ML20203A353
| ML20203A353 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 01/29/1998 |
| From: | Westreich B NRC (Affiliation Not Assigned) |
| To: | NRC (Affiliation Not Assigned) |
| References | |
| NUDOCS 9802240007 | |
| Download: ML20203A353 (63) | |
Text
_ _ _ _ _ _ _ _ _ - _ _ _ _ _ _ _ _ _ _ _ _
4 January 29, 1998 3
LICENSEE: Southern California Edison Company FACILITY: San Onofre Nuclear Generating Station
SUBJECT:
MEETING WITH SOUTHERN CALIFORNIA EDISON TO DISCUSS THE MECHANICAL NOZZLE SEAL ASSEMBLY RELIEF REQUEST On December 23,1997, a meeting was held with Southern California Edison (SCE) and the Nuclear Regulatory Commission (NRC) to discuss SCE's request to use mechanical nozzle
, seal assemblies (MNSAs) as an alternate repair method for inconel 600 instrument nozzles j
' attached to the RCS at the San Onofre Nucleal Generating Station (SONGS). Discussions related to the methodology used to perform design calculations, locations which may require installation of the MNSA, installation procedures, plant conditions required for installation, and post installation monitoring. The meeting slides included as Attachment 2 outline the topics discussed.
A notice of this meeting was issued by the staff on December 9,1997. The meeting was held at the NRC Headquarters offices in Rockville, Maryland. Attachment 1 is a list of meeting attendees.
Original Signed By Barry C. Westreich, Project Manager Project Directorate IV 2 Division of Reactor Projects lil/IV Office of Nuclear Reactor Regulation Docket No. 50 361 and 50-362 Attachments: 1. List of Meeting Attendees
- 2. SCE Meeting Slides DISTRIBUTION:(wlencls.1 and 2)
Docket Files PUBLIC I PDIV 2 r/f B. Westreich M. Fields P. Gwynn, RIV /
K. Perkins, RIV/WCFO DlSTRIBUTION: wlencl. 2)
S. Collins (SJC1) F. Miraglia (FJM) 0 Ft>l R. Zimmerman (RPZ) E. Adensam (EGA1)
W. Bateman (WHB) E. Peyton (ESP)
NRC Meeting Attendees DOCUMENT NAME:SONGSMTG. SUM To receive a copy of this document. indicate in the box: "C" = Copy without enclosures *E" = Copy with enclosures "N" = No copy 0FFICE PMW2/PM PDIV-2/LA NAME- CBWktreich EPeyton T -r -, c m me DATE 01hp /98 0168/98 - - '
V O. e L d Cid I 0FFICIAL RECORD COPY 9002240007 980129 P" ^= "*m2 ll ll lIllI,II,1 lllll
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2 cc w/encis:
Mr. R. W. Krieger, Vice President Resident inspector / San Onofre NPS Southem California Edison Company clo U.S. Nuclear Regulatory Commission San Onofre Nuclear Generating Station Post Office Box 4329 P. O. Box 128 San Clemente, California 92674 San Clemente, Califomia 92674-0128 Mayor Chairman, Board of Supervisors City of San Clemente
- County of San Diego - - 100 Avenida Presidio 1600 Pacific Highway, Room 335 San Clemente, Califomia 92672 San Diego, California 92101 Mr. Dwight E. Nunn, Vice President Alan R. Watts, Esq. Southem California Edison Company Woodruff Spradlin & Smart San Onofre Nuclear Generating Station 701 S. Parker St. No. 7000 - P.O. Box 128 Orange, California 92668-4702 San Clemente, Californla 92674-0128 Mr. Sherwin Harris Mr. Harold B. Ray Resource Project Manager Executive Vice President Public Utilities Department Southern Califomla Edison Company City of Riverside San Onofre Nuclear Generating Station 3900 Main Street P.O. Box 128 Riverside, California 92522 San Clemente, California 92674 0128 Regional Administrator, Region IV U.S. Nuclear Regulatory Commission Harris Tower & Pavilion 611 Ryan Plaza Drive, Suite 400 Arlington, Texas 76011-8064 Mr. Terry Winter Manager, Power Operations San Diego Gas & Electric Company P.O. Box 1831 San Diego, Califomia 92112-4150 Mr. Steve Hsu Radiologic Health Branch State Department of Health Services Post Office Box 942732 Sacramento, Califomia 94234
O MEETING ATTENDANCE
(
I DECEMBER 23,1997 SAN ONOFRE NUCLEAR GENERATING STATION l
MECHANICAL NOZZLE SEAL ASSEMBLY RELIEF REQUEST NRC/SCE EDUTHERN CALIFORNIA EDlSON COMPANY R. Oashu E. Schoonover S. Shaw J. Rainsbery ABB/ COMBUSTION ENGINEERIN_G E. Siegel K. Haslinger J. McGarry M. Wade O. Hedden NRC M. Fields B. Westreich B. Hermann K. Wichman J. Strosnider W. Bateman E. Sullivan B. Elliot D. Wessman K. Manoly M. Hartzman
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Southern California Edison
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San Onofre Units 2 and 3 December 23,1997 3l l
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Agenda
+ Purpose
+ Introduction
+ Background I
+ Design and Installation
+ Monitoring
+ Summary 2
Purpose
+ Describe SONGS Alloy 600 instrument and sample (approx.1" diameter) nozzle program
+ Discuss MXSA design and qualification Discuss answers to NRC RAI
- Discuss answers to other NRC questions 3
Introduction
+ SCE is experiencing stress corosion cracking (SCC) ofits Alloy 600 RCS instrument nozzles in both Units 2 and 3
+ Replacement of Alloy 600 with the improved Alloy 690 is being implemented as quickly as possible. The up-coming mid-cycles will be used to begin this process
+ Since it is possible some instrument and sample nozzles '
will have identified leakage before replacement can be performed, SCE is proposing to use these mechanical seals as an interim replacement method
+ Further, in a limited set of cases SCE is proposing long term use of the mechanical seal 4
Introduction - Replacement Plan RCS Nozzle Status + All primary nozzles are being replaced by the next refueling Instrument Unit 2 Unit 3 .
'**" " nozzies aiioy eso) aiioy s,o outage with Alloy 690 Pressurizer
+ During mid-cycles, MNSAs
- Instrument Nozzles 7 5 4 will be used as an interim steam senerators 8 o o measure for inaccessible hot leg Primary Loops locations found leaking
- Hot leg nozzles 32 1 to + 5 remaining Alloy 600 d i
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. E 5 Pressurizer nozzles will be
- Cold Icg nozzles 12 o 1 MNSAs Totals: 59 6 15
+ 4 MNSAs are being staged as a contingency for SG nozzles should any be found leaking 5
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Introduction - Expected MNSA Use during Mid-cycle Outages
+ All instrument and sample nozzles will be visually inspected
+ RCS Hot Legs - expect a few inaccessible nozzles will be replaced with interim MNSAs
+ RCS Cold Legs - no MNSAs are needed
+ Pressurizer - 5 MNSAs will be preemptively installed as long term replacements
+ Steam Generators - no MNSAs are expected to be required, however,4 have been staged as contingencies. If used, we envision these will be long term replacements 7
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Introduction - Use of MNSAs
+ Significant ALARA savings for the 5 pressurizer nozzles (3-4 person rem per nozzle)
+ Eliminates the difficulties ofinstalling Alloy 690 ;
nozzles in the pressurizer
+ Avoids the unnecessary handling of fuel assemblies and reactor disassembly /re-assembly of a core offload to gain access to certain hot leg nozzles 9
Introduction - MXSA Design
+ Code design rules for replacement were utilized without exception
+ Design uses materials and concepts previously used in RCS applications
+ SCE believes the design may be implemented under 50.59 ;
without prior NRC approval l
+ However, in the absence of a formal code guidance, a relM request was provided to afford the NRC staff the opportunity to concur based on their preliminary assessment the MNSA may not meet the code
+ Code committee in agreement that this is Code construction 10
Introduction - MNSA Monitoring
+ Like the nozzles themselves, each MVSA will be monitored for indications of external leakage '
during refueling outages
+ No leakage is allowed
+ Augmented monitoring 11
Background - Typical Nozzle n
Location of PWSCC 304 SS Cladding >
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J-Weld I-600 Nozzle Base Material RCS Hot Leg 12
I Background - Nozzle Leakage
+ Caused by stress corrosion cracking of Alloy 600
+ Leads to very small leaks during operation l
+ Although the leakage is not significant to reactor safety, operation with RCS pressure boundary leakage is not !
allowed by license
+ Replaced with updated material (Alloy 690) or by installation of a mechanical nozzle seal as.sembly (MNSA) 13 l
Purpose
+ Explain the design of the MNSA and answer NRC questions Design Objectives ASME Code
- Qualification Degradation 14
MNSA Design Objectives
+ Provide a mechanical de' ' 2 for replacement of small bore I-600 nozzle '
pressure boundary
+ Use proven sealing technique similar to other RCS mechanicaljoints
+ Can be used as preemptive measure (before leakage detected) or as permanent replacement if weld or nozzle fails
+ Can be installed on leaking nozzle without draining RCS is
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Mechanical Joints in RCS
+ Examples of RCS mechanicaljoints Reactor vessel closure head Steam generator and pressurizer manway Incore instruments Reactor head instrument and control rod drive nozzle seal assemblies (CETNA/CSCA)
+ Similarity of design All are bolted closures All rely on a compressed gasket for sealing All are qualified by analysis and/or testing i6
Grafoil Seals
+ Grafoil seals are used in many RCS closures Incore Instruments
. Core Exit Thermocouple Nozzle Assembly (CETNA)
Canopy Seal Clamp Assembly (CSCA)
Mechanical Sealing Sleeves (MSS) for CRD stub tubes in a BWR Reactor 17 l
Incore Instrumen+ Mechanical Joints i
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II Tr F 3 F, 1 Grafoil Seal
- Grayloc Seal Ring Grayloc Clamp o ,; 9/
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CETNA/CSCA
+ Approximately 40 plants have these modifications installed on several nozzles
+ Considered a permanent fix for a 40 year qualified life
+ Some have been in operation for over 10 years without leaks
+ Thousands of hours of operating experience 21
ma-Mechanical Sealing Sleeves (MSS)
+ Installed inside a BWR on CRD stub tubes I
and are subjected to severe reactor operating conditions of high ternperature, high pressure, severe thermal transients and radiation 1
+ Grafoil seals used in groove on the MSS
+ 35 MSS installed at Santa Maria de Garona in Spain since 1982 Failure of Grafoil seal would be detected by leakage from the reactor vessel 22
--no-MNSA Design Requirements
+ Designed in accordance with ASME Section l III Class I
+ Full function replacement forj-groove weld Prevents external leakage in the event of weld or i nozzle crack l Prevents nozue ejection in the event of full circumferential crack of weld or nozzle
+ Qualified for full pressure and temperature conditions
+ Tested for seismic loadings u
RTD Xozzle MVSA L
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GR AFOIL SEAL 7 l l 25 l .___________ ____ _ -
e e NRC Question on MNSA Fabrication
+ Why is bottom pressurizer MNSA Lower Flange flat when Pzr surface is curved?
MNSA designed such that the only contact between the MNSA and the Pzr is at the Seal Retainer Need tight clamping force on Seal Retainer o MNSA designed to have gap between Lower Flange and Pzr Contact with Pzr surface at cater edge of Lower Flange would reduce clamping force at Seal Retainer and could allow seal extrusion Making Lower Flange curved to match Pzr surface would serve no utilitarian purpose o Curved Lower Flange would never contact curved Pzr surface Making Lower Flange curved would increase manufacturing cost without any benefit 26
weu-I ASME Code -
+ MS'SA designed and fabricated in 1 accordance with AS ME Code Section III, Class 1, XB-3300, XB-3600
+ MS'SA is installed in accordance with AS ME Code Section XI, IWB 7000 as a replacement activity 27
osamuu-ASME Code (cont'd)
+ ABB submitted inquiry to AS ME Code regarding MXSA design:
i
-- "Is a bolted connection designed in accordance with the requirements of NB-3200, while not in conformance with the dimensional I
requirements of ANSI B16.5 as recommended l in NB-3262, acceptable for connection of external piping to a vessel?"
28
swou-ASME Code (cont'd)
+ ASME response divided into two parts:
Is a bolted connection designed in accordance with the requirements of NB-3200 acceptable for connection of external piping to a vessel?
- Reply: Yes Are bolted connections designed to NB-3200 required to meet the dimensional requirements of ANSI B16.5 as recommended in NB-3262?
- Reply: No 29
Qualification of MNSA
+ ASME Section III, NB-3671.1 requires that Joint design make provision to prevent separation under all Service Loadings Be accessible for maintenance, removal, and replacement after service Either of the following:
Prototypejoint has been subjected to performance tests under simulated service conditions to prove joint is sufficiently leak tight Joints are designed in accordance with the rules of NB-3200 30
Qualification of MNSA (cont'd)
+ MNSA is designed in accordance with NB-3200 1 - Code analyses were performed on the MNSA, RCS piping, and RCS vessels
+ Prototype testing was performed to prove the !
sealing capability under seismic conditions
+ Additional thermal cycle and hydrostatic tests performed to validate integrity of seal design 31
Analysis
+ MNSA analyzed in accordance with the requirements of ASME Section III Class 1
- Stresses and fatigue usage factors calculated to be within Code allowable limits i
32
Analysis (cont'd)
+ Attachment locations in piping or vessel analyzed in accordance with requirements of ASME Section III Class 1, NB-3334
- After bolt holes are drilled, the reinforcement area around each instrument nozzle location exceeds the required reinforcement area The fatigue usage factor at each instrument nozzle location, considering the stress concentration due to the attachment stud holes, is below the Code allowable limit 33
Analysis (cont'd)
+ Pressurizer shell reinforcement area remains satisfactory after removal of material for bolt holes W
Required Area of "ll-~ '
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MNSA Bolt Available Area for einf mem nt 11 ole Lower Level Nozzle Penetration
Analysis (cont'd)
+ RCS piping reinforcement area remains satisfactory after removal of material for bolt holes ,
l RTD NozzicPenetration MNSA Bolt Hole j v /
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Minimum Wall 35
- O Hydrostatic Pressure Test
+ Nozzle not attached to test plate to simulate 360 degree circumferential crack
+ Test performed at 3175 50 psig, ambient temperature
+ Acceptance criterion was no leakage from !
seal
+ Test results were satisfactory .
I 36
NRC Question on Hydrotest
+ Why wasn't hydrotest done at plant operating conditions?
ASME Code does not require that hydrotest be done at operating temperature Hydrotest performed at 1.25 times operating pressure as required by NB-6221 37
ll XRC Question on Hydrotest
+ Why did pressure decay 100 psig?
- Reported pressure decay over 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> of 100 psig was from test apparatus, which is not atypical No leakage past seal was observed i - Test duration of 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> exceeds NB-6223 ,
requirement of 10 minutes 38
Thermal Cycle Test
+ Nozzle not attached to test plate to simulate 360 degree circumferential crack
+ Test r'g heated from ambient to 650 F i 10 F at 150 F/hr and 2500 psig
+ Temperature held for minimum of 60 minutes, then test rig cooled to less than 200 F
+ Three thermal cycles performed to verify
! design and integrity of the seal
+ Acceptance criterion was no leakage from seal
+ Test result.s were satisfactory 39
Seismic Test
+ Nozzle welded to test plate for 180 of the circumference to simulate crack
+ Test specimens equipped with weights that simulated RTD heads and valves existing in field installations 40
NRC Question on Seismic Test
+ Why was 180 deg crack used in seismic test rather than 360 deg crack?
For the seismic test, a 180 deg crack simulating a cracked nozzle but not fully ejected was judged to pose a more severe vibration potential failure condition for the MNSA than a 360 deg crack Hydrotest and autoclave test were conducted with a 360 deg crack to demonstrate seal integrity and leak tightness under complete weld failure which is considered the worst scenario for continuous operation 41 t
NRC Question on Seismic Test
+ Seismic test fixture is flat rather than curved. Why is that valid?
Design of MNSA provides a gap between shell and bottom plate in actual installation Test arrangement provided a gap between MNSA and test fixture
- Therefore adequately simulates installed configuration of MNSA 42 o
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Seismic Test (cont'd)
+ Specimens tested at hydrostatic test pressure to envelope operating conditions
+ Seismic tests based on SONGS response frequencies
+ Shaker table tests simulating 5 OBEs and 1 DBE were conducted 43
NRC Question on Seismic Test
+ Is SRSKHH program qualified?
Yes, this program has been V and V'd in
- accordance with Class 1 requirements of ABB CENO's QA program as referenced in the test report
- This program is an application code which translates time history output from the shaker table to seismic response spectrum 44
Seismic Test (cont'd)
+ Acceptance criteria
- No leakage from seal No loss of structural integrity
+ Resonant frequency of shaker table and MNSA test assembly nearly coincident Actual input excitation at lowest nozzle resonance exceeded required input by a factor .
of 5 45
Seismic Test (cont'd)
+ Test results were satisfactory No seal leakage No loss of pressure
. No structural damage to specimens 46
Degradation
+ Four degradation mechanisms considered
- Boric acid corrosion oflow alloy steel
- - Galvanic corrosion of low alloy steel
- Sealleakage PWSCC of MNSA fasteners i
47
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! Boric Acid Corrosion of Low
- Alloy Steel ,
+ Testing has shown generally low corrosion rates (<5 mils /yr) for low alloy carbon steel submerged in high temperature borated l' water 3
48
Boric Acid Corrosion of Low Alloy Steel (cont'd; l
+ Expected corrosion environment at MNSA Ifj-weld or nozzle cracks the annulus will fill .
l with borated water
- Low alloy steel in the presence of high ,
temperature borated water will experience minor general corrosion
- Grafoil seal prevents continuous flow; water in annulus will not be replenished
- pH of water will rise as boric acid is depleted; general corrosion will be halted 49
Galvanic Corrosion of Low Alloy Steel
+ Galvanic corrosion might occur due to the higher electric potential between low alloy steel (cathode) and graphite in Grafoil seal (anode) in the presence of borated water (electrolyte) .
50
Galvanic Corrosion of Low Alloy Steel (cont'd? .
+ Expected galvanic corrosion environment at MNSA i Ifj-weld or nozzle cracks the annulus will fill with borated water
- Grafoil seal prevents continuous flow; water in annulus will not be replenished Buildup of corrosion products and boric acid crystals in annulus will reduce wetting of Grafoil seal minimizing galvanic corrosion 51 I _
nM -
Seal Leakage
+ Sealing action provided by compression of Grafoil
+ Seal compression collar makes metal-to- 1 metal contact with lower flange No relaxation of seal during heatup and cooldown
+ Grafoil exhibits very low creep relaxation, even at high temperature .
Reference:
Union Carbide paper " Flexible Graphite Nonasbestos Gasketing Material", P.S. Petrunich,1986 52 r _ ___ _ _____
NRC Question on Grafoil
+ Justify statement that Grafoil is not affected by boric acid.
- "Grafoil is resistant to attack by all common organic and inorganic fluids except highly oxidizing mineral acids ..." such as sulfuric acid, nitric acid and agua regia.
Reference:
Union Carbide paper " Flexible Graphite Nonasbestos Gasketing Material", P.S. Petrunich,1986
- Operating experience 53
Seal Leakage (cont'd)
+ L.oss of seal preload unlikely to occur Locking tabs on washers prevent loosening of bolts
+ Even in the unlikely event of seal leakage, corrosion of tapped holes unlikely Bolt holes outboard of seal region Water would flash to steam before reaching bolt holes Dry boric acid crystals do not cause low alloy steel corrosion ,,
PWSCC of MXSA Fasteners
+ MNSA fasteners are SA-453 Gr. 660, austenitic nickel chromium stainless steel Material can be susceptible to SCC when stressed to near yield strength and exposed to primary water !
Material yield strength = 85 ksi 55
PWSCC of MNSA Fasteners (cont'd?
+ PWSCC of MNSA fasteners not expected because
- Stress in fasteners is low Bolt stress from preload i.s low (22.5 ksi)
Tie rod stress for normal operating condition is zero
. Tie rod stress for upset condition (nozzle failure) is low (32 ksi impact; 6.3 ksi after impact)
- Liquid environment unlikely Grafoil seal leakage unlikely Fasteners outside seal region and any leakage would flash to steam before reaching them
- Concentration ofimpurities (chlorides) does not reduce stress required to induce PWSCC 56
Monitoring Boric Acid Corrosion of Nozzle Bore
+ During Unit 2 mid cycle outage, SCE will remove one Alloy 690 nozzle installed in 1993
+ The bore in the RCS pipe will be inspected to verify the expected boric acid corrosion rate (<4 mils / year) l
+ A new Alloy 690 nozzle will be installed j
57
Monitoring Performance of Grafoil Seal
+ Refueling interval visual inspections are performed per our Section XI and boric acid programs All noz7les will be visually inspected during subsequent outages for indications of RCS leakage as evidenced by boric acid accumulation All nozzles including installed MNSAs will be included in the ISI program and receive VT exams as required by the Code
+ On-line leakage monitoring 58
4 On-line Monitoring
+ Monitoring during operation
- SCE monitors for RCS leakage through a combination of water inventory balances, sump level and containment radiation monitoring
- Technical Specifications limit unidentified leakage to 1 l gpm
+ MNSAs are part of the RCS pressure boundary; consequently leakage frora seal is not allowed 59
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