ML20086R335
| ML20086R335 | |
| Person / Time | |
|---|---|
| Site: | Limerick  |
| Issue date: | 12/19/1991 |
| From: | Beck G PECO ENERGY CO., (FORMERLY PHILADELPHIA ELECTRIC |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9201020005 | |
| Download: ML20086R335 (8) | |
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PIllLADELPillA ELECTRIC COMi%NY NUCLEAR GROUP HEADQUARTERS
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955-65 CHESTERBROOK BLVD, WAYNE, PA 19087-5691 (21.' % 6000 NUCLEAR ENGINEERINO & SERVICE $ DEPARTMENT December 19, 1991 Docket No. 50-352 License No. NPF-39 U.S. Nuclear Regulatory Commission Attn: Document Control Deck Washington, DC 20555
Subject:
Limerick Generating Station, Unit 1 Request for Relief From ASME Code Section XI Requirements Concerning a Temporary Repair of a Class 3 Pipe in a Moderate Energy System Gentlemen:
This letter is being submitted in accordance with 10 CFR 50.55a(g)(5)(111) to request relief from the American Society of Mechanical Engineers (ASME), Boiler and Pressure Vessel (B&PV)
Code,Section XI, Subsection IWA and IWD requirements; regarding the repair or replacement of an ASME Section III Code Class 3 pipe installed at Limerick Generating Station (LGS) Unit 1.
During a Unit 1 High Pressure Coolant Injection (HPCI) system walkdown, a through wall (i.e., pin hole) flaw was identified in a section of 3" diameter Emergency Service Water (ESW) system piping that services the HPCI room unit cooler.
The ESW system at LGS is a moderate energy safety-related system consisting of two independent loops ccmmon to Units 1 and 2.
The system is designed to supply cooling water to selected Emergency Core Cooling Systems (ECCS) equipment on both units in the event of a Loss of Offsite Power (LOOP) or Loss of Coolant Accident (LOCA).
The portion of ESW piping containing the flaw is carbon steel ASME Section III Code Class 3 pipe and cannot be isolated.
The flaw is suspected of being the result of localized corrosion caused by Microbiologically Induced Corrosion (MIC).
In order to perform a permanent Code repair or replacement, both LGS units would be required by Technical Specifications to shut down within six (6) hours since Code repair activities would render a number of ECCS components inoperable.
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-Nuclear Regulatory Commission December 19, 1991 Document Control Desk Page 2 NRC Generic Letter (GL) 90-05, " Guidance for Performing Temporary Non-Code Repair of ASME Code Class 1, 2,
and 3 Piping," provides guidance for evaluating the structural integrity and the use of temporary non-code repairs on ASME Section III Code Class 3 piping.
Specifically, this GL states that temporary non-code repairs on ' lawed moderate energy Class 3 piping may be acceptable if the piping cannot be isolated without a plant shutdown and the structural integrity of the pipe has not been irreversibly effected.
Additionally, a temporary non-code repair may be considered acceptable until the next oce.eduled outage exceeding 30 days, but no later than the next scheduled refueling outage.
However, the NRC must grant relief for the temporary non-code repair.
An engineering evaluation was performed and concluded that the affected ESW piping system stresses remain within Code allowables.
Since the flaw is suspected of being caused by MIC, and leakage is minimal, the flawed section of pipe will be left "as-is" until a Code repair or replacement is implemented.
The pipe repair or replacement wil.'. be performed during the scheduled March 1992 Unit I refueling outage, since there are no planned outages greater than 30 days between no, and the next Unit 1 scheduled refueling outage.
In the interim, this section of pipe will be monitored in accordance with the guidance specified in GL 90-05 and the necessary actions taken in the event further pipe degradation occurs.
Therefore, we are requesting that the NRC approve the attached relief request which contains specific information and details concerning the ASME Code requirements from which relief is requested, the basis for this relief, and alternate provisions.
If you have any questions or require additional information, please do not hesitate to contact us.
Very truly yours, fb0f f'w (f2 S
G. }[ eck Manager Licensing Section Attachment cc:
T.
T.
Martin, Administrator, Region I, USNRC (w/ attachment)
T.
J.
Kenny, USNRC Senior Resident Inspector, LGS (w/ attachment)
MS - 1 4
ISI Program 4
REQUEST FOR RELIE7 FROM ASME SECTION XI REQUIREMENTS RELIEF REQUEST IN ACCORDANCE WITH THE GUIDANCE SPECIFIED IN GENERIC LETTER 90-05 FOR LIMERICK GENERATING STATION, UNIT 1 I.
IDENTLELQhnQN OF COMPOJEHf8 This Relief Request involves an unisolable portion of the "B" loop of the Emergency Service Water (ESW) system at Limerick Generating Station (LGS), Unit 1.
The component consists of a 3 inch nominal diameter carbon steel, ASME Section III, Class 3 pipe leading from the outlet of the High Pressure Coolant Injection (HPCI) room unit coolers, to the common Unit 1/ Unit 2 ESW return header.
This piping is categorized in Generic Letter 90-05, " Guidance for Performing Temporary Non-Code Repair of ASME Code Class 1,
2, and 3 Piping," as moderate energy piping (i.e., less than 200 degrees F and less than 275 psig maximum operating conditions),
and uses untreated water as the process fluid.
II.
CODE RR{LUJR_ERENTS FROM WHICH RELIEF 18 REOUESTED ASME Section XI 1986 Edition, Articles IWA/IWD-4000 and IWA/IWD-7000 provide requirements for the repair or replacement of ASME Class 3 pressure retaining components with defects exceeding the acceptance standards of ASME Section XI, Article IWA-3000.
Relief is requented to defer the ASME Section XI repair or replacement required by IWA/IWD-4000 or IWA/IWD-7000 until the next scheduled outage exceeding 30 days, but no later than the end of the next refueling outage which is scheduled to begin in March of 1992.
This request for deferment follows the guidance specified in NRC Generic Letter 90-05, since isolation of this portion of the piping is not practicable at this time.
III. BASIB FOR RELIEF A through wall (weeping) leak in the Unit 1 ESW system, 3 inch diameter return header piping from the HPCI room unit coolers, was found during a walkdown of the system of December 3, 1991.
Since there are no isolation valves between the leak and the Unit 1/ Unit 2 connon ESW return header, an ASME Code Section XI repair would require a two-unit shutdown and is, therefore, impractical at this time.
LGS - 1 l'
ISI Program Isolation of the defect to implement a Code repair or replacement requires a two-unit shutdown in accordance with LGS Technical Specifications (TS) since the loss of the "B" loop of ESW makes the Unit 1 and Unit 2 HPCI room unit coolers inoperable, and therefore, the associated HPCI systems inoperable.
The HPCI TS Limiting Condition for Operation requires that all other Emergency Core Cooling System (ECCS) components be operable with HPCI inoperable.
However, since the "B"
loop of ESW supplies cooling water to half of the diocel generators and half of the ECCS room unit coolers for both units, loss of this loop causes a subsequent loss of the associated diesel generators and ECCS systems. This condition requires that both units be in at least "Startup" within six (6) hours, at least " Hot Shutdown" within the following six (6) hours, and at le nt " Cold Shutdown" within the subsequent 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
Attachment I to this Relief Request shows the location of the leak.in relation to the rest of the ESW system and Attachment 2 shows the specific section of piping involved.
The condition of the defect has been characterized and sized using ultrasonic testing (UT) and was determined to be a small pit originating at the inside surface of the pipe.
Preliminary examination suggests that the defect has been caused by localized Microbiological 1y Induced Corrosion (MIC).
This assertion is substantiated by comparison with a similar leak that occurred on an isolable portion of the same pipe.
When the similar leak was replaced and examined, it was noted that the failure was caused by MIC.
The structural integrity of the flawed pipe has been evaluated using the "through-wall flaw" technique which is detailed in NRC Generic Letter 90-05 and verified by a second method.
Both techniques provided acceptable results.
These evaluations used actual UT results, and assumed a maximum hole diameter of 1/8 inch (much greater than the actual pin hole size).
Additionally, an evaluation of the effects of wall thinning on the existing calculated pipe stresses has been performed, and the results indicate that the measured thicknesses provide adequate structural integrity to maintain the piping stresses below the AS)-
code allowables for all required loading conditions.
Interaction between the flawed pipe and other systems has been considered for ef fects of flooding, spraying, loss of flow, etc..
Based on the nature and location of the flaw, it is expected that there will be no adverse effects to any other component or system.
We request relief to allow the flawed pipe to be left "as-is" since the leak rate is less than one drop a minute and clamping or covering may facilitate MIC.
This form of corrosion may be perpetuated because the anaerobic bacteria (i.e.,
living in the absence of free oxygen) that causes MIC !
1
LGS - 1 ISI Program would multiply if the leak were to be covered.
The defect will be monitored as described in Generic Letter 90-05 which specifies a qualitative visual examination on a weekly basis and either UT or Radiographic Testing (RT) on a 3-month interval until the pipe is repaired or replaced.
IV.
BhTREFAUVE PROVISIONS An augmented examination program, as described in Generic Letter 90-05, has been initiated to assess the nndition of the ESW system.
Five (5) additional pipe locations, each determined to have a similar susceptibility and degradation mechanism as the flawed pipe, have been chosen and examined using UT methods. The results of these examinations show that the additional locations are acceptable regarding minimum wall thickness requirements; and therefore, maintain adequate structural integrity when compared to the input and results of previous calculations for this particular piping system.
Therefore, no additional inspection sites are required at this time.
During the monitoring program, if the conditions of the pipe further degrade and the structural integrity cannot be assured, an emergency repair would be required.
Since the flawed pipe section does not have a valve between it and the common ESW system return header, freeze seals or other isolation mechanism would be required to implement the repair.
This method would require prior NRC approval and submittal of an emergency TS change, or temporary waiver of compliance, to avoid a two-unit shutdown. - _
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