ML20211B597
| ML20211B597 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 07/31/1999 |
| From: | WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| To: | |
| Shared Package | |
| ML20211B576 | List: |
| References | |
| SG-99-07-006, NUDOCS 9908250039 | |
| Download: ML20211B597 (44) | |
Text
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SG-99-07-006 PRAIRIE ISLAND UNIT-1 CYCLE 20 VOLTAGE-BASED REPAIR CRITERIA 90-DAY REPORT July 1999 O
Westinghouse Electric Company LLC Nuclear Services P.O. Box 158 Madison, Pennsylvania 15663-0158 gge2koo$ os g2
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SG 99-07-006 l
PRAIRIE ISLAND UNIT-1 i
CYCLE 20 VOLTAGE-BASED REPAIR CRITERIA 90-DAY REPORT ,
July 1999 q:\spc\ nap 9 Nap 90 day. doc
PRAIRIEISLAND UNIT-1 CYCLE 10 VOLTAGE BASED REPAIR CRITERIA 90-DAY REPORT TABLE OF CONTENTS Pace No.
1.0 Introduction 1-1 2.0 Summary and Conclusions 2-1 3.0 EOC-19 Inspection Results and Voltage Growth Rates 3-1 3.1 EOC-19 Inspection Results 3-1 3.2 Voltage Growth Rates 3-2 3.3 Probe Wear Criteria 3-3 3.4 Assessment of RPC Confirmation Rates 3-4 3.5 Probability of Prior Cycle Detection (POPCD) 3-5 3.6 NDE Uncertainties 3-6 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 ProbabilityofDetection 6-1 6.2 Cycle Operating Time 6-2 6.3 Predicted EOC-20 Voltage Distributions 6-2 6.4 Comparison of Predicted and Actual EOC-19 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-19 (Condition Monitoring Assessment) 7-1 7.3 Projected Leak Rate and Tube Burst Probability for EOC-20 (Operational Assessment) 7-2 8.0 References 8-1 q:\apc\nsp99\nsp90 day. doc i
PRAIRIEISLAND UNIT-1 CYCLE 10 VOLTAGE BASED REPAIR CRITERIA 90-DAY REPORT
1.0 INTRODUCTION
This report provides the Prairie Island Unit-1 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 20 as outlined in the NRC Generic Letter 95-05 (Reference 8-1). The 2-volt repair criterion was implemented at the beginning of the last cycle (Cycle 19).
Information required by GL 95-05 is provided in this report including projections of bobbin voltage distributions, leak rates and burst probabilities at the end of the current cycle (EOC-20). The methodology used in these evaluations is described in Reference 8-2 and is consistent with the NRC SER for 2-volt repair criteria (Reference 8-3) as well as the methodology used in the last 90-day report (Reference 8-4).
The application of the 2-volt repair criteria for outside diameter stress corrosion cracking (ODSCC) indications at TSPs in the Prairie Island Unit-1 SGs involves a complete,100% Eddy Current (EC) bobbin coil inspection of all TSP intersections in the tube bundles of both SGs and plugging of tubes with' TSP indications greater than 2 volts that are confirmed by a Rotating Pancake Coil (RPC) probe. In the EOC-19 inspection, all TSP intersections with a Distorted Signal Indication (DSI) were RPC inspected due to the possibility of wastage in combination with ODSCC indications. The Prairie Island Unit-1 EC inspection plan also calls for RPC testing of TSP locations with the following types of bobbin signals: dents 22 volts, mixed residual indications (MRIs) 22 volts, copper deposit (CUD) signals 21 volt, scale deposit (DEP) signals 22 volts, indications not reportable (INRs) 22 volts, cold leg thinning 240% through wall, and cold leg thinning <40% through and 21.5 volts.
No MRI, DEP or CUD bobbin signals were detected during the EOC-19 inspection.
Eddy current and repair data for TSP indications from the EOC-19 inspection are provided in Section 3. The actual EOC-19 voltage distributions as well as the leak rates and tube burst probabilities calculated for these distributions are compared with the projections for EOC-19 conditions performed after the last (EOC-18) outage. Leak rates and burst probabilities projected for the end of the current cycle (EOC-20) are reported in Section 7 and compared with allowable limits.
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2.0
SUMMARY
AND CONCLUSIONS SLB leak rate and tube burst probability analyses were performed for both SGs based i on their actual measured EOC-19 voltage distributions and the results compared with the projections performed after the last (EOC-18) outage. The total number of indications found at TSPs in each SG during the current inspection ad the measured 3
peak voltages are less than those projected using a constant POD of 0.6 per the Generic Letter 95-05 requirements. Also, SLB leak rates and tube burst probabilities calculated using the actual measured voltages are equal'to or below the projected values.
For the actual EOC-19 bobbin voltage distribution, the largest SLB leak rate is calculated for SG-11 which had the larger number ofindications among the 2 SGs as well as the largest indication detected in this inspection. Its magnitude is 0.021 gpm (at room temperature), and it was calculated using the same leak and burst database used for the projections performed after the last outage. A voltage dependent leak rate correlation based on the latest pulled tube database for 7/8" tubes was applied.
The limiting SLB leak rate based on the actual EOC-9 voltages is almost a factor of 50 below the current allowable SLB leakage limit (1 gpm at room temperature), and it is also below its projected value. The largest conditional tube burst probability based on' the measured voltage data is also calculated for SG-11. Its magnitude is 1.9x104, 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 l requirements for continued Cycle 20 operation.
i I
A total of 466 indications were found in the EOC-19 inspection, and the largest indication detected had 1.85 volts. Only one indication was found on the cold leg side -
in both SGs combined, and it had a bobbin voltage of 0.75 volts. Over 50% of the indications in both SGs are under 0.5 volts, which explains the low values of SLB leak l rates and tube burst probabilities calculated using the measured EOC-19 voltages. "
All TSP indications detected were RPC inspected; only one indication had a single volumetric indication and the rest showed no degradation. The tube with the volumetric indication was repaired. The RPC inspection did not find any axial '
indication extending outside a TSP intersection.
An augmented RPC inspection was performed consistent with the GL 95-05 requirements. All TSP intersections with a dent over 5 volts were RPC inspected in this outage; no PWSCC axial or circumferential cracks were detected. The augmented RPC inspection plan also called for inspection of all MRIs 22 volts, scale deposit signals 22 volts and copper deposit signals 21 volt. No bobbin signals indicating MRIs, scale deposits or copper deposits were detected during this q:\apc\nsp99\nsp90 day. doc 2-1
inspection.
Of the 466 indications detected,110 were new indications and the beginning of cycle voltages were not available for new indications. Therefore, growth rate statistics for Cycle 19 were established using the data for the 356 indications detected in both EOC-18 and EOC-19 inspections. Both SGs show a small negative value for the average Cycle 19 growth indicating that most indications had only a modest growth during the cyclejust completed.
SG-11 is projected to have a larger number ofindications among the two SGs as well as the largest indication at EOC-20 conditions. The EOC-20 leak rate projection was performed using a 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-20 SLB leak rate projected for SG-11 is 0.46 gpm, which is less than one-half of the current licensed limit of 1 gpm. All leak rate values quoted are equivalent volumetric rates at room temperature. The limiting EOC-20 tube burst probability, 2.5x10- 5, is calculated for SG-12, and it is more than two orders of magnitude 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 20 are met.
q:\apc\nsp99\nsp90 day. doc 2-2
t l 3.0 EOC-19 INSPECTION RESULTS AND VOLTAGE GROWTH RATES l
3.1 EOC-19 INSPECTION RESULTS l
In ' accordance with the guidance provided by the NRC Generic Letter 95-05 (Reference 8-1) for application of voltage-based repair criteria, the EOC-19 inspection
- of the Prairie Island Unit-1 SGs consisted of a complete,100% EC bobbin probe, full
!. length e ; amination of the tube bundles in both SGs. A 0.720 inch diameter probe was used to inspect all hot and cold leg TSP intersections where the 2-volt repair i criterion was applied.
Prairie Island provided summaries of the EC data for both steam generators via e-mail, References 8-5 through 8-8, which were analyzed to develop the database applicable to the voltage based repair criteria. The results of these analyses are summarized in Table 3-1 and are discussed below.
. All inaications detected during the present inspection were under the 2-volt repair l )imit, and only 10 indications exceeded 1 volt. Only one TSP indication was detected on the cold leg side in both SGs combined, and it had a bobbin voltage 0.75 volts.
t Over 50% of the indications in both SGs were under 0.5 volts, and therefore SLB leak rates and tube burst probabilities based on these voltages are expected to be small.
All TSP indications detected were RPC inspected; only one indication had a 1 volumetric indication and.the rest showed no degradation. The tube with the volumetric indication was repaired. The RPC inspection did not find any indication ;
extending outside a TSP intersection. i
! An augmented RPC inspection was performed consistent with the GL 95-05 l requimments. All TSP intersections that had dents 2 2 volts were RPC inspected in l
this inspection; no PWSCC axial or circumferential cracks were detected. The Prairie Island Unit-1 EC inspection plan for TSP intersections also includes RPC testing of MRIs 2 2 volts CUDS 21 volt, DEPs 22 volts, INRs 22 volts, cold leg thinning 240%
L through wall, and cold leg thinning <40% through and 21.5 volts. No MRI, DEP or i CUD bobbin signals were detected during the EOC-19 inspection.
A summary of eddy current signal voltage distributions for all steam generators is
- j. shown on Table 3-1, which tabulates the number of field bobbin indications, the l number of these field bobbin indications that were RPC inspected, the number of !
RPC confirmed indications, and the number ofindications removed from service due i to tube repairs.~ The indications that remain active for Cycle 20 operation is the difference between the observed and the ones removed from service. Only a total of
- 7 indications were taken out of service due to tube repairs in both SGs combined. Of these 7. indications, only the single. volumetric indication in SG-11 was repaired q:\ ape \nsp09\nspn90 day. doc 3-1 l
' because of potential ODSCC concerns, and the remaining 6 indications were in tubes repaired for reasons other than TSP indications. Figure 3-1 shows the actual bobbin voltage distribution for tubes that were in service during Cycle 19, as determined from the EOC-19 EC inspection. Since only 7 (out of 466) indications were taken out i of service due to tube repairs, the bobbin voltage distribution ofindications returned to service at BOC-20 is essentially the same as the actual bobbin voltage distribution shown in Figure 3-1. A review of Table 3-1 indicates that more indications (a quantity of 315, with 9 indications above 1.0 volt) were returned to service for Cycle 20 operation in SG-11 than in SG-12. Therefore, SG-11 is likely to be the limiting SG at EOC-20.
]
The distribution of EOC-19 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 (336 of the 466 PIs, or about 72%, '
occurred in the first two hot leg TSPs), although the mechanism does extend to higher TSPs. Only one TSP bobbin indication was reported on the cold leg side in both SGs combined. The occurrence of a majority of ODSCC indications in the first two TSPs on the hot leg side in Prairie Island Unit-1 shows predominant dependency on temperature, which is consistent with that observed at other plants.
3.2 Voltage Growth Rates For projection ofleak rates and tube burst probabilities at EOC-20 condition, voltage growth rates were developed from EOC-19 and EOC-18 inspection data. Of the 466 indications detected,110 are new indications and beginning of cycle voltage was not available for new indications. Therefore, growth rate statistics for Cycle 19 were established using only the data for the 356 indications detected in both EOC-18 and EOC-19 inspections.
Table 3-3 summarizes growth data for Cycle 19. The average Cycle 19 voltage growth rates for both SGs have a small negative value indicating that the magnitude of growth during Cycle 19 was small and comparable to NDE uncertainties. Indications with a BOC bobbin voltage above 0.75 volta show a larger negative gmwth than those i below 0.75 BOC volts in both SGs, which, although unusual, is not meaningful l because of the small growth magnitude.
Table 3-4 pmvides a comparison of average growth data for the last 3 operating cycles !
for Prairie Island Unit-1, and the data shows that TSP indications in both SGs had l
- only a modest growth during the last 3 cycles. The average growth rates were below l 3%/EFPY in all 3 cycles. Table 3-5 shows the cumulative pmbability distribution
-(CPDF) of growth rate per EFPY for each steam generator during Cycle 19. The grtwth CPDF data are also plotted in Figure 3-3. The curve labelled ' cumulative'in qdape\nsp99\nspn90 day. doc 3-2
Figure 3-3 represents averaged composite growth data from both SGs. The average growth rate distribution for Cycle 19 is compared with that for the last cycle (Cycle 18)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 that Cycle 18 growth distribution is slightly more limiting than the Cycle 19 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 18 growth data will be applied to obtain EOC-20 projections.
Accc. Jing to the Westinghouse tube integrity analysis methodology presented in 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 above, Cycle 18 growth rates should be used to perform EOC-20 projections as they are higher than the Cycle 19 growth rates. Since the Cycle 18 growth rates for SG-11 are below the composite growth rate (see Table 3 3),
the composite gmwth rate is applied to SG-11 to provide a conservative basis for predicting EOC-20 conditions. Cycle 18 growth rates for SG-12 are slightly higher than the composite growth rate; however, SG-12 gmwth distribution is based on fewer indications than the minimum number (200) required by GL 95-05.
Nevertheless, EOC-20 predictions for SG-12 were 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 BOC voltage. To determine if Prairie Island Unit-1 exhibited a similar trend during Cycle 19, growth rate data for Cycle 19 were plotted against BOC voltage, and the resulting plot is shown in Figure 3-5. It is evident that the Cycle 19 growth decreases with increasing BOC voltage. The indications with the top 3 growth values had a BOC voltage under 0.4 volts. Thus, it is conservative to assume growth is independent of BOC voltage.
~
Table 3-6 lists the top 30 indications from the standpoint of growth during Cycle 19.
This data shows more clearly that there was only a modest growth during Cycle 19 since all but two of these 30 indications had growth under 0.5 volts. All 30 indications were detected during the last (EOC-18) inspection (i.e., they are not new indications).
8.8 Probe Wear Criteria An alternate probe wear criterion discussed in Reference 8-9 was applied during the EOC-19 inspection. This criterion was also applied during the last inspection for Prairie Island Unit-1. When a probe does not pass the 15% wear limit, this
- q:\apc\nsp09\nspn90 day. doc 3-3
e alternate criterion requires that all tubes with indications detected 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.
l Only one tube had an indication greater than the 1.5 volt needed to trigger a retest j
condition for worn probes and that tube was tdsted with a good (non worn) probe.
l Therefore, no tubes had to be retested because of probe wear. The alternate probe l wear criteria used in the EOC-19 inspection is consistent with the NRC guidance l provided in Reference 8-5.
8.4 Assessment of RPC Confirmation Rates Generic Letter 95-05, upon NRC approval, allows inclusion of only a fraction of indications for which RPC detected no degradation (NDD) in performing the Monte Carlo analyses for leak rate and tube burst probability. The _ fractional value appropriate for voltage-based repair criteria evaluations is the largest RPC confirmation rate for prior cycle RPC NDD indications from the last two outages.
This section tracks the 1997 EOC-18 indications left in service at BOC-19 relative to RPC inspection results in 1999 at EOC-19. The composite results for both SGs are given in Table 3-7. For 1997 bobbin indications left in service, the indications are tracked relative to 1997 RPC confirmed,1997 RPC NDD,1997 bobbin indications not RPC inspected and 1997 bobbin indications with no indication found in 1999. Also l
included are new 1999 indications. The table shows, for each category ofindications, l the number ofindications RPC inspected and RPC confirmed in 1999 as well as the
{
percentage of RPC confirmed indications. One hundred and ten (110) new indications '
were detected during the EOC-19 inspection, and only one of them was confirmed by ,
RPC (a single volumetric indication). Forty-nine prior cycle (EOC-18) indications l wem called NDD by bobbin in the present inspection. 1 All 356 RPC NDD indications left in service at BOC-19 were RPC tested at EOC-19 and none of them were confirmed. Therefore, EOC-19 RPC confirmation rate for prior cycle RPC NDD indications is 0%. Since all indications left in service at the beginning of the current cycle are RPC NDDs, this RPC confirmation rate data suggest that only a small fraction of BOC-20 indications need to be considered in the tube integrity evaluations for the EOC-20 condition. However, since NRC approval for using a fraction of RPC NDD indications in tube integrity evaluations has not been obtained, all indications left in service were included in the EOC-20 tube integrity evaluation reported here.
I f q:\apc\nsp99\nspn90 day. doc 3-4
3.5 Probability of Prior Cycle Detection The inspection results at EOC-19 permit an evaluation of the probability of detection (POD) at the prior EOC-18 inspection. This evaluation provides data to support a voltage-dependent POD distribution. For voltage-based repair criteria applications, the important indications are those that could significantly contribute to EOC leakage or burst probnidlity. These signi6 cant indications can be expected to be detected by bobbin and confirmed by RPC inspection. Thus, the population ofinterest for POD 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-18 inspection can then be defined as follows.
EOC-18 cycle reported + Indications confirmed indications confirmed by and repaired in EOC-18 RPC in EOC-19 inspection inspection POPCD =
(EOC-18) { Terms in the Numerator) + New indications RPC confirmed in EOC-19 inspection POPCD is evaluated at the 1997 EOC-18 voltage values since it is an EOC-18 POPCD assessment. The indications at EOC-18 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-19. 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-19 indications not reported in the EOC-18 inspection. The new indications include EOC-18 indications present at detectable levels but not reported, indications present at EOC-18 below detectable levels and indications that initiated during Cycle 19. Thus, this definition, by including newly initiated indications, differs from the traditional POD de6nition. Since the newly initiated indications are appropriate for voltage-based repair criteria applications, POPCD is an acceptable definition and eliminates the need to acUust the traditional POD for new indications.
As shown in Table 3-7 there were 110 new indications in the EOC-19 inspection, and only one of them was confirmed by RPC. (All new indicatione were RPC inspected in this inspection.). So, on the basis of RPC inspection there was only one new indication in the EOC-1P inspection. With the exception of the single new indication, the above definition of POPCD yields 100% detection, i.e., POPCD value is unity at q:\ ape \nsp09\nspn90 day. doc 3-5
all voltages except in the voltage range containing the single RPC confirmed indication (0.3 to 0.4 volts). The single RPC-confirmed indication in the EOC-19 inspection and 42 out of.44 RPC confirmed indication repaired in the EOC-18 inspection were interpreted as volumetric indications, and they may or may not contain an ODSCC component. Therefore, the present POPCD for Prairie Island Unit-1 should 'not be included in the EPRI POPCD database which is intended for ODSCC indications only.
3.6 NDE Uncertainties The NDE uncertainties applied for the EOC-19 voltage projections in this report are same as those used in the last tube integrity evaluation for Prairie Island Unit-1 (Reference 8-4). 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 standarti. The analyst variability uncertainty has a standard deviation of 10.3%
about a mean of zero with no cutoff. These NDE uncertainty distributions presented in Table 3-8 as well as graphically illustrated in Figure 3-6. The NDE uncertainty distributions are included in the Monte Carlo analyses used to project the EOC-20 voltage distributions.
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Table 3-6 Prairie Island Unit-1 April 1999 Summary of Largest Voltage Growth Rates for BOC-19 to EOC-19 Steam Generator Bobbin Voltage RPC New SG Row Col Elevation EOC BOC Growth Confirmed ? Indication ?
12 15 43 OlH 0.97 031 0.66 N N 12 36 69 02H 0.27 0.51 N 0.78 _ N 12 29 17 O1H 0.84 038 0.46 N N 12 19 30 O1H 0.84 0.46 038 N N 12 20 10 OlH 0.59 0.25 034 N N 11 18 41 O1H 0.86 0.56 03 N N 11 22 82 O1H 0.64 035 0.29 N N 11 38 65 02H 0.72 0.45 0.27 N N 12 42 28 03H 0.64 038 0.26 N N 12 33 45 O1H 0.76 0.5 0.26 N N 11 18 68 OlH 0.76 0.51 0.25 N N 11 13 39 02H 1.85 1.6 0.25 N N 12 11 51 04H 0.51 0.26 0.25 N N 11 13 91 03H 0.68 0.45 0.23 N N 11 15 79 03H 0.95 0.72 0.23 N N 11 32 33 OlH 0.69 0.48 0.21 N N _
11 41 45 02H 0.87 0.67 0.2 N N 11 43 36 01C 0.75 0.56 0.19 N N 11 18 39 05H 0.65 0.48 0.17 N N 11 8 50 02H 0.75 0.58 0.17 N N 11 42 50 O1H 0.76 0.59 0.17 N N 12 26 85 OlH 0.53 036 0.17 N N
, 12 33 32 O1H 0.49 032 0.17 N N 11 22 82 02H 0.84 0.68 0.16 N N 12 28 42 O1H 0.51 035 0.16 N N 12 19 51 O1H 0.64 0.48 0.16 N N 12 17 9 OlH 0.42 0.26 0.16 N N 11 5 72 02H 0.55 0.4 0.15 N N 11 9 82 0$H 0.65 0.5 0.15 N N . _ _
11 18 16 OlH 0.58 0.43 0.15 N N Growth Tables .72769 8:13 PM 3-12
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No Cutoff Cutoff at +/- 15%
Value Cumul. Prob. Value Cumul. Prob.
-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.N324
-30.0% 0.00179 -11.0% 0.05804
-28.0% 0.00328 -10.0% 0.07656
-26.0% 0.00580 -9.0% 0.09927
-24.0% 0.00990 -8.0% 0.12655
-22.0% 0.01634 -7.0% 0.15866
-20.0% 0.02608 -6.0% 0.19568
-18.0% 0.04027 -5.0% 0.23753
-16.0% 0.06016 -4.0% 0.28385
-14.0% 0.087N -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
-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 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 NDEuncert Table 3 7 7/2099 232 PM 3-14
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4.0 DATA BASE APPLIED FOR LEAK AND BURST CORRELATIONS The correlations applied to project SLB leak rate and tube burst probability for the EOC-20 conditions are based on the latest voltage-based repair criteria database for 7/8" tubes approved by the NRC (Reference 8-10); it is documented in Reference 8-
- 11. NRC recommends including Model Boiler specimen 542-4 and Plant J-1 pulled tube R8C74, TSP 1 in the database. The correlations for probability ofleakage and leak rate as a function of bobbin voltage presented in References 8-11 include these datapoints.
Data for the tube segments pulled from the Prairie Island Unit-1 SGs during the EOC-18 outage are not yet included the database. The database update to include this data has been completed, but the updated database is not yet reviewed and approved by the NRC. However, as noted in that last 90-day report (Reference 8-4),
addition of the Prairie Island Unit-1 data to the reference database would not meaningfully change the burst pressure, leak rate and the probability of leak correlations to the common logarithm of the bobbin amplitude.
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 8-11 for 7/8" tubes.
< s Leak Rate (l/hr) = 10 s The above leak rate correlation was used to perform EOC-20 SLB leak rate projections for the limiting SG.
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 and measured at room temperature). Therefore, SLB leak rate calculated using the leak rate correlations provides a volumetric rate at room temperature. l q:\apc\nsp99\nsp90 day. doc 4-1
5.0 SLB ANALYSIS METHODS Monte Carlo analyses are used to predict the EOC-20 voltage distributions and to calculate the SLB leak rates and tube burst probabilities for both the actual EOC-19 voltage distributions and the predicted EOC-20 voltage distributions. These methods are described in the generic methods report of WCAP-14277, Revision 1 (Reference 8-2), and are in accord with NRC Generic Letter 95-05 (Reference 8-1). Leak rates calculated with the WCAP-14277 methodology provide a volumetric leak rate at room temperature, and they are compared with allowable volumetric leak rate at room temperature.
In general, the methodology involves application of correlations for burst pressure, probability of leak and leak rate to a measured or calculated EOC distribution to estimate the likelihood of tube burst and primary-to-secondary leakage during a postulated SLB event. NDE uncertainties and uncertainties associated with burst pressure, leak rate probability and leak rate correlations are explicitly included by considering many thousands of voltage distributions through a Monte Carlo sampling process. Voltage distribution projections at the end of an operating cycle are obtained by applying gmwth data to the BOC distribution. The BOC voltage distributions include an achustment for detection uncertainty and occurrence of new indications, in addition to the adjustments for NDE uncertainties. Comparisons of projected EOC voltage distributions with actual distributions after a cycle of operation have shown that the Monte Carlo analysis technique yields conservative estimates for EOC voltage distributions and as well as leak and burst results based on those distributions. Equation 3.5 in Reference 8-2 was used to determine the true BOC voltage.
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 mee. ting the Generic Letter 95-05 requirement. Therefore, leak rate analysis for the EOC-20 condition was carried out using the leak rate vs. bobbin correlation shown in the previous section.
( q:\ ape \nsp99\nspn90 day. doc 5-1
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 prebability of detection, as described in the following paragraphs. Other input used for predicJng 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. Where appropriate, adjustments are also made 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 of indications aturned to service, defined as:
N-Nra nts = py - Nn.p a + Nw .
where:
Nrotars = 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 = ProbabilityofDetection.
Nn,p.imi = Number of Ni which are mpaired (plugged) after the last cycle.
N4.piuss.a = Number of previously-plugged indications which are deplugged after the last cycle and are returned to service.
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\nsp99\nsp00 day. doc .
6-1
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 8-11. POPCD data as a function of bobbin voltage is illustrated graphically in Figure 6-1. It is evident from Figure 6-1 that the NRC recommended POD of 0.6 is too conservative above about 0.5 volts. 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 OperatingTime The following operating period values are used in the voltage projection calculations:
Cycle 19 = 440 EFPD Cycle 20 = 598 EFPD (estimated) 6.3 Predicted EOC-20 Voltage Distributions Bobbin voltage projections start with a cycle initial voltage distribution which is projected to the corresponding cycle final voltage distribution, based on the growth rate adjusted for the anticipated cycle operating time period. The overall growth rates for each of the Prairie Island Unit-1 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 18) growth rates slightly exceed those of the 1997 - 1999 (Cycle 19) operation and are used to predict the EOC-20 bobbin voltage distributions. Further conservatism for the EOC-20 bobbin voltage prediction is provided by the use of the larger of the composite growth rate for both SGs and the SG-specific growth rate in pmjecting 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 ofindications being returned to service for the next cycle (BOC-20) is derived from the actual EOC-19 inspection msults adjusted for tubes that are taken out of service by plugging. Table 6-1 shows l EOC-19 bobbin voltage indications, the subsequent plugged indications (which were in service for Cycle 19 and then taken out of service, albeit not all for reasons of ODSCC at TSP), and the BOC-20 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 distribution is shown in Figure 6-1.
Table 6-2' provides the EOC-20 voltage distributions predicted using the BOC-20 q:\apc\nsp99\nsp00 day. doc 6-2
4 4
voltage distribution shown in Table 6-1. As anticipated, a larger number of indications is predicted for SG-11, about 526 indications for a constant POD of 0.6, than for SG-12. The assumed BOC-20 and predicted EOC-20 bobbin voltage frequency distributions for both SGs are also graphically illustrated on Figures 6-2 and 6-3. The largest bobbin voltage predicted for EOC-20 is in SG-11 (assuming a constant POD of 0.6), and its magnitude is 2.4 volts 6.4 Comparison of Predicted and Actual EOC-19 Voltage Distributions The actual EOC-19 bobbin voltage distributions and the corresponding predictions presented in the last 90-day report (for EOC-19 inspection, Reference 8-4), are compared in Table 6-3 and on Figure 6-4. SG-11 was predicted to be limiting for EOC-19 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-11, as predicted. The total number ofindications for SGs 11 and 12 are overpredicted by 47% and 129%, respectively, in the licensing-basis analysis with a POD of 0.6, and the voltage population over 1 volt are overpredicted by even greater percentage. The overprediction of indications in virtually every voltage size range demonstrates conservatism in the projection methodology.
q:\apc\nsp99\nsp90 day. doc 6-3
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i Table 6-2 Prairie island Unit-1 April 1999 Voltage Distribution Projection for EOC - 20 Steam Generator 11 Steam Generator 12 Voltage Projected Number of Indications at EOC - 20 Bin POD = 0.6 POPCD POD = 0.6 POPCD 0.1 _
0.41 0.68 0.16 0.27 0.2 10.56 16.37 4.48 6.93 0.3 _
37.13 48.21 18.11 23.62 0.4 64.43 75.13 31.66 36.95 0.5 87.47 92.77 38.71 41.60 0.6 _ 88.52 86.13 39.50 38.66 0.7 76.89 69.46 32.83 29.76 i 0.8 56.47 48.34 25.89 22.37 0.9 37.48 30.85 18.77 15.68 1.0 22.92 18.43 12.77 10.39 1.1 14.10 11.23 8.13 6.49 1.2 9.45 7.51 4.80 3.86 1.3 6.80 5.40 3.03 2.60
, 1.4 4.70 3.68 2.00 1.75 1.5 _
3.02 2.34 1.26 1.07 1.6 1.79 1.36 0.83 0.51 1.7 1.07 0.78 0.08 0.00 1.8 0.68 0.48 0.70 0.70 1.9 0.49 0.17 0.00 0.30 2.0 0.26 0.00 0.30 2.1 0.00 0.70 2.2 0.70 0.00 2.3 0.00 0.30 2.4 0.30 TOTAL 525.64 520.32 244.01 243.51
>1V 43.36 33.95 21.13 17.28
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Table 6-3 Prairie Island Unit-1 April 1999 Comparison of Predicted and Actual EOC-19 Voltage Distributions Steam Generator 11 Steam Generator 12 Number of Indications EOC-19 Prediction EOC-19 EOC-19 Prediction EOC-19
" **"' Actual POD = 0.6 POD = 0.6 i 1
0.1 0.20 0 0.37 0 0.2 6.28 10 9.50 4 0.3 30.52 42 32.15 22 ,
0.4 61.55 59 53.32 34 0.5 76.87 73 57.71 26 0.6 81.69 49 52.21 26 l
0.7 72.11 40 41.74 18 i 0.8 52.43 23 30.62 8 0.9 32.71 6 21.43 9 1.0 18.78 5 14.75 2 1.1 10.88 4 10.09 1 1.2 6.86 4 6.67 0 1.3 4.71 0 4.26 0 1.4 3.06 0 2.67 0 1.5 1.84 0 1.83 0 1.6 1.09 0 1,44 0 1.7 0.67 0 1.22 0 1.8 0.08 0 1.00 0 1.9 0.70 1 0.75 0 2.0 0.00 0 0.26 0 2.1 0.30 0 0.00 0 2.2 0.00 0 0.70 0 2.3 0.00 0 0.30 0 TOTAL 463.33 316 344.99 150
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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 the actual measured EOC-19 voltage distributions and projected EOC-20 voltage 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. The latest leak and burst correlations approved by the NRC were applied for the EOC-20 projections, and they are documented in Reference 8-11. The leak rate vs. bobbin voltage correlation used is shown in Section 4.0. The EOC-19 projections (presented originally in Reference 8-4) also used the latest leak and burst correlations available to the NRC at the time of those calculations wers performed (documented in Reference 8-12). The calculated leak rates are volumetric rates at room temperature and they should be compared with allowable leak rates at room temperature.
7.2 Predicted and Actual Leak Rate and Tube Burst Probability for EOC-19 (Condition Monitoring Assessment)
Monte Carlo analyses were -performed to calculate SLB leak rates and tube burst probabilities based on the actual bobbin voltage distributions at EOC-19, including NDE uncertainties. The results are shown on Table 7-1. Projected EOC-19 results for both SGs originally 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 19)was 1 gpm (at room temperature).
Comparisons of the EOC-19 actuals with the corresponding projections indicate the following:
a) SG-11 was projected to be the limiting steam generator for EOC-19 based on !
EOC-18 data, and SG-11 was confirmed to have the limiting leak rate based ;
on the actual bobbin measurements for EOC-19 (0.02 gpm at room ;
temperature). All estimated EOC-19 leak rates are well below the acceptance limit for Prairie Island Unit-1 (1 gpm).
b) The tube burst probabilities based on the actual voltage distributions are equal to or less than the projections with POD =0.6. The highest tube burst probability based on the measured voltages was calculated for SG-11 (1.9x104) which had the largest indication detected in this inspection. Tube q:\ ape \nsp99\nsp90 day. doc 7-1
burst probability results for both SGs are well below the NRC reporting guideline of10-2, In summary, the limiting SLB leak rate (0.02 gpm at room temperature, applying voltage correlation) and tube burst probability (1.9x104) calculated using the actual measured EOC-19 bobbin voltage distributions are more than 2 orders of magnitude below the corresponding allowable limits. The results meet the Generic Letter 95-05 requirement for continued Cycle 20 operation.
7.8 Projected . Leak Rate and Tube Burst Probability for EOC-20 (Operational Assessment)
Using the methodology previously described, calculations were performed to predict the EOC-20 conditions of both steam generators in Prairie Island Unit-1, and the results are summarized in Table 7-2. EOC-20 bobbin voltage distributions as well as the leak rates and tube burst probabilities based on these distributions are predicted.
As mentioned earlier, EOC-20 leak rates and tube burst probabilities are calculated using the latest burst and leak correlations presented in Reference 8-11. The j projected leak rates are compared with the allowable leak rate at room temperature ;
(1 gpm). The leak rate vs. bobbin voltage correlation shown in Section 4.0 is pplied. l Since the ~gmwth rate for Cycle 18 is higher than that for Cycle 19, Cycle 18 growth I data were used in the EOC-20 projection analysis.
The predicted EOC-20 SLB leak rate and burst pmbability for both SGs are shown in Table 7-2. It is evident that the projected maximum voltage and SLB leak rate at the EOC-20 condition for SG-11 are slightly higher than those for SG-12; while the tube burst pmbability is slightly higher for SG-12. However, the difference in EOC-20 l
tube burst probabilities for SGs 11 and 12 am small (about 6x104) and is attributable to the differences in the random number sequences used in the Monte Carlo analyses.
Thus, SG-11 is once again projected to be limiting at the end of the current cycle (EOC-20). The limiting EOC-20 SLB leak rate predicted for SG-11 based on constant POD of 0.6 is 0.46 gpm (room temperature) which is less than half of the current licensed limit of 1 gpm at room temperature. The limiting EOC-20 burst probability with POD =0.6, predicted for SG-12, is 2.5x104; it is better than 2 orders of magnitude '
below the NRC acceptance limit of10-2 The results based on the voltage-dependent POPCD also show similar margins. Thus, the projected EOC-20 results meet the voltage-dependent repair criteria requirement for continued operation.
' The pmjected EOC-20 leak rates are about 7 to 9 times the projected EOC-19 leak rates (shown in Table 7-1), although the pmjected number of indications and peak voltages for the two EOC conditions are comparable. The main source of the differences in these EOC leak rate pmjections is the database for the leak and burst q:\ ape \nsp09\nsp00 day. doc 7-2 l
correlations applied. The EOC-19 projections are based on the database presented in Reference 8-12 and the EOC-20 projections are based on the database presented in Reference 8-11. Both these databases were the latest database submitted to the NRC at the time these projections were performed.
In summary, SLB leak rates and tube burst probabilities projected for EOC-20 for both SGs using the NRC-mandated POD = 0.6 meet the SER limits for Prairie Island Unit-1. Results based on voltage dependent POPCD show even a greater margin between EOC-20 predictions and acceptance limits.
q:\apc\nsp99\nsp90 day. doc 7-3
l l
1 :
Table 7-1 Prairie Island Unit-1 1999 EOC-19 Outage Summary of Calculations of Tube Leak Rate and Burst Probability Burst Probability SLB Steam POD No.of Max.
Generator Indic. Volts Mor 1 Tuk Ra ationsm Tubes (gpm)
EOC - 19 Projections (POD =0.6)
- 11 464 2.1 1.9x10-5 1.9x10-5 0.064 Leak rate Correlation 12 0.6 345 2.3 APPlied 2.5x10-5 2.5x106 0.055 Based on Actual Measured EOC - 19 Voltage Distributions
- 11 316 1.9 1.9 x 10 5 1.9 x 10-5 0.021 Leak rate Correlation i 12 150 1.0 APPlied 1.2 x 10-5 1.2 x 10-5 0.004 Eples (1) Achusted for POD.
(2) Volumetric leak rate adjusted to room temperature.
(3) Leak and burst database shown in Reference 8-12 applied.
I I
q:\ ape \nsp99\nsp90 day. doc 7-4
Table 7-2 Prairie Island Unit-1 Summary of Projected Tube Leak Rate and Burst Probability for EOC-20 (Based on projected Cycle 20 length 598 EFPD)
Steam POD No. of Max.
Comments Generator Indic. Volts One or Leak ationsm 1 Tube More Rate Tubes (EPmy2>
Leak and Burst Database and Correlations Reported in Reference 8-11 Applied 1 11(8) 526 2.4 1.9x104 1.9x104 0.46 12(* 244 2.3 2.5x104 0.40 te 2.5x104 C 1 i9 11(s> 520 2.0 1.2x104 1.2x104 0.19 applied 12(o POPCD 244 1.9 1.9x104 1.9x104 0.16 Notes (1) Number ofindications adjusted for POD.
(2) Volumetric leak rate adjusted to room temperature.
(3) All SG composite Cycle 18 growth rate distribution applied.
(4) SG-12 specific Cycle 18 growth rate distribution applied.
q:\apc\nsp99\nsp90 day. doc 7-5
t
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, August 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 NRC Safety Evaluation Related to Amendment Nos.133 and 125 to Facility Operating License Nos. DPR-42 and DPR-60, Northern States Power Company, Prairie Island Nuclear Generating Plant Unit Nos. I and 2, Docket Nos. 50 282 and 50-306, November 18,1997.
8-4 SG-98-03-002, " Prairie Island Unit-1 Cycle 19 Voltage Based Repair Criteria 90-Day Report," Westinghouse Nuclear Services Division, March 1998.
8-5 E-Mail from S. Redner (PINGP) to V. Srinivas (Westinghouse), "DSI Data,"
May 5,1999.
8-6 E-Mail from S. Redner (PINGP) to V. Srinivas (Westinghouse), "EOC-19 Bobbin Data," May 9,1999.
8-7 E-Mail from S. Redner (PINGP) to V. Srinivas (Westinghouse), " Entire EC Database," May 10,1999.
I 8 E-Mail from S. Redner (PINGP) to V. Srinivas (Westinghouse), " Final Plug l Status," May 17,1999.
]
8-9 Letter from B. W. Sheron, Nuclear Regulatory Commission, to A. Marion, Nuclear Energy Institute, dated February 9,1996.
8-10 " 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 j 20,1998. !
1 8-11 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.
8-12 " Updated ODSCC ARC Correlations for 7/8" Diameter Tubes (Project No.
689)," R. Clive Callaway (NEI) to U.S. Nuclear Regulatory Comminaion, December 29,1997.
l q:\apc\nep99\ asp 00 day. doc '
8-1