ML20212J914
| ML20212J914 | |
| Person / Time | |
|---|---|
| Issue date: | 09/29/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20212J900 | List: |
| References | |
| NUDOCS 9910060038 | |
| Download: ML20212J914 (6) | |
Text
i U.S. NUCI FAR REGULATORY 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-108724 l
1.0 INTRODUCTION
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1.1 Background
l By letter dated March 6,1998, as supplemented by letter dated March 3,1999, the Boiling Water Reactor Vessel and Internals Project (BWRVIP) submitted the Electric Power Research Institute (EPRI) proprietary report TR 108724 "BWR Vessel and Internals Project, 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 evaluation (l&E) of the attachments, with the objective to ensure the long term integrity of the safety related components. A secondary objective is to recommend additional inspections 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 significance of the flaws that are detected during the examinations. The l
intent of the BWRVIP-48 guidelines is that BWRVIP members will adopt the inspection j
l recommendations as a replacement for General Electric (GE) Safety information Letters (SILs).
By letter dated March 21,1999, the staff forwarded its initial safety evaluation (SE) of the BWRVIP-48 report to the BWRVIP. This SE had several open items, repeated below, and requested that the BWRVIP address these issues in a timely manner. By letter dated April 30, 1999, the BWRVIP responded to the open items in the staff's initial SE.
1.2 Puroose l
l The staff reviewed the BWRVIP-48 report, as supplemented, to determine whether its revised guidance addressed the open items in the staff's initial SE, and if it will provide acceptable levels of quality for inspection and flaw evaluation (l&E) of the subject safety-related reactor pressure vessel (RPV) intemal components. The review considered the consequences of component failures, potential degradation mechanisms and past service experience, and the ability of the proposed inspections to detect degradation in a timely manner.
l 1.3 Organization of this Report i
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Because the BWRVIP-48 report, as revised, is proprietary, this SE was written so as not to repeat proprietary information contained in the report or its revision. The staff does not discuss l
in any detail the provisions of the guidelines nor the parts of the guidelines it finds acceptable.
ENCLOSURE 9910060038 990929 PDR TOPRP EXIEPRI C
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I A brief summary of the contents of the BWRVIP-48 report is given in Section 2.0 of this SE, with a detailed evaluation in Section 3.0. The conclusion is summarized in Section 4.0. The presentation of this evaluation is structured according to the organization of the BWRVIP-48 report.
2.0
SUMMARY
OF BWRVIP-48 The BWRVIP-48 report addresses the following toples in the following order:
Bracket Attachment Confiaurations -The attachments for the core spray piping and jet o
pumps have been classified as safety related, whereas other BWR vessel attachments are not safety-related. The BWRVIP-48 report describes in detail, with reference to a collection of figures, the various designs of attachments for various components and BWR plant types. The particular bracket designs used at each of the U.S. BWR plants are identified, along with safety classification o' 9e component supported by the brackets.
Susceotibility Factors - The primary damage mechanisms for vessel ID attachment welds o
are fatigue and stress corrosion cracking. However, to date, plant operating experience has provided no evidence of significant fatigue degradation. Various factors, including materials, stress state, and environmental conditions, that dect 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 of hydrogen water chemistry 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 cracking, with specific incidents discussed in detail. 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 plants.
Potential Failure Locations - The potential failure locations are identified only in general o
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 of the materials and welding parameters for the welds. While there have been no observed cases to date of stress corrosion cracks propagating into the vessel wall, the proposed inspection strategy is designed to address this possibility.
Backaround and Inspection History-Data on service related failures of components are o
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. Other 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 vessel 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 moment due to an improperly positioned seismic block) or cracking within clad material.
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t BWRVIP insoection Guidelines - The guidelines for inspecting bracket attachments are l
o 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 same VT-1 and VT-3 methods used for past inspections in accordance with ASME Section XI. All inspections 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.
Acceptance Criteria and Reportina - The guidelines state the inspection results should be o
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 report can also be performed to determine the suitability of a degraded bracket for continued plant operation, Loads - This section describes the methodology and the lods to be used in fracture o
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 or jet pumps) to ensure consistency with other structural evaluations 4
Load Combinations - Loads should be combined in a consistent manner. The BWRVIP-o 48 report notes that the loads as specified by Genoral 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. In the urJikely case that a flaw might propagate into the vessel wall, it would be evaluated using the requirememts of ASME Section XI. It is recommended that a failure cause (e.g., vibration, stress corrosbn crccking, 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 5 x 10 inches per hour (3.528 x 10-e cm/sec) for stress 5
corrosion cracks. Allowable crack sizes should be based on applied stresses and stress allowables. The detailed evaluations per Paragraph IWB-3600 would be performed only when the flaw sizes exceed the acceptance criteria of ASME Section XI.
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3.0 NRC STAFF EVALUATION The staff's March 21,1999, initial SE provided three open items. The BWRVIP, in its letter of April 30,1999, addressed these items, which are discussed belov?. Issue 3.1.2, VT Inspection Effectiveness, was previously addressed in the BWRVIP's Revision 1 to the BWRVIP-03 report, which the staff found to be acceptable in a letter dated July 15,1999. No further review of this issue is required.
- 3.1 issue 3.2.1 Baseline inspection The staff's March 21,1999, initial SE stated:
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 addresa the staff's concerns 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 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 (Flaw 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.
BWRVIP's April 30,1999, Response:
The BWRVIP is in agreement with the NRC position on this issue and proposes to modify the BWRVIP-48 report as follows:
In the first line of the first bul!sted paragraph, delete the words "which are classified as safety related." These words are not necessary since any weld to the RPV, and thus the reactor coolant pressure boundary, are safety-related. Note that the attached component may or may not be safety-related. This will be addressed in a revision to Section 4.3, in the first line of the second bulleted paragraph, delete the words "which are classified as non-safety related." These words are not correct since any weld to the RPV, and thus the reactor coolant prest.ure boundary is safety-related.
In the last paragraph on page 3-3, delete the words " Paragraph IWB-3600." The revision to Section 4.3 provides the path to IWB-3600. However, if a flaw can be sized by UT and determined acceptable per IWB-3510, no analysis is needed and IWB-3600 would not be used.
' Staff's Evaluation:
The staff finds that the BWRVIP's response adequately addressed this item.
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3.2 lasue 4.3 Flaw Evaluation Anorocch The staff's March 21,1999, initial SE stated:
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 l
attachment welds. Therefore, flaws for specific attachment designs to address various j.
flaw locations and orientations will need to be evaluated on a case-by-case basis.
The reference to the detailed methodology of ASME Section XI is appropriate for the i
evaluation of flaws that extend into the vessel base material. However,Section XI does j
not provide specific guidance for flaws whose extent are limited to the attachments l
themselves. For such flaws, fracture mechanics approaches will need to be developed on j
a case-by-case basis to address the attachment geometries and flaw configurations of l
concem. It is expected that the calculational methods and acceptance criteria would be l
consistent with approaches now prescribed in Section XI for flaws in vessel and piping l
components.
This review finds 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 following:
A discussion that supports the rationale for not developing detailed flaw evaluation e
methods for inclusion in the BWRVIP-48 report. This discussion should emphasize the large number of attachment designs, and the uncettainties regarding the
!ccations and orientations of flaws that could be detected by future inspections.
A statement that ASME Section XI does not specifically address the evaluation of o
flaws in attachment welds, other than aspects dealing with flaws extending into the base material 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 structuralintegrity 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 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 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 BWRVIP-48 report.
BWRVIP's April 30,1999, Response:
Methods for evaluating indications in the attachment weld and base metal are described in the Code. The BWRVIP intends to address issue 4.3 by re-writing Section 4.3 as described in the April 30,1999, response.
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.... v Staff's Evaluation:-
The staff finds that the BWRVIP's response adequately addressed this item.
4.0 CONCLUSION
S The staff has completed its review of the BWRVIP-48 report, as revised, and finds that the licensee's implementation of the revised guidelines, with the staff's final comments addressed above, will provide an acceptable level of quality for examination of the safety-related components addressed in the BWRVIP-48 document.
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