ML20093G284
| ML20093G284 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 10/11/1995 |
| From: | Shell R TENNESSEE VALLEY AUTHORITY |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9510190008 | |
| Download: ML20093G284 (7) | |
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HA Tennessee Valley Authonty Post Othee Box 2000. SoddpDag,y. Tennessee 37379 October 11,1995 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, D.C. 20555 Gentlemen:
In the Matter of
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Docket No. 50-327 Tennessee Valley Authority
)
SEQUOYAH NUCLEAR PLANT (SON)- REQUEST FOR APPROVAL OF ALTERNATIVE TO AMERICAN SOCIETY OF MECHANICAL ENGINEERS (ASME) CODE REQUIREMENTS - CANOPY SEAL WELDS - UNIT 1 Pursuant to 10 CFR.50.55a(a)(3),TVA is requesting the use of alternatives to the ASME code for repair of three lower canopy seal welds. While in the Unit 1 Cycle 7 refueling outage, boric acid residue was noticed on a control rod drive mechanism (CRDM) during reactor vessel disassembly. A closer inspection indicated that two CRDMs (E-13 and L-13) and one thermocouple penetration (A-5) have signs of minor leakage (boric acid residue) at the lower canopy seal weld.
Repair options have been evaluated and it was determined that the most appropriate repair was the use of weld buildup rather than removing the defect and performing a weld repair. Weld buildup is considered by TVA to be an acceptable repair technique because the canopy seal weld does not provide structuralintegrity or act as a pressure retaining boundary for the threaded joint. Even though the subject welds do not provide structural strength or serve as a pressure boundary for the threaded joint, the weld buildup over the canopy sealis considered a repair under the rules of ASME Section Xi, IWA-4000, because welding is performed on pressure retaining i
components.
SON's CRDMs and thermocouple penetrations are Class A vessels as defined by ASME Section lil,1968 Edition. N-518.4 of the 1968 Edition of the ASME Section lli code and NB-5271 of later editions of the Section lil code require seal welds to receive either a magnetic particle or liquid penetrant examination. TVA has evaluated performance of these examinations and has determined that either examination would be impractical. The affected canopy seal welds are located in a high radiation area (2 rem / hour on contact and 1000- 1500 millirem / hour at 1 foot) and access to the welds is difficult due to the limited clearance between the adjacent CRDMs.'
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1-Acco' dingly, TVA has used an alternative examination technique. ' The alternative r
examination involved using a remote video camera with a magnification of -
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approximately 8X to 10X to perform a vis'ual examination of the final weld at the
. enhanced magriification. In addition, fracture mechanic analyses demonstrates that.
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. critical flaw size would be sufficiently large for detection using the enhanced visual -
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' examination..
'Please direct questions concerning this issue to D. V. Goodin at (423) 843-7734.
Sincerely, lY*
i R. H.~ Shell Manager.
SON Site Licensing
- Enclosures cc (Enclosures):
Mr. D. E. LaBarge, Froject Manager Nuclear Regulatory Commission
- One White Flint, North 11555 Rockville Pike Rockville, Maryland 20852-2739 NRC Resident inspector Sequoyah Nuclear Plant 2600 Igou Ferry Road Soddy-Daisy, Tennessee 37379 3624
. Regional Administrator U.S. Nuclear Regulatory Commission Region ll ~-
101 Marietta Street,- NW, Suite 2900 Atlanta, Georgia 30323-2711-
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ENCLOSURE
. Unit:
1 1
System:
Reactor Coolant - System 68 j
Class:
1 Function:
Vertically position a control rod in the nuclear core by raising or lowering an 4.
interconnecting drive shaft.
I Code Requirement: American Society of Mechanical Engineers (ASME)Section XI,1980 Edition through the 1981 Winter Addenda, Paragraph IWA-4120, " Repairs shall be performed in accordance with the Owner's Design Specification and Construction Code of the component or system. Later editions of the Construction Code or of Section Ill, either in the entirety or portions thereof, i
may be used. If repair welding cannot be performed in accordance with these requirements, the following may be used: (a) IWB-4000 for Class 1 i
components."
l-
Background:
During the Unit 1 Cycle 7 refueling outage activity of disassembling the reactor vessel, boric acid residue was noticed on a control rod drive mechanism (CRDM). Further inspection shows that two CRDMs and one j
thermocouple penetration have started leaking at tha lower canopy seal 4
weld (sos Attachments 1 and 2).
The CRDMs and a thermocouple penetration are part of the nuclear steam supply system procured from Westinghouse Electric Corporation under Contract 91934. The 1968 Edition of ASME Section lli and its addenda-i establish the design specification and construction code for the CRDMs.
l The CRDMs and a thermocouple penetration are part of the Class A vessel as defined by ASME Section ill,1968 Edition.
The CRDMs and a thermocouple penetration are fabricated in sections with threaded joints providing the pressure-retaining capabilities. Since the threaded joint provides pressure retention, the canopy seal weld is not pressure retaining and is for leakaga control. The 1968 Edition of ASME Section 111 does not specifically address (either allowing or disallowing) these types of joints. Later editions of ASME Section 111 address tnreaded joints and do not allow them as the only seal as described in NS-367.1.3.
NB-3227.7 addresses the design of canopy seal welds. NB-5271 requires that seal welds receive either a magnetic particle or liquid penetrant examination.
2-Impractical ~
Requirement: Due to physical space limitations and in consideration of as low as is reasonably achievable (ALARA), the flaws found by visual inspection cannot be removed. In addition, if the flaws were removed, it would be impossible i
to reproduce the configuration of the material as required by the design in order to restore the canopy seal back to its original design condition as required by IWA 4000 and IWB-4000.
Alternative Requirement: TVA's Code of Record for Repairs and Replacements is ASME Section XI, 1980 Edition through the Winter 1981 Addenda. TVA proposes to use Code Case N-389, which allows the use of later code editions and addenda of Section XI. Code Case N-389 is approved for use in the latest revision of Regulatory Guide 1.147. IWB-3640 and Appendix C of the 1989 Edition of ASME Section XI is used to perform the required fracture mechanics and to design a weld overlay repair of the flawed canopy seal weld. Code Case N-504-1 is also used for guidance. Code Case N-504-1 allows repair by addition of weld material without removal of the underlying defect to be considered as a code repair, lWB-3640 provides criteria for acceptance of flaws without repair in ductile, austenitic materials. The basis for such acceptance is the evaluation of the structural adequacy of the flawed component after considering the predicted flaw growth over the evaluation period. The acceptance criteria is based upon the net section collapse (limit load) criteria which are defined in detailin Appendix C of Section XI. Also, NUREG-0313, Revision 2 is used for guidance. The repair design is based upon conservative treatment of applied stresses, and includes allowance for continued flaw growth, as required by Section XI.
The material used for the repair is Alloy 625 weld material, which is stronger than the underlying base material, more resistant to degradation mechanisms such as stress corrosion cracking, and is highly ductile. The load carrying capability of the repaired location is greater than the original component, t
l Dye penetrant examinations that are required by NB-5271 will not be j
performed due to the space limitations, which prevent examiners the needed access to successfully perform the examination. As an alternative examination, TVA will use a remote video camera with a magnification of approximately 8X to 10X and perform a visual examination of the final weld at the enhanced magnification. The entire process of the repair will be recorded on video tape. The basis for this approach is that postweld dye l.
penetrant examinations are surface examination.s, and provides minimal assurance of repair integrity when compared to an enhanced visual examination. Additionally, fracture mechanics analyses demonstrate that
I i,
3 the critical flaw size (i.e., the flaw size, which would lead to the incipient 4
collapse of the repair under code allowable applied stress conditions)is significantly larger than a flaw that would be reliably detected by the enhanced visual examination.
The fracture mechanics analyses assume that an initial flaw is completely through the repair membrane. Thus, there is a large margin in the analyses.
TVA considers the fracture mechanics analyses, coupled with the enhanced visual examination, suitable to provide an acceptable alternative to the code required dye penetrant examination.
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ENCLOSURE 1 6
. w h e RECEPTACLE VENT PLUG
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geELECTRICAL LEAD TUBE V
ROO TRAVEL HOUSING h
ORIVE ROO ASSEMBLY l
PRESSURE BOUNDARY ASSEMBLY LATCH HOUSING s
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ELECTROMAGNET h{-
h LIFT j
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ASSEMBLY
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COIL STACK
)!ft I
i ASSEMBLY f
MOVABLE GRIPPER l 1 'I ELECTROMAGNET l
ASSEMBLY
( J, q[ g MOVABLE GRIPPER
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STATIONARY GRIPPER 4
t d
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ELECTROMAGNET J
ASSEMBLY
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f-STATIONARY GRIPPER 1,
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L gwSt. C4.9 SP Y f!4L (^/A O j
\\ 'L CONNECTION TO
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SPIDER OF ROD l I CONTROL CLUSTER (RCC)
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Figure 3.21. Cutaway of a Typical Control Rod Drive Mechanism
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ENCLOSURE 2 j
ATTACHMENTS i
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1-TUNGSTEN ELECTRODE N
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J LWELD TORCH. GAS CUP ILLER METAL INSERT '
"Y" OR "J" SHAPED) f HEAD ADAPTER i
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L Figure 2-1.
Schematic 111ustration of Canopy Seal Wald Configuration s
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