ML13308B220

Monticello - Enclosure 4 - Structural Integrity Associates, Inc. Evaluation File No. 1300180.301 - Evaluation of Shear Capacity of Monticello Shroud Welds H8 and H9

ML13308B220
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Site:	Monticello
Issue date:	04/04/2013
From:	Wong W Structural Integrity Associates
To:	Office of Nuclear Reactor Regulation
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ML13308B206	List: L-MT-13-102, Cycle 26 Inservice Inspection and In-Vessel Visual Inspection Summary Reports ML13308B217 ML13308B218 ML13308B219 ML13308B220 ML13308B221
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L-MT-13-102 1300180.301
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ENCLOSURE 4 Monticello Nuclear Generating Plant Structural Integrity Associates, Inc. Evaluation File No.: 1300180.301 Evaluation of Shear Capacity of Monticello Shroud Welds H8 and H9 (Non-Proprietary)

(11 pages follow)

Structural Integrity Associates, Inc.! File No.: 1300180.301 Project No.: 1300180 CALCULATION PACKAGE Quality Program: 0 Nuclear El Commercial PROJECT NAME: Evaluation of the Monticello Shroud Support Plate Uplift Load with Indications in the H8 and H9 Welds CONTRACT NO.: 00001005 CLIENT: PLANT: XCEL Energy Monticello Nuclear Generating Plant CALCULATION TITLE: Evaluation of Shear Capacity of Monticello Shroud Welds H8 and H9 Document Affected Project Manager Preparer(s)

&Revision Pages Revision Description Approval Checker(s)

Signature

& Date Signatures

& Date 0 1 -11 Initial Issue Preparer: Wilson Wong Aparna Alleshwaram 03/27/13 03/27/13 Checker: Jim Wu 03/27/13 1 1 -11 Responded to Client Preparer: Comments-Wilson Wong Editorial Only Aparna Alleshwaram 04/02/13 04/02/13 Checker: Jim Wu 04/02/13 2 1 -11 Changed Section XI Preparer: Code Year- W Slight Safety Factor Modification IWilson Wong Aparna Alleshwaram 04/04/13 04/04/13 Checker: Jim Wu 04/04/13 Page 1 of 11 F0306-OIRI IClotW qdly Assocatesj 1xi Table of Contents 1.0 IN TRO D U CTION ...................................................................................................

4 2.0 TECHN ICA L A PPRO A CH .....................................................................................

4 2.1 Lim it Load A pproach ...................................................................................

4 2.2 Evaluation Cases ...........................................................................................

5 3.0 A SSU M PTION S ........................................................................................................

5 4.0 D ESIGN IN PU TS ......................................................................................................

6 5.0 CA LCU LA TION S ....................................................................................................

6 5.1 W eld Lengths .................................................................................................

6 5.2 Total Load for Shear ......................................................................................

7 5.2.1 Uplift Load ...................................................................................................

7 5.2.2 Vertical Seism ic Load ....................................................................................

7 5.2.3 Acoustic Loading ..........................................................................................

7 5.3 Crack G rowth ...............................................................................................

7 6.0 RESU LTS O F AN A LY SIS ......................................................................................

8 7.0 CO N CLU SION S A N D D ISCU SSION S .................................................................

8 8.0 RE FEREN CES ......................................................................................................

9 File No.: 1300180.301 Revision:

2 Page 2 of 11 F0306-O1RI List of Tables Table 1: Load Sum m ary .....................................................................................................

10 Table 2: Material Properties at 550 'F [8] ..........................................................................

10 Table 3: Pressure Differential across the Shroud Support Plate [6] ..................................

10 Table 4: Uplift Load on Shroud Support Plate ...................................................................

10 Table 5: V ertical Seism ic Load ..........................................................................................

10 Table 6: Total U pw ard Shear Force .........................................................................................

11 Table 7: Limit Load Evaluation Results for Compound Crack Profile ..................................

11 Table 8: Limit Load Evaluation Results for Surface Crack Profile ........................................

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$WnVW ht tgIly ASSOCafS, incg

1.0 INTRODUCTION

During the Spring 2013 outage, visual inspection of the Monticello shroud support plate weld H8 and weld H9 will be performed.

This calculation is performed to determine the minimum required percentage of H8 and H9 welds that need to be free of through-wall indications in order to maintain an adequate structural margin for at least one cycle of operation.

Plastic collapse in shear is assumed to be the applicable failure mode because the most significant loading is the uplift load due to the vertical seismic load, acoustic load, and pressure difference across the shroud support plate.2.0 TECHNICAL APPROACH 2.1 Limit Load Approach The technical approach used for this evaluation is based on the BWRVIP-76, limit load approach [1].The limit load analysis for shear failure is developed based on the approach for determining the limit load for an axial crack in the shroud as presented in [1] and summarized below. Consistent with the BWRVIP methodology in [1], the failure mode of the Alloy 600 and corresponding weld materials is considered to be net section (plastic) collapse, because of the high ductility of these materials at reactor operating temperatures.

Also, the fluence in this region is not high enough to impact the material ductility.

The limit load for an axial crack assuming tensile failure of the remaining ligament is expressed in Section E.1.2 of [1] as: By similar approach, the shear failure limit load for a crack in a circular weld can be expressed as: (SF)S = a-IfLt (2)where: S = shear force due to uplift load SF = safety factor Gsf = shear flow stress L = length of uncracked circumference in circular welds (H8/H9)t = thickness of shroud support plate.Using the maximum shear theory or Tresca criteria, the maximum shear stress at yield is half the maximum yield strength.

The shear flow stress can be expressed as: File No.: 1300180.301 Page 4 of 11 Revision:

2 F0306-OIRI 11h0tegril~Y ASSOciaes, Inc Osf = O/2 Per Reference9, required minimum safety factors of 2.4 for normal/upset (Level A/B) conditions and 1.4 for emergency/faulted (Level C/D) conditions are used in this evaluation.

2.2 Evaluation

Cases Two crack profiles are used to evaluate the structural margin retaining the shroud support plate weld H8 and weld H9. Each of these profiles contain significant margin, which compensate for any uncertainty in the flaw depths. It is important to note that BWR shroud cracking history, of which there is a significant amount, has shown that cracks in shroud welds typically grow to approximately two-thirds of the shroud wall and then appear to become essentially inactive.

This is particularly expected for Monticello's case because of the excellent water chemistry experienced in the vicinity of the indications on the lower side of H8 and H9. The two crack profiles considered in this evaluation are: (a) Compound Crack: A through-wall crack is postulated in the uninspected regions and a remaining ligament of 1/2 of the plate thickness is postulated in the inspected region. The 1/2 plate thickness is used as the remaining ligament because assuming all inspected regions are cracked two-thirds of the weld would be overly conservative.(b) Full Circumferential Surface Crack: A surface crack at the bottom plate surface extending along the circumferential length of Weld H8 and H9 with a crack depth at 75% of the support plate thickness is postulated.

This corresponds to a remaining ligament of 0.625 inches in the support plate.3.0 ASSUMPTIONS The following assumptions are used: a. Loading in weld H8 and weld H9 is assumed to be pure shear because the most significant loads are the vertical seismic and the pressure difference across the shroud support plate.b. Material properties of Alloy 600 compatible weld metal are assumed to be the same as the Alloy 600 base metal.c. The shear load is assumed to be evenly distribution between the weld H8 and weld H9.d. The shear load is assumed to be evenly distributed in the remaining ligament of each weld.e. For uninspected regions, through-wall cracking is assumed. This is conservative since no credit is taken for the uninspected regions (Only applies to compound crack case).f. The SSE accelerations are twice as large as the OBE accelerations.

g. The vertical flexural shear from the moment induced by the horizontal acceleration due to the jet pump weight on the support plate is assumed to be negligible.

It was estimated that this upward flexural shear is less than 5% of the total uplift shear load.h. The material is considered to behave in an elastic-plastic manner, which is consistent with BWRVIP methodology for reactor internals in low fluence regions.i. Crack growth in the depth direction is not considered since the assumed crack depth is based on flaw depths observed from BWR fleet operating experience.

Subsequent growth is minimal due to excellent Monticello water chemistry conditions in the lower plenum.File No.: 1300180.301 Page 5 of 11 Revision:

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j. Seismic and LOCA are combined accordingly

[11] in order to provide added margin to the evaluation and further justify the maximum flaw depth based on BWR fleet operating experience.

4.0 DESIGN

INPUTS The following dimensions are used in the evaluation:

Reactor vessel inside diameter (ID): 17.167 ft [2]Shroud plate thickness:

2.5 inches

[3]Shroud ID: 159.75 inches [4]Shroud thickness:

1.75 inches [4]Design AP across shroud support plate for Levels A through D 100 psi [2]Per Reference 2, the vertical earthquake acceleration is 0.06g.The vessel internal component loads and water loads are obtained from Reference 5 and summarized in Table 1.The maximum AP for Level A/B is 29.03 psid, and the maximum for Level C/D is 47 psid from the 113% OLTP [11]. The pressure differentials across the shroud support plate are summarized in Table 3.Per Reference 2, the Code of Construction isSection III, 1965 with Summer 1966 Addenda [6]. The allowable stress intensity (Sm) is 23.3 ksi [2]. The material yield strength (Sy), ultimate strength (SJ)and allowable stress intensity for Alloy 600 are obtained from Reference 7 at 550 'F for conservatism and summarized in Table 2. As compared to the allowable Sm from Reference 6 stated in Reference 2, the Sm from different Code Editions remains the same.The shroud support plate material is Alloy 600 [8].One cycle of operation is 24 months [2].5.0 CALCULATIONS 5.1 Weld Lengths The lengths of weld H8 and H9 are calculated below.Length of weld H8 = 21TRi=2*iT(159.75/2+l.75)=

2*"c(81.625)

= 512.865 in Length of weld H9 = 21TrRo=2*Tr(17.167*

12/2)= 2*1T(10 3.00 2) = 647.18 in To simplify, an average length for welds H8 and H9 is used for evaluation.

Average weld length for welds H8 and H9 = (512.865+647.18)/2

=580.02 inch Using the average weld length, the following are obtained for each weld: File No.: 1300180.301 Page 6 of 11 Revision:

2 F0306-OIRI Total inspected length = Inspection Rate*580.02 Total uninspected length = 580.02 -Total Inspected Length 5.2 Total Load for Shear 5.2.1 Uplift Load The uplift load is due to the pressure difference across the shroud support plate. The AP uplift area (UA) is calculated as: UA = I(Ro2-R 2)=T(103.0022

-81.6252)

= 12,399.15 in 2 The uplift loads due to the pressure difference for Level A/B and C/D are calculated and shown in Table 4.5.2.2 Vertical Seismic Load In Table 1, it is shown that the total weight of the jet pumps is 10 kips. Also, the maximum water weight of 1,080 kips from Table 1 (b) is selected.

Thus the total weight due to internal structure

&periphery fuel, jet pumps and water weight is: Wt = 189+ 10 + 1080 =1,279 kips The vertical seismic acceleration is 0.06g. This is assumed to be for OBE.The total vertical seismic load for OBE and SSE is summarized in Table 5.The total upward shear force is summarized in Table 6.5.2.3 Acoustic Loading The total shear force on the H8 and H9 welds due to acoustic loading is 2,200 kips, per Reference 11.This loading is applicable only for the Level C/D condition, and applied as 4,400 kips (doubled) as required by the memo in Reference 11.5.3 Crack Growth Crack growth in the depth direction is not included since the depth used in the evaluation is consistent with the depths based on BWR fleet operating experience.

In addition, due to Monticello's excellent water chemistry in the lower plenum, subsequent crack growth will not be significant.

Therefore, only crack growth in the circumferential direction is considered.

Per Reference 1, the crack growth rate is 5x10 5 in/hr. This is used for conservatism regardless of plant specific water chemistry.

For 10 uninspected regions, with 2 crack fronts for each region since a through-wall crack is used, the total crack growth Alfor 24 months is: A/= I0*2*5x10 5*2*365*24

= 17.52 inches File No.: 1300180.301 Page 7 of 11 Revision:

2 F0306-01 R I IVhS&OWNly ASSiMOctS, kic?Therefore, the remaining length of un-cracked circumference at the end of one cycle of operation is One cycle of operation uncracked Length =Total Inspected Length -17.52 6.0 RESULTS OF ANALYSIS The limiting shear force due to limit load failure criteria can be calculated using Eq. (2). The flow stress is taken as 3 Si per Reference 9 as used in Reference 10.In order to meet or exceed the minimum required safety factor for the H8 and H9 welds considering the two crack profiles described above, 18% of the total weld length is required to be free of through-wall indications.

7.0 CONCLUSION

S AND DISCUSSIONS An analysis was performed to determine the minimum required percentage of the H8 and H9 welds to be free of through-wall indications in order to maintain an adequate structural margin for at least one cycle of additional operation.

For a 24 month cycle of additional operation, 18% of the total weld length is required to be free of through-wall indications for the two conservative flaw configurations analyzed.It must be noted that during the Spring 2013 inspections, an average of 34% of the H8 and H9 welds were inspected and found to be free of indications

[11]. Using the industry accepted crack growth rate of 5x 10-- in/hr per Reference 1 combined with the same calculation method above, structural margin is still maintained after a period of 12 years.File No.: 1300180.301 Revision:

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8.0 REFERENCES

1. BWR Vessel and Internals Project: BWR Core Shroud Inspection and Flaw Evaluation Guidelines (BWRVIP-76), EPRI, Palo Alto, CA, BWRVIP 1999, TR-1 14232.2. Xcel Energy Design Information Transmittal (DIT), "Shroud Support Plate Uplift Analysis," Tracking Number EC, Date 5/2/2011, DIT No. 3, SI File 1100626.207.

3. Chicago Bridge & Iron Co. Drawing 35 Rev 5, "Plan of Shroud Support 17'2" ID, 63'-2" INS Heads Nuclear Reactor," No. NX-9310-28, SI File 1100626.201.

4. General Electric Drawing 886D487, "Reactor Vessel," No. NX7831-7-2, SI File 1100626.201.

5. Monticello Drawing No. NX7831-7-7 (GE Drawing No. 886D482, Revision 10), "Reactor Vessel," SI File No. 1100626.201.

6. ASME, Boiler and Pressure Vessel Code,Section III, 1965 Edition with Addenda to and including Summer 1966 Addenda.7. ASME Boiler and Pressure Vessel Code,Section II, Part D, 1998 Edition with no Addenda.8. Xcel Energy DIT No. EC 18095, "Shroud Support Plate Uplift Evaluation," Rev. 1, SI File 1100626.206.

9. ASME Boiler and Pressure Vessel Code,Section XI, 2007 Edition with Addenda through 2008.10. SI Calculation, "Evaluation of the Monticello Shroud with Indications at Welds H8 and H9," SI File 1100560.301, Rev. 0.11. Xcel Energy Design Information Transmittal (DIT), "2013 Shroud Support Plate Uplift Analysis," EC 21839, Date 3-29-2013, DIT No. 21839-1, SI File 1300180.208.

File No.: 1300180.301 Revision:

2 Page 9 of 11 F0306-O1RI 1CbrOftO&gI Intgry AS~SOciesr, WOc.Table 1: Load Summary (a) Loads Supported by Internal Shroud Support [5]Component Weight (kips)Internal Structure

& Periphery Fuel 189 Guide loads (all fuel, drives, control rods, guide tubes) 397 for horizontal earthquake loads only Jet Pumps 10 (b) Water Loads [5]Operating Conditions Water Weight (kips)Normal Full Power 353.1 Hot, Stand By 381.4 Cold Vessel, Full 729.2 Refueling (water level 927") 1080 Refueling (water level 655") 651 Table 2: Material Properties at 550 'F [7]Alloy 600 Base Metal Yield Strength (ksi) 30.1 Ultimate Strength (ksi) 80 Stress Intensity Sm (ksi) 23.3 Table 3: Pressure Differential across the Shroud Support Plate [111]Level Pressure Differential (psid)A/B 29.03 C/D 47.0 Table 4: Uplift Load on Shroud Support Plate Level Area (in 2) Pressure Differential (psid) Up Force (lbs)A/B 12399.15 29.03 359947.27 C/D 12399.15 47.00 582759.95 Table 5: Vertical Seismic Load Level Coefficient Total Wt (kips) Up Force (kips)A/B 0.06 1279 76.74 C/D 0.12 1279 153.48 Table 6: Total Upward Shear Force File No.: 1300180.301 Revision:

2 Page 10 of 11 F0306-OIRI IC -SbwcOunW Intgry AS~oifSWM Wc PressureToa Vertical Seismic Total Level Differential L Acoustic Load (kips) (kips)A/B 359.95 76.74 0 436.69 C/D 582.76 153.48 4400 5136.24 Table 7: Limit Load Evaluation Results for Compound Crack Profile Level A/B Level C/D (1) EoEP uncracked length (in) 86.88 86.88 (2) Support plate thickness (in) 2.50 2.50 (3) Remaining ligament (in) 1.25 1.25 (4) Available shear area (in 2) (=(1)*(3))

108.61 108.61 (5) Total applied shear load (kips) 436.69 5136.24 (6) Applied shear in each weld (kips) (=(5)/2) 218.34 2568.12 (7) Tensile Flow Stress (ksi) 69.90 69.90 (8) Shear Flow Stress (ksi) 34.95 34.95 (9) Shear limited load (kips) (=(8)*(4))

3795.75 3795.75 (10) Safety Factor (=(9)/(6) 17.38 1.48 (11) Required Safety Factor 2.4 1.4 Table 8: Limit Load Evaluation Results for Surface Crack Profile Level A/B Level C/D (1) EoEP uncracked length (in) 580.02 580.02 (2) Support plate thickness (in) 2.50 2.50 (3) Remaining ligament (in) 0.63 0.63 (4) Available shear area (in ) (I=)*(3))

362.51 362.51 (5) Total applied shear load (kips) 436.69 5136.24 (6) Applied shear in each weld (kips) (=(5)/2) 218.34 2568.12 (7) Tensile Flow Stress (ksi) 69.90 69.90 (8) Shear Flow Stress (ksi) 34.95 34.95 (9) Shear limited load (kips) (=(8)*(4))

12669.87 12669.87 (10) Safety Factor (=(9)/(6) 58.03 4.93 (11) Required Safety Factor 2.4 1.4 File No.: 1300180.301 Revision:

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