ML20195K343

Safety Evaluation Granting Licensee Request to Use Guidance of GL 90-05 to Repair Flaws in ASME Class 3 Salt Svc Water System Piping for Plant

ML20195K343
Person / Time
Site:	Pilgrim
Issue date:	06/15/1999
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20195K314	List: ML20195K307 ML20195K343
References
GL-90-05, GL-90-5, NUDOCS 9906220067
Download: ML20195K343 (6)

Text

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c =ta v p _. -94 UNITED STATES s# [ NUCLEAR REGULATORY COMMISSION

1. WASHINGTON, D.C. 20555 4 001

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' SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REQUEST FOR APPROVAL TO REPAIR FLAWS IN ASME CODE CLASS 3 SALT SERVICE WATER PIPING BOSTON EDISON COMPANY PILGRIM NUCLEAR POWER STATION DOCKET NO. 50-293 1.0 (NTRODUCTION

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Title 10 of the Code of Federal Reaulations (10 CFR) Part 50.55a(g) requires nuclear power facility piping and components to meet the applicable requirements of Section XI of the American Society of Mechanical Engineers (ASME) Boiler and Pmssure Vessel Code (the Code).Section XI of the Code specifies Code-acceptable repair methods for flaws that exceed Code acceptance limits in piping that is in-service. A Code repair is required to restore the structuralintegrity of flawed Code piping, independent of the operational mode of the plant when the flaw is detected. Those repairs not in compliance with Section XI of the Code are non-Code repairs. However, the implementation of required Code (weld) repairs to ASME ,

Code Class 1,2, or 3 systems is often impractical for licensees since the repairs normally  !

require an isolation of the system requiring the repair, and often a shutdown of the nuclear power plant.

Attematives to Code requirements may be used by licensees when authorized by the Commission if the proposed alternatives to the requirements are such that they are shown to provide an acceptable level of quality and safety in lieu of the Code requirements (10 CFR 50.55a(a)(3)(i)), or if compliance with the Code requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety (10 CFR 50.55a(a)(3)(ii)).

A licensee may also submit requests for relief from certain Code requirements when it has determined that conformance with certain Code requirements is impractical for its facility (10 CFR 50.55a(g)(5)(iii)). Pursuant to 10 CFR 50.55a(g)(6)(i), the Commission will evaluate determinations of impracticality and may grant relief and may impose attemCive requirements as it determines is authorized by law.

Generic Letter (GL) 90-05, entitled " Guidance for Performing Temporary Non-Code Repair of ASME Code Class 1,2, and 3 Piping," and dated June 15,1990, provides guidance for the staff in evaluating relief requests submitted by licensees for temporary non Code repairs of Code Class 3 piping. The staff uses the guidance in GL 60-05 as its criteria for making its safety evaluation of reibf requests for temporary non-Code repairs of Code Class 3 piping.

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9906220067 990615 DR ADOCK 050002 3 Enclosure

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2.0 BACKGROUND

in letters dated April 7,1998, and March 17,1999, Boston Edison Company (BECo/ licensee) reported to the NRC that through-wall leaks have been identified on piping in the salt service water (SSW) system to the turbine building closed cooling water (TBCCW) heat exchanger.

. The April 7,1998,' letter noted that there are two discrete through wall pits. A temporary non-Code repair was performed, however, by conference call on February 18,1999, BECo informed

the staff that a pin-hole leak had developed in the same area. The SSW system is a moderate energy system and provides the ultimate heat sink for containment heat removal. ~l he licensee requested, under the provisions of 10 CFR 50.55a(g)(6)(i), a relief from the ASME Code,

'Section XI requirements to perform Code repair or replace the degraded piping. The relief was sought until the next refueling outage.- The plant is currently in an outage and replacing the degraded piping. At that time, the licensee is planning to make a permanent ASME code repair. The licensee based its request for relief on ths results of a "through-wall flaw" evaluation that was performed in accordance with the guidelines and acceptance criteria contained in GL 90-05.

3.0 LICENSEE'S RELIEF REQUEST 3.1 ' Comoonent for Which Relief is Recuested

The piping on SSW spool JF29-9-5 in) mediately downstream of butterfly valve MO-3805 valve has developed through-wall leaks. The leaks are Macent to the pipe slip-on flange that mates

with the valve.L This location is downstream of the TDCCW heat exchanger. The original degradation was two discrete through-wall pits. Subsequently, a pin hole leak developed in the flange hub adjacent to the stainless steel patch installed to repair the two through-wall pits. The ,

. through-wall leak can be attributed to delamination of the rubber lining exacerbated by localized 1

- high flow velocities from throttling the upstream butterfly valve. The 12 inch spool is 300 series

' stainless steel having nominal thickness of 0.375 inches. The line is designed to take 100 psi s pressure. However, the line is open ended and there is usually a small vacuum m the pipe at I this location depending on the changing tides.  ;

3.2 Section XI Edition for the Pilorim Plant L The applicable Edition of Section XI of the ASME Code for Pilgrim is the 1980 Edition including the Winter 1980 Addenda.

3.3 ' ASME Section XI Code Reauirement

' The ASME Code Section XI requires inat repairs or replacements of ASME Code Class

components be performed in accordance with rules found in Articles IWA;4000 or IWA-7000, '

respectively. :The intent of these rules serves to provide an acceptable means of restoring the structural integrity of a degraded Code C! ass system back to the original design requirements.

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3.4 ~ Content of the Relief Reauest' Relief is sought from performing a Code repair or replacement of the SSW system piping per the requirements of Article IWA-4000 or IWA-7000, respectively. Relief is being sought until the next refueling outage which has started. The relief is being sought because performing a Code repair during plant operation was detemiaed to be impractical. The licensee is performing a permanent Code repair for the atfected piping.

3.51ggis for Relief Request for relief has been submitted and alternatives to the Code requirements have been proposed by the licensee. The licensee has evaluated the piping in accordance with the guidance provided in GL 90-05. Based upon the evaluation,-it was established that the piping is degraded but it is operable. The piping also satisfies the criteria for non-code repair as described in GL 90-05 and performing permanent repairs in accordance with the ASME Code during plant operation would constitute an undue burden upon the licensee since the repairs would have necessitated a Unit shutdown.

3.6 Licensee's Alternative Proaram 3.6.1 April 7.1998. Reoalr The licensee has proposed a temporary Code repair to maintain the structural integrity of the

. piping until the piping is replaced during an outage of sufficient duration. Th a temporary repair consisted of two 3/8 inch stainless steel cover plates that encapsulate the 10-inch pipe. The plates were welded to the 12-inch pipe and flange where the erosion'and cor rosion occurred.

~ The welding procedures and the welders were qualified using the guidance provided in ASME

+ . Code Case N-562. Although ASME Code Case N-562 is written as guidance for the weld overlay repair method, it 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 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 RFO12.

. . The cover plates are acceptable for up to 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

!avel because the line discharges to the sea. The line's 100 psi design specification was

, selected at Pilgrim's construction to make it uniform with other parts of the system that are subjected to higher pressures; therefore,100 psi is a conservative value for tha application.

.- The cover plate method has less potentialimpact on the existing rubber lining of the pipe because it exposes the pipe to less heat from the welding process. Existing procedures for wc! ding the cover plate to a water-backed pipe at a qualified.

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. The cover plate method will not affect plant operations l

1 3.6.2 March 17.1999. Reoair This repair required welding the pin hole in accordance with an approved ASME Section IX procedure for P1 to P8 material for the fillet welds. The structuraI weld build-up is to be 1/8 to 1/4 inch thick. The surface was examined by the visual test method prior to welding and satisfies the surface examination acceptance criteria of ND-4000. Because this weld has been done on water-backed piping, this visual examination will not be performed sooner than 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after weld completion, and an in-service leak check is required.

4.0 STAFF EVAIAAllON AND CONCLUSIONS 4.1 Operability Determination. Root Cause Analysis and Structural Intearity Evaluation The structural integrity of comparable 18-inch piping from the reactor building closed cooling l water (RBCCW) heat exchanger was evaluated using tae guidance of GL 90-05. The results of that evaluation which justified a non-code repair to tnat piping was provided to the NRC in a BECo letter dated July 7,1997. The staff approved the non-code repair. The evaluation bounds the conditions currently existing in the 12-inch pipe. The method used evaluates the stress intensity factor "K" in the pipe with the limiting 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 GL 90-05 fracture toughness criteria of K,c = 35 ksi(in)o.s. The structural evaluation was performed for a design pressure of 100 psi which is much greater than the maximum operating pressure. The operating conditions for the 12-inch piping of concern in this request are similar, and the R/t ratio for the 12-inch piping is 15 versus 24 for the 18-inch pipe.

Therefore, the 12-inch pipe is stronger, and the allowable pit size is greater. Stress levels for

- the 12-inch pipe are less than that for the 18-inch pipe. Current ultrasonic testing (UT) thickness measurements indicate, with the exception of the two identified leaks, the minimum pipe wall thickness at any single loc.ation (pit) is yproximately 0.1 inch. This meets the Code

' minimum wall limit. The pipe wall thickness within about 1 inch of the leaks ranges from 0.12 inch minimum to 0.37 inch based on UT readings. As a result, a large portion of the degraded pipe stub is still at or above the design thickness. The piping spool piece condition was analyzef by the licensee and was found to be within the allowable stress limit of SJ ksi. The

. licensee pedormed an operability determination of the SSW system in the "as found" condition and the system was determined to be operable. The system was constructed in accordance with the requirements of the ASME Code, Class 3.

The root cause of the piping degradation was attributed to delamination of aging rubber pipe

. lining oxacerbated by localized high-flow velocities from throttling of the upstream butterfly valve. Rubber lined piping flaws experience accelerated erosion and corrosion where the rubber lining has delaminated. Wnere the lining remains intact, the pipe remains at its nominal full-wall thickness." Hence, the wall erosion is local to the areas where lining has de!aminated.

This conclusion was also confirmed by the results of the uitmsonic examination of five

. additional pipe locations which identified no other type of operationally caused defects. These

. five locations were reinspected following the discovery of the pin hole leak in February 1999.

The licensee evaluated the structural integrity of the piping using the guidance of GL 90-05.

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f t Based upon the evaluation, it was determined that the two through wall pits and the one pin

" hole leak are within the stress criteria allowable pit size and the integrity of the piping wil: be

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maintained and that the degraded piping satisfied the criteria of GL 90-05.

h Augmented inspection -

iTo assess the overall degradation of the SSW system, augmented ultrasonic examinations were performed on five additional locations as described in the April 7,1998, letter. The r

locations that were examined are similar locations to the SSW system piping location where

. the leaks occurred. All augmented inspection results at these locations found values greater

. than the manufacturers minimum pipe wall tiiickness. These five locations were reinspected

, following the discovery of the pin hole leak. All augmented inspection results were still greater

~ than the manufacturers minimum pipe wall thickness.

4.3 Proposed Temoorarv Non-code Roosir and Monitorina Provisions The licensee proposed temporary Code repairs to maintain the structuralintegrity

. of the pip;ng until the piping is replaced during an outage of sufficient duration. The licensee

installed stainless steel cover plates to the pipe at the degraded locations and welded the pin hole as temporary repairs.- The cover plate will be ultrasonically examined periodically until the pipe can be replaced. In addition, plant operators will visually monitor the pipe for any changes once per shift dur!ng operator tours until the pemianent ASME Code repair can be completed.

Further, bi-weekly monitoring (ultrasonic testing) of th3 stainless steel cover plate material will continue until test results show the test frequencies can be changed.

4.4 Staff Evaluation The pressure at the repair location is low. It ranges from a slight vacuum to a slight positive '

. pressure. No safety-related components are within the proximity of the piping pitting location.

1 - that would be directly affected due to a leak. In addition, the leakage would be accommodated

. by th's design of the o'u xiliary bay. Air in-leakage has a negligible effect on the flow rate through the heat exchangers. The staff has determined that the licensee's flaw evaluation has been

' consistent with the guidelines and acceptance criteria of GL 90-05. Therefore, based on the above, the staff finds the licensee's structural integrity and operability assessments to be . '

s acceptable.- The licensee welded stainless steel cover plates to encapsulate the degraded ,

piping spool piece and welded the pin hole as temporary repairs. During the period of plant operation and until a permanent Code repair is acconiplished, the repair area will be monitored by plant personnel. The licensee has evaluated the f emoorary repair and determined that the SSW is operable.

' The RBCCW is required for plant operation. To petform a Code repair would require removing

one loop _ of SSW system which would place the plant 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. "Since the Code repair would take 4 to 5 days, the plant would have to be shut down

. to perform the reoair.' '

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5.0 CONCLUSION

Shutting the Unit down is not in the best interest of plant safety, given the magnitude of the flaw and the licensee's alternative program. Since the Code repair would have necessitated a Unit shutdown, the staff finds that performance of an immediate Code repair during plant operation

- is impractical. The staff, therefore, grants the licensee's request for relief from performing the -

Code repair, pursuant to 10 CFR 50.55a(g)(6)(i), and finds that implementation of the licensee's altamative progrsm is authorized by law and will not endanger life or property or the common  :

. defense and security, and is otherwise in the public interest, giving due consideration to the burden upon the licensee and facility that could have resulted if the Code requirements were -  :

imposed on the facility. The altemative program is authorized.

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' Principal Contributor: A. Wang Date: JJune'15, 1999 1

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