suggest that the ob- ii

served lovels were lover than those that would have been expected had this

prophylactic measure not been taken. (INSAG, 1986, p. 93). The use of kI by the

uAledar Stolwr, personal coaunication, July 29,1986.

7-8
-Pipa pulatio9n in particular vas -redited wih pris beioiecotn

(ess than 30 ra)fudin 971 of th 0 cest'ste at:one. relocation

cetr Sndr, 96,.~5.E~is: a - - important- toý note -that noserious ....center Li.. tg(andei••:i, 198gi6,ýP.ý).i• in .

side effects of KI use have been reported to. da INSAG, 1986,.p. 93; Sanders,

p.5;:WarmAn, 1984,b,ý ',3,).

.......... Subsequentdecisioins S regarding evacua tion were ,bas-ed- on. increasingradiation e vels in a surrounding lreas t. e herndbyl plant hen o, ..

radiation levels In ritpyat started rising. and it soon became apparent that the

lower intervention level for evacuatc-- and eventually the upper intervention

level could- be exceeý.e. ifthe gio.pulati•on•,rieained in their homes (INSAG,

1986, p. 78).

Evacuation-.of .Pripyat did not -comence until--.4bbut 367- hourts••8after the. accident

at Chernobyl because of this delayed increase ie ra 9..at .i.t.onleve Prip.yat - an theneed fon~rdntz h eeded 'logistical risoqrces,, 'and prepiarin,

~ vauaio rutesý.--Ad ýiýoc cuation plnhdtbjrjaed icentall

.existing arrangements" coud be applii-ed(INSAG, 1986 p." 78). Arrangements

for transportations ing up reloation centers, providing radiation monitor- ..ing and decont e f e povidpinrelacement."clothing and,

other necessitiesa, tiyp aId m ica ,ueti facilities, are some of

the things that had to,'be donein ordr -to carry out an effective evacuation.

These actions were _plannd and put intoý-place ýduring the roughlyr36 hours from

the time of the accident to th.. start of -,the evacuation (Sanders, 1986, pp. 3-4).

Time was also needed to take precautions along the evacuation, routes that had

been contaminated above. pmsible levels. This was done by using a polymer

substance to cover land, areas alongthe roads used for the e.acuation (Sanders,

1986,1p. 4).

The population of .Priyt :vas eacuated :priarily by buses obtained from Kiev

approximately.80 km '(50. i) aw•ay, s.ince there were very few private automobiles in the Chernobyl area (Vsma !g6bp.p2). At noon on April 27, permissior

was given to people d their own cles to evacuate, and the general .- evacuation of PripyA PW-M~tigant . 1:00 .44i.-oýArl.- 27 when : the buse• ariefo rApri ivedfrom ,

Kiev I(Wa rman, 196,11' p0) -TeA 000-.residenits of Pripyat- were. evacuated i

3hours (Sanders, 19S6- .).-

:IZvacuation of an 40,0 0additioa ihaitsALithin the 30-km (18.6-mi) zone

S•atsed several days-, l•ter Iatnd was met -Colc.a d until a week after the accident had occurred (War ; 19Mb, p. 1)..- Altogetr, the Soviets announced.

that 135,000 people had Lee•.evacusted from the 30-km ares (USSR, 1986, p. 38)..

A major difficulty that "as Oreporte in -carrying out this evacuation was that

many peasants refused:,to- bandon their animals, so an evacuation of animals was

ordered to convince thosepeasants to leave (Warman, 1986a, p. 6). This forced

the ad hoc planning of livestock e'acuation described in Section 7.1. Another

problem dealing with the evacuation was the fact that the evacuation route apparently coincided with. te plume centerline .for a fairly larse distance, r

sultin8 in high exposures received by some of the bum drAvers (Warmn, 1986a,

Generally, the behavior of evacuees was reported by the Soviet official who

supervised the evacuation to be exemplary. No panic was observed, although

7-9
.mbe psychological problems required overnight hiostpitaliz2ations of a few diatraught persons te calm' th down" (War man, ..1986ap 4).

Other protective, action measures- that were reported. dring te m e•rgency in

cluded decontamination (see Section 7.8) and measures taken to. prevent or minimierdiation ex-our vaheiesion pathway .11i Decontamiination activtie

have been extensive in the 30""'•- zone and other messures. have extended beyond

that zýone,_particularily to the 'city of Kiev (see, b~elowv). Another account,

... given a~t the Vienna eting, indicatesi that the primary. contaminatn of eva

uated dairy cows was surface -contamination and most animals were washed* own.. Those animals which had not been washed down or were injured dWring evacuation.

were slaughtered (Warman6, 1986a, p. 7).-In addition, intervention levels for'

1-131 in milk, 75 of.wich"is 'expor~ted f rom the area (Warman, 1986a, p. 7), as

well as leafy vegetables, were established with the object of limiting the dose

toa -child's thyroid to 30 r-ep per year.., Other, unidentified standards were

selcte 113 fr inmea, ou~yeggs, andberries. -Later, still. more

foods rwere included and.an overal•,dse. limit of 5 rem for anid.idual in the

first year was -established :(IIISAG, 1986- p.- 96). 'No specif ic--inforaion was

available on how or .to.hat extent these'limits were enforced.

,The Ukrainian Riallhifinister,,HAnatouy Y1 26manenko, appeared- on`eeiso

and told the peopleodf -Kiev- that they were in- no danger, but advised. the• to

keep children indoors,. -to w4ashtheir hair daily, to wipe the dust indoors with

.wet cloths, and to take. . ev1eral o:-ther, precautions (NY Times, 1986e).

There is concern by the.Soviets, about ,potential contamination of the groundwater

and surface water in- the area of the 'Chernobyl power station. These water supplies are being monitored and remedial work has been done. In the early stages

of the accident, as a.preventive measure, -the residents of Kiev used well water

rather than surface water. ,,The public water supply has been used subsequently, but with co.astant ping-'(Vara-,•.. 186as," p.2 of section-on specific radiological matters). :-The:highest levels-of .- 131 concentrations observed in the

:Kiev reservoir, the-source-of most ouck, i,-ere 3x10-8 curies/iliter on Pay 3, 1986 (Sanders, 1986,0"14p., 7). The -a sis alo reported that in June, con-

.struction of a series aof hydraulic ,,emgiering structures was initiated in orderltoý ' a-- ,p- .stati 

orertoprotc zthekoundwater and -surface ý,water In the ýChernobyl -pover station area f rom.cumoiaati-oW ( , 3). In-addition, a unique|-•1s nique aspect

of Soiet te emer eyospopse was the sefing of cld by' Aircraft tobrak

them up and prevent. traiinfall th• er:ai number of ,eks. after the

accident (Warman, [•1986b,-p." 2)-. -This -clou aeeding was accomplished by spraying with silver iodide (INSAG, 1986, p. ÷ .".

.Although much is now known about the protective measures taken as a result of the accident at Chernobyl, -little-i, known about the details ofthe .evacuation,

particularly the additional evacustion-of ;the l90 ,000. people after the initial

Pripyat evacu st ion (Sanders 1986, p. A •),and any spontaneous or unordered

evacuation. Also, little-is know. bout how people were advised of ingestion

pathway protective measures andhbow• food cosumption restrictions were enforced.

7.5 Radiolooical Honitorina and.ksure Contrul

_-x

•:F•

I:t•

* 4i

.:Ii

II-.

Available information indicates that radiological monitoring was conducted by

the Soviets at the Chernobyl power station st the time of the accident, and

subsequently in nearby and outlying areas affected by the radioactive releases

7-10
(-IISAG,i 9186, p.. 67-691, The Soviet working-. document, presented. in" August

- vienna(USSRr, 1986, p. 7ý35) 1Indica~testa:

at

.hen t e accident occurred, the official meteorological, radiation,

an 4pblic health monitoring egan to operate on an emergency

footing-. As soon as the scale of the accident became evident, the

-mon-i origa system was "widened-to bding in add ....ia .. roup.--of.experts an, cchnicians. In the first days after the accident.., the

monie ntorigytem began. to be extended to cover long-term problems

also41 8ng.- t organizations involved in the establishment of the

systm ethe"S•ate C ittee on Hydrometeorology and Environmental

rotectio6n, the inistries of Health of the USSR. and; of the Union

Republics, the Academies of Science of the USSR, the Ukrainian SSR

andth.e usian SSRthe StateCCo ttee on e IJtIizaton of:,.-

- -Atomic,,, Enrg nd teSteAoidustria ii teeL.

In addition, repor~ts. indicate that- dosimetry for radioilogical exposure: control

was used y-emergencyvorkers. uring the early stages of the accident response.

One-:Sovi'et pressr~s• •olyikov, 1986, -p..5)describes-the use-of dosimeters

ýby th Ieicopte~r: piloti 2

-"Everyone can determine-at any moment with -the help of an, individual

-dosimeterwhat •doseof radiation. he has received," said Major General

of•Aviation V.•Kobyakow,- meber of the Military Council and chief of

th district Air:F.o6rce Political Department. "And yet even we senior

comrades sometimes need, to have recourse to monitoring. Certain

pilots are very reluctant to report the dose received and are afraid

that it will be recognized as high and that they will be taken off.

flightsa-amdý;roved ffrom the -region. have to explain: us wilYou 

.be 1replaced.zat oncebby.another crew, a fresh one - don't' worry..."

,The use 'ofitheword-"hi•gh:"relating-to doses received, vith an associated impact

described .as-bein aken-.ff- flights and removed from the region" implies so

system 'of radiologicel. e-sure control for- these emergency workers. As-,, mentioned inSBectio ., S t. d t entitled, "Radiation Safety Standa ~ ~ ~ & ~ ytmo ~ ~ 4 .so,~tbih limits for nuler ioer -planits, abuti

7isntmoe wee rwter are w i~aads for operational wresol ras

for -emergencyx wrkers. .J•-ifo ation "as-presented at the Vienna meeting reg.,,a.rding anyraiolog8cal expose:econtrol system for emergency workers, although

,there was discussion of strict, dosimetric .monitoring of all transport and of

transferring working personnel ifrom one vehicle to another at three, surveillance

zone bcandlrits within then30-km zone (Waorsn, 1916b, p. 2).

-Whatever -the radiological .exposure control system, certain pilots were reluctaut to report -doses0* d may have received doses above those usually allowed.

This may have been alloyed because of the critical importance of their mission

to people in the region.

I

.4

A similar situation

explosion and fire.

of dosimsttry by the

their mission:

exirted for the firefighters vho responded to the initial

Another Soviet press report (Aliov, 1986) noted the use

firemesi, but also noted the extreme life-saving nature of

?

7'-11
In this menacing situation, when the fate of the power station -' ane not only the power station - was beingdecided, none of the

7 firemen faltered or gave way. They all understood clearly and ".

consciously what they were going into - by that time the dosimeter operators had already given the terrible warning -

radiation! But there was simply no other way out. They knew

what was at stake in their struggle against the fire.

The article quoted the chief of the Chernobyl fire unit, who was hospitalized

in -erious condition from radiation exposure, as saying: "We only knew one

thing, we must stay to the end. That was our duty to [the] people."-

The above examples indicate that for these critical emergency workers at .east,

the magnitude of the disaster hindered and in some cases forced abandonmeLt Of ... radiological exposure control. However, another'Soviet press report.(Zhukovsk~y,

98 6 •,p p-3)• stated .that when an operat ona, he•dq. aters waset up at the- ''

ý.F.Pr'ipyat., citymi~itia- station early i.n hiccident-. responsel ý",i: miliitiamen on .

their- way 'to the posts were additionally armed wth dosimeters, indicatingj

practice of exposure control under a less critical immediate situation.

In the period following the initial response to the explosion and fire at t

Chernobyl, available information indicates that radiological monitoring of food

and of the environment has been extensively used to determine the extent of

-radiological contamination. In Gorbachevis public address on Soviet television

cn Nay 14 (Pravda, 1986, p. 1) he stated:

Organizations of the meteorological service are constantly monitoring the radiation situation on the ground surface, on water,

and in the atmosphere. They have at their disposal ther necessary

technical systems and are using specially equipped planes, heli-

.copters, and ground monitoring stations.

Tables 7.1 and 7.2 present contamination levels of various agricultural products

and milk, respectively, which the*Soviets measured in "he aftermath of the acci-,

dent (US!,R, 198, Annex 7, pp. 58 and 60).

According to the Soviets, "it proved possible to keeppopulation exposures

within the established limits" (USSR, 1986, p. 38). As noted in Section 7.1,

che 45,000 evecuees from Pripyat received an average whole-body dose of 3.3

rem. This reflects the effectiveness of the evacuation mentioned in that section and the relatively low dose rates during the first day. Additional information provided by the Russian delegation at Vienna indicates that:

The 90,000 evacuees residing within 3 to 30 km (1.9 to 18.6 mi)

received an average whole body dose of 16 rem and an average

thyroid dote of less than 30 rem. The 24,000 person subgroup

within 3 to 15 km (1.9 to 9.3 mi) received an average dose of

43 rem. These large doses reflect the fact that many of these

persons were not evacuated until late in the first week following the accident, despite the fact that fuel fines were widely

deposited in that area (Warman, 1986b, p. 4).

The higher dostrt resulting from the longer evacuation times were also partly

due to delays c---'d by pear-ants refusing to evacuate until their cattle were

7-12
Table 7. 1 Agrclul products i hc h emte

' -radioactive contamination was fou'4nd' to.be exceeded .

Food products an.d proportions (I) hich

did not comply with regulations

~. .. " ilk &

--Heat produce . Reublic Region ,Vege.

Greens tables • erries Figi

Byelorussia Minskaya

Gcmelskaya

:Brestska.a

Hogilevskaya

:Grodhenskaya

10

20

10

20

.5 . -

30 15

510 5

10

10

3

5 '90

R"FSR" .. Tul skays

Brynakays

Kalujakays

Kurikays

Orlovakays

20

-• 130 - .

The Ukraine -.-Iievskaya .... -- 10 20 20

Source: USSR, 1986, Ainex 7z, p.-58,

Table 7.12... arison of esti-iated and actual levels j f mAilk -contanina tion by 1-.131. in Hay 198.6 in. 10.. regons,.subjected to the greatest -radio- ..-ctve contamination by Chernobyl accidenta 1 rlease prodct. ';*I/1iter

44

* . 

-4>4.,,;..

~

!~

Regions 1⁄2 upit..dlels .Actual. miasuzimests'

... Oilkays i. :0.2 1 0.02- 14

"Kievskays ,0'. 06', .7.3-

Pyarskaya , 0.00 - 3.0 0.02 - 1.3 Jitosirskaya .,03 3.3 -

Mogilevskays .02 - 2.3 0.02 - 2.0 Orlovskay& 0.02 " 2.3 0.02 " 0.8 tbernlovsksaya 002 -. 2.3 - .Tulskaya . 0.02 - 2.0 0.06 - 6.5

Cberkasskay& 0.01 - 1.5 -

Brestskaya 0.01 - 1.3 0.2 - 9.0

Source: USS~t, 1936, Annex 7, p. 60.

7-13
ý_:also evacuated (see. Sectionsý 7. 1, And- 7.4). overall, te. Soviet report presented atiea(US.1986, Ahnex, 7, p. 58) stated that: `Theý do~e` levels: of e'x- t~ernalr•gama radiation from the c'loud of effluent and rdioactiive fallout 0 for the, ýmjority Qf the populatio•n•-did notý e~xceed 25 r....." "-

Altogether, the dose t o the 135:,000 evacuees, was estimated at- 1.6x10-8 Irem: (QI.NSAG, 1986, p. 5) Ao e 135, Oýpe'rson- 000: evacuees "eamined by d•ctos,.

"nurses, and other medical .rsonnel for clinically manifest signs of acute radatonsydroenocaeswere found (INSAG,. 1986, p.-)~ozio~ andA. .eamation of injuredadcontam ed Chernobyl site workers- is described in Section 7.6.

Chapter 8 of this report provides exteive Amounts of information onadio-

, tive logicýl -information monitoring 

on 

of the. people, use 

food, and the environment. Despite the descrip- of doslimetry by emergency workers, 'little is. knowr abouti the;.Soviet,::radiological exposure costrol system for emergency• • rkers and

,. 

~whe~the:the +syst i•ded .in, keepin worker doise,, levels down. More infomation" on.ogrker ss.-ent rotat;os,. etc., Could ••ovetvaluable to radiological -emergny panners. 7-ý~-"

7.6 •,edical Treatment •

Medical . treatment by the Soviets was extensive during the response to the Chernobyl emergency. The Soviet written report and presentations in Vienna on :the medical response to the Accident were -extensive and open. Mluch of the dis- cussipnh dealt with the handling of -the 203 plant and response personnel who suf- fered acute radiation sickness :.(Sanders, 1986, p. 5). By the time of the Vienna meeting (August 25), there had been 31 fatalities and 30 persons remained hospi- talized (Warman, 1986b, p. 3). Two sources reported that two workers at the plant died imediately as a result of the accident, but not from radiation in- juries:.' One died from severe heat':burns, ý-and the other died when part of che

patients 

reactor building collapsed (WHO, 19"6.; USSR, 1986, p. 39). The majority of the suffering from acute radiatioc. sickness :had made a clinical recovery by the end of. June. The Soviets--attributed,.ýthir sucess in diagnosis and treat- ment to previously acquired experience .and recomendations made by international radiology centers (Sanders, 19864 p. 5).

The "medical -aid health s11'ect~ism V"i~1gb pla-wsinfozredof -the, accident

at about 02:00 on April 06;(USI # - liese :7, p. I). These medical persnon- Bel assisted the first 29 vict/aiivithin +the first r30 to 40 minutes, ýsending them immediately to the hospital ,,,(USSR, 1986, Annex 7, p. 1). As au indicator

Df the speed and extent of ithe emergency medical response, the Russians reported that by 06:00 on April 26, 108 people had been hospitalized and an additional ?4 were admitted during the day. .-After initial diagnosis in local or regional %ospitals, 129 patients were sent to :a.pecialized hospital in Moscow and 72 atients were sent to clincial -institutes in liev. All of these patients suf- Fered from acute radi ation sickness (Sanders, 1986, pp. 5-6). Teams of spe- :ialists, arrived withic 12 hours; these team consisted of physicists, radiol- Sgy therapists, laboratory assistants, and hematologists. Within 24 hours, .hese specialized medical team had examined some 350 persons and performed ibout 

%isting 

1000 blood analyses (USSR, 1986, Annex 7, p. I). This suggests that pre-

,xi sted. plans for medical assistance requests and ambulance support may have '. .

7-14
According to. Soviet off._c4•Ials, a detailed diagnosit8 and treatment regimen was

implented' (USSR, 1986, A4nex 7, p -170). -The victims. *re ca-t'tio.ed as having any of four degrees f cute radiation syndrome, basedon a, number of diagnostic criteria, wit t•! ourth, egree being the most severe and the fir-st

- degree being the least severe(USSR-, 1986-, Aex 7, pp. 3-4). Autopsiest vere

:performed on those who died (USSR,1-986, Annex 7, p. 7).

Acc0rding to Soviet officitls, of the 129 victims who were sent to Moscow in

.the •firsttwodays -of the accident., 8-4. w*.re diagnosed.- as isufferting•f row degrees

II-IV of acute radiation syndrome ad 27 were diagnosed as suffering. from

degree I of acute radiation syndrom (USSR, 1986, p. 39). -Of patients treated

in Kiev, 17 were diagnosed as being afflicted: with degrees II-IV, and 55 with

degree I (USSR, 1985, p..39).

Simately, 203 persons were treated for acute radiation syndrome resulting frowm-:. ga "ad beta-ray exposure .(USSR, 1986,Annex-7,, p.-!;•INS•G•1986,

p.r ,8) -rA eWpsure, resultdi eeesi un n-8vcis (INSAG,

g4iv~in bone marrow t anslnsto-3 f.tsevorcied btaytia "s. -_ i I

'ion exposures, but 12of thse did notsu e (INSAG, 1986,"p. 91).

Dr. Angelina Gus 'kova,: oe of th sia- representatives at the Vienna meeting, expressed the oiimthat_ý bonmrrwta patainvuldjiot be

expected to play a significant role :`in iany•future major accident (Warman, 1986b, p. 3). Many of the-deaths were .hastened by burns resulting from beta'

exposure (iNSAG, 1986,. 90).

An Israeli specialist, biophysiciistý Tair ,]esner, who worked with American and -1

Soviet doctors in performing: bone marr-w transplants, said that there were deLays in testing victimb' blood I which made it impossible to determine how such

radiation they had been exposed to or to find suitable donors (post, 1986b). However, another source-praised the blood..testing as a "very efficient and

adequate" method, while. recognizing that-between 48 and 55 hours is needed to

culture the blood samples (INSA,, 198,-p. 9)).

Despite what ever problems y ha veoc•cred, .a vid-- variety of treatment.s was

used. In addition-.to bone mrlants,.transfusions of platelets,--hemo- -.therapy,. adinistratio .sand infusions of concentrated . . n-.sJ• globulin "e i.eAG, b mr ..sfficisc( .twlth -W •nrow 

1986, P. 91). Intstia OWi sl a treated . ce-fuly i a :n : "'-

.ber of cases with artificia rAv..os .... "fedifg and measures to prevent bacteriesia and septicemia f ,Including in•tensive antiseptic

measures to prevent infect1 61986,.P. 91). -.-Burn therapy was used on

;patients with extensive beta -exposure, !but this was.aot generally successful

"in cases of very extensive involvement .of the teguments" (INSAG, 1986, p. 91).- - _--

ZIn addition, medical care for workers Involved in 'eliminatina the consequences I

of the accident" was provided for st -a polyclinic with four 24-hour first aid

brigades set up in Chernobyl .(USSR, 1986, Annex 7, :. 67).

Reports have also indicated that medical attention and treatment were provided

for evacuees. In order to provide medical care for those people evacuated during the first few days after the Chernobyl accidenit. 450 brigades of doctors,

7-15
nuss, assistants, h ih th ,physicists wer 'oiie nd providdwtabulances to care for evacuees.- Including rotations ba sed'o radiation conditions, 1240 physicians,' 920 nursesý-, 360ý physicians' assistants, 2720' assistants

:with s'econdary scoo edction, 720 satudnt., fom media inttts and r large~_`1 gru of4 mebr o cenii eerh institutes; prcwiided' meic care

-After beiug decontaminated, all evacuees were examined by physicians and re-,

ceived compulso _y dos etric monitorig and lboratoryblood -a i testsl. in" 

.tions were repeated where necessaryý (USSR, 1986, Annex 7, p. 67).

Evacuees with health irregularities wereshospitalized in "special sections" set

up at central regional hospitals (USSR, 1986, Annex 7, p. C7). LDr. eonid 1lyin,

head.of Moscow Hospitsl, which treated the most seriously injured, said that after the evacuation that followid the accident, 18,000 people reported to has-.

pitals for checkups (1 Tims, 1968f). 'Dr. :Ilyin, who was a member of the

Russian delegation at te Vienna meeting, indicated that evacuees were suffer- .ingfrom~headaches, coughing, respiratory trouble, -and some were itting up

od. r i that they hadh iffer•rla-i oi'i d isore r "-

8'-(,00Ad 000 [986, p. prblm." eus, - and T " hbo wede resuater .e -ase a d 9-os,,

18tflizAboton 8109h000h insdfiren, inc . udetringehlrn eha ased iigip6i.the' ifrequncs

near the 30-4m zone, h ave beenexamined(SR 1986,Annex 7, p. 67).67 so: ca

of acute radiation syntdrome -0re agnosed ang theri 3 5,0o0 evacuees from the

30u-k zone, which is consistentucit the exposthatleeposures amongthis grou did ;,ot reach the. threshold fojrclinically manifest sifgns of t~hi's disorder

(INSAG, 1986, p. 91).

The distribution of nl to the eevacues did not result in a single case of hos- pitalization, although -insuf ficient time has passed, to determine the frequency

of thyrotoxicosis which may have been induced (INSAG, 1986, p. 93). At the

same time, measurements of 1-131 activity in the thyroid glands of evacuees

suggest. that the use of El re duced the exposure levels of the thousands who

used' it below what would otherwise have been expected (INSAG, 1986,1 p. 93).

7.7 Soviet~ Guidance on-Acceptable Levels of'Public Radiation Exposure

The decision to evacuas •-was based: onpr#-ezisting intervention levels, which

are summarized in Table 7.;3, •in hich.Leveli A appears to -be a: point above which

,protective actions reýoptiý o a 5e s -the Point at whiche

evacuation is mandatoi(aap. 2). h In additioo,n•rofessor -lyin indicated during th.e ... m .l ton.Cheiin.. a.t Viennasthat the6 iet

,Union has had these protecttve actton guides* Lo place since 1969. N.e ilso indicated that they were -the. basis --forl protective action decisions regarding

sheltering, evacuation, and-ll distribution following the accident (Sanders,

1986). Protective action decisions were also based on dose projections which

were modeled daily during the accident (Warman, 1986a, p. 1).

It appears that these criteria were -followed in deciding to evacuate Pripyat.

Although the radioactive plme initially bypassed Pripyat (Warman, 1986a,

pp. 3-4), the situation chanked during the night of April 26, when radiation

levelr there reached I R/hr (INSAG, 1986, p. 78; USSR, 1986, p. 38). This

triggered the decision to evacuate Pripyat the next afternoon (INSAG, 1986,

p. 78).

7-16
-Tble 7.3 Criteria for Oaki0 :decisions for protecti o pop ula o.

: ii a t i o n o r "e . . . . . '". . ......... t...... A*.5~:ro~"">•~Protetiomeasures

External 

,radiatione " iadiation dose) rem. 25 75I and liiting the time

7 min tiopngs e tim .. e. to thyroid resulted .. prophylaxis, . of. radioactive tempo- iodine . -25 m 250 rarily shel '~~ .::t'ogInaain - .. .:•i":i. 

through inhalation . rrl h-terin8, and . .: evacuation (children,

Zntesrated specific - Ciiliter

a.ct•"ai-vity in the air:

C•'Children , 20

~Adults. 2020

-

Total 

3ood consumption of - Ci P . ' lmntn r lmt

Ing the consumption of ___ ••-contaminated food, re- .... ro• cating -dairy, attle * to uncontaminated pasMaxiaimuý contamination of ia/itr C fres milk or daily -i/do 0.0.06 O.6

food ration

Initial density of 141-11.1-3 disposition -Pci/N2 0.4 4 On ••rstures

*Level A: If a, dosage does Dot.exceed, this level, there is, no need to perform- urSent.measUreswhi:ch will temporarily .disrapt normal life of thep:L ution.

-Level b: fadoaeaceereedstis ,0ev, urgentmawstures have

fe42andecnm developments Ma i -.partic-

.be 

If 

made 

a dosage ezceeds LevelX.A but not moe. reach Level 3, decisic,ns ,hould i accordance with.a rcocrete situation and local conditions.

Source: ISavov, 1985.

7-17
7.8 Decontaminti' on

The Soviets initiated d -econt nation Activities. durig the eary stages qf

emergeincy remedilwr i 0 n decotaminaion contonua ltteCroblower, -station itself, amog evacuees, and in outlying areas ecedbal

contanination.

Evacuees were decontanated as-they arrived: at the reception 'enters to which

the evacuation " buses tk tem. Accordin toS•oviet of fiialg, 'each e.acu ..

showered and was given new.clothing. _.e old- Cloth-ng was, destroyed (Warman,

1986a, p. 6).

After the accident, three •urveillance zones were established: a special 3-km

zone, a 10-km zonezone. trict radiological monitoring of all

, 6k. rI,1 w•tz..as established in thl 9-.t1'.:transpor, ese zones as we.ll.a at decontamination points.

At ech oneboudary. wrkers transferred from one- Vehicljetathrore

his May. 14 public. address. ve t ,ievson (-Pravda:, -.1986): 'as.:foll.ows: .:•!:-: "

• Extensive, and 'long--work stil-l ..lies ahead. The level of,.radiation .. in the stations zone and on teterritory in the iediate vic-o

nity still remains hdangerous for.bm "health. The top-prority

task as of today, operations #therefore,is to deal with the

effects of the accident.., large-scale program for the decontami-

* nation of the territory of the electric power station and the

So settlement, of buildings and structures has been drawn up and-is

-being implemented. :The noecessary manpower, material,: and techni-'

cal resources have ,been concentrated for that purpose..

.Some reports shed light onu- .the implementtion of these-plan.. One source indicates that y pproximately 1000 emergency workers eloyed in5-hour shifts have

•fanned out over the plant .site beyond *d t locat he ad encapsulate the most

highly radioacti.ve d~b~ria- ?. se•:crews re applying a .decostaminattag film of .

• •unidenti fied c• s~iti•:ost theet- f •240,0.0 squa re ..yard8s a day-throughout..:. -. the territory . Other - edtpes repors

indicate that a radiationsisolatia abstance: described as "liquid glass" is

also being applied tothe ' herefor, plant.'s, omerous buildigs (wt, .1986). A

repc.-t from the WorldHidealth Or*anizeatos (l , 1986b) states that "work-to decontaminate the territory, buildings ad facilities of the power station, -s

well as the motor roads and other facilities located in the nearby terrain has

begune on a large scale wi-ththeuse ofmup-to-date materials and technical means."

Decontamination of buildings withinthe 30-km th .6-mi)es evacuation zone hsou been

proceedina. Approximately 'te half of released-ma.erisl was deposited wCAhin

this 3-k zone o, (INSAG, 198, p. 81). -According to Soviet officiaas, the contamination level of such. structures was found to fluctuate greatly (P,5.5., 1986,

Annex 3, p. 5). The sethod of decent~amlnatlc'n has b~'c: tc: spray t.Y0 1',- hogin

surfaces wi th "deccontsmaton solution" froP "auta•, dtco fI]r, . ,. at ofa 

fioi rate of 1O-c . liter/114 . As a-result, the dore rate fro tt 4- buildings

hwa redured to background levels, with the beta cntamhnrt-ve nrA greater

1i000 n eta ptrtcle/csm. asauaed owever, this t as "liqoid geass" i

7-18
,. • .. .. ....

the eart~i aloug tevla to in~ae y2 to25 ~s eesttn e!

buorial or remval ft hais: earth.

Extens'ive plans also have been made for-Aecon'taminatifti. t~he agricultural areas- within the 30-k(. evac.a 1tion zone, sAthough s'pecifict actions must aWait

more detailed data, (Uiss, 1986,"Alnex 3, p. 4). These include fixing radio- nuclides in the soil'b spraying sorbents (clayey suspension and zeolites) on ...he ~.• soi tprevent uptake by, vegetation, adding lime .-and-: mineral•fert•l•zers .... s nts following te-hrvest aad of perennial grasses and winter crops to

increase soil fertility,_ -reoving a contaminated surface layer of turf (either by mechanical means- or, after consolidation following: application of latex emulsion SKS-65 gp), and.restricting the extent of dust-producing cultivation, the uses of the crops-eat -re bing harvested, and the types of crops and

processing that will be permitted in the future. Heanwhile, "agricultural ýhar ting work--,..in, the eva cua tion zone and,, in the.,s4.ri'Ct- cnro'l zone. ý.4'is beiptg Carried ..outa•s •nrmal. inaccordance'evith.the specialVmeasuresworked out togethr with the " tat" "-ricu ural .- Progr' of' .he R and th"- Ukrainia SSRe SRMnityo elh;(SR -now 96 nnx3 p -)

The contaminated-forts are an area of concern because they act .asccumulators ,of radionuclides, `."firti t he.• .iwnmmd then in. the lit ter'• ... S. 1986, Annex

p. 5). Consequently', fie prevention measures. are being strengtheind (USSR, 1986, Annex 3, p. ..5).-1 -".T:

Although the descriptions• s- "ven aveprovide much information on Soviet decon- •tamination efforts to date,. little can be known yet about their.overall success ic achieving decontamination throughout the 30-ku area because of the ongoing nature of this work.

7.9 Site Recovery

3,

4

.;~

p 4~

~, ,~'

- - p-. ~

PA

This section addresses tbe•radiological aspects of site recovery. -It is not intended to cover the*accadentscenario,..-radionuclide release, :-reactor shutdown,

or off-site actions. '- lAs a riesiult,.essentially none of the preaccident, publi- cations describe or the literature actalresul~~~to are directly relevant. This section .ill.attempto ciostake during the crisis stage and those

•actions taken a.-fter;heao d te were secured in order.4o-parmit.: site '•.

-recover and, resart b teumacjd is

The actions taken to6 -'odite a:n• edivided into three. classes: 'initial -responses to stabilize the situation, -actio .--taken -to imobilize radioactive materials, and actions taken to relocate radioactive materials (decontaminate).

Shortly after the initial 44nlosoe6, 'several fires we.re identified within the

reactor complex. They-vere:in the cable .spaces and turbine hall of the reactor building, on the roof (70-(.1230-ftJ level) of the reactor building, and in the walls separating Unite 3 and 4. -These fires were extinguished within hours in spite of fields of extreme radiation.

From April 27 to May 10, more than 5000 tonnes of lead, sand, clay, boron, and dolomite were dropped from helicopters onto the reactor core, as the graphite moderator continued to burn. This appears to have been effective in helping to extinguish the reactor fire, in shielding the exposed core, and in providing

7-19
a. f iltering ad' cdsin pateoiut :i i iiunton for radioc tive, m r being re

leased, fr e ( R,19.6,p..27- INS. , . 186 piidioa:t• 

maerial (dispese from5thie -core)- that fl ontegound' 66erby wasinob

izdwhen the are was sryd italiud plymer material.

In early Maywokrenrdte lover spaces of Unit 4to, 4:.-%in watt: fromý

the. suppression pools.to avoid- the potetil of a, Steam explosion in theeveent 1:_

.-the core melted through. Sobon a fteirwkad, iners began tunnelling under the:

iteactor-building- to6ul--cnrt basemetaand.install a. itogen cool-ing,

system to help coolth lwer portions of te core, to prevent oxygen from

reaching it, and to freeze the-fground bneath the suppression- pool (INSAG,

1986, p. 65). This worik- as.' ;comletd in, 1. liate June. A "makeshift flat heat

exchanger" also was -bui•lt idra•he- reactor: building to help cool- the ore:

(INSAG, 1986, p.6)di. -an r~ o a : i

~The 1oý ge'r term Sovet plan ,for 'the asit ,includerdcnaintopnd-lc

in ri tive. atri (USSR, 1986,pp., -) r , tr .or 

*4vent 'radioactive-dust from speing, t1e rofo hh~ub all ad h

z<lshbul ders of-0roadsi were' sprayed- with -ft.4ioulmrzn YO(US1 Ksolutioa 6SR, '.-

1986, p. 30). :Second, th 8iteit lf vas....ide into separate zones in order

to facilitate decontai•nio*. 0). -Third, deco ition of

the site has begu, ce ad contamnate e int

from the site," decon tm n b"ildingsufaces91, removing a-, 5-- t Mo

soil layer (and its coai eri•zedstrae in a w4aste repo ory -ithe .fifth

unit), laying concrete slabs em a filling sow areas th clean soil,

and coating various concrete or skoil rea• , ith-, fil-forming compounds" (USSR,

1986, pp. 30-31). Consequently, -the "bt.a ga,-m background in the area of

the [damaged reactor) unit" ]has been reduced to 20-30 mR/hr, mainly dueto continued radiation from.the reactor Itself (USSR, 1986, p. 31).

Another source discussed the: site 'recovry-effort within the various buildins

S(INSAG, 1986, pp81.-82). to the measures -Idaddition described above, sprayIng with water, spraying cithdcont sti'n -solvents, steam eerion, -polymer

covers,and washing the s a b d) hvebeen used (INSAU, 7,986, p. 82). Aii

As a result, the dose rate .withinh units hbas ..dropped from 100-600 mR/hr to

,2-10 mR/br (INSAG, 4986,p. i82).

The damaged runclear cr. lowe-• i order to•e radiation

levels to norm and topm sftivemateria ls iR

1986, p. 31; IISAS, 1986 p. 70). inrates jeot exceeding S mR/hr at-the roof

and I mR/hr at the Wall's oftestrut&ure ree* m-being Sought (INSA, 1986, p.71).

It is not clear whether this-4s.tombeet .is .intended for penmanent disposal of

the debris and fission products in tAh damaged reactor unit ,(INSAG, 1986, p. 71).

More than ten options for carrying out the entombment were studied before the

final selection was made (INS , 1M956, p. 71). 1n addition, other areas are to

be sealed with concrete ((US8R, 1986, p. 30). Many of the concrete wells are

to be I a or more tVick (1S8, 1986, p. 31).

Much information is noa available about site recovery operations, specifically

in relation to the damaged No. 4 power unit. lovever, details are still lacking *

on the extent of contamination in the other three units and the specific problems associated with their decontamination.

7-20
7.1.0 oRelocationi •ad Reien fSite)••f 6)

Athog soe esidents have aled eentered some' pop'r'tions ''Ir"aI tat er

evacuaed, tiqn activities further 

are r'een'try finished. is bein- delaye ni oitrn and'dcnaia ,It -now appears, t reentry-to certain areas

•iw~ilbe suspended indefinitely adi thtresidents who formerly lived in these

a reas will be relocated eisewhere permanently.

-Since the radiation conditi.OS- continue to change, and various radionuclides

are resettling still being redistributed ver .parts of he. 30-ks area, the question of the population will not-b addressed Until the radiation situation

_over -the whole area has be .stabilized.& Such reentry must also await the entombment of the damaged reactori,-econtamination of the plant site, and imiobi- -- lization of the radioactivity in offsite-places where there are high levels of contmintion (USSR, 1986, p 3. -3-

on the return of the •people evacuated fo te areasvithn th.3.-. .zn

•(Sanderi, 19~86, p. 8),..Accordin Svet official.,."the radiaton conditions

will am high Continue contamination to chng lvlradiet '!sigzifc (.S .i Or 1- .1986, ars Annex :partcularly 3,,3p. 3 ,in•. rgions vith

t h • .. . .. . . • . . .:

To addresstue containationfer paccident,-the the area's within ahe 30-k zradus

of the Chernobyl power plant. has been divided into three zones: :a.m special zone of about 4-5 ka around the:plantiwhere re-entry .f the general -population is not anticipated in the near future and where activity will be restricted to that at the power station itself; a 5-to-10-ks zone where partial re-entry and some special activities may be allowed after an unspecified period of time; and a l0-to-30-km zone which .thaegeneral.lpopulation may eventually be allo6wed to

re-enter and in which agricultural work may be- resumed under a "strict program of radiological surveillance":;(-INSAG, 1986,, p.,;7,9).

As described in more detailiin:7Section 78, spcial agro-engineering and decon- tamination- procedures ;have -.been -establihed and 4 are now being implemented in order to return the 1A to••oics•.proceduresiinucle changes to the previously usedtsyste f so•qiof special materialsfOr dut %suppress ion, and modcifations :ifte havetj Andco.rcssn ehd (USSR, 1986, p.33).

Until allowed to return to .evacuated areas, ,evacuees have been resettled in surrounding areas. The Soviet report gave no inforsation about details of this relocation effort, although there are numerous -unverified accounts in the press about relocation areas and the ,construction of njew housing. 1ecause the cleanup is an ongoing project, little..Ai known' bout the timing of re-entry of former residents into selected areas /Ln the 30-km evacuation zone.

7.11 Public Education and Information 1rograms'

Given the available information, it is not known-if the Soviets have a coordina- ted public education *ad information program on radiological emergency response. As discussed in Section 7.12, the residents of Pripyat apparently had recei, L-d

some. type of emergency preparedness guidance before the accident. However, It is not known whether the "test and training" received before the accident by

I

~-A- i~

7-21
Pripyat rsidenfts (Warman.,; '198a:,

adinformtation campaign-.

p4)were.:Atcbmpanied: by; Wulcedcto

4 ) • .. ... ..... i t.i 

.. indication .,at least some #atsof the pb may ot hav

1osly educated' about the danersý of radiation was the refusal of some peasants. .inthe 30-ke zone.to ooions to es roy milk from. ",privately kept cows" after the accideni-t.' "ese'i'nstnces resulted.in the highest doses of

yaitoz recorded. amon~g evacuee.-J IAddition, .. the. refuisal-6 pofeasaints to h

evacatedunles th ir ar animals vere, also-. evacuated mAy iniae lck of

education about the dangirs .Ofq diation•among this- segment of the population.

: n.the other hand, ~virtually al..peasants enthusiastically took" -potassium . .:.odide• tablets, .whch couldindicate e~ther; public education or sily trust of

,e~vacuation offi~cials (arman 96,a, p. ) I]L T-he. Soviets have been criticized widely in the media for ieodato about-I Ai. being, slowl to.,release 4~~1 h istdy o , 1hroyacdnepcali the

• about " the accident wee icomlet an ur • out to be incorrect (UPI, 1986).

. .. :Several weeks ,-fte h cietadaoel rtczdth easi eot

ing. th.iatr" adsae htpblcaami h firs fe ,as a~fter the " S disaster "came ... ucertanty,w.ich '.... ".f~rom :w.as ca!'•used :t a.. times by .delayed •informa- ... tion about the real.sitution atthsite of the accident" .(Y. Times, 986b) .

S.Pravda also reported.tht "inthe first days, shifts in people's moods came from

wuncertaintythat was-soetIesd promotediby -telated informatione on.the ma real state

of affairs aaccdent" at the site of the (LAw Times, 1986a).

In his public address on Soviet television (Pravda, 1986), Seviet leader

..... Gorbachev said : .. ::::.!:{:_:::

The seriousness of the situation was ovious. It was necessary to

n e-aluate it urgently and omeatsntly.s;nd aM soon as we dreceived

Sreliable initial-infiomtio.-i•cct wsi- e availabe to Ithe Soviet

Padpeople and sentethio" diplomaý, ticchaels to the gover .nets of mr om

An, effecive't p h site oafmthes ateideW as. l imen, by9he8 kai)a.Hat

Minister Anatomy ,Y-ddmes o .wenh aparion Ki ev television (Yova Times,a.

1986e) after the wind fted a'dit adticn.ws dust began I blowing toard the

city. In that appea~rance,?he=;asure the;: pqpple of Kiev that they were in no danger, but advised the tor keepI •hidrenindoors, sto wash their hair daily,

to wipe the dust iwith-et indoors cloths, aid to take several other precautions.

"Similarly, there is arevijen•ce that SVi• Officialsmade efforts to combat

the many rumors which werespread. tumo.ii r, c~trol efforts included-articles in.

the Soviet press, .someof which .resulted. from interviews granted to the .press/

by Soviet government officials :for the purpoe of dispelling rumors (NY Times,

'986d and e; Post, 1986e and .f). •,.,. .. '.-

7.12 Traintno Prorrae

joreign .countries

On the basis of the available infonmation, it 6 is ot known ho, much radiolog6cal

emergency response training was receivedb before the accident by personnel

A

it

.v 4

7-22 "
affected by and involve'd'.i the emergency. icti;titiis, during 's'the Chernobyl -'

dsse. Apparently the- residents of Pripyat 6ad receiveid- some. type of ener-

,gency preparedness gudnergadu vcuat~ion before-1 te_ accident,.Th-ý.--'1% Soviet official who st Ulerviedthe eva'u'i hi tpersonel "responded ratd -

according to the test and training and mjority accepted the situation. lie 5e

also noted that, besideste'iiItnio-dozesorderly

manr (Warman, 1086.,~ ).......vllgseau na re

Similarly, although during te fire at Chernoby, many firen aid workers received high radiation exposures: when.. they,, fougbt the fire, 4t appears, these

exposures were due to iinsufficient equi t and procedures fora disaster of - -

such magnitude. Effectivepirevio•s tra n was deotrated-b the accolish-

•.ent of the primary objective obflplreVetig te.fire, which was rapi-dly aining

:i:-n strength, from spreading along thetop. of the machine hall to the adjoining.

-.•"•.',third:reactori unit (USSR,. 1986, ,p..2) i..Watern._was effectively -'.used. 'to extin- ..A I 

~ ush zhe~achine. hall ie~n.t u~h rpiebok ?ofadcb

with -foam sp aysued -n arL wih.l ble'mater~ omsry eeb

~l~vedt-have contributed'Ab, iei bion fth rsuspensio of"-raio-w

nuclides (INSAG, 1986, p. 'owever, 6"). it appears that, the.fire and: radiation

release were of such a etrm intensity tha" ee thouhhefrigts

knew the r~adiation:1was, preet e xitn prcdrsadprot ct e ip nt,

usually considered adequatew;ere . in .. fficient in this instance. In such a,

case,. exposure to- high lqevelsof-radiatinws un idble unless the firefighters retreated fm teie._- Inste .adtefrsoth mainhalof3

andaUnit 4 were localize by fie i•ghson-April 26 at,02:10 an•d'02:30,

respectively, and the fire was quen at000 (USSR, 19'6, p. 25; INSAG,

1986, p. 63).

It is also possible that the director and the chief engineer of the Chernobyl

power. station performed.poorly,.because they ad not received sufficient training.

garding the eacatioan.::.ý, telannin and . measures tak (see Section 7. 1) evolved :ad. hoc. beca-use, i t aill !existing arrangements could be applied" (INSAG, 196, .p. 7:..8). The: ability'to:-AM-rovise and, still .conduct the

.evacuation may reflectpio46i _, siblyli:for civil.defee.

Zowever, -decoutmninattonef qcssapeare4&, to be bae uoseCiaie

training. All evacuiees 'to, sa& re aid recmit newt cloi as t r .,at their destinatio k. :bl, a~t"t~hei~r et~into"s i. dble facilities uplied by

chemical detachments. of th oitNms~yfees.Te prsonnel eo

tamination was accompanied.b:y osietr au ents, ad old clothing was . .4

destroyed by the military etachments and:civilian officials administering the

%reception centers. (Warms -,1986•,.p.&6). Similarly, the notification of residents of Pripyat and distribution,'of potassium iodide tablets to them by the

system of Soviets-and ayoungCnnst mePamtbe rs was apparently efficient

(see Section 7.2) -and may reflect prior training.

There are also accounts of lack of training and experience of medical personnel

at local medkal hoapitalls (Post, 4986b and c). In tn interview with the

Soviet Litera.y. Gazette Ol. -Shchepin was quoted as saying (Post, 1986c):

7-23
Unfortunately lclly, there rea few peiltsin 'the f ie'ld of

medical railgbut the. maj ority of medical-workers are not well

p-repared in thil- regard- ýandno~t velv inft& d Tbis it one of the

... -i fig, ......... ... .. +,+ : . + -++ ..... 

serious gaps n the.tri.ig.fa n. i...ur healt . . y. tm-.

Thsrsle nmovement of heavily exposd pesons, to-, a-raditio treatment

center in Moscow, where specialists were available for diagnosis and' treatment

The foregoing infrai, i" it saeec ions;ýrseems to 'indicatefetv . perfor•-n6es of emergenc p e activities an to.reflect prior tining. , Additional informatiogn po ii"rt ea ht proved.t°bemost

" useful to the Soviets deuring hls rpons could prove quite hepful to, othr ... radiological emergencyplanners.

gandizion-l of o tJ:•} 'k, ties r '"at d toý ; m 

7. 13 S ew. O f•

Imite isenformate'o Uisavilabletr1 a Sovien t'idibl•gicA, P ree y'*plxning

at Chernobyl. The: SOviet&.ad developed a framework tor the p,00ninp and org iing30 onsitead. .of o t rcors tr mukeuar power jlant accidenta. prThey

also had established BAe3e i

prohibited new. factorydevelopmen between ;And. 140,e free- the plant.

I 1969, they hadp stablit-r f k d s e,. measuesfr roetigth pbiclving in the- iinity of nuclear ,power pl.pants.

It has been asserted, tht Soviet e1Merency planning actually is quite etenive,

tied very closely to civil, defens.• Th• evac tion of some 135,000people within 30 km (18.6 hNobf the reactor, sktort e:ý 36 hours after the accident wars pre

t bceded by sheltering of 450I00 people in Pripyatoand coverin evacuation route

land areas with a -polymer s6bstne h eauto was accomlished with a

great deil of ad hoc platr ad the requ A mobilization of enormous resources in a relatively sriut perd deaed 'ttio lggests that .prior .cldnt

fling, perhaps for civie l def ensh, provdn Atef(i.

The available infoth atio indicatest teat msargaentions and -04.oa. The s

took pa rt, in the- zsponset ree h blacdn. Asealcmission" very Simila, i dsrbdi tmrg cylana aewk..

,hindered the -- sp bwluing slc ft qupets facilities

and o ndereitination 4ofthe sb rit of the t by plant personnel sid local

officials.. Asua result Sarmnothpe, e difficities, the directoreand chief

engineer of the Chernoby power station were.#iumissed. After the plant was

stabilized, Soviet General Seacretary.. GorbacheV ýmade a, statement in which he said

that in the future, grXeateir attention will zbe~aid to the reliability of ,equipment and "questions of discipl9"ine., order, ýanw~organizstion" At- nuclear power

plants.

Initial notification in Tripyat was delayed intentionally because the accident

occurred at night and tb~kotc'tiVe 'Action wia.,to shelter io place. -when notification was given at '08. 00 that mrning (April 2t), it was by dooi-to-door

visits, using the-system of Soviets in each apartment house and block. The

notification to evacuate by lbsis at 14:00 the n0ext day (April 27,) also relliA

on radio announcements. Apparently people with private vehicles were given

7-24
phes . s .- to oe• ts :.eIvacut-e at 12:00 on" AproI il r27. Seither *-siren system nor-.- te 'ewee:u~ for~ noi cto.One Sovýiet off ki~iil suggested after the

41 4~'

xtensive protective action's based- on existing exposr citeria were taken duar-.

i".ng thenemerency.inc.fluding shelteringl, dayst ati b ,eon e o otiooumn. dide(i,.

30.ncy in Lod ii•. ....

- vcuation. y" rot.appro, .And: .easures-'4 7tprevedt o-:ias exposure.i in the

an itlhwa •Iti !Y•~i ii- Ptes yaAl'3• acom .een.;dn ,eacs~n..;.L:

...... P b I-- ., -".-

ing:"mes,.l. tesad.ere.>noes1o.tion pwi y aco ed b or-t-ondvi-.

tribution of the enerals pui. the chosen-sprotective action from

th.-eJ begini 'fothe. accident 'hcideyto Aprill26 toug 12:00ho April 27. I .hen

radatin oeef-eai iigi rpyat, a0 tOrn 0ih- 6uato oAbouti

to ensued .... Ti .. was olloed svera day laterPby evac-uatioon ofv a.end disti- i•

btio of0Xit apoial9,0eatstin 71 velscithin 30 km of

the-A "traom fevcted 4 fre tahe3d zpln one b - 4

causenpeasatsi -refused -t Ae•~~trhou effots altgenher.ll ~-Theresponsebofthea~ --- : 

ev•acuees vmst re oftea::rdib- o voiea exofficia to: be.-.: exemplary. Some e...auees,'''" .:.

were 00 o 3.cl .icystr ad maor .•el- ehment-in the .e.acn

toarwn a stie a O ts w vering -hi f leud ara s lalon r o a y .used of ati - d .. r

withb sh lim tsuapprot a ,ttlye p9 oblem thata o villar- dr ithea v.cuto

waftat thovet .dose apparetly folloedth evac-ation.rou. for a: lagd

tan-ce; as abu -rsult am.e btsdrivers'-, recive hihexours -': . "

tea reectaor.,o protect the . i. patha inclued.stt.in -i

priervestiom le•els f asra t oiegvableson

metie r edolta eggs tree, ad. y",, cA overtl j of 5 emefor an iivda

iThe firstyews -t esas ab lsed.As- reau. i.ona ur

of~ se ~~~~" ei Kiv 'atrrahe t thei norma surac wter suply or

sceply 1was bu.teecraeiwaer -e4md Uas monitored ostnlforai.

logicl contamination

11011respns woete taernoby accient people land, and ftdhv be i

torved for, radiological_*rbs e vntaiaaties. Dosimtry wats` utilie-yarec b

pwreats #Iry -- seggsa ;."Id eris ,u then -mssverl scale of theemfo accidiidenuad

Inthe firstiaeatr wasestabise.t Ancaivprescausedar reasuctneo the prt nt .. -.

seof Ke sdosietryythese rathe othan teiergy noruaIesrfac water 'supplyn fo~re a

abrodt ho-immh readings potutia ualld oemva -r~ the, Wer ace. wte~ rr

als moitoed.lsvath majoridty of the pouatf~.

suppl with vease do.leesufr acepwate was montor--cos':- for xd<2.

laIN ponwas 1661 ptersnrw -pic eopl Sax -6 hae.be 

established liitV ciowlevcný;~ - Doie irt ,.Uize-,,y

of~4i~v thel ofitdselmt. -th -ec

inKe~v, iitt som WC 'it ca'undreucane ,o ,he' i

Ofrist..ns e cuatenrov .. e short ndo ce irefgt 'a th-lnt

Abotensie medical-A tratmentia.eol I m h: o

vastonsatermedioal resp0%), theff 

1-25
.Ad ý,was supplemented by teams of s1pecialists within about 12 hours. Sm

2~xosed emergency .workers and: Chernobyl plant #ersonnel were diagnosed .and2

cattegorized into one of four degrees of acute radiation syndrome. Althoughi"

1$bone marrow transplants were .performedJ, 1.2 of thtse patients ha 'd died by., the

... , - ul 9 6 o sr c r . .. - si . ý. f om

-efid of Jul 1961 oa of 31 fatalities had been rcddasresm ling: fro

.t.. . Chernobyl accident as of the time of the Vieana meeting (August. 25"1., -.. Burns:-,-

... ,.-oused y et-irrediaion were noted asr a d ult &ron 1cee,"ia fordin

A'number of deaths. The evacuees, though' aff licted with minior medical- probleaýi;-s

were also treated. No cases of acute radiation syendrome among evacuees wereý e-.,

,prted. The use of KI by evacuees to prevent radioiodine uptakebl hi

.ithyroids d appe• ars to have reduced ther exposure levels of the thousands who: use.

tof below what would otherwise have been expected. None were hospitalized

foCrside effects from ingesting this drug.

The adeciso trea cuateowas based one pr-existing indtervention levauees.ls...hi

;iiclude6 one point above which protective actionare-optional (25-rem wholebdy dose and 25-rem-thyroid dose), "andanother level above which protective

:"._avt-ions are mandatory (75-rem whole-body dose aud 250-re. thyroid dose)....: ,

ring the accident, dose projections were modeled daily and used:to decide:

:..p~totective actions. •1 a •t.. . . ,he :, e r./

e Soviet decontamination activities began during the early stages of"the emer-

. ency response and are now taking place at te Chernobyl power station itse, 4

a.ong evacuees, aud in outlying areas that w.-re contaminated. Each evacuee

s:.. owered and was given new clothing upon arriving at a reception center.:.

.Veicles entering and then exiting the 30-ku zone were scrubbed upon leaving;.

.Various measures are being used to decontaminate buildings in this area,.and-ex

...tensive plans have been laid and are being implemented for decontaminating.th" frm!land and forests, including spraying with film to preveLt radioactive particlate resuspension, fixing radionuclides in the soil, .increasing fert.l ty,

riemving P contaminated surface turf layer, and restricting the typesof crops,

c•ultivation, avd processing that will be permitted in the future. Approxiately-

:._1000 workers are decontaminating the plant site itself, and decontamination measures have been taken in Kiev, as well. Soviet officials admit that some treas

.near the reactor may never be decontaminated..

Site recovery activities hLve included initial responses to stabilize the stt--

tion, actions taken to immobili.ýe radioactivc eaterials, and actions taken to.

dispose of radioactive materials. laitiaily, actions st the site focused on

extinguishing the radioactive fire in the reactor, including droppinp more than

5000 tonnes of lead. sc:., clay, boron, and dolomite on the reactor to shield'.

it and sufiocate I i, ct. In May, a concrete basement and nitrogen cooling

system were buLilt. .Aftcr :.pra ing buildings >n the site with polymers to preve-nt resuspens io:n e! di-.t, more permanent decontamination measures were begun .•th: iri - ::.....,,.Finally, the damaged reactor is being entombed, although

it is uW! I-;ir w-ther this is intended for permanent disposal of the radioat iVw i• is aricf ission products.

5o.tvit'-. oulfi. als have decided that re-entry cin be considered only after the

ra-!i;,tion iituation has stabilized, the damaged reactor has been entombed, the

piianrritt has been decontaminated, and radioactivity in other areas has been

1M-,o(l)1izcd. Most of those who were evacuated have been relocatod to rural

ar-eas outr-dt the 30-km evacuation zone, and new housing and harns are being

7-2(,
.constructed for them. Soviet. officials have ',tated that stable radiation con,-

ditions - re of the pre-conditionis for re-eL.y by residents.- are not expected

for I to 2 years, particularly in-highly contaminated areas. -

..It is not known whether the Soviet Union has a coordinated *ublic education and

Snf o rma tion, program- on- radioloýical- ergnyesoe.ierefusal<o pe Asnts

generally to be evacuated unless their farm animals were also evacuated, and the

-..refusal of some peasants to destroy contaminated milk from their cows, .jay Lave

resulted from lack of public education -about the dangers of radiation. H, wev._r,

virtually all peasants are reported to have "enthusiastically" taken ;.:tassium

iodide tablets. Also, the 45,000 residents of Pripyat evacuated within a 3-hour

period, which may be an indication ti it c public education program existed .prior

.to the accident. -

.The Soviets ..were widely criticized for being slow about releasing information':-

aboutt,-"the _Z CherobylV acciident,-O especially yd in the-first days ef cer -i&i-lh gen- -CyW"- The Soviets have indicated that the first reports Moscow received from plant

management were incomplete and turned out to be incorrect, causing substantial

delays in the availability of detailed, factual information which could be rel.eased to the public. The Soviet media reported public uncertainty and alarm

in the first few days after the accident because information about the situation

at the accident site was delayed.

The Ukrainian Health Minister appeared on Kiev television to provide protective

action information to city residents when the wind changed and began to blow

radioactive dust toward Kiev. Many rumors were spread and instances- were found

.where newspapers and public officials disseminated information to refute rumors.

It is not known how much radiological emergency response training was received

before the accident by personnel affected by and involved in the emergency activities during the Chernobyl disaster. Apparently, the firemen and'workers who

extinguished the fire at the-reactor were trained well and accomplished their

goal; the large number of deaths was caused by inadequate equipment and procedures. According to one Soviet official, the residents of Pripyat received some

guidance, which he referred to as "the test and training," regarding evacuation.

Similarly, the ability of evacuation planners to prepare evacuation routes and

evacuate despite the fact that existing arrangements were sometimes inapplicable

my reflect prior training, possibly for civil defense. It seems that deconLamination of evacuees was based upon specialized training, but that medical

personnel in some nearby locations were not experienced enough, especially in

medical radiology. Heavily exposed persons were therefore moved to a radiation

treatment center in Moscow where specialists were available for diagnosis and

treatment.

7.14 References

Alimov, 1986 Alimov, G., "By the Laws of Courage; Fulfilling His Duty:

After the Accident at the AES He Was One of the First To Go

Into Danger," Izvestiya, p. 3, May 10, 1986.
Bekrestnov, 1981,

Chicago

Tribune, 1986

Egavov, 1985

ERjlE 1986r

Gubarev, 1986

Illesh, 1986

INSAG, 1986

LA Times, 1986a

NY Times, 1986b

NY Times, 1986c

NY Times, 1986d

NY Times, 1986e

NY Times, 1986f

NY Times, 1986g

Bekrestnov, N. V., andý V. Ph. Xozlov, "Aspects of Accident

.Management at Nuclear Power Plants in the: USSR," in J. W.*

Lathrop (ed.), Planning for Rare Events: Nuclear Accident

Preparedness and Management, International Institute for

Applied Systems Analysis Proceedings Series, Vol. 4,

pp. 147-151, Pergamon Press, 1981.

Chicago Tribune. "Soviet Disaster Plan Lacking," May 23 .

1986.

Egavov, Yu. A. (Ed.), "Criteria for Making Urgent Decio..ons

on Measures To Protect Population in Case of Accidents at

Nuclear Power Plants," Radiation Safety and Protection of

Nuclear Power Plants - Collection of Articles Issue #9,

Moscow,, Energoatomizdat, 1985.

iEngineeringNews Record, "Chernobyl Cleanup Proceeds,"

p. 12, June 5, 1986.

Gubarev, V., and H. Odinets, "Spring of Alarm and Courage.

Our Special Correspondents Report From Kiev," Pravda,

p. 6, May 9, 1986.

Illesh, A., "The Situation Is Under Control. Izvestiya's

Special Correspondent Reports From the Chernobyl AES Area,"

Izvestiya, p. 6, Hay 7, 1986.

International Nuclear Safety Advisory Group (INSAG), "Summary Report on the Pc.:t-Accident Review Meeting on the

Chernobyl Accident," Ail- .-. 30-September 5, 1986,

GLC(SPL.I)/3, IAFA.. September 24, 1986.

Los Angeles Times, "k nvda- Information Delay Caused

-Widespread Anxiety," May 18, 1986.

New York Times, S. Schmemann, "Soviet Mobilizes a Vast

Operation To Overcome the Disaster," May 19, 1986.

New York Times, T. Shabad, "Soviet Revises Rules of Operation for Nuclear Plants After Disaster," May 25, 1986.

New York Times, "Refugees' Plight After Chernobyl Perplexes

Soviet," May 26, 1986.

New York Times, "Cloud of Chernobyl Chills a Lush Kiev

Spring," Nay 31, 1986.

New York Times, "18,600 Chernobyl Evacuees Given Checkups

at Hospitals," June 4, 1986.

New York Times, "Chernobyl Chief Ousted for Erring During

Accident," June 16, 1986.

7-28
Polyakov, 1986 Polyakov, A., "The Soviet Character.

.the Chernobyl AES," Krasnaya Zvezda,

From the Region of

p. may 9v1986.

Post, 1986a Washington Post, "Gorbachev Says 9 Died From Niiclear Acci-

- dent; Extends Soviet Test Ban," Hay 15, 1986.

Post, 1986b Washington Post, "Soviet-Doctors' Inexperience Cited,"

~Hay2O,1986.

Post, 1986c Washington Post, "Chernobyl Explosion Said To Come During

Test," Hay 22, 1986.

..Post, 1986d Washington Post, C. Bohlen, "Chernobyl Death Toll Put at 19,

Hay 27, 1986.

Ps, 86 Washington Post, "Kiev: Its" Pl~aygtroun ds V.acant,ScusO,

June 9, 1986.

Post, 1986f Washington Post, "2 Elderly Women Evaded Exodus from

Chernobyl," June 11, 1986.

Pravda, 1986 Pravda, H. S. Gorbachev's Address on Soviet Television,

p. 1, May 15, 1986.

Sanders, 1986 Sanders, Marshall, Federal Emergency Management Agency,

testimony prepared for the U.S. Senate Subcommittee on

Nuclear Regulation of the Committee on Environment and

Public Works, (hearing cancelled), September 29, 1986,

Washington, DC.

Semenov, 1983 Semenov, B. A., "Nuclear Power in the Soviet Union," IAEA

Bulletin 25:(2), 53-54, 1983.

UPI, 1986 United Press International, "Chernobyl Technicians Responded 'Haphazardly' to Disaster," p. 1, May 12, 1986.

USSR, 1986 USSR State Comittee on the Utilization of Atomic-Energy,.

"The Accident at the Chernobyl Nuclear Power Plant and Its

Consequences," Information compiled for the IAZA Experts' Meeting, August 25-29, 1986, Vienna, 1986.

Warman, 1986a Warman, Edward A., Senior Consulting Engineer, Stone and

Webster Engineering Corporation, Notes of Private Discussions With Soviet Delegates at Vienna Meeting, September 5,

1986.

Warman, 1986b Warman, Edward A., Senior Consulting Engineer, Stone and

Webster Engineering Corporation, "Facts and Observations

From the Vienna Meetings on Chernobyl, September 23, 1986.

WHO, 1986a World Health Organization, "Chernobyl Reactor Accident,

Report of a Consultation," p. 13, May 6, 1986.

7-29
WHO, 1986b

Zhukovskiy, 1986

World Health Organization, "Chernobyl Reactor Accident, Information Received ov IPblic Health Measures as of

May 29, 1986," May 29, 1986.

Zhukovskiy, V., V. Itkin, and L. A. Chernenko, "Battle Line Without a Front Line - Resolute Actions Performed by Staff

Members of Internal Affairs- Or•anisat-ion -at. the TlFst-ý Hours-,- and on the First Days of the AccidtAt at the Chernotyl AES," Kiev Radyanski Ukraina, p. 3, hay 8, 1986.

7-30
CHAPTER 8

HEAI TH AND ENVIRONMENTAL CONSEQUENCES ..

Tne accident at the Chernobyl Nuclear Power Station in the Soviet Union released (in addition to radioactive noble gases) about 50 MCi of various radionuclides into the environment. The first sections in this chapter provide

background information on the major pathways of human exposure to radiation

from the release, and on the types of health effects that could result.:, N',N.xt

.acute iradiation'.doses and effects are reported for those on -site at thetie of•

,.the aic'c'ident. Collective: dose"s"'and potential health. effe'cts-" atre thOn rxepote or etued-for:the-populations V within 30 kmoi tesiei thEuoeaipr

of •the Sovie~tUnin, in Europe, and in the United States. Finally, ther'e is a

discussion of the effett of the Chernobyl accident on agriculture in general

and on ecological systems.

8.1 Pathways of Human Exposure

Following an airborne release of radionuclides, there are several pathways that

can result in radiation exposures and doses to humans. During the passage of

a radioactive cloud, -?arby individuals can receive doses by direct irradiation

from the cloud and by inhalation of airborne radionuclides. However, except

for locations near the source, the more important exposure pathways are usually

direct exposure to gamma rays from radionuclides deposited on the ground.and

ingestion of radionuclides that enter the .food chain. Inhaled radioiodines

may, however, significantly contribute to the thyroid dose.

8.1.1 External Dose From Radionuclides Deposited on the Ground

A major route of radiation exposure from the accident is external irradiation

by gam rays from radionuclides deposited on the ground. These radionuclides

may-gradually leach into the soil. Nevertheless, sow radiation will penetrate.

the overlying soil layer and the walls of structures, irradiating people indoors

as well as outdoors. INuman intervention. (e.g., plowing agricultural land or

washing city streets) may -educe the gamm ray flux. Moreover, this flux will

naturally decrease over t-me as the deposited radionuclides are removed through

radioactive decay and weathering.

The relative contribution of each deposited radionuclide to the total activity

will vary over time, depending mostly on its radioactive half-life. This point

is illustrated for southern Finland in Table 8.1, where relative values calcu-:

lated for the total dose and the doses sccumulated for 1-day and 1-year exposure

periods are displayed. Initially, most of the radiation exposure resulted from

1-131, 1-132, and La-140; howeyer, over the long term, most of the exposure

will result from Cs-134-saTt-13s. 7.

J. Puskin, C. Nelson, N. Nelson, D., Jazes and 8. Myers of the U.S. Environment*.

Protection Agency (EPA) compiled this chapter

8-I
Table 8. 1 Radionuclide '•ntribut)ons 1t

extterrt dose "ased on spectrometiic measurements in

southern Finland on May 6-

and 7, 1986.

Relative doses .for speci:fied

p'periods following deposition

Radionucl ide

Ru- 103

1-131

Te-132, 1-!132

Cs- 134

Cs-137ý

la-140 ,La-140

Totals

1 day 1 year All time

0.02

0.14

0.35

0.17

",0'.21

1.0

1.1

1.6 1..6

52

39

99

1.1 ̧

1.6

1100

Notes: Doses have been normalized to a

total of one for the 1-day period.

and rounded to 2 significant places for the remaining perivds.

1-132 and La-140 are presumed to

be in equilibrium with Te-132 and

Ba-140, respectively.

Environmental removal rate coefficient is presumed to be 0.02 y-1.

Source: Data are from VIUK, 1986, Table 2.

.f ;J.

The only radionuclides in Table 8.I that contribute appreciably to the external

dose after the first year are Cs-134 and Cs-137. Cs-137 contributes about 84%

of the total dose even though it a :ounts for only 117 of the dose in, the day

following deposition. Since both the total activity and tf'e relative proportins of deposited radionuclides can vary substantially from one lo ation to

.another, the relationship between the total external dose and the 7ose in the

first day will vary accordingly.

After the Chernob)l accident, the geographic distribution of drposited radioactivity was highly irregilar. Both the magnitude of the release and Its composition varied over a period of days. Wind directions, which varied as a

function of height, shifted frequently, carrying part of the initial release

northward over northeastern Poland and parts of Sweden and Finland, but later

over large portions of central and soutbern Europe (se Chapter 6).

Initial estimates of the depasition pat ern of radloac ivity were calculated

using large-scale dispersion and deposi ton models (AR4,•MfSOS. and GRID)

(LIJNL, 1986; WHO, 1986b). Although some broad featurea of the dispersal of

mattrial from the accident can be deduce4 from the~e pr lminary calculationg,

\ - 2

.;"

~-1
they do not provide an; aJequate basis for estimating: population: doses. If . subsequent calculations incorporate improved deposition modeling as wel asrelease data for the ent$.re 10-day period following the accident, the results

should be more consistedt with observed patterns of dispersion and: deposition.

Exposures to radionuclides deposited in rainfall greatly augmented the exposure

due to dry deposition in some locations. On May 9, in Sweden, for example,

the; te` 6rna-l •F epit osu r rate near Givi e wa s , est ted to be: more than 300 ,/h r,

while the exposure rate in Stockholm, about 160 km away, was only about 30 fR/hr.

J. 0. Snihs (NIRP. 1986) attributes much of the variation to differences-in

deposition of radionuclides in precipitation. This wide variability in deposition between areas separated by relatively small distances occurred throughout

Europe.

'Deposition in the United States was generally .much :1ower than Ain. Europe, becaus .e o-fadditionalýdilution of ýthe cloud ,and b cause of •the action of removil

7"-processes .(radioactive decay and6deposition iAnSpassage. in As Europe, d:positioL varied substantially bet-veen,.nearby. 1s tioniTFý r -exileiTable 8.2

sumarizes some depositinn measurements rsde by the Department of Energy's

Environmental Measurements Laboratory (EML) at Chester, New Jersey, and New

York City for the period of Hay 5 through June 2 (DOE, 1986). The Chester

station is about 60 km west of the New York City station. M staff attributed

the higher deposition per unit area at the Chester station to differences, in

local meteorology (DOE, 1986).

Table 8.2 Radionuclide deposition in tvo U.S. areas

Totals for period

5/6/86 - 6/2/8E Chester, N.J. New York, N.Y.

Precipitation (m) 45.4 28.2

Deposition (pCi/n2 ).

1 12380 1-200

Cs-137 650 260

iCs-134 290 140

tu- 10.3 720 230

Source: DOC, 1986.

8.1.2 Internal Dose Yrom Radionuclides In Food

Food pathways provide an additional route for radiation exposure from deposited

radionuclides. A fraction of the deposition is directly deposited on plant

surfaces. The radionuclides may then be tranulocated from the surface to other

parts of the plant. Processes suca as washing (by rain) remove about 5% per

day of the deposited materials. F. Ads harvested shortly after the accident-

.. especially leafy vegetables or other produce subject to surfarc contanminatin -

would be expected to sbhw higher levels of contamination than produce which

would not be ready for harvesting until months later..

8-3
Radionuclides that deposit oa the ground ca- subsequently; transfer:to plants. through their root systems. The concentrations in plants as a. result f root' uptak-e are generally much lower than those which result from direct interception, but may remain significant for long periods of time. Therefore, highly

contaminated soil in locations near the, release may be unsuitable for prodbuction of food crops for many years. Only longer-lived radionuclides, such as

Cs-137, would be expected to enter the food pathway in any significant quantities as 8 result of thisprcs.

Contaminated animal feeds can contaminate-meat and dairy products. Grazing

animals can consume considerable quantities of freshly deposited radionuclides

from pasture or other feeds. Appreciable fractions of ingested iodine and

cesium are transferred to milk in dairy animals. As a result of the Che,/-.obyl

accident, milk concentrations (especially of 1-131), were high in areas where -

cows grazed on contaminated pasture. In Lapland, the concentration ýo of: esif .um

radonulids i renderseat. rose rapidly becaus~e the lichens ou which the

- -einee grze btrcet a apreciable fractio of teAepositxiig radio~

nulde~ :uL aea low-mass- per unit area of gunsufce--

Predicting the radionuclide intake that results from ingestion of contaminate..

food is subject to considerable uncertairty. S4oage time and preparation

methods can substantially affect radionuclide levels in food. Differences in

diet (a significant part of the Laplanders' food is reindeer meat, for example)

can appreciably affect the radionuclide intake for particular groups of people.

As a rule of thumb (if no protective action is taken), doses from ingestion may

be similar in magnitude to those from direct exposure. However, a credible

assessment of food pathways will require detailed data on levels of contamination and patterns of food consumption.

8.2 Health Effects

Health effects in humans may result from celluar and tissue damage caused by

ionizing radiation. As the radiation penetrates the body, energy. is deposited,

causing damage. The damage depends on the amount of radiation deposited per

unit mass (absorbed dose), the type of radiation, and the time over which the

dose is delivered. If the do&e to certain tissues is very high, the indiid, d.I

may become sick, or even die, noon after the exposure (acute effects). Even ot

much lower doses, however, health effects may manifest tiemselves many years

later (long-term effects).

8.2.1 Acute Health Effects

At high doses of ionizing radiation, many cells will be killed or functionally

compromised, possibly damaging the individual severely. The effects of such

?&mage appear rapidly, and at very high doses may include death. These health

effects are comonly referred to as nonstochastic effects. For these effects,

the incidence and severity increase with the dose of radiation received. Moreover, there are levels of exposure below which these effects are not expectjd

to occur. All of the dose response relationships noted in the material that

follows apply in cases where the radiation dose is delivered in a short period

of time, usually in much less than a day. 11 the exposure is protracted, then

the response for a given dose will usually be less severe. In addition,

medical rupportive treatment may be able to reduce the severlty of response for

exposures lower than the minimum lethal dose (WHO, 1961; NKR.B, 1983).

8-4
After total body exposures of the magnitude indicated btýlow, the following

effects would. be expected: at greater than 50 rad, radiation sickness including nausea, vomiting, weakness, etc,j With 100% incidence of radiation sickness

expected at about 200 rad; at greater than 150 rad, in addition to radiation

sickness, start of hematopoietic sy:lrome with blood and immune system problems

and some deaths within 60 days; at 300 to 500 rad, death in 50% of those exposed within 60 days; and at over "/00 rad, nearly 100% mortality is expec.ted

I• ,1983) •. Some. organs; are:. atprticular risk. For-example,",a IS-trad dose

to the testes can temporarily reduce fertility; at higher exposures, the severity and duration of reduced fertility is increased until at 300 to 700 rad,

permanent sterility may result. In the ovary, a dose of 200 to 450 rad may

cause sterility. A 0O-rad dose to the lens of the eye may cause cataracts.

A 250--rad dose to the skin may cause erythema, 700 rad - loss of hair, and more

than 2000 rad - severe dermatitis (radiation "burns"). !

.The thyroid presents a special :cagse because: i•t .-concentrates radioiodines which -.

have, been inhaled or i•ngested... Theidose fromtheseiradionuclides in the ýthyroid_- can greatly augment the1thyroiddose-received-from rxternal irrad iation and

from other internal emitters. Total doses of 200 rad may cause impaired function, but loss of function is more likely for doses greater than 3000 rad.

Complete destruction of the ihyroid requires doses of 100,000 rad or more.

8.2.2 Long-Term Effects -

Energy deposited in a cell by ionizing radiation may not immediately affect,

vital cell functions but may damage the cell's genetic material, leading to

an adverse effect eir ressed at some later time. These effects are often

referred to as stochastic effef.ts. A stochastic effect is one for which the

probability of occurrence in a person is proportional to the radiation dose,

received, but the severity is not. The p:ime example of a stochastic effect is

radiation-induced cancer. Thus, for instance, the probability of ilducing a

bone cancer is proportional to the radiation dose to the boneý, but each bone .

cancer induced is equally severe or life threatening. Cancer induction and

induction of genetic effects are the. two types of stochastic effects associated .

with radiation exposure. (Note: In discussing stochastic effects, doses are

sometimes expressed in "rem" rather than "rad" units. 'For the case of the I

Chernobyl accident, doses essentially all came from low linear energy transfer

(low-LET) radiation., i.e.,.X-rays, gama-rays, or beta particles. As a result,

these units -can be used interchangeably, as is done in this chapter.)

Cancer: hany types of cancer are known to be inducible ey ionizing radiation.

Induced leukemias and bone cancers would occur in the first 30 years after

exposure, whereas all other induced cancers could occur at any time during the

remsin4 ng lifespan following exposure after about a 10-year minimn latent

period.

The most important data for assessing the risk of inducing a cancer by radiation are derived from epideLiological studies of people exposed to radiation

from the atom bombs at Hiroshima and Nagasaki or to medical radiation. Although

the epidemiological data are extensive, inadequacies exist which limit the

accuracy of risk estimates. In particular," for staorbed doses below about

10 rad any excess risk of cancer is too small to be detected dircctly in the

expobrd populations. Therefore, at lower doses, risk estimates Tepresent extrapolations ba~ed on b-heoretical models. The choice of model for this purpose

8-5
is generally based on exPet p Judgpent, taking into account laboratory. experiments on animals as well. as studies onm cellular and subcelluiar systems.

For illustrative purposes in this- chapter, the staff used a risk factor of

2x10- 4 fatal cancers per rad of (low-LET) radiation to the whole bo6y, corresponding approximately to the linear-quadratic, relative risk model described

in the National Academy of Sciences "BEIR IiI" report (#AS, 1980). With minor

modif~istion , this. del has -recently, been adopted- by,--,two panels ofýexperts as .-

providing a reasonable central estimate of the risk from low-level radiation

(NRC, 1985a; NIH, 1985). [Others might recommend a risk factor that is up to

3 times higher or lower. Hence, if I million people were eAposed to one rad

of whole-body ,'adiation, about 70 to 600 fatal cancers would be projected.]

Genetics: If the cell damaged byradiation is a reproductive cell, i.e., ,a

cell-, giving, rise tor-ova or sperm, it may be involved in .the.-conceptýi6n.of a... .:.,

chld oei. ha a. erious, heritable disorder. . ca.s.. et hi isa stocatic,.

ef~ctthe raiatin dose, influenres the robityf curee.rthrta ti e !s i~ of-theeffeect. In estimating .enetfic efects,-only radiation exposures that occur before age 30 are considered, since that is about the average

age parents conceive their children.

Current human genetic risk estimates are extrapolated, from animal studies.

According to the BEIR III report, for every red of radiation exposure to the

parents, there is a risk of about 260 (60 to 1100) serious, heritable disorders

per million liveborn infants (NAS, 1980).. About 10% of the effects are

expected to occur in the first generation born after the exposure, the rest in

all succeeding generations. These serious, heritable disorders are genetic

disorders and traits which would cause a serious handicap at some tiie durinS

a lifetime.

Teratogenesis: There is an additional type of rzdiation effect which occurs

under certain circumstances, when the radiation injury occursa in the developing

.fetus. Whether teratogenic effects should be classified as acute or long term,

as stochastic or nonstochastic, is not yet clear. At present, the only

radiation-induced teratogenic effect that is quantified in man Is severe mental

-retardation. There is a window from the 8th te the 15th week of gestation durIng which the risk of inducing severe mental retardation is estimated to be

-hi0O- per red (Otake, 1984).. The data on which this risk estimate was based

included an elevated risk in the 1-rad to 9-rad dose group and were consistent

with a linear nonthreshbPld model.

Other types of teratogenesis have been 2bserved in aniail studies following

radiation doses as low as 5 rad, but there are no corresponding human data. Although it is suggested that teratogenesis may occur during the first trimester

of 8estctien following "*ses of 25 red or less, most human date are case reports

in which the exposure Wab 00 roentgen or more, or unknown but higb.

8.3 Radiological Effects on the Soviet Union

At the International Atomic Kneegy Agency (IAZA) Experts' Meeting in Vienna,

held August 25-29, 1986, Soviet representst Ieh presented their report (USSR,

1986) on all aspects of the accident, including information on radiation exposures and doses in different regions of the European part of the Soviet

i-tu
Uaion. The Soviet report and. add 4ionaii disu ssions which tooki pace at the

mAe.etigg have been swinarized in a repor by the International N clear SA fety

Advisory Group (INSAG,. 986). The di scussion here is largely baSed- on. these

two sources.... ..-.. : ,

8-.3.1 Acute Effects in Onsite Personnel..

After the Chernobvl accident, acute radiation effects were diagnosed in 203

~j~i~viuals- *l~osiho Vere',eithert workingi 'tea etth W .. to deal with the emergency. Twenty-nine persons, were reported to have shown .. some acute effects within the first 30 to 40 minutes after the accident, and

within the first 36 hours, acute radiation sickness was diagno sed iv 203 individuals. Estimates of radiation doses received were based- on clinical cri-

• teria, not on dosimetric data. Of 22 persons estimated to have received more

than 600 rad, 21 died: in 4 to 50 days; 1 died later. Of 23- estimated. .to have " ,received400 t'600 rad, 7 have died. All3 53 persons estimated to have:received

00t 40ad and all 105-estimated to have- received- 80'.to' 200 -rid -,have suri-i vi .At~ thi time',,:29 pe son have, d ei:,:a s a .resdltc of t~hei z.,expoue;i

: .di-ioni;diperson~died of severe burns and'another was killed when part ofl

the reactor building collapsed. Assuming that the Soviet dose estimates are

reasonably accurate, these data (above) suggest a median lethal dose above

400 rad.

AAs supportive therapy, transplantation of marrow or fetal liver cells was of

marginal utility;, fresh unpooled platelets were reported to have had significant efficacy in combating some aspepts of the acute radiation sybdroae.

8.3.2 Late Effects in the Population Near Chernobyl

A much larger population is. at risk from delayed effects of the radiation,

including cancers,-genetic mutations, and teratological effects. Apart from

workers at the reactor site, the largest doses were received by the 135,000

people who lived within 30 km of the plant. According to the Soviet report,

Sthe 45,000 inhabitants of the town of Pripyat were evacuated on April 27. The

other 90,000 were said to .be evacuated "•in the firs: few days after the acci-

,dent," but, elsewhere in the report it is asaid t-at the evacuation Look place

after 9 to 10 days, .on. May 4-5.

The Soriet report states that doses "for the vast majority of the population .did not exceed 25 rem,".although some-:people in the most contaminated areas may

have received: 30. to 0 rim. Koiever, .Table 7.2.2 in the Soviet report indicates

that some inhabitants who remained in the area for 7 days or more before being

-evacuated would have received at least 60 to 80 rem from external radiation alone.

In Table. 7.2.3, moreover, the average dose for inhabitants of the zone 3 to 7 hk

-around the plant is estimated to be 54 rem. That no acute radiation sickness

symptoms were observed despite doses in excess of. 50 rem might be explained by

the fact that doses were protracted over a period of days. O nthe other hand,

the tabulated values are once referred to as "maximum estimates," and this say

help to explain some Ipparent inconsistencies - i.e., 30 to 40 rem may be intended

as a more realistic estimate of the maximum dose received off site. The report

1dJicates thAt the dose estimates for thb evacuee: are preliminary -and that

more accurate estimates will be fortbcomin$.

8-7
The maximum collective ekternal doseý delivered to týe 135,•000b eacueesbydirect ` 

radiation from the effluent cloud. and-' by radionuclid-es deposited on the ground

wasý estimated: by the Soviet offmfici als to be 1.6xlO person-rem, an average -of.

about 12 rem per person. -On the batsis of that collective dose estimate and

assuming a cancer risk factor of about 2xlO- 4 /rerA, one would calculate about 320

excess fatal cancers in the population attributable to the external radiation

Pxposure. It would, however, be very difficult to detect this excess since,

accord ing--to ýý,the `Saviet-report -n-h Jcu~f- noma event -abu 2o'W.....

evacuees, (approximately 16,000): will normally die of cancers from other causes.

Although inhabitants of the 30-km zone were given potassuim iodide (KI) to

.minimize uptake of radioiodines, the population received appreciable doses to

the thyroid through inhalation and ingestion of 1-131 and 1-132. The uptake of

1-131 in the 30-km zone ano elsewhere along the path of the radioactive, cloud* . . - was monitored` extensie-lý(- y,,: particu•ar y in ch.ldren (100,000 childrenn were mea Isured in all Ds"es to chlre••dren are f special, concern, primarily because

thersma ler rthyroid gndc. reOeive higher doses. for a givejimintakee and j --

bica eir h. consumpti of milk, a food:in which -13, f allout is

likely to become concentrated. From the monitoring, it was estimated that most

doses to the thyroid from inhaled or ingested radioiodine were less th-n 30 rad,

although a few children may have received doses as high as 250 :red., Children

receiving more than 30 rad to the thyroid w*ere put under continuing medical

observation, but the risk of hypothyroidism appears to le negligiblt below

about a 1000-rad thyroid'dose froei 1-131 (NRC, 1985b).

If the average dose to the gland were about 30 rad, the collective thyroid dose

would be about. 4x10 6 person-rad. The number of excess cancers and benign tumors

resulting from such a dose is highly uncertain. The most serious uncertainties

relate to: (1) the possible reduced effectiveness per unit dose from 1-131 (as

compared to X-rays. and gSma-rays) in causing thyroid cancers and benign tumors

and (2) the long,-term risk to those irradiated as children.

If one adopts the thyroid risk coefficients presented in re,.ent reports published by the Nuclear Regulato-y Commission (NRC, 1985a; NRC. 1985b), and the

above ccilective thyroid dose (presumed mostly from I-131),.about 100 excess

thyroid cancers in the population, 10 of them fatal,. are estimated. This

estimate is based on an absolute; risk model and assumes that beta irradiation

of the thyroid by 1-131 has about one-third the carcinogenic potency per unit

dose as external X-rays.

In addition, a somewhat larger number of benign thyroid nodules might occur as

a result of the irradiation. Thyroid effects, being generally nonfatal, are

less important than other radiation-induced tumors in the population. However,

the excess incidence of thyroid cancers and benign nodules may be detectable since the background incidence rate is expected to be low.

Also of corcern in the evacuated population are possible radiation-induced birtL.

defects, particularly mental retardation for fetuses irradiated in the critical

8- to 15-week period of gestation. There is e'.-idence that such a fetus may have

an excess risk of about 4x0- 3 /rad of being seveely mentally retarded. and that

this risk is proportional to dose, at least down to-a few, rad. Thus, a fetus

receiving 25! rad would have perhaps a 10% risk of mental retirdatio. The rask

may, however, be significantly lower because this estimate was based on studies

8-8
of the atomic bomb survivors .t Hiroshima and Nagasaki, ho r•eceived their doses

almost instantaneously.; in contrast, the exposure in the case of the evacuated

Russian, op- ulztion was: spread out over diys.

The estimated average individual dose'to the evacuated population wasi. 12 rad;

thus, unless therapeutic abortions we.-e performed on pregnant women in the popu-

.lation,_ about 5% of all fetuses in the pcpulation who were in the critical 8-

-t~oAlS-weekperiodat the time of the accident -born untaslyl rasere 

a result. Typicaliy, a population of 135,000 in an industrialized society

would, at any time, include about 300 women in the critical period of pregnancy,

although the number would naturally vary depending on the age structure of the population and other factors. Hence, if therapeutic abortions were not performed, about 300 children who are at increased risk of being mentally retarded ..ý:because- of. their in ntero radiation exposure may be_ born to theevacuees.

8.33 xpsue athay ad oss n texpouroe-ay oito Unhe:-eacee.

Extrapolating fr om the experience wioth-tie Japadnese atomic bombu iseurivors,

approxi'matl 15i of, these children '(5%) may B&everely ,reta rded- 0saruto

thexposu of nx thin magnitudeohould bel reradil detec

'fewer than 'Rof all children in developed countrieOs) sffer rom severe

mental retardation.

8.3.3 Exposure Pathways and Doses in the European Soviet Union

The Soviet report also attempts to assess the collective radiation impact on

.the nbtIOL. -The bulk of the exposure was determinedto occur in the Europ an

part of the Soviet Union, more particularly, in the Ukrainian 5511, the

Byelorussian SER, the Moldavian SSR, and the Russian SF511. The size of the

exposed population is about 75 million people.

'As discussed above, the most important exposure pathways for thi. population

Will be.: (1 external irradiation by radionuclides deposited on ta e ground and

(2) internal irradiation by ingested radionuclides. There will the some

contribution to doses from inhaled radioactivity resuspended into the air from

the ground on which it was deposited.

-As prev.ouuly discussed, most of the lona-term dose - both interal -and

.external - will be from Cs-137 and Cs-134 -however, in the first few weeks after

ý'the accident J-431 was am important contributor.to the ingestion dose.

Immediately after the accident, intervention levels were:estwblished for the

concLntration of 1-131 in milk and milk products and in leaff vegetables.

Methods of ensuring cocqliance with these levels were introduced and enforced.

The levels were based on the principle that the dose to a thyroid of ; child

should not exceed 30 m per year. In addition, standards were set goerninga

1-131 content in meat, poultry, berries, and raw materials used for medical

purposes. Later, when the activity from radioiodine had decayed away and

Cs-137 and other longer-lived nuclides became predominant, intervention levels

were set for these radionuclides based on the principle that the effective dose

to an individ&al should not exceed 5 rem in the first year (INSAG, 1986).

gstimates of the external dose were provided in the tovietireport for various

regions of the European part of the Soviet Union, for urban and rural dwellers,

respectively. Estimated doses tended to be higher for people in rural areas

because they spend more time outdoors and eat sore locally produced food. The

8-9
17

individual doses for the year 1986 ranged from 3: Mrad to about I rad. The

collective dose to all 75 million people over the next 50 years was estimated

to be 29x0 6 person-rad, about 307 of which would be rece.ved in 1986.

The Soviet report also estimates exposure through the food pathway. For food

produced in 1986, there will be contamination from a variety of radionuclides,

espcially 1-131,. Ru-1,06,- Ce.-14.4,- C17, anMs14 ost of ti otmn

ti0n will resultiomt•• i direct deposition of airborne-radioactivity onto

vegetation.. More important _ver.the long term will be contamination of food by . ground-deposited Cs-134 and Cs-137, which are taken up from the soil by crops.

Uptake of Sr-90 from soil may also be important, but the data on Sr-90 deposition were considered to be'too scanty to draw any conclusions at this time;

hence, its contribution to the collective dose was neglected. The hig.est

doses from Cs-134 and .Cs-137 were believed to be in the Ukrainian and Byelorussian relions of Poles'ye wheýre an estimated 100 Ci of C -1047 as depo

sited (iabout 'I07.:0f *that released -n the accident). A critical consideration

in the alculaton was hat theso~i'l in this region ;is schthtpaelno plants was=expected to be enhanced 10 or even 100 times over what it would be

in other soils.

8.3.4 Collective Dose and Health Effects in the Soviet Union

The collective dose delivered through the food pathway to the population of the

Poles'ye region for a period of 70 years after the accident was estimated to

be 2.lxlO person-rad. Experts at the IAEA Review feeting questioned this

figure because previous experience in estimating collective doses fro& release

of cesium to the atmosphere (e.g., from nuclear weapons tests) suggests that

the dose via food consumption is roughly equal to that from external exposure. Furthermore, preliminary whole-body scanning measurements suggest that cesium

transfer through the food chain may be only about 10% of what was predicted for

the region. According to the U.S. attendees of the TAEA meeting, there was - general agreement among both Soviet and Western experts there that the estimated collective dose given in the report was probably too high, perhaps by

about an order of magnitude. A more definitive assessment of the collective

dose via the food patbvay must await further measurement of cesium and strontim concentrations in *oil and uptake into food throughout the contauinated

areas of the European Soviet Union, including those outside Poles'ye. The

Soviets have initiated a program for carrying out suc! measurements. A major

purpose of that program is to help formulate measures to reduce the population

dose.

The estimated effect of the Chernobyl accident on the exposed population of

75 million is, from the standpoint of potential health effects induced, quite V,

substantial. Even if thO Scviet report overestimates the dose via the food

pathway by an order of magnitude, one estimates a total collective doce of

about 5al? person-rem. Assuming a risk factor of 2z.•-2/rem, about 10,000

fatal cancers (plus a comparable number of nonfatal cancers) would be projected

over the next 70 years. fititgatin measures will reduce the collective dose,

bat final consideration of the enhanced cesium ,-ptake by ciops in the Poles'ye

region as well as inclusion of neglected sources of exposure (e.g., from contaminated crops in other parts of the European Soviet Union, from uptake of

Sr-90, and from inhalation of resuspended radioactivity) may substantially

increase the final estimate.

8-10
The radiation exposure a*y also, induce severe genetic disorders. These have beea estimatc1 to: 6ý comparable in number to the excess fatal. cancers. Although they will, be dit-ributed over alI fture generations,-roughly 10% are " .. expected to occur. in. the next geniration. Any postulated. induction of.mental .. retardation in the fetus at the very low incremental doses and dose rates caused by the fallout from Chernobyl over most of the European Soviet Union would ih'reosld. be extremely doSe-response speculative. In any case, e*ven.•s•-sing , .linear.,non-7-.. . , relationship s appicble during the sensitive -week

period, the number of such cases would be expected to .be very small compared wWt1, the number of excess fatal cancers.

Although the number of excess, fatal cancers- predicted on the basis of the" Soviet report is very large, -these will be widely distributed over a-popula- tion of 75 million people and over decadeE. ::T-he--eSoviet report indicates that S -about 9.5 -million cancer fatalities w'oulde•,predited :for t•he,,,population over,

- for 0yer.Tis is- almost a. factorof:: 'tv efiwertb~nh ' -ou-d'b at predicted. .U.S.. population .0of that-sie, b e-tig"•niý• gh- to render usdetectable

the excess due to the accident..

Corr'spondingly, the risk to- an average individual in the population owing to the accident-is relatively small. Assuiing that the original Soviet estimate

of dose received through the contaminated food pathway is high by a factor of 10, the estimated average individual dose from external and internal pathways would be about 0.67 rad, roughly equivalent to the dose received from back round radiation over a period of 7 .years.- Based again on a risk factor of 2xO-f/rad,- this dose would give an estimated lifetime risk of 0.013%, which is only about .0.1% of the stated Soviet baseline risk of fatal cancer (12-13%). Individual doses and risks could, however, oe substantially-higher for some inhabitants of the Poles'ye region whose diets, despite intervention measures taken by the

government, my still consist larg.ly of locally 3rown food.

8.-4 Radiological Effects on Europe Outside the Soviet Union

8.4.1 Exp o•sure Pathways and Doses

Beginning with the initial detection.at, the Formeark nuclear power. tsttion in Sweden on April 28, of .radiation from the release, radioactivity mas monitored A in air, on the ground, and in food throughout Europe. The quality aomd

pleteness of these data, however, vary greatly. byc.ountry.

Data 

collected 

on radiation 

by the. World 

levels 

Realth 

in Europe 

Organization 

for the period 

(HO) sad 

following 

published 

the 

in 

accident 

a series 

were 

of

V

-reports. A full amary of all the data is contained in the final report of

June 12 (WBO, 1986a). Every country in turope reported that levels of radio- -activity had increased because of the accident. Table 8.3S sumrizes the maxi- mal values reported by coustries outside the Soviet Uuion.: Included in the table are only those countrie, reporting either ground exposure levels of

100 pR/hr or 1-131 levels in milk exceeding 15,000 pci/liter. The former level

represents about 10 times normal background, the latter is the Preventive Action Guide prepared by the U.S. Food and Drug Administration, above which actions for mitigation are relom ded.

8-1I
Table 8. 3 i mum radiationb leyels: found' in Europe following/ -

accident, by country

aGround exposure rate 1-131-in cows ilIk

Country (pR/hr) (pCi/liter)

Austria 240 41,000

3450- 0Finland- -

Hungary 70,000

Italy 160,000

Poland 1000 54,000

Romania •350 78,000

Sweden 500 78,000

-Switzerland 150 50,000

Turkey 100

Uni ted Kingdom- 100 ...... 1 ,00

West Grmany. 20-4:,0

Yugloslavia 150

Source: WHO, 1986a.

More than 10 European nations have produced reports on the effects of the

Chernobyl accident. Among the more detaile,. reports which-attempt to assess

collective long-term doses in their respective countries were- those published

by Italy (ENEA-DISP, 1986), Finland (STUK, 1986), and Sweden (NIRP, 1986).

.-The 'Iltalian report focuses chiefly on the food pathway; other exposure routes

(including direct irradiation by ground-deposited radioactivity) were judged

less important ,in Italy. Taking into account the measures adopted to restrict

the consumption of contaminated food, it was estimated that the committed

population thyroid dose up to the 25th of May wasel0) person-rad. It was fur-

.ther estimated that the dose would have been 3.5 times higher had the restrictive measures not been taken. The collective committed effective dose for the Ssameperiod was estimated to be about 5xlO Sperson-rem, or about 10 arm per

person. Inclusion of the projected dose over all future time would incresse

this figure by less than a factor of 2.,

Average individual doses estimated for Finlpnd and Sweden Were considerably .higher than for Italy. The Finnish report projects am average dose from external irradiation of 20 mrea and 160 mrmn for I year and over all time,

respectively. -The average internal wbole-body equivalent dose was estimated to

be 50 arem for the first year; the total internal dose over all time vas not

calculated. Thus, the averase committed dose o aerall time was estimated to be

210 mrea plus any additional committed Internal dose from ingestion of radio-,

auclides after the first year. ?be Swedish report estimated a collective dose

of 300,000 person-te and an average dose of 40 armn for the -first year, primarily from external and internal irradiation by Ca-134 and Cs-137. The maximum individsial doses are expected to be about an order of magnitude higher.

Estimates of lifetime doses attributable to the accident have also be"n reported

(GSF, 1986) for the area around Winich, West Germany, where signiticant deposition of ra-4oactivity in rain occurred. The average internal dose was estimated

8-12
to be 100 to 250 mrem and 50ý to 200 mrem, for children, and, adults, respectiv•ely.

Comparable ranges for external irradiation by ground-deposited cesium are 200:

to 300 mrem and 100 to 200 mrem, respecti"'ely.

8.4.2 Estimation of Collective Dose and Health Effects

The assessýme~nt,"ýý'611b-of bth ýshot a g ter c ecti-ve external doses',equfres . a population-weighted sum of doses from ground-deposited, radionuclides over

geographical areas. Consequently, the deposition pattern for Europe had to

first be determined. Efforts to, do this began soon after the accident, but

everything published so far in this regard has been very preliminary (for example, see WHO, 1986b). Estimates of the deposition pattern should ultimately

be based on both measurements of local deposition and computer models describing

atmosphericdispersion of material from the source and incorporating detailed

weather -information,.' ,-Ith is importantt-::that the areal grid used todefinethe- .. depositionbe fineenough to relect-the significant variations•.iresl:t-ing •fro ..,..-, widand-rainfl pat.rs Aspeviously noted, because o

tion, deposition often varied enormously over rather small distances.

The assessment of long-term doses received through the food pathwaywill require data on deposit4z-%u, particularly of Cs-137, and on agricultural patterns,

coupled through a emvsti tical model to estimate contamination levels in food.

Assessment of doses received through ingestion of radionuclides in the first

few weeks after the accident (primarily from 1-131) will probably also have to

rely heavily on model calculations. These calculations should, where possible,

be compared against measured levels of radionuclides in -0ood and in people.

Estimates based on model calculations or measurements of food must be adjusted,

moreover, to take into- account any mitigative measures that decreased human

intake.

Until the analysis outlined above is completed, any estimates of collective

dose outside the Soviet Union must be regarded as highly tentati". - rough

approximation can be made, however, by projecting from locations 'here a fairly

complete analysis has been performed. For example, in the area around Munich,

the average dose over all time wa estimated in the GSF report to be a few

hundred millirem, about 20% of which would be received in the first year.

The GSF report alsc indicated that the maximsm level of radiation recorded

there was about 100 p1/hr. An examination of the World Health Organization

final report (WHO, 19566) would seem to indicate that (excluding Spain. Portugal, Ireland, England, Denmark, and most of France, all of which received very little of the fallout) bhe maxium levels of outdoor exposure recorded at most

locations in Europe were generally in the range 10 to 1000 pR/hr above background ievels. Most locations would seem to fall in the lower portion of this

range (10 to 40 pl/hr), although higher readings were fairly widespread (see

Table 8.3). On the basi's of an examination of the data, it is estimated that

the maximum exposure from the fallout, averaged over all locations in Europe

(outside the Soviet Union and the countries listed above), is 20 pR/hr (to within

about a factor oi 2). Extrapolating from the It nicb area data, this would

imply an average lifetime dose of about 60 mrem (12 ores in the first year).

A similar extrapolation based on the data on Sweden, where the iaximau exposure

was 500 pR/hr (WHO, 1986a) and the maximum first-year dose was a few hundred

ores (NIRP, 1986), would be consistent with this analysis. Nevertheless, it

should be noted that the g8ographical variability it the relative activities of

9-13
deposited radionuclides, among other factors, may introduce appreciable error

into such extrapolations.

Thus, as a tentative approximation, the avera&e individual in Europe (outside

the Soviet Union and the other countries named above) will receive a 60-mrem

dose from the accident, this dose being spread over a period of years. For

comparison, this individual will receive about 100 mrem each year from background radiation. Using thisest-imated average dose and a total-ou

about 350 S i'•ion people in.that part of Europe bei.ng considered, a collective ,dose• ofi 2X0 7 person-rem is calculated. Based again on a risk factor of

2xlO- 4 /rem, about 4000 excess cancer deaths outside the Soviet Union may be

calculated to result from the accident. These deaths would be completely

masked by the 70 million or so cancer deaths predicted in the population over

the next 70 years.

8.5 RadiolOgical Effects on the United States

- adionclide concentrations n the United States as a resul•,of thei Chernobyd .

accident were generally lower than those in Europe and the resultant intakes

and exposures were lower accordingly. Using preliminary data, C. R. Porteer*

has estimated the total U.S. deposition of radionuclides of iodine and cesium

to have been approximately 4=1 0' Ci and I04 Ci, respectively. Table 8.4 summarizes Porter's estimates of maximum individual doses to groups in the United

States from exposures or intakecs in the year following the accident, calculated

using data obtained from DOE (1986), WHO (1986a), and other sources.* Thyroid

doses were dominated by the intake of 1-131 in milk and show a strong age

dependence. Effective dose equivalents were dominated by external irradiation

from deposited Cs-134 and Cs-137 and show no significant age dependence. For

comparison, a typical background annual effective dose in the United States is

about 100 arem. After Chernobyl, C. R. Porter* estimated the U.S. collective

dose to the thyroid from 1-131 in pasteurized dairy milk .to be lO person-rad

(within a factor of 3). Using this collective thyroid dose and the thyroid

risk estimates noted earlier, the projected thyroid effects in the United

S: eas would be about two excess thyroid cancers. For comparison, based on

recent cancer statistics,, roughly 200,000 thyroid cancers are expected to occur

among current residents of the U.S. over the remainder of their lifetimes.

8.6 Global Effects on Agriculture and Food

The Chernobyl accident .disrupted agriculture and other segments of the fo,.d

industry. Some of this disruption will continue-well into the future. Land A

close to the accident site was determined to be so badly contaminated that its.

cultivation may be impossible for years to come. Land between 5 ka and 10 ka

from the power station site falls into this category (INSAG, 1986). In other

locations within the European Soviet Union, it may be necessary to modify farm-.

ing practices in order to minimize huan exposures through the food paLhwsy.'

In particular, the zone from 10 km to 30 km distant from the reactor site falls

tnto this category (INSAG, 1986). With some exceptions, however, problems with

respect to contamination of agricultural land outside the Soviet Union are much

less severe, and extensive decontamination or control of farming practices Will..

*C. R. Porter, private communication of preliminary data submitted by the U.S.

Envlronmental'Protection Agency to the Nuclear Energy Agency (OECD), U.S. EPA

Eastern Envirormental Radiation Facility, ?Iuntgowery, Alabama, September 1986.

8-i4
Table 8.4 Maximum individuai d.Ises (urem) to groups in the United

States due to exposures or intakes in the first year

after the accident

4l~*'

Radionuclides

thay Iodne Cesium Other

ExternalC

Immersion 1.6x10-6 2.0xlO-? 2.OxlO-6

Irradiation from

deposited activity 4. 4x•10-. -

?iIhalation d, a-8.'i0'• ' ." 12• -5 . . 1. O10-4

Ingestion' (mill and

milk products )

Adults 5-.OxlO- 1 1.0110O i -

Children (under 10 years) 3.0 9.01O-3 -

Infants (under I year) 1.4x•10 1 -4.Ox10- 1

•

a Thw^4 fl A .o "wamf

b)

c)

d)

e)

Effective dose equivalent.

Within an order of magnitude.

Within a factor of 3.

Based on maximum total intake of 1-131 from drinkivg pasteurized

milk as reported for 65 U.S. cities.

Source: C. R. Porter, private communication of preliminary data submitted by the U.S. Environmental Protection Agency to the

Nuclear Energy Agency (OC0), U.S-. EPA Eastern Environmental

Radiation Facility, Montgomery, Alabama, September 1986.

not be required. Novever, elevated levels of cesium and strontium isotopes in

crops are expected to persist for a number of years.

Fallout from the accident deposited on crops over much of Europe, and in many

places fruits and vegetables were removed from the marketplace. Initially,

dairy products derived from animals grazing on contaminated pastureland were

most severely affected, especially by 1-131 (cf. Table S.3). Within a matter

of weeks, radioiodine levels bad dropped to acceptable levels, but contamination of seat and other foods with Cs-137 and Cs-134 remains a concez in some

places. Following the accident, many nations, including the United States,

began to monitor food products for radioactivity. As a result, some imports

into the United States were deemed unacceptable.* Some countries in Western

*As of mid-December 1986, the Food and Drug Administration (FDA) has measured

radionuclide concentrations in 690 samples of shipments prebented for import

into the United States. Of these, 4 have exceeded the FDA's level of concern.

8-I5
Europe ~anned importatifn of agticuiliyral. pro uc from laste 'European

coantr:es. Total osser to agriculthre vii the Nmdre s of millions of dol'lars.• I

One localized problem that persists in Lapland con es he hieh concentration

of CL-134, and Cs-137 in rpindeer beat. Concen atons up to 2 ,000 Bq/k•

(540,000 *Cf/kg) have been meassred - well in ec; of t~e 30 8qBkZ reference ••,••levetl-~used in Sweden as the 66kimaim-a c t•r•t iii sld for -.

human consn tion (NIRP, 1986). .Te:c6.6oentrat as re expecte to persist

above the *eference level for sevetal yea~s. On cc sequeýce a this situation

is the dis •, priori of an impoFtant Industrd of th La andegs, th Oroduction of reindeer mePt. A second is that tho doses to th Lap andeis (fo whom reindeer

meat is an Lportant dietary item) coUld be as hi h a I re- per ear or more

(NIRP, 1986)..,

8.7 Ecoi. i. al .ffects

With the kzce tion of a~hieiafe~ai~im d"~ eý d ge recor (0 to

30 km from th• site), no direct adverse ecosystem fect a expe ted. In the

reactor coolin• pond.aid parts of the Pripyat River exp ure• of quatic biota

way be high en ugh to bars some Individual organis or •s i ive cies.

Within the 30-' zone around the'site, .discrefe are of ig~h on ination miy

show significant changes in radiosensitive species. Howe r, ven ev,

effects may not \e detectable.

In addition to p\tential effects o# fireihvater'ecosys ems a ted abov , the

possibility of long-tez- effects on terrestrial.and a uatic ape ies nd

ecosystem effects at both high and low doses has been raise ( Go 986).

The Soviet report listed a number of z~search programs which ds be initiated

to determine if there will be long-tkrk e-ological iff cts, p rt cula ly on

aquatic biota.,

Although transport of radionuclides through ecosystems y not af ect the ecosystem Itself, there still may be an effect on man. Fo ezamp , the concentrations of cesium and its transaissoia to man in reind r in. p•land will not

affect the tundra biome, but man wviii 1e. affected if tbe food in is not

controlled. Likewise, trvasport of radionuclides to 'ro dvatLe and to the

water supply for man must be controlled even though t$ete is no effect on

ecosystems. A -si..,r problem in this regard is the potential con tion of

the Kiev reservoir, which the Soviets hsVe taken measures to prev nt.

8.8 References

DOE, 1986 U.S. Dept. of Energy, Environmental Mleasurements La ratory,

"A Comeendium of the Lavironmental Measurements Lsbo story's

Research Projects Related to the Chernobyl Nuclear cident,"

VML-460, New York, October 1986.

INY.A-DISP, Italian Agency for Nuclear and Alternative Energy - Directorate

1986 for Nuclear Safety and Realth Protection, External Relations

and Information Department. "The Chernobyl Accident: Radiolo8ical 1986 Consequences in Italy. Re.port to 27th of May 1986,"

DOC./DI(8016)2, kois, June 19896

8-16
GSF, 1986 Gesellschaft fur Strahlea-und'-ýUmweltfors~chung,- "'meltradioaktivitat und Strahlenexposition in Sudbayern

durch de- Tachernobyl-Unfall," Bericht des Instituts fur

Strobkilnsch.Utz, Hunc~hen-Ne'uhe'rberg.-i June15196

INSAG, 1986 International Nuclear Safety Advisory Group, "Suin"r Report

on the eost-Accident Review Meeting on the Chernobyl Accide.t,_"

- AgSeptemb -Seer2,.I -... ,.....-C -. 3 , ...IAEA,- en n..-.

September, 24, 1':,86-. -

LLNL, 1986 Lawrence Livermore National Laboratory, "ARAC Response to the

Chernobyl Reactor:Accident," UCID-20834, July 1986.

NAS, 1980 National Academy of Sciences, "'The Effects on Populations of

JZposure -to Low.Level S of Ion , izing Radiation-: 1198 70 (BEIR- 11-1

-National, Academy Press,. Washington,-D D.,C'.,1 1980. ~

NIH -185. 7 tiomal Institutes, of Health, ."Re~port of he National'n ltiites

of Health Ad Hoc Working Group To-Develop Radioepidemiol-ogical

Tables," NIH Publication No. 85-2718, January 1985.

NIRP, 1986

NRC, 1985a

NRC, 1985b

NIm, 1983

Otake, 1984

S11K, 1986

USSR, 1986

National Institute of Radiation Protection, J. 0. Snihs, "Consequencei in Sweden Chernobyl Accident. lABA General Conference,

Scientific ,Programme for Safety, 2-3 Octob#.r 1986,"-SSI-report

86-21, Sweden, September 30, 1986.

U.S. Nuclear Regulatory Commission, E. Gilbert, "Late Somatic

Effects" in Health Effects Model for Nuclear Power Plant

Accident Consequence Analysis, NUREG/CR-4214, 1985.

U.S. Nuclear Regulatory Comission, H. Maxon, S. A. Book,

C. R. Buncher, V. Zertzberg, and S. R. Thomas, "Thyroid Effects,"

in Health Effects Model for Nuclear.Power Plant Accident

Consequence Analysis, :NJREG/CR-4214, 1985.

NationalsRadiation Protection Board,:H. Smith, "Dose-Effect

Relationships for Early Response to Total, ody Irradiation,"

PS-4-R139, Chilton,. England, 1983.

Otake, N.,:andV. J. Schull, "in ,Utero Exposure toA-Bomb

Radiation and Mental Retardation;, A Reassessment," Brit. J.

J/adiol. 57, 409-414, 1984.

Siteilyturvakeskus Strilsikerhetacentralen, Second Interim Report.

*Radiation Situation in Finland from 5 May 1986," SU-B-VALO 45,

Joint report of Finnish Centre for Radiation and Nuclear Safety -

and NBC Defence Office of the General Headquarters Rescue

Department of the Ministry of the Interior, Helsinki, May 1986.

USSR State Comittee on theb Utilization of Atomic Energy, "The

Accident at the Chernobyl Nuclear Power Plant and Its Consequences." Information compiled for the IARA Experts' Meeting,

August 25-29 1986, Vienna, 1986.

8-17
WHO, 1961

WHO, 1986a

World Health Organization, S. A. Killmann, E. P. Cronkite,

V. P. Bond, and T. H. Fliedner, "Acute Radiation Effects on Han

Revealed by Unexpected Exposures," pp. 151-164 in Diagnosis and

Treatment of Acute Radiation Injury, Columbia University Press,

New York,. 1961.. .

World Health Organization, Regional Office for Europe, "Summary

'Review -of. Xeasuzement•7ftx#AUA*$pMse Assessment,", -

Update Revision No. -7,- Copenhagen,- June-12,-. -1986'........... .

YMO, 1986b World Health Organization, Regional Office- for Europe,. "Su2mmary

Report of Working Group on Assessment of Radiation Dose Commitsent in Europe Due to the*Chernobyl Accident," Bilthoven,

June 25-27,, 1986, Copenhagen, ,September 8, 1986.

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