ML20056D642
| ML20056D642 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 07/27/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20056D640 | List: |
| References | |
| NUDOCS 9308170244 | |
| Download: ML20056D642 (6) | |
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- J g UNITED STATES -
t i E NUCLEAR REGULATORY COMMISSION !
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WASHINGTON D.C. 20065-0001
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i SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION j t
RE00ESTS FOR RELIEF AND ALTERNATIVE FROM/TO !
ASME BOILER AND PRESSURE VESSEL CODE RE0UIREMENTS i l
SOUTHERN CALIFORNIA EDISON COMPANY l j
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SAN ON0FRE NUCLEAR GENERATING STATION. UNITS 2 AND 3 DOCKET NOS. 50-361 AND 50-362 i
1.0 INTRODUCTION
l Pursuant to 10 CFR 50.55a(f)(4), American Society of Mechanical Engineers (ASME) Code Class 1, 2, and 3 components (including supports) shall meet the requirements, except the design and access provisions and the preservice 1 examination requirements, set forth in the ASME Code,Section XI, " Rules for !
Inservice Inspection of Nuclear 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
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ten-year. interval comply with the requirements in the latest edition and '
addenda of Section XI of the ASME Code incorporated by reference in 10 CFR -
50.55a(b) on the date twelve months prior to the issuance of the operating ,
license, subject to the limitations and modifications listed therein. The? :
applicable edition of Section XI of-the ASME Code for San Onofre Nuclear i Generating Station (SONGS) Units 2 and 3 first 10-year ISI interval'is the -j 1977 Edition through Summer 1979 Addenda. The components (including supports) ,
may ineet the requirements : set forth in subsequent editions and addenda of the ;
ASME Code incorporated by reference in-10 CFR 50.55a(b) subject to the -!
limitations and modifications listed therein. 1 Technical Specification 4.0.5 for SONGS Units 2 and 3, states that inservice '
inspection and testing of the ASME Code Class 1, 2,' and 3 components-shall be .>
performed in accordance with~ Section XI of the ASME Boiler and Pressure Vessel !
Code and applicable Addenda as required by 10 CFR 50.55a(g), except where l specific written relief has-been granted by the Commission pursuant' to 10 CFR 1 50.55a(g)(6)(i). By_a rule change effective September 8,_1992, inservice- i testing requirements are now in 10 CFR 50.55a(f). Paragraph.10 CFR l 150.55a(a)(3) states that alternatives to the requirements of paragraph (f)'may l*
be used 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 1 the level of quality and safety. l, I
9308170244 930727 i PDR- ADOCK' 05000361 <
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s In a letter dated March 5, 1993, Southern California Edison (SCE or the licensee) submitted two relief requests, B-10 and B-11, to use alternative requirements to the 1977 Edition, through Summer of 1979 Addenda, of the ASME ,
Boiler and Pressure Vessel Code at SONGS, Unit 2 and 3. In a letter dated '
August 24, 1992, the licensee requested approval for the use of helical coil '
threaded inserts in steam generator manway and handhole stud holes at SONGS, ;
Units 2 and 3. Specifically, the licensee seeks to install these inserts :
under the aegis of ASME Code Case N-496, which was approved on March 14, 1991, ,
but has not yet been endorsed by the U.S. Nuclear Regulatory Commission. i 2.0 NEW RELIEF RE0 VESTS 2.1 Relief Recuest B-10 The licensee is requesting relief from performing a VT-1 visual examination on the internal surface of the reactor coolant pump (RCP) casing required by the 1977 Edition of the ASME Code,Section XI, Table IWB-2500-1, Examination Category B-L-2, Item 12.20.
2.1.1 Licensee's Basis for Recuestina Relief The licensee's basis for Relief Request B-10 is as follows:
(A) The 1989 Edition of the ASME Code (approved in 10 CFR 55.55a),
Table IWB-2500-1, Category B-t-2 requ' res this inspection only be performed when the pump is disassembled for other reasons.
(B) The dose associated with the disassembly of an RCP to perform the required inspections is estimated to be approximately 50 person-REM. Eliminating the need to disassemble a RCP for the sole purpose of performing the internal visual inspection will limit unnecessary dose to plant workers (ALARA) and prevent possible damage to pumps.
(C) Combustion Engineering Owner's Group Report CEN-412 provides technical justification supporting the relaxation of the Code Section XI, Category B-L-1 requirements to perform a volumetric inspection for the reactor coolant pumps operated at San Onofre Units 2 and 3. Therefore, since the CEN-412 evaluated the safety and serviceability of the pump casing, the results can be applied to support the relaxation of the Category B-L-2 visual internal inspection.
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2.1.2 Alternate Testina Perform a VT-1. internal visual inspection of the internal surfaces whenever a RCP is disassembled for maintenance.
. i 2.1.3 Staff Evaluation '
The staff has approved the 1989 Edition of the ASME Code, which includes the !
elimination of the requirement to disassemble pumps for the sole purpose of-performing examinations of the internal surfaces. Since the licensee's ,
alternate testing commitment complies with the approved 1989 Edition of the -l ASME Code, the staff concludes that SCE's proposed alternative provides an acceptable level of quality and safety. Therefore, pursuant to 10 CFR .;
50.55a(a)(3)(i), the licensee's proposed alternative examination may be authorized. 1 2.2 Relief Reauest B-11 The licensee is requesting, pursuant to 10 CFR 50.55a(g)(5)(iii), relief.to >
use the 1992 Edition of the ASME Section XI, Article IWA-4500 for the -
automatic / machine Gas Tungsten' Arc Welding (GTAW) process when performing a temperbead weld repair on ferrous base materials. The 1992 edition of the ASME Code,Section XI, Article IWA-4500, permits the use of the automatic / machine GTAW process and F43 filler metal with the temperbead technique without requiring impact testing on the weld deposit of the procedure qualification test assembly; however, the 1992 Edition of the ASME ,
Code has not yet been endorsed by the NRC.
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2.2.1 Licensee's Basis for Recuestino Relief -
SCE may need to replace some small' bore Inconel 600 nozzles in the reactor !
coolant system during their next reYueling outage, which will require welding -
on ferrous base material without a post-weld heat treatment (i.e., the temperbead technique) in conjunction with non-ferrous F43 Inconel' filler.- SCE is committed to the ASME Code,Section XI, 1977 Edition / Summer 1979 Addenda, which only provides for using the Shielded Metal Arc Welding (SMAW) process. >
SCE prefers the use of the automatic / machine GTAW process more so than the manually operated SMAW process for temperbead repairs. The automatic / machine ,
GTAW process is inherently superior for controlling important process variables that affect the overall quality of the weld. Additionally, the automatic / machine GTAW process is less affected by adverse field environments than manual welding.
SCE is aware that Code Case N-432 provides for the use of the automatic / machine GTAW process for the temperbead technique weld repairs on Class I components. However, full compliance with this Code Case would require the performance of impact testing (i.e., dropweight and Charpy V-Notch
- testing) on the weld deposit of the procedure qualification test assembly. In recognition of the low temperature fracture toughness properties of the nickel 3 base alloys, the ASME Code,Section III, Articles NB-4335 and NB-2310 exempt i impact testing of non-ferrous materials for procedure qualifications.
Although the 1992 Edition of the ASME Code has not been approved by the NRC, the 1992 Edition of the ASME Code,Section XI, Article IWA-4500, provides for using the technique without requiring impact testing on the weld deposit of
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,. the procedure qualification test assembly. Therefore, SCE requests permission to implement the 1992 edition of the ASME Code for the use of the automatic welding process.
2.2.2 Alternate Weld Process Automatic / machine Gas Tungsten Arc Welding process and F43 filler metal with the temperbead technique as provided for in the 1992 Edition of the ASME Code,
- Section XI, Article IWA-4500. ,
2.2.3 Staff Evaluation The staff has reviewed the licensee's request for relief as a request to use an alternative pursuant to 10 CFR 50.55a(a)(3)(i) since the request is to use a part of a ASME Code edition not yet incorporated by reference in 10 CFR '
50.55a(b). :
The staff agrees with the licensee that the automatic / machine GTAW process is inherently superior for controlling important process variables that affect the overall quality of the weld. The staff further agrees that, in recognition of the low temperature fracture toughness properties of the nickel base alloys, exempting impact testing of non-ferrous materials for procedure qualifications is acceptable.
The staff has reviewed the licensee's submittal and has concluded that the use t of the provisions of the 1992 Edition of the ASME Code,Section XI, Article IWA-4500 in place of ASME Code,Section XI,1977 Edition / Summer 1979 Addenda, which only provides for using the Shielded Metal Arc Welding (SMAW) Process, is an alternative which provides an acceptable level of quality and safety.
Therefore, pursuant to 10 CFR 50.55a(a)(3)(i), use of licensee's proposed alternative is authorized as requested.
3.0 CODE CASE N-496 Case N-496 of the ASME Boiler and Pressure Vessel (B&PV) Code documents the approval for use of helical coil threaded inserts provided certain conditions concerning the purchase, installation, maintenance, and documentation are met.
Code Case N-496 has not yet been approved by the NRC and does not appear in Regulatory Guide 1.85, " Materials Code Case Acceptability-ASME Section III, Division 1,". Revision 28, dated April 1992; nor in Regulatory Guide 1.147,
" Inservice Inspection Code Case Acceptability-ASME Section XI Division 1,"
Revision 9, dated April 1932.
3.1 Licensee's Basis for Reouestina Use of Code Case N-496 Steam generators (SGs) are provided with sccess for periodic inspection and maintenance. Access is gained through manways located in areas of interest.
The manway cover plates are secured using a number of threaded studs and hexagonal nuts. The base of each threaded stud is tightened into a matching threaded hole in the base of the manway. After the manway cover is removed, <
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the studs are removed to provide free access to the generator and protect the studs from possible damage.
Damage to stud hole threads is caused by galling, which is associated with stud seizure on the threads, and minor drill misalignment during the removal of a seized stud. Repairing damaged stud holes by installing a helical coil involves minimal drilling and thread tapping of the damaged stud hole. This increases the hole diameter just enough to install a helical coil insert on the new threads and use the same size stud.
The two thread repair methods currently specified in the ASME B&PV Code are:
(1) drilling and thread tapping and (2) welding.
The more viable of these two techniques is drilling and thread tapping, which removes additional metal and forms a larger threaded hole (oversized). As a result, if a helical coil insert is not used, tr.is repaired stud hole requires an oversized stud. The use of nonuniform studs p')ses procurement and logistic difficulties and complicates subsequent maintenance operations.
The other viable repair technique, which is less desirable, restores metal to the hole by welding and reforms the hole by drilling and tapping. Welding on a pressure vessel in a field environment presents numerous difficulties. i These difficulties include significant expenditure of engineering planning ;
resources, base metal preparation for welding, protection of the gasket '
seating (sealing) surface from welding process damage, and heat treatment.
Also, welding would generally take significantly longer to perform than installation of helical coil inserts. Because the dose rates with the best available shielding are approximately 100 mrem / hour to a welder performing repairs on the primary manway and secondary handhole, the potential exists for significantly increased radiation exposure when using this technique in these two areas.
The helical coil repair technique involves less metal removal from the stud .;
hole than the other two repair techniques currently available. Therefore, repair using helical coils minimizes the potential need to add metal to the component by welding to repair any future damage to the same threads.
Without the installation of a helical coil insert, the repaired hole threads are in direct contact with the stud threads and the hole threads are subject i to further galling or damage during future repairs due to drill misalignment.
The helical coil, which can be easily replaced, bears the risk of galling and provides a buffer for minor drill misalignment during any subsequent seized -
stud removal. Accordingly, the longevity of a repair without a helical coil insert may be shorter than that achievable using the insert.
ABB-Combustion Engineering (CE), SCE's qualified supplier for helical coil inserts, completed Design Report CP.-9448-CSE92-1106, " Southern California -
Edison - San Onofre Steam Generator Manway and Handhole Stud / Stud Hole Repair Evaluation, Units 2 and 3." This report identifies and justifies the minimum i thread engagements for the use of helical coil inserts in steam generator
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. f primary manways, secondary manways, and secondary handholes for Units 2 and 3, and concludes that the helical coil inserts meets ASME B&PV Code standards in terms of stress loads, thermal expansion and material compatibility.
3.2 Staff Evaluation :
Helical coil inserts are being used in other plants and have demonstrated ~
I acceptable results. For example, in San Onofre Unit 1, helical coil inserts have been used in the SGs since 1984 and have been installed in 45 SG stud holes. The licensee's experience at San Onofre Unit I has shown all of the inserts to have performed as intended.
San Onofre Unit 1 is constructed to Section VIII of the ASME B&PV Code for material requirements, whereas San Onofre Units 2 and 3 are constructed to
- Section III requirements. There were and are provisions in ASME Section VIII that allowed the use of parts such as helical coil inserts when performing Section XI replacements at Unit 1. Therefore, NRC approval was not required to install helical coil inserts at Unit 1.
The helical coil threaded inserts have demonstrated an acceptable level of quality and safety through both evaluation and plant experience. Repairs of seized studs without the use of helical coil inserts lead to unusual difficulties including complicating maintenance procedures and possibly .[
reducing the longevity of repairs, with corresponding increases of doses to .i the workers. f Based on a review of the information provided, the staff has concluded that i use of helical coils according to ASME Code Case N-496 for the steam generator primary manways, secondary manways, and secondary handholes at San Onofre :
Units 2 and 3, is an alternative which provides an acceptable level of quality l and safety. Therefore, NRC approval for the use of Code Case N-496 is {
-granted, pursuant to 10 CFR 50.55a(a)(3)(1).
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4.0 CONCLUSION
Pursuant to 10 CFR 50.55a(a)(3)(1), the staff finds that the proposed I alternative examination as discussed in Relief Request B-10 and B-11, and the .
use of Code Case N-496 as discussed above, are authorized and will provide an !
acceptable level of quality and safety for San.Onofre Nuclear Generating i Station, Units 2 and 3.
Principal contributors: T. K. Mclellan, EMCB/NRR J. Davis, EMCB/NRR M. Khanna, ECMB/NRR ;
Date: July 27,1993 i i
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