ML20210L445
| ML20210L445 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 07/31/1999 |
| From: | WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| To: | |
| Shared Package | |
| ML20210L433 | List: |
| References | |
| SG-99-07-009, NUDOCS 9908090129 | |
| Download: ML20210L445 (48) | |
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~ 1 ATTACHMENT SG-99-07-009 SEQUOYAH UNIT-2 l
CYCLE 10 VOLTAGE-BASED REPAIR CRITERIA i 90-DAY REPORT l i
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July 1999 6
Westinghouse Electric Company LLC Nuclear Services P.O. Box 158 Madison, Pennsylvania 15663-0158 9908090129 990802 PDR ADOCK 05000328 P PDR
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m
l SG-99-07-009 SEQUOYAH UNIT-2 CYCLE 10 VOLTAGE-BASED REPAIR CRITERIA 90-DAY REPORT July 1999 l
SEQUOYAH UNIT-2 CYCLE 10 VOLTAGE-BASED REPAIR CRITERIA 90-DAY REPORT TABLE OF CONTENTS Page No.
1.0 Introduction 1-1 2.0 Summary and Conclusions 2-1 3.0 EOC-9 Inspection Results and Voltage Growth Rates 3-1 3.1 EOC-9 Inspection Results 3-1 3.2 Voltage Growth Rates 3-2 3.3 Probe Wear Criteria 3-3 3.4 Probability of Prior Cycle Detection (POPCD) 3-3 3.5 Assessment of RPC Confirmation Rates 3-5 3.6 NDE Uncertainties 3-5 4.0 Data Base Applied for Leak and Burst Correlations 4-1 5.0 SLB Analysis Methods 5-1 6.0 Bobbin Voltage Distributions 6-1 6.1 Probability of Detection 6-1 6.2 Cycle Operating Time 6-2 6.3 Predicted EOC-10 Voltage Distributions 6-2 6.4 Comparison of Predicted and Actual EOC-9 Voltage Distributions 6-3 7.0 Tube Leak Rate and Tube Burst Probabilities 7-1 7.1 Calculation of Leak Rate and Tube Burst Probabilities 7-1 7.2 Predicted and Actual Leak Rate and Tube Burst Probability for EOC-9 (Condition Monitoring Assessment) 7-1 7.3 Projected Leak Rate and Tube Burst Probability for EOC-10 (Operational Assessment) 7-2 8.0 References 8-1 1
I q:\ ape \ ten 99\ ten 90 day. doc i
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SEQUOYAH UNIT-2 CYCLE 10 VOLTAGE-BASED REPAIR CRITERIA 90-DAY REPORT
1.0 INTRODUCTION
This report provides the Sequoyah Unit-2 steam generator (SG) tube support plate (TSP) bobbin voltage data summary, together with postulated Steam Line Break (SLB) leak rate and tube burst probability analysis results. These results support continued application of the 2.0 volt repair criteria for TSP indications during Cycle 10 as outlined in the NRC Generic Letter 95-05 (Reference 8-1). Information required by the Generic Letter is provided in this report including projections of bobbin voltage distributions, leak rates and burst probabilities for Cycle 10 operation. The methodology used in these evaluations is consistent with the Westinghouse generic methodology described in Reference 8-2 as well as the methodology reported in the prior 90-day reports for Sequoyah Unit-2 (References 8-3 and 8-4).
The application of the 2-volt repair criteria for outside diameter stress corrosion cracking (ODSCC) indications at TSPs in the Sequoyah Unit-2 SGs involves a complete,100% Eddy Current (EC) bobbin coil inspection of all TSP intersections in the tube bundles of all four SGs and plugging of TSP indications greater than 2 volts which are confirmed by a Rotating Pancake Coil (RPC) probe. RPC inspections are also performed at certain locations exhibiting dent voltages and mixed residual signals. The measured bobbin signals are used to predict SG tube leak rate and probability of burst during a postulated SLB and show that they are within the allowable regulatory limits. l 1
1 Eddy current and repair data for TSP indications from the EOC-9 inspections are provided in Section 3. The actual EOC-9 voltage distributions as well as leak rates l and tube burst probabilities calculated for these distributions are compared with the projections for EOC-9 conditions performed using the EOC-8 data. Leak rates and burst probabilities for the projected EOC-10 voltage distributions are reported in Section 7 and compared with allowable limits.
I q:\ ape \ ten 99\ ten 90 day. doc 1-1 L, -
l 2.0
SUMMARY
AND CONCLUSIONS SLB leak rate and tube burst probability analyses were performed for all four SGs based on their actual measured EOC-9 voltage ~ distributions and the results compared with the projections performed at the beginning of the cycle. The total number of
-indications found at TSPs in each SG during the current inspection and the actual peak voltages are less than those projected at-the beginning of the cycle using a constant POD of 0.6 per the Generic Letter 95-05 requirements as well as those based on the voltage-dependent POPCD. Also, with the exception of the EOC-9 tube burst probability for SG-3, leak rates and tube burst probabilities calculated using the actual measured voltages are equal to or below those prcjected with both a constant POD of 0.6 as well as voltage-dependent POPCD. The projected EOC-9 tube burst probability for SG-3 with POD =0.6 is underestimated by 2.5x104, although the value predicted with POPCD is not underpredicted. However, a difference of this magnitude is attributable to the different random number sequences used in the Monte Carlo analysis, and therefore the results are acceptable. SG-4 was predicted to be the limiting SG at EOC-9 and was found limiting for leakage based on the actual measured EOC-9 voltage data.
For the actual EOC-9 bobbin voltage distribution, the largest SLB leak rate is
- calculated for SG-4, and its magnitude is 0.58 gpm. This leak rate value was calculated using the same leak and burst database used for the projections performed at the beginning of Cycle 9. Although a voltage dependent leak rate correlation can now be applied for 7/8" tubes, leak rates for all SGs based on the actual EOC-9 voltages were obtained assuming that leak rate is independent of bobbin voltage so that they can be compared with the EOC-9 projections which used voltage independent leak rates. However, the SG-4 SLB leak rate was also calculated using a voltage-dependent leak rate correlation; its magnitude is 0.06 gpm which is an order of magnitude below that obtained assuming the leak rate is independent of voltage (0.58 gpm). All leak rate values quoted are equivalent volumetric rates at room temperature. The SLB leak rates based on the actual voltages are substantially lower than the current allowable SLB leakage limit, and they are also below their corresponding projections. The largest conditional tube burst probability based on the measured voltage data was predicted for SG-3 which had the largest indication detected in this inspection. Its magnitude is 3.7x104, which is more than 2 orders of magnitude below the NRC reporting guideline of 10-2 Thus, the results meet the voltage-based repair criteria requirements for continued Cycle 10 operation.
A total of 659 indications were found in the EOC-9 inspection, and the largest indication detected had 2.03 volts. Only 7 indications were found on the cold leg side in all 4 SGs combined and the largest one had a bobbin voltage of 0.55 volts. Twenty-four indications, including the 2.03 volt indication in SG-3, were RPC inspected, and q:\ ape \ ten 99\ ten 90 day. doc 2-1
j all but three were confirmed as flaws. The largest number of bobbin indications,407 indications, was found in SG-4; 16 of those were inspected by RPC, and 13 were confirmed as flaws. The RPC inspection did not find any axial indications extending
.outside a TSP intersection.
An augmented RPC inspection was performed consistent with the GL 95-05 j l requirements. All TSP intersections that had dents over 5 volts during the last
{
inspection and dents between 2 and 5 volts up to the 3"i TSP and 20% sample at the l
l 4th TSP on the hot leg side were RPC inspected in this inspection; no PWSCC axial or circumferential cracks were detected. All mixed residual signals were reviewed for potential to mask a 1-volt OD indication, and there were no mixed residuals that required RPC testing. Two cold leg indications in SG-2 had copper deposit signals; they were inspected with RPC and no degradation was detected.
l SG-4 is projected to have the largest number ofindications as well as the highest SLB
- leak rate.and tube burst probability at EOC-10 conditions and SG-3 is predicted to l have the largest indication. The EOC-10 leak rate projection was performed using a I
leak rate versus bobbin voltage correlation meeting the Generic Letter 95-05 requirement. Using the NRC mandated constant POD of 0.6 and the latest leak and burst database for 7/8" tubes, the limiting EOC-10 SLB leak rate projected for SG-4 is 0.94 gpm (room temperature), which is significantly below the current licensed limit of 2.7 gpm (room temperature). The limiting EOC-10 tube burst probability,5.3x10-5, is also calculated for SG-4 (with POPCD), and it is more than two decades below the NRC reporting guideline of 10- 2 Thus the GL 95-05 requirements for continued implementation of the 2-volt repair criteria during Cycle 10 are met.
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3.0 EOC-9 INSPECTION RESULTS AND VOLTAGE GROWTH RATES l 3.1 ' EOC-9 INSPECTION RESULTS In accordance with the guidance provided by the NRC Generic Letter 95-05 1 (Reference 8-1) for application of voltage-based repair criteria, the EOC-9 inspection of the Sequoyah Unit-2 SGs consisted of a complete,100% EC bobbin probe, full 3 i
length examination of the tube bundles in all four SGs. A 0.720 inch diameter probe was used to inspect all hot and cold leg TSP intersections where a 2-volt ,
repair criteria was applied. Only one indication in SG-3 slightly exceeded the 2 volt I repair limit (the actual voltage was 2.03 volts), and all other indications detected were under 2 volts. Only a total of 6 TSP indications detected were on the cold leg l side in all 4 SGs combined and the largest one had a bobbin voltage 0.55 volts.
Twenty-four indications were inspected with a RPC probe in all 4 SGs combined, and all but three were confirmed as flaws. The single indication over 2 volts was confirmed by RPC as a flaw and was repaired. Three other TSP indications were also removed from service because they were present in tubes repaired for other reasons.
There were no indications extending outside the TSPs.
An augmented RPC inspection was performed consistent with the Generic Letter 95-05 requirements. The augmented RPC inspection using the + Point probe included examination of all TSP intersections in all four SGs with a dent voltage over 5.0 volts in the last inspection. Also, dents between 2 and 5 volts (in the last inspection) at intersections up to the 3"1 TSP and 20% sample at the 4th TSP were also tested with a
+ Point probe. The RPC inspection did not find any PWSCC axial or circumferential 1 indications. Two cold leg indications in SG-2 had copper deposit signals; they were inspected with RPC and no degradation was detected.
A summary of eddy current signal voltage distributions for all steam generators is shown on Table 3-1, whieb tabulates the number of field bobbin indications, the number of these field bobbin indications that were RPC inspected, the number of RPC confirmed indications, and the number ofindicationc removed from service due to tube repairs. The indications that remain active for Cycle 10 operation is the difference between the observed and the ones removed from service. No tubes were deplugged in the current inspection with the intent of returning them to service after inspection. Figure 3-1 shows the actual bobbin voltage distribution for tubes that were in service during Cycle 9, as determined from the EOC-9 EC inspection.
l Since only 5 indications were taken out of service due to tube repairs, the bobbin voltage distribution of indications returned to service at BOC-10 is essentially the l same as the actual bobbin voltage distribution shown in Figure 3-1. A review of l Table 3-1 indicates that more indications (a quantity of 406, with 27 indications above 1.0 volt) were returned to service for Cycle 10 operation in SG-4 than any q:\ ape \ ten 99\ ten 90 day. doc 3-1
II~ ,'
other SG. Therefore, SG-4 is likely to be the limiting SG at EOC-10.
l l: The distribution of EOC-9. indications as a function of support plate elevation, summarized in Table 3-2 and illustrated on Figure 3-2, shows the predisposition of ODSCC to occur in the first few hot leg TSPs (534 of the 659 PIs, or about 81%,
occurred in the first two hot leg TSPs), although the mechanism does extend to higher !
TSPs. Only 7 TSP bobbin indications were reported on the cold leg side in all 4 SGs I combined. The occurrence of a majority of ODSCC indications in the first two TSPs l on the hot leg side in Sequoyah Unit-2 shows predominant dependency on ;
temperature, which is consistent with that observed at other plants, i
3.2 Voltage Growth Rates f I'
For projection of leak rates and tube burst probabilities at the end of Cycle 10 operation, voltage growth rates were developed from EOC-9 inspection data and a j reevaluation of the same indications from the EOC-8 inspection EC signals. Table 3-3 '
shows the average growth rate for each SG during Cycle 9. It is evident that the !
absolute magnitude of voltage growth rates during Cycle 9 are small for all four SGs.
The average growth for indications with a BOC bobbin voltage above 0.75 volts is smaller than indications with a BOC voltage below 0.75 volts for all 4 SGs.
Table 3-4 provides a comparison of average growth data for the last 3 operating cycles i i for Sequoyah Unit-2, and the data shows a decreasing growth trend. The average I composite voltage growth rate from four SGs during Cycle 9,9.7%/EFPY, is below that for Cycle 8 (18.9%/EFPY). Part of the decreasing percentage growth rate is due to the increasing BOC voltage for the later cycles. From Table 3-3, AV per EFPY has decreased by about a factor of 2 while percent growth decreased by a factor of 5.
Table 3-5 shows the cumulative probability distribution (CPDF) of growth rate per f EFPY for each Sequoyah Unit-2 steam generator during Cycle 9. These growth data
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are also plotted in Figure 3-3. The curve labelled ' cumulative' in Figure 3-3 q represents averaged composite growth data from all four SGs. The average growth rate distribution for Cycle 9 is compared with that for the last cycle (Cycle 8) in Figure 3-4. The growth data are presented on an EFPY basis to account for the difference in the length of the two operating periods. It is evident from Figure 3-4 I
that Cycle 8 growth distribution is more limiting than Cycle 9 growth distribution.
The NRC guidelines require that the more conservative growth distribution for the last two operating periods be applied for projecting the next cycle voltage distributions. Therefore, Cycle 8 growth data will be applied to obtain EOC-10 projections, g
According to the Westinghouse tube integrity analysis methodology presented in l-q:\ ape \ ten 99\ ten 90 day. doc i
3-2 l
Reference 8-2, the larger of the composite growth rate for all SGs and the SG-specific growth rate should be used in projecting SLB leak rate and tube burst probability for individual SGs. .'As noted earlier, Cycle 8 growth rates would be used to perform EOC-10 projections as they are higher than the Cycle 9 growth rates. Since the Cycle 8 growth rates for SGs 1 to 3 are below the composite growth rate (see Table 3-5), the composite growth rate is applied to those three SGs to provide a conservative basis for predicting EOC-10 conditions.- EOC-10 predictions for SG-4 are obtained using its own growth rate since it is higher than the composite rate.
In the past, some plants 'with 3/4" tube SGs experienced growth rates that are dependent on the beginning of cycle (BOC) voltage. To determine if Sequoyah Unit-2 exhibited a similar trend during Cycle 9, growth rate data for Cycle 9 was plotted against BOC voltage, and the resulting plot is shown in Figure 3-5. It is evident that the Cycle 9 growth data do not show a trend to increase with BOC voltage. The indications with the top 3 growth values had a BOC voltage under 0.4 volts.
Table 3-6 lists the top 30 indications from the standpoint of growth during Cycle 9.
This data shows more clearly that there was only a modest growth during Cycle 9 as 25 out of these 30 indications had growth under 0.5 volts. Half of these 30 indications are identified as new indications for Cycle 9.
3.3 Probe Wear Criteria An alternate probe wear criterion discussed in Reference 8-5 was applied during the EOC-9 inspection. This criterion was also applied during last inspection for both Sequoyah units. When a probe does not pass the 15% wear limit, this alternate criterion requires that all tubes with indications above 75% of the repair limit since the last successful probe wear check be reinspected with a good probe. Accordingly, only tubes containing indications for which the worn probe voltage is above 1.5 volts need to be inspected with a new probe.
Only one ODSCC indication above the threshold was detected with a probe that failed the wear check subsequent to a successful wear test. That indication was in SG-1 and had a bobbin voltage slightly above the 1.5 volts threshold for retesting (1.53 volts).
The tube containing this indication was retested with a new probe and 1.23 volts was assigned to the indication in the retest. No other indications were detected in the tube retested. The alternate probe wear criteria used in the EOC-9 inspection is consistent with the NRC guidance provided in Reference 8-5.
3.4 Probability of Prior Cycle Detection (POPCD)
The inspection results at EOC-9 permit an evaluation of the probability of detection gnape\ ten 99\ ten 90 day. doc 3-3
e at the prior EOC-8 inspection. For voltage-based repair criteria applications, the
'important indications are those that could significantly contribute to EOC leakage or burst probability. These significant indications can be expected to be detected by bobbin and confirmed by RPC inspection. Thus, the population ofinterest for POD i assessments is the EOC RPC confirmed indications that were detected or not detected
! at the prior inspection. The probability of prior cycle detection (POPCD) for the EOC-8 inspection can then be defined as follows.
l l
EOC-8 cycle reported + Indications confirmed indications confirmed by and repaired in EOC-8 RPC in EOC-9 inspection inspection POPCD =
(EOC-8) { Numerator) + New indications RPC l confirmed in EOC-9 inspection POPCD is evaluated at the 1997 EOC-8 voltage values (from 1999 reevaluation for growth rate) since it is an EOC-8 POPCD assessment. The indications at EOC-8 that were RPC confirmed and plugged are included as it can be expected that these indications would also have been detected and confirmed at EOC-9. It is also appropriate to include the plugged tubes for voltage-based repair criteria applications since POD adjustments to define the BOC distribution are applied prior to reduction of the EOC indication distribution for plugged tubes.
It should be noted that the above POPCD definition includes all new EOC-9 l indications not reported in the EOC-8 inspection. The new indications include EOC-8 l indications present at detectable levels but not reported, indications present at EOC-8 l below' detectable levels and indications that initiated during Cycle 9. Thus, this ;
definition, by including newly initiated indications, differs from the traditional POD l
definition. Since the newly initiated indications are appropriate for voltage-based ;
repair _ criteria. applications, POPCD is an acceptable definition and eliminates the !
need to adjust the traditional POD for new indications.
The above definition for POPCD would be entirely appropriate if all EOC-8 indications were RPC inspected. Since only a fraction of bobbin indications are generally RPC inspected, POPCD could be distorted by using only the RPC inspected indications. Thus, a more appropriate POPCD estimate can be made by assuming that all bobbin indications not RPC inspected would have been RPC confirmed. This definition is applied only for the 1999 EOC-9 indications not RPC inspected since ;
inclusion of.the EOC-8 repaired indications could increase POPCD by including indications on a tube plugged for non-ODSCC causes which could be RPC NDD indications. In addition, the objective of using RPC confirmation for POPCD is to q:\ ape \ ten 99\ ten 90 day. doc 3-4
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distinguish detection of an indication at EOCna that could contribute to burst at i EOCo so that the emphasis is on EOCn RPC confirmation. This POPCD can be obtained by replacing the EOC-9 RPC confirmed by RPC confirmed plus not RPC inspected in the above definition of POPCD.
I The POPCD evaluation for the 1997 EOC-8 inspection data is summarized in Table 3- l 7 and illustrated on Figure 3-6. Because of relatively low bobbin voltages, only a small fraction of the indication population was RPC inspected in the EOC-9 inspection. Therefore, as evident in Table 3-7, there is insufficient data for defining POPCD based on RPC confirmed only indications. Only POPCD data based on RPC confirmed plus not RPC inspected indications are shown in Figure 3-6. Also shown in the figure is a generic POPCD distribution developed by analyses of 18 inspections in 10 plants and presented in Table 7-4 of Reference 8-7. It is seen from Figure 3-6 that the predicted POPCD distribution for Sequoyah Unit-2 is better than the generic POPCD distribution, and it reaches unity at about 1.5 volts.
In summary, the Sequoyah Unit-2 EOC-8 POPCD supports a POD value of 1.0 above about 1.5 volts, and it is higher than the generic POPCD in all voltage ranges.
3.5 Assessment of RPC Confirmation Rates This section tracks the 1997 EOC-8 indications left in service at BOC-9 relative to RPC inspection results in 1999 at EOC-9. The composite results for all SGs are given in Table 3-8. For 1997 bobbin indications left in service, the indications are tracked relative to 1997 RPC confirmed,1997 RPC NDD,1997 bobbin indications not RPC i inspected and 1997 bobbin indications with no indication found in 1999. Also I included are new 1999 indications. The table shows, for each category ofindications, the number ofindications RPC inspected and RPC confirmed in 1999 as well as the percentage of RPC confirmed indications.
Four out of the 35 RPC confirmed indications left in service at BOC-9 were RPC tested during the EOC-9 inspection, and all were confirmed. Only one RPC NDD indication was left in service at BOC-9 and it was not RPC tested at EOC-9.
Therefore, no RPC confirmation data for prior cycle RPC NDD indications is available from this inspection. All indications left inservice were included in the EOC-10 tube integrity evaluation reported.
3.6 NDE Uncertainties The NDE uncertainties applied for the EOC-9 voltage projections in this report are those given in the prior Sequoyah Unit-2 voltage-based repair criteria reports q:\ ape \ ten 99\ ten 90 day. doc 3-5
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l-(References 8-3 and 8-4). The probe wear uncertainty has a standard deviation of 7%
about a mean of zero and has a cutoff at 15% hased on implementation of the probe wear standard. The analyst variability uncertainty has a standard deviation of 10.3% about a mean of zero with no cutoff. These NDE uncertainty distributions I presented in Table 3-9 as well as graphically illustrated in Figure 3-7. The NDE 3 l
uncertainty distributions are included in the Monte Carlo analyses used to project the l
EOC-9 voltage distributions.
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Table 3-1 Sequoyah Unit 2 May 99 Outage Summary ofInspection and Repair For Tubes in Service During Cycle 9 Steam Generator 1 Steam Generator 2 In-Service During Cycle 9 RTS for Cycle 10 In-Service During Cycle 9 RTS for Cycle 10 Voltage rnid "'""'d "d are arc indicanon. Ali "
Bin m RPC RPC indic.oons All
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i,,pecied conrumed nepawed indic.nons fg(($ ,7" inspecied conrunna nepared indic nons [jyd ,
0.1 0 0 0 0 0 0 0 0 0 0 0 0 0.2 3 0 0 0 3 3 2 0 0 0 2 2
~~D T 17 0 0 0 17 17 4 0 0 0 4 4 04 12 0 0 0 12 12 9 0 0 0 9 9 DT- -'7 0 0 0 7 7 12 0 0 0 i2 - 12 -
0.6 '
11 0 0 0 11 11 10 0 0 1 9 9 0'.7~- 6 2 2 0 6 6 12 1 1 0 12 12 0.8 5 0 0 0" $ 5 5 0 0 1 4 4 7.9- 4 1 1 0 4 4 3 0 0 0 3 3 1 3 0 0 0 3 3 4 0 0 0 4 4 1.1 0 0 0 T)- 0 0 2 0 0 0 2 2 1.2 -0 0 0 0 0 0 0 0 0 0 0 0 1.3 1 0 0 0 1 1 1 1 1 0 I i !
1.4 0 0 0 0 0 0 -'-3 1 1 0 3 3 1.5 0 0 0 0 0 0 0 0 0 0 6 0 1.7 6 0 0 0 0 0 0 0 0 0 0 o 1.8 0 0 0 0 6 0 0 0 0 0 0 0 21 0 0 0 o o o 0 0 0 0 0 0 Total 69 3 3 0 69 69 67 3 3 2 65 65
>lv i 0 0 0 1 I 6 2 2 0 6 6 Steam Generator 3 Steam Generator 4 In-Service During Cycle 9 RTS for Cycle 10 In Service During Cycle 9 RTS for Cycle 10 Voltage neid conrumed rieid gpc gpc g,j,, g g ,g y conrumed Bin j inspecied conrrn=4 nep ired gi, indicanons [(QQ,j ,${" inspecied contemed Repawed Indicanons 0.1 0 0 0 0 0 0 1 0 0 0 1 i
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I Table 3-6 Sequoyah Unit 2 May 1999 Summary of Largest Voltage Growth Rates for BOC-9 to EOC-9 Steam Generator Bobbin Voltage RPC New SG Row Col Elevation EOC BOC Growth Confirmed ? Indication ?
D 5 39 0211 0.86 0.14 0.72 N1* Y B 8 92 OlH 1.03 0.34 0.69 NI Y C 8 56 OlH 1.69 1.16 0.53 NI N D 9 19 02H 0.8 0.27 0.53 NI Y D 32 69 0 111 1.03 0.51 0.52 NI Y D 23 66 02H 1.02 0.53 0.49 Y N A 28 43 02H 1.28 0.85 0.43 NI Y D 8 85 02H 0.63 0.2 0.43 Y Y D 8 34 OlH 1.28 0.9 0.38 NI N D 20 69 OlH 1.27 0.89 0.38 NI N B 6 43 02H 1.03 0.66 0.37 NI N C 27 26 02H 1.36 0.99 0.37 NI N I B 30 55 OlH 0.97 0.61 0.36 NI Y D 25 41 0 111 0.79 0.43 0.36 NI N D 9 42 OlH 0.84 0.48 NI Y
_ 0.36 B 14 48 02H 0.68 0.33 0.35 NI Y D 32 64 OlH 0.64 0.29 0.35 NI N D 31 71 0111 _ 0.7 0.3$ 0.35 N1 Y C 7 14 02H 0.82 0.48 0.34 NI Y D 8 27 OlH 0.58 0.24 0.34 N1 Y D 12 45 OlH 1.45 1.12 N 0.3 3_,,_ __ NI D 22 47 0211 1.34 1.02 0.32 NI N D 26 68 OlH 1.49 1.17 0.32 NI N
-D 20 46 03H 0.63 0.31 0.32 N1 Y D 20 50 OlH 0.81 0.5 0.31 NI N B 38 45 02H 0.86 0.56 0.3 NI N D 23 69 OlH 0.55 0.25 0.3 NI Y D 15 50 0iH 0.64 0.34 0.3 NI N D 45 51 04H 0.47 0.17 0.3 NI Y A 3 34 OlH 0.42 0.13 0.29 NI N N1*= Not Inspected Growth Tabic 5 7/22/991:32 PM 3-12
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[ j Table 3-9 Probe Wear and Analyst Variability - Tabulated Values Analyst Variability Probe Wear Variability Std. Dev = 10.3% Mean = 0.0% Std. Dev = 7.0% Mean = 0.0%
No Cutoff Cutoff at +/- 15%
Value Cumul. Prob. Value Cumul. Prob.
l -40.0% 0.00005 < - 15.0% 0.00000
-38.0% 0.00011 -15.0% 0.01606
-36.0% 0.00024 -14.0% 0.02275
-34.0% 0.00048 -13.0% 0.03165
-32.0% 0.00095 -12.0% 0.04324
-30.0% 0.00179 -11.0% 0.05804
-28.0% 0.00328 -10.0% 0.07656 l -26.0% 0.00580 -9.0% 0.09927
-24.0% 0.00990 -8.0% 0.12655 l -22.0% 0.01634 -7.0% 0.15866
-20.0% 0.02608 -6.0% 0.19568 1 -18.0% 0.04027 -5.0% 0.23753 l -16.0% 0.06016 -4.0% 0.28385
- 14.0% 0.08704 -3.0% 0.33412
-12.0% 0.12200 -2.0% 0.38755
-10.0% 0.16581 -1.0% 0.44320
-8.0% 0.21867 0.0% 0.50000
-6.0% 0.28011 1.0% 0.55680 j -4.0% 0.34888 2.0% 0.61245
-2.0% 0.42302 3.0% 0.66588 0.0% 0.50000 4.0% 0.71615 2.0% 0.57698 5.0% 0.76247 4.0% 0.65112 6.0% 0.80432 l 6.0% 0.71989 7.0% 0.84134 8.0% 0.78133 8.0% 0.87345 10.0 % 0.83419 9.0% 0.90073 12.0 % 0.87800 10.0 % 0.92344 14.0% 0.91296 11.0% 0.94196 16.0 % 0.93984 12.0 % 0.95676 18.0% 0.95973 13.0 % 0.96835 20.0 % 0.97392 14.0 % 0.97725 22.0 % 0.98366 15.0 % 0.98394 24.0 % 0.99010 > 15.0% 1.00000 26.0 % 0.99420 28.0 % 0.99672 30.0 % 0.99821 32.0 % 0.99905 34.0 % 0.99952 36.0 % 0.99976 38.0 % 0.99989 40.0 % 0.99995 I I
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e 4.0 DATA BASE APPLIED FOR LEAK AND BURST CORRELATIONS The leak and burst correlations applied to project SLB leak rate and tube burst
- probability for EOC-10 are based on the latest voltage-based repair criteria database for 7/8" tubes approved by the NRC (Reference 8-6), and it is documented in . Reference 8-7. For the SLB leak race correlation, the NRC recommends that Model Boiler specimen 542-4 and Plant J.1 pulled tube R8C74, TSP 1 be included in the database. This database is referred to as the NRC-approved EPRI database and l the correlations for probability of leakage and leak rate as a function of bobbin voltage presented in Reference 8-7 include those datapoints.
l Sequoyah plant pulled tube data from 1993 (Unit-1) and 1996 (Unit-2) inspections 1 are included in the database applied. The data for the 2 tube segments pulled from the Unit-2 SGs during the EOC-7 outage are not yet included the database. The i database update to include this data has been completed, but the updated database is not yet reviewed and approved by the NRC. However, an evaluation of the effects l of adding the latest Sequoyah Unit-2 data to the reference database in Reference 8-7 indicates that the burst pressure, leak rate and the probability ofleak correlations to the common logarithm of the bobbin amplitude would not be meaningfully changed.
A leak rate correlation can now be applied to 7/8" tubes based on the p-value for the ;
slope of the leak rate correlation on a one-sided basis meeting the Generic Letter 95- !
05 requirement. The following leak rate correlation is developed in Reference 8-7 for 7/8" tubes. !
l < ,
Leak Rate (l/hr) = 10 s The above leak rate correlation was used to perform EOC-10 SLB leak rate l projections for the limiting SG.
l The leak rate data in the database represent a room temperature measurement of leakage at prototypic SLB conditions (i.e., leakage at SLB conditions was condensed
- l. and measured at room temperature). Therefore, SLB leak rates calculated using the I leak rate correlations provides a volumetric rate at room temperature.
q:\ ape \ ten 99\ ten 90 day. doc 4-1
5.0 SLB ANALYSIS METHODS Monte Carlo analyses are used to predict the EOC-10 voltage distributions and to calculate the SLB leak rates and tube burst probabilities for both the actual EOC-9 voltage distributions and the predicted EOC-10 voltage distributions. These methods are described in the generic methods report of WCAP-14277, hvision 1 (&ference 8-
- 2) and the prior reports for Sequoyah Unit-2 (hferences 8-3 and 8-4), and are in accord with NRC Generic Letter 95-05 (hference 8-1). Leak rates calculated with the WCAP-14277 methodology provide a volumetric leak rate at room temperature, and they are compared with an allowable volumetric leak rate at room temperature.
At the time of performing projections for the EOC-9 conditions in 1997, a leak rate versus bobbin voltage correlation could not be applied for 7/8" tubes since the leak rate database for 7/8" tubes did not satisfy the requirement applied then for a SLB leak rate versus bobbin voltage correlation (p-value for the correlation slope parameter calculated on a two-sided basis less than 5%). Therefore, EOC-9 leak rate projections were carried out using a distribution ofleak rate data that is independent of voltage. In order to ensure consistency in the comparison ofleak rates estimated !
with projected EOC-9 voltages with those based on the actual measured voltages, l leak rates based on the actual measured EOC-9 voltages were also calculated without applying a leak rate correlation As mentioned in the previous section, a leak rate correlation can now be applied for 7/8" tubes based on the p-value for the slope of the leak rate correlation calculated on a one-sided basis meeting the Generic Letter 95-05 requirement. Therefore, the leak rate analysis for the EOC-10 condition was carried out using the leak rate vs. bobbin correlation shown in the previous section.
l l
l q:\ ape \ ten 99\ ten 90 day. doc 5-1
,- l l
6.0 BOBBIN VOLTAGE DISTRIBUTIONS This section describes prediction of the EOC voltage distribution used for evaluating the SLB leak rate and tube burst probability at the end of the operating period. The calculation consists of establishing the initial conditions (i.e., the bobbin indication population distribution) based on eddy current inspection data and projecting the indication growth over the operating period. Since indication growth is considered proportional to operating time, the limiting tube conditions occur at the end of any given time period or cycle.
The bobbin voltage distribution established for the BOC conditions is adjusted for measurement uncertainty using a quantity termed probability of detection, as described in the following paragraphs. Other input used for predicting the EOC voltage distribution and the results are presented below.
6.1 Probability of Detection The number of bobbin indications used to predict tube leak rate and burst probability is obtained by adjusting the number of reported indications to account for measurement uncertainty and confidence level in voltage correlations. This is accomplished by using a POD factor. Adjustments are also made for indications either removed from 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:
Ni Nr. ars - Na,p.i,,o + Noepios,co ,
POD where:
NrotnTs = Number of bobbin indications being returned to service for the next cycle.
Ni = Number of bobbin indications (in tubes in service during the previous cycle) reported in the current inspection.
POD = Probability of detection.
Nrepairea = Number of Ni which are repaired (plugged) after the last cycle.
Naepioggea = Number of previously-plugged indications which are deplugged after ,
the last cycle and are returned to service.
l There were no deplugged tubes returned to service in the recent inspection.
The NRC generic letter (Reference 8-1) requires the application of a constant POD =
0.6 to define the BOC distribution for the EOC voltage projections, unless an alternate POD is approved by the NRC. A voltage-dependent POD known as POPCD q:\apc\ ten 99\ ten 90 day. doc 6-1
F .
i 1 has been established using data from 18 post-1992 inspections at 10 different plants.
It takes into account newly initiated indications that are important for voltage-based repair criteria application. The development of POPCD and supporting data are presented in Reference 8-7. POPCD data as a function of bobbin voltage is illustrated graphically in Figure 6-1. It is evident from Figure 6-1 that below about 0.4 volt the NRC recommended POD of 0.6 is non-conservative while it is too conservative above about 0.5 volt. It is ofinterest to apply POPCD for sensitivity analysis and compare the results for the case with a POD value of 0.6.
6.2 Cycle Operating Time The following operating period values are used in the voltage projection calculations:
Cycle 8 = 450 EFPD Cycle 9 = 509.2 EFPD Cycle 10 = 483 EFPD (estimated) 6.3 Predicted EOC-10 Voltage Distributions Bobbin voltage. projections start with a cycle initial voltage distribution that is projected to the end of the anticipated cycle using growth rates adjusted for cycle operating time period. The overall growth rates for each of the Sequoyah Unit-2 steam generators during the last two operating periods, as represented by their CPDFs, are shown on Table 3-5. A Generic Letter 95-05 requirement is that limiting growth rate for the past two cycles of operation should be used in the projections. As noted in Section 3.2, the 1996 - 1997 operation (Cycle 8) growth rates slightly exceed those of the 1997 - 1999 (Cycle 9) operation and are used to predict the EOC-10 bobbin' voltage distributions. Further conservatism for the EOC-10 bobbin voltage prediction is provided by the use of the larger of the composite growth rate for all SGs and the SG-specific growth rate in projecting EOC voltages for each SG. The methodology used in the calculations of EOC bobbin voltage distributions is described in Reference 8-2.
For each SG, the initial bobbin voltage distribution of indications being returned to service for the next cycle (BOC-10) is derived from the actual EOC-9 inspection results adjusted for tubes that are taken out of service by plugging. The Cycle 10 bobbin voltage population data is summarized on Table 6-1. It shows EOC-9 bobbin l voltage indications, the subsequent plugged indications (which were in service for i
Cycle 9 and then taken out of service, albeit not all for reasons of ODSCC at TSP),
and the BOC-10 indications corresponding to a constant POD value of 0.6 as well as the voltage dependent generic POPCD. POPCD distribution is developed based on bobbin and RPC data from 18 EC inspections at 10 different plants, and its q:\apc\ ten 99\ ten 90 day. doc 6-2 I
. 1
- l l
1 distribution is shown in Figure 6-1.
Table 6-2 provides the EOC-10 voltage distributions predicted using the BOC-10 voltage distribution shown in Table 6-1. As anticipated, the largest number of ;
indications is predicted for SG-4, about 677 indications for a constant POD of 0.6.
The assumed BOC-10 and predicted EOC-10 bobbin voltage frequency distributions for all four SGs are also graphically illustrated on Figures 6-2 to 6-5. The largest bobbin voltage predicted for EOC-10 is in SG-3, and its magnitude is 2.6 volts for a constant POD of 0.6 i
6.4 Comparison of Predicted and Actual EOC-9 Voltage Distributions The actual EOC-9 bobbin voltage distributions and the corresponding predictions I presented in the last 90-day report (for EOC-9 inspection, Reference 8-3), are compared in Table 6-3 and on Figures 6-6 and 6-7. SG-4 was predicted to be limiting for EOC-9 based on the total number ofindications and it was confirmed to have the highest number of indications. Also, the largest indication was found in SG-3, as predicted. The total number ofindications for all SGs is overpredicted by 15% to 38%
in the licensing-basis analysis with a POD of 0.6, and the voltage population over 1 volt are overpredicted by an even greater percentage. The overprediction of indications in virtually every voltage size range demonstrates conservatism in the projection methodology. EOC-9 voltage distributions based on the voltage-dependent j POPCD also yields conservative results, and the extent of overprediction in all voltage ranges is comparable to the predictions with POD =0.6. Thus it is concluded .
that the voltage-dependent POPCD yields conservative results. !
l q:\ ape \ ten 99\ ten 90 day. doc 6-3 k
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1 7.0 TUBE LEAK RATE AND TUBE BURST PROBABILITY 7.1 Calculation of Leak Rate and Tube Burst Probabilities This section presents the SLB leak rates and tube burst probabilities obtained using f the actual measured EOC-9 voltage distributions and projected EOC-10 voltage i distributions. The calculation utilizes correlations relating bobbin voltage amplitudes (either measured or calculated) to free span burst pressure, probability ofleakage and '
associated leak rates for ODSCC indications at TSP locations. The methodology used is documented in Reference 8-2, and is consistent with NRC criteria and guidelines of References 8-1. Leak rates based on the actual measured voltages are calculated using a leak rate correlation independent of voltage, and the leak rate calculations based on the projected EOC-10 voltages utilize the leak rate vs. bobbin voltage correlation shown in Section 4.0. The latest leak and burst correlations approved by the NRC were applied for the EOC-10 projections, and they are documented in Reference 8-7. The same database used earlier for the EOC-9 projections, documented in Reference 8-8, was applied for the actual EOC-9 conditions. The l
calculated leak rates are volumetric rates at room temperature and they should be j compared with allowable leak rates at room temperature. j l
7.2 Predicted and Actual Leak Rate and Tube Burst Probability for EOC-9 l (Condition Monitoring Assessment)
Analyses were performed to calculate SLB leak rate and tube burst probability for the actual bobbin voltage distribution at EOC-9 previously presented in this report. The results of Monte Carlo calculations performed based on the actual voltage distributions including NDE uncertainties are shown on Table 7-1. Projections for EOC-9 conditions for all four SGs presented in the last 90-day report are also included for comparison in Table 7-1. The allowable SLB rate for the last operating cycle (Cycle 9) was 2.7 gpm (at room temperature).
Comparisons of the EOC-9 actuals with the corresponding projections indicate the following:
a) The actual number of indications found during EOC-9 inspection in all SGs are significantly below those projected at the BOC-9 using POD =0.6 as well as with POPCD. The peak voltages measured for all four SGs are also lower than projected with both POD distributions.
b) SG-4 was projected to be the limiting steam generator for EOC-9 based on EOC-8 data, and SG-4 was confirmed to have the limiting leak rate based on the actual bobbin measurements for EOC-9. All estimated EOC-9 leak rates q:\ ape \ ten 99\ ten 90 day. doc 7-1
V .,
+
are well below the acceptance limit for Sequoyah Unit-2.
1 c) The tube burst probabilities based on the actual voltage distributions are equal to or less than the projections with POD =0.6 as well as POPCD for all SGs, l except for SG-3 where the projected value for POD =0.6 is lower than the actual by 2.5x104; however, a difference of this magnitude is not significant and can be attributed to the difference in the random number sequences used in the Monte Carlo analysis. The highest tube burst probability was calculated for SG-3 which had the largest indication detected in this inspection. The overall EOC-9 projections bound the actual burst probabilities since the projected SG-4 burst probability of 5.3x104 is higher than the SG-3 actual burst probability of 3.7x104- All calculated tube burst probabilities are well below the NRC reporting guideline of10 2, The limiting SLB leak rate (0.58 gpm) was obtained assuming that the leak rate is independent of bobbin voltage because the EOC-9 projection utilized that assumption.
Since a leak rate vs. bobbin voltage can now be applied for 7/8" tubes, EOC-9 SLB leak rate for the limiting case (SG-4) was also repeated using the leak rate correlation for 7/8" tubes presented in Reference 8-7. As shown Table 7-1, application of the leak rate correlation leads to an order of magnitude reduction in the limiting leak rate; therefore, there is more margin in the SLB leak rate than implied by the results in Table 7-1. A leak rate correlation is applied in the operational assessment analysis l for EOC-10.
1 In summary, the limiting SLB leak rate (0.06 gpm at room temperature, applying voltage correlation) and tube burst probability (3.7x104) calculated using the actual measured EOC-9 bobbin voltage distributions are nearly 2 orders of magnitude below the corresponding allowable limits. The results meet the Generic Letter 95-05 requirement for continued Cycle 10 operation.
7.3 Projected Leak Rate and Tube Burst Probability for EOC-10 (Operational Assessment)
Using the methodology previously described, calculations were performed to predict the EOC-10 conditions of all 4 SGs in Sequoyah Unit-2, and the results are summarized in Table 7-2. EOC-10 bobbin voltage distributions as well as the leak rates and tube burst probabilities based on these distributions are predicted. As mentioned earlier, EOC-10 leak rates and tube burst probabilities are calculated using the latest burst and leak correlations presented in Reference 8-7. The projected leak rates are compared with the allowable leak rate at room temperature (2.7 gpm).
The leak rate vs. bobbin voltage correlation shown in Section 4.0 is applied. Since q:\apc\ ten 99\ ten 90 day. doc 7-2
E growth rate for Cycle 8 is higher than that for Cycle 9, Cycle 8 growth data were used in the EOC-10 projection analysis.
The predicted EOC-10 SLB leak rate and burst probability for all four SGs are shown in Table 7-2. It is evident that the projected maximum voltages, SLB leak rates and tube burst probabilities for the EOC-10 condition for SGs 1 to 3 are in a narrow range, while SG 4 is predicted to have a relatively higher SLB leak rate and tube burst probability. SG-3 is predicted to have a slightly higher peak voltage The limiting EOC-10 SLB leak race predicted for SG-4 based on constant POD of 0.6 is 0.94 gpm (room temperature) which is only a 1/3d the current licensed limit of 2.7 gpm at room temperature. The limiting EOC-10 burst probability with POD =0.6, also predicted for SG-4, is 4.7x104; it is better than 2 orders of magnitude below the NRC acceptance limit of 10 2 The results based on the voltage-dependent POPCD also show similar margins. Thus, the projected EOC-10 results meet the voltage-dependent repair criteria requirement for continued operation.
In summary, SLB leak rates and tube burst probabilities projected for EOC-10 for all four SGs using the NRC-mandated POD = 0.6 meet the SFR limits for Sequoyah Unit-2. Results based on voltage dependent POPCD show even a greater margin between EOC-10 predictions and acceptance limits.
)
q:\apc\ ten 99\ ten 90 day. doc 7-3 m
l Table 7-1 Sequoyah Unit-2 1997 EOC-9 Outage )
Summary of Calculations of Tube Leak Rate and Burst Probability
]
Number SLB Steam POD of Max. Burst Probability bak Generator Indicationsu) Volts Rate (2) 1 Tube 1 or More (epm)
Tubes EOC - 9 PROJECTIONS (Based on Leak and Burst Database Presented in Reference 8- 8 Leak Rate Correlation Not Used) 1 85 1.7 1.2x104 1.2x104 0.09 2 83.3 2.1 3.1x10-5 3.1x104 0.15 3 159.7 2.4 1.2x104 1.2x10-5 0.39 4 470.0 2.3 5.3x104 5.3x104 1.22 1 97.2 1.6 1.2x10-5 1.2x10-5 0.09 j 2 84.5 2.0 1.9x104 1.9x104 0.13 3 169.6 2.3 3.7x104 3.7x10-5 0.33 POPCD 4 492.5 2.2 4.3 x 10-5 4.3x104 1.10 EOC - 9 ACTUALS (Same Leak and Burst Database as Used in the Above Projections Leak Rate Correlation Not Used) 1 1 69 1.3 1.2 x 10-5 1.2 x 10- 5 0.02 2 1 67 1.4 1.9 x 10-5 1.9 x 10-5 0.05 3 1 116 2.0 3.7 x 10-5 3.7 x 10-5 0.16 4 1 407 1.6 1.9 x 10-5 1.9 x 104 0.58 Leak Rate Correlation Presented in Reference 8-7 Applied SG-4 0.6 470.0 2.3 3.1x10-5 (8) 3.1x104 (3) 0.14(a)
Pro _iected POPCD 492.5 2.2 3.7x10-5 (3) 3.7x104 (a) 0.12(a)
SG-4 1 407 1.6 1.2x104 (3) 1.2x104 (3) 0.06<a>
Actual Eqtes (1) Adjusted for POD.
(2) Volumetric leak rate adjusted to room temperature.
(3) Leak and burst database shown in Reference 8-7 applied.
q:\apc\ ten 99\ ten 90 day. doc 7-4
r.
.o Table 7-2 Sequoyah Unit-2 '
Summary of Projected Tube Leak Rate and Burst Probability for EOC-10 (Based on projected Cycle 10 length 483 EFPD)
Steam POD No. of Max.
Comments Generator Indic. Volts One or Leak ationsm 1 Tube More Rate Tubes (epm)*
Leak and Burst Database and Correlations Reported in Reference 8-7 Applied 1(3) 115.0 1.9 1.2x10-5 1.2x10-5 0.09 2(3) 0.6 110.7 2.1 1.9x10-5 1.9x10-5 0.12 3<a) 191.3 2.6 1.2x10-8 1.2x10-5 0.24 4(o 677.3 2.4 4.7x10-5 4.7x10 5 0.94 Leak rate Correlation 1(8) 118.5 1.8 1.2x10-8 1.2x10-5 0.07 applied 2(3) POPCD 101.2 2.1 1.2x10-8 1.2x10-5 0.10 3(3) 186.4 2.4 3.1x10-5 3.1x10-5 0.20 4(o 623.7 2.3 5.3x10-5 5.3x10-5 0.77 Nntes (1) Number ofindications adjusted for POD.
(2) Volumetric leak rate adjusted to room temperature.
(3) All SG composite Cycle 8 growth rate distribution applied.
(4) SG-4 specific Cycle 8 growth rate distribution applied.
i l
I l
q:\ ape \ ten 99\ ten 90 day. doc 7-5
f
/
8.0 REFERENCES
8-1 NRC Generic Letter 95-05, " Voltage-Based Repair Criteria for Westinghouse Steam Generator Tubes Affected by Outside Diameter Stress Corrosion Cracking", USNRC Office of Nuclear Reactor Regulation, Augurt 3,1995.
8-2 WCAP-14277, Revision 1, "SLB Leak Rate and Tube Burst Probability Analysis Methods for ODSCC at TSP Intersections," Westinghouse Nuclear Services Division, December 1996.
8-3 SG-98-01-002, "Sequoyah Unit-2 Cycle 9 Alternate Plugging Criteria 90-Day Report," Westinghouse Nuclear Services Division, January 1998.
8-4 SG-96-08-010, "Sequoyah Unit-2 Cycle 8 Alternate Plugging Criteria 90-Day Report," Westinghouse Nuclear Services Division, August 1996.
8-5 Letter from B. W. Sheron, Nuclear Regulatory Commission, to A. Marion, Nuclear Energy Institute, dated February 9,1996.
8-6 " Evaluation of Proposed Update to SGDSM Database and Modifications to the Methodology to Assess Steam Generator Tubing Outside Diameter Stress Corrosion Cracking," G. C. Lainas (USNRC) to D. J. Modeen (NEI),
November 20,1998. j 8-7 Addendum-2 to EPRI Report NP-7480-L, " Steam Generator Outside Diameter Stress Corrosion Cracking at Tube Support Plates - Database for Alternate Repair Criteria," April 1998.
8-8 Letter from S. C. Jain, Duquesne Light Company, to U.S. Nuclear Regulatory Commission, " Beaver Valley Power Station Unit No.1, Docket No. 50-334, License No. DPR-66, Steam Generator Pulled Tube Data (Supplemental)
Supporting Alternate Tube Plugging Criteria Implementation," dated March 27,1996.
l q:\ ape \ ten 99\ ten 90 day. doc 8-1 m