ML20236K553

SER Accepting Request for Change in ISI Commitment on Protection Against Pipe Breaks Outside Containment

ML20236K553
Person / Time
Site:	North Anna
Issue date:	07/07/1998
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20236K548	List: ML20236K545 ML20236K553
References
NUDOCS 9807090367
Download: ML20236K553 (4)

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NUCLEAR REGULATORY COMMISSION o WASHINGTON, D.c. 2006M201 l

SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REQUEST FOR CHANGE IN ISI COMMITMENT ON PROTECTION AGAINST PIPE BREAKS OUTSIDE CONTAINMENT VIRGINIA ELECTRIC AND POWER COMPANY NORTH ANNA POWER STATION. UNITS 1 AND 2 DOCKET NOS. 50-338 AND 50-339

1. BACKGROUND General Design Criterion (GDC) 4 requires that structures, systems, and components important to safety shall be designed to accommodate the dynamic and environmental effects of postulated high energy line breaks. In the North Anna Power Station, Units 1 and 2, pipe breaks were postulated in high energy lines in accordance with specified criteria, and devices to mitigate the consequences, such as pipe whip restraints and jet impingement shields, were installed. However, the originallicensing commitment did not require break postulation at portions of Main Steam and Feedwater piping in the Mechanical Equipment Room located outside the containment, and therefore, no mitigation devices were installed in the area.

Instead, an augmented inservice inspection (ISI) program for welds at the postulated break locations was required and a leak detection system was provided in the area to ensure piping structural integrity and to detect leaks prior to pipe rupture.

The augmented inspection is done in accordance with the rules of ASME Section XI, subsection IWC (Class 2), edition and addenda corresponding to the current approved ASME Section XI program (currently the 1983 edition and Summer 1983 addenda for North Anna Unit 1 and the 1986 edition for North Anna Unit 2). The current Auqmented ISI Program consists of 100%

ultrasonic testing (UT) examination of all welds in the area. The program requires three full inspections in the first 10 years after the start of plam operation, and once every 10 years afterwards.

In addition, another inspection program activity is in place under the Secondary Piping and Component inspection Program (SPCIP) for detecting flow-accelerated corrosion. This program specifically covers the portion of Main Steam and Feedwater piping located in the Mechanical Equipment Room.

2. _ LICENSEE'S INITIALLY PROPOSED COMMITMENT CHANGE

a. The initially proposed change:

In its first letter to NRC dated September 23,1996, the licensee proposed to terminate the augmented ISI program stated above.

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' b. ' The' basis:

The licensee indicated in the letter that, based on at least 15 years experience on each unit since initial licensing and plant operation, the augmented inservice weld inspections - ;

performed on the Main Steam and Feedwater piping have not identified any unacceptable weld indications. Thus, the licensee concludes that weld failure due to operational stress or material defects not initially detected in the manufacture of these high energy lines is

unlikely. The licensee states that, since the most likely failure mechanism associated with carbon steel piping is flow-accelerated corrosion, the existing SPCIP, which is specifically designed for that purpose, is adequate to ensure piping structural integrity.

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3. STAFF EVALUATION OF THE INITIAL PROPOSAL

, The staff reviewed the licensee's initially proposed licensing commitment change and had j discussions with the licensee. The staff found the following:

L a. The current Augmented ISI Program for portions of Main' Steam and Feedwater piping in the Mechanical Equipment Room area consists of 100 percent ultrasonic testing (UT) l examination of all welds in the area. The program requires three full inspections in the l first 10 years after the start of plant operation, and once~every 10 years afterwards. Unit

! 1 started operation in 1978 and Unit 2 started in 1980. Thus, each unit had three or four 100% UT examinations already. The staff found that the number of welds to be inspected, as required by the program, could be reduced, because so far all welds were

found acceptable and these welds will continue under the same environmental effects.

L UT examination of a lesser number of welds may be adequate to ensure piping structural integrity.

b. The staff does not agree with the licensee's initial proposal to terminate the Augmented l lSI Program for the following reasons:

1. If the augmented ISI is terminated, the integrity of specific welds will not be l monitored under the existing SPCIP. This is not acceptable, since the SPCIP is specifically designed to monitor flow-accelerated corrosion and the locations to be examined under the SPCIP do not coincide with the postulated break locations (high stress areas, terminal ends, etc.) currently examined under the augmented ISI program.

ii. The pipe break postulation criteria are based on levels of stresses and cumulative fatigue usage factors calculated in piping design. The implied piping l degradation mechanisms are different from the consideration of flow-accelerated corrosion. Especially, the fatigue effects are cumulative in nature. Piping welds found acceptable in previous inservice inspections may not ensure that degradation will never occur.

iii. Since no Main Steam and Feedwater line breaks are postulated and no mitigation devices are installed in the area of the Mechanical Equipment Room

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located outside containment, for meeting the intent of GDC 4, continued L monitoring of all postulated break locations is prudent. Thus, the central issue is how much continued inspection is adequate.

4, LICENSEE'S REVISED PROPOSAL In ite !::tter to NRC, dated February 5,1997, the licensee proposed that, in addition to the i

~ SPCIP and leak detection system,25% of the potential break point weld locations in the Main Steam and Jeedwater Systems piping located in the Mechanical Equipment Room will be {

. examined t;ach inspection interval in accordance with the rules of ASME Section XI,  ;

Subsection IWC.

5. STAFF EVALUATION OF THE REVISED PROPOSAL The staff finds the above program inadequate since the number of welds inspected was low, and in addition, spread over a long interval (25% of the welds in 10 years) for reasons given in 3.b.i and 11. This was conveyed to the licensee on March 16,1998.

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6. LICENSEE'S FINAL PROPOSAL As a response to previous discussions, in its letter to NRC dated March 25,1998, the licensee i--

modified its proposed commitment change as follows:

Twenty-five percent of the potential break point weld locations in the Main Steam and Feedwater Systems piping located in the Mechanical Equipment Room will be examined each inspection period in accordance with the rules of ASME Section XI,

- Subsection IWC (Class 2).

7. STAFF EVALUATION OF THE FINAL PROPOSAL The staff reviewed the licensee's revised licensing commitment change as stated in its letter to the NRC on March 25,1998, as briefly described in Section 4. above. The evaluation found the following:

a. As discussed in Section 3.b above, the Augmented ISI Program is needed for meeting the intent of GDC 4 and should not oe terminated, but the number of welds to be inspected under the existing program could be reduced. According to ASME Section XI, the licensee's revised proposal of reducing the number of welds to be inspected in each inspection interval from 100% to 75% (25% per period) is equivalent to three times the percentage currently required for ASME Class 1 or 2 piping. Thus, the proposed percentage is adequate and acceptable.

b. The licensee's revised proposal indicated that the method of inspection is in accordance with the rules of ASME Section XI, Subsection IWC, which requires volumetric examination of piping welds; thus, the method committed to by the licensee in its augmented inspection program is adequate and acceptable.

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c. The existing leak detection system and the Secondary Piping and Component Inspection Program are not included in the revised commitment change, and hence will remain. These programs will continue to provide additional assurance to structural

integrity of the portions of the Main Steam and Feedwater piping in the Mechanical i

Equipment Room located outside the containment.

8. CONCLUSION Based on the evaluation stated above, the staff concludes that VEPCO's proposed change in licensing commitment on augmented inservice inspection program for portions of Main Steam and Feedwater line in the area of the Mechanical Equipment Room located outside containment, which is described in detail in its letter to NRC dated March 25,1998 (Serial No.98-123), would provide an acceptable level of quality and safety. Consequently, pursuant to 10 CFR 50.55a(a)(3)(i), the revised change in licensing commitment is authorized.

I Principal Contributor: Shou-nien Hou Date: July 7,1998

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