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MINUTES OF THESECOND MEETING OF THE

PROGRAMME’S WORKING GROUP 2ON COMPREHENSIVE ASSESSMENT

TECHNIQUES

LENINGRAD NPP, SOSNOVY BOR, RUSSIA14-16 MARCH 2001

EXTRABUDGETARY PROGRAMME ON MITIGATION OF INTERGRANULARSTRESS CORROSION CRACKING IN RBMK REACTORS

INTERNATIONAL ATOMIC ENERGY AGENCY

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CONTENTS

1. INTRODUCTION......................................................................................................... 2

2. MEETING SUMMARY ............................................................................................... 2

3. ACTION ITEMS........................................................................................................... 6

4. CONCLUSION ............................................................................................................. 6

Appendix I. LIST OF PARTICIPANTS ........................................................................... 7

Appendix II. AGENDA................................................................................................... 13

Appendix III. TASK 1 - GROUP MEETING ................................................................. 16

Appendix IV. TASK 2 - GROUP MEETING................................................................. 18

Appendix V. TASK 3 - GROUP MEETING.................................................................. 20

Appendix VI. TASK 4 - STATUS.................................................................................. 22

Appendix VII. FINAL REPORT OUTLINE .................................................................. 26

Appendix VIII. PROPOSED BENCHMARK PROBLEM, TASK 2 ............................. 27

Appendix IX. PRESENTATIONS HANDOUTS........................................................... 30

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1. INTRODUCTION

The second meeting of Working Group 2 on Comprehensive Assessment Techniqueswas held at the Leningrad Nuclear Power Plant, Sosnovy Bor, Russia, 14-16 March 2001. Themeeting successfully achieved its major objectives:

�� finalize and approve the minutes of the previous Working Group 2 meeting;�� the review the status of activities of task 1, 2, 3 and 4;�� stimulation of further fruitful co-operation and work in the frame of task activities;�� consideration of Steering Committee’s recommendations;�� adjustment of the task programmes and development of detailed list of future actions,

deliverables and schedules of the four tasks in order to obtain their goals;�� definition of action items and deliverables for the Steering Committee to consider;�� deciding on the venue and dates of the future Working Group 2 meetings.

The list of second Working Group 2 meeting participants is provided in Appendix I, the

Agenda in Appendix II. List of future actions for Task 1, 2, 3 & 4 are presented inAppendices III – VI. An outline of the Working Group 2 Final Report is given in AppendixVII. In Appendix VIII., a proposed benchmark problem statement is given. In Appendix IXavailable handouts of presentations are given. A summary of the meeting is provided in thefollowing sections of this report.

2. MEETING SUMMARY

On behalf of Leningrad plant, Yu. Zakharzhevsky welcomed all the participants to the

second meeting of Working Group 2. The Working Group 2 leader, B. Brickstad, opened themeeting. All the participants introduced themselves. The list of participants is provided inAppendix I. B. Brickstad presented the agenda and described the goals of the meeting. After ashort discussion, the agenda was approved with minor changes.

The Working Group 2 leader reminded the objectives and tasks of Working Group 2 and

shortly described Working Group 2 related issues raised at the Steering Committee Meeting inVienna, December 5-7, 2000. The minutes of the 1st Working Group 2 meeting were approvedby the Working Group 2 meeting participants.

B. Brickstad stressed the need of unprejudiced co-operation and openness in respect of

sharing the damage data and reports as a prerequisite of achieving the objectives ofWorking Group 2.

A. Petrov described the situation at Leningrad plant. The first signs of IGSCC were

observed in 1987. 1997-98 more general stress corrosion cracking in 300 mm stainless steelpiping was found. Laboratory metallographic examinations have been made.

S. Polyanskih did not attend the meeting and therefore there was no presentation of

status at Kursk plant. A. Parshin, Smolensk plant, stated that the defects left without repair were sized both in

respect of crack length and depth. Unit 1 has outage for reactor retubing. During 1997 and

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1999, 100% UT was performed covering all the welds in group 1 and 2 pipe systems. Repairswere done by replacement. No through wall defects were detected.

J. Saburov, Ignalina plant, reported that no significant events have occurred since lastWorking Group 2 meeting. Ignalina’s unit 2 will be inspected in April 2001. 643 welds in300mm piping will be inspected. 18 welds with detected defects have been left without repairsince 2000. Automatic inspections are used extensively at Ignalina. Prometey Institute hasperformed the metallographic investigations. New IGSCC cracks in some repaired weldswere found after two years of operation (also at Leningrad). However, a new repair weldingprocedure resembling heat sink welding with running water inside the pipe and argon gasduring the root pass is starting to be used at Ignalina plant. No new defects have (yet) beendetected in these welds with the new repair technique.

Electro chemical potential (ECP) is planned to be measured at Ignalina plant. V. Torop, Ukraine described the investigations at Chernobyl plant unit 3. In some cut-

out welds, defects were found also in the weld metal. R. Kilian confirmed the possibility ofcracks along the dendrites in general welds.

U. Ehrnsten and R. Kilian described the task 3 status. Subtask 3.1, Collection of data

should be finalized by the end of September. Subtask 3.2, Evaluation of data would follow.

Furthermore, U. Ehrnsten and R. Kilian stressed the importance of Task 1 DamageDatabase for obtaining statistical data on occurred damages in relation to different influencingparameters.

U. Ehrnsten also presented a description of the methodology for root cause analysis ofRBMK cracking that the Task 3 group is going to use, see handout in Appendix IX.

A. Arzhaev suggested a revised Task 3 which would even include analytical work. U.Ehrnsten judged that the Task 3 would not manage to finish such a work within the remainingtime of the project, i.e. until March 2002.

V. Abramov presented NIKIETs’ root cause efforts. A lot of metallographicexaminations showing cracks in welds were presented. He also described some fractographicexaminations. The crack growth rate for cracks with depth a = 5.9 – 12 mm and length l = 70– 400 mm was estimated to 0.29 – 0.86 mm/year.

V. Abramov was positive to reveal relevant information and to extend the relevantreports to Task 3. A. Arzhaev opposed this offer with reference to his agreement with A.Petrov, Minatom.

A. Letzter, DNV, described status of Task 1. Discussions and suggestions from the lastSteering Committee meeting were reported. All arguments for creation of common RBMKdamage database were reviewed once again as well as the optional offer of joining theOECD/NEA piping failure data exchange project.

SIP, Sweden has sponsored adaptation of the “OECD/NEA database shell” by BengtLydell/RSA Technologies. The adaptation has been made in co-operation with Task 3

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members to enable and secure future support to Task 3 activities focused on finding the root causes of IGSCC in RBMK environment. The database shell format (MS Access) and the corresponding report were delivered to all Task 1 members during the task group meeting.

The reluctance of delivering the damage data by the NPPs as well as opposition by A. Arzhaev caused A. Letzter to express a warning to Working Group 2 of not being able to give supporting information to other Tasks and Working Groups (particularly Working Group 1) in case of Task 1 failure. Other Western experts supported this warning. Further discussion of the future of this task was submitted to the Task 1 group meeting the next day.

Y. Morishita presented the Japanese procedure for break preclusion.

A. Arzhaev and V. Kiselyov described the Russian procedure for safety assessment of detected defects and LBB criteria.

B. Brickstad commented that these procedures do not use ASME XI, Appendix C, Z-factors or do not take any other account for less ductile materials. NIKIET response was that cracks are not located in the weld material but rather in base material or HAZ and net section collapse is assumed to be valid. However, this is contrary to the opinion of the ASME XI code and most other flaw assessment codes used in the Western countries. It should be noted, that the definition of flow stress, σf = 0.42(Rp0.2 + Rm) in the Russian procedure is more conservative than common definitions used in other procedures.

B. Brickstad proposed a benchmark problem, presented in Appendix VIII, to compare and enhance understanding of the national flaw assessment procedures in the participating countries.

A. Arzhaev opposed the crack growth rate equation based on the local stress intensity factor in the benchmark problem, since only constant crack growth rates equal to 1mm/year in depth and 20mm/year in length are currently used in Russian procedures.

T. Taylor, leader of Working Group 1, described the activities in Working Group 1 as well as the interactions with Working Group 2. Determining the target flaw size i.e., the largest flaw size that is allowed to be barely undetected during an inspection, with still a sufficient safety margin until the next inspection (or the maximum flaw size that could be missed), would help Working Group 1. The RBMK damage database would also give valuable information on crack size distribution, bounding conditions, and flaw distribution in the plants/systems.

T. Taylor stressed that NDE is probabilistic in nature. Reasonably high probability of detection (POD) for a crack depth of 4-5mm (a/t = 0.30) would be about 70%.

Working Group 1 would need the following input from Working Group 2: a set of flaw size distributions (in high and low stressed regions) to be considered by Working Group 1 for performance demonstration. The benchmark problem, as proposed by B. Brickstad, may give some additional insight to this question.

L. Poulter discussed different opinions for target flaw sizes and inspections requirements.

A. Klimasauskas presented LEI’s and Ignalina plant’s work on collection of data for Task 2.4. These data could in fact be used in all Tasks. The two cut-out welds with cracks (a

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= 2.7-4mm) imply crack growth rates in the range of 1.75 – 2.7mm/year which is larger thanthe crack growth rates presented by the Russian representatives.

B. Brickstad presented status of Task 4, which follows the time schedule. Appendix VIcontains a Program Status and future plans for Task 4 until March 2001. All data for Task 4has now been collected and the risk evaluations are about to begin. B. Brickstad also gave adetailed presentation of the data collection of Task 4 and the RBI procedures to be used forthe pilot study. The handout is included in Appendix IX.

The earlier recommendation to invite Prof. Speidel to Task 3 was transferred to the taskgroup for decision.

Working Group 2 is supposed to deliver a final report of the whole Working Group 2work. An outline of this report is given in Appendix VII with headlines and responsiblepersons. A draft of this report should be finished by December 31, 2001. The Working Group2 final report should be finished by March 31, 2002.

The task groups met and discussed status, activities and future plans and schedules. Theresults are presented in Appendices III – VI.

The individual task group meetings were quite successful with respect of co-operationas well as regarding the problem of sharing data, reports and other information needed forsuccessful completion of Working Group 2 work and achieving the Working Group 2objectives.

Task leaders reported future plans, schedules, deliverables, delivery dates andresponsibilities to the Working Group 2 meeting participants.

The members of Task 2 have not come to an agreement on the issue of target flaw sizes.The discussion will continue until the next meeting. DNV will update the report of theSwedish flaw assessment procedure with a description of how the question of target flaw sizeis defined in the Swedish Regulations and how it is used in the Swedish NPPs.

The Task 3 members have chosen V. Abramov as an additional task co-leader.

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3. ACTION ITEMS

The following general action items resulted from the meeting discussion.

1. B. Brickstad and A. Letzter (DNV) will draft the minutes of the second Working Group2 meeting to be sent to R Havel (IAEA) by March 31, 2001.

2. B. Brickstad in cooperation with T. Taylor to consider how to incorporate RBITechnology to the EBP and report to the Steering Committee meeting in May 2001

3. All task leaders to deliver summary of the discussion in their task groups includingaction lists by March 26, 2001.

4. V. Torop, Ukraine, to provide Working Group 2 with information concerning Chernobylleak incident from December 2000.

5. R. Havel to contact Smolensk/Kursk plants and ask them to host the next WorkingGroup 2 meeting.

See also the action lists from the individual task groups, Appendix III, IV, V and VI.

Future meetings of the Working Groups are planned to be held as follows:

The third Working Group 2 meeting: Smolensk plant (first priority) or Kursk plant;September 12–14, 2001. This meeting should be coordinated with the Working Group 3meeting.

The fourth Working Group 2 meeting; Ignalina plant, Visaginas, March 13–15, 2002and will be preceded by an IRBIS (task 4) Workshop on March 12, 2002 also at Visaginas.

4. CONCLUSION

Finally, the Working Group 2 leader wants to emphasize to all Working Group 2participants, that without their active involvement, the objectives of this programme will bedifficult to reach. However, through the discussions with many experienced specialists fromall countries during this second Working Group 2 meeting, the Working Group 2 leader B.Brickstad and Working Group 2 secretary A. Letzter are optimistic that this will be asuccessful project. Please remember your scheduled work until the next Working Group 2meeting.

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Appendix I.

PARTICIPANTS LIST

FINLANDMs. Ulla EhrnstenVTT Manufacturing TechnologyKemistintie 3P.O.Box 1704Espoo02044 VTT FinlandTel.: +358 9 456 6860, +358 40 5354057Fax: +358 9 456 7002E-mail: [email protected]

Mr. Rauli KeskinenRadiation and Nuclear Safety AuthoritySTUKP.O.Box 14Helsinki00881 FinlandTel.: +358 9 7598 8327Fax: +358 9 7598 8382E-mail: [email protected]

GERMANYMs. Renate KilianFramatome ANP GmbH Task 3 LeaderNT1 (provisional)Freyeslebenstrasse 1Postfach 3220Erlangen91050 GermanyTel.: +49 9131 18 97363Fax: ++49 9131 18 95395E-mail: [email protected]

GERMANYMr. Hermann SchaeferGesellschaft fuer Anlagen- undReaktorsicherheit (GRS) mbH Task 2ForschungsgelaendeGarching85748 GermanyTel.: +49 89 32004 483Fax: +49 89 32004 306E-mail: [email protected]

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JAPANMr. Yoshitsugu Morishita Task 2International Cooperation and TechnologyDevelopment Center, ICTDC / JNC Task 2FBR Training and Planning Group1 Shiraki, Tsuruga-shiFukui9 319-1134 JapanTel.: +81 770 39 1031Fax: +81 770 39 9227E-mail: [email protected]

LITHUANIAMr. Juri Saburov Task 1 Co-leader &Ignalina NPP, INPP Task 4 Co-leaderVisaginas4761 LithuaniaTel.: +370 66 286 68Fax: +370 66 60188, 3706628818E-mail: [email protected]

Mr. Arturas Klimasaukas Task 2 & 4Lithuanian Energy Institute (LEI)3 Breslaujos Str.Kaunas3035 LithuaniaTel.: +370 7 40 1919/+370 87 70 724Fax: +370 7 35 1271E-mail: [email protected]

RUSSIA

Mr. Alexey I. ArzhaevNIKIET / RDIPE Task 2 Co-LeaderP.O.Box 788Moscow101000 RussiaTel.: +7 095 263 74 51Fax: +7 095 264 40 10E-mail: [email protected]

RUSSIAMr. Vladimir Abramov Task 3 Co-leaderENES, NIKIET/RDIPEP.O.Box 788Moscow101000 RussiaTel.: +7 095 268 9389Fax: +7 095 264 7934E-mail: [email protected]

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RUSSIAMr. Vladimir Belous Task 3 provisionalENES, NIKIET/RDIPE memberP.O.Box 788Moscow101000 RussiaTel.: +7 095 263 7443Fax: +7 095 264 7934E-mail: [email protected]

Mr. N. Karpunin Task 2SEC NRS of Gosatomnadzor(NTC / GAN)Taganskaya st., 34Moscow109147 RussiaTel.: +7 095 912 7986, +7 095 275 5548Fax: +7 095 912 40 41, +7 095 275 55 48E-mail: [email protected]

Mr. Vitaly A Kiselyov Task 2ENES, NIKIET / RDIPEP.O.Box 788Moscow101000 RussiaTel.: +7 095 263 74 49Fax: +7 095 264 79 34E-mail: [email protected]

Mr. Valentin MakhanevINSC/RINSCP.O.Box 873Moscow101000 RussiaTel.: +7 095 263 7309Fax: +7 095 264 4010E-mail: [email protected]: TOK 781449

Mr. Alexander M. Parshin Task 1Smolensk NPPSmolensk regionDesnogorsk216532 RussiaTel.: +7 08153 7 93 14, +7 095 992 12 11 62314Home: +7 08153 72727Fax: +7 08153 7 47 69E-mail: [email protected]: TOK 781449

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RUSSIAMr. Anatoly A. PetrovLeningrad NPP Task 1Leningrad regionSosnovy Bor188537 RussiaTel.: +7 812269 6 1207Fax: +7 812269 6-2518, -1207E-mail:

Mr. Sergei A. Polyanskikh (absent at 2nd WG2 meeting)Kursk NPP Task 1Kursk regionKurchatov307239 RussiaTel.: +7 0731 5 33 26Fax: +7 07131 41819; +7 07131 53190E-mail: [email protected]

Mr. Alexander TereshchenkoCentral Research Institute of Structural MaterialsPROMETEY Task 3 Co-LeaderShpalernaya 49St. PetersburgRussiaTel.: +812 274 1134Fax: +812 274 1707E-mail:[email protected]

SWEDENMr. Bjorn Brickstad WG2 LeaderDet Norske Veritas (DNV) Task 4 LeaderFracture Mechanics, Technical ConsultingWarfvinges väg 19BBox 30234Stockholm104 25 SwedenTel.: +46 8 587 940 57Fax: +46 8 651 70 43E-mail: [email protected]

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SWEDENMr. Adam Letzter WG2 Secretary &Det Norske Veritas Task 1 LeaderDNV Nuclear Technology ABWarfvinges väg 19BBox 30234Stockholm104 25 SwedenTel.: +46 8 587 94 281, +46 70 561 18 47Fax: +46 8 651 70 43E-mail: [email protected]

UKRAINEMr. V. Chugunov (absent at 2nd meeting)Chernobyl NPP Task 1Slavutych, Kiev. Obl.255190 UkraineTel.: +38 044 93 43343Fax: +380 44 79 25943E-mail: [email protected]

Mr. Vasyl Torop Task 2Institute for Problems of Strength ofUkrainian Academy of ScienceIPP-Centre Ltd2, Timityazevskaja strKiev01014 UkraineTel.: +38044 296 3957Fax: +38044 296 3957E-mail: [email protected]

UNITED KINGDOMMr. Laurence PoulterAEATechnology Task 2RD3 Risley, WarringtonCheshireWA3 6AT United KingdomTel.: +44 1925 254255Fax: +44 1925 254629E-mail: [email protected]

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USAMr. William H. KooU.S. Nuclear Regulatory CommissionNRC NRR/DE Task 2 Leader11555 Rockville PikeMail Stop O-9H4Washington, DC 20555-0001USATel.: +1 301 415-2706Fax: +1 301 415-2444E-mail: [email protected]

IAEAMr. Radim HavelNSNIWagramer Str. 5P.O. Box 100Vienna1400 AustriaTel.: +43 1 2600 22013Fax: +43 1 2600 29649E-mail: [email protected]

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Appendix II.

AGENDA

March 14, 2001, Wednesday

09.00 Opening remarks from Leningrad NPP and IAEA Yu. ZakharzhevskyR. Havel

09.20 Adoption of Agenda B. Brickstad

09.40 Short Review of Working Group 2 objectives and tasks. B. Brickstad

10.00 Review of Minutes of the 1st Working Group 2 meeting inVisaginas. B. Brickstad

10.30 Issues raised at the Steering Committee Meeting in Vienna, B.. BrickstadDecember 5-7, 2000. R. Havel

11.00 Coffee break

11.30 Short status of IGSCC in Leningrad plant A. Petrov

11.40 Short status of IGSCC in Kursk plant. S. Polyanskih,

11.50 Short status of IGSCC in Smolensk plant. A. Parshin

12.00 Short status of IGSCC in Ignalina plant. J. Saburov

12.10 Short status of IGSCC in Chernobyl plant. V. Chugunov

14.00 Task 1, Program status. A. Letzter

14.30 Task 2, Program status. A. ArzhaevW. Koo

15.00 Task 2.2, Japanese procedure for break preclusion. Y. Morishita

15.30 Coffee break

16.00 Task 2.1, Russian regulatory procedure for flaw assessment. A. Arzhaev,V. Kiselyov

16.30 Task 2.3 Suggested benchmark problem for Task 2 B. Brickstad

16.50 Task 2.6, Inspection requirements and target flaw sizes. T. Taylor Report from Working Group-1 activities. L. Poulter

18.00 Adjourn

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March 15, 2001, Thursday

08.30 Task 2.4, Existing data on stabilised steels at Ignalina plant. A. Klimasauskas

09.00 Task 3, Program status. U. Ehrnsten

09.30 Proposed methodology for root cause analysis of RBMK U. Ehrnstencracking. R. Kilian

10.00 NIKET presentation of root cause analyses V. Abramov

10.30 Coffee break

11.00 Task 4, Program status. B. Brickstad

11.30 RBI procedure for Ignalina plant. B. Brickstad

12.30 Lunch break

14.00 Outline of Working Group -2 Final Report. All/B. Brickstad

14.15 Individual Task Group Meetings. AllComposition of Task Group members.Adjustment of plans, milestone reports and time schedules.Planning of contributions to Working Group-2 Final Report.

17.00 Discussion on specific needs, allocation of homework. All/B. Brickstad

18.00 Adjourn

…. Homework

March 16, 2001, Friday

09.00 Task 1, future plans and schedules. A. Letzter

09.30 Task 2, future plans and schedules. A. ArzhaevW. Koo

10.00 Task 3, future plans and schedules. U. Ehrnsten

10.30 Task 4, future plans and schedules. B. Brickstad

11.00 Coffee break

11.30 Finalise Task Group plans and schedules. Task leadersB. Brickstad

Corrective actions to get back on time schedule.

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12.30 Lunch break

14.00 Deliverables needed for Steering Committee consideration All/B. Brickstad

15.00 Action items and assignments All/B. Brickstad

16.00 Next meetings. All/B. Brickstad

16.30 Adjourn

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Appendix III.

TASK 1 - GROUP MEETING

RBMK DAMAGE DATABASE

Task 1 group met twice during the second Working Group 2 meeting. The followingpersons participated in the discussions and compiled the future action plan:

1. A. Letzter2. R. Keskinen3. J. Saburov4. A. Petrov5. V. Torop6. A. Parshin

A. Letzter gave a brief summary of the Task 1 activities and discussion of the taskobjectives and scope during the second Steering Committee meeting.

R. Keskinen described the Finnish databases and their different applications in Finlandas well as the STUK opinion and national requirements.

A. Petrov and J. Saburov expressed their concerns regarding the confidentiality of thedata, resources (man and computer power), the opinion of their management and otherRussian organizations.

The database MS Access shell format and the corresponding report were handed over toall Task 1 participants. A. Letzter described the database shell format together with its scopeand made a demonstration of its function. The parameter list and the focus of the databasewere briefly discussed.

During the succeeding discussion, joined by R. Havel, a successful agreement wasachieved to proceed with Task 1 according to a new work plan:

WORK PLAN

1. All participating NPPs are to start collecting damage data directly after the meeting. Thecollected data will be entered locally at each plant into local damage database based on theMS Access provided at the Task 1 group meeting.Goal: To have local Damage Databases at each plant by September Working Group 2meeting.Responsible: All NPP representatives/members of Task 1.

�� All NPP representatives in Task 1 group are:to contact their plant’s management to allocate manpower for the data collectionwork

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�� to get the acceptance of data sharing between the plants and merging the localdatabases into one common “RBMK Pipe Damage Database”

�� to organize the database administration and appoint a responsible manager for thecommon “RBMK Pipe Damage Database”Co-ordinator: J. Saburov, Ignalina plantTime limit: 4 weeks

2. All NPPs are to reach an agreement on the rules for exchanging and sharing the damagedataResponsible: A. Petrov, Leningrad plantTime limit: 4 weeks

3. Translation of the parameter tableResponsible: A. Parshin, Smolensk plant for translation and sending the translated table toA. Letzter before April 1, 2001A. Letzter, to check up and, if needed, to make the parameter definitions clearer andunivocal before April 15, 2001.

ACTIONS

1. R. Havel and A. Letzter will write letters to chief engineers of all NPPs to promote Task1’s goal of the common “RBMK Pipe Damage Database”.

2. A. Parshin to contact S. Polyanskikh.

3. V. Torop to contact V. Chugunov.

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Appendix IV.

TASK 2 - GROUP MEETING.

SUMMARY OF TASK 2 FUTURE PLAN AND SCHEDULE

(Development of a Break Preclusion Methodology)(03/29/2001)

The Task 2 action items that were discussed during the second Working Group 2meeting are summarized below:

1. For the national practices of break preclusion methodology (integrity assessment) foraustenitic stainless steel components used in boiling water or RBMK nuclear powerplants, L. Poulter, V. Torop, H. Schaefer and A. Klimasuskas will provide a description ofthe methodologies including a sample calculation (desirable to have) that are used in theirrespective countries. L. Poulter will also provide a brief comparison of the R-6 methodand the limit load method (net section plastic collapse). Y. Morishita will try to provide asample calculation based on the practice used by the utilities in Japan.

2. The summary and comparison of various national practices (Russia, Ukraine, Lithuania,

Germany, UK, Sweden, Japan and USA) for flaw assessment will be provided in tableform with brief descriptions on key elements of the methodologies. Recommendations forimprovement of the Russian flaw assessment procedure will be provided. W. Koo will co-ordinate this effort.

3. A. Arzhaev, A. Klimasauskas and V. Torop will co-ordinate the collection of existing data

on stabilized stainless steel. 4. L. Poulter, V. Kiselyov and A Arzhaev will co-ordinate the input for the applicability of

LBB for RBMK stainless steel piping. 5. L. Poulter and A. Arzhaev will co-ordinate the input for inspection requirements. 6. One of the objectives for Working Group 2 is to recommend target sizes for flaws that

need to be detected during ISI depending on the inspection interval. This recommendationof target flaw sizes is requested by Working Group 1 (T. Taylor). A. Arzhaev stronglyobjected to providing simplified recommendations in this case without clear understandingof:

a) what national regulatory documents have requirements to specify “target flaw size”b) what is a regulatory definition of “target flaw size”c) what is corresponding level of probability of detection.

In current practices of most countries inspection intervals are defined according toregulatory prescriptions based on engineering judgement. In the case of RBMK IGSCCissues, it is not too simple to recommend approaches which are not in use in countries with

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large experience in IGSCC damages - USA, Japan, Germany etc. This objection wasexpressed at the meeting.

During T. Taylor’s presentation to Working Group 2 regarding target flaw sizes, W.Koo inquired about the other alternatives to target flaw sizes such as the maximum flaw sizesthat the ultrasonic testing (UT) could miss and the uncertainties associated with the UT sizingmeasurements. This information is useful and would provide acceptable solutions indetermining the in-service inspection interval. B. Brickstad indicated that he may bring upthis issue to the next Steering Committee meeting.

7. A. Klimasauskas will summarize the input to the benchmark problem submitted by Task 2members in accordance with the reporting format stated in the benchmark problem. Basedon a discussion among the Task 2 members about the input data and the language of theproblem, the recommended modifications of the benchmark problem are listed below:

(i) The stress conditions given in the benchmark problem appear to be on the low side. Anadditional case for high stress weld is recommended for consideration with the followingstress conditions:P(m)= 40MPaP(b)= 30MPaP(e)= 55MPa

One may work only on the case of high stress weld. However, working on both cases(high stress case and the case given in the problem) is highly recommended.

(ii) To improve the understanding of problem (6), the "target flaw size/depth/length" may be

replaced with "maximum flaw size/depth/length that could be missed". (iii) Each one should use the crack growth rate that is prescribed in the national practice. Use

of the crack growth rate given in the problem is optional. (iv) Discussion of the benchmark problem among the team members and with B. Brickstard is

encouraged, so that a common understanding of the intent of the problems can beachieved.

8. The target date is August 15, 2001 for all the deliverables (report/input).

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Appendix V.

TASK 3 - GROUP MEETING

Task 3 ‘Root causes of IGSCC in RBMK plants’, discussed the topic during two taskgroup meetings and additionally in smaller groups during the Working Group 2 meeting atLeningrad. The following persons participated in the discussions:

1. U. Ehrnsten Task 3 leader2. R. Kilian3. G. Sund4. V. Abramov Task 3 co-leader5. A. Arzhaev6. V. Makhanev7. V. Belous8. N. Karpunin9. V. Torop10. A. Tereshchenko Task 3 co-leader

The discussions were constructive and very helpful in addition to the lectures presentedduring the Working Group 2 meeting. The constructive atmosphere by partners is gratefullyacknowledged. A revised work plan was sent out by the Task 3 leader U. Ehrnsten on11.12.2000 for comments. The comments have been included as well as in the final work planfor Task 3. A. Abramov was appointed as additional co-leader for Task 3.

During the discussions, A. Arzhaev raised a need for further experimental work. Thematter was discussed and it was decided that a proposal on urgent matters based on theknowledge today would be prepared for the next Steering Committee meeting in Vienna to beheld in May. This issue is included in the Action list. Based on these discussions, theobjectives of Task 3 were stated as well as a list of deliverables and an action list wasprepared.

An action to consider the use of Western experience in analysing the root causes ofRBMK cracking and to consider the possibility to involve experts from ANL (USA) andInstitute of Metallurgy (Switzerland), had been given to Task 3 Group by the SteeringCommittee. The Russian Task 3 Group members stated that the knowledge of Task 3 issatisfactory and thus, there is no need to involve more Western experts into Task 3.

Objectives

�� To collect data of destructive examinations form all RBMK plants, both official andinternal reports

�� To use also the data base to be developed in Task 1�� To perform engineering judgement as well as use the experience from e.g., Germany and

USA.

Deliverables

− Presentation on the issue at the next Working Group 2 meeting

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− Summary report on destructive examinations from all plants (date to be set at the latest atthe next Working Group 2 meeting)

�� Evaluation report on root causes including need for further work. Engineering judgementincluding experience from e.g. Germany and USA (date to be set at the next WorkingGroup 2 meeting).

Action list

Action responsible deadline Proposal from all on urgent matters to be sent to U. Ehrnsten([email protected]) as a basis for a proposal for the nextSteering Committee meeting

All 30.03.2001

Preparation of a proposal based on the informationsubmitted for the Steering Committee Meeting

U. Ehrnsten 12.04.2001

Summary reports on destructive examinations from allplants, separately:

• Chernobyl, Kursk, Smolensk, Leningrad plants• Ignalina plant

A. Abramov A. Tereshchenko

16.04.2001 16.04.2001

List of destructive examinations performed per plant to besent to Task 3 leader

A. Arzhaev 30.3.2001

Translation of main content of Russian reports G. Sund as soon aspossible

Deliver reports on destructive examinations, both officialand internal to Task 3 leader

• NIKIET summary report• Prometey summary report• Chernobyl reports• Results on hardness and EPR measurements• Summary and reports on destructive investigations

for Kursk and Smolensk plants

A. AbramovA. TereshchenkoV. ToropA. AbramovA. Arzhaev

both delivered atthe meeting30.3.200130.3.200116.04.2001

22

Appendix VI.

TASK 4 - STATUS

PROGRAM STATUS, MARCH 2001 FOR THE PROJECT IRBIS, IGNALINA RISKBASED INSPECTION PILOT STUDY (TASK 4 IN WORKING GROUP 2)

The project IRBIS (Ignalina Risk Based Inspection pilot Study) started in May 2000 andwill be completed in December 2001. The aim is:

�� To perform a risk analysis of selected pipe systems at Ignalina plant, unit 2�� To use the results of the risk analysis (in terms of core damage frequency) in order to

define a new risk based inspection program where the focus is set on the high risklocations

�� To account for the cumulative radiation exposure to the Ignalina plant inspectionpersonnel when suggesting a new inspection program

�� To recommend improvements and further investigations that may increase the safety of theoperation of Ignalina plant, unit 2.

The pilot study involves approximately 1240 welds in the relevant 300mm stainlesssteel piping at Ignalina plant unit 2. This includes the water equalising pipes, blowdown andcooldown system, pressure header, suction header, pressure tubes, group distribution headerand downcomers. The primary damage mechanism in these pipe systems is IGSCC. Thedetailed content of the project is described below.

The main contractor is Det Norske Veritas (DNV), Sweden with Ignalina plant aspurchaser. Project leader is B. Brickstad from DNV. So far, the project group has had 5official project meetings in Vilnius and in Stockholm since the start of the project.

The progress until March 5, 2001 involves the following:

1. RBI course: Completed in September 2000 with 18 participants from Lithuania, Russiaand Ukraine.

2. Data collection: This task was completed in February 2001.3. Initial defect lengths: A report of the defect distributions for initial crack lengths used for

IRBIS has been issued by DNV.4. PFM development: This task is completed by DNV.5. Inspection: The assumptions regarding the effectiveness of the inspection methods used at

Ignalina plant unit-2, are based on a classification of the accessibility of the welds duringinspection and on the different techniques used for inspection.

6. Material: The material data used in IRBIS is reported. Some issues are still to be resolved.7. Weld residual stresses: This task is completed by DNV and a report of the numerical weld

simulations has been issued.8. System barriers: This task is completed and the results are given in the data collection

report.9. Risk analysis: This task will start during March 2001.

23

10. Definition of a new risk based inspection program: This task will be initiated in May 2001as soon as the risk evaluations are completed.

11. Sensitivity analysis: The influence upon the risk for core damage from key parameterssuch as vibrations, system barriers with dynamic effects and inspection intervals will beinvestigated during the summer of 2001

12. Reporting: A final report (in English) describing all assumptions, results and conclusionsshall be issued after the completion of the work. This is scheduled for December 2001.

The IRBIS-project consists of the following items:

1. RBI course. This includes a 3-day course (in English) on RBI and it’s practicalapplications held in Visaginas in September 2000 during the starting phase of theproject. The course will give insights of the theory and applications of RBI for nuclearpiping components and how these procedures can be used to optimize both theselection of components to be inspected and the length of the inspection intervals. Thecourse is expected to contain the following items:

�� General information on RBI-procedures for nuclear power plants�� Damage mechanisms�� Probabilistic fracture mechanics PFM�� Monte Carlo and FORM/SORM methods�� Statistical treatment of fracture toughness and crack size data�� Software for PFM�� PSA and consequence analyses�� Evaluation of risk�� Risk ranking procedures�� Definition of a risk based inspection program�� Application on Nuclear Power Plants�� Exercises of PFM and risk evaluations using validated PFM software

2. Data collection. In this task all the necessary information for performing the risk

evaluation shall be collected. This involves approximately 1240 welds in the relevant300 mm stainless steel piping in Ignalina plant unit 2. This includes the waterequalising pipes, blowdown and cooldown system, pressure header suction header,pressure tubes, group distribution header and downcomers. This includes e.g. data fordamage mechanisms, pipe and weld geometries, stress state, material data, inspectioninformation including radiation exposure and system barriers for LOCA events. Thistask does not only involve collection of existing data but also separate investigationsfor e.g. determination of pipe operating stresses.

3. PFM development. This task involves special development of the probabilistic fracture

mechanics (PFM) software to be able to treat the kind of IGSCC that has been observedin some of the welds in the 300 mm stainless steel primary circuit piping of Ignalinaplant. It includes the treatment of stress corrosion cracks that may arrest at the weld capat some distance from the outer pipe surface before leakage occurs.

24

4. Inspection. For the estimation of the risk reduction by using a non destructiveevaluation method, an assessment of the effectiveness (detection capability) of eachused inspection method is needed. This should preferably be quantified in terms ofprobability of detection (POD) for the damage mechanism IGSCC.

5. Material. In this task the material data for the pipe systems (base materials and welds)

shall be specified at the operating temperature and at the hydrotest temperature. Thematerial data includes:�� Yield strength and ultimate tensile strength.�� Fracture properties Jc at initiation and at 2 mm of stable crack growth.�� Fatigue threshold for vibration fatigue.�� Crack growth rate for IGSCC at HAZ for the stainless steel base materials.

6. Weld residual stresses. The special weld geometries and weld procedures used in the

300 mm stainless steel primary circuit piping at Ignalina plant, will make it necessary toperform a separate investigation to determine the weld residual stresses. This shall beperformed by numerical finite element method simulations based on information of thespecific welding specifications for the pipes.

7. System barriers. This task shall quantify the conditional core damage frequencies

(consequences) for different postulated LOCA events. These system barriers will inmost cases be taken directly from the Ignalina plant PSA-evaluation.

8. Risk analysis. In this task the risk for core damage (per reactor-year) for each weld will

be evaluated by combining the failure probabilities with the system barriers. The riskusing different inspection alternatives shall be investigated including the case of noinspections at all, the current inspection selection and a future risk based selection.

9. Definition of a new risk based inspection program. Based on the results of the risk

analysis and the risk profile for all the selected pipe systems, a new risk basedinspection program shall be suggested. The risk for core damage and radiation doseexposed to plant personnel for each inspection alternative shall be accounted for. Thenew inspection program shall include recommendations for suitable inspectiontechniques, inspection effectiveness and inspection intervals.

10. Reporting. A final report (in English) describing all assumptions, results and

conclusions shall be issued after the completion of the work. The results shall also bepresented at a technical seminar, to be held in Vilnius at the end of the project. All thesoftware developed during the project will be made available to Ignalina aftercompletion of the project.

Deliverables

Upon the completion of the project the following items shall be delivered to the Ignalinaplant:

25

(i) A full report written in English describing all assumptions, results and conclusionsfrom the evaluations. This includes a suggestion for a new risk based inspectionprogram for the studied pipe systems at Ignalina plant unit 2.

(ii) An Excel file (or a similar database) containing the basic input data for the riskanalysis for each studied component.

(iii) An Excel file (or a similar database) containing the basic results (failureprobabilities, system barriers, risk for core damage) for each studied component.

26

Appendix VII.

FINAL REPORT OUTLINE

1.0 WORKING GROUP 2 SUMMARY B. Brickstad

2.0 WORKING GROUP 2 OBJECTIVES B. Brickstad

3.0 TASK 1. DAMAGE DATA BASE

3.1 Data base structure and format A. Letzter

3.2 Information from collected damage data A. Letzter

4.0 TASK 2. DEVELOPMENT OF A BREAK PRECLUSIONMETHODOLOGY4.1 Description of the Russian flaw assessment procedure A. Arzhaev

V. Kiselyov4.2 Summary of national practices for flaw assessment W. Koo

4.3 Existing data on stabilised stainless steel A. ArzhaevW. KooA. Klimasauskas

4.4 Applicability of LBB for RBMK stainless steel piping W. KooA. Arzhaev

4.5 Inspection requirements L. Poulter

4.6 Benchmark problem A. ArzhaevW. KooA. Klimasauskas

5.0 TASK 3. ROOT CAUSES OF IGSCC IN RBMK PLANTS5.1 Collection of data on IGSCC cracking U.Ehrnsten

R. Kilian5.2 Evaluation of collected data U.Ehrnsten

R. Kilian5.3 Issues for further investigations U. Ehrnsten

R. Kilian

6.0 TASK 4. IGNALINA RBI PILOT STUDY

6.1 Summary report of the IRBIS project B. Brickstad

Draft Report scheduled for January 31, 2001Final Report scheduled for March 31, 2002

27

Appendix VIII.

SUGGESTED BENCHMARK PROBLEM FOR WORKING GROUP 2, TASK 2

A circumferential surface crack has been detected in a 300mm stainless steel pipe weld.The crack is located along the fusion line of the weld and the cause of the cracking is assumedto be IGSCC. The length of the detected crack is 80mm and the estimated crack depth is 5mmusing ultrasonic inspection techniques. The following tasks are to be performed usingdifferent national flaw assessment procedures:

1. Determine the acceptable surface crack size (depth and length) using appropriate nationalsafety factors. Account shall be taken to the growth of the crack by IGSCC which also isaffecting the shape of the crack. Is the pipe acceptable for continued operation without repair? 2. Determine the critical crack size (depth and length) for the surface crack accounting forthe growth of the crack by IGSCC. Critical condition is in this case, wall penetration. 3. Determine the critical through wall crack assuming crack fronts parallel with the piperadius. Critical condition is in this case, rupture of the pipe. 4. Determine a suitable inspection interval until the next inspection. (1 year is assumed tocorrespond to 8000 hours of operation). 5. Assuming that only the length of the crack is known (l = 80mm), and the crack depth isunknown, is the pipe still acceptable for continued operation without repair? If yes, suggest asuitable inspection interval until the next inspection. 6. Assume that it is possible to define a target flaw size that with the used inspectiontechnique, would be detected with a certain (high) probability. If no crack is detected, thelargest flaw that can possibly still exist is then assumed to be the target flaw size.

Determine an appropriate target flaw depth (assuming that the target flaw length is80mm) if the desired inspection interval is 4 years and that no crack has been detected withthe used inspection technique. After 4 years (32 000 hours), the final crack size (after possiblestress corrosion crack growth) should still be acceptable using appropriate national safetyfactors.

28

GEOMETRY

Detected crack length l = 80 mmDetected crack depth a = 5 mmInternal radius Ri = 146.5 mmWall thickness t = 16 mm

STRESS CONDITIONS AT OPERATING TEMPERATURE 285ºC

Primary membrane stress due to internal pressure (6.9 MPa): Pm = 30.0 MPaPrimary global bending stress due to deadweight: Pb = 10.0 MPaThermal expansion global bending stress: Pe = 20.0 MPaWeld residual stress in the axial direction along the fusion line: σ = 150 MPa at the

at the inside of pipe(local bending stress) σ = -150 MPa at the

outside of pipe

It is assumed that no other significant stresses exist.

MATERIAL DATA AT OPERATING TEMPERATURE 285ºC

Austenitic stainless steelYield stress σy = 221 MPa (lowest of base metal, HAZ and weld metal)(Yield stress at 20ºC, σy = 300 MPa)Ultimate tensile strength σu = 464 MPa at both 20 C and at 285 CAllowable design stress Sm = 155 MPaIntiation value of J-integral in weld metal: JIc = 73 kN/m (KIc = 120 MPa m )Value of J-integral in weld metal after 1 mm of stable crack growth: Jc = 253 kN/m(Kc = 224 MPa m )Elastic modulus: E = 180 000 MPaPoisson’s ratio: ν = 0.30

CRACK GROWTH RATE AT OPERATING TEMPERATURE 285ºC

The crack growth rate (mm/s) in every direction is assumed to be controlled by the localstress intensity factor at the crack front KI, measured in MPa m , of the following form:

da

dtK= ⋅ −4 5 10 12 3 0. .

I mm/s

Note that this crack growth law is here considered to be an upper bound estimation.

29

REPORTING FORMAT

Each participant dealing with the benchmark problem, should write a short summaryreport of the evaluations including stating the procedures and assumptions used and giving theapplicable safety factors according to each national regulation. Appropriate references shouldbe given. The result of the benchmark exercise should be reported at the 3rd Working Group 2meeting in September 2001.

30

Appendix IX.

PRESENTATION HANDOUTS

IAEA Extrabudgetary Program onMitigation of Intergranular Stress Corrosion Cracking in RBMK Reactors

RUSSIAN MINATOM ENGINEERING CENTER OF NUCLEAREQUIPMENT STRENGTH, RELIABILITY AND LIFETIME

The Second Meeting ofWorking Group 2 on Comprehensive Assessment Techniques

Leningrad Nuclear Power Plant, March 14-16, 2001

Task 2.1: Application of Russian RegulatoryProcedure for Flaw Assessment ofRBMK Austenitic Piping Systems

Vitaly Kiselyov, Alexsey Arjaev

ENES, Moscow

Table of Contents

Page

Error! No table of contents entries found.

March 2001

Application of Russian Regulatory Procedure for Flaw Assessment of RBMKAustenitic Piping Systems

KIS 07.03.01/Procedure.doc Page �

1 Defect Tolerance Evaluation

1.1 Procedure

The discovery of IGSCC in 300mm austenitic piping systems raised a problem regarding flawevaluation by fracture mechanics analysis.

To fully preclude catastrophic failure due to defect developments in weldments, conservative flawevaluation regulatory procedure /KIS 98/ based on generation the plant-specific As an illustrationStructural Integrity Diagram (SID) was applied to a RBMK austenitic downcomer pipework. Theprocedure (see Fig.1) requires that crack growth should be calculated, and the flawed welds are tobe acceptable for further operation only for the time period that the flaw remains rather small andadequate safety margins against failure are provided. Crack growth rates estimated in the procedurewould be used as inputs into predicting the ISI interval for RBMK downcomer pipework with adetected flaw.

For development of the plant-specific SID defining the critical flaw sizes, it is assumes the existence ofa circumferential part-through wall surface flaw in sensitised austenitic material so that crack growth dueto IGSCC can occur under sustained loads. Determination of the acceptance flaw sizes is based on acritical flaw size evaluation and also includes a safety margins on service loads as per the flawevaluation procedure laid down the procedure /KIS 98/.

The dominant failure mode in austenitic pipe with circumferential flaws is plastic instability and collapse.

Austenitic straight pipes with circumferential flaws under combined loading by internal pressure andbending moment were evaluated using the verified Net-Section-Collapse (NSC) analysis /NRC 94/.The NSC analysis is a plastic limit-load analysis which assumes that the material toughness issufficiently high that failure is governed by plastic instability which occurs when the stress in entirenet section at the crack reaches the material’s flow stress (or collapse stress) and is not sensitive tothe material’s toughness. The flow stress is a value between the material’s yield strength andultimate strength and represents an average critical net-section stress reached throughout the flawedligament of the structure. Also, crack stable crack growth is not taken into account, i.e. crack initiationconditions are not evaluated.

For a pressurised pipe with a circumferential through-wall and part-through-wall surface flaw

subjected to bending, that is, combined loading by axial tension and membrane stress, σm , and

bending stress, σb, the plastic limit conditions are:

�VLQW

D�VLQ��

�

)Eθβσ

πσ , (1)

�5

5�

W

D��

� L

)

P

σσ

πθπβ for θ+β≤π . (2)

Here: θ is half angle of circumferential crack in straight pipe; β is angle from bottom of pipe toneutral axis; a is depth of internal surface crack; t is wall thickness; Ri is inner radius, R is meanradius of pipe.

The NSC analysis uses only the primary stresses (membrane and bending). Weld residual stressesand other secondary bending stresses do not contribute to net-section collapse of circumferentiallyflawed straight pipe and are not included in the analysis. No specific fracture mechanics data areneeded for NSC analysis only yield strength and ultimate strength from relevant stress-strain curve isused. For these purposes, plant-specific materials properties are the most preferable.



For austenitic pipes, a lower-bound stress-strain curve should be used in NSC analysis regardless ofwhether the weld or base metal is limiting, even in the case where the crack is located in the weldmaterials. That is because of higher mechanical properties of weld materials for which the ASME codeZ-factors /ASME XI/ should be taken into consideration. In addition, a lower-bound toughness (weldmetal or base metal) should be used in case of application of more sophisticated elastic-plasticfracture mechanics based on tearing instability.

Flow stress (or collapse stress) was defined as follows: σF = 0.42 (Rp0.2+ Rm), i.e. more conservativelythan the average of the yield and ultimate strengths of the material, or 3Sm as recommended inASME code /ASME XI/. Relevant mechanical properties obtained from actual material testing at350oC / PRO 98 / were used in the analysis.

Structural integrity diagrams defining the combination of crack length and depth at which collapseoccurs at the most loaded weld were developed for stress levels expected for normal operatingloads, hydraulic test (HT) and safe shutdown earthquake (SSE). Piping loads of interest are normaloperating loads (pressure, dead weight and system thermal expansion) and inertia seismic loads andmoments. Maximum piping stresses, axial tensile and bending produced by a combination of theseloads were obtained from plant-specific stress analysis of whole piping system considered. Althoughthe residual stress is assumed to be the same for all welds, the applied service stresses vary fromweld to weld; therefore, bounding allowable analysis curve was generated for the most loaded weld.

1.2 Example of Analysed Downcomer Pipework

The purpose of this study is only evaluation of critical crack sizes for actual plant loading conditionsand material properties.

For example, to ensure break preclusion under plant-specific operating loads in a 300mm RBMKaustenitic stainless steel downcomer pipework the following objectives were addressed to thisdefect-tolerance study by fracture mechanics:

- Generation of a Structural Integrity Diagram (SID) defining the combination of crack length anddepth at which collapse occurs;

- Generation of a flaw acceptability diagram defining the maximum allowable end-of-inspectionperiod flaw size.

The geometry of straight pipe, material data, and loading conditions used for the flaw evaluations inthe 300mm austenitic downcomer pipework are summarized in Table 1.

1.3 Results

Fracture mechanics analysis for the 300mm austenitic pipework was done using valid computer code“Fracture 1.0” /KIS 00/ and input data from Table 1.

Fig.2 shows Structural Integrity Diagrams generated for normal operation (N-load), hydraulic test(HT-load) and maximum accident (N+SSE-load) conditions. The calculations were carried out usingactual material properties.

The following values of the critical length of through-wall crack (TWC) were obtained:

( 1 ) 2cc = 388mm for N-load;

( 2 ) 2cc = 404mm for HT-load;

( 3 ) 2cc = 353mm for (N+SSE).

As it followed from Fig.2, the critical length of surface crack is larger than 500mm for all loadingconditions if the crack depth is less than 80% of wall thickness and 300mm stainless steel pipes can



tolerate large circumferential cracks without failure. For example, a through-wall crack not extending40% of the pipe circumference will not lead to collapse under maximum normal operating load.

Fig.3 summarises Flaw Acceptability Diagrams developed for excessively high loads. For generationof allowable analysis curve for normal operation, the explicit margin of 3 on all normal (N) stresses(axial tensile and bending) was included in the NSC analysis. Similar allowable analysis curves weredeveloped for emergency (HT) and maximum accident (N+SSE) conditions with safety factors of 2.0and 1.4 on all stress components, respectively. In the last case, corresponding stress componentsarising from calculated inertia seismic loads were added absolutely to normal loads.

Regarding to 300mm austenitic downcomer pipework, flaw acceptability line generated for normaloperating load using safety factor of 3 on all stress components was identified as bounding analysiscurve in comparison with acceptable analysis curves generated for emergency or maximum accidentloads using safety factor of 2 and 1.4 on all stress components, respectively. It follows also fromconsideration of maximum service stresses given in Table 1.

Due to the fact that flaws exceeding 50% of the circumference are equivalent to a 360o

circumferential crack and through-wall cracks can not be tolerated (even though sufficient structuralmargins can be demonstrated), and taking into account some uncertainty in sizing IGSCC(especially, accuracy and reliability in determination of through-wall defect depth by UT), themaximum allowable crack size is conservatively limited. The acceptance crack length is limited to50% of critical length of through-wall crack, and the maximum allowable crack depth is limited to two-thirds (67%) of the pipe thickness regardless of part through-wall flaw length. Also, from a practicalperspective it may be more cost effective to repair the weldments with such large cracks even thoughadequate structural margins can be shown.

Fig.4 shows the SID for the 300mm austenitic pipework. The acceptance flaw sizes correspond tosafe area under the bounding allowable analysis curve covering all service loads.

The results plotted allow evaluating a detected flaw. It should be note that the acceptance criteriarepresent the maximum flaw size that can be tolerated at the end of the next inspection interval. Todetermine the maximum admissible flaw sizes for a surface flaw at the previous inspection, it isnecessary to subtract the expected crack growth during the stipulated period, e.g. 1 year or 4 year.

The crack growth consideration is required in order to estimate the period for which the flaw can beleft unattended. In compliance with the statements /KIS 98/, the anticipated crack propagation in300mm stainless steel downcomer pipework due to IGSCC is limited by upper bound crack growthrates. For crack depth less than 2/3 of the wall thickness, the following conservative estimate ofcorrosion-related crack growth was made: 1 mm/year in the depth direction and 10 mm/year in eachlength direction.

The admissible flaw size ( ap , 2cp ) is the maximum size to which a detectable flaw ( a0 , 2c0 ) can beallowed to grow within the need for repair. In other words, it is the maximum allowable end-of-inspection period flaw size. Thus, the SID represents a simple tool for assessment of the results ofnon-destructive examination of 300mm RBMK austenitic pipework.

2 Conclusions

Conservative Structural Integrity Diagram based on net-section collapse analysis was generated for300mm RBMK austenitic pipework to be used in assessment of IGSCC flaws revealed by ISI. Topredict instability conditions for flawed austenitic downcomer pipework the flaw evaluation procedurebased on Net-Section Collapse (NSC) analysis was used.



References

/ASME XI/ ASME Boiler and Pressure Vessel Code. Section XI, Rules for In-service Inspection ofNPP Components. New York, 1995.

/KIS 98/ V. Kiselyov. Flaw Evaluation Procedure Based on ISI Results.IAEA Regional Workshop. Environmentally Assisted Cracking of NPP Austenitic Piping,Slavutich, 22-26 June 1998.

/KIS 00/ V. Kiselyov, M. Dobrov. Computer Code )racture 1.0 for Flaw Stability Analysis andCrack Opening Area Calculation. User’s Guide. Moscow, 2000.

/NRC 94/ Nuclear Regulatory Commission. Stability of Cracked Pipe under Inertial Stresses.USNRC, NUREG/CR-6233, BMI-2177, Vol.1, 1994.

/PRO 98/ TSNIIKM ‘Prometey’. Analysis of Experimental Results of Material Tests of MechanicalProperties and Fracture Resistance Characteristics of Austenitic Type Stainless Steelsand Their Weldments of NPP Piping. Report. St.P., 1998.

Tables and Figures

Table 1 Input Data Used in the Fracture Mechanics Analysis of a RBMK DowncomerPipework

Figure 1 Current Flaw Evaluation Procedure for 300mm Austenitic Pipework

Figure 2 Example of Structural Integrity Diagrams for 300mm Austenitic Pipework under

Normal Operation, Hydraulic Test and Maximum Accident (NOC+SSE) Conditions

Figure 3 Example of Flaw Acceptability Diagrams for 300mm Austenitic Pipework

under 3×N-load, 2×HT-load and 1.4×(N+SSE) load

Figure 4 Example of Structural Integrity Diagram for 300mm Austenitic Pipework under

Normal Operating Loads



Table 1 Input Data Used in the Fracture Mechanics Analysis of a RBMK Downcomer Pipework

Geometry

Outer diameter

Wall thickness

Crack location

Crack shape

325 mm

15 mm

heat affected zone (HAZ) of circumferential welds

(i) circumferential semi-elliptical crack on the inner surface

(ii) circumferential through-wall crack

Plant Specific Material Data (Base metal or HAZ) at 350oC /PRO 98/

Material

Actual tensile yield strength, Rp0.2

Actual ultimate tensile strength, Rm

Minimum specified yield strength, Rp0.2

Minimum specified ultimate strength, Rm

Young’s modulus, E

Austenitic stainless steel, type 08Ch18Ni10T

226 MPa

530 MPa

180 MPa

415 MPa

182 GPa

Loading ConditionsNormal operating

conditions(NOC)

Emergency,Hydraulic test

(HT)

Maximumaccident

(NOC+SSE)

Design Temperature, ToC

Design pressure, p, MPa

Maximum membrane stress, σm , MPa

Maximum bending stress, σb , MPa

285

6.9

41.5

83.6

59.3

46.2

41.6

108.0



�� Detectable flaw size ( a0 , 2c0 ) from ISI results

�� Transients and degradation mechanisms

�� Operation loading spectrum ( Pd ) and piping stress analysis

�� Evaluation of critical flaw sizes by NSC analysis: ( ac , 2cc ) = f ( Pd )

�� Generation of failure analysis diagrams for design loading conditions

�� Safety margins on service loads:

np = 3.0 for Normal and Upset loads;

np = 2.0 for Abnormal and Emergency loads;

np = 1.4 for Maximum accident loads, e.g. NOC+SSE

�� Evaluation of acceptance flaw sizes by NSC analysis, e.g. ( ap , 2cp ) = f ( 3Pd ) for NOC

�� Generation of bounding allowable curve, i.e. Structural Integrity Diagram ( SID )

�� Flaw growth analysis:

- Corrosion fatigue (∆aCF, 2∆cCF ) = (da/dN, dc/dN)w = gcf(∆KI) = 10(da/dN, dc/dN)air

- IGSCC ( ∆aSCC , 2∆cSCC ) = gscc (KI ), e.g. = [ 1 mm/yr, 20 mm/yr ]

�� Flaw size evaluation for the end of the next inspection period (τ, Nτ), e.g. τ =1 year:

at = a0 +∆aΣ = a0 +∆aCF +∆aSCC = a0 + ( da/dN )w ⋅Nτ + 1 (mm/yr)⋅τ

2ct = 2c0 +2∆cΣ = 2c0 +2∆cCF +2∆cSCC = 2c0 + 2 ( dc/dN )w⋅Nτ + 20 ( mm/yr )⋅τ

�� Evaluation of the cracked weldments:

( at , 2ct ) ≤ ( ap , 2cp )

↓ yes ↓ no

- Permission for further - Repair or replacement

limited time operation - Shorter inspection interval

- LBB case evaluation - Other urgent measures

- Long-term measures

Fig.1 Current Flaw Evaluation Procedure for 300mm Austenitic Pipework



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2cc=388mm

2cc=388mm

Application of Russian Regulatory Procedure for Flaw Assessment of RBMK Austenitic Piping Systems


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TASK 3. ROOT CAUSE ANALYSIS

NIKIET RESULTS

by V. Abramov

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