ML20216F477
| ML20216F477 | |
| Person / Time | |
|---|---|
| Issue date: | 09/16/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20216F457 | List: |
| References | |
| NUDOCS 9909220057 | |
| Download: ML20216F477 (8) | |
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U.S. NUCLEAR REGULATORY COMMISSION DIVISION OF ENGINEERING OFFICE OF NUCLEAR REACTOR REGULATION SAFETY EVALUATION OF EPRI TOPICAL REPORT TR-108823 BWR VESSEL AND INTERNALS PROJECT. BWR SHROUD SUPPORT INSPECTION AND FLAW EVALUATION GUIDELINES (BWRVIP-38)
1.0 INTRODUCTION
By letter dated September 15,1997, the Boiling Water Reactor Vessel and Internals Project (BWRVIP) submitted both the proprietary and non-proprietary versions of the report, "BWR Vessel and Internals Project, BWR Shroud Support inspection and Flaw Evaluation Guidelines
- (BWRVIP-38)," for staff review and approval. The s'aff requested additional infonnation (RAl) in a letter dated April 8,1998. The BWRVIP responded to the RAI in a submittal dated November 24,1998.
The BWRVIP-38 report contains generic guidelines to BWRVIP members on inspection and flaw evaluation of BWR shroud supports. These guidelines considered degradation susceptibility, degradation mecharJsms, loads, and inspection strategies for shroud supports.
The intent of the report, when approved by the NRC, is to provide inspection and flaw evaluation guidance to BWRVIP members that can be used to assure adequate BWR shroud support integrity.
The BWRVIP-38 report allows for plant-specific analysis to be performed for a given weld location. These plant-specific analyses are not addressed in the scope of this report, and the NRC approval must be obtained on a case-by-case basis, until such time as they are approved i for general usage by the staff.
1.1 Puroose The staff reviewed the BWRVIP-38 report to determine whether its guidance would provide adequate assurance of the structuraiintegrity and fun:: tion of the BWR shroud supports.
1.2 Oraanization of the Reoort Because the BWRVIP-38 report is proprietary, this safety evaluation (SE) was written so as not to repeat information contained in the report. This SE gives a brief summary of the general contents of the report in Section 2.0 and the detciled evaluation in Section 3.0 below. The SE does not discuss in any detail the provisions of the guidelines nor the parts of the guidelines that the staff finds acceptable.
I Enclosure 9909220057 990916 PDR TOPRP EXIEPRI C PDR I
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2.0
SUMMARY
OF BWRVIP-38 REPORT The BWRVIP-38 report addresses the following topics in the following order: '
o Shroud Suncort Desians and Sureotibility Information
- Shroud support designs
- Susceptibility information
- Potential for shroud support leg or gusset weld cracking o Shroud Sunoort insoection
- Baseline inspection strategy
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- Rainspection strategy o Loads and Load Combinations
- Shroud support loads
- Load combinations
- Dynamic response evalus. tion j o Stress / Flaw Evaluations )
- Plant ranking tables
- Flaw tolerance for supports and legs
- Flaw tolerance for supports with gussets
- BWR-2 conical shroud support The BWRVIP-38 report also contains appendices on (A): Stress / Flaw Evriluations and (B) BWR Shroud Support Demonstration of Compliance with the Technical Information Requirements of -
the License Renewal Rule, (10 CFR 54.21). Appendix B to the BWRVIP-38 report is not evaluated in the SE report, but will be evaluated under a separate license renewal review.
3.0 STAFF EVALUATION The inspection guidelines provided by the BWRVIP-38 repod are comprehensive in dealing with the in-service inspection -requirements of the shroud supports. 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. Implementation of the recommendations are intended to provide a basis for baseline inspections, re-inspections, and structural evaluations of the shroud support structure.
With the exception of issues described below, this review finds that the guidance provided in the subject report to be acceptable.
3.1 Inerection and Materials issues 3.1.1 Residual Stresses The BWRVIP-38 report references BWRV!P-14 to support the discussion on residual stresses in shroud support welds. In the GE on BWRVIP-14, dated June 8,1998, the staff expressed a ;
need for evaluations of applied and reeldual stress used by the licensee to determine the acceptability of an assumed crack growth rate and that tesiMI strees determinations must
3 include repairs tnd any other relevant factors. The discussion on residual stress in the staff's BWRVIP-14 SE is also applicable to the BWRVIP-38 report, and licensees should perform a plant-specific evaluation for their core shroud support weld crack growth determinations, using appropriate applied and residual stresses.
3.1.2 Lower Plenum Insoections Because of the difficulty in performing an inspection on the legs in the lower plenum, the BWRVIP-38 report states that the lower plenum is not part of the guideline inspection strategy.
The BWRVIP should revise the BWRVIP-38 report such that, when the inspection tooling and methodologies are developed that allow the welds in the lower plenum to be accessible, the guidelines will state that licensees should inspect thess welds with the appropriate NDE method, in order to establish a baseline for these welds. An appropriate re inspection schedule should be proposed by the BWRVIP in a revised BWRVIP-38 report.
3.1.3 Crack Growth Rate By letter dated December 23,1998, the BWRVIP developed a crack growth rate correlation
.' fnodel for nickel base alloys in BWR type environments that was submitted to the staff as BWRVIP-59. Until such time as the sta5 et 1 review and evaluate the crack growth rate correlation model, licensees should use s ; conservative bounding crack growth rate of 5 x 10-5 in/hr for their normal plant operating condi. ion.
3.1.4 Vertical Annulus Welds The BWRVIP is currently developing guidelines for inspections of shroud support cylinder vertical welds. These webs are in the annulus area and fall within the guideline area of inspection. In the absence of BWRVIP guidance that has been reviewed by the staff, the inspection of vertical welds in the shroud support cylinder should be to the same criteria as gussets.
3.1.5 Gusset Welds The BWRVIP-38 report is vague regarding the minimum length of accessible gusset welds that should be examined for a baseline inspection. The report should establish a minimum weld length criterion to be examined, and evaluation criteria to be used in a plant-specific evaluation for licensees that cannot satisfy the minimum inspection criterion specified by the BWRVIP-38 report. By using the statement from page 3-9, the vagueness surrounding what constitutes a minimum percentage of cumulative weld lengths (total length) becomes a specific number that may be unique for each plant. Plant-specific evaluations should include a technical basis regarding the quality and safety of the un-examined welds.
3.2 Load and Load Combinations The various loads and load combinations that need to be considered to determine the primary and secondary stress levels appropriate for various operating conditions are discussed in Section 4.0 of the BWRVIP-38 report. These loads were used in the flaw tolerance evaluation addressed in Appendix A of the BWRVIP-38 report and could be used in plant-specific flaw
4 evaluations. In either case, the loads used in plant specific evaluations should be consistent with the plant's design basis.
I' This loading includes horizontal and vertical forces and moments acting on the shroud support structure due to seismic excitation of the core and internals. For Mark ll and lli conteinment '
plants, this loading also includes effects on the internals of annulus pressurization Icads and hydrodynamic loads on the vessel and containment. The location where the equivalent static loads are applied to the shroud support is through the attachment to the core shroud (weld H7).
Generally, the shear forces and overtuming moments are available at the shroud support elevation from previous analyses such as goveming dynamic evaluations. Vertical seismic accelerations are generally available in the safety analysis report (SAR). For Mark ll and ill containment plants, vertical forces and accelerations are available at the shroud support elevation from the applicable vertical dynamic loads analyses. If only the vertical accelerations are available, forces may be obtained by multiplying the acceleration by the appropriate mass.
The square-root-sum-of-the-squares (SRSS) method of combining dynamic loads is ;
appropriate, unless otherwise specified in the plant SAR. '
The horizontal dynamic loads may be specified in either the east-west or north-south (X- or Z-) !
directions. The most conservative direction of loading should be assumed in the stress and flaw evaluations. For the vertical dynamic loads, this consideration should include the effects of deadweight and buoyancy. For stress and flaw evaluation purposes, the stress from this loading is treated as primary.
There are thermal stresses induced into the shroud support structure as a result of thermal transients experienced within the RPV. These stresses are the result of temperature ;
differences within the shroud support structure itself, as well as the presence of different I materials for components attached to the shroud support structure (i.e., RPV and shroud). '
Fluid temperatures during the transient conditions are based on the information provided on the vessel thermal cycle diagrams. The loads produced by these transients are secondary in nature, but are conservatively treated as primary in the stress and flaw evaluations documented herein.. Loads that are secondary in nature, are only considered for normal / upset conditions, as secondary loads are excluded from consideration during emergency / faulted conditions.
Differential thermal growth of the RPV, as well as pressure dilation of the RPV, induces additional loadings on the shroud support structure. These loads should also be considered in the stress and flaw evaluations of the shroud support structure. As with the thermal loads, vessel-induced loads are secondary in nature, but are conservatively treated as primary in the stress and flaw evaluations documented herein. As stated previously, loads secondary in nature are only considered for normal / upset conditions.
The load combinations used in the evaluation should be consistent with the requirements of the plant SAR or related licensing basis documentation.. Load combinations used to analyze reactor intemals vary, depending on the plant vintage. There are two major categories of plants: those with Mark-Il or Mark-Ill containment where hydrodynamic events cause vessel ;
intemals loads, and those with Mark l containment where hydrodynamic effects in the torus do l not cause loads on the vessel internals.
The dynamic response evaluation is discussed in Section 4.3 of the report. The potential cracking in the shroud support legs or gussets was assumed to be 50 percent through-wall, j l
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5 Thus, based on the difficulty of access discussed previously, the structural and flaw analyses discussed in Appendix A of the BWRVIP-38 report used the assumption of evenly distributed cracking of 50 percent through-wallin the support legs as a bounding assumption.
The 'dynamic loads used in the stress analyses of the shroud support structure come from the plant-unique dynamic analyses. Since a change in the sdffness of the shroud support structure due to partial cracking of the legs or gussets could significantly effect the dynamic response of this structure and the resultant loads used in the subsequent stress analyses, a generic j evaluation was performed by GE (Reference 1). The conclusion of this study was that the l
dynamic loads to use for evaluation of BWR vessel shroud support structures for the bounding !
assumption of evenly distributed cracking of 50 percent through-wall are the same as those used for the evaluation of the uncracked structures.
The results of the analysis in Reference 1 indicate that due to the interaction with the shroud support plate, postulated 50 percent cracking in the legs or gussets resatted in a decrease of between 20 percent and 30 percent in the overall rotational (i.e., overturning) stiffness of the shroud support structure. The frequency decreased by less than 10 percent for the first and dominant rotational mode, which is at approximately 6 Hertz.
The effect of the decrease in the 6 Hertz ho:izontal mode is only important for the seismic response, since the selsraic response input peaks in this range. The safety / relief valve (SRV) actuation and LOCA loads have response spectra peaks above 8 to 10 Hertz, and the effect on the shroud support loads due to the frequency decreases discussed above is negligible. The effect on the shroud support loads due to the slight change in the vertical natural frequency at 30 Hertz is also negligible for the SRV and LOCA loads.
A generic evaluation of the dynamic response of the vesselinternals, using both the Regulatory Guide (RG) 1.60 (Reference 2) and Housner spectra, indicated that postulated 50 percent cracking in a typical BWR bhroud support would cause an increase in horizontal seismic loads of less than 6 percent, and an insignificant change in vertical seismic loads.
A seismic time history analysis for a typical BWR plant with shroud support legs assumed 50 percent cracked, and using time histories based on the RG 1.60 spectra, Housner spectra, Taft earthquake, and El Centro earthquake, showed some small increases in the seismic loads compared to the case with no cracks. In a comparison of the loads for cracked cases with those for uncracked cases, for all four earthquakes, less than 30 percent of the equipment locations showed increased loads, while the other equipment locations showed either the same or decreased loads. Of the loads that increased, the increases were all less than 10 percent.
Since it is industry practice to consider increases in dynamic response due to changes in a finite element model, of less than 10 percent insignificant, the evaluation was not carried further. The_10 percent criterion on increases in the dynamic response of a finite element model are used in the NRC Standard Review Plan (Reference 3), Subsection ll.E.2.a(1) and is considered reasonable and acceptable.
Comparison of the shroud support stiffness and fundamental rotational frequencies for twelve BWRs showed that the cases evaluated are representative of all BWRVIP plants. The plants included small and large BWR 3,4,5 and 6 plants.
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6 The structural analyses which were performed for various shroud support structure configurations are discussed in Appendix A of the BWRVIP-38 report. The ASME Code I requirements used for the analysis in accordance with Subsection NG are sumrnarized in 1 Section A.1.2 of the BWRVIP-38 report. The objective of the structural analysis was to verify )
structural acceptability in accordance with design-basis requirements and to provide flaw tolerance estimates.
For all shroud support configurations except the support leg design, verification of structural acMptability in accordance with design basis requirements was not necessary, since this objective was accomplished for all plants in the original ASME Code, Class 1 RPV stress and fatigue analysis reports. For the leg design, additional analyses were performed to demonstrate acceptability for design-basis conditions. These evaluations were undertaken because the condition of 50 percent cracked legs was interpreted as a design change, and
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therefore, re analysis in accordance with the design basis was deemed appropriate. This process had an objective of demonstrating that the support leg design met all design basis .
requirements with the support legs possessing a minimum cross-section that was 50 percent of that assumed in the original plant design.
. For the leg design, detailed shroud support analyses were performed in accordance with the methodology of ASME Code, Section ill, Subsection NG,1989 Edition to demonstrate the i analytical muthodology. Plastic analysis techni-ques were used. The stress analyses were performed using detailed 180* three-dimensional finite element models of the lower shroud, shroud support plate and cylinder, access hole covers, jet pump holes, and legs or gussets, as appropriate. The finite element analyses utilized non-linear material properties for elastic-plastic evaluation. The analysis for normal and upset conditions were performed in accordance with the requirements of Section ill, Subsection NG-3228 of the ASME Code. The limits on primary membrane stress intensity P., local primary membrane stress intensity Po, and the primary bending stress intensity Po, were in accordance with Subsection NG-3228.2.
The average primary shear stress across a section loaded irs pure shear satisfied the requirements of NG-3227.2. Finally, the displacements were checked against control rod insertion requirements and were determined to be acceptable.
No analysis is required for emergency conditions, since these are bounded by faulted conditions. The analysis for faulted conditions satisfied Level D service limits stipulated in Subsection NG-3225 and Appendix F. The evaluation of the core support structures met the requirements stipulated in F-1440. The limits on the primary membrane Pt stress intensities satisfied F-1331.1(a) and F-1331.1(b) requirements respectively. The static loads did not exceed 90 percent of the limit load analysis collapse load using a yield which is lesser of 2.3 S, and 0.7Soin accordance with F-1331.1(c)2 requirements. The displacements were checked against control rod insertion requirements.
Detailed analyses were performed for both upset and faulted conditions in accordance with these criteria using a " generic" finite element model that did not iricluoe either support legs or gussets. These evaluations demonstrated the upset condition to be limiting from a structural standpoint. Therefore, the detailed analyses described in the report for each shroud support design (legs or gussets) evaluate only the upset (limiting) condition.
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Based on its review, as discussed above, the staff finds the development of the loads and load combinations for shroud support flaw evaluations acceptable because they are in accordance with previously-accepted NRC guidelines and consistent with plant SAR and related licensing basis documentation. As discussed above, the analytical methodology is in accordance with ASME Section 111, Subsection NG criteria, and is therefore, acceptable.
4.0 CONCLUSION
The staff has reviewed the BWRVIP-38 report and RAI response and finds that the guidance contained therein is acceptable for inspection of the subject safety-related BWR shroud supports except where the staff's concems differ from the proposed guidance, as discussed above. This finding is based on information submitted both originally and in response to the staff's RAI that clarified the guidance in the BWRVIP-38 report. The staff has concluded that licensee implementation of the guidelines in BWRVIP-38, with modifications to address the staff's concerns above, will provide an acceptable level of quality for examination of the safety-related components addressed in the BWRVIP-38 document. The staff requests that the BWRVIP review and resolve the issues raised in the enclosed SE, and incorporate the staff's concerns into a revised BWRVIP-38 report. Please inform the staff in writing as to this resolution.
5.0 REFERENCES
- 1. GE Report No. GENF.-B13-01805-71,"BWR Shroud Support Structure Dynamic Loads Evaluation," February 1997
- 2. U.S. Nuclear Regulatory Commission, Regulatory Guide 1.60," Design Response Spectra for Seismic Design of Nuclear Power Plants," Revision 1, December 1973
- 3. NUREG-0800, Revision 2, Standard Review Plan, Section 3.9.2, " Dynamic Testing and Analysis of Systems, Components, and Equipment," U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation, July 1981
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Cari Terry 2-Please contact C. E. (Gene) Carpenter, Jr., of my staff at (301) 415-2169 if you have any further questions regarding this subject.
Sincerely, original signed by:
Jack R. Strosnider, Director
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Division of Engineering l Office of Nuclear Reactor Regulation I
Enclosure:
As stated I cc: See next page I l
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