IAEA-TECDOC-1042 nun iiiui XA9848108 Safety analysis of nuclear power plants durin wgpol owednra shutdown conditions m INTERNATIONAL ATOMIC ENERGY AGENCY LIZ/—\ y The originating Section of this publication in the IAEA was: Safety Assessment Section International Atomic Energy Agency Wagramer Strasse 5 P.O. Box 100 A-1400 Vienna, Austria SAFETY ANALYSIS OF NUCLEAR POWER PLANTS DURIW NPOGOL WD ESNRAH UTDOWN CONDITIONS IAEA, VIENNA, 1998 IAEA-TECDOC-1042 ISSN 1011-4289 ©IAEA, 1998 Printed by the IAEA in Austria September 1998 The IAEA does not normally maintain stocks of reports in this series. However, microfiche copies of these reports can be obtained from IIM1S Clearinghouse International Atomic Energy Agency Wagramerstrass5e P.O. Box 100 A-1400 Vienna, Austria Orders shoue labdc companie ybpd repaymenf oAt ustrian Schillings 100, in the form of a cheque or in the form of IAEA microfiche service coupons whice bh oyam rdered separately from SI NCI eht learinghouse. FOREWORD The Technical Committee Meeting on Safety Analysis of Nuclear Power Plants during Low d PoSnwhauetr down Conditions, nheiVldi enna, A5 us-Dt1reiac ,ember sa1w997 , organized by the IAEA within the framework of the Technical Co-operation Project RER/9/046. The 23 participants, representing 14 Member States, reviewed recent developments and discussed directions for future efforts in the area of safety analysis of nuclear power plants durin wgopl owed nras hutdown (LPS) conditions. Durine ghtm eeting8 1t, echnical papers were presented, devoted to various aspects of LPS conditions: probabilistic safety assessment studies, descriptif oonp articular phenomena, calculational analysf oiis ndividual events, contents of safety reports, hardware modifications, experience from plant operations, etc. A number of events at nuclear power plants, as well as results from probabilistic safety assessment (PSA) studies for NPPs, have indicated that events occurring during shutdown modesy am contribute significantlye ht ot overall risk associated with PPN operation. It si recognized thata great dealf o work ovee rhtp ast yeare hts wni orldwide nuclear power commus nfaoihtcy used upon redue crihinstgk associatedS owPpiLethr atiy oabnns alysing specific phenomena occurring d SuPcroLinngd itions, improving analysis methodology, implementing additional administrative med ahnsaurardews are modificd asnttiaoalnlisn ,g incorporation of the LPS conditions to the safety analysis report. The IAEA expresses its sincere thanks and appreciation to the experts who contributed to this publicatione hT.o ffice heIt frAo EA responsibe hlTt reof EC ehDDt fo nOiiLv .CC siaws ion of Nuclear Installation Safety. EDITORIAL NOTE In preparing this publication for press, staff of the IAEA have made up the pages from the original manuscript(s). The views expressed do not necessarily reflect those of the IAEA, the governments of the nominating Member States or the nominating organizations. Throug ehthoetxut t namf oMes ember Se tarrteaetsa is natehde y were sw eahthweetxn t compiled. Te hsoue f particular designationf osc ountrie rost erritories dot eoinsm p ynjlauy dgemenybt the e plheugbta loils etshIh AtesatErat ,uAs , of such countrr itoeesr ritories, of their authodrintiaes institutions or of the delimitation of their boundaries. The mention of names of specific companies or products (whether or not indicated as registered) does not imply any intention to infringe proprietary rights, nor should it be construed as an endorsement or recommendation on the part of the IAEA. CONTENTS 1. INTRODUCTION...................................................................................................................! 1.1. Background......................................................................................................................! 1.2. Vulnerability of NPPs at low power and shutdown conditions....................................... 1 1.3. Objective and scope of the meeting................................................................................. 2 1.4. Pae rmtihceid tecp toanifutnioargosn. e ..........................................................................2 2. SUMMARY OF SITUATION IN MEMBER COUNTRIES ................................................3 2.1. Armenia...........................................................................................................................3 2.2. Bulgaria...........................................................................................................................3 2.3. Czech Republic...............................................................................................................4 2.4. Finland............................................................................................................................6 2.5. France..............................................................................................................................? 2.6. Germany..........................................................................................................................9 2.7. Hungary.........................................................................................................................10 2.8. Lithuania....................................................................................................................... 10 2.9. Romania........................................................................................................................11 2.10. Russian Federation....................................................................................................1..1. 2.11. Slovakia........................................................................................................................12 2.12. Slovenia...................................................................................................................4..1.. . 2.13. Ukraine......................................................................................................................... 15 2.14. USA.......................................................................................................................5....1.. . 3. SELECTED PHENOMENA OCCURRING DURING LOW POWER AND SHUTDOWN CONDITIONS ...............................................................................................17 3.1. Hydraulic phenomena....................................................................................................71 3.1.1. Cavitation phenomena ......................................................................................... 17 3.1.2. Depriming vortex................................................................................................. 17 3.2. Degradatiof onn atural circulation ..............................................................................7..1. 3.3. Boron dilution................................................................................................................ 19 3.3.1. Homogenous deboration....................................................................................9.1. 3.3.2. Heterogenous deboration.....................................................................................20 3.4. Pressurised thermal shock..............................................................................................21 . S4AFETY ISSUD SW PEHONSUOW RTAEDELRLO AWOTNTED CONDITIONS ......................................................................................................................22 4.1. PSA................................................................................................................................22 4.2. Accident analysis methodology .....................................................................................23 4.3. Operational safety issues................................................................................................24 4.3.1. Administrative improvements.............................................................................24 4.3.2d .t rParionncinedgau..r..e .s..................................................................................28 4.3.3. Hardware improvements.....................................................................................29 4.3.4. Feedback of operating experience.......................................................................30 4.4. Safety analysis reports....................................................................................................30 5. CONCLUSIONS AND RECOMMENDATIONS ..............................................................32 ABBREVIATIONS...................................................................................................................34 CONTRIBUTORS TO DRAFTING AND REVIEW ..............................................................35 1. INTRODUCTION 1.1. BACKGROUND A number of events at nuclear power plants (NPP), as well as results from probabilistic safety assessment (PSA) studies for NPPs, have indicated that events occurring during shutdown modes may contribute significantly to the overall risk associated with NPP operation. Ie nih mstp pfioote eir hstast unfecoe, safety rd ugnlueaisd elines applicabn mlie ost countries do not contain detailed requirements concerning the safety analyses specific for shutdown conditions. Only recently systematic e aahnctacli ydfsoeens ts during shutdown conditions have been undertaken. The current operating procedures used to cope with shutdown acct iodpner noodtvs ide sufficient ot snud eperaptao dirnltasy eb, sdu fficient analytical results. Since many significant activities are ongoing in all areas related to low power and shutdown (LPS) conditions, exchanf egoxe perience among Member Stas tceios nsidered very useful and beneficial. The IAEA convened a Consultants Meeting on Accidents During Shutdown Conditions for WWER NPPs in November 1995, and a second meeting in October 1996 to develop Procedures for Analysis of Accidents in Shutdown Modes for WWER NPPs (IAEA-EBP- WWER-09). The above two meetings emphasized a deterministic approach. Two other meetings had previously been organized by the IAEA to deal with probabilistic approaches to shutdown safety anal ltlayy rpsof iea fosrc s e soaawncst eounrlOtsa. nts meetino dgt evelop guideliner sof shutdown risk assessmey anM nti1 99e h3t dnoa tha e sarwT echnical Committee Meeting on Procedures for Probabilistic Safety Assessment for Shutdown and Low Power Operating Modes in November 1994 in the Netherlands. 1.2. VULNERABW IPLOOIF D TWLNSOY NHPTE APAURs TDOWN CONDITIONS Events occurring during shutdown operational modes represent a significant contribution to P Pe rNhfti eoasht teckudt that both preventiv denma itigative capabilitie ehpt fsol aebn natc degraded, e.g. by: • Failuf raoe utomatic startu fsopo me safety systems; • Equipmenn mit aintenann cri reeo pair; « Reduced amount of coolant in primary as well as secondary circuit for some modes; • Instrumentation and measurements switched off or non-functionable; • Open primary circuit; • Open containment, etc. Extensive maintenance work together with a large number of people present due to refueling, preved ncntoiavrre ective maintenance, modifications, surveillance ted sintnia-nsg ervice inspe nciatniccornes ase conside en rlhiaikannbtealayl d ihfv ooeoorttde n te uevDent . inherent belief tha ehttu an n'iis stia fe shutdown modee hto' perating personenb yeaml ue pnhoattwe f rnahoortiieg fah ler risk during various shutdown conditions. The number of events occurring during LPS conditions is considerably higher than might be expected compae rhnte oudt mbef ore vents occurring during power operational modensI. many cases higher S pePrvLoes b nipnat aabri tlfiioatyl ly coe mhsltpo eywnblsy arotefd e evao ceonrt e eo( ipdh.eea.rma tltaoognhresg) , t tipmosesibt ly gfi vdinegveoloping na ture longer time ot respone dht oet vent.n I some instancese ,ht operatory same vene b ableseu ot external equipment not available at the plant to mitigate the accident. However, to properly account for all the various plant conditions and operator actions, an LPS PSA should be performed. ? 1.3. OBJECTIVE AND SCOPE OF THE MEETING The main objectives of the meeting were to exchange information on the studies performed and countermeasures takenr of coping with accidents duringw ol powed rna shutdown conditions, and to define the necessity and directions of further activities which may promote safety improvements for NPPs. Participants from east European Member States were requeste odt present their activitid enasr esultn os this too tp dpinaca rticipeahtt nei discussions. Experts from selected western countries were requested to summarize the most important results achieved in LPS work in their countries. The scope of the meeting included: • Summaryf o activitiesn i evaluatioS PnLc fo onditions (completed, ongoingr ,o planned); • Safety analysS PeLc fsoo nditions (results, methodology, including deterministic and/or probabilistic, typical examplesf o analysis, etc.); • Lessons led amrnneaaesd ures tr acokoepfni ng withS c aoPcncLdidi teitonants s; • Statu fotse chnical specificatiod ennmas ergency operating procedS cPuoLr nreodfs itions. A broad spectrum of approaches and results for various types of reactors operated in the European region (WWER, RBMK, CANDU, Westinghouse PWR, BWR) were coevhet rneid presentations. 1.4. PARTICIPATID OCNNOA UE MHRTES FEOE TING Eighteen participants (regulators, operators, engineering institutions) from ten Member States operating NPPs in the eastern European region — namely Armenia, Bulgaria, the Czech Republic, Hungary, Lithuania, Re oRmhuastnsiiaa ,n Federation, Slovakia, dSlnovaeni a Ukrained na — five experts frome ht USA, Finland, Franced na Germany attendedeht meeting. Thus,a total4 1 foc ountries were represented. Altoget8 ht1eecr h,nical papers were presented, covering variouS scP oaLnsdp eiftcoiots ns: PSA studies, description of particular phenomena, calculational analysis of individual events, contents of safety reports, hardware modifications, plant operational experience, etc. Copies ofl la papers presentee dht tma eetinge ra available upon request frome htD ivisionf o Nuclear Installation Safety. In addition to the presentation and discussion of the technical papers, the participants prepared this technical report, which includes information in addition to their presentations. Se erchett ipfooo 2nr t summarize eahstn alyd smnias easures taken nci ountries representede ht ta meetingo t reduce risk duringS PL conditions. Section3 explainsdna discusses selected physical phenomena which are important to LPS conditions: cavitation and depriming vortex during mid-loop operationf o NPP, degradationf o natural circulation, boron dilution and pressurized thermal shock. In Section 4, selected safety issues relevant to LPS conditions are discussed from the point of view of their significance, their causes, and actions planr ntoaeo krd eetse n i eosehlhsxvutpTe eesr .ts' opinions were prepy awrbeodr king groups and were later accepted at the plenary meeting. Conclusions and suggestions, as formulated by the experts presene htt tam eeting,e ra includedn i Section.5 . S2UMMAF SROITY UATI NOMINE MBER COUNTRIES The summary of activities related to safety analysis of shutdown and low power operating mode rocsf ountries participati ehnmt ngie etings ip resen ethfeto ndil lowing sections. 2.1. ARMENIA At the Armenian plant, under the low power and shutdown conditions, organizational and technical measures have been implemented to meet the following objectives: • Avoid reducf btoioorn ic acid cone creehantctrt aontri iocn oolant system (RCS), makeup d snaafety injection systed msnyass, temf oos rganized leakages; • Ensure essential power supplyo t safety related equipment; • Ensure reliable service water supply; • Ensure naturar ofl orced circuS lcCaRot eoihotl fano nt; • Ensure heat removal from secondary system; • Avoid formation of air-gas bubbles in different RCS parts; • Avoid contaminatin Sgc oCompRepno nents with foreign material; • Avoid overexposure of personnel; • Avoid human errors in the course of activating and shutting down equipment; • Avoid cold overpressuref o reactor vessel. Events which have oce cAhutrrm rteaedn iat nao tpdhlnearan t plants worldwe triadake en into accn oruienv tiewing sd yosntpeaemras tions, developing hardware modifidcantiaon , refining emergency operating procedures. Technical requirements for plant systems and equipment are defined in "Process operational procedures for the Armenian Unit 2". Since there is no technical support organization in Armenia, the engineering support for the Armenian ps lpairno ty vfiodbreedi gn organizations, my aVibnNlHy AEeSh, Gti ddronpraes s Kurchatov Institute. The Armenian plant is operated according to Russian standards. However, these standards do not specify requirements for safety analyses during low power and shutdown conditions. Th ehrti essia soy hsnwu ch safety analysis ht aebyv teoene n done. e hATrmenian plant plo atnus pgrade these ste ahnltedva oertdl s correspondointg international practice. 2.2. BULGARIA Bulgarias ah performed Level1 PSAsl la rof unitsf o Kozloduy PPN during full power operation. In the framework of the reconstruction programmes for Units 5 and 6 with WWER-1000 type reactorsr of dnaU nits4 ot 1 with WWER-440 type reactors, Kozloduy PPN included eht task of performing Level 1 PSA analyses for these units during shutdown conditions. Basede ht no incident reportsl la rof units, Kozloduys ah PPN completede ht identificationfo initiating events which occurred during shutdown conditions for inclusion in the PSA studies. These initiating even ertas classified into five groups accore dhtin IotAg EA recommendatione shmT. ost important events are: • Non-condensables ag injection into RCS; • Lossf o power supply;