ML20205F025
| ML20205F025 | |
| Person / Time | |
|---|---|
| Issue date: | 03/21/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20205F021 | List: |
| References | |
| NUDOCS 9904060086 | |
| Download: ML20205F025 (8) | |
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m U.S. NUCLEAR RF%t_CORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION SAFETY EVALUATION OF "BWRVIP VESSEL AND INTERNALS PROJECT. VESSEL ID ATTACHMENT WELD INSPECTION AND FLAW EVALUATION GUIDELINES (BWRVIP-48)
EPRI REPORT TR 108727. DECEMBER 1997 i
1.0' INTRODUCTION
1.1 Background
By letter dated March 6,1998, and supplemented by letter dated March 3,1999, the Boiling
. Water Reactor Vessel and Intemals Project (BWRVIP) submitted the Electric Power Research institute (EPRI) Proprietary Report TR-108724, *BWR Vessel and Intemals Project, BWR Vessel [ inner Diameter] ID Attachment Weld Inspection and Flaw Evaluation Guidelines (BWRVIP-48)," for NRC staff review. The BWRVIP-48 report presents generic guidelines for the inspection and evaluat;on (l&E) of the attachments, with the objective to assure the long term integrity of the safety-related components. A secondary objective is to recommend additionalinspections of non safety related attachment welds to assess the possibility that cracks which initiate at the attachments might propagate into the vessel wall. The guidelines provide recommendations for nondestructive evaluation (NDE) methods, inspection locations and inspection frequencies. The BWRVIP-48 report also recommends acceptable methods for use in evaluating the structuralintegrity significame of the flaws that are detected during the examinations. The intent of the BWRVlP-48 guidelines is that BWRVIP members wiil adopt the inspectbn recommendations as a replacement for General Electric (GE) Safety Information Letters (Sils).
1.2 Purpose The staff reviewed the BWRVIP-48 report to determine whetherits guidance will provide acceptable levels of quality forl&E of the subject safety related reactor pressure vessel (RPV)
Intemal components. The review considered the consequences of component failures, potential degradat,'on mechanisms and past service experience, and the ability of the proposed inspections to detect degradation in a timely manner.
1.3 Organization of this Report Because the BWRVIP-48 report is proprietary, this safety evaluation (SE) was written so as not to repeat proprietary information contained in the report. The staff does not discuss in any detail the provisions of the guidelines nor the parts of the guidelines it finds acceptable. A brief summary of the contents of the BWRVIP-48 report is given in Section 2, with a detailed evaluation in Section 3. The conclusions are summarized in Section 4. The presentation of the evaluation is structured according to the organization of the BWRVIP-48 report.
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2.0
SUMMARY
OF BWRVIP-48 REPORT The BWRVIP-48 report addresses topics in the order as follows:
Bracket Attachment Confiourations - The attachments for the core spray piping and jet pumps have been classified as safety-related, whereas other BWR vessel attachments j
are not safety-related. The BWRVIP-48 report describes in detall, with reference to a.
colleWn of figures, the various designs of attachments for various components and BWR l
plant types. The particularbracket designs used at each of the U.S. BWR plants are identified, along with safety classification of the component supported by the brackets.
Susceptibilltv Factors -The primary damage mechanisms for vessel ID attachment welds are fatigue and stress corrosion cracking. However, to date, plant operating experience j
has provided no evidence of significant fatigue degradation. Various factors, including materials, stress state, and environmental conditions, that affect stress corrosion cracking of components are described as they apply to vessel attachment welds. The most susceptible weld materials for stress corrosion cracking are identified along with other materials that are less susceptible. The conductivity and electrochemical corrosion potential.of the BWR coolant are identified as important environmental factors to be addressed on a plant specific basis. It is noted that the use cf hydrogen water chemistry l
can reduce the likelihood of stress corrosion cracking for those plants that have implemented this practice. The BWRVIP-48 report cites a list of historical occurrences of attachment weld cre.cking, with specific incidents discussed in dotail. In no case has there been cracking that has propagated into the vessel base material. The most susceptible welds are those for Alloy 182 attachments, and the only cracking of these welds has been at non-GE foreign p' ants.
Potential Failure Locations - The potential failure locations are identified only in general j
terms as being in attachment welds or in the adjacent heat affected zones. It is implied that each plant would identify specific welds for inspection based on knowledge cf the materials and welding parameters for the welds.- While there have been no observed
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cases to date of stress corrosion cracks propagating into the vessel wall, the proposed inspection strategy is designed to address this possibility.
Backarour.d and insoection Historv - Data on service-related failures of components are summarized. Inspections have been performed at all plants in accordance with ASME Section XI requirements. In the beltline region, the bracket attachment welds have been examined using the visual examination (VT 1) requirements specified in the BWRVIP-03 report. Otner attachments have been examined using VT-3 Inspections. Degradation has been observed at only five plants, with no cases of cracks that have extended into the
- vesss! base material. Two of the plants were foreign non-GE plants, where IGSCC cracks were found in Alloy 182 welds. Three U.S. plants experienced cracking by either fatigue
- (caused by a high mome'nt due to an improperly positioned seismic block) or cracking with!n clad material.
BWRVIP insoection Guidelines -The ge delines forinspecting bracket attachments are a
summarized in Table 3-1 of the BWRVIP-48 report. Inspections are limited to visual examinations. With some notable exceptions, the recommended NDE methods are the
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. seme VT-1 and VT-3 methods used for past inspections in accordance with ASME Section XL Allinspections of the brackets for core spray piping and jet pumps are by the modified VT-1 examination, as specified in the BWRViP-03 report. The examination volumes are limited to the attachment weld and the adjacent heat affected regions of the vessel clad. Unless specifically stated, the selection of welds for inspection are the same as in the plant's current ASME Section XI program. ' Additional examinations are required for brackets associated with core spray piping, jet pump risers, steam dryers, and feedwater attachments. The additionalinspections are performed in two steps. First, there is a set of baseline inspections, which address essentially all of the identified critical welds. Once the baseline examination is completed, reinspections are to be performed at intervals ranging from once every four refueling outages to once in 12 years.
Acceotance Criteria and Reoortina - The guidelines state the inspection results should be reported to the regulatory authorities having jurisdiction at the owner's facility. Flaws may be accepted on the allowable sizes given for category B N-2 of IWB-3520 of ASME Section XI. A detailed fracture mechanics evaluation following the approach described in Section 4.3 of the BWRVIP-48.eport can also be performed to determine the suitability of a degraded bracket for continued plant operation.
Loads - This section describes the methodology and the loads to be used in fracture mechanics evaluations that address the consequences of detected flaws on structural integrity. The evaluation should begin by considering the design loads originally specified by the NSSS supplier, and should be updated to be consistent with the current plant-
. licensing basis, if the failure is known to be the result of unanticipated loads, such as vibration, these service loads should be included in the evaluation. The evaluation should also review loads used in recent evaluations of the attached components (core spray piping orjet pumps) to ensure consistency with other structural evaluations Load Combinations - Loads should be combined in a consistent manner. The BWRVIP-48 report notes that the loads as specified by General Electric have already included load combinations, and do not need special consideration.
Flaw evaluation methodologies for dispositioning indications are addressed briefly in the BWRVIP-48 report. Flaws that propagate into the vessel wall should be evaluated using the requirements of ASME Section XI, Paragraph IWB-3600. It is recommended that a failure cause (e.g., vibration, stress corrosion cracking, wear, overload, etc.) be identified. Crack growth between inspections should be estimated using fatigue crack grow rates from ASME Section XI, or a bounding rate of 5x10 1nches per hour (11.133 mm/ year) for stress corrosion 4
cracks. Allowable crack sizes should be based on applied stresses and stress allowables.
The detailed evaluations wouM bs performed only when the flaw sizes exceed the acceptance criteria of ASME Section XI, IWB-3520.
3.0 EVALUATION The inspection guidelines cover the " baseline inspection," the re-inspection requirements, and
' scope expansion. Inspection acceptance criteria are provided along with the reporting of inspection results. The items covered in the bracket attacSment inspection recommendations include the jet pump riser brace (primary and secondary brace attachment), the core spray U
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4 piping, the steam dryer support and feedwater brackets, and all other attachments such as the steam dryer, guide rod, feedwater sparger and surveillance sample holder.
The inspection strategy provided in the BWRVIP-48 report is based on an evaluation of data from priorinspections, consideration of expected degradation mechanisms, material and wolding characteristics, and the safety importance of the individual brackets. Differences in the design and operating experience for the various types of BWR reactors (BWR/2, BWR/3 5 and BWR/6) are reviewed and taken into account as part of the inspection strategy. The recommended inspection strategies address the number of inspection locations, frequency of the inspections, and the examination methods. The recommended inspections of BWRVIP-48 focus on welds with special attention given to detecting stress corrosion cracking.
The guidelirles allow NDE inspections to be conducted with ultrasonic inspections (UT), VT-1, enhanced VT-1 (EVT-1), and VT-3. The core spray piping bracket and the jet pump riser brace attachments are classified as safety related and the recommended inspection is to be performed with an EVT-1 visual method. The areas to be inspected are the attachment weld and the adjacent heat affected region of the vessel clad.
j For all indications that are detected during the EVT-1 inspections, UT inspections will be performed to determine if the indication has propagated into the reactor vessel base material.
For any flaws that are found to have propagated into the vassel base material, an evaluation
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will be performed in accordance with the requirements of ASME Section XI, Paragraph IWB-3600.
Scope expansion will occur if one or more flaws are found during either the baseline inspection
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or re inspection. Then all of the remaining locations of the same type should be inspected during the same outage unless the licensee can correlate the flaw to a specific event, which would not affect otherlocations, i
With the exception of the issues described below, this review finds that the guidance provided in the subject report acceptable. The recommended strategy for inspection is based on ine current industry-wide experience, and is intended that the inspections focus on those i
component locations where degradation is considered to be most likely to occur, and components whose failure will have the greatest impact on safety. The BWRVIP is regoested to resolve the following issues and revise the subject document in accordaisce with the stated recommendations.
issue 3.1.2 VT Insoection Effectiveness j
in the BWRVIP-48 guidelines, the specified inspections for vessel ID bracket attachment inspections include UT, VT-1, EVT-1, and VT-3. The VT methods are identified to be employed for flaw detecton and length sizing of the flaws. UT can be used for confirming the flaw length, for sizing the depth of the flaw, and for determining if a flaw has propagated into the vessel wall.
Based on inspection history and the kinds of degradation modes that the material employed in attachments are susceptible to, the most likely flaw mechanisms are IGSCC and fatigue.
These damage mechanisms have been detected during inspections of some attachments.
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5 However, other degradations have also been found, including slag, impact wear, and not removing a tack weld; Therefore, IGSCC and fatigue processes are the most likely service induced degradation that needs to be reliably detected. The nature of these degradation processes should lead to a crack initiating in the weld or in the adjacent heat affected zones.
Furthermore, the crack should initiato from the surface of the weldment area that is in contact with the reactor coolant. Thus, these surface-breaking cracks should be detectable when using effective surface inspection methods.
Most of these bracket attachment welds will be a challenge to performing effective volumetric ultrasonic examinations. Some of the reasons for this difficulty include the design, access, inspection complexity associated with a fillet type weld, complex welds involving several welding processes and those welds made wif.h materials that are coarse grained and acoustically anisotropic. Therefore, this review concludes that UT methods do not offer particular advantages over visual methods for flaw detection.
In the BWRVIP-48 guidelines, fourinspection methods are recommended. The implementation requirements and definitions of these methods are to be found in the BWRVIP-03 report, which is presently l'..ng revised and updated in accordance with comments made in the NRC staff's SE dated June 8,1998. The requirement identified for the EVT-1 calls for a visual system capable of achieving a 1 mil wire resolution. The BWRVIP informed the staff during a public meeting on December 17,1998, that committee members are working on revisions to the BWRVIP-48 and -03 reports, and plan to make the EVT-1 a visual exam capable of achieving a % mil wire resolution. This change would be an improvement, and would be consistent with the requirement in BWRVIP-03 for core shrouds, which uses a % mil wire resolution standard.
There is presently no mention of the requirements in the BWRVIP-48 report for assuring that an effective visual examination can be conducted, other than that the utilities should assess whether cleaning would be required in order to obtain meaningful / reliable results. The 1
resolution capability of video visual examinations can vary from excellent to poor, depending on the equipment, environmental conditions and the skliis of the inspector. Some of the parameters that need to be controlled include: the quality of the video equipment, the adjustment of the camera, camera positioning, lighting (intensity, color and position), water clarity, water temperature, level of radiation fields, the effects of radiation on the camera lens and camera imaging device, cleanliness of the component needing inspection, geometry
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constraints of the component, etc. Specifics are needed for many of these parameters, and are expected in the revised BWRVIP-03 document.
Relying on inspectors to receive training, develop the necessary skills, and the associated variability !n inspection reliability, are some of the reasons that performance demonstration testing has been developed, placed into ASME Section XI Appendix Vill, and is being endorsed by the NRC through rule action. A paper by Baker and Hoomissen,1994 (Reference 5), conceming core shroud inspections, provides a summary of the advantages arid disadvantages of both VT and UT, it is significant to note that they state the three disadvantages of VT as being: 1) the inspection surfaces must be cleaned,2) there is no through-wall sizing information, and 3) VT has low repeatability. They did not provide any data i
on the level of repeatability other than to describe it as low. For UT there is a wealth of information that has been created through the use of round-robin studies to quantify the i
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'6 effectiveness of procedures, techniques, equipment, and personnel.- However, there is virtually no similar studies or information in the literature conceming VT.
A visual examination can potentially be very effective in finding degradation; however, the current versions of the BWRVIP-03 and 48 reports do not provide adequate detail to insure that effective visualinspections will be consistently performed. These revisions will need to
- provide sufficient guidance for inspection equipment, procedures, and inspector training / skills to insure that a reliable inspection will be performed or that inspectors, equipment and -
procedures in combination will demonstrate their effectiveness through a performance test type of test.
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By letter dated March 3,1999, the BWRVIP proposed revisions to Section 3.1.2 to address the concems ever the level of visual examinations which were discussed with the BWRVIP during a December 17,1998 public meeting. This revision addresses the NRC staff's above comment on EVT-1 exams. Therefore, no further actions are needed to resolve this issue, other than those the BWRVIP has previously proposed in the December 17,1998, public meeting regarding modifications to the BWRVIP-03 report.
Issue 3.2.1 Baseline insoection-The recommendations in the BWRVIP-48 report for the inspection of bracket attachments make distinctions between safety-related and non-safety-related attachments. Additional information provided by the BWRVIP acknowledges that any weld to the reactor coolant pressure boundary is safety-related as per 10 CFR 50.2 and 10 CFR 50.55a.
By letter dated March 3,1999, the BWRVIP proposed revisions to Section 3.2.1'to address the staff's concems which were discussed with the BWRVIP during a December 17,1998 public meeting. The proposed changes delete references to safety related and non safety-related in the two bulleted items of Section 3.2.1, and should recommend EVT-1 inspections for core spray and jet pump riser brackets on the basis of the safety function of the components that i
are supported by the brackets.
in Section 3.2.1 (second to last paragraph) the reference to ASME Section XI Paragraph IWB-3600 should be deleted, and discussions of Section XI evaluations to Section 4.3 (Flew Evaluation Approach) should be limited. These changes will avoid the suggestion that an lWB-3600 evaluation is always required, since Section XI permits flaws to be accepted solely.
on the basis of the criteria of IWB 3510.
Issue 4.3 Flaw Evaluation Acoroach The title and outline of the BWRVIP-48 report includes the topical area of flaw evaluation methodologies in an effort to be consistent with the standard format adopted for the series of similar BWRVIP reports on intemal components. However, Section 4.3 of the current report is very brief (about half a page) and serves only to provide broad guidance on methodologies appropriate for the evaluation of flaws detected during examinations of attachment welds.
Therefore, flaws for specific attachment designs to address various flaw locations and orientations will need to be evaluated on a case-by-case basis.
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3 The reference to the detailed methodology of ASME Section XI is appropriate for the evaluation of flaws that extend into the vessel base material. However,Section XI does not provide specific guidance for flaws whose extents are limited to the attachments themselves.
For such flaws, fracture mechanics approaches will need to be developed on a case-by-case basis to cddress the attachment geometries and flaw configurations of concem. It is expected that the calculat;onal methods and acceptance criteria would be consistent with approaches now prescribed in Section XI for flaws in vessel and piping components.
This review fir ds the overall flaw evaluation approach of the BWRVIP-48 report to be generally acceptable. However, the staff recommends that the BWRVIP consider expanding this section to include the fo' lowing:
A discussion that supports the rationale for not developing detailed flaw evaluation e
methods forinclusion in the BWRVIP 48 report. This discussion should emphasize the large number of attachment designs, and the uncertainties regarding the locations and orientations of flaws that could be detected by future inspections, A statement that ASME Section XI does not speu;fically address the evaluation of flaws in e
attachment welds, other than aspects dealing with flaws extending into the base material i
of the vessel. Acceptable approaches should be based on margins, fracture mechanics methods, consideration of loads and operating conditions, and estimates of material properties and crack growth rates which provide conservative predictions of structural integrity in a manner consistent with ASME code methods and methods adopted for application to other BWRVIP documents. The BWRVIP 48 report should also state that 1
flaws evaluated by alternative methodologies would require review and approval of regulatory authorities.
The flaw evaluation approach will determine re-inspection frequencies as they are i
e governed by flaw growth rates. It should be clearly stated that the frequencies of inspections for degraded components will be established independently of the inspection intervals of Section 3.0 of the BWRVlP-48 report.
4.0 CONCLUSION
S The staff has reviewed the BWRVIP-48 report and finds that the guidance of the report is acceptable for inspection and flaw evaluation of the subject safety-related core intemal components, except where the staff's conclusions differ from the proposed guidance, as discussed above. The staff requests that the BWRVIP review and resolve the issues raised above, and incorporate the staff's conclusions into revised BWRVIP-48 report. Please inform the staff in writing as to this resolution.
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5.0 REFERENCES
1.
Letter from BWRVIP with submittal of the report "BWR Vessel ID Attachment Weld Inspection and Flaw Evaluation Guidelines (BWRVIP-48)," EPRI TR 108724, dated
. March 6,1998.
2.
BWR Vessel and Intemals Project, "BWR Vessel ID Attachment Weld Inspection and Flaw Evaluation Guidelines (BWRVIP-48)," EPRI TR-108724, February 1998.
3.
B'VR Vessels and intemals Project, " Reactor Pressure Vessel and intemals Examinations
' Guideline (BWRVIP-03)," EPRI Report TR-106696, October 1995.
I 4.
"BWR Vessel and Intemals Project, Vessel Intemals inspection Summary," April 1997.
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i 5.
Baker, A. and J. Van Hoomissen, BWR Core Shroud inspection Program. presented at EPRI Vessel & Intemals inspection Conference, San Antonio, TX, Juiy 11-15,1994.
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