ML20217K944
| ML20217K944 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 10/15/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20217K937 | List: |
| References | |
| NUDOCS 9910270034 | |
| Download: ML20217K944 (3) | |
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8 NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20566-0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO THE SECOND 10-YEAR INSERVICE INSPECTION PROGRAM REllEF REQUEST RR ENG-2-4 i
STP NUCLEAR OPERATING COMPANY SOUTH TEXAS PROJECT. UNITS 1 AND 2 DOCKET NOS 50-498 AND 50-499
1.0 INTRODUCTION
By letter dated June 9,1999, STP Nuclear Operating Company (the licensee) submitted a request for relief from the American Society of Mechanical Engineers (ASME) Code,Section XI, nondestructive examination requirements applicable to South Texas Project (STP), Units 1 and 2, reactor vessel closure head nuts (Relief No. RR-ENG-2 4). The licensee proposes to i
perform an alternative ultrasonic examination from the outside and end surfaces of these nuts.
The licensee's relief request is applicable to the second 10-year inservice inspection (ISI) interval for STP, Units 1 and 2.
2.0 BACKGROUND
ISI of the ASME Code Class 1,2, and 3 components shall be performed in accordance with Section XI of the ASME Boiler and Pressure Vessel (B&PV) Code and applicable addenda as required by 10 CFR 50.55a(g), except where specific written relief has been granted by the Commission pursuant to 10 CFR 50.55a(6)(g)(i). The regulation at 10 CFR 50.55a(a)(3) states that alternatives to the requirements of paragraph (g) may be used, when authorized by the NRC, if (i) the proposed alternatives would provide an acceptable level of quality and safety or (ii) compliance with the specified requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
Pursuant to 10 CFR 50.55a(g)(4), ASME Code Class 1,2, and 3 components (including supports) shall meet the requirements, except the design and access provisions and the preservice examination requirements, set forth in the ASME Code,Section XI," Rules for inservice inspection of Nuclear Power Plant Components," to the extent practical within the limitations of design, geometry, and materials of construction of the components. The regulations require that inservice examination of components and system pressure tests l
conducted during the first 10-year interval and subsequent intervals comply with the requirements in the latest edition and addenda of Section XI of the ASME Code incorporated by
-reference in 10 CFR 50.55a(b) 12 months prior to the start of the 120-month interval, subject to j
the limitations and modifications listed therein. For STP Units 1 and 2 the applicable edition of Section XI of the ASME Code for the second 10-year ISI intervalis the 1989 Edition.
Enclosure 9910270034 991015 PDR ADOCK 05000498 P
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3.0 :
LiQfEEEE'S REQUEST Relief No. RR-ENG-2-4 Code Reauiremnt ASME Code,Section XI, Table IWB-25001, Examination Category B-G-1, item No. B6.10,
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requires a surface examination (i.e., magnetic particle) of the inside (threaded) surface of the reactor pressure vessel closure head nuts.
Liconsee's Basis fEr Reauestina Relief from Code Reauirements (as stated)
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L ASME Section XI requires surface examination of the rear.cor pressure vessel nuts (see Table IWB-2500-1, Examination Category 8-0 1, item No. B6.10).
l Although there is no description or figt ' reference in f action XI depicting the area of the examination, the requircmo nas been int.rpreted to include all surfaces of the nut (i.e., top, bottom, owside, and insic a threads). Typically, a fluorescent magnetic particle examination is performeu on throaded areas
. because of its suitability for discrimination of relevant indications in the root of the thread. However, due to the poor viewing angle and the limited access to the threaded surface with the magnetic partic'e equipment and black light, a technically adequate magnetic particle examination cannot be performed on the teactor pressure vessel nut threads.
In accordance with the provisions of 10 CFR 50.55a(a)(3)(i), the South Texas Project requests relief from the Section XI requirement to perform a surface examination on the inside surface (i.e., threaded portion) of the reactor pressure vesse! closure head nuts.. This relief request proposes application of an
" alternative ultracnic examination in lieu of the magnetic particle examination of tt.a threaded area of each nut.
Licensee's Procosed Alternative Examination (as stated) in lieu of the magnetic particle _ examination of the threaded swfaces of the reactor pressure vessel nut, the South Texas Project will perform an altcrnative ultrasonic examination of the threaded region from the outside and end surfaces of the nut. This ultrasonic examination will provide coverage of the thread root area in two directions:
A 0-degree ultrasonic examination is performed 360 degrees around the nut end surface to examine the threaded area for circ..mferential flaws (see Figura 1). No limitations are' encountered with the 0-degree ultrasonic examinations.-
1 A _43-degree ultre. sonic (UT43) examination from the outside surface is I
used to examine the threaded area in clockwise and counterclockwise
- directions to detect axial flaws (see Figure 2). Limitations are -
encountered with the UT43 examination in the area of the six wrench l
slots located on the upper end of the outside diamoter. These slots l
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ep, reduce the UT43 examination coverage of the threaded region by approximately 7.5 percent.
A spare reactor pressure vessel nut will be used for the calibration biock (see Figure 3). Circumferential notches cut 0.115 inch deep and an axial notch cut through the threads to the thread root serve as bases for tf sensitivity of the ultiesonic examinations.
- Magnetic particle examinations will continue to be conducted on the outside and end surfaces of the reactor pressure vessel nuts.
4.0 EVALUATION The applicable code requirements for the licensee's seconc' 10-year ISI interval requires a surface examination of all surfaces associated with the reactor vessel head closure nuts (top, bottom,inside threads, outside diameter). Performing a surface examination on the threaded area of the nuts is hindered by the viewing angle and limited access. As an afternative, the licensee' proposes to use ultrasonic' testing to examine the inside threaded region of the nuts.
The licensee proposes to use a 0-degree ultrasonic examination technique on the nut end surface to examine the threaded area for circumferential flaws. No limitations are encountered with the 0-degree ultrasonic examinations. In addition, a 43-degree ultrasonic examination fechnique will be used on the outside diameter in clockwise and counterclockwise directions to l
detect axial flaws in the threaded region. Limitations are encountered in the 43 degree l
examination'in the area of the six wrench slots located on the upper edge of the outside diameter. The wrench slots reduce the 43-degree examination coverage of the threaded region by approximately 7.5 percent.
The code-required magnetic particle examination is intended to provide detection of surface Ecrack initiation, but does not provide information on the depth of the cracks. The proposed ultrasonic examination would be expected to reveal cracking before it becomes significant and provides an esti oate of crack depth. The ultrasonic examination can be performed on the exterior surf %s of the ree.ctor vessel head closure nuts and provides indication of discontinudas throughout the volume of the' material including indications of cracking that t
initiate in the threaded region of the nut.- Therefore, the staff finds the licensee's proposed alternative of performing an ultrasonic examination on the exterior surfaces of the nuts will provide acceptable assurance of structuralintegrity by identifying any significant indications in the threaded region of the nut.
5.0 CONCLUSION
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' Based on the above evaluation, the staff has determined that the code requircJ examination is I
not feasible on the interior threaded region of the nuts. The staff concludes that the licensee's proposed attemative to the code requirement provides an acceptable level of quality and safety.
Therefore, pursuant to 10 CFR 50.55a(a)(3)(i), the alternative proposed in relief request RRa ENG-2-4 is authorized for the second 10-year ISI interval at STP, Units 1 and 2.
Principal Cont;ibutor: A. Keim Date: October 15, 1999 x
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