ML20154A635

Steam Generator U-Bend Tube Fatigue Evaluation

ML20154A635
Person / Time
Site:	San Onofre
Issue date:	07/31/1988
From:	Hall J, Hopkins G, Hu M WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
To:
Shared Package
ML13330B376	List: ML20154A593 ML20154A635
References
WCAP-11906, NUDOCS 8809130036
Download: ML20154A635 (14)

Text

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WESUNCHOUSE CLASS 3 WCAP 11906 .

l San Onofre Unit 1 }

l Steam Generator U Bend Tube 4

i Fatigue Evaluation l t

i L

4

+ J. M. Hall .

G. W. Hopkins  :

M.H.Hu

M. R. Patel  !

I H. W. Yant  !

t

)  !

July less i i f i

i Westinghouse Electric Corporation j

' Service Technology Department '

P. O. Box 3377 '

f00280ck$$88$j6 152 PN0 '

e 1.0 SU MARY AND CONCLUSIONS

. The San Onofre Unit I steam generators have been evaluated for the susceptibility of unsupported U bend tubing 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. The evaluation used Eddy Current Test (ECT) data supplied by Southern California Edison Company.

1.1 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 instablitty. This condition is believed to have prevailed in the R9C51 tube since 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 (AVB), had a higher flow field due to local flow peaking as a result of non uniform insertion depths of AVBs, had reduced damping due to denting 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.

1.2 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 uplitude (to < 4.0 ksi) for a Row 9 tube in the North Anna 1 steam generators (SG's). 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 40 year fatigue life objective. This same fatigue criteria is applied as the principal criteria in the evaluation of San Onofre tubing.

C24?m/0715SS:4M

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The fluidelastic stability ratio is the ratio of the effective velocity divided '

]

by the critical velocity. A value greater than unity (1.0) indicates [

instability. The stress ratio is the expected stress amplitude in a San Onofre j

tube divided by the stress amplitude for the North Anna 1, R9C51 tube, t

! {

. Displacements are computed for tne unseported U bend tubes in Rows 14, 13, 12, i

! and in some cases, Rows smaller than 12, using relative stability ratios to l

] 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 stiffness add frequency and, therefore, different stress  ;

and fatigue usage 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  ;

i fatigue usage for the San Onofre tubing for the reference fuel cycle analyzed.  ;

Therefore, a stress ratio less than 1.0 provides the next level of acceptance I criteria for unsupported tubes fr which the relative stability ratio,  ;

including flow peaking, exced 0.9.  !

l 1 e

) The stability ratios for San Onofre tubing, the corresponding stress and [

l-amplitude, and the resulting cumulative fatigue usage must be evaluated relative to the ruptured tube at Row 9 Column 51, North Anna 1, Steam i

Generator C, for two reasons. The local effect on the flow field due to l f j various AVB insertion depths is not within the capability of available analysis j i

techniques and is determined by test as a ratio between two AVB [

i configurations. In addition, an analysis ::.d examination of the ruptured tube l j at North Anna 1 provided a range of initiating stress amplitudes, but could ,

e only bound the possible stability ratios that correspond to these stress l

4 amplitudes. Therefore, to minimize the influence of uncertainties, the j

evaluation of San Onofre tubing has been based on relative stability ratios,  !

t relative flow peaking factors, and relative stress ratios.  !

i i t

i The criteria for establishing that a tube has support from an AVB and therefore 1 .

eliminate it from further considerations is that at least one sided AVB support f I

is present at the tube centerline. Since the San Onofre AVBs have a round j cross section, the centerline of the AVB must be at, or below tha centerline of [

1 a given tube to provide tube support. Test results show that one sided AVB f

I ewmnmm.,

); ,

2 i

support is sufficient to 11.ait the vibration amplitude for fluideiastic excitation. AVB support is established by analysis of eddy current (EC) measurements and is a key factor in the determining the local flow peaking factors. The local flow peaking produces increased local velocities which cause an increase in stability ratio. A small percentage change in the stability ratio causes a significant change in stress amplitude. The relative flow peaking futors for San Onofre tubing without direct AVB support have been determined by test. These flow peaking factors normalized to the North Anna R9C51 peaking, are applied to relative stability ratios determined by 3 0 tuM bundle flow analysis, to obtain the combined relative stability ratio used in the stress ratio determination.

1.3 Denting Evaluation The analyses of eddy current (EC) measurements show numerous tubes with denting induced deformation at the top support plate. The majority of the tubes show the presence of top tube support plate corrosion and magnetite in the crevice.

Therefore, for conservatism in the evaluation, all of the tubes evaluated are postulated as being dented. The effect of denting on the fatigue usage of the tube hcs been conservatively maximized by assuming the maximum effect of mean stress in the tube fatigue usage evaluation and by incorporating reduced damping in the tube vibration evaluation.

1.4 AVD Insertion Depths The San Onofre Unit 1 SGs have two sets of AV8s. The original design AVBs

extud into the tube bundle approximately as far as Row 14, and provide a nomir.al total clearance between a tube without ovality and the surrounding AVBs of ( ]a,c inch. For the inner row tubes of interest to this study, significant tube ovality from tube bending exists and it is estimated (based on comparable tube data for Model 51 SG) that the nominal gap is less than

( Ja.c inch. The supplemental AVB's, which were field installed, have a square cross section, extend into the tube bundle approximately as far as Row 21, and provide a nominal tube to AVB clearance comparable to or less than the l

original AV35. Since the purpose of this analysis is to evaluate the potentially unsupported tubes at or near the point of maximt.m AVB insertion, only the dimensions and EC data pertaining to the original AVB's is required.

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The eddy current data, supplied by Southern California Edi.on Company, wero l

reviewed to identify the number of tube /AVB intersections and the location of l these intersections relative to the apex of a given tube. This information was  ;

j ,

used in calculations to determine the deepest penetration of a given AVB into  ;

i the tube bundle. For the area of interest in the San Onofre steam generators. [

the AVB suppcrt of the tube can normally be verified if EC data shows both legs of an AVB, one on each side (hot leg cold leg) of the U bend. This data, i indicated by a listing of two or more AVBs, in the insertion depth plots, is

] the method of choice for establishing tube support. l l

If only the apex of an AVB assembly is near, or touching the apex of a tube  :

U bend, only one AVB signal may be seen. In this case, adequate tube support l cannot be assumed withcut supplemental input. Support can be determined if  !

! ' projection' calculations, based on AVB intercepts of higher row number tubes  !

l in the same and adjacent columns, verify insertion depth to a point below the l tube centerline. Maps of the AVB insertion depths are shown in Figures 1, 2, r and 3. These AVB maps list the results of the ' projection' calculations where l

this information contributes to understanding of the AVB insertion depth.  !

I I

l- 1.5 Flow Peaking Factors f

i

,- Tests were performed modeling San Onofre, Series 27 SG tube and AVB geometries l to determine the flow peaking factors for various AVB configurations relative l to the North Anna R9C51 peaking factor. The test results were used to define an upper bound of the ratio relative to the R9C51 configuration. It was found [

j that the worst case flow peaking results for the San Onofre AVB patterns were  !

less than the peaking factor determined for R9C51 (by more than 15 percent).

1 l 1.6 Tube Vibration Evaluation The calculation of relative stability ratios for San Onofre makes use of detailed tube bundle flow field information computed by the ATH0S steam

. generator thermal / hydraulic ant. lysis code. Code output includes threw-j dimensional distributions of secondary side velocity, density, and void fraction, along with primary fluid and tube wall temperatures. Distributions l- (

l i

• 24 N C71H4:43 4 h

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of thest parameters have been generated for nyery tube of interest in the San Onofre tube bundle based on recent full power operating conditions. This l information was factored into the tube vibrat. ion analysis leading to the relative stability ratios.

1 . Relative stability ratios of San Onofre (Row 12 through Row 16) tubing versus l R9C51 of North Anna 1 are plotted in Figure 4. These relative stability ratios include relative flow peaking factors. The stress ratios in Figure 5 also L 2

include the relative flow peaking effect. Fcr all three steam generators, with the exception of Row 15 tubes in Columns 4 ar.d 97 (that is, a total of six tubes), the stress ratios for all tubes in Rcw 15 and below is less than 1.0, l even when the tubes are assumed to be unsuppcrted. l

]

The original San Onofre AVB design has a round cross section and a nominal gap

] of 0.024 inch adjusted for inner row tube ovality. Therefore, even if a tube is considered supported based on the EC data, depending on'how the various  ;

~

l design tolerances stack up, it may undergo large vibrating amplitude prior to (or without) contacting the AVBs. Hence for "supported" tubes which exceed a j stress ratio of 1.0, an additional requiremer,t is that the nominal gap plus l l tolerances will limit the tube amplitude to a value corresponding to less than I l 4.0 ksi alternating stress. j i  !

l Assuming random design tolerances for the AVil, tube, TSP tube hole diameter and l

j pitch and nominal gap, the maximum apex displacement of a tube without AVB l

,'t .c inch (corresponding to the a

contact in Row 15 is calculated to be (  !

i maximum gap of approximately ( Ja,c inch at the AVB) in the absence of  !

any tube wear at the AVB. It can be noted that the maximum gap would have to I occur on both sides of the tube or the smallor gap would limit the vibration l amplitude. The resulting maximum bending stress at the top of the TSP is  !

l approximately( Ja.c ksi, which is less than [ ]a.c ksi corresponding l to the stress ratio of 1.0. With the exce) tion of the tube at R15C97 in SG C, i which has been previously plugged, none of the other five tubes in Row 15 with -

I j

stress ratio above 1.0 currently show any indication of wear at the AVS, The  ;

, vibration amplitude of those tubes is therefore limited to a value '

corresponding to an alternating stress of ( Ja.c ksi or less. Hence, the j tubes are acceptable for continued service, f

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In order for tubes in Row 16 and above to reach the limiting stress of 4.0 ksi, the gap at the AVB would have to exceed ( Ja. cinch,morethantwicethe nominal design gap adjusted for tube ovality. Again, incorporating tolerance l

. affects, the tube would have to undergo a wear of ( Ja,c inch or greater on both sides to reach a stress amplitude of 4 ksi. Based on Tech. Spec.

- plugging criteria, this magnitude of wear would lead to tube plugging '

regardless of the AVB support condition. With the exception of the tube at R16C93 in SG A, which has an indication of wear and the tube at R16C97 in SG C

(

which has previously been plugged, none of the tubes that exceed a stress ratio of 1.0 when assumed to be unsupported, currently show any indication of wear.

The alternating stress in these tubes will be limited to less than 4.0 ksi and  ;

therefore they are acceptablG.

The two tubes in SG C, at R15C97 and R16C97, were plugged because of restriction at the first support plate, and had no prior history of tube wear at the AVB. ihe tube at R16C93 in SG A is active with the wear indication acceptable per Tech. Spec. requirements, indicating that on the basis of wear, the stress amplitude is less than 4 ksi. Therefore, no further actions are required for these tubes. Overall, it can be expected that tube plugging due to wear of AVBs would occur before a 4 ksi stress amplitude would be exceeded.

A sumary listing of the remaining unsupported critical tubes evaluated is  !

given in Table 1. For those tubes, the maximum relative stability ratios and j stress ratios occur in Row 14 tubes of Columns 36 in SG A 65 in SG B and 34,

[

37 and 61 in SG C. Tube R14C36 in SG A is currently plugged.

L The maximum cumulative fatigue usage for a 30 year operating period is i calculated to be 0.088 for'the highest stressed, unsupported tube with a stress l ratio of 0.7. This is less than 1.0. Therefore, the tube is acceptable for ,

continued service. The fatigue calculation utilizes plant oporting history to  !

date and assumed future operation at 100% availability with current fuel cycle parameters.  :

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i Examination of Figure 5 and Table I shows that none of the unsupported tubes I

• f exceed the limiting criteria, and thus none are recommended for plugging.

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, e l 1.7 Overall Conclusion l The analysis described above, using data supplied by Southern' California Editon fI i

j, Company indicates that the San Onofre tubes are not expected to be susceptible

] to fatigue rupture at the top support plate in a manner similar to the rupture ,

, which occurred at North Anna 1. Therefore, no modification, preventive tube  !

i plugging, or other measure to preclude such an event is judged to be necessary.

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SCE- STRESS RATIO SCE/NAR9C51 (MEVF DEP ZETA, MTH DENTING) 2 ,

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1.9 -

1.8 -

1.7 -

1.6 -

I 1.5 -

, 1.4 -

l 1.3 -

9 1.2 -

h 1.1 -

l m 1- .

3. b o.9 -

o e E 15 0.8 -

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, oi 0.7 -

jp o.6 -

0.5 -

o.4 -

o.3 -

o.2 -

O.1 -

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~3 O 20 i / tot 7 i 40/61 i . COLUMN NUMBER e o e v.- - -,-

Table 1 San Onofre Unit 1 - Critical Tube Summary

• RELATIVE RELATIVE RELATIVE FLOW STABILITY STRESS *

, EG RQW COLUMN PEAKING RATIO RATIO

, s A 14 36 a,c ,94 ,7o All other inner row tubes s.85 1 41

(< Row 16)

B 13 69 .85 .47 14 65 .94 .70 All other inner row tubes 1 85 1 41

(< Row 16)

. C 13 10 .85 .46 22 .69 .14 25 .69 .14 14 34 .94 .70 37 .94 .70 61 .94 .70 All other inner row tubes -

1 85 s.41

(< Row 16)

• All relative stress ratios less thar or equal to 1.0 are acceptable.

024FM/071548:49 8 (6