ML20077R325
| ML20077R325 | |
| Person / Time | |
|---|---|
| Site: | 05200004 |
| Issue date: | 01/31/1995 |
| From: | SOUTHERN NUCLEAR OPERATING CO. |
| To: | |
| Shared Package | |
| ML20077R321 | List: |
| References | |
| PROC-950131, NUDOCS 9501200146 | |
| Download: ML20077R325 (10) | |
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'1 BWR Vessel and Internals Project Program Plan January 1995 i
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.9501200146 PDR 950113 1 ADOCK 05200004 l 4
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BWR Vessel and Internals Project Introduction '
The BWR Vessel and Internals Project (BWRVIP) is an association of utilities owning and operating boiling water reactors. The project is focused exclusively on reactor vessel and vessel internals issues in operating plants.
Objectives of the BWRVIP are to lead the BWR industry toward generic resolution of vessel and internals integrity and operability issues; to identify or develop generic, cost-effective strategies from which each operating plant ,
will select the most appropriate alternative; to serve as the focal point for the regulatory interface with the industry on BWR vessel and internals integrity and operability issues; and to share information among members.
The BWR Vessel and Internals Project has been organized into four technical tasks: Inspection, Assessment, Mitigation and Repair. An Integration task coordinates the work. Each task is managed in wordance with the BWRVIP charter by a committee of BWR utility representativos. EPRI manages the technical program. ,
Background
Intergranular stress corrosion cracking (IGSCC) limits the service life of susceptible materials and components in BWR water environments. Several remedy or repair strategies for IGSCC in BWR piping have been developed and applied since the early 1980s. These include local stress reduction, local repair, corrosion-resistant cladding, piping replacement with resistant materials, control of impurities in the reactor coolant, and hydrogen water chemistry (HWC) to reduce the oxidizing potential of the BWR water <
environment. Development of these countermeasures for pipe cracking was supported from 1979 to 1988 by the BWR Owners Group for IGSCC Research.
This organization of BWR owners was disbanded in 1988 following publication of NUREG 0313 Rev. 2, which outlined the pipe inspection requirements for plants utilizing various of the remedial measures listed above.
There are many similarities in terms of materials, fabrication and environment between BWR piping and BWR internal components.
Recognizing the potential susceptibility of vessel internals to IGSCC, a three-party program was outlined in 1989 to address the vessel and internals issues as they were then defined. Since 1989, EPRI has pursued generic !
developments including the extension of HWC technology to protection of internals, generation of materials data for component assessment, improved materials for repair and replacement, weld repair technology suitable for in-2 l
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vessel applications, and inspection methods suitable for detection of in-vessel SCC. The BWR Owners Group has evaluated the likelihood and consequence of IGSCC in specific components, as a basis for inspection prioritization. The NSSS vendor, General Electric, has developed inspection and repair methods for specific components.
Events in 1993 and 1994 confirmed that IGSCC is a significant issue for BWR internals. Core shroud cracking has been found in BWRs worldwide, and repairs have been planned or implemented in several of these. As a result, the industry focus is shifting from contingency planning and technology development to implementation of more specific plans for inspection, remedy and repair.
In this context, US BWR executives formed the BWR Vessel and Internals Project in June of 1994 to address integrity issues arising from service-related degradation of these key components, beginning with core shroud cracking in the near term. BWRVIP activities from June to September of 1994 have produced standards and criteria for inspection, evaluation and repair of core shrouds, to be utilized in fall outage planning for several plants. BWRVIP has assumed responsibility for the regulatory interface formerly managed by BWROG. BWRVIP has recently reviewed the current EPRI program, the BWROG project plan, and other mid-term and long-term needs, for the purpose of defining a program plan. This program plan for work during and beyond 1994 reflects the next phase of the transition to proactive issue management under direction of BWR owners through the BWRVIP.
Needs in the near term, the BWR industry needs a cost-effective program for maintaining the structural integrity of reactor internals susceptible to stress corrosion cracking. This program must begin with satisfactory resolution of the core shroud integrity issues noted above. The industry must anticipate and make preparations to contain the impact of SCC in other susceptible components. Finally, comprehensive guidance is needed to allow utilities to manage all forms of degradation assdciated with the reactor vessel and internals.
A general strategy for developing this guidance may be summarized as follows:
The components and locations potentially susceptible to degradation will be systematically identified, making use of amiable information, experience and technology. Future service experience and inspection results will be utilized to strengthen or modify assessments of susceptibility.
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. l Priorities will be established for addressing components and flaws based on susceptibility to degradation, on the safety consequence and cost impact of severe degradation, and on other practical and economic factors.
Inspection methods and repair alternatives will be defined and qualified
! on a schedule consistent with established priorities. Goals for inspection and repair include reliability, cost-effectiveness and regulatory acceptance.
Generally, components can be replaced or reinforced when frequent inspection would otherwise be required. Preemptive repair strategies will be developed for components for which service life is clearly limited and inspection is expensive.
If repair or replacement is prohibitively expensive, more cest-effective countermeasures for service-related degradation will be developed in paw.iel with inspection and repair strategies.
BWRVIP will prsue several tasks, detailed in the program plan, to support more realistic assessments of seniceability and remaining life through irnproved technology and through utilization of plant inspection data. '
BWRVIP Goal .
The goal of the BWRVIP is to develop a comprehensive program that allows utilities to manage degradation assoc'ated with the reactor vessel and internal components. This program will address structural evaluation, inspection and repair, and other countermeasures for sen> ice-related degradation. Due consideration will be given not only to the risk associated with degradation but also to the economic impact and to regulatory perspectives.
Under the auspices of the BWROG, GE has developed a draft "Model Inspection Program" which lays the groundwork for determining the susceptibilty and safety consequences nf component cracking. Substantial benefits can be realized by expanding the "Model Inspection Program" to include repair and replacement considerations as well as mitigation options so that the most appropriate course of action for an individual utility may be developed consistent with generic guidance.
Initially, this product will prioritize the progression of components for which evaluation. inspection, and repair methods and techniques must be defined.
Informatk,n on degradation obtained from a BWRVIP developed industry database program will provide much of the required data to prioritize issues.
The iinal product will integrate all of the methods, techniques and 4
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This Guide will provide utilities with the necessary information to make cost-effective decisions to manage degradation of the reactor and its internals.
BWRVIP Tasks Task 1. Integration The goal of the Integration Committee is to develop the overall BWRVIP issue management strategy for resolution of vessel and internals integrity issues. To implement the strategy, this Program Plan has been jointly developed by the Integration Committee and the Technical Committees. The Program Plan defines work to be performed in each of four technical tasks:
Inspection, Assessment, Mitigation and Repair; each under the direction of a Technical Committee. As the mrk progresses, the Integration Committee will coordinate the activities, products, priorities and schedule of the work in each task, evaluating proposed and completed workscope to ensure consistency with the overall strategy.
The Integration Committee will serve as the focal point for regulatory issues and NRC communications, and will be responsible for ensuring that regulatory commitments are understood and met.
The Integration Committee will recommend technical and business practices to ensure that the data, information and technology supporting strategic issue management are available, utilized and appropriately controlled.
Task 2. inspection The broad goal of the Inspection task is to determine the condition of BWR vessel welds, internals, vessel attachments and penetrations that are potentially susceptible to service-related degradation, through effective and predictable inspection techniques. The measures of success in this task are the availability, reliability, and cost-effectiveness of methods for required inspections.
Methods and techniques will be identified, qualified and implemented to detect, discriminate and size indications in BWR 2 through 6 plant types.
Consensus stanaards will be developed as appropriate to prescribe technique, ,
operator training requirements and qualification requirements. A program and facilities will be established to support NDE service vendor qualification 5
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and to demonstrate applications, including evaluation of remote inspection devices.
The Inspection +ask will implement a plan to be developed in conjuncti.m with the Assessment task and endorsed by BWRVIP. The Inspection Program Plan will prioritize and schedule the progression of components and locations for which inspection methods and techniques must be defined. The Inspection Program Plan will also provide guidance on the size and location of significant indications that must be reliably detected and characterized.
The Inspection Task will use the input from the Inspection Program Plan to assess access requirements, inspcction methods, at:d scanning and robotic equipment in order to evaluate performance against the inspection criteria. ;
Uncertainties will be evaluated. If necessary, new technologies will be t recommended to reduce error or uncertainty in the measurements and / or to reduce cost. When technologies are established mock-ups will be developed, training and qualification programs willl be established and initial training and qualification will be performed.
Additionally, under the BWRVIP Inspection task, plant-specific inspection data will be compiled and evaluated to characterize damage typical of generic component types and locations, and to characterize the progress of damage development over time. This information will be utilized within BWRVIP to sharpen the focus of the Inspection Program Plan, to improve predictive models of SCC susceptibility, to support assumptions made for structural assessments, to benchmark the long-term effectiveness of countermeasures, and to guide the development of preemptive repair methods. The Integration Committee will be responsible for information exchange.
Task 3. Assessment .
The broad ;;oal of the Assessment task is methodology for evaluation of vessel and internal components in support of decisions for operation, inspection, mitigation or repair. Success in this task will lead to realistic and defensible assessments of serviceability and service life, avoiding unnecessary or premature repair / replace decisions and supporting timely implementation of countermeasures for in-service degradation.
The Assessment task will develop the analytical basis and criteria for structural evaluation of degraded components. This requires information about the present condition of the component; data on strength and toughness of materials in service; selection of analysis methods appropriate for those materials; definition of service Joadings and design basis loads; definition of acceptance criteria for serviceability; estimation of corrosion-assisted crack growth rates; and remaining life predictions. If the available i
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data and interpretations are excessively conservative in specific applications, the Assessment task may undertake testing or data analysis as necessary to support more realistic evaluations.
The Assessment task will also support evaluations of plant data and applications of materials and chemistry technology to produce degradation susceptibility rankings and component life predictions. Such predictions are difficult but necessary to support inspection planning, run/ repair decisions, and application of countermeasures. The premise for investment in countermeasures is that component performance can be predicted and measureably improved, leading to recovery of investment through increased service life. The real service life of safety-related components depends in part on the credibility of predictions of susceptibility and remaining service life.
For these reasons, it is important to apply available resources of information and technology to forecasting and prediction.
Needed resources of information for SCC ranking and forecasting include component-specific inspection results and fleetwide trends; component-specific information on materials and fabrication; plant-specific water :
chemistry history; derived data on stress, local water chemistry and radiation exposure; and data relating material and exposure to SCC damage.
End products of the Assessment task will include:
Generic evaluations of structural integrity and operability of degraded components, and associated flaw acceptance criteria supporting run-repair decisions.
Acceptable methods to determine minimum residual service life of degraded components, supporting reinspection interval or scheduling l of repairs.
Safety consequence of severe degradation supporting risk-based inspection prioritization, j SCC susceptibility, generic groupings and plant rankings, supporting risk-based inspection prioritization.
Experience-based estimates of component service life, supporting plant-specific strategies for inspection timing and implementation of countermeasures.
Rapid evaluation of the implications of new information for experience-based estimates of component service life.
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+ l Post-mitigation flaw evaluation criteria, enabling plants with prior l I
damage to derive benefit from countermeasures such as HWC.
Quantification of post-mitigation service life, developed in cooperation with the Mitigation task and supporting inspection relief.
Task 4. Mitigation The goal of the Mitigation task is to develop and demonstrate countermeasures for service-related degradation. The Mitigation task will be successful if owners implement cost-effective countermeasures for stress ;
corrosion cracking that significantly reduce the requirements for repair.
Presently, the only countermeasure shown to be technically viable is hydrogen addition to modify BWR water chemistry. Hydrogen Water Chemistry (HWC) for internals is relatively expensive to implement; it can protect many but not all components; and there remain open questions as to the degree of effectiveness for certain material-environment conditions.
However, it is effective for lower areas of the core, where repairs will be extremely costly. The first objective of the Mitigation task is to provide the information required by utilities to optimize the cost-effective application of HWC.
The use of noble metal coatings to minimize IGSCC has been demonstrated in principle, but not in plant. A practical and effective alternative to HWC, such as cathodic protection or another water chemistry treatment, has not yet been identified by exploratory studies. The second objective of the Mitigation task is to demonstrate practical and cost effective advanced mitigation techniques.
The Mitigation task will complete work in progress to quantify HWC cffectiveness. A comprehensive study of 10 representative BWRs will be completed as part of the evaluation of current capabilities of HWC to mitigate cracking of reactor internals. BWR Water Chemistry Guidelines will incorporate new results on material susceptibility to SCC vs. local water chemistry, flow velocity and radiation flux. The 1996 guidelines will also reflect results of recent work on local water chemietry vs. hydrogen addition rate for key locations in several BWR plant types. In addition, new n ork on local flux mapping will permit better assessment of HWC effectiveness as well as improved estimates of component radiation levels. The Mitigation task will address all HWC application issues, which include control of shutdown and operating radiation exposures, and fuel clad surveillance to identify any evidence of adverse effects of high hydrogen levels in the coolant.
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A surface deposit of noble metal can improve the efficiency of HWC mitigatior., extending protection to components for which hydrogen alone is not effective, or reducing the hydrogen requirement for a specific component.
A method for solution deposition of noble metal coating will be demonstrated in plant under the Mitigation task,if current development produces satisfactory results. Other new technology that is proposed for control of stress corrosion cracking will be reviewed, and alternative techniques will be demonstrated in plant where there is adequate justification.
Task 5. Repair The goal of the Repair task is to assure the availability of cost-effective repair alternatives for BWR internals, vessel attachments and penetrations damaged by service-related degradation. Success in this task means that repair applications have been anticipated, repair technology is available, and repair design criteria have been defined so that repair service vendors can respond proniptly to plant-specific needs. The schedule for repair development should be planned to keep pace with the inspection schedule as defined by the Inspection Program Plan.
The concept of preemptive repair has been proposed as a cost-effective alternative to inspection. Core shroud reinforcement is planned in some plants to eliminate the need to inspect several circumferential welds. Generally, components that are susceptible to SCC damage can be replaced or reinforced when frequent inspection would otherwise be required. The Repair task in cooperation with the Assessment and Inspection tasks will identify other components for which service life is clearly limited, inspection is expensive, and preemptive repair is justifiable Mechanical repairs have been applied thus far to BWR internals damaged by SCC. Weld repairs have been made to nozzle safe ends and thermal sleeves.
The Repair task will review other pressurc boundary components and internal components to determine what repair method is appropriate and whether currently available repair technology can be readily adapted to each application.
The Repair task will identify technology developments, for example the extension of weld repair to radiation hardened materials, which may be needed to assure a viable repair option for each application.
Materials for repair and replacement have been improved in recent years. Some of these improved materials have been recently introduced and others are under ;
development, or under review in Code commit +ees. The Repair task will ;
monitor ongoing developments and assure that the materials specified for repair 1 and replacement will perform well in the BWR environment. The Repair task will coordinate with the Assessment task to identify BWR repair / replace ;
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applications, for example in high radiation locations, where improved material performance is needed.
Standard repair design criteria are needed to facilitate regulatory review. The Repair task will develop and propose criteria for consensus support and regulatory approval.
The cost-effectiveness of repairs depends in part on post-repair inspection requirements for the repaired component and for new repair hardware. The Repair task will coordinate with the Assessment and Inspection tasks to define post-repair inspection requirements and to evaluate the feasibility and cost of performing necessary inspections.
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