ML20196F232

Rev 0 to NSP-31Q-302, Evaluation of Limiting Flaws for Structural Adequacy on Lower Full Length CRDM Canopy Seal Weld

ML20196F232
Person / Time
Site:	Prairie Island
Issue date:	11/21/1998
From:	STRUCTURAL INTEGRITY ASSOCIATES, INC.
To:
Shared Package
ML20196F208	List: ML20196F203 ML20196F232
References
NSP-31Q-302, NSP-31Q-302-R, NSP-31Q-302-R00, NUDOCS 9812040223
Download: ML20196F232 (12)

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CALCULATION FILE No: NSP-31Q-302 STRUCTURAL PACKAGE PROJECT No: NSP-3io INTEGIUTY Associates, Inc.

PROJECT NAME: Repair of Prairie Island Unit 2 Lower Full Length CRDM Canopy Seal Weld 1

Cl. TENT: Northern States Power Company CALCULATION TITLE: Evaluation of 1.imiting Flaws ihr Structural Adequacy in Lower Full Length CRDM Canopy Seal Weld.

! PROHLEM STATEMENT OR OBJECTIVE OF THE CALCULATION:

Demonstrate that the through wall flaw size detectable by visual cxamination is less than the critical fhw size for both axially and circumferentially oriented flaws.

Project Mgr.

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1.0 INTRODUCTION

in support of the use of visual examination rather than dye penetrant examination of the completed wcld overlay repair to the lower full length CRDM canopy seal weld at Prairie Island Unit 2, Structural Integrity Associates (SI) performed several analyses to determine the critical flaw size in the repaired location. The pumosc of these analyses is to demonstrate that a through wall flaw could be detceted by visual examination having a size which is sufficiently smaller than the critical flaw si7e, thus assuring sufficient safety margins. NSP will review the critical flaw xi/es determined in this calculation to confirm that the resulting sizes are detectable with margin by the visual technique, 2.0 GEOMETRY j

The design geometry of the repair is illustrated in Figurc 1. The geometry is obtained from i

Reference 1. For the purpose of the present evaluation the component was modeled as a pipe with outside radius equal to the distance from the drive centerline to the canopy seal wcld (3.225 inches) and wall thickness equal to the minimum anticipated overlay thickness (0.36 inches), as shown on Figure 1. Through wall axial and circumferential flaws were evaluated. These geometries are considered to be reasonable representations of the actual design geometry. 'nic j

model geometries are shown in Figure 2.

3.0 APPLIED STRESSF.S For conservatism, the applied stress was assumed to act as a membranc stress at the Code allowable membranc stress magnitude (P,,, = S,,). No distinction was made between the hoop and axial directions in this regard, although realistically, the axial direction should be half of this value. liased upon discussions with plant personnel. then: are no bending loads present.

l Therefore, bending stresses were not considered.

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4.0 MATERIAL PROPERTIES The allowable stress, S, was taken to be 16.2 ksi at 650 F which is typical of 304 stainless steel.

The Alloy 625 to be used in the weld overlay repair has a significantly higher allowable stress at q

this temperature so use of the stainless steel value is censervative. The flow stress for this component was taken as 3(S.).

For linear clastic fracture mechanics evaluations, the Kw was assumed as 135 kni Jin, which is very conservative for this material at this temperature (the weld metal will be applied using an automated gas shielded process).

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.4.0 ANALYTICAL APPROAC11 l

Two analysis methodologics were employed. The limit load (net section collapse) method is considered most appropriate for evaluation of through wall flaws in this very ductile material.

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'lhis method is described in Appendix C of ASME Section XI [2]. For comparison, linear elastic fracture mechanics (LEFM) methods were also applied. This approach is very conservative for this material, due to its ductile behavior.

Four cases were studied. These were:

1. Thmugh wall axial flaw: Limit load.

2. Through wall axial flaw: LEFM

3. Through wall circumferential flaw: Limit load.

4. Through wall circumferential flaw: LEFM.

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l-No Code safety margins were included in this evaluation, since the objective is to get a rcasonable l

view of the relationship between detectable and critical flaw sizes.

The results from cach case are summarized below.

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i 6.0 RESULTS l

6.1 Through Wall AxialFlaw: Limit Load i

This case follows the methodology underlying ASME Section XI,IWB-3641 and Appendix C.

The SI program pc-CRACK [3] was used to perform the analysis. The pc-CRACK results are attached in Appendix A to this report. The conclusion is that an axial flaw could be at least 4.8 inches long before leading to incipient collapse. This is much longer than is physically I

achievable,.since cracking would be expected to be confined to the weld overlay material and vicinity, which, in the axial direction, extends approximately I inch.

i 6.2 Through Wall AxialFlaw: LEFM j

This analysis assumes that brittle failure is the operative mechanism. The pc-CRACK program is used with this analysis. A fracture mechanics model of a through wall crack in a cylinder under internal pressure was used, together with an assumed fracture toughness Kg. - 135 ksi din. The pc-CRACK results are shown in Appendir. B. The c(melusion is that for this set of assumptions, the critical flaw length is greater than 5.05 inches.

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6.3 Through Wall Circumferential Flaw: Limit Load This analysis used calculations using the methods of Secticn XI Appendix C. The spreadsheet in i

i Appendix C of this report was used to perform the analysis of the configuration. The analysis Revision 0

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assumed a through wall circumferential flaw, and detennined the critical flaw length using lim load techniques. The conclusion is that such a flaw could be 132"around the cylinder beibre reaching a critical size. This corresponds to a flaw approximately 7.43 inches long. Comput output is included in Appendix C of this report.

6,4 Through Wall Circumferentini Flaw: LEFM

'lhis analysis assumed that the failure mode was brittic failure. The pc-CRACK program was used with a through-wall circumferential flaw in a cylinder under remote tension fracture mechanics model. A K,,- 135 ksi 6 was conservatively assumed. The conclusion of this analysis was that the critical flaw length for this set of assumptions was approximately 7.31 inches. The computer output is included in Appendix D.

7,0 CONCLUSIONS The above results demonstmte that, under a variety of cc7scrvative assumptions, the critical flaw size predicted for the repair geometry is in all cases significantly longer than the flaw length which is expected to be detectable by a visual examination under magnification, as proposed by NSP. This will be confirmed by NSP.

8.0 REFERENCES

1.

S1 File No. NSP-31Q-201, "CRDM Geometry".

2.

ASMt! Boiler and Pressure Vessel Code,Section XI,1989 F.dition. No Addenda.

3.

Structural Imegrity Associates, pc-CR ACK, for Windows, Version 3.0, March 1997.

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ATTACHMENT 2 9

SUMMARY

OF CAMERA TESTING WELDING SERVICES INCORPORATED canopyseatrogrelef1998 doc

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l lll1 Welding Services Inc.

2225 GKYLAND COURr NORCa00% OcorGA 300n gg<szoms June 21,1995 Mr. Dick Cooper Northern States Power Company 1717 Waknaade Ddve East WcIch,MN 55089 FAX: 612-330-7603

SUBJECT:

PrairicIsland Nuclear Plant i

WS1 ReferenceNo.: 35049-2

DearMr. Cooper:

i Per your request, we have perfonned several tests to evaluate the capabilities of the camera system used in the performance of the wcid repair ofPrairie Island's CRDM Seals. De intent of this testing is to provide data to be v' sed by NSF to evaluate the adequacy of this camera fot the performance of adequate visual inspection orthe wcld overlay. The testing descdbed was not performed as a safety related procedure. He testing was performed as follows:

1. The video fmnt end of the WSI weld head was connected to a VCR and monitor of the same make and model as the system used on site.

2. A mockup of a canopy seal housing similarin configuration to the Prairie Island design was ovedayed in a similar configuration as the mpair performed at the site. A.0005 inch diameter wire and a.001 inch tilamder wire each,.4 inches long, were taped to the surface of the wcld overlay on the housing.

3. %c two win:s were filmed using the weld head fmnt end, and the weld head lighting for illumination.

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4. WSrs site QC representative, Gary Caul, reviewed the tape and was able to e both wires on the surface of the weld.

A copy of the tape, samples of the two wires used, and additional camera information me included with this letter for your myiew. Please !ct me know ifyou need addit!onal information.

Sincerely,

% g.44 PedmE. Amador SeniorProjectManager l

3504%2 DOC

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