ML20237E986
| ML20237E986 | |
| Person / Time | |
|---|---|
| Site: | Pilgrim |
| Issue date: | 08/26/1998 |
| From: | Ted Sullivan BOSTON EDISON CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| BECO-2.98.109, GL-90-05, GL-90-5, NUDOCS 9809010379 | |
| Download: ML20237E986 (6) | |
Text
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10 CFR 50.55a Generic Letter 90-05 Boston Edison Pilgrim Nuclear Power Station Rocky Hill Road Plymouth, Massachusetts 02360 August 26,1998 l
BECo Ltr. 2.98.109 T.A. Sullivan General Manager Production and Station Director U. S. Nuclear Regulatory Commission Attn.: Document Control Desk Washington DC 20555 Docket No. 50-293 License No. DPR-35 Reauest for NRC Review of a Proposed Non-ASME Code Pipe Repair This letter reports degradation of a spool piece associated with Pilgrim Nuclear Power Station's (PNPS) salt service water (SSW) system. This moderate energy system provides the ultimate heat sink for process building heat removal.
In addition, a discussion of a temporary repair to be performed in accordance with Generic Letter (GL) 90-05 and, in part, ASME Code Case N-562 is provided. Relief to perform this repair is requested from the NRC under the purview of 10CFR50.55a (g)(6)(i).
Description The piping in the SSW system to the reactor building closed cooling water (RBCCW) heat exchanger has through-wall leaks due to localized delamination of the rubber lining and subsequent erosion and corrosion of the carbon steel pipe. The leak location is in an elbow downstream of the RBCCW heat exchanger. There are two discrete through-wall pits; all are within the stress criteria allowable pit size.
Evaluation The structural integrity of the 18 inch elbow from the SSW system to the reactor building closed cooling water (RBCCW) heat exchanger was evaluated using the guidance of GL90-
- 05. The method used evaluates the stress intensity factor "K" in the pipe with the limiting f
circumferential length removed based on the pipe stresses from existing Pilgrim piping analysis for combined loads, including seismic deadweight and thermal expansion. The maximum allowable pitting length was calculated using the GL90-05 fracture toughness iTp' criteria of Kic = 35 ksi(in) 5 The structural evaluation was performed for a design pressure of
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100 psi which is much greater than the maximum operating pressure. Current ultrasonic testing (UT) thickness measurements indicate two areas with thin. walls, one area is 1 inch by Page 1 of 5 h
AD 0
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i Based on the known operating history, inspection, maintenance and test requirements for the SSW system as validated through interviews with the design engineers, system engineer, and OC/ISIinspectors, the root cause of the through-wallleaks is attributed to delamination of the rubber lining. Steel piping will experience accelerated erosion and corrosion where the rubber lining tias delaminated. Where the lining remains intact, the pipe remains at its full wall thickness. Hence, the wall thinning is local to the areas where lining has delaminated, while elsewhere there is no effect. Therefore, the through-wall leaks in this piping are due to localized delamination of the lining resulting in areas of erosion and corrosion.
PNPS performed an analysis using a hydraulic model of the SSW system to evaluate the actual pressure at the subject location in the SSW piping. This analysis showed the pressure at this location is slightly negative except at high tides. At high tides this location has a slight positive pressure, resulting in service water leakage. No safety-related components are within the proximity of the piping pitting location that would be directly affected by this leakage. The leakage would be accommodated by the design of the auxiliary bay.
There is usually a small vacuum in the pipe at this location related to the changing tides. Air in-leakage has a negligible offect on the flow rate through the heat exchangers.
_._,Q Therefore, the piping is structurally sound and capable of performing its design function.
Conclusion of Evaluation The above discussion and associated operability evaluation demonstrate the pipe's structural integrity is acceptable. The effect from SSW leakage into the auxiliary equipment bay and/or air in-leakage into the flow stream (i.e., when the pressure is negative at this location) are acceptable. Therefore, the system associated with the degraded spool piece is capable of performing its safety function; hence, it is operable.
The root cause determination indicates the pitting can be attributed to delamination of the rubber lining.
Monitorina Measures I
immediate compensatory measures are not required to assure system operability or safe operation because the piping is currently structurally sound and leakage does not adversely impact system operability.
Pipe monitoring using ultrasonic testing (UT) will be performed bi-weekly on the carbon steel material adjacent to the cover plate, and once every 3 months on the stainless cover plate material; These test frequencies will continue gntil test results show they can be changed.
In addition, GL90-05 requires that a minimum of 5 locations be subject to augmented inspections to evaluate other system locations for similar degradation. To address this, 9 SSW pipe locations were inspected in accordance with GL 90-05 guidance. All augmented inspection results at these locations found values greater than the manufacturer's tolerance.
Page 2 of 5
Rman fer Nsn-Ced, Tcmporrry Repiir The impact a code repair would have on plant operation has been assessed. The code repair methods require removing one loop of the SSW system from service and cross tying the RBCCW systems during power operation. The removal of one loop of the SSW system would place P'ilgrim in a 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> limiting condition for operation (LCO) under Technical Specification section 3.5.B.3. The code repair we considered viable (spool replacement) requires removing a loop from service for greater than the LCO's 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> (preliminary installation schedule estimates are 4 to 5 days), resulting in a plant shutdown. Hence, we are requesting relief in accordance with the guidance of GL 90-05 for a non-code temporary repair that can be executed with the loop in-service.
Description of Proposed Temporary Repair A temporary non-code repair is proposed to stop the leak and maintain structural integrity until the piping spool can be replaced during an outage of sufficient duration. The proposed temporary repair will be stainless steel cover plates that encapsulate the 18 inch elbow. The plates (elbow pieces) will be welded to the pipe at the leak location.
The guidance provided in GL 90-05 applies to this temporary modification. The 1/2 inch cover plates (elbow pieces) will be welded to the 18 inch SSW elbow and flange where erosion and corrosion have occurred. ASME Code Case N-562, although written as guidance for the weld overlay repair method, will be used as a technical guide to attach the cover plate. The cover plate method was selected as the preferred temporary repair instead of the overlay method for the following reasons:
The cover plate repair method will stop the leak with less risk of enlarging the pitting l
than the overlay method. All other guidance of N-562 will be followed as applicable.
For example, the cover plate will be UT examined periodically for erosion until the pipe is replaced in RFO#12.
The cover plates are acceptable for 100 psi, although the pressure at the leak's location ranges from a slight vacuum to a slight positive pressure; it is dependent on tide level because the line discharges to the sea.
The line's 100 psi design l
specification was selected at Pilgrim's construction to make it uniform with other parts I
of the system that are subjected to higher pressures; therefore,100 psi is a conservative value for this application.
The cover plate method has less potential impact on the existing rubber lining of the pipe because it exposes the pipe to less heat from the welding process. Existing
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l procedures for welding the cover plate to a water-backed pipe are qualified.
The cover plate method does not affect plant operations.
l Repair's Safety impact Pilgrim has performed a safety impact evaluation of this proposed repair which determined the following:
l The safety-related functions of the SSW system remain qualified for plant design bases loads after completion of this temporary repair.
Page 3 of 5
The proposed temporary repair does not increase the probability of occurrence or consequences of an accident or malfunction of equipment important to safety. The possibility of creating an accident or malfunction other than those evaluated in the
,UFSAR is not increased because the temporary modification does not introduce any interaction with other safety-related systems.
This temporary repair does not create the possibility of a new type of accident or malfunction because no new failure mechanisms are introduced.
Commitments This letter makes the following commitments:
Operators will visually monitor for changes once per shift during operator tours until the permanent ASME code repair / replacement is completed.
1 Bi-weekly monitoring (ultrasonic testing) of the carbon steel material adjacent to the I
cover plate, and ultrasonic testing of the stainless cover plate material every three l
months, will continue until test results show the test frequencies can be changed.
The permanent ASME code repair / replacement will be performed at the next l
l '
scheduled outage exceeding 30 days and no later than startup from our next scheduled refueling outage (RFO#12).
Should you require further information on this issue, please contact P.M.Kahler at (508) 830-7939.
AID T.. S livan
~ PMK/dcg/298109 l
i Attachments: Pictorial Depiction of the Repair j
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l Page 4 of 5 I
I E_______.________.__.-._._____.
cc: Mr. Alan B. Wang, Project Manager U.S. Nuclear Regulatory Commission Project Directorate 1-3 Region i Office of Nuclear Reactor Regulation 475 Allendale Road Mail Stop: OWFN 1482 King of Prussia, PA 19406 U. S'. Nuclear Regulatory Commission 1 White Flint North l
11555 Rockville Pike Rockville, MD 20852 Senior Resident inspector Mr. Peter LaPorte, Director Pilgrim Nuclear Power Station Massachusetts Energy Management Agency 400 Worcester Road P.O. Box 1496 Framingham, MA 01701-0313 Attn: Mr. James Muckerheide i
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