ML20056E821

Documents Actions Taken by Util to Evaluate,W/O Immediate Code Repair,Small Leak in Svc Water Piping & Requests Relief from Requirement to Perform Immediate Code Repair,Per GL 90-05

ML20056E821
Person / Time
Site:	Vermont Yankee File:NorthStar Vermont Yankee icon.png
Issue date:	08/18/1993
From:	Tremblay L VERMONT YANKEE NUCLEAR POWER CORP.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
BVY-93-85, GL-90-05, GL-90-5, NUDOCS 9308250210
Download: ML20056E821 (5)

Text

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VERMONT YANKEE N,. CLEA.R POWER CORPORATION I

I g Ferry Road, Brattleboro, VT 05301-7002 N w---

• ENGINEERING OFFICE f

• 580 MAIN STRE ET GOLTON MA 01740 (508) 779 4 711 United States Nuclear Regulatory Commission August 18,1993 A'ITN: Document Control Desk BVY 93 - 85 Washington, DC 20555

References:

a.

b. License No. DPR-28 (Docket No. 50-271)

USNRC Generic Letter 90-05, " Guidance for Performing Temporary Non-Code Repair of ASME Code Class 1,2 and 3 Piping", dated 6/15/90.

Subject:

Temporary Non-Code Repair of Service Water Piping Dear Sir-submitted. to document actions taken by Vermont Yankee repair, a small leak in the service water piping at the Vermont Yankee Nuclear Po to request relief from the requirement to perform an immediate codes were repair These action discussed with members of the USNRC staff on August 12,1993. .

On August 11,1993, during a routine Operations Department tour of the plant leakage Reactor Building Closed Cooling Water System (RB contains a 4-inch globe valve (V70-92D), which is used as a bypass for 12-in conditions oflow heat load. Refer to Figure 1 attached a diagram and arrangement of these components.

Upon inspection, the leak originated from several pin-hole perforations of the of the flange accumulating immediately at the rate of 90 to 100 drops downstream per minute. of V70-92D. The leak consisted of a l water Dow through the "A" heat exchanger was secured, stopp ,

Thinning was observed in an area on the welded neck fl .

92D. The thinned zone is limited to the weld, and the welded It does not neck flange fitting extend to the downstream pipe. The thinned zone is limited to a 180 degre circumference of the fitting, with the ma positions and a maximum at 90 degrees.gnitude of thinning minimal at the 0 degree and 180 degree The thinned zone corresponds to the area above the seat of the upstream globe valve. The piping downstream of the flange weld neck sh wall loss. Based on the UT results we have preliminarily concluded that the proba mechanism depicts is erosion a cross-sectional by flow view through the impingement flange weld neck. resulting from the turbulent throttl

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VERMONT YANKEE NUCLEAR POWER CORPORATION l United States Nuclear Regulatory Commission l A0 gust 18,1993 j Page 2 i

. Engineering analyses were performed using design basis values for deadweight, pressure, thermal and seismic loading. Structural margin was determined to be adequate to ensure the  ;

integrity of the piping system. The code required minimum wall thickness is on the order of 0.020  !

inches. Other than the pin-hole leaks the wall thickness ranges from 0.050 inches to 0.300 inches.

The analyses have been documented via a formal calculation (VYC 1207) in accordance with our approved QA program.

The flange weld neck in the redundant train was inspected with no evidence of a similar

! condition; some minor loss of pipe wall, down to 0.180 inches, was detected in a few localized areas.

! As noted previously, closing "A" RBCCW heat exchanger outlet valves V70-92A and V70-l 92D terminated the leak, even though the leak still remains within the pressure boundary of the l common SW outfall line with the "B" RBCCW heat exchanger. This is due to the configuration of l the piping system and the relatively high elevation of the affected area (elevation 303 ft. of the reactor building), resulting in an intemal pressure at the location of the leak which is approximately equal to atmospheric pressure. Derforming a code repair would necessitate a plant shutdown, since l flow to a number of other components would need to be isolated and piping in the vicinity of the

leak would need to be isolated by use of temporary seals (i.e. freeze plugs). Such action is not l considered prudent with a scheduled refueling outage less than 2 weeks away and given the fact l that the leak has been terminated.

A plant walkdown was conducted to assess the impact of potential flooding or water spray I

should the leaking pipe rupture. All floor penetrations, except for the floor drains, stairway and elevator shaft, are bermed to prevent uncontrolled cascading to lower levels. This ensures that flooding is not a concem (as previously evaluated and documented in the Vermont Yankee flooding study). Additionally, we have concluded that there is no critical equipment within the vicinity of the leak that could be negatively impacted by water spray.

In summary, the leak is located in a moderate energy Safety Class 3 piping system (design temperature of 150*F and a design pressure of 125 psig, with normal operating pressure at this elevation in the reactor building bemg less than 30 psig). Im alementation of a code repair would require a plant shutdown. Shutting valves V70-92A & D has stopped the leak. Engineering evaluations have demonstrated that the structuralintegrity of the piping system remains unaffected; even if the pipe were to fail there would be no adverse affect on plant safety equipment. The operability of the service water system would not be impacted.

Vermont Yankee concludes that relief from immediate code repair will present no undue risk to the health and safety of the public. We request that the USNRC grant relief to allow deferral of a code repair until the 1993 refueling outage, which is scheduled to begin on August 28,1993. A code repair will be implemented prior to startup from the refueling outage. In addition, we will perform a root cause evaluation to determine the cause of erosion in the affected area and implement any corrective steps that are deemed necessary to prevent or mitigate recurrence, i

i VERMONT YANKEE NUCLEAR POWER CORPORATION United Sbtes Nuclear Regulatory Commission August 18,1993 Page 3 Should you have any further questions or require additional information with regard to this issue, please contact this office.

Very truly yours, i T.RMONT YANKEE NUCLEAR POWER CORPORATION Leef014 Leonard A.Tremblay,Jr. \

.u.

Senior Licensing Engineer

! Attachments- Figures 1 and 2 cc: USNRC Region I Administrator USNRC ResidentInspector- VYNPS USNRC Project Manager- VYNPS 1

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