ML20205S907

Steam Generator U-Bend Tube Fatigue Evaluation

ML20205S907
Person / Time
Site:	Diablo Canyon
Issue date:	10/31/1988
From:	Hall J, Hu M, Alexandra Smith WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
To:
Shared Package
ML16341E840	List: ML20205S907
References
WCAP-12033, NUDOCS 8811140047
Download: ML20205S907 (21)

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' ATTACHMENT 2 NCAP-f2033. "Diablo Canyon t.vit 2 Steam Generator U-Bend Tube Fatigue Evaluation" (Non-Proprietary) s 5

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WESTINGHOUSE PROPRTETARY CLA55 3 l

WCAP 12033 l

DIABLO CANYON UNIT 2 STEAM SENERATOR U BEND TURE FATIGUE EVALUATION October 1988 AUTHOR $t J. M. Hall N. H. Hu ,

A. C. Smith j APPROVED: Id ~b T. A. PITilRLE, MANAGER STEAM GENERATOR ENGINEERIN3 This document contains information proprietary to Westinghouse Electric Corporation. It is submitted in confidence and is to be used soley for the purpose for which it is furnished and is to be raturned upon request. This document and such information is not to be reproduced, transmitted, disclosed or used otherwise in whole or in part without written authorization of Westinghouse Electric Corporation Power Systems Business Unit.

WESTINGHOUSE ELECTRIC CORPORATION SERVICE TECHNOLOGY DIV!510N '

P. O. 80X 3377 PITT5 BURGH, PEM 5YLVANIA 15230 2

02994:49/102788 1 i

DIABLO CANYON UNIT 2 STEAM GENERATOR U BEND TUBE FATIGUE EVALUATION The Diablo Canyon 2 steam generators have been evaluated for the susceptibility of unsupported U-bend tubes with denting at the top tube support plate to a fatigue rupture of the type experienced at Row 9 Column 51 (R9C51) of Steam Generator C North Anna 1.

1.0 BACKGROUND

The initiation of the circumferential crack in the tube at the top of the top tube support plate at North Anna 1 has been attributed to limited displacement, fluid elastic instability. The unstable condition prevailed in the R9C51 tube when the tube experienced denting at the support plate. A combination of conditions were present that led to the rupture. The tube was not supported by an anti-vibration bar (AYB), had a higher flow field due to local flow peaking as a result of no uniform insertion depths of nearby AVB's, had reduced damping due to centing at the top support plate, and had reduced fatigue properties due to the environment of the all volatile treatment (AVT) chemistry of the secondary water and the additional mean stress from the denting.

2.0 EVALUATION CRITERIA The criteria established to provide a fatigue usage less than 1.0 for a finite period of time (i.e., 40 years) is a 10% reduction in stability ratio that provides at least a 58% reduction in stress amplitude (to < 4.0 ksi) for a Row 9 tube in the North Anna 1 steam generators. This reduction is required to produce a fatigue usage of < 0.021 per year for a Row 9 tube in North Anna and therefore greater than a 40 year fatigue objective. This same criteria is being applied as the principal criteria in the evaluation of Diablo Canyon tubing.

The determination of stability ratio is the evaluation of a ratio of velocities, the effective velocity divided by the critical velocity. A value grenerthanunity(1.0)indicatesinstability. The stress ratio is the 0299u:49/102788 2

expected stress amplitude in a Diablo Canyon tube divided by the stress amplitude for the North Anna 1. R9C51 tube.

A stress amplitude that corresponds to a stability ratio equivalent to 10%

lower than R9C51 of North Anna 1 is required of Row 9 tubes in Diablo Canyon 2 as we'.1 as Row 8 and smaller U bend radius tubes. Displacements are computed for these tubes using relative stability ratios to R9C51 of North Anna 1 and an appropriate power law relationship based on instability displacement versus flow velocity. Different U bend radius tubes will have different stiff.. ass and frequency and, therefore, different stress and fatigue ussge per year than the Row 9 North Anna tube. These effects are accounted for in a stress ratio technique. The stress ratio is formulated so that a stress ratio of 1.0 or less produces acceptable stress amplitudes and fatigue usage for the Diablo Canyon 2 tubing for the reference fuel cycle analyzed.

The stability ratios for Diablo Canyon 2 tubing, the corresponding stress and amplitude, and the resulting cumulative future fatigue usage must be evaluated relative to the ruptured tube at Row 9 Column 51, North Anna 1, Steam Generator C, for two reasons. The local effect on the flow fleid due to various AVB insertion depths is not within tne capability of available analysis techniques and is determined by test as a ratio between two AVB configurations. In addition, an analysis and examination of the ruptured tube at North Anna 1 provided a range of initiating stress esplitudes, but could only bound the possible stability ratios that correspond to these stress amplitudes. Therefore, to minimize the influence af uncertainties, the evaluation of Diablo Canyon 2 tubing has been based on relative stability r&tios, relative flow peaking factors, and relative stress ratios.

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The criteria for establishing that a tube has support from an AVB and therefore eliminate it from further considerations is that at least one sided support is present to the tube centerline. Test results show that one sided AVB support is sufficient to limit the vibration amplitude for fluidelastic excitation.

AVB support at the tube centerline is established by analysis of eddy current (EC) measurements and is a key factor in the assessment af fatigue failure susceptibility since the AVB positions determine the local flow peaking factors. The local flow peaking factor is a direct factor on the apparent c299N49/ir"Se 3

stability ratio and a small percentage change causes a significant change in stress amplitude. The relative flow peaking factors for Diablo Canyon 2 tubing without direct AVB support have been determined by instability tests. These factors have been applied to relative stability ratios determined by 3 D tube bundle flow analysis using Diablo Canyon 2 conditions and the combined relative stability ratio used in the stress ratio deterutnation.

3.0 DENTING AND WEAR EVALUATION No denting has been found in any of the Diablo Canyon 2 steam generators among the rows subject to the present evaluation. Some deposits in crevices are present based on eddy current inspection and thus those tubes with deposits are conservatively censidered to be clamped for tube fatigue considerations.

The eday current test data indicates no small radius tubes with wall thinning at AVB locations. Thus, it is unlikely that t' te tubes with AVB support have been unstable.

4.0 AVB INSERTION DEPTHS

The eddy current data were reviewed to identify the location of the

intersections of the AVBs with the tubes and to provide angular data on AVB l locations for use in calculations to determine the deepest penetration of the l AVBs. For an AYB supported tube in the Diablo Canyon steam generators, one leg of each AYB assembly should be identified on each of the hot and the cold leg l side of the U bend. Thus, if both legs of an AVB assembly are identified in the EC e"aluation, AVB support at the tube centerline may bt assumed, thus l limiting potential tube vibration amplitudes. If the bottom of an AVB assembly is near a tube, only one AVB signal may be seen (which can represent the i

continuous contact of the curved lower end of the AVB) near the apex of the l U bend, in this case, tube support cannot be confirmed without supporting projection calculations based on AYB intersections of higher row number tubes in the same column. A typical eedy current record, indicating the presence of an AVB, is shown in Figure 1.

0299e:49/102738 4

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eddy current inspection data from the 1987 and 1988 inspections were utilized for this evaluation. The 1987 inspection covered nearly 100% of the tubes in i Rows 8 to 12 for each steam gener' or. Based on the 1987 data, PGLE provided AV8 position data. Tubes potentiJly susceptible to tube fatigue were ide3tified from the 1987 data and these tube locations were reinspected in the 1988 inspection to resolve AVB position uncertainties. The 1988 inspection yield somewhat less noisy data with alternate frequencies used to enhance AYB detectability at the identified tube locations. AYB arc length data were obtained at the identified locations for use in projections of AVB insertion depths.

Based on the Diablo Canyon 2 tube vibration analysis, very large flow peaking factors would be required in Row 8 and Row 9 tubes to be of concern for tube fatigue considerations. For example, Rows 8 and 9 tubes would require flow peaking factors on the order of [ ]a c and [ la.c. respectively, to be of ccncern for tube fatigue. Therefore, emphasis on establishing AVB insertion depth accuracy emphasized Row 10 or larger tubes and Row 9 tubes potentially subject to high flow peaking factors. For example, the 1 AVB indications in Figure 3. Row 9. Columns 41 to 53 do not need to be resolved for AV8 depth as the potential peaking factors given any resolution would be less than[ Ja.c, The evaluation of Tube R9C60 in SG2 (See Figure 3) can be used as an example of the use of AYB projection methods to resolve tube support and flow peaking factor considerations. Projected AVB depths [

Most tubes do not require this detail to evaluate tube fatigue 0299mua9/102788 5

susceptibility so that this depth of analysis is not always performed. This l example indicates the use of projections to resolve uncertainties on AVB l positions and depth.

Maps of the AVB insertion depths are shown in Figures 2 through 5. All of the key unsupported tubes potentially subject to tube fatigue are listed in Table 1.

5.0 FLOW PEAKING FACTORS i

Tests were performed to determine the flow peaking factors for Diablo Canyon 2 AVB configurations relative to the North Anna R9C51 peaking factor. The tests modeled the AVB design comon to Diablo Canyon 2 and North Anna 1 and focused on representative patterns of the variations of the AVB depth of insertion.

The flow peaking factors used for the Diablo Canyon 2 evaluation are she a in Figure 6.

6.0 TUBE VIBRATION EVALUATION The calculation c/ relative stability ratios for Diablo Canyon 2 makes use of detailed tube bundle flow field information computed by the ATHOS steam generator thermal / hydraulic analysis code. Cod 6 output includes three-dimensional distributions of secondary side velocity, density, and void fraction, along with primary fluid and tube wall temperatures. Distributions of these parameters have been generated for every tube of interest in the Diablo Canyon 2 tube bundle based on recent full porer operating conditions.

This information was factored into the tube vibration analysis leading to the relative stability ratios. Actual stability ratios are calculated with the FASTVIB computer code. This code makes use of the ATH0S Senerated 7/H data and also uses void fraction dependent damping data obtained from tests.

Relative stability ratios of Diablo Canygn 2 tubing for Rows 8 through 12 (relativetoR9C51ofNorthAnna1)areplottedinFigure7. These relative stability ratios include relative flow peaking factors and assume the tubes in each colum are unsupported. Stress ratios are plotted in Figure 8 and Figure

9. These plots include the relative flow peaking effect. Figure 8 shows 02994t49/102788 6

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stress' ratio plots vs column for Rows 8 through 12 for the ' dented' condition providing a visual example of the stress ratios w/ denting column from Table 1.  ;

F.igure g shows stress ratio plots for the undented con (ition (Ref. Table 1 also),

it is to be noted that the stability and stress ratio plots in Figures 7 through 9 are composites of all four generators. That is, any peaking effect shown on these plots corresponds to the maximum peaking effect of the four steam generators in a given column location. These figures should therefore be used in conjunction with Table 7 to identify the critical tubes in individual I steam generators.  ;

i As can be observed in Table 1 Figure 8, and Figure g, a total of five tubes, all of them in SG 3, namely R9C37. R10C37 R10C43. R11C43, and R12C43: exceed the allowable stress ratio value of 1.00. It is recomended that these tubes be removed from service. It may be noted that the condition of exceeding the stress ratio criteria is independent of whether the tube / TSP interface is

' dented' or not. All other tubes meet the stress ratio acceptance criteria even assuming denting with tube deformation.

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The tube remaining in service with the largest stress ratio, that does not exceed the 1.0 stress ratio criterion, is the R9C35 tube in $6 2. This tube f

has a stress ratio of 0.85 without denting, and 0.95 if dented. The calculated l

cumulative usage for this tube, assuming denting, and future operation at l

l current cycle parameters with 100% availability is 0.56. This indicates that i there is sufficient margin in these tubes to allow continued operation of these tubes for the remainder of the plant license period.

7.0 CVERALL CONCLUSION l

The Diablo Canyon Unit 2 tube fatigue evaluation identified five tubes requiring corrective action under dented or clamped tube / TSP interface conditions. These tubes are all in steam generator 3, and are R9C37. R10C37 R10C43. R11C43 and R12C43. Of these tubes, R9C37 R10C43, and R11C43 show evidence of crevice " gnetite by eddy current inspection, and should be removed from service during the current outage. Tubes R10C37 and R12C43 show no 0299m 49/102784 7

evidence of crevice corrosion. However, as a preventative measure relative to potential future crevice corrosion, it is recomended that R10C37 and R12C43 also be removed from service during the current outage. These tubes may be l removed from service using either sentinel plugs, or by installation of cable stabilizers and conventional tube plugs.

8.0 SELECTED CORRECTIVE ACTION The corrective action selected for Diabic Canyon Unit 2 is shown in Figure 10. t This includes installation of cable dampers and solid mechanical plugs in the four tubes with significant stress ratios (R10C37. R10C43. R11C43, and R12C43). Potentially susceptible tube RgC37 has a sentinel plug installed. In addition, the remaining tubes ' boxing in' the tubes with installed cable dampers are removed from service using ser.linel plugs. This approach supplements the cable installation by providing capability for identifying propagation of cracks in tha event that any of the tube fatigue sensitive tubes should rupture, and impact surrounding tubes. The installation of solid plugs with cable dampers also minimizes the potential for tube leakage and unplanned outages in the event that a crack is initiated in the susceptible tubes and does not propagate to a rupture, or does not lead or affect adjacent tubes.

The cable damper selected for installation in the Diablo Canyon Unit 2 is a <

. ,a,c cable design selected on the basis of tube damping tests comparing three different cable designs. On the basis of ' pluck tests" to measure tube I

damping, the ]cabledesignwasfoundtoyieldthelargestincreaseintube damping of the designs tested. Shaker tests to measure tube damping under constant amplitude conditions are on-going to finalize the long tera quantification of tube damping with the cable installed. The added damping obtained with the cable installed significantly reduces the potential for tube rupture of the susceptible tubes and thus further reduces the potential for a controlled leakage event based on sentinel plug leak rates on the order of 270 i

gallons per day.

02994:49/102788 a

Row Column Flow Fbw Relative Stress RED Peaking Peaking Stabiltty Factor Rat'c Ratio WIDenting W/O Derting a.c SG 1 8 31 0.62 0.16 0.14 35 0.65 0.19 0.17 64 0.62 0.16 0.14 7

9 81 0.72 0.29 026 SG 2 8 60 0.62 0.16 0.14 64 0.70 0.31 0.28 9 12 0.70 0.25 0.22 13 0 70 0.24 0.22 35 0.89 0.95 0.55 SG 3 8 31 0.70 0.31 0.28 9 37 0.95 1.43 1.28

. 43 0.71 0.26 0.23 10 37 1.20 >10.00 >10.00 43 1.22 >10.00 >10.00 1 11 43 1.40 >10.03 >10.00 12 43 1.60 >10.00 >10.00 ,

SG 4 8 31 0.62 0.16 0.14 I 35 0.62 0.16 0.14 l 64 0.70 0.31 0 28 ,

9 60 0.65 0.17 0.15 69 0.89 0.93 0.83 4

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Table 1 ,

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ATTACHMENT 3 f Westinghouse Anathorization Letter CAN-88-120.  !

Proprietary Information Notice, and Accompanying Affidavit CAN-88-106 }

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