ML20206E874

SER Determined That Flaw Evaluation Meets Rules of ASME Code & Assumed Crack Growth Rate Adequate for Application

ML20206E874
Person / Time
Site:	Vermont Yankee File:NorthStar Vermont Yankee icon.png
Issue date:	04/29/1999
From:	NRC (Affiliation Not Assigned)
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Shared Package
ML20206E868	List: ML20206E864 ML20206E874
References
NUDOCS 9905050271
Download: ML20206E874 (3)

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It UNITED STATES g ,j NUCLEAR REGULATORY COMMISSION o ~c WASHINGTON, D.C. 2C565 4001

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. . m FETY EVALUATION BY THE OFFICE OF NUCLEAR R2 ACTOR REGULATION I

VERMONT YANKEE NUCLEAR POWER STATION JET PUMP RISER INSPECTION RESULTS AND THE FLAW EVALUATION VERMONT YANKEE NUCLEAR POWER CORPORATION DOCKET NO. 50-271

1.0 INTRODUCTION

By letter dated March 29,1999, Vermont Yankee Nuclear Power Corporation (the licensee) submitted, for NRC review, its revised flaw evaluation for the detected flaws on the jet pump riser (JPR) circumferential welds. These flaws were detected during the 1998 refueling outage.

The current submittal differs from the submittal on the same subject dated May 4,1998, in two respects: (1) the current flaw evaluation referenced the revised General Electric (GE) report, GE-NE-B13-01935-02, Rev.1, " Jet Pump Assembly Welds Flaw Evaluation Handbook for Vermont Yankee,"instead of GE-NE-B13-01935-LTR, Rev.1, " Jet Pump Riser Welds Allowab!e Flaw Sizes Letter Report fo- Vermont Yankee," and (2) the current flaw evaluation is i for two fuel cycles instead of one cycle. The key feature of GE-NE-B13-01935-02, Rev.1 is that the loading has been revised using a finito element model for the entire jet pump assembly.

The characterization of the detected flaws stays the same in this submittal: four flaws exist in the circumferential weld connecting thermal sleeve to the riser elbow (RS 1 weld), and the maximum flaw size is 2.82 inches.

2.0 EVALUATION 2.1 Licensee's Evaluation Based on the maximum flaw size of 2.82 inches for the detected flaws in the RS 1 welds and a flaw measurement uncertainty of 0.191 per flaw end, the licensee calculated the initial flaw size to be 3.20 inches. The licensee then assumed that the flaw was through-wall and performed a flaw evaluation applying GE-NE-B13-01935-02, Rev.1 to determine the allowable crack size for the limiting detected flaw. The flaw evaluation methodology of GE-NE-B13-01935-02, Rev.1 is limit load enalysis consistent with the latest Appendix C (1996 Addenda) of Section XI of the )

i American Society of Mechanical Engineers (ASME) Code. The limit load analysis used a safety i factor of 2.77 for Normal and Upset and 1,39 for Emergency and Faulted conditions and a Z- j factor for submerged arc welds (SAW). The normal load includes dead weight, hydraulic loads, j flow induced vibration (FIV), and thermal loads. The faulted load includes the normal load plus i the safe shutdown earthquake inertia load. In determining the predicted crack size at the end j

'of two fuel cycles, the licensee used a bounding intergranular-stress-corrosion-cracking ,

(IGSCC) growth. rate of SX104 inch / hour. The fatigue crack growth due to FIV under the I normal condition has also been considered and determined to be insignificant.

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The licensee added the crack growth corresponding to two fuel cycles, or 24,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> of operation (0.60 inch per crack end per fuel cycle), to the initial crack size (3.20 inches), and obtained the final crack size (5.6 inches). On the other hand, the licensee applied GE-NE-B13-01935-02, Rev.1 and determined that the allowable crack size is 18.62 inches in length. Since -

the predicted crack size at the end of two fuel cycles is less than the allowable crack size with an adequate margin, the licensee concluded that JPR integrity and performance will not be compromised for the next two fuel cycles.

2.2 NRC Staff's Evaluation 2.2.1 The Limit Load Analysis of the 1996 Addenda The licensee performed a flaw evaluation for the detected flaw at the RS-1 weld of JPRs. The flaw evaluation was based on the limit load analysis of the latest Appendix C (1996 Addenda) of Section XI of the ASME Code. Previously, the Code required the user to set the pipe diameter in the Z-factor equations to be 24 inches for any pipe having a diameter less than 24 inches.

The 1996 Addenda had removed this additional conservatism based on results from extensive pipe fracture experiments on austenitic SAW and SMAWs. The staff accepts the limit load analysis of the 1996 Addenda, because it is supported by numerous test data documented in l NUREG/CR-4878. Further, since the 1996 Addenda only involves a minor modification in Z-factor calculation while keeping the main body of the limit load analysis intact, the staff determined that relief is not needed for the licensee to use the limit load analysis of the1996 Addenda in this application.

2.2.2 The Flaw Evaluation j The staff evaluated the licensee's allowable crack size evaluation and determined that the limit load analysis meets the rules of the ASME Code (1996 Addenda), and therefore, is acceptable.

As to the predicted flaw size estimation, the staff determined that the use of the bounding IGSCC growth rate is conservative, and the use of FIV for the fatigue crack growth calculation is adequate. The UT measurement uncertainty of 0.191 inch for each crack end was determined in accordance with BWRVIP-03, "BWR Vessel and Internals Project, Reactor Pressure Vessel and internals Examinations Guidelines." This report was approved by the i NRC on June 8,1998. The UT technique was partially demonstrated at the EPRI NDE Center l in 1997 and was then completed at Peach Bottom Atomic Power Station in the same year. The i staff examined the margin (Margin 1) between the allowable flew size (18.62 inches) and the  ;

predicted flaw size (5.6 inches) and the margin (Margin 2) between the FIV threshold flaw size  ;

(8.16 inches) and the predicted flaw size (5.6 inches), and found both margins are acceptable.

Margin 1 is used to justify the continued operation of the unit for two fuel cycles with the detected flaws in the JPR welds. Margin 2 is used to support that FIV contributes to no fatigue growth during the intended fuel cycles. The FIV threshold flaw size is defined in the submittal as the flaw size beyond which the applied stress intensity factor difference (6K) would be large enough (exceeding 5.0 ksi (in)") to contribute to fatigue crack growth.

3.0 CONCLUSION

S The staff has reviewed the licensee's submittal and determined that the flaw evaluation meets <

the rules of the ASME Code and the assumed crack growth rate is adequate for this application. Since the predicted final flaw size at the end of two cycles (5.6 inches) is far less i

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than the allowable flaw size (18.62 inches)' rom the limit load analysis, the staff determined that continued operation for Vermont Yankee without repair is acceptable for two fuel cycles.

Although JPR welds do not belong to the examination categories of the ASME Code, the successive inspection requirement of the Code should be applied to the JPR welds to maintain the same level of safety for the welds in the ASME examination categories. Hence, the i licensee should reinspect these flaws during the 2001 refueling outage (approximately in line l with the rules of the ASME Code regarding successive inspections) and reevaluate the flaws at that time, in the submittal, the licensee indicated that it is anticipated that these flawed welds will be reinspected during the 2001 refueling outage and further evaluated.

I Principal Contributor: S.Sheng l

Date: April 23,1999 l l

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