ML20212A744
| ML20212A744 | |
| Person / Time | |
|---|---|
| Site: | Braidwood  |
| Issue date: | 09/10/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20212A704 | List: |
| References | |
| NUDOCS 9909170122 | |
| Download: ML20212A744 (6) | |
Text
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\' $4 J1 UNITED STATES f j NUCLEAR REGULATORY COMMISSION
$ t WASHINGTON, D.C. 20555 0001
%:4.....f SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO THE SECOND 10-YEAR INTERVAL INSERVICE INSPECTION PROGRAM PLAN REQUEST FOR RELIEF NO.12R-07 COMMONWEALTH EDISON COMPANY BRAIDWOOD STATION. UNIT NOS.1 & 2 DOCKET NOS. STN 50-456 AND STN 50-457
1.0 INTRODUCTION
The Technical Specifications (TS) for Braidwood Station, Units 1 and 2, state that the inservice inspection (ISI) of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (Code) Class 1,2, and 3 components shall be performed in accordance with Section XI of the ASME Code and applicable Addenda as required by 10 CFR 50.55a(g), except where specific written relief has been granted by the Commission pursuant to 10 CFR 50.55a(g)(6)(i).10 CFR 50.55a(a)(3) states that alternatives to the requirements of paragraph (g) may be used, when authorized by the NRC, if (i) the proposed alternatives would provide an acceptable level of quality and safety, or (ii) compliance with the specified requirements would result in hardship or unusual difficulties without a compensating increase in the level of quality and safety.
Pursuant to 10 CFR 50.55a(g)(4), ASME Code Class 1,2, and 3 components (including supports) shall meet the requirements, except the design and access provisions and the preservice examination requirements, set forth in the ASME Code,Section XI, " Rules for Insen/ ice Inspection of t.Juclear Power Plant Components," to the extent practical within the limitations of design, geometry, and materials of construction of the components. The regulations require that inservice examination of components and system pressure tests conducted during the first 10-year interval and subsequent intervals comply with the requirements in the latest editico and addenda of Section XI of the ASME Code incorporated by reference in 10 CFR 50.55a(b) 12 months prior to the start of the 120-month interval, subject to the limitations and modifications listed therein. The applicable edition of Section XI of the ASME Code for Braidwood Station's second 10-year ISI interval is the 1989 Edition.
l Pursuant to 10 CFR 50.55a(a)(3)(i), if the licensee determines that the alternative would provide an acceptable level of quality and safety, information shall be submitted to the Commission in ENCLOSURE 9909170122 990910 PDR ADOCK 05000456 G PDR I
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support of that determination and a request made for relief from the ASME Code requirement.
After evaluation of the determination, pursuant to 10 CFR 50.55a(a)(3)(i), the Commission may grant relief and may impose alternative requirements that are determined to be authorized by law, will not endanger life, property, or the common defense and security, and are otherwise in the public interest, giving due consideration to the burden upon the licensee that could result if the requirements were imposed.
By letter dated April 17,1998, Commonwealth Edison Company (Comed. the licensee),
submitted Relief Request 12R-07 as part of the second 10-year interval ISI plan. Revisions 1 and 2 to the ISI plan were submitted by letters dated August 3 and September 2,1998, respectively. I 1
2.0 EVALUATION The information provided by the licensee in support of the relief request has been evaluated and I the bases for disposition are documented below. I Relief Reauest 12R-07. Limited Volumetric Examination of Residual Heat Removal (RHR) Heat Exchancer Nozzle-to-Shell Welds and Nozzle inner Radii l Code Recuirement: Table IWC-2500-1, Examination Category C-B, Item C2.21 requires volumetric and surface examination of the nozzle-to-shell welds, and item C2.22 requires volumetric examination of the nozzle inner radius of the regions described in Figure IWC 2500-4(a) or (b), for nozzles without reinforcing plate in vessels >% inch nominal thickness. Examinations shall be conducted on nozzles at terminal ends of piping runs selected for examination under Examination Category C-F each inspection interval. In cases of multiple vessels of similar design, size, and service, the required examinations '
may be limited to one vessel or distributed among the vessels.
Licensee's Code Relief Recuest: The licensee requested relief from the Table +
IWC-2500-1, Examination Category C-8, Item C2.21 and C2.22 requirements to perform volumetric examination of the Residual Heat Removal Heat Exchanger nozzle-to-shell welds and nozzle inner radii for the second 10-year interval currently scheduled to commence July.29,1998, and to be completed July 28,2008, +/- 1 year Interval extension as allowed by paragraph (d) of IWA-2430.
Licensee's Basis for Reauestina Relief (as stated):
Pursuant to 10 CFR 50.55a(a)(3)(i), relief is requested on the basis that the proposed alternative would provide an acceptable level of quality and safety.
The Residual Heat Removal Heat Exchangers are approximately 7/8 inch nominal wall thickness with nozzles of 14 inch diameter and approximately 3/8 inch in nominal wall thickness. [T]he subject joint is best characterized as a fillet welded nozzle using an internal reinforcement pad and, as such, is not accurately represented by Figures IWC-2500-4(a) and 4(b), which depict full penetration butt welded nozzles. The inner
I surface of the nozzle reinforcement pad is representative of the nozzle inner radius requiring Section XIinspection.
Due to the inherent geometric constraints of the nozzle design, performance of the Code required ultrasonic examinations of the nozzle inner radius cannot be accomplished.
Outside geometrical constraints also limit the ultrasonic examination of the nozzle-to-vessel welds.
The Braidwood RHR Heat Exchangers were manufactured by Joseph Oats Corporation in 1975 per the requirement of ASME Section Ill,1974 Edition, Summer 1975 Addenda, Subarticle NC-3200, Alternate Design Rules for Vessels. The nozzles and shell are fabricated from SA240 type 304 stainless steel material. The RHR heat exchangers' tube side is Code Class 2 and the shell side is Code Class 3. The nozzle to shell welds were not required to be volumetrically examined during fabrication; only liquid penetrant examinations were performed on the final surface
- of the weld.
During the preservice inspections of the Byron and Braidwood components, relief was requested from performing volumetric examinations of the nozzle to vessel welds due to inherent geometric constraints.
The fillet weld located directly above the nozzle-to-shell weld is an obstruction to the proper movement of the inspection instrumentation transducer. These constraints limited the ability to perform a meaningful UT. These relief requests, NR-14 for Byron Unit 1, NR-13 for Byron Unit 2,1NR-12 for Braidwood Unit 1, and 2NR-12 for Braidwood Unit 2 were approved by the NRC in Byron SSER 7, page 16 and Braidwood SSER 5, page 6-2.
Relief requests NR-12 for Byron and NR-12 for Braidwood were included with the First 10-Year Interval ISI Program Submittal. These relief requests sought the same Code inspection exemptions for the nozzle to shell welds as did the preservice relief requests.
Relief for the nozzle to shell weld examination was denied and NRR requested a best effort UT of the nozzle-to-vessel welds be conducted.
The initial UT inspection performed in 1991 on the Braidwood Unit 2 "A" RHR heat exchanger identified indications which exceeded the ASME Section XI 1983 Edition, Summer 1983 Addenda, Article IWC-3000 allowable limits. The indications which exceeded the acceptance standards of IWC-3000 were subjected to further evaluation in accordance with ASME Section XI Subarticle IWB-3600. The required Fracture Mechanics Analysis was submitted to the NRC [ letter fro,m T.W. Simpkin (Comed) to T.
E. Murley (NRC), "Braidwood Station Unit 2 Flaw Evaluation for RHR Heat Exchanger Nozzle to Shell Welds," dated November 13,1991] and the indications were found to be acceptable for continued service (letter from R.M. Pulsifer (NRC) to T.J. Kovach (Comed), " Residual Heat Removal Heat Exchanger Nozzle to Shell Welds (TAC No.
M82087)," dated November 21,1991]. Additional examinations were performed in 1992 for Byron Unit 2 and Braidwood Unit 1 heat exchangers, in 1993 for Byron Unit 1 and Braidwood Unit 2 heat exchangers, in 1994 for Braidwood Unit 2 heat exchangers, and
l 3 t in 1995 for Byron Unit 2 heat exchangers. All examinations confirmed the existence of !
fabrication flaws in the nozzle to vessel welds.
The examination results from the inspections performed in 1994 at Braidwood Unit 2
{
included flaws on the outlet nozzle weld of the 2B RHR vessel which exceeded the l 60 percent acceptance criteria. The size change from previous inspections was l
attributed to enhancement in the volumetric examination technique. An ASME Section XI repair by excavation was completed; the unacceptable flaws were removed.
I After completion of the best effort UT requested by the NRC, Relief Request NR-23 was submitted by Comed for review, proposing a surface examination in lieu of the Code Required volumetric examination of the nozzle-to-vessel welds. Relief was granted per j
[ letter from R.A. Capra (NRC) to D.L. Farrar (Comed), " Residual Heat Exchanger Nozzle (
Welds, Byron Station, Units 1 and 2, and Braidwood Station, Units 1 and 2 (TAC Nos.
M94393, M94394, M94395 and M94396)," dated February 29,1996) with the provision that a UT examination of one RHR Heat Exchanger nozzle-to-vessel weld per unit will be performed in accordance with IWB-3500 during the next inspection interval. {
l Licensee's Prooosed Altemative:(as stated)
The nozzle-to-vessel welds on the A and B Heat Exchangers for Braidwood Units 1 and 2 wil1 receive a Section XI surface examination each inspection period.
In addition, a UT examination of one RHR Heat Exchanger nozzle-to-vessel weld per unit will be performed in accordance with IWB-3500. This UT examination is based on
[a] letter dated February 29,1996, R. A. Capra, USNRC to D. L. Farrar (Comed) (letter from R.A. Capra (NRC) to D.L. Farrar (Comed), " Residual Heat Exchanger Nozzle Welds, Byron Station, Units 1 and 2, and Braidwood Station, Units 1 and 2 (TAC Nos.
M94393, M94394, M94395 and M94396)," dated February 29,1996).
A Visual (VT-1) examination of the nozzle inner radii in one heat exchanger will be performed either directly or remotely, to the extent practical, if disassembly of a heat exchanger is required for maintenance purposes. These examinations will only be performed once.per inspection interval. In addition, a visual examination (VT-2) shall be performed each inspection period on all pressure retaining components.
Evaluation:
The Code requires volumetric and surface examinations of the subject RHR heat exchanger nozzle-to-vessel we(ds and a volumetric examination of the nozzle inner radius. In lieu of the Code-required examinations, the licensee has proposed to perform a surface examination once each inspection period for the RHR heat exchanger nozzle-to-vessel welds, a best-effort ultrasonic examination of one nozzle-to-vessel weld per unit, and a visualinspection of the nozzle inner radii if disassemb.ly of a heat exchanger is required for maintenance purposes.
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During the first 10-year ISI interval, the licensee conducted volumetric examinations of all Byron, I Units 1 and 2, and Braidwood, Units 1 and 2, RHR heat exchanger nozzle-to-vessel welds.
There are a total of 16 nozzles for both stations, two heat exchangers per unit and two nozzles per heat exchanger. These examinations revealed indications that were determined by the licensee to be fabrication flaws (slag). The results of these examinations were submitted by two letters dated November 8,1994, and by letters dated January 23,1994, and January 20,1995.
The licensee either removed indications that were greater than ASME Code,Section XI, Subsection lWB-3500, or provided fracture mechanics analysis and a finite element analysis in order to justify leaving these indications in service. During the first ISI interval the licensee conducted some follow-up UT examinations and did not observe significant crack growth of the indications left in service.
i The RHR heat exchanger welds are within a Class 2 system, on a moderate energy line which operates at a pressure of about 400 psig. This operating pressure is well below the design pressure (600 psig).
The RHR heat exchanger nozzles are fabricated from type 304 austenitic stainless steel which has a very high fracture toughness. This material has a large flaw tolerance because of material ductility, flexibility (thin walled), and the reinforcement provided by the fillet weld (nozzle design).
All RHR heat exchanger welds were hydrostatically tested by the manufacturer at a pressure of 800 psig. Another hydrostatic test was performed in the field, after installation, at a pressure of 775 psig for Byron, Unit 1, and 800 psig for Byron, Unit 2, with no leakage noted from these regions. Since the pressure is the dominant load on the nozzle weld, the hydrostatic tests have demonstrated that these nozzle welds can withstand almost double the operating pressure, without structural failure, despite the presence of fabrication flaws in the weld.
During the first IS! interval, the licensee's fracture mechanics and finite element analyses were submitted to the NRC for review and approval. The fracture mechanics analysis used full design pressure (600 psig) for the allowable flaw size calculations in the ana'ysis. The actual induced piping loads on the nozzles are less than 60 percent of the design loads used by the allowable flaw size calculations. The results of the finite element analysis also show that the inside diameter of the nozzle is in compression and the outside (OD) is in tension. Consequently, any service induced flaw would be expected to initiate at the OD of the nozzle where the weld membrane stresses are in tension. The staff concluded that the liquid penetrant test (PT), in this case, would be likely to detect service induced flaws on the surface of these nozzle welds. A VT-2 examination is being conducted on all RHR heat exchangers once per inspection period as required by ASME,Section XI, Code item C2.33.
The staff concludes that performance of PT examinations each inspection period as proposed by the licensee will provide an acceptable means for detection of the service induced flaws on the surface of the nozzles and should identify any significant service induced crack growth.
Performance of the Code required UT examinations for the RHR nozzle-to-vessel weld would provide additional monitoring capabilities for service induced flaw growth beyond that provided by the PT examination. However, because the licensee has UT examined all the nozzles at least once, has removed and/or analyzed existing indications, has concluded that the indications
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- are fabrication flaws, and because any crack growth would be expected to be slow due to the compressive stresses on the inside surface of the nozzles, additional UT examinations in this service period will not add significant assurance that the indications are not growing in service.
By letter dated February 29,1996, the staff appros :;d a similar relief request for Braidwood Station's first ISI interval. As part of that approva the staff required the licensee to perform UT examinations on a sample of RHR nozzle-to-vessel welds (one nozzle per unit) during the second 10-year inspection interval to provide additional assurance that the fabrication indications have not grown and that no new service induced indications have developed. Relief Request 12R-07 is consistent with the actions that were previously reviewed and approved by the staff. The licensee's technical basis for the relief request is acceptable.
3.0 ' CONCLUSION l
The staff concludes that the alternatives contained in Request for Relief 12R-07 provide an acceptable level of quality and safety and, therefore, the requests for relief are authorized
, pursuant to 10 CFR 50.55a(a)(3)(i).
Principal Contributor: S. Bailey Date: September 10, 1999 e