ML20216H034

Cycle-19 Voltage Based TSP Alternate Repair Criteria 90-Day Rept

ML20216H034
Person / Time
Site:	Prairie Island
Issue date:	03/31/1998
From:	WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
To:
Shared Package
ML20216H015	List: ML20216H009 ML20216H034
References
SG-98-03-002, SG-98-3-2, NUDOCS 9803200159
Download: ML20216H034 (62)

Text

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a SG-98-03402 Prairie Island Unit 1 Cycle -19 Voltage Based TSP Alternate Repair Criteria 90-Day Report March 1998 1

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Westinghouse Electric Company  ;

Nuclear Services Division i P.O. Box 158 Madison, Pennsylvania 15663-0158 l 1

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Table of Contents

't.0 Introduction 2.0 Summary and Conclusions 3.0 Pulled Tube Examination Results and Evaluation 3.1 Pulled Tube Examination Results 3,1,1 Introduction 3.1.2 Non-Destructive Examinations 3.1.3 Leak, Burst and Tensile Data 3.1.4 Destructive Examinations

--3.1-.S Conclusions 3.2 Evaluation of Pulled Tube Data for ARC Applications .

3.2.1 Eddy Current Data Review 3.2.2 Prairie Island -1 Data for ARC Application 3.3 Comparison of Prairie Island-l Pulled Tube Data With Existing ARC Correlations 3.3.1 Suitability for Inclusion in Data Base 3.3.2 Burst Pressure vs. Bobbin Amplitude 3.3.3 Probability of Leak 3.3.4 Leak Rate vs.~ Bobbin Amplitude 3.3.5 General Conclusions 4.0 Results EOC 18 Inspection Results and Voltage Growth Rates 4.1 EOC-18 Inspection Results 4.2 Voltage Growth Rates

.g ~ 4.3 NDE Uncertainties 5.0 Data Base Applied for ARC Correlations 6.0 SLB Analysis Methods

)" 7.0 Bob' bin Voltage Distributions 7.1 Probability of Detection (POD) 7.2 Cycle Operating Time 7.3 Calculation of Voltage Distributions -

7.4 Predicted EOC-19 Voltage Distributions -

, 8.0 Tube Leak Rate and Bu' rst Probabilities i

N 9.0 References

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!l 1.0 Introduction' .

Prairie Island Unit 1 implemented the Voltage Based Repair Criteria for ODSCC at Tube Support Plates (TSP)in November 1997 according to NRC GL 95-05, Reference 1, during the regular refueling outage after operating cycle 18. Implementation of the ARC was approved by the NRC SER, Reference 2, which permits indications of up to 2 volts to remain in service, allows a total leakage from the failed

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steam generator under postulated main steam line break (MSLB) conditions of 1 gpm at room temperature j conditions, and further requires that the EOC probability of burst be less than lx104. This report is the . l first "90 day report" following initial implementation of the TSP ODSCC ARC. j i

In accordance with the requirements of GL 95-05 Section 6.b, this report provides the results of the l metallurgical examination of tubes pulled in conjunction with implementation of the ARC (Section 3), as- i measured voltage distributions at EOC-18 and the results of voltage growth rate analysis (Section 4), and results of the analyses for the SLB leak rate and probability of burst for the mnsured EOC-18 distribution j and the projected EOC-19 distribution (Section 7). Summaries of the applica:ie approved data base and I the applicable correlations ofleakage and burst probability with bobbin voltage are contained in Sections )

5 and 6. I 2.0 Summary and Conclusions Analyses were performed to determine the predicted leak rate under postulated SLB conditions and to estimate the probability of tube burst for Prairie Island Unit 1 in support ofimplementation of attemate

! repair criteria for ODSCC at the TSP intersections in accordance with NRC GL 95-05. The analyses were  ;

performed based on the actual EOC-18 distribution and the projected EOC-19 distribution. The input basis of these analyses was the measured bobbin voltage distribution at the end of operating cycle 18, and lookback analysis of the EC data from the inspection at the end of operating cycle 17 to support evaluation of the bobbin voltage growth rates.  !

The limiting leak rate at EOC-19 for the two steam generators was predicted for SG-11 at 0.064 gpm, and the limiting probability of burst was predicted for SG-12 at 2.52x104 . SG-11 exhibits the limiting leak rate as expected due to the larger number ofindications found in this SG The predicted leak rate for SG-12 is 0.055 gpm, Similarly, it was expected that SG-12 would exhibit the limiting probability of burst since the highest voltage indication was found in SG-12. The predicted probability of burst for SG-11 is 4

<l.90x10 . The NRC SER approved allowable values for Prairie Island Unit I are I gpm leak rate and 4

lx10 probability of burst. Thus, the predicted limiting values ofleak rate and probability of burst for Prairie Island Unit 1 operating cycle 19 are well within the specified limits.

The analyses performed utilized the latest data base for ODSCC at the TSPs. The data base includes the data from the 1997 plant A-1 and A-2 pulled tubes. Also, the analysis for leak rate utilized leak rate data correlated to bobbin voltage since the NRC requirements for a correlation are now satisfied.

During the inspection, potential indications at the TSPs were called distorted support plate indications (DSI). All except two of the reported bobbin signals (DSI) were tested using the + Point MRPC. These

two tubes were removed from service because it could not be stated with certainty that volumetric 1

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indications did not exist in these two tubes. All of the MRPC confirmed indications were removed from service for one of three reasons: 1) the indications included a volumetric component,2) the tube was repaired for reasons unrelated to the ARC, or 3) the tube was pulled for ARC purposes.

The final population of DSIs includes 278 indications in SG-11 and 207 indications in SG-12. The tubes that included volumetric components were conservatively retained in the population ofindications for the leak rate and burst probability analyses due to the possibility that the indications are ODSCC or ODSCC with shallow thinning as found for pulled tube R32C41 (Section 3). If thinning is present, the voltages would be cornervatively high for the ODSCC component. The maximum voltage DS1 in SG-11 was 1.6V and was not confirmed by RPC. The maximum voltage DSI in SG-12 was 1.74 V, confirmed by RPC to be an imiication with a volumetric component.

The lookback analysis for the growth rate study yielded 250 tubes in SG-11 and 130 tubes in SG-12 with bobbin voltage calls in both the EOC-17 and EOC-18 inspections. Leak rate and burst probability analyses were performed using the combined population of 380 indication growth rates to address the GL 95-05 guidance that a minimum of 200 points should be available for a valid growth rate. Separate ,

analyses were performed for SG-11 using the combined population (380 indications) growth rate and the SG-11 population (250 indication) growth rate to confirm that the combined population growth was the more conservative (see Table 8.1).

Destructive metallographic examinations were performed on four TSP intersections contained on three tubes pulled from Prairie Island Unit-1. The EC d sta acquired during the field inspection were re-analyzed as part of the destructive examination of the pulled tubes, which also included leak and burst tests of the TSP intersections. The results of the des ructive examinations show that the degradation morphology of all of the intersections is consistent with the morphology required by GL 95-05 to be inc'9ded for applicaticn of the ODSCC ARC. All of the intergranular cormsion was confined to the TSP crevice region. One of the intersections, Tube R32C41, TSP 1, also included a zone of :ransgranular wastage, separated from the IGA within the crevice region. The wastage was found to have a maximum depth 58%. The presence of the wastage conservatively increases the bobbin voltage for the intersection, without a commensurate decrease in the burst capability of the tube. The measured voltage for this intersection was 0.85 volts, and the measured bur:t pressure for this intersection was 9.93 ksi, when normalized to 68.78 ksi flow stress. The predicted mean burst pressure for a 0.85 volt indication is rpproximat:ly 7800 psi (see Figure 3-11).

The data from two of the intersecNus (R32C41, TSP 3 and R19Cl1, TSP 1) are included in the EPRI pulled tube data base correlations for the ODSCC ARC since they pass all exclusion criteria. Intersection r R32C41, TSP i is excluded by criterion la, which excludes intersections whose eddy current signals may be corrupted due to exuaneous bobbin voltage effects other than ODSCC. Consequently, the presence of wastage excludes this intersection. Intersection R27C21, TSP 1 is excluded on the basis of criterion 2a which applies to atypical ligament morphology. Under this criterion, cracks less than 60% deep with s2 uncorroded ligaments are excluded. The R27C21, TSP 1 indication has a maximum depth of 28%, and has only one uncorroded ligament.

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The following conclusions were reached based on the analyses and examinations performed:

• The predicted limiting SLB leak rate of 0.064 gpm is less than the specified allowable leak of I gpm.

Both leak rates are stated at room temperature conditions.

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• The predicted limiting probability of burst,2.52x10 is less than the allowable probability of burst of lx10-2 ,

e Destructive examination of pulled tube intersections confinns the morphology as ODSCC and/or shallow IGA patches at the TSPs for three of the four intersections examined. The fourth intersection was found to include shallow thinning and ODSCC/ IGA with a maximum depth 58% for both types ofdegradation.

• Two of the four pulled tube intersections examined were added to the ODSCC ARC database. The other two data points were excluded based on the database exclusion criteria.

• No indications were found that extend beyond the span of the TSPs.

• : No crack-like circumferential indications were found at any TSP intersection.

. None of the indications was attributable to primary water stress corrosion cracking.

• Prairie Island Unit 1 meets the requirements for operation under the licensed basis (Ref 2) for the tube ODSCC at TSP ARC.

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3.0 Prairie Island-1 Pulled Tube Examination Results And Evaluation 3.1 Prairie Island-1 Pulled Tube Examination Results 3.1.1 Introduction Sections fmm three steam generator tubes from Prairie Island Unit I were sent to the Westinghouse Science and Technology Center for evaludion of corrosion indications detected by field eddy current inspections during the fall of 1997. The received hot leg side tubing included tubing from the first tube support plate (TSP 1) region of Tubes R19C11 (S/G 12), R27C21 (S/G 11), and R32C41 (S/G 11). The TSP 3 region of Tube R32C41 was also received. j i

Field eddy current inspections, using bobbin and + Point probes, of the TSP 1 regions of Tubes R19C11 and R32C41 showed distorted bobbin indications with a single axial indication (FAI) for Tube R19C11  :

and a volumetric call for Tube R32C41 based on pancake and + Point coil data. .n the case of the TSP 1 region of Tube R27C21, only the bobbin probe showed an indication signal while thgancake and + Point coils showed no indication signals. The TSP 3 region of Tube R32C41 had no detectabh degradation by the field eddy current inspection calls.

LA review of field eddy current data from previous years suggested that the indications either have not  !

grown or have experienced negligible growth in recent years.

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• I 4 3.1.2 Nondestructive Examinations Table 3.1 presents a summary of nondestructive examination data gathered in this examination. Both original field eddy currem calls and reevaluated field eddy current data are presented. Other than for some adjustments in signal voltages, the only differences between the original cddy current indication calls and the reevaluated data were: the + Point and pancake coil (PC) calls for TSP 1 region of Tube R19Cl1, where the original call of a SAI became a multiple axial indication (MAI) call; and the bobbin call for the TSP 3 region of Tube R32C41, where the NDD call became a small voltage (0.12 volts) distorted indication (DI) call. The + Point probe indication signals for the TSP 1 regions were a SAI call for Tube R19C11 and a volumetric call for Tube R32C41.

In the laboratory, no NDE was performed on the TSP 1 region of Tube R19C11 in order to obtain expedited destructive examination information. For the other three specimens, the tube pull apparently introduced significant eddy current dent signals in the TSP regions as large dent signals were observed in the laboratory that were not present in the pre-pull field data. (However, these large dent signals were also observed in the post-pull EC data obtained in the field.) This observation is not unusual as TSP crevice deposits can shift within the crevice region and temporarily act as an " extruding die" until the j crevice deposits are abraded. As a consequence of the dents, laboratory eddy current indication signals j i

were hidden within the dent signals to the extent that laboratory bobbin data was of no value. The 11boratory + Point data for the TSP 1 region of Tube R32C41 produced similar indication signals as did the field + Point data. The laboratory + Point analyses of the modest indications showed a stronger circumferential component response than an axial componept response.

Laboratory UT confirmed the original field bobbin DI call in the TSP 1 region of Tube R27C21 by fmding the presence of shallow OD axial and circumferential aim indication signals which could be volumetric cellular or IGA corrosion. For the TSP 1 region of Tube R32C41, UT found both shallow tube wall thinning and shallow OD degradation signals which also could be volumetric cellular or IGA corrosion. (Both types of degradation subsequently were found by destructive examination.)

. Radiography detected wastage (tube wall thinning) indications in the TSP 1 region of Tube R32C41 and possible wastage signals in the TSP 1 region of Tube R27C21. However, the latter werejudged moie likely to have been caused by the edges of spalled surface deposits.

3.1.3 Leak, Burst and Tensile Data Room temperature leak testing was performed on the TSP 1 regions of Tubes R19Cl1, R27C21 and R32C41 at differential pressures ranging from normal operating conditions (NOC) to steam line break (SLB) conditions. No leakage was recorded for the specimens.

Table 3.2 presents a summary of room temperature burst test results for the pulled tubes, including burst test data for free span sections from the pulled tubing where no corrosion was expected. All TSP locations received were burst tested. All burst specimens developed axial burst openings. For the three TSP crevice regions with original field eddy current indication calls, all burst openings were centered in t the crevice regions. For the TSP 3 region of Tube R32C41, which did not have an original field ind: cation call, the burst opening occurred entirely below the TSP crevice region in a free span region.

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All burst tests for spcimens with original field NDE indications had slightly reduced burst strength and ductility data, as expected. However, all burst pressures exceeded 10,000 psi, consistent with shallow corrosion degradation. All burst strengths were well above those required by safety guidelines. Table 3.2 clso presents a summary of room temperature tensile data obtained on free span sections of the pulled tubing. All tensile strength data were normal for mill annealed tubing of this vintage and manufacture.

Following burst testing, maps were created of any secondary cracks or wastage zones visually (20X stereoscopic examination) observed en the specimens. Figures 3-1 through 3-4 present sketches of these cracks or wastage zones with some modifications based on later destructive examination data. Both shallow wastage and corrosion cracks were observed in the TSP 1 region of Tube R32C41. Only j corrosion cracks were observed in the TSP 1 region of Tubes R19C11 and R27C21. No degradation was i visually observed in the TSP 3 region of Tube R32C41, although later metallographic data did find i random shallow IGA patches within the TSP crevice region.

3.1.4 Destructive Examinations Scannint 'lectron Microscope (SEM) fractography was performed on all burst opening cracks to provide detailed uack depth profiles and ductile ligament data. Table 3.3 presents a summary of the data. For the OD origin intergranular corrosion at the TSP 1 crevice regions of Tubes R19C11 and R27C21, the intergranular corrosion macrocracks were composed of numerous intergranular microcracks that were interconnected by ligaments. Most of these ligaments only had intergranular corrosion features indicating that the microcracks had interconnected by corrosion during plant operation. Some of the ligaments had primarily tensile tearing (ductile) features indicating that they tore during tube pulling or subsequent burst testing or destructive examination specimen preparation. The OD origin macrocracks at these TSP 1 crevice locations were axial cracks. The burst opening corrosion macrocrack at the TSP 1 region of Tube

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R19ClI was 0.618 inch long with a maximum depth of 50% and an average depth of 22%. The burst opening corrosion macrocrack at the TSP 1 region of Tube R27C21 was 0.714 inch long with a maximum depth of 28% and an average depth of 13%. All intergranular corrosion was confm' ed to the TSP crevice i regions.

At the TSP 1 region of Tube R32C41, only transgranular wastage was found on the burst opening fracture face'. The axially oriented fracture face cut through a circumferentially oriented zone of shallow wastage that visually appeared to cover approximately half of the tube circumference. See Figure 3-3. The axial extent of the wastage varied from approximately 0.03 to 0.25 inch with all wastage confined to the TSP ,

crevice region. However, much of this wastage zone was so shallow (barely enough to remove circumferential tube manufacturing marks, i.e., less than 0.0001 inch) that it could not be measured in the SEM fractography or subsequent metallography. Only wastage greater than 0.001 inch could be successfully measured. The measured wastage on the burst opening had a maximum depth of 8% with an cverage depth of 4%. This wastage occurred over a zone that was 0.056 inch high.

' Very shallow intergranular features w ere observed on the tube OD surface adjacent to the fracture face in one area of the wastage zone..

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The burst opening for the TSP 3 region specimen of Tube R32C41 occurred below the TSP crevice region. SEM fractography showed that the center of the burst opening occurred in a shallow axial gouge that resulted from the tube pull. The gouge had a maximum depth of 0.002 inch 2 Table 3.4 presents a summary of metallographic data obtained from each of the four TSP crevice regions.

OD origin intergranular corrosion was found at the TSP 1 regions of Tubes R19Cl1, R27C21 and R32C41, as well as the TSP 3 region of Tube R32C41. It was difficult to classify the corrosion morphology for the observed intergranular corrosion since it varied from two dimensional intergranular stress corrosion cracking (IGSCC), through intergranular cellular corrosion (ICC), to three dimensional-intergranular attack' (IGA). The TSP 1 region of Tube R19ClI had axial IGSCC typical of that in the EPRI database developed for altemative repair criteria at TSP crevice regions. For the TSP 1 region of Tube R19C11, some of the intergranular corrosion had IGA patch characteristics. The OD intergranular corrosion morphology in the TSP 1 and TSP 3 region of Tube R32C41 frequently was more typical of patch IGA similar to the shallow IGA found in some tubes in the EPRI database. However, the R32C41, TSP 1 degradation differs from that in the EPRI database due to the presence of thinning. The thinning would be expected to increase the voltage response relative to that of only cracking. For the TSP 1 region of Tube R27C21, the corrosion morphology was a mixture of axial IGSCC and IGA patches, which is similar to some tubes in the EPRI database. Overall, these indications add additional axial IGSCC with  !

shallow ICA indications to the database. Figure 3-5 shows an example of the IGSCC morphology (burst testing opened the cracks) from the TSP 1 region ofTube R19Cl1. Note that the individual stress corrosion cracks have different depths. Figure 3-6 shows an example of patch IGA morphology from the TSP 1 region of Tube R27C21. Note that the crack fronts all have the approximate same depth, effectively forming a patch. With an extensive IGA morphology, all of the grains within the patch would be degraded. In this case, the called IGA patches had many nondegraded grains. However, the closeness of the degradation, along with the relatively uniform depth of degradation, approximates " pure" three dimensional, shallow degradation.

From metallographic data, estimates ofIGSCC crack density and the extent ofIGA found in association with individual IGSCC can be made for the purpose of comparing the observed corrosion morphology with that observed at other plants. In this examination, only the TSP 1 region of Tube R19ClI was defined as having a dominant morphology ofIGSCC. It had a moderate crack density and a moderate association ofIGA with its IOSCC as measured by D/W ratios'.

Radial metallography was performed on the TSP 1 regions of Tubes R19Cl1, R27C21 and R32C41 for the purposes of assessing the degree ofICC that was associated with the axialIGSCC in locations with

-2 The surface of the gouge had shiny metal; an older gouge would have been covered with deposits.

' f These corrosion morphology differences are notjust artificial classifications. The extremes ofIGSCC and IGA have different growth rates, with IGA having a significantly Swer stress dependency.

• A D/W ratio is a measurement of crack depth divided by crack width at its mid-depth, producing a measurement of the cmount ofIGA associated with a given crack. D/W ratios in the range of 3 to 20 suggest a moderate association ofIG/. with IGSCC, D/W ratios less than 3 suggest a high association ofIGA with IGSCC, and D/W ratios greater than 20 suggest a low association ofIGA with IGSCC Corrosion morphology is also characterized by crack density estimates. A high crack density is defined as greater than 100 cracks around the circumference, a moderate crack density as 25 to ICC cracks, and a low crack

' density as less than 25 cracks around the circumference.

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relatively high crack densities. For Tube R19Cl1, where an IGSCC morphology was prnent, only minor ICC structures were identified and they were more shallow than the axial IGSCC. For Tube R27C21, where a mixture ofIGSCC and patch IGA morphologies were present, more significant ICC structures j were present in local areas. Again, the ICC was more shallow than the contiguous axial IGSCC. Figures 3-7 and 3-8 present photomicrograohs showing ICC along with the more dominant axial IGSCC at different depths from the OD surface. They show a relatively larger decrease in the ICC morphology within the crack structure as a function of depth. For Tube R32C41, where a patch IGA morphology was I called from the transverse metallography, only minor ICC components were present in a local area with a f relatively high density of short axial cracks that, attematively, could have been defined as an IGA patch.

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(Note that it is difficult to distinguish between IGA and a local area with a high density of axial cracks fromjust radial metallographic data.)  ;

I Finally, metallography of the TSP 1 region of Tube R32C41 showed that the shallow wall thinning present in the wastage zone was indeed transgranular wastage. The maximum depth observed by ]

metallography was 4%. (Note that the maximum wastage depth by SEM fractography was 8%.) Figure 3-  !

9 shows a photomicrograph taken from an axial section cut through the wastage zone. Both transgranular wastage and IGA were found in the TSP 1 region of Tube R32C41. In general, the transgranular wastage and the IGA did not occur in the same location within the crevice region, as observed in both the I metallographic and SEM data. In one area of the transgranular wastage zone, minor intergranular penetrations were observed. If this was caused by the same corrosion mechanism as the IGA, then the wastage probably preceded the IGA.

3.f.5 Conclusions All four TSP region specimens had OD intergranular corrosion. In the case of the TSP 1 and TSP 3 region of Tube R32C41, the intergrmular corrosion was very shallow (< 8% for both locations) and had a corrosion morphology more typical ofIGA patches than ofIGSCC. The intergranular corrosion in the TSP 1 crevice regions of Tubes R19Cl1 and R27C21 was deeper and had an axial IGSCC morphology in the case of Tube R19Cl1 and a mixed corrosion morphology of axial IGSCC and patch IGA in the case of Tube R27C21. All of the deeper corrosion in the TSP 1 region of Tube R27C21 was that of axial IGSCC, as would be expected. The burst test opening intergranular coirosion macrocracks were 0.618 inch long with an average depth of 22% and a maximum depth of 50% for the TSP 1 region of Tube R19C11 and 0.714 inch long with an average depth of 13% and a maximum depth of 28% for the TSP 1 region of Tube R27C21. These burst opening intergranular macrocracks were composed of a large number of intergranular microcracks joined together primarily by intergranular ligaments. A few ductile ligaments were also present. The morphologies of the Tube R19Cl1 and Tube R27C21 corrosion degadation are characteristic of axial ODSCC with shallow IGA in the EPRI ARC database for ODSCC at TSP intersections.

I Not all observed corrosion was intergranular. In the case of the TSP 1 region of Tube R32C41, shallow  !'

transgranular wastage was found in a relatively narrow circumferential band across the crevice region.

The maximum depth of wastage was 8%, the same maximum depth as the intergranular corrosion. In general, the wastage and IGA corrosion in the TSP 1 region of Tube R32C41 did not occur in contiguous zones. Based on the observation of one isolated occurrence ofintergranular penetrations in the l 7

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transgranular wastage zone, it is suspected that the wastage preceded the IGA. The field eddy current data from these very shallow indications show that the corrosion indications are not actively growing. It is possible, if not probable, that the transgranular wastage is old wastage from the early days of phosphate water chemistry in the secondary side of the steam generators. Its appearance is certainly very similar to phosphate induced thinning.

The IGSCC present in the TSP 1 region of Tube R19C11 wa : typical of that in the EPRI database gathered in support of TSP region alternative plugging criteria. (Moderate crack density and moderate D/W ratios.) The relatively few axial cracks (IGSCC) present in the TSP 1 region of Tube R27C21 clso were similar to degradation in the EPRI database with a high IGA component to the cracks and with the widespread presence of shallow IGA patches. The corrosion present in the TSP 1 of Tube R32C41 was not typical of that in the EPRI database, as the dominant corrosion morphology was very shallow .

transgranular wastage. In the case of the TSP 3 region of R32C41, very shallow IGA patches were found which is typical of only a few indications in the EPRI database.

The field eddy current inspections from the current outage were excellent in predicting the shallow corrosion that was present in the pulled tubes. Even when the degradation was no more than 8% deep, eddy current indication signals were called (a field call in the case of TSP 1 of Tube R32C41 and a reevaluated bobbin call in the case of TSP 3 of Tube R32C41, both of which had a maximum degradation depth of 8%). Detection of d gradation this shallow is related to the volumetric characteristics of the degradation, both the wastage and IGA patch forms. The volumetric characteristics increase the bobbin voltage response compared to that ofIGSCC of comparable depth.

Leak tests conducted on selected tubes all produced no leak rates, as expected for shallow corrosion. All specimens with degradation had burst test strengths that exceeded 10,000 psi, well above safety guidelines. All burst specimens had axial burst openings.

3.2 Evaluation of Pulled Tube Data for ARC Applications This section evaluates the pulled tube examination results described above for application to the EPRI ,

database for ARC applications. The eddy current data is reviewed, including reevaluation of the field j data, to finalize the voltages assigned to the indications and to assess the field NDD calls for detectability under laboratory analysis conditions. The data for incorporation into the EPRI database are then defined L and reviewed against the EPRI database exclusion criteria to assure acceptability for the database.

3.2.1 Eddy Current Data Review l l

Table 3.6 provides a summary of the eddy current data evaluations for the Prairie Island-l pulled tubes.

There is little difference in the bobbin voltage calls between the field EC data and the laboratory ,

reanalysis of the field data. For inclusion of the data in the EPRI database, it is desirable to' minimize analyst variability in the vol: age calls since this variable is separately accounted for in ARC applications  ;

as an NDE uncertainty. Most of the pulled tube EPRI database has been analyzed by the same analyst that performed the laboratory reevaluation of Table 3.5. Thus, the reevaluated field bobbin voltages are used for application to the ARC correlations.

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1 I e The field bobbin data for the field NDD call was reevaluated to derive the most appropriate amplitude l measurements, where possible, for these very small signals. This review indicated that the field NDD )

indication for R32C41, TSP 3 could be assigned a bobbin flaw voltage of 0.12 volt. The bobbin analysis i for this indication is shown in Figure 3-10 (0.15 volt in figure is prior to correction for cross calibration to laboratory standard). Both the field + Point data obtained prior to the tube pull and the laboratory + Point inspection for the pulled tube were NDD for this intersection.

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The tube pulling operations for R27C21 and R32C41 resulted in denting at the TSP intersections and no indications could be identified in the post pull bobbin inspection. No laboratory NDE data were obtained for R19C11 as the tube examination for this tube was expedited to obtain burst and leak rate data as well as crack morphology information.

i The + Point results for R32C41, TSP 1 are of particular interest since the field call was volumetric, the reevaluation of the field data gave a circumferential call and the post pull evaluation indicated both an axial and a circumferential call. As noted above, the destructive examination yielded a shallow (8% 1 maximum depth) wastage indication of significant circumferential extent (close to 180 ) with very l shallow axial ODSCC.

1 3.2.2. Prairie Island-l Data for ARC Application I The Prairie Island-l pulled tube results, as developed above, are summarized in Table 3.6. None of the three indications leak tested resulted in leakage at the reference SLB conditions (pressure differential of  ;

2560 psi at a temperature of 616 F). The measured burst pressures are adjusted to the nominal flow stress of 68.78 ksi , the reference flow stress for the EPRI database, Reference 3, for 7/8 inch tubing at operating temperature.

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i The (lata of Table 3.6 are used in Section 3.3 to assess their influence on the EPRI ARC burst pressure and SLB probability ofleakage and SLB leak rate versus voltage correlations. The Prairie Island-l pulled  !

tube results were evaluated for patential exclusions from the database against the EPIU data exclusion criteria (Appendix E of Reference 9). Criteria le to le apply primarily to unacceptable leak rate measurements, tube damage from tube pull forces and indications without Icak test measurements. These criteria do not apply to the indications of Table 3.6 and would not lead to any exclusions from the database. Criterion 2b applies only to indications > 20 vohs which is not applicable to the indications. j EPRI Criterion 3 has not been approved by the NRC and is not applicable since none of the indications  !

had SLB leakage. I Criterion la applies to eddy current signal corruption due to extraneous bobbin voltage effects other than j ODSCC. The R32C41 TSP 1 indication had wastage with a maximum depth of 8% in addition to very l shallow axial ODSCC (<8% maximum depth). It is expected that the bobbin voltage of 0.85 volt for this I indication is dominated by the wastage response. The ODSCC for this indication is a modest number of shallow microcracks (See Figure 3-3) for which the degree of degradation is similar to that found at TSP 3 (See Figure 3-4) which had a 0.13 volt bobbin signal. Consequently, data exclusion Criterion 1a is ,

9 Q APC\NsP97W0 rep. doc:03/10/98

-a

r_

applicable to this indication and the R32C41 TSP 1 indication is excluded from the EPRI database for all correlations.

Criterion 2a applies to atypical ligament morphology and states that cracks having s 2 uncorroded ligaments in shallow cracks < 60% deep shall be excluded from the database. The R27C21 TSP 1 indicction is 0.714"long with a maximum depth of 28% and only one uncorroded ligament present. This indication satisfies exclusion Criterion 2a and is excluded from the database. Exclusion criterion 2a is intended to identify indications that would have higher voltages than other indications in the database due l to the absence ofligaments in shallow cracks. This effect is present for R27C21 as its burst pressure is almost that of an undegraded tube although having a voltage of 0.62 volt and the burst pressure would lie well above the mean of the burst pressure correlation.

Criterion Ib would exclude indications with inadequate or inappror iate burst pressure. The R32C41 TSP 3 indication burst outside of the TSP and might be considered an inappropriate burst pressu.e. Ilowever, as shown in Table 3.6. this indication burst at the same burst pressure as the undegraded fret span section of tubing. Therefore, the burst outside the TSP did not lower the burst pressure by any signhicant amount 'l and the burst pressure can be accepted as that corresponding to the shallow (8%) degradation at this intersection. Criterion Ib would not be applicabic to this indication. Since this indication burst cutside the TSP, fractography data are not available to evaluate remaining ligaments against exclusion Critvion 2a. Ifowever, this indication had not developed into a macrocrack through coa!cscence of microcracks (See Figure 3-4) and ligaments remain between the microcracks even though the ligaments were not quantified. Therefore, Criterion 2a should not be applied for the R32C41 TSP 3 indication. The morphology for this indication was that of shallow IGA patches which had not progressed to significant l

ODSCC depths. The more common occurrence ofIGA in the EPRI database is IGA in common with deeper SCC cracks. This difference in morphology does not provide a basis for excluding R32C41 TSP 3 from the database since it may represent an earlier stage ofIGA involvement than other indications in the l database. Therefore, it is concluded that the R32C41 TSP 3 indication should be included in the EPRI database for the burst and probability ofleakage (POL) correlations. Ahhough no leak test was perfonm  !

for this indication, it is clear from the 8% depth th:.t the indication would not leak at SLB indicatica and can be included in the POL cortclation.

The indication at R19Cl1 TSP 1 does not satisfy any of the criteria for exclusion from the database. The 50% maximum depth indication has 4 uncorroded ligaments and thus does not satisfy Criterion 2a.

Criteria la to le are also not applicable to this indication. Therefore, this indication is meluded in the EPRI database for the burst and probability ofleakage correlations. Since the indication did not leak, it cannot be considered for the leak rate coredation.

liased on the above assessment, the indications at R19Cli TSP 1 and R32C41 TSP 3 are included in the EPRI database for the burst and probability ofleak correlations. The indications at R27C21 TSP 1 and R32C41 TSP 1 are excluded from the database and all correlations.

10 QAK%V9M0 rep doc: 03/10/98

n -

3.3 Comparison of Prairic Island-l Pulled Tube Data With Existing ARC Correlations This section reports on the evaluations performed which utilized the results ofleak rate and burst testing of the tube sections which were removed from Prairie Island Unit 1 in 1997 (R19Cl1, TSP 1 and -

R32C41, TSP 3). The results from the destructive examinations of the tubes are recorded in another section of this report. The Prairie Island 1 pulled tube data germane to the ARC correlations, and the bobbin amplitudes for ARC applicetions, are illustrated on Figure 3-11. The results of the destructive examinations, e.g., burst t nd leak tests, are compared to the database of similar test results for 7/8" outside diameter steam generator ti;bes. In addition, the effect ofincluding the new test data in the reference database was evaluated. In summary, the test data are consistent with the database relative to the burst pressures, the probability ofleak as a function of the bobbin amplitude. Neither of the specimens exhibited leakage at SLB conditions, hence there is no effect on the database of ODSCC leak rates. The comparisons and evaluations are discussed below.

The database consisted of the EPRI recommended database as described in EPRI NP7180L, November 1996, modified by the NRC staffletter to NEI of January 20,1998. The staff did not concur with a recommendation to remove the French data from the database, hence the French data have been retained for the analyses documented herein. Only those French specimens that burst were retained in the burst database. The database used for the evaluations for this report is the same as that to be provided to the NRC staff during the first quarter of this year to satisfy the provisions of the NEI/NRC protocol for updating the database.

3.3.1 Suitability for Inclusion in the Database The report information on the destructive examinations of the tube sections was reviewed relative to the' EPRI guidelines for inclusion / exclusion of tube specimen data in the alternate plugging criteria (APC) database. This review revealed no infor mation that would lead to a conclusion that the data should not be included in the database. Therefore, the resulting correlations should be considered applicable to the use e of APC for indications in 7/8" diameter tubes in Westinghouse SGs.  ;

3.3.2 Burst Pressure vs. Bobbin Amplitude The result from burst tests, performed on tube specimens which exhibited a nonzero bobbin amplitude at a TSP elevation location, were considered for evaluation. Plots of the burst pressures of the Prairie Island I specimens are depicted on Figures 3-11 and 3-12 relative to the burst pressure correlation developed using

' the reference database.

1.

~ A visnti examination of the data relative to the EPRI database indicates that the measured

. burst pressurer, fall within the scatter band of the reference data, see Figures 3-11 and 3-12. a

~

1 2.' . . The c'ata points fall within a 95% confidence band for 90% of the populatian (5% in each )

tail) about ti e regression line, hence no statistical anoma'y is indicated see Figure 3-11.

l I

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l The net result is that the visual e xamination doesn't indicate any significant departures from the reference database, although the burst presures are greater than would have been expected from such indications since the pressures are above th e mean of the data as given by the regression line. Based on the placement of the new data, it can be judged that there would be no significant effect on the analysis of the residuals of the regression; either on the scatter plot of the residuals as a function of the predicted burst pressures or on the normal probability plot of the residuals.

Since the Prairie Island 1 burst pressure data were not indicated to be from a separate population from the reference data, the regression analysis of the burst pressure on the common logarithm of the bobbin amplitude was repeated with the additional data included. A comparison of the regression results obtained by including these data in the regression analysis is provided in Table 3.7. Regression predictions obtained by including these data in the regression analysis are also shown on Figure 3-12. A summary of the changes is as follows:

1. The intercept of the burst pressure, PB , as a linear function of the common logarithm of the bobbin amplitude regression line is increased by 0.2%, or about 12 psi. This has the effect of increasing the predicted burst pressure as e function of the bobbin amplitude.

2. The absolute slope of the regression line is increased by 0.5%, i.e., the slope is more steep.

This has the effect of decreasing the burst pressure as a function of bobbin amplitude for large indications.

3. There is a decrease in the standard error of the residuals of 1.0%. The effect of this change is reflected in a slightly smaller deviation of the 95% prediction line from the regression line.

The net effect of the changes on the SLB structural limit, using 95%/95% lower tolerance limit material properties, is to increase it by 0.1 V,i.e., from 8.3 to 8.4 V. The increase of the intercept and the decrease in the standard error, coupled with the fact that the structural limit is also increased, indicate that the probability of burst would also decrease for bobbin indications over the structural range ofinterest. Based on the relatively small change in the structural limit, the change in the probability of burst would also be expected to be small. Predicted values on the probability of burst of a single indication as a function of the bobbin amplitude are illustrated on Figure 3-13. The probability of burst is reduced slightly up to an amplitude of about 10 V. Beyond that value, the probability of burst is increased almost imperceptibly.

3.3.3 Probability of Leak The Prairie Island I data were examined relative to the reference correlation for the Pol as a function of the common logarithm of the bobbin amplitude. Figures 3-14 and 3-15 illustrate the Prairie Island I data relative to the reference correlation. The specimens exhibited expected Pot behavior consistent with the reference model predictions. Based on the data examination, there is no significant evidence ofirregular g results, i.e., outlying behavior is not indicated.

In order to assess the quantitative effect of the new data on the correlation curve, the database was expanded to include the Prairie Island 1 data point and a Generalized Linear Model regression of the pol 12 QAPGNSr'9'r@0tep doc: 0.1/MH

on the common logarithm of the bobbin amplitude was repeated. A comparison of the correlation .

parameters with those for the reference database is shown in Table 3.8. These results indicate:

1. An essentially 0.0% change in the logistic intercept parameter.

2. A <0.0% change in the logistic slope parameter.

3. The values of the elements of the covariance matrix of the parameters changed 0.0%.

Examination of Figures 3-14 and 3-15 indicate that there is no visually perceptible change in the Pot for any indication, hence, the impact on the 95% confidence bound on the total estimated leak rate from a single SG would not be expected to be significant.

4. The mean square error (deviance divided by number of degrees of freedom) decreased by 1 0.9%.

In order to confirm thejudgment that the changes are not significant, the reference correlation and the new correlation were also plotted on Figures 3-14 and 3-15. An examination of the figures reveals no significant change in the correlations. It is noted that when the total leak rate is determined using the leak rate to bobbin volts correlation, as is supported for the analysis of 7/8" diameter tubes, the resulting value can be quite insensitive to the form of the pol function. So, the effect of the changes in the parameter values and variances would be expected to be small or insignificant relative to the calculation of the 95%

confidence bound of the total leak rate from a SG.

3.3.4 Leak Rate vs. Bobbin Amplitude f Neither of the specimens leaked at SLB temperature and pressure difference conditions; hence the pulled tube specimens from Prairie Island I have no effect on the leak rate correlation. It is appropriate to report the results of the regression analysis of the leak rate data because of recent NRC staff concurrence with the use of a one-sided p value test of the significance of the regression. The correlation of the leak rate to bobbin voltage exhibits a one sided p-value of 3.8% for the slope parameter using the reference database (French data included). A summary of the regression analysis results is provided in Table 3.9. Figure 3-  !

16 illustrates the reference database relative to the distribution predicted median and mean leak rates from j the regression equation. Figures 3-17 and 3-18 are included for information to illustrate the distribution of )

the residuals about the regression line and the same distribution relative to the expected distribution based on the regression associated assumption of normality. Based on the requirements stipulated in NRC Generic Letter 9505, the regression equation and error distribution of the leak rate as a function of the bobbin amplitude may be used to perform Monte Carlo simulations to estimate the total leak rate.

3.3.5 GeneralConclusions The review of the effect of the Prairie Island I data indicates that the burst pressure and the probability of

- leak correlations to the common logarithm of the bobbin amplitude would not be meaningfully changed by the inclusion of the data. Therefore, it is likely that the conclusions relative to EOC probability of burst and EOC total leak rate based on the use of the reference database would not be significantly I

13

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1 affected by the addition of the Prairie Island I data from R19C11 and R32C41. The increase in the Pol l would be at least partially offset by decreases in the leak rate.

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Steam Generator 11 R27C21 1 0.86 NDD 0.62 Di NDD Dent NDD R32C41 1 0.87 0.56 0.85 DI 0.20 Dent 0.11 Ax.

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1. Bobbin voltage data include cross calibration of ASME standard to the reference 1 laboratory standard. Cross calibration normalization voltages were 2.21 volts for  !

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(2) Degrees of freedom.

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Figure 3-1 Sketch of the OD origin intergranular crack distribution found at the TSP 1 region of Tube R19C11.

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Figure 3-3 Sketch of OD origin transgranular wastage and intergranular crack distribution found at the TSP 1 region of Tube R32C41. Also shown is the location of the burst opening, which occurred through a circumferentially oriented zone of shallow transgranular wastage in the TSP crevice region. The burst opening extended beyond the TSP crevice region, but the wastage was confined to the crevice region. I While no OD origin intergranult.r corrosion occurred on the burst fracture face, isolated patches of OD j IGSCC were observed at many locations within the crevice region.

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' 4.0 EOC-18 Inspection Results and Voltage Growth Rates

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- 4.1 ' EOC-18 Inspection Results

.With regard to implementing Voltage Based Repair Criteria for ODSCC at the TSPs, the inspection of the Unit-1 steam generators consisted of a complete,100% Eddy Current Test (ECT) utilizing the '

i bobbin probe for a full length inspection of all hot leg (HL) and cold leg (CL) TSP intersections in the tube bundle. A 0.720 inch diameter probe was used for a!! bobbin inspections at intersections where the

. Voltage Based Repair Criteria apply. In addition, all except two of the TSP intersections with bobbin DSI calls (SG-11, R40C56 and R30C53, both removed from service) were inspected using the + Point j

rotating pancake coil (RPC).

Prairie Island provided summaries of the data for both steam generators via e-mail, References.4 through 7, which were analyzed to develop the database applicable to the voltage based repair criteria.

The results of these analyses are summarized in Table 4.1 and are discussed below.

)

In SG-11,278 indications (DSI) were recorded as potential ODSCC indications. The maximum voltage among these indications (DSI) was 1.60V. Twenty-two of the bobbin indications were confinned by RPC, and all were interpreted as volumetric (VOL) indications. The two tubes not inspected with the RPC (R40C56 and R30C53) are included in the data base. A prior cycle RPC indication (R10C51) was NDD RPC, a non-reportable (INR) indication. The INR classification applies to indications reported in the previous inspection with no flaw indication in the current inspection. The maximum voltage indication (1.60V, R13C69) was not among the population of confirmed indications.

In SG-12,207 bobbin indications (DSI and INR) were recorded as potential ODSCC indications. The maximum voltage indication in the SG12 bobbin database was 1.74V (R43C59). Forty-four of the bobbin indications were confirmed by RPC, including 42 volumetric (VOL) indications, I single axial indication (SAI) and I multiple axial indication (MAI). 'Ihe maximum bobbin voltage indication is included in the population of confirmed volumetric indications.

All of the confirmed indications were removed from service. The SAI and MAI indications were removed from service for reasons unrelated to the requirements of GL 95-05. The volumetric indications were removed from service based on the NRC position that no qualified sizing technique was available for combined ODSCC and thinning, although it would be expected that this "ombination

~

would lead to conservative voltage calls with respect to the ARC. These tubes may be candidates to be retumed to service at a future time.

. Table 4.1 summarizes the voltage distribution of the bobbin indications found during the inspection at EOC - 18 for both of the steam generators, together with the voltage distribution of the confirmed indications and the voltage distribution of the indications retumed to service. Figures 4-1 and 4-2 provide a graphical representation of the distribution of bobbin voltages for SG-11 and -12 respectively.

40 YiWALOOMAPC.ShamAPCWSP9h90-Day Rept. doc;03/1098 ~

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l No indications were found to extend outside the span of the TSP at any intersection. No crack-like circumferential indications were found at any TSP intersection. None of the indications were attributable to primary water stress corrosion cracking. j 4.2 Voltage Growth Rates i

The prior inspection (EOC-17) EC data provided by Reference 8 were reviewed relative to the current j inspection data to extract the population of tubes with DSI calls in both inspections for voltage growth i rate analysis. The EC signals from the EOC-17 inspection were re-evaluated for tubes represented on -

the EOC-18 bobbin voltage indication summary but not represented on the EOC-17 bobbin voltage

{

indication summary. For SG-11,250 tubes of the 278 tubes in the current population were found to j have DSI ca!!s in both inspections. For SG-12,130 of the 207 tube population were found to have DSI {

calls in both inspections. Thus, these tubes represent the data base for developing the growth rate for op rational assessment of the tubes vis a vis the ODSCC ARC.  ;

The growth rates developed as noted above meet the requirements of GL 95-05, paragraph 2.b.2(2), i which specifies that growth rates should be evaluated only for those intersections at which bobbin {

indications can be identified at two successive inspections, unless an indication changes from NDD to I a relatively high voltage, e.g. 2 volts. in SG -11, the largest voltage indication that was previously NDD was 0.85 V, and in SG-12, the largest indication that was previously NDD was 1.03 V. These values are well within the GL 95-05 guidelines.

GL 95-05 requires a minimum population of 200 data points for developing growth rates.

l Consequently, since there are only 130 data points for SG-12, a combined growth rate was developed  !

utilizing both the SG-11 and SG.12 data for a total of 380 data points. The mean and standard deviations of the individual distributions are very similar, indicating that they can be combined into a single larger population. The average combined data growth rate is 0.021 volts for the 565.5 FPD i operating cycle. The average individual SG growth rates are 0.018 and 0.027 volts over the 565.5 FPD l operating cycle for SG-11 and SG-12, respectively. Table 4.2 summarizes the individual SG and combined growth rate data for EOC -l 8 and EOC-17 (see below) to provide a historical perspective of the average voltage growth rate over these two cycles. It is apparent that the bobbin voltage growth has been small, and that the average voltages and voltage growth rate is similar for both SGs for cycle

18. For cycle 17, SG-11 exhibited essentially zero growth, while SG-12 exhibited a growth rate of approximately 5.6%/EFPY.

Table 4.3 summarizes the individual and combined SG voltage growth rate voltage distributions.

Table 4.3(a) expresses the growth rates in terms of the delta-voltage from BOC to EOC 18,i.e. over the 565.5 days of operation, and Table 4.3 (b) presents the same data in terms of delta volts per EFPY.

Figures 4-3 through 4-5 are histograms of the growth rates for the individual generators and for the

. combined growth rate. Figure 4-6 shows the cumulative distribution function of the individual and combined population growth rates. Based on prior experience, the growth rate with the longest tail provides the most conservative projections; therefore the combined, or SG-12, growth rate is the appropriate data for the operational assessment for the ODSCC ARC. However, SG-12 did not meet '

the required 200 indications; thus, the combined growth rate is the acceptable limiting growth. This 41 '

tSWALDO@APCJharciAPC\N5P9h90-Day Repth;03/10/98

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-.was confirmed in an analysis for SG-11 for which both the combined and individual growth rates are available;in which use of the combined growth rate data resulted in a higher predicted EOC leak rate.

- A separate lookback analysis was performed for the EOC -17 bobbin voltage indication population to evaluate the voltage growth rate for the preceding cycle to address the GL 95-05 requirement that the more conservative of the growth rates from the prior two cycles should be utilized in the operational-assessment. As noted above, experience has shown that the growth rate cumulative distribution function with the longest tail provides the most conservative results; thus the BOC-EOC cycle 17

. growth rate was evaluated vs. the BOC-EOC cycle 18 growth rate in this manner. Figure 4-7 shows the results of the growth rate comparison for these two cycles expressed in AV/EFPY, and confirms that the use of the BOC-EOC Cycle 18 growth rate is conservative.

'4.3 NDE Uncertainties Consistent with the requirements of GL 95-05, the NDE uncertainties utilized for the operational assessment analyses are those recommended in Reference 9, Sections 2.4.1,2.4.2 and D.4.2.3.

Reference 10, Section 3.4, further explains the treatment in regard to the voltage variability measurement distribution. Reference 10 has been accepted by the NRC as appropriate statistical

' methods for application to the ODSCC ARC. The probe wear uncertainty has a standard deviation of 7% about a mean of zero, and has a cutoff at 15% based on implementation of the probe wear standard.

The analyst variability has a standard deviation of 10.3% about a mean of zero with no cutoff. These NDE uncertainties are included in the Monte Carlo analysis to project the EOC-19 voltage distributions.

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Table 4.1 Summary of Voltage Distributions; EOC-18 Steam Generator 11 In-Service During Cycle 18 Cycle 19 Voltage Bin Field Bobbin RPC RPC Indications Retumed to Indications Inspected Confirmed Repaired Service 0.2 5 5 0 0 5 0.3 29 29 2 2 27 0.4 55 55 5 5 50 0.5 46 45 5 5 41 0.6 60 59 3 3 57 0.7 40 40 3 3 37 0.8 22 22 3 3 19 0.9 11 11 1 1 10 1.0 3 3 0 0 3 1.1 2 2 0 0 2 1.2 4 4 0 0 4 1.3 0 0 0 0 0 f 1.4 0 0 0 0 0 j l.5 0 0 0 0 0 j 1.6 1 1 0 0 I l Total 278 276 22 22 256 Steam Generator 12 I

In-Service During Cycle 18 Cycle 19 Voltage Bin Field Bobbin RPC RPC Indications Returned to Indications Inspected Confirmed Repaired Service J 1

0.2 9 9 1 1 8 0.3 30 28 2 2 28 0.4 44 42 6 6 38 0.5 33 32 8 8 25 0.6 33 32 8 8 .25 '

O.7 19 18 4 4 15 0.8 15 15 7 7 8 0.9 9 9 3 3 6 1.0 4 3 2 2 2 1.1 5 4 1 1 4 1.2 2 2 0 0 2 1.3 1 1 1 1 0 1.4 0 0 0 0 0 1.5 0 0 0 0 0 1.6 2 2 0 0 2 1.7 0 0- 0 0 0 1.74 I I I i 0 Total 207 198 44 44 163 ,

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Table 4.2 Prairie Island Unit i Voltage Growth History No. Avg. Volts Average Voltage Growth Percent Growth 7 Indications BOC Per Cycle Per EFPY Per Cycle Per EFPY EOC-18 (Cycle Length 565.5 EFPD)

Combined 380 0.520 0.021 0.014 4.0% 2.7%

i SG-11 250 0.521 0.018 0.012 3.5% 2.3%

SG-12 130 0.519 0.027 0.017 5.2% 3.3%

EOC-17 (Cycle Length 535.9 EFPD)

Combined '250 0.542 0.017 0.011 3.1% 2.1%

SG-11 168 0.519 -0.002 -0.001 -0.4% -0.2%

SG-12 82 0.592 0.055 0.037 9.3% 6.3%

44 Q2pc:nsp97/904y tep. doc

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Table 43 EOC 18 Voltage Growth Rates (a) Prairie Island Unit i Voltage Growth Rttte for Cycle 18 (565.5 EFFD)

~

1 SG 11 SG-12 Combined Voltage . No. of CPDF No. of 'CPDF No. of - CPDF  ;

Bin 1:<dications Indications Indications 1

~

-0.5 l 0.00400- 0 0.00000 1 0.00263

-0.4 C~ l 0.00400 0 0.00000 0 0.00263

-0.3 _ 2 0.01200 2 0.01538 4 0.01316

-0.2 2 0.02000 4 0.04615 6 0.02895

-0.1 - 2i 0.10400 19

~

0.19231 40 0.13421 0 100 0.50400 40 0.50000 140 0.50263 0.1 77 0.81200 44 0.83846 121 0.82105 02 38 0.96400 8 0.90000 46 0.94211 0.3 4 0.98000 4 0.93077 8 0.96315 04 l ,, 0.98400 5 0.96923 6 0.97895

'l 0.5 0 u.98200 1 0.97692 1 0 98158 0.6 1 0.98800 1 0.98462 2 0.98684 0.7 1 0.99200 0 0.98462 1 0.98947 '

0.8 2 1.00000 1 0.99231 3 0.99737 0.82 0 1.00000 1 2.00000 1 1.00000 Total 250 130 380 (b) Prairie Island Unit i Voltage Growth Rate for Cycie 18 on EFPY Basis SGil SG-12 Combined Voltage No. of CPDF No. of CPDF No. of CPDF Bin Indications Indications Indications

-0.3 1 0.00400 1 0.00263

-0.2 2 0.01200 2 0.01538 4 0.01316 0.1 6 0.03600 7 0.06923 13 0.04737 0 117 0.50400 56 0.50000 173 0.50263 0.1 105 0.92400 49 0.87692 154 0.90789 0.2 14 0.98000 7 0.93077 21 0.96316 0.3 1 0.98400 5 0.96923 6 0.97895 0.4 l 0.98800 2 0.98462 3 0.98684 0.5 3 1.00000 0 0.98462 3 0.99474 0.53 2 1.00000 2 1.00000 Total 250 130 380 45

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i 5.0 Data Base Applied for ARC Correlations GL 95-05 requires the use of a NRC approved data base for correlating burst pressure and leak rate to the measured bobbin voltage. GL 95-05 further requires that the data base be continually updsad through the addition of new data from periodic tube pulls required for plants that implement the ODSCC ARC. Consequently, the originally approved data base is periodically updated to include the new data. Reference 11 transmitted the most recent update data for 7/8" diameter tubing to the NRC.

This revision to the data base includes the tube pull data from Plants A-1 and A-2. This data base was i applied to the analyses performed for Prairie Island Unit 1. I 6.0 SLB Analysis Methods j Reference 12 approved the use of a correlation between leak rate and voltage, using the data base as noted in Section 5, above, based on the use of a one-sided p-test for the leak rate vs. voltage correlation. Accordingly, the voltage correlated leak rate, as documented in Reference 11 is utilized in 1 the analysis for the predicted MSLB leak rates.

i 7.0 Current Cycle EOC Bobbin Voltage Distributions 1

Calculation of the current EOC 19 leakage for a postulated SL3 event and the probability of burst <

based on the bobbin voltage distribution requires prediction of the EOC-19 bobbin voltage distrioution. l The current cycle BOC conditions are the measured bobbin voltage distribution from the last EC  ;

inspection adjusted by application of an appropriate probability of detection (POD) factor. NDE uncertainties and th bobbin voltage growth rate from Section 4 are applied to the current BOC conditions, taking into account the applicable cycle length for which the growth rate was developed and the estimated cycle length for the current operating cycle.

7.1 Probability of Detection (POD)

The number of bobbin indications used to predict leak rates and burst probability is obtained by adjusting the number of reported indications to account for measurement uncertainty and confidence level in the voltage correlations. This is accomplished by utilizing the Probability of Detection (POD) factor. Where appropriate, adjustments are made to the number of BOC indications for tubes either removed from service or returned to service. The calculation of projected bobbin voltage frequency distribution is based on a net total number ofindications returned to service, defined as:

N N,n = - N. ,,,,+ N,,,%.,,

where:

N.yn = number of bobbin indications being returned to service for the next cycle; N, = number of bobbin indications (in tubes in service during the previous cycle) reported in the current inspection; 53 snimmee shmwearv7so-o.yit pi4xsiow 1

POD = probability of detection N,,,,,, = number of N fwhich are repaired (plugged) after the last cycle; N ,,y,,,, = number of previously plugged indications which are de-plugged after the last cycle and are returned to service in the current cycle.

For Prairie Island Unit 1, no tubes were de-p'ugged and returned to service.

Absent an NRC approved alternate POD definition, NRC GL 95-05, Reference 1, requires the application of POD = 0.6 to the end of cycle measured bobbin voltage distribution to define the beginning of cycle bobbin voltage distribution for the subsequent operating cycle. The POD value of 0.6 was applied to the analyses performed for Prairie Island Unit 1.

7.2 Cycle Operating Time The cycle lengths for operating Cycle 18 and Cycle 19 were provided by Prairic Island Plant in references 13 and 14. At the request of Prairie Island, the cycle length used for the operational assessment is conservatively assumed to be slightly longer than the estimated operating cycle length of 48.1. EFPD. The following are the applicable cycle lengths used in the analyses for Prairie Island Unit 1:

Operating Cycle Cycle Length (EFPD) 18 565.5 19 500 l

7.3 Calculation of Voltage Distributions The EOC voltage distributions are developed from the as-measured bobbin voltage distributions at the l

BOC, adjusted by the POD factor of 0.6 as noted above, and by the voltage growth rates discussed in l

Section 4. Voltage growth rates specific to Prairie Island were developed based on lookback analyses i for the tubes exhibiting bobbin indications at the last inspection. The EOC voltage distribution is developed by Monte Carlo sampling of the POD adjusted BOC voltage distribution, NDE uncertainities and the growth rate distribution as described in Reference 10. Since no deplugged tubes were returned to service, the application of the growth rate is straightforward, in that only the single ;

growth rate can be applied to the entire population of tubes described in Section 4.

The voltage based repair limits were first applied at the EOC-18 inspection. Consequently, there are no prior voltage distribution projections for EOC-18 to compare with the measured distribution.

l 54 ,

"5 Waul 000 APC_ Siwe AIONSP9790-Day Rept . doc; 01/1898

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1

7.4 Predicted EOC-19 Voltage Distributions -

Table 7.1 summarizes the EOC-19 predicted voltage distributions for both of the SGs of Prairie Island Unit 1. Figures 71 and 7-2 show the BOC-19 and predicted EOC-19 voltage distributions for Pmirie

' island Unit i SGs -11 and -12, respectively. SG-11 exhibits the larger number ofindications, while SG 12 includes the highest predicted voltage indication at 2.3 volts. The highest predicted voltage indication in SG-11 is 2.1 volts. Further, the predicted voltage distributions indicate that 0-1 tubes in SG .-lI and 1 to 2 tubes in SG-12 are predicted to exceed 2 volts at the EOC-19. These values are well below the structural limit of the tubes, which is given in Reference 15 as 8.3 volts.

Table 7.1 Prairie Island Unit !

Voltage Distribution Projection for EOC-19 SG -11 SG -12 Volts No. of Indications No. of Indications 0.1 0.20- 0.37 0.2 6.28 9.50 0.3 30.52 32.15 j 0.4 61.55 53.32  !

0.5 76.87 57.7I l 0.6 81.69 52.21 I 0.7 72.I1 41.74 0.8 52.43 30.62 0.9 32.71 21.43 1.0 18.78 14.75 l 1.1 10.88 10.09 4 1.2 6786 6.67 1.3 4.71 4.26 1.4 3.06 2.67 1.5 1.84 1.83 1.6 1.09 1.44 1.7 0.67 1.22 i 1.8 0.08 1.00  !

l.9 0.70 0.75 2.0 0 0.26 2.1 0.30 0 2.2 0 0.70 2.3 0 0.30 <

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-8.0 Tube Leak Rate and Tube Burst Probabilities

' Predicted total leak rates and tube burst probabilities were calculated based on the measured EOC-1R

~

and projected EOC-19 voltage distributions utilizing correlations ofleak rate and free span burst

' pressure with bobbin voltage. The methods of these calculations are described in Reference 10, and are consistent with NRC criteria and guidelines of References 1 and 2.

8.1 SLB Leak and Burst Analyses for the Actual EOC-18 Distribution Table 8.1 Summarizes the SLB leak rate and probability of burst for the actual measured EOC-18 bobbin voltage distributions for SGs -11 and -12. For this analysis, POD was set to 1, and no growth .

rate was utilized. Leak rate was correlated to voltage, consistent with the methods used for projecting

- leak rate and burst probability for EOC-19, below. The calculated leak rates and probabilities of burst

-are well within the allowable limits specified in Reference 2.

8.2 SLB Leak and Burst Analyses for the Projected EOC-19 Distribution

- Table 8.1 summarizes the predicted EOC-19 leak rates and probabilities of burst for SG-11 and -12 of Prairie Island Unit 1. SG-11 exhibits the higher predicted total leak rate as expected, while SG-12 exhibits the higher predicted probability of burst as expected from the higher upper limit of the predicted voltage distribution. In both cases of predicted total leak rate and probability of burst, the predicted values are very small compared to the allowable values.' The SLB leak rates for SG-11 and -

12 are 0.064 gpm and 0.055 gpm, respectively, compared to an allowable SLB leak rate of I grm

'(predicted and allowable both at room temperature conditions), and the probabilities of burst are

<l.90x10'8 and 2.52x10, respectively, compared to the allowable of lx10 2, i

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gp 4 xa y 9.0 References -

~(1) US-NRC Generic Letter 95-05; Voltage Based Repair Criteria for Westinghouse Steam Generator

<u Tubes Affected by, Outside Diameter Stress Corrosion Cracking; dated August 3,1995.

"(2)'NRC Safety Evaluation Related to Amendment Nos.133 and 125 to Facility Operating License Nc 3. DPR-42 and DPR-60, Northern States Power Company, Prairie Island Nuclear Generating Plant Unit Nos.1 and 2, Docket Nos. 50-282 and 50-306, November 18,1997

' (3) EPPJ Repor' NP 7480-L, Volume 1, Revision 2 " Steam Generator Tubing Outside Diameter Stress

- Cctrosion Cracking at Tube Support Plates Database for Alternate Repair Limits" (4) E-mail, Redner/Pearson (PINGP) to Lagally'(Westinghouse); DSI Spreadsheets; 11/2/97.

- (5) E-mail, Pearson (PINGP) to Lagally (Westinghouse); ET Database; 11/8/97.

(6) E-mail, Redner (PINGP) to Lagally (Westinghouse); Additional DSIs; 11/12/97.

(7) E-mail,' Redner (PINGP) to Lagally (Westinghouse); P.I. Final Repairs; 11/21/97.

(8) E-mail, Redner (PINGP) to Lagally (Westinghouse); APC Data; 9/4/97 (9) EPRI Report TR-100407, Revision 2A, Draft Report January 1995, "PWR Steam Generator Tube

Repair Limits- Technical Support Document for Outside Diameter Stress Corrosion Cracking at

Tube Support Plates"

~ (10) Westinghouse Report WCAP-14277, Revision 1, December 1996; :SLB Leak Rate and Tube

- Burst Probability Analysis Methods for ODSCC at TSP Intersections".

(11) Letter, NEI (Callaway) to US-NRC_ Document Control Desk, dated December 29,1997; Updated ODSCC ARC Correlations for 7/8" Diameter Tubes (Project No. 689).

(12) Letter, US-NRC (Sullivan) to NEI (Modeen) dated January 20,1998; EPRI Report: " Steam

- Generator Tubing Outside Diameter Stress Corrosion Cracking At Tube Support Plates Database for Alternate Repair Limits." NP-7480-L, Addendum 1, November 1996.

- (13) E-mail, Pearson (PINGP) to Lagally (Westinghouse), Dose Letter and EFPD; dated 10/1/97 (14) E-mail, Pearson (PINGP) to Lagally (Westinghouse), Nevt Cycle EFPD; dated 11/12/97.

(15) Westinghouse Letter NSD-EPRI-l175 to EPRI(Thomas), dated January 9,1998; EPRI

, . Agreement WOS550-17, entitled " Database Maintenance for ODSCC at TSP Intersections ARC:

E Updated ODSCC ARC Correlations for 7/8" Diameter Tubes". ,

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