ML20211J256
| ML20211J256 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 08/31/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20211J228 | List: |
| References | |
| NUDOCS 9909030100 | |
| Download: ML20211J256 (4) | |
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- NUCLEAR REGULATORY COMMISSION r
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION
- AUGMENTED INSERVICE INSPECTIONS OF PRESSURIZER SPRAY LINES NORTH ANNA POWER STATION. UNITS 1 AND 2 4
VIRGINIA ELECTRIC AND POWER COMPANY (VEPCO)
DOCKET NOS. 50-338 AND 50-339
1.0 BACKGROUND
General Design Criterion 4 of Title 10 of the Code of Federal Reaulations (10 CFR) Part 50, Appendix A [1] requires postulation of breaks in high energy piping in nuclear power plants and protection of safety-related equipment, structures and components from dynamic and other
- effects from the postulated pipe breaks. Section 3.6.2.3(2) of the Updated Final Safety Analysis Report [2] currently requires an augmented inservice inspection (All) program for the pressurizer spray lines at both Units 1 and 2. The All examinations were implemented at postulated rupture locations at Unit 1 as an alternative to the installation of pipe whip restraints orjet impingement shields which were deemed impractical to install at these locations. The All examinations were also implemented et Unit 2, although protective measures such as pipe rerouting, pipe whip restraints and jet impingement shields were implemented as required.
The pressurizer spray lines are Class 1 nominal 4-inch or smaller diameter lines located inside the containment, between the main reactor coolant loops, the charging system and the pressurizer, and as such are subject to the ASME Section XI [3] Class 1 inspection requirements.
The current All program requires that each postulated rupture location be examined three times during each 10-year interval, or 12 times under the current ASME Code Section XI q
requirements during the life of the plant. VEPCO has stated that these examinations have been performed at both units, but at considerable personnel exposure risk. VEPCO has therefore -
performed a reanalysis and a reevaluation of the affected prmurizer spray piping to eliminate rupture locations from the All program and thus reduce the personnel exposure [4).
1 2.0 EVALUATION In'accordance with the USNRC standard review plan (SRP) Section 3.6.2 and Mechanical Engineering Branch Position 3-1 (BTP MEB 3-1) [5), VEPCO postulated breaks in the
' rressurizer spray lines but failed to provide protection to mitigate the dynamic effects of these
. breaks. (These effects are mitigated by installing pipe whip restraints and/or jet impingement
' shields at the postulated locations.) The All was implemented because VEPCO did not provide any protection at the postulated breaks in Unit 1 and protection at some locations in Unit 2.
9909030100 990831 PDR ADOCK 05000338 P
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, BTP MEB 3-1 requires that breaks in Class 1 piping be postulated at the terminal ends and at intermediate locations where the following criteria are applicable:
1.
The primary plus secondary stress intensity range Sn between any two load sets, as calculated by Equation 10 of ASME Section Ill (6) Subsection NB-3653.1 for Class 1 piping, exceeds the threshold value of 2.4Sm. Sm is the Class 1 code allowable design stress intensity value. The value 2.4Sm represents 80% of the allowable stress range value of 3.0Sm.
i 2.
The cumulative usage factor (CUF), as calculated in NB 3653.2 through NB 3653.6, should not exceed the threshold value of 0.1.
1 The Class 1 fatigue analysis at North Anna was performed under the piping design rules of ASME Section ill,1976 Edition, NB-3650 (6). These rules were modified in the Summer of 1981 addenda to the ASME Section 111 Code, in which the stress range component of Equation 10 l
caused by the linear temperature gradient across the pipe wall (the DeltaT1 term) was removed from this equation. This was justified on the basis that the linear temperature gradient introduces overconservatism in the design of piping. Thus, this term was reclassified from the secondary stress to the peak stress category, although it has the characteristics of a secondary stress. Since breaks must be postulated at locations where the 2.4Sm threshold is exceeded, the effect of removing this term is to reduce the number of postulated breaks. Using logic based on GL 87-11 [7), VEPCO eliminated the requirement for postulating arbitrary intermediate breaks. The CUF is also reduced, since the calculation of the peak stress ranges beyond the elastic range depends on Sn. The staff has accepted this modification by endorsing the ASME Section ill Code in 10 CFR 50.55a beyond the 1980 Edition, through the 1989 Edition.
In Unit 1, VEPCO identified 16 locations on the pressurizer spray line at which Sn exceeded the threshold value 2.4Sm for austenitic steels, based on 1976 design calculations. VEPCO then re-calculated the secondary stress intensity ranges Sn at these locations without the Delta T1 term. This reduced the Sn values at 13 locations below the 2.4Sm threshold. For the ' remaining three locations, Sn still exceeded the threshold value. The All of these locations will therefore be continued as before. VEPCO also reevaluated the corresponding CUF at each location to determine compliance with the second requirement of MEB 3-1, CUF<0.1. The 1976 calculation showed seven locations where the CUF >0.1. The reevaluation calculations showed two locations with CUF>0.1. However, one of these locations corresponded to a value of Sn exceeding the threshotd criterion. Therefore, the reevaluation of the CUFs indicated only one additional location where the All should be continued. The proposed reduction of 16 to 4 locations in the pressurizer spray line at Unit 1 is acceptable because the revised calculations of Sn stress intensity ranges and the CUFs meet the requirements of references 5 and 7.
Because VEPCO provided protective measures at Unit 2 such as pipe rerouting, pipe whip restraints and jet impingement shields as required, the All program is not needed. Therefore,
. the staff finds it acceptable to eliminate the All program for pressurizer spray lines at Unit 2.
3.0 CONCLU$QN The staff has reviewed the revised calculations of the Sn stress intensity ranges and the CUFs in the pressurizer spray line at North Anna Unit 1 and finds them in accordance with the rules stated in (5) and (7), and Perefore acceptable. The staff therefore finds the proposed reduction
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- of 16 to 4 locations on the Unit 1 kne subject to All acceptable. The staff also finds the proposed elimination of the All locations on the pressurizer spray line at Unit 2 acceptable, since VEPCO stated that there are preventive measures in place to mitigate any potential pipe breaks in this line.
4.0 References 1.
10 CFR Part 50 Appendix A, General Design Criterion 4, " Environmental and Missile Design Basis" 4
2.
North Anna Updated Final Safety Analysis Report, Section 3.6 3.
. ASME Boiler and Pressure Vessel Code Section XI,1986 Edition 4.
VEPCO Civil Engineering Technical Report CE-0082, " Review of Augmented ISI Frequency for Pressurizer Spray Lines, May 1996 5.
NUREG-800, Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants, LWR Edition, July 1981 6.
ASME Boiler and Pressure Vessel Code Section Ill, Subsection NB,1976 Edition 7.
USNRC Generic Letter 87-11: Relaxation in Arbitrary Intermediate Pipe Rupture Requirements, June 1987 Principal Contributor: M. Hartzman Date: August 31, 1999 l
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J. P. O'Hanlon August 31, 1999 Our safety evaluations are enclosed. This completes our effort on this issue, and we are closing TAC Nos. MA2301 and MA2302.
Sincerely, Original signed by:
Gordon E. Edison, Senior Project Manager, Section 1 Project Directorate 11 Division of Licensing Project Management Office of Nuclear Reactor Regulation Docket Nos. 50-338 and 50 339
Enclosure:
As stated cc w/ encl.: See next page Distribution:
Docket File RScholl (e-mail SE only)
PUBLIC OGC PD 11-1 R/F ACRS CHawes
. LPlisco, Ril GEdison SSheng MHartzman
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