EXPERTS DISCUSSION ON FOLLOW- UP ACTIVITIES ON IGSCC OF RBMK REACTORS AUSTENITIC STAINLESS STEEL PIPING

Transcription

1 IAEA-EBP-IGSCC-P02 IAEA-TC-8141 LIMITED DISTRIBUTION EXPERTS DISCUSSION ON FOLLOW- UP ACTIVITIES ON IGSCC OF RBMK REACTORS AUSTENITIC STAINLESS STEEL PIPING IAEA, VIENNA, AUSTRIA OCTOBER 1998 TC PROJECT RER/9/052 DEPARTMENT OF TECHNICAL CO-OPERATION DEPARTMENT OF NUCLEAR SAFETY INTERNATIONAL ATOMIC ENERGY AGENCY

2 CONTENTS 1. INTRODUCTION 2. OBJECTIVE 3. PRIORITIES 4. MEASURES REQUIRED TO ADDRESS THE ISSUE 5. CURRENT INTERNATIONAL ASSISTANCE PROGRAMMES 6. PROPOSAL OF FOLLOW-UP ACTIVITIES 7. CONCLUSIONS APPENDIX I. STATUS UPDATE FOR LITHUANIA APPENDIX II. STATUS UPDATE FOR RUSSIA APPENDIX III. REVISED OUTLINES OF FOLLOW-UP ACTIVITIES ABBREVIATIONS REFERENCES CONTRIBUTORS TO DRAFTING AND REVIEW 2

3 1. INTRODUCTION In 1997, intergranular stress corrosion cracking (IGSCC) was detected in the heat affected zone of austenitic stainless steel piping welds in several RBMK NPPs. Similar degradation has been observed earlier at a number of vessel type boiling water reactors (BWR) and recognized as a generic safety issue for reactors operating with BWR type water chemistry, being a new safety issue for RBMK reactors. Recognizing the importance of the issue and upon invitation of the Government of Ukraine, the IAEA organized a Workshop on Environment Assisted Cracking of NPP Austenitic Stainless Steel Piping in Slavutych, June 22-26, The objective of the Workshop was to provide a forum for the exchange of experience. The Workshop was organized in the frame of the IAEA TC Project RER/9/052. The Workshop concluded, that actions to address the issue for RBMK reactors have been initiated but still need to be completed. Further exchange of experience and international co-operation in this area is of a high importance. Follow-up activities to address safety concerns associated with this issue for RBMK reactors should be established as a matter of urgency. In order to develop comprehensive well balanced proposals for follow-up activities necessary to address the issue of IGSCC that were consistent with the conclusions of the Workshop [1], the IAEA convened an experts meeting at its headquarters in Vienna, October 27-30, During the meeting, experts performed a final review of the Workshop report [1] and developed detailed outlines for several follow-up activities necessary to address the issue of IGSCC. 16 experts from countries operating RBMK reactors, countries involved in the international assistance programmes and the IAEA participated in the meeting. The basis for the discussion during the meeting was the draft report of the Workshop [1], international experience on the IGSCC issue and the experience accumulated to date in Lithuania, Russia, and Ukraine. Based on the discussion during the meeting this report was developed by the experts. Section 2 of this report indicates its objective. Section 3 deals with priorities for compensatory and corrective measures to address the issue. Section 4 provides an overview of technical measures to address the issue. Section 5 summarizes the status of ongoing co-operation programmes based on information available during the meeting. In Section 6 seven follow-up activities are outlined and in Section 7 the conclusions of the meeting are given. In Appendix I, update of the status in Lithuania as presented during the meeting is given. Appendix II contains the status update for Russia as well as some views of the RBMK designer as provided. In Appendix III., revised and condensed description of follow-up activities drafted by Russian specialists after the meeting is given. 2. OBJECTIVE The objective of this report is to present the outlines of the safety related activities judged by the experts participating in the meeting as necessary to address the issue of IGSCC in RBMK reactors. Each outline includes the objective, scope, tasks, milestones, product and participants required to implement the follow-up activity. In developing the activity outlines given in this report, safety, technical rationale, and practical feasibility were considered. The activities presented in this report represent to the extent possible the consensus of the group of experts who participated in the meeting. The implementation of all of the follow-up activities outlined is a matter to be addressed in the frame of the national safety improvements programmes and bilateral and international co-operation programmes. 3

4 3. PRIORITIES During the discussion, the following priorities for actions to address the IGSCC issue were proposed by specialists from countries operating RBMK reactors: 1. Improvement of the reliability of the in-service inspection (ISI) performed on site; 2. Implementation/improvement of leak detection systems, improvement of the mechanistic understanding of the degradation, and comprehensive integrity assessment; 3. Improvement of repair techniques; 4. Adaptation of corrective measures to regulatory requirements and their on-site application. It should be noted that some experts participating in the meeting did not agree with these priorities and it was not possible to establish consensus during the meeting. In connection with the priorities given above, it was stated by RDIPE experts that these are based on the discussion held during the Slavutych Workshop [1], international experience available [7], IAEA recommendations on RBMK pressure boundary integrity [3, 4] and current status of implementation of measures at RBMK NPPs. 4. MEASURES REQUIRED TO ADDRESS THE ISSUE Measures required to address the issue of IGSCC for RBMK reactors can be summarized as follows. Improvement of ISI capability and reliability Improving in-service inspection capability in RBMKs can be accomplished by improving four areas in ISI: Developing ISI techniques that improve inspection of welds with limited or no access; Establishing a regional ultrasonic ISI qualification program; Transferring ultrasonic technology for inspection of welds repaired using the weld overlay technique; Application of automated UT with state-of-art data acquisition systems and software for ISI results evaluation. Improvement of leak detection systems capability This could be achieved through review of performance of existing leak detection systems and their subsequent improvements or installation of new leak detection systems. Comprehensive safety and integrity assessments A comprehensive safety and integrity assessments should be performed to ensure that all potential issues associated with IGSCC are addressed. The comprehensive assessments should include the following elements. A study of leading damage causes which influence different level of damage at specific Units, Improvement of on-site water chemistry parameters monitoring and control system, Establishing a comprehensive design, manufacturing and operating history data base, A comprehensive failure analyses, including laboratory testing using state-of-the-arttechniques, analyses of on-site and R&D data, Analyses of possible transients and assessment of loads introduced due analysed transients, 4

5 Analyses of criteria for integrity assessment and data on their engineering validation, comprehensive integrity assessment based on verified criteria, loads assessment and results of leading causes of damage. Improvement of repair techniques Improving the repair process for IGSCC can be accomplished by improving techniques for on-site weld edges preparation, transferring automated welding techniques for on-site repair, transfer of technology for optimized welding, transfer of technology for overlay welding repair, transfer of technology for thermal treatment (solution heat treatment on-site, induction heating). Corrective measures Corrective measures include in particular the transfer of technology for mechanical stress improvement process application, and transfer of technology for water chemistry improvement with respect to IGSCC (e.g. conductivity, noble metals or other specific addition).the use of materials more resistant to IGSCC should be also considered. Regarding the approach to the implementation of the measures to address the issue, it was stated that the issue would be most effectively addressed on a regional basis involving regulators, utilities, plant designers and other technical support organizations A Russian side proposal formulated after the meeting (text given next in italics) is that a regional team involving three working groups should be established: Group 1: Representatives of regulatory authorities and technical support organisations; Group 2: Representatives of appropriate ministries and organisations of NPP technical support, Chief Designer of RBMK; Group 3: Utilities operating RBMK reactors. All international programs of general activities (as comprehensive root cause analysis, integrity analysis, inspection qualification requirements harmonisation) and technology transfer which needs adaptation to regulatory requirements should be performed by Group 2 efforts co-ordinated with Group 1 because there is no sufficient differences in regulatory practice up to now for welds of austenitic piping. All programs of direct delivery of equipment to RBMK NPPs should be supervised by Group 3 in co-ordination with Group 2. Further, the Russian experts noted the following. General safety problems of RBMK NPPs with respect to design basis accidents have been analysed in the framework of IAEA Extrabudgetary Programme on the Safety of WWER and RBMK NPPs ( ) [2-4], CEC studies on RBMK NPPs safety improvement [5] and numerous bilateral studies. As prioritized for RBMK NPPs earlier [3, 4] the issues of ISI improvement are one of the most important when pressure boundary integrity is discussed. Following recommendations of [6] on the role of ISI in proper addressing of IGSCC damages tremendous amount of inspection was fulfilled by UT during at all RBMK NPPs [1] and made it possible to have a picture on IGSCC damage levels at specific sites. Increased efficiency of inspection by UT instead of radiography was demonstrated obviously at Chernobyl NPP Unit 3 where flaws have been revealed in 182 welds in addition to 268 flawed welds after radiography [1]. But still there are 5

6 some zones in dia.300 mm piping not accessible for the UT or with limited access. Another aspect of UT improvement is elimination of human factor influence during inspection by application of automatic and semiautomatic UT tools/procedures with data recording to PC memory. Taking into consideration common features of design of RBMK austenitic piping (piping diameter and wall thickness, geometry of weld edges preparation, base and weld materials, etc.) there is opportunity to establish a Regional Center for personnel training and inspection qualification. It comes out from the discussion that all RBMK countries have in the current moment very similar requirements to operation and repair of austenitic piping. The later makes it possible to perform adaptation of known technologies in the area of inspection and repair of RBMK austenitic piping on generic basis for all RBMKs. Strategies to address IGSCC issues by appropriate technical measures are very similar in all RBMK operating countries as stated in discussion by all RBMK experts from utilities, regulators and technical support organizations. 5. CURRENT INTERNATIONAL ASSISTANCE PROGRAMMES There are several ongoing bilateral and international assistance programmes, some of which are directly related to this issue, other indirectly. It is however recognized that much more extensive and concerted effort would be required to address the IGSCC safety issue in a reasonable timeframe. Next, the information provided by individual experts participating in the meeting on the current situation regarding IGSCC relevant co-operation programmes is given. USA International Nuclear Safety Program Regarding Ukraine, in March 1997, the United States delivered state-of-the-art manual ultrasonic pipe inspection systems to all five nuclear power plants in Ukraine. Also in March, inspection personnel from each plant attended a six-day training course at the Khmelnitsky plant, led by U.S. experts. In April 1998, specialists from the Pacific Northwest National Laboratory conducted a two-week workshop at Khmelnitsky on ultrasonic examination of pipes fabricated from austenitic steel. Representatives from all five Ukrainian plants participated. The United States is supporting increased training in nondestructive examination. With U.S. support, Energoatom and the National University of Ukraine have established a central training and certification facility for nondestructive examination. U.S. and Ukrainian experts are developing a process for certifying technicians as nondestructive examination specialists. These requirements conform to international standards. The centre is also developing a procedure for inspection of austenitic welds, 1 st draft of which has been completed by mid Regarding Russia, the United States delivered both hand-held and remotely operated ultrasonic test equipment to Kursk plant in Kursk personnel received training in the use of both types of equipment in October and November In January 1999, the United States delivered a manual ultrasonic inspection system to each Russian plant. U.S. specialists trained technicians at each site to use the equipment. The training will be completed in line with the schedule agreed by both sides. 6

7 The International Nuclear Safety Programme (INSP) also sponsors a programme that is directly related to RBMK maintenance and titled RBMK Maintenance project. This project has created an RBMK Advisory Board that sets priorities for maintenance issues related to RBMKs, which could also be a central point of contact in coordinating efforts related to IGSCC repair. The US Department of Energy point of contact for this project is Gregory Trosman. The project manager is Tom Vehec from Pacific Northwest National Laboratory. Sweden and Lithuania Within the Lithuanian-Swedish bilateral programme, a broad support is given in the field of ISI to Ignalina NPP. Up to now, concerning IGSCC in downcomers the following support was provided by SIP to Ignalina plant in performance of 100 % ISI on Unit 1: special manipulator was developed by ABB TRC for application of automated UT with application of Tomoscan and P-scan data acquisition equipment for UT of the 325 mm pipelines; TomoView software for assessment of automated UT results was provided; implementation of specially developed UT techniques ( KRAB ) and special UT probes for defect characterization (AMDATA probe) was provided; root causes analysis and comprehensive material investigation of IGSCC defects was performed, follow-up activities were planned within development of Reactor Cooling System Safety Case which is under development in Ignalina NPP (Safety justification with respect to the current ISI results, consideration of improved repairing techniques, improvements of LDS, etc.). Russia No direct activities to address IGSCC issues via international/bilateral programmes of technical co-operation and assistance have been realised in for RBMK NPPs in Russia. Some previous activities, which were not planned to address directly IGSCC issues, could be useful to improve current situation. Their current status is as follows: 1)TACIS Project R1.05/94 for Smolensk NPP (equipment for NDE, including means for dia.300 mm piping): January endorsement of ST-TOR; June finalising the bid among participating companies; October 30, no contract for NDE equipment delivery have been signed yet. 2)TACIS Project R1.05/95A for Smolensk NPP (implementation of leak detection system): ST-TOR is not prepared and agreed yet. 3)EBRD Project B-3 for Units 1-4 of Leningrad NPP (Implementation of leak detection system by humidity monitoring): equipment is in stage of assembling at Unit 1. 4)EBRD Project A.1.2. for Units 1-4 of Leningrad NPP on UT non-destructive examination equipment contract awarded to Tecnatom, Spain UT equipment to be delivered by December 1998 Ukraine The equipment for ISI and weld preparation is being supplied through funding by Nuclear Safety 7

8 Account EBRD Grant only. The ultrasonic and visual test equipment (EBRD Project A.1.2. for Chernobyl Unit 3) will be provided by Tecnatom, Spain in November 1998.The weld preparation equipment was provided by Furmonoit, UK. In addition, the Chernobyl plant procured itself automatic welding device from Polisud, France. 6. PROPOSAL OF FOLLOW-UP ACTIVITIES This section presents seven follow-up activity outlines that were developed during the meeting. These activities are to be implemented on a regional basis wherever applicable, involving experts from all 3 countries operating RBMK reactors (regulatory bodies, plant operators, technical support organizations, and plant designer). It is recognized that a strong co-ordination with the other relevant programmes is required to avoid duplication of effort. As a first step to initiate these activities, the IAEA should convene for each of them a narrow focus specialized technical meeting with the objective to develop the activity proposals given in this section into a detailed workplan for each. Experts participating in these meeting should have a detailed knowledge in the subject discussed and will be expected to form an activity experts group. Russian experts pointed out that in addition to these seven activities, which are aimed at addressing the root cause of the safety issue, there is an urgent need to implement immediate mitigation measures. These include direct supply of equipment for automated non-destructive testing, automated weld preparation equipment (edge cutting devices), automated welding equipment and technology for improvement of properties of existing welds to the operating plants. These mitigation measures would reduce the dose to the personnel and improve the quality of maintenance and inspection. It should be noted, that Russian RBMK specialists also developed after the meeting a condensed and partly revised version of the follow-up activities outline. This revised version, which was not discussed and agreed upon during the meeting but which contains useful and well structured information is provided in Appendix III Safety assessment Background IGSCC is known to be a cracking mechanism capable of producing long cracks that will not necessarily exhibit LBB behavior. The performance of RBMK systems with extensive IGSCC, either under accident conditions or with continuing growth under normal operating conditions is not well understood. Furthermore, the existing ISI results have not been well explained in terms of the extent of cracking at particular plant and weld types. Objectives Identify the overall safety concerns associated with the presence of IGSCC in RBMKs. Consider the capabilities of safety systems to recover from accidents involving austenitic piping failure. This would include identification of limiting transients. Further consider the value of defense in depth concepts such as LBB to reduce the risk posed by IGSCC. Perform limited LBB analysis to identify the stability of cracks allowed to remain in service and to determine the leak detection systems requirements. Collate existing ISI information and plant operational conditions to explain 8

9 observed behavior and predict possible future development. Identify priorities for ISI and leak detection systems Scope The scope of this activity includes developing requirements and priorities for safety improvements, that allow IGSCC to be managed safely. Identify the relative benefits of possible safety improvements to plant or maintenance processes e.g. better leak detection, inspection improvements, stress improvement etc. In addition, results from other activities given next will provide important input and feedback and will be therefore provided to and discussed on a regular basis in the frame of this activity. Tasks 1. List possible accident scenarios and comment on relative likelihood. 2. Identify safety systems and safety arguments available to make defense in depth arguments against IGSCC related accidents. Suggest a basis for regulatory review of safety assessment. 3. Identify requirements for leak detection systems and targets for ISI effectiveness (including performance of LBB analysis). 4. a) Collection and analysis of ISI experience from each RBMK. b) Identify conditions that may be leading/have led to IGSCC at each NPP. c) Determine critical factors for IGSCC and likely future trends. 5. Prioritize improvements to ISI implementation, leak detection and other systems. Milestones 1. Preliminary report (3 months) 2. Preliminary report (6 months) 3. Preliminary report (12 months) 4. Preliminary report (12 months) 5. Final report (12 months) Product Recommendations on priorities for system/maintenance procedure upgrades. Participants RBMK safety experts Regulatory body experts RF, UA, LT staff from NPPs Plant design organizations 6.2. Improvement of ISI techniques Background In-service inspection of nuclear power plants is a diagnostic tool that helps ensure the structural integrity of primary coolant and safety engineered piping systems. In-service inspection detects defects that can compromise the safe operation of nuclear power plants. The IAEA organized since 1990 a programme to assist countries in Eastern Europe and the former Soviet Union to evaluate the safety of their nuclear power plants by identifying major design and operational safety issues. As part of the IAEA TC programmes and of this programme, a regional workshop that evaluated 9

10 environmentally assisted cracking of austenitic piping has been organized in June A panel of international experts that attended the workshop concluded that environmentally assisted intergranular stress corrosion cracking (IGSCC) is a generic safety issue for reactors that operate with a BWR type water chemistry. IGSCC has recently been detected in RBMK type reactors in Lithuania, Russia and Ukraine. In-service inspection is vital to the proper management of IGSCC. Several specific ultrasonic inspection techniques have been developed in other countries earlier (both Europe and the USA). These inspection techniques have played a valuable role in helping manage piping degradation due to IGSCC. This follow-up activity to the Regional Workshop on Environmentally Assisted Cracking of NPP Austenitic Piping proposes that several regional workshops and scientific exchanges be sponsored that transfer the following ultrasonic inspection techniques. Ultrasonic inspection of weld overlays Ultrasonic inspection for single side (limited) weld access Ultrasonic search unit design for welds that have only 20-40mm scanning access Objective The objective of this engineering study is to organize the relevant experts and conduct a series of workshops and scientific exchanges to transfer ultrasonic inspection technology in the following areas. Ultrasonic inspection of weld overlays Ultrasonic inspection for single side (limited) weld access Ultrasonic search unit design for welds that have only 20-40mm scanning access Scope The scope of this activity addresses the transfer of technology through workshops and scientific exchanges. These workshops will concentrate on ultrasonic examination of primary coolant and safety engineered system piping welds. Tasks The following tasks are required to complete this engineering study. 1. Select appropriate experts for the following technical issues in ultrasonic inspection. Ultrasonic inspection of weld overlays Ultrasonic inspection for single side (limited) weld access Ultrasonic search unit design for welds that have only 20-40mm scanning access 2. Conduct three one week workshops to address the specific topics listed above in Task Sponsor three one week scientific exchanges to follow-up and reinforce the information presented during the workshops in Task Provide topical reports that summarize the information presented in workshops and the success of the technology exchange as determined by the follow-up scientific exchanges. 10

11 Milestones Completion of each of the tasks listed above represents a major milestone. This activity is estimated to require one year to complete. The estimated time duration for each task is listed below, however, activity funding levels may significantly alter the estimated duration for the tasks. 1. Select appropriate experts for ultrasonic inspection. This should be complete within two months after the start of the activity. 2. Conduct three one week Workshops. This should be complete within six months after the start of the activity. 3. Sponsor three one week scientific exchanges to reinforce and follow-up Workshop presentation. This document should be complete within three months after completion of the initial workshop. 4. Topical Reports. The topical reports should be completed within two weeks of the follow-up exchange. Product The product from this activity is initial and rapid technology transfer of critical ultrasonic inspection technology to regional experts. Participants The following organizations should participate as members of the activity team. Representatives from regulatory bodies operating RBMK nuclear power plants. Ultrasonic inspection specialists from RBMK power plants. RBMK designer Appropriate ultrasonic inspection experts from western organizations that have developed ultrasonic techniques for the following areas. Ultrasonic inspection of weld overlays Ultrasonic inspection for single side (limited) weld access Ultrasonic search unit design for welds that have only 20-40mm scanning access Relationship to other projects The US Department of Energy, the European Union and other organizations sponsor projects to improve the safety culture and safe operation of eastern European nuclear reactors. This technology transfer project leverages the funding and safety improvement efforts. This project also fosters international collaboration and coordination of both the US and Europe to improve inservice inspection in Eastern Europe ISI systems qualification Background In-service inspection of nuclear power plants is a diagnostic tool that helps ensure the structural integrity of primary coolant and safety engineered piping systems. In-service inspection detects defects that can compromise the safe operation of nuclear power plants. IAEA organized a programme to assist countries in Eastern Europe and the former Soviet Union to evaluate the safety of their nuclear power plants by identifying major design and operational issues. As part of 11

12 this program, IAEA sponsored a regional workshop that evaluated environmentally assisted cracking of austenitic piping. A panel of international experts that attended the workshop concluded that environmentally assisted intergranular stress corrosion cracking (IGSCC) is a generic safety for reactors that operate with a BWR type water chemistry. IGSCC has recently been detected in RBMK type reactors in Lithuania, Russia and Ukraine. In-service inspection is vital to the proper management of IGSCC. International ultrasonic inspection reliability programmes have shown that one of the key elements of ensuring that reliable in-service inspection is possible involves qualifying in-service inspection processes using both performance demonstration and technical justification. IAEA has published recently a report on Methodology for qualification of in-service inspection systems for WWER nuclear power plants, IAEA-EBP-WWER-11 [7]. These guidelines for qualifying in-service inspection processes include developing a technical justification as well as experimental trials. The European countries experience with respect to the implementation of ENIQ guidelines should be considered as well. This follow-up activity proposes that a regional engineering study be conducted that develops practical guidelines to address the generic issue of reliable piping examination in RBMK nuclear power plants. Objective The objective of this engineering study is to organize the relevant experts into a activity team to develop practical qualification criteria and implementation documentation for an ISI qualification center. Scope The scope of this activity addresses adapting the qualification guidelines already developed by IAEA into a clear concise report that outlines the technical criteria and implementation requirements for a regional center for qualification of ultrasonic examination of piping. The qualification center approach will be based upon a combination of technical justification and experimental trials. The experimental trials will provide assurance that the combination of equipment, procedures, and personnel are capable of detecting defects that can potentially compromise the structural integrity of primary and safety related piping. Tasks The following tasks are required to complete this study. 1. Selection of participants for the activity team 2. Develop criteria for selection of location of the regional performance 3. Develop Technical Justification Documentation - This document outlines the basis for determining: The technical topics that must be addressed in the procedure technical justification. The rationale for the distribution of pipe diameters and wall thicknesses used in the trials. The rationale for defect sizes and numbers of defects used in the experimental trials The accept/reject criteria for both blind and non-blind trials Technical Support provided to candidates during testing 4. Develop and document Protocols for demonstration 5. Develop and document Specifications for the test specimens 6. Develop and document Quality assurance requirements for the ISI qualification center. 12

13 Milestones Completion of each of the tasks listed above represents a major milestone. This activity is estimated to require two years to complete. The estimated time duration for each task is listed below, however, activity funding levels may significantly alter the estimated duration for the tasks. 1. Selection of activity team participants. This task should be complete within three months after the start of the activity. 2. Criteria for selection of regional ISI qualification center. This task should be complete within six months after the start of the activity. 3. Develop technical justification documentation. This document should be complete within one year after the start of the activity. 4. Develop and document protocols for demonstration. This task should be complete within one year after the start of the activity. 5. Develop and document specifications for the test specimens. This document should be complete within nine months, however, the work on the document cannot start until the completion of Task Develop and document quality assurance requirements. This document should be complete within eight months after the start of the activity. Product The product from this engineering study is a report that provides detailed engineering specifications and complete ISI qualification testing protocols including a complete quality assurance plan for the ISI qualification center. Participants The following organizations should participate as members of the activity team. Representatives from regulatory bodies operating RBMKs Plant Design Organization Representatives from Utilities operating RBMKs Representatives from RBMK NPP metals control department Representatives from western organizations that have implemented ISI qualification programs Representatives from the US International Nuclear Safety Program (INSP), EU, etc. Representatives from IAEA Relationship to other projects The US Department of Energy, the European Union and other organizations sponsor projects to improve the safety culture and safe operation of eastern European nuclear power plants. This engineering study accomplishes two objectives. First, the study outlines the technical requirements for a regional center that improves in-service inspection by proving a standard metric for organizations that conduct ultrasonic in-service inspection of piping in eastern European nuclear power plants. A regional ISI qualification center is the only economically feasible way to introduce ISI qualification into eastern European nuclear power plants. Second, by creating an international project team, the study fosters international collaboration and coordination of both the US and Europe to improve in-service inspection in eastern Europe. 13

14 6.4. Leak detection systems Background The LBB concept is based on the assumption that a crack in large diameter high energy piping will grow through wall and lead to a reliably detectable leak well before a catastrophic break could occur. This is demonstrated through the LBB analysis for the candidate piping system. The LBB concept has been recognised as a viable methodology to demonstrate integrity of the large diameter high energy piping. A successful application of the LBB concept enables to exclude specific considerations for the dynamic effects associated with primary circuit large diameter piping breaks for a number of PWRs. It should be noted, that the LBB concept was not successfully applied and approved for BWR piping to date, mainly due to the concerns associated with the IGSCC. The IAEA within the frame of its Extrabudgetary Programme on the Safety of RBMK NPPs organised activities on the LBB Concept Application to the RBMK NPPs, reports RBMK-SC-034 and RBMK-SC-057. In these reports, the importance of the leak detection system (LDS) was presented. Reliable leak detection is one of the main pre-condition for a successful LBB concept application. The implementation of a reliable leak detection system would lead to a direct improvement of information about the operational status of the piping. The leak detection system should be therefore implemented in the short term at operating plants independently of the whole LBB concept application. In connection to the Regional Workshop on Environmentally Assisted Cracking of NPP Austenitic Piping the question of applicability and implementation of a leak detection system was newly discussed. Objective A team of relevant experts should be organised to develop the general requirements for application of leak detection systems including conditions and methodologies. The document should take into account the general requirements of the LBB and above mentioned documents and IAEA Guidance on the LBB concept application, IAEA-TECDOC-774 as a basis. Scope The scope of this activity is to address possible leak detection system (LDS) implementation to RBMK plants taking into account the typical environmental conditions and specific geometry of compartments of various generations of the plants. Extension of the existing leakage surveillance with sophisticated system should be than proposed. The conditions and a basis for such implementation should be stated in the document including general requirements for verification and qualification of such systems. Tasks Behind the proposal of work plan mentioned below the two main tasks should be presented as the basis for the activity Task 1 - Criteria for postulation of leakage size through wall-flaw (including e.g. evaluation of material data and crack growth rate); Task 2 - Assessment and analysis of efficiency of existing system and applicability of systems and methods of leakage detection including location of the systems their sensitivity and other aspects as geometry and environmental conditions, etc. The following subtasks are supposed to be included in the work plan: 14

15 Preparation of Terms of Reference and time schedule To build-up the team of experts Development of Technical Specifications and description of current position Develop document dealing mainly with conditions for LDS system application (material properties, classification and sources of leakage, etc.) Principal methods of leak detection Requirements for Implementation of LDS Qualification of LDS Peer-review of the draft document. Finalization of the document Milestones Each of the Tasks mentioned represents a milestone which should be presented in a time schedule in connection to the Subtask 1. The estimated duration of the Subtask 2 6 of the activity should not exceed 12 month. Product The product of the work of the team of experts will be a document providing a general guidance and description of the proposal for application (improvement) of leak detection systems to the RBMK plants. Participants The following parties should participate in the team of experts: Representatives of design organisations Representatives of regulatory authorities of countries operating RBMKs Representatives of Utilities and NPPs operating RBMKs Representatives of western NPPs, regulatory authorities and Utilities that have implemented LBB and LDS. Representatives from similar International Programmes related to this issue Representatives from IAEA 6.5. Comprehensive integrity assessment Background IGSCC is known phenomenon that has been studied extensively in the U.S. and other countries operating BWR type reactors. This phenomenon has only recently been observed in RBMK type reactors (1997). RBMK operators and regulatory authorities are still evaluating the extent and severity of the problem. IGSCC is a corrosion damage mechanism resulting from the combined action of tensile stress, a susceptible material condition and a corrosive environment. The tensile stress required for cracking is easily achieved during the welding process without post-welding heat treatment. The susceptible material condition for austenitic stainless steels is termed sensitization. Sensitization describes a material condition where Cr has been depleted from areas adjacent to the grain boundary with a subsequent increase in brittle phase of Cr carbide Cr 23 C 6. The corrosive environment can be something as benign as oxidizing species (e.g. O 2 in water). Different degree of RBMK piping degradation at individual units is not well understood or characterized. This follow-up activity proposes a study that will aid in understanding IGSCC crack 15

16 initiation and propagation processes in RBMK piping systems. One of the ways to mitigate this phenomenon is to maintain strict control of favorable water chemistry and to limit contaminants. New methodologies are being investigated (Zinc addition, noble metal additions and isolating coatings) and should be evaluated for RBMK application. Objectives 1. To obtain a comprehensive understanding of leading causes contributing to different level of damage, crack initiation and propagation processes for the austenitic materials currently used at RBMKs. This goal should be accomplished through the use of additional comprehensive failure analyses using state of the art techniques and equipment. The results of the failure analyses and the updated ISI inspection data will be combined to establish a database for IGSCC in RBMKs. The ultimate objective will be a thorough understanding of crack growth rates and could be used for further risk assessment in order to improve ISI programme. 2. To determine and recommend the optimum water chemistry condition necessary to mitigate instances of IGSCC in RBMKs. Scope Program to set requirements and priorities for safety improvements and allow IGSCC to be managed. Tasks Tasks of metallurgical and water chemistry issues can run concurrently or separately. A) Metallurgical regime 1. Select participants for the activity team 2. Preparation of a guideline how to handle crack assessment 3. Collection and correlation of existing data Collect ISI results from each RBMK Collect water chemistry data Collect metallurgical data Collect other relevant data, e.g. about decontamination procedures Identify conditions that may be leading/have led to IGSCC at each NPP. Determine critical factors for IGSCC and likely future trends. 4. Additional comprehensive failure analyses to include: state of the art techniques detailed crack length and depth measurements 5. Take the results of the failure analyses and combine them with the existing data base to capture all the different crack morphologies and growth patterns/locations. 6. Experimental program establishing a model starting from the beginning and taking into account the results of task 3, 4 and 5 7. Recommendations for improvement of ISI on the bases of task 5 and 6 results Milestones Completion of each of the tasks listed above represents a major milestone. This activity is estimated to require three years to complete. The estimated time duration for each task is listed below, however, activity funding levels may significantly alter the estimated duration and scope for the tasks. 16

17 1. Preliminary report (3 months) 2. Preliminary report (2 months) 3. Preliminary report (12 months) 4. Preliminary report (12 months) 5. Preliminary report (4 months) 6. Preliminary report (18 months) 7. Preliminary report (2 months) The final reports for each task will be presented 3 months after preliminary report. Depending on the results of the assessment of the current decontamination procedures at RBMKs the activity on Decontamination should be started. Product A comprehensive risk analyses based on the metallurgical aspects of IGSCC in RBMK environment. This will be in form of a report. B) Water Chemistry Issues 1. Select participants for the activity team 2. Comprehensive evaluation of current RBMK versus other water chemistry standards 3. Reviewing the current data base of RBMK water chemistry. Results should be taken into account, to be discussed and included in the metallurgical regime. 4. Evaluation of water chemistry monitoring practice at RBMKs 5. Feasibility study on the use of western water chemistry modifications and RBMK applicability 6. Recommendations based upon results of tasks 2, 3, 4 and 5 7. Identify locations and attributes for monitoring by on site measurements and laboratory testing 8. Providing recommendations for total water chemistry regime and modification Milestones Completion of each of the tasks listed above represents a major milestone. This activity is estimated to require 18 months to complete. The estimated time duration for each task is listed below, however, activity funding levels may significantly alter the estimated duration and scope for the tasks. 1. Preliminary report (3 months) 2. Preliminary report (2 months) 3. Preliminary report (2 months) 4. Preliminary report (2 months) 5. Preliminary report (3 months after task 4) 6. Preliminary report (6 months after task 5 completion) 7. Preliminary report (12 months) 8. Preliminary report (4 months after task 7) The final reports for each task will be presented 3 months after preliminary report. Product A comprehensive water chemistry evaluation for operation of RBMKs. This will be in form of a report. 17

18 Participants The following organizations should participate as members of the activity team. Countries operating RBMKs ƒ Representatives from regulatory bodies ƒ Representatives from utilities and plants ƒ Representatives from design organizations Representatives from western organizations having relevant experience Representatives from TACIS and the US International Nuclear Safety Program (INSP) Representatives from IAEA Note In connection with this task, the following note has been pointed out by Russian specialists: proposed division of metallurgical studies and water chemistry issues which is perhaps correct in academic studies is not of practical value for current situation at RBMKs and emphasis on safety improvement on site. It is well known that such complex parameter as electrochemical corrosion potential (ECP) could indicate clearly situation with potential to IGSCC damages at each NPP Unit. No high temperature ECP measurements have been performed at any RBMK NPP site [1]. So as a matter of urgency high temperature water chemistry and corrosion monitoring should be foreseen at reference NPP sites to indicate potential for IGSCC damages of austenitic piping welds and give valuable input to understanding of different level of damages at different reference NPP Units. IAEA could provide additional input to this project from on-going Coordinated Research Program on high temperature corrosion monitoring WACOL Repair techniques Background IGSCC is known phenomenon that has been studied extensively by the U.S. and other countries operating BWR type reactors. This phenomenon has only recently been observed in RBMK type reactors (1997). RBMK operators and regulatory authorities worldwide are still evaluating the extent and severity of the problem. When a flaw in an austenitic pipe is revealed during an inspection, decisions will be made to either repair the flaw, or keep the component (flawed) in service for some time interval. The repair method chosen may remove the flaw entirely or provide reinforcement to maintain the original load bearing/carrying ability of the pipe. Four methods of repair have been applied to weld repair of austenitic stainless steel piping containing IGSCC indications: 1. Complete removal of the original flawed weld and replacement with a piece of original piping, resulting in two new (potentially susceptible) welds being made. 2. A whole section of similar piping (spool piece) low carbon grade or more resistant material may be installed. (US BWR and German technology). 3. The defect/flaw may be removed by local repair (excavation and backfilling) with or without reinforcement - Chernobyl Unit The flaw may be left in situ and a structural overlay, automatically welded over the original circumferential weld may be installed. US-BWR technology. 18

19 Objectives To develop a program of improved RBMK repair techniques and essential variables that may have a bearing on IGSCC. This goal should be accomplished through the thorough evaluation of current and proposed repair schemes and welding optimization schemes for RBMK repairs of IGSCC. This objective may be initiated through the organization of workshops, technical meetings, scientific meetings and seminars Tasks 1. Evaluate the issues needed to be resolved to incorporate Western repair/welding technology into RBMK reactors. This should include both new weld installation and repair (weld overlay) techniques. This can be accomplished through the use of workshops, equipment demonstrations, and technical seminars. The information to exchange should include: Collection and correlation of existing data Evaluate all repair welds made for RBMK, IGSCC repairs to date Collect ISI results on these repairs from each RBMK Identify any instances of recurring IGSCC in repair welds Determine critical factors for recurring IGSCC 2. Form a Regional Team to develop draft feasibility study for processes to be used for RBMK repairs using input from Western Experts and governing regulatory authorities. This could take the form of 1 or 2 extended Workshops of the necessary experts. 3. Demonstrate the effectiveness of the developed technical feasibility study to provide adequate weld repairs and new welds at a Western manufacturer on RBMK material. This should include developing the following information: Residual stress measurement Degree of sensitization of the repair weld area Stress improvement/modification techniques Heat input/weld weave Applicability for potential for new material substitutions 4. Purchase appropriate automatic welding equipment and train all RBMK welding operators. (A minimum of 5 welding set-ups is estimated at $250K-$400K/set-up.) 5. Implement a unified welding repair program at all RBMK sites. Milestones Completion of each of the tasks listed above represents a major milestone. The estimated time duration for each task is listed below, however, activity funding levels may significantly alter the estimated duration and scope for the tasks months 2. 5 months months months after Task 3 demonstration 5. immediately at the end of Task 4 The final reports for each task will be presented 3 months after completion of Task. 19

20 Product A comprehensive set of generic repair procedures (based on good metallurgical and welding principles and adequate inspectability) for IGSCC in RBMK environment. This will be in form of a report. Participants The following organizations should participate as members of the activity team. Countries operating RBMKs ƒ Representatives from regulatory bodies ƒ Representatives from utilities and plants ƒ Representatives from design organizations Representatives from western organizations having relevant experience Representatives from TACIS and the US International Nuclear Safety Program (INSP) Representatives from IAEA 6.7. Decontamination Background IGSCC is known phenomenon that has been studied extensively by the U.S. and other countries operating BWR type reactors. This phenomenon has only recently been observed in RBMK type reactors (1997). RBMK operators and regulatory authorities worldwide are still evaluating the extent and severity of the problem. IGSCC is a metallurgical phenomenon resulting from the combined action of tensile stress, a susceptible material condition and a corrosive environment. Since IGSCC repairs and inspection may significantly increase the personnel dose of RBMK stuff decontamination becomes an important issue for this reactor type. Some decontamination processes have initiated a significant number of IGSCC events, so care is required to ensure compatibility of the decontamination process with the material and water chemistry requirements of RBMKs. Objectives To exchange information on decontamination principles and procedures at RBMKs which are necessary to reduce dose rate for both repair and inspection personnel Scope Decontamination principles considering ALARA at RBMKs. Tasks 1. Select participants for the activity team 2. Comprehensive evaluation of current decontamination practice worldwide 3. Feasibility study on the use of decontamination practices to RBMKs 4. Recommendations based upon results of tasks 2 and 3 5. Initiate a laboratory program (carry out screening tests) to verify compatibility of decontamination process with RBMK materials and water chemistry 20