ML20205K758

Safety Evaluation Supporting Alternative Proposal for Reexamination of Circumferential Welds with Detected Flaw Indications in Plant RPV

ML20205K758
Person / Time
Site:	Vermont Yankee
Issue date:	04/07/1999
From:	NRC (Affiliation Not Assigned)
To:
Shared Package
ML20205K757	List: ML20205K753 ML20205K758
References
NUDOCS 9904140008
Download: ML20205K758 (3)

Text

U pa " Ecog ye yi UNITED STATES i S NUCLEAR REGULATORY COMMISSION (21f WASHINGTON. c.c- 20555 4 001

%4 8 4

.....,o SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION )

i VERMONT YANKEE NUCLEAR POWER STATION j VERMONT YANKEE NUCLEAR POWER CORPORATION p_OCKET NUMBER 50-271 l'

1.0 INTRODUCTION

On January 28,1999, the Vermont Yankee Nuclear Power Corporation (the licensee) submitted a request seeking approval of an alternative examination to replace that specified in the American Society of Mechanical Engineers (ASME) Code regarding successive reexamination of the area containing flaw indications in circumferential welds of the reactor pressure vessel (RPV) for Vermont Yankee Nuclear Power Station. IWB-2420(b) of Section XI requires three ,

successive reexaminations of the areas containing flaws or relevant conditions that have been i approved for continued service. The licensee is seeking approval under the provisions of I Section 50.55a(a)(3)(i) to Title 10 of the Code of Federal Regulations [10 CFR 50.55a(a)(3)(i))

for an alternative inspection schedule that would eliminate the successive reexaminations for the current inspection interval. Section 10 CFR 50.55a(a)(3)(i) allows licensees to propose i alternatives to the inservice inspection (ISI) requirements when the proposed alternatives would i provide an acceptable level of quality and safety in lieu of implementing the requirements.

2.0 BACKGROUND

I "BWR Vessel and Internals Projects, BWR Reactor Vessel Shell Weld Inspection  !

Recommendations (Technical Report BWRVIP-05)

By letter dated September 28,1995,' the Boiling Water Reactor Vessel and Internals Project (BWRVIP), a technical committee of the BWR Owners Group (BWROG), submitted the proprietary report, "BWR Vessel and Internals Project, BWR Reactor Vessel Shell Weld  ;

Inspection Recommendations (BWRVIP-05)." This report proposed to reduce the scope of inspection of the BWR RPV welds and to eliminate the successive reexamination requirements j in the ASME Code regarding welds with detected indications. The three conditions for the elimination of the successive reexamination r3quirements are:

1 As supplemented by letters dated June 24,1996; October 29,1996; May 16,1997; June 4,1997; June 13, 1997; and December 18,1997.

~

9904140008 990407 PDR P ADOCK 05000271 PDR Enclosure

f t . l 2

(1) The flaw is characterized as subsurface in accordance with Figure 9-1 (of the BWRVIP-05 l Report).

(2) The NDE technique and evaluation that detected and characterized the flaw as originating from material manufacture or vessel fabrication shall be documented in a flavt evaluation report.

(3) The vessel containing the flaw is acceptable for continued service in accordance with IWB-3600 and the flaw is demonstrated acceptable for the intended senrice life of the vessel.

The staff accepted the proposal regarding successive reexaminations in the final safety evaluation (SE) dated July 28,1998, for the BWRVIP-05 report. l 3.0 EVALUATION Proposed Alternative to the Successive Reexamination Requirements for Circumferential Welds in the Vermont Yankee RPV 3.1 Reauest for Relief The licensee is requesting relief from performing successive reexaminations of the area containing flaw indications in circumferential welds of the Vermont Yankee RPV for the current inspection interval.

3.2 A_poticable Reauirement Pursuant to the requirements of 10 CFR 50.55a(g)(4), ASME Code Class 1,2, and 3 components mue>t meet the requirements, except the decign and access provisions and the preservice examination requirements, set forth in Section XI, " Rules for Inservice Inspection of Nuclear Power Plant Components," of ASME Code (Section XI) to the extent practical within the limitations of design, geometry and materials of construction of the components.2 The regulations require that all inservice examinations and system pressure tests conducted during the first 10-year interval and subsequent intervals on ASME Code Class 1,2, and 3 components must comply with the requirements in the latest edition and addenda of Section XI incorporated by reference in 10 CFR 50.55a(b) on the date 12 months prior to the start of the 10-year interval.

If components with detected flaw indications from the ISI are accepted for continued service in accordance with the ASME Code, Paragraph IWB-2420 requires that the areas containing flaws or relevant conditions shall be reexamined during the next three inspection periods listed in the schedule of the inspection program of IWB-2400.

3.3 Basis for Relief Section 10 CFR 50.55a(a)(3)(i) allows licensees to propose alternatives to the IS! requirements when the proposed alternatives would provide an acceptable level of quality and safety in lieu of 2 Except for design and access provisions and preservice inspection requirements.

l l

l L

3 complying with the requirements. The licensee demonstrated that the proposed alternative has I an acceptable level of quality and safety by employing the approved criteria (reproduced in Section 2.0 of this SE) in the BWRVIP-05 report for eliminating successive reexaminations of subsurface flaws in RPV circumferential welds.

3.4 Proposed Alternatives The licensee proposes to eliminate the successive reexaminations of subsurface flaws in RPV circumferential welds fo. the current inspection interval.

3.5 Evaluation i

It was stated in the relief request that the detected flaws in the RPV circumferential welds, I which were approved for continued service, meet all three criteria of BWRVIP-05 for the elimination of the successive reexamination requirement of the ASME Code. As mentioned, these proposed criteria for the circumferential welds were accepted by the staff. This I determination was based on the low failure probability of 8.2 X 104 /yr from the staff's

. independent calculation for the limiting plant-specific analyses (32 EFPY) using a low l temperature transient with an event frequency of 1 X 10 /yr. In the current evaluation, the staff l confirmed that (1) the detected flaw is characterized as a subsurface flaw, which was accepted by the staff in another SE dated October 11,1996, (2) the NDE technique and evaluation that detected and characterized the flaw are documented in the flaw evaluation report previously i submitted, and (3) the vessel containing the flaw was approved (the same SE dated October 11,1996) for continued service without repair through end-of-license for Vermont Yankee. Hence, all three criteria in the BWRVIP-05 report for eliminating successive l reexaminations of detected flaws have been satisfied.

It should be noted that the bounding failure probability of 8.2 X 104/yr from the staff's independent evaluation of the BWRVIP-05 report is for the limiting Babcock and Wilcox vessel. t The corresponding value for the limiting Chicago Bridge and Iron (CB&l) vessel (Vermont i Yankee's RPV was manufactured by CB&l) is an order of magnitude lower as indicated in the final SE for the BWRVIP-05 report. This provides additional assurance that an acceptable level of quality and safety will be maintained for the alternative inspection requested by the licensee.

4.0 CONCLUSION

S The staff has reviewed the licensee's alternative proposal for the reexamination of the ,

circumferential welds with detected flaw indications in the Vermont Yankee RPV. The staff has determined that the licensee's alternative proposal for the circumferential welds meets the conditions in the BWRVIP-05 report approved by the staff for the elimination of the successive reexaminations for the circumferential welds with detected flaw indications. Hence, the staff has determined, pursuant to 10 CFR 50.55a(a)(3)(i), that the alternative inspection provides an acceptable level of quality and safety relative to assuring the structural integrity of the subject circumferential weld.

Principal Contributor: S. Sheng Date: April 7,1999 l