Text

Cycle 21 Steam Generator Tube Support Plate Voltage Based Repair Criteria 90-Day Report
	ML030720401
Person / Time
Site:	Prairie Island
Issue date:	03/06/2003
From:	Solymossy J; Nuclear Management Co
To:	; Document Control Desk, Office of Nuclear Reactor Regulation
References
	GL-95-005, L-PI-03-031
	Download: ML030720401 (31)
	v • d • e

MC Committed to NuclearExcellence Praire Island Nuclear Generating Plant Operated by Nuclear Management Company, LLC L-PI-03-031 March 6, 2003 Generic Letter 95-05 U S Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555 PRAIRIE ISLAND NUCLEAR GENERATING PLANT DOCKET NO. 50-282 LICENSE NO. DPR-42 UNIT 1, CYCLE 21 STEAM GENERATOR TUBE SUPPORT PLATE VOLTAGE BASED REPAIR CRITERIA 90-DAY REPORT During the Unit I refueling outage following operating cycle 21, the Voltage Based Repair Criteria for Outside Diameter Stress Corrosion Cracking at Tube Support Plates (TSPs) was used in accordance with the requirements of NRC Generic Letter (GL) 95 05 and Amendment Number 133 to Facility Operating License Number DPR-42. GL 95 05, Attachment 1, Section 6.b, requires information to be submitted to the NRC staff within 90 days of each restart following a steam generator (SG) inspection. The attached report includes the required information.

The report notes the maximum voltage distorted signal indications were 1.73 Volts in SG 11 and 1.43 Volts in SG 12. The report concludes that present results support continued application of the 2.0 Volt repair criteria for TSP indications per GL 95-05.

This letter contains no new commitments and no revisions to existing commitments.

Please contact Jeff Kivi (651-388-1121) if you have any questions related to this letter.

ice Presiden , Prairi* Island Nuclear Generating Plant CC Regional Administrator, USNRC, Region III Project Manager, Prairie Island Nuclear Generating Plant, USNRC, NRR NRC Resident Inspector - Prairie Island Nuclear Generating Plant ,

-p Attachment 1717 Wakonade Drive East e Welch, Minnesota 55089-9642 Telephone: 651.388.1121 A-DOI

Aoe~n FA - PI - 016 February, 2003 Revision 0 90-Day Report Voltage-Based Repair Criteria Prairie Island, Unit 1, EOC 21, BOC 22 FINAL J. A. Begley, Framatome ANP, Inc.

T. F. Begley, ForeLine Associates, LLC C. J. Begley, TCA Solutions, LLC 226 Paul Street Pittsburgh, PA 15211 (4121481-3363 Phone 14121481-3365 Fax

TABLE OF CONTENTS Section Title Pacqe

1. Introduction 2

2. EOC 21 Inspection Results and Growth Rates 4

3. EOC 22 Projections 16

4. Leak and Burst Analysis Methodologies 20

5. Leak and Burst Analysis Results 24

6. Summary and Conclusions 28

7. References 29 FA-PI1-016 ForeLine Associates, LLC I i FA - PI1 - 016

Section 1 INTRODUCTION This report describes the Bobbin Voltage Alternate Repair Criteria for steam generator tubing applied at Prairie Island Unit 1, EOC 21. Bobbin voltage data at tube support plates is summarized, with analyses of leak rate and probability of tube burst under postulated steam line break conditions both in terms of condition monitoring at EOC 21 and operational assessment for cycle 22. Projected bobbin voltage distributions for the conclusion of the next operating cycle are included along with calculated probabilities of burst and projected leak rates at postulated SLB conditions. The Bobbin Voltage-Based Repair Criteria has been applied since cycle 17 at Prairie Island Unit 1, and the methodologies used in preparing this report are consistent with NRC Generic Letter 95 051, the Safety Evaluation by the Office of Nuclear Reactor Regulation Related to Amendment Nos. 133 and 125 to Facility Operating License Nos. DPR-42 and DPR-60 for Prairie Island Nuclear Generating Plant 2, previous 90-day reports 3' 4 and standard industry practice 5 ' 6.7 Present results support continued application of the 2.0 volt repair criteria for TSP indications per NRC Generic Letter 95-05.

Eddy current examination requirements for application of the Bobbin Voltage Alternate Repair Criteria are captured in Prairie Island Nuclear Generating Plant Maintenance Procedure D27.25 8 . A 100% eddy current inspection of all tube support plate intersections was performed for both steam generators at Prairie Island Unit 1. All distorted signal indications (DSI) were inspected by rotating pancake coil (RPC) probe to identify possible instances of wastage at ODSCC locations. No such indications with voltages greater than the 2.0-volt limit were found at EOC 21. RPC inspection was also performed for dents (DNT) greater then 5.0 volts, mixed residual indications (MRI) greater than 2.0 volts, indications not reportable (INR) greater than 1.5 volts, and cold leg thinning greater than 1.5 volts with a depth less than 40 percent through wall. All indications at TSP locations found at EOC 21 are included in Section 2 along with voltage growth rates. The largest voltages observed at EOC 21 are 1.73 volts in SG 11 and 1.43 volts in SG 12.

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The latest available data for burst pressure and leak rate correlations has been applied 6 . This is discussed in Section 3. For burst probability calculations, the procedure is straightforward, new correlation coefficients are applied in performing Monte Carlo analyses according to a standardized procedure. Since a "p" value greater than 0.05 was obtained for the updated correlation of bobbin voltage with SLB leak rates at 2560 psi, several methodologies were used to obtain projected SLB leak rates incorporating a lack of correlation effect. These approaches are consistent with a methodology previously presented to the NRC 9 and are described in Section 4. Leak rate and burst probability results are presented in Section 5.

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Section 2 EOC 21 INSPECTION RESULTS AND GROWTH RATES The distributions of bobbin voltage indications of axial ODSCC at tube support plate intersections are presented in Figures 2.1 and 2.2. This data is summarized in Table 2.1 below. As expected from past results, the distribution of EOC bobbin voltages is about the same in both steam generators but the number of indications is greater in SG 11 compared to SG 12. The as found numbers of indications are less than those Table 2.1 EOC 21 Inspection Results Summary SG 11 SG 12 Top Number Number of Number of PP Top Number Number of Number of PP of Bin in Bin New Indications Confirmations of Bin in Bin New Indications Confirmations 0.1 1 0 0 0.1 1 0 0 0.2 21 2 0 0.2 10 2 0 0.3 44 2 0 0.3 31 12 1 0.4 71 9 0 0.4 50 13 4 0.5 76 7 0 0.5 35 5 1 0.6 62 6 0 0.6 26 3 2 0.7 37 2 0 0.7 23 5 0 0.8 29 1 1 0.8 10 2 0 0.9 9 0 0 0.9 7 2 0 1 8 1 0 1 3 0 0 1.1 2 0 0 1.1 4 1 0 1.2 1 0 0 1.2 2 0 0 1.3 1 0 0 1.3 0 0 0 1.4 1 0 0 1.4 1 0 0 1.5 1 1 0 1.5 1 0 0 1.6 0 0 0 1.6 0 0 0 1.7 0 0 0 1.7 0 0 0 1.8 1 0 0 1.8 0 0 0 1.9 0 0 0 1.9 0 0 0 2 0 0 0 2 0 0 0 Total Total Total Total Total Total 365 31 1 204 45 8

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expected from using a POD value of 0.6. There are 365 indications in SG11 compared to a projected EOC 21 value of 572. In SG 12 these numbers are 204 and 272 respectively. The progression of degradation in both steam generators is relatively mild in terms of number of indications. The severity of degradation is also relatively mild.

The largest EOC 21 bobbin voltage in SG 11 was 1.73 volts which is comparable to a value of 1.8 volts for EOC 20. In SG 12 these maximum voltage values are 1.43 volts at EOC 21 compared to 1.27 volts at EOC 20. The number of indications greater than 1.0 volts is 7 in SG 11 at EOC 21 versus 10 at EOC 20. For SG 12 there were 8 indications greater than 1.0 volts at EOC 21 compared to 4 at EOC 20. Another measure of degradation severity in the number of indications that are confirmed with the Plus Point probe. There were a total of 9 confirmations with the Plus Point probe at EOC 21 compared to 11 confirmations at EOC 20. In each of the last two cycles of operation, the maximum Plus Point confirmation voltage was 0.17 volts. From a database in the latest revision of the EPRI In Situ Testing Guidelines 1 °, the expected maximum depth of axial ODSCC is 50% TW at a Plus Point voltage of 0.17 volts. The upper 90 %

predictions interval value is 63% TW. From the above considerations, the instances and severity of degradation at TSP intersections at Prairie Island Unit 1 continues to be mild.

One OD axial Plus Point indication was found in SG 12 at 01H with no corresponding DSI bobbin signal. It exhibited a Plus Point voltage of 0.11 volts. Using the Prairie Island data of Figure 2.3, it was assigned a bobbin voltage of 0.73 volts, the top of the scatter band, and included in the burst probability and SLB leakage calculations.

The results of bobbin voltage growth rate analyses over the last two cycles of operation are presented in Figures 2.4 through 2.7. Voltage growth rates are modest, as shown in the Figures 2.4 and 2.5. About 50% of the time, voltage growth is negative. Growth rates distributions are about the same at EOC 21 and EOC 20. The maximum observed growth rate is 0.420 volts / EFPY. Although SG 12 exhibits fewer indications than SG 11, the voltage growth is somewhat faster. Hence, a composite growth rate distribution was used for analyses for SG 11, even though there were 334 growth data ForeLine Associates, LLC 5 FA - P11 - 016

points for SG 11. Although there were 160 growth data points for SG 12, compared to the standard minimum value of 2005, the growth distribution for SG 12 EOC 21 was used because it is more conservative than the composite growth rate distribution. The voltage growth rate distributions for Cycle 21 are marginally more adverse than for Cycle 20. Average voltage growth rates as a percentage of the average BOC voltage are presented in Table 2.2. This information supports the description of growth rates at Prairie Island Unit 1 as modest.

Figures 2.6 and 2.7 show plots of voltage growth rates versus BOC voltage for SG's 11 and 12 over the last two cycles of operation. There is no evidence of voltage accelerated growth rates. Degradation at Prairie Island Unit 1 has not evolved to the point where this may be an issue. The relatively low number of indications, particularly the low number of indications greater than 1.0 volts and the trending of observed indications less than the projected numbers based on a POD of 0.6 signifies than the possible onset of voltage dependent growth is at least several cycles into the future.

Table 2.2 Average BOC Voltages and Average Voltage Growth Rates SG 11 SG 12 Composite Average Average Average Average Average Average Average Average Average Cycle Voltage Growth, All Growth, >=0 Voltage Growth, All Growth, >=0 Voltage Growth, All Growth, >=0 (volts) %/EFPY %/EFPY (volts) %/EFPY %/EFPY (volts) %/EFPY %/EFPY Cycle 21 0.498 -0.54 10.24 0.488 0.37 15.37 0.494 -026 11.94 Cycle 20 0.498 -0.74 10.84 0484 -0.91 17.97 0.494 -0.79 12.75 FA-PI1-016 ForeLine Associates, LLC LLC 66 FA - P11 - 016

The actual binned voltage growth rate distribution data used in structural and leakage integrity calculations are presented in Figure 2.8. The data used to construct these plots was derived from binning of Cycle 21 growth rates. A composite Cycle 21 growth rate distribution was applied to SG 11, while a Cycle 21 generator specific growth rate distribution was applied to SG 12. These binned growth rates are listed in Table 2.3 Table 2.3 Binned Voltage Growth Rates SG 11 SG 12 Composite Data SG Specific Data Cycle 21 Cycle 21 Top of Growth Cumulative Top of Growth Cumulative Rate Bin Fraction Rate Bin Fraction (volts/EFPY) (volts/EFPY) 0 0.5162 0 0.5063 0.1 0.9089 0.1 0.8625 0.2 0.9777 0.2 0.9625 0.3 0.9939 0.3 0.9813 0.4 0.9980 0.4 0.9938 0.5 1.0000 0.5 1.0000 No tubes were plugged at EOC 21 due to exceeding the bobbin voltage ARC repair limit of 2.0 volts at TSP locations. Several tubes were plugged due to indications of other degradation mechanisms. Table 2.4 lists the distribution of TSP bobbin voltage indications returned to service at BOC 22.

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Table 2.4 Number of Indications Found, Plugged and Returned to Service SG 11 SG 12

- .. . .... . . ~ Y ............ . .. ............

Top Number Number Number at Top Number Number Number at of Bin I at EOC 21 Plugged BOC 22 of Bin I at EOC 21 Plugged n m BOC 22

-I I 0.1 1 1 0.1 I 1 0.2 21 21 0.2 10 10 0.3 44 44 0.3 31 31 0.4 71 70 0.4 50 46 0.5 76 76 0.5 35 32 0.6 62 61 0.6 26 25 0.7 37 37 0.7 23 20 0.8 29 29 0.8 10 9 0.9 9 9 0.9 7 6 1 8 8 1 3 2 1.1 2 2 1.1 4 4 1.2 1 1 1.2 2 2 1.3 1 1.3 0 0 1.4 1 1.4 1 1 0 1 1.5 1 1 1.5 1.6 0 0 1.6 0 0 1.7 1 0 1.7 0 0 1.8 00 1 1.8 0 0 1.9 0 0 1.9 0 0 2 0 2 0 0

-4. * -- --I- i 4 Total Total Total Total Total Total 365 2 363 204 14 190 J. .1. I i -

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80 70 60 50 0,

z 0

40 0z _____SG____________12_______

301 10 0 01 02 03 04 05 06 07 08 09 1 11 12 13 14 15 16 17 18 BOBBIN VOLTS Figure 2.1 Bobbin Voltages Observed at Prairie Island Unit 1, EOC 21 9 FA-PI1-016 Associates, LLC ForeLine Associates, LLC 9 FA - PI1 - 016

140 120 100 z

o 80 z

40 0

01H 02H 03H 04H 05H 06H 07H 01C 03C 07C TUBE SUPPORT PLATE Figure 2.2 Distribution of ODSCC TSP Indications by Location 10 FA-PI1-016 ForeLine Associates, LLC LLC 10 FA - PI1 - 016

1 0.9 0.8 0.7

> 0.6 "0.5 "0.4 0.3 0.2 0.1 0 I 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 Plus Point Voltage, volts Figure 2.3 Bobbin Voltage versus Plus Point Voltage, Prairie Island Unit 1, EOC 21. Top of Scatter Band Used to Assign a Bobbin Voltage to a 0.11 Volt Plus Point Indication 11 FA-PI1-.016 ForeLine Associates, LLC 11 FA - P11 - 016

0.9- 1 fiW 0.8_

0.7, 0.6SG 11, Cycle 21 0 0.6 S* - Composite, Cycle 21 SG 12, Cycle 21 U

E M.

I

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 Voltage Growth Rate, volts I EFPY Figure 2.4 Observed Bobbin Voltage Growth Rate Distributions at Prairie Island Unit 1, Cycle 21 12 FA - P11 016 ForeLine Associates, LLC 12 FA - P11 - 016

a--*-SG 12, Cycle 20 r

0 -- SG 11, Cycle 20 (U

Composite, Cycle 20 L.

a)

E C.,

-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 Voltage Growth Rate, volts I EFPY Figure 2.5 Observed Bobbin Voltage Growth Rate Distributions at Prairie Island Unit 1, Cycle 20 13 FA-PIl -016 ForeLine Associates, LLC LLC 13 FA - P11 - 016

SG 11 0.6 0.4 a

0.2 I

L_

0 CD-0.2 4D CO 0

-0.4

-0.6 0 0.5 1 1.5 2 BOC Voltage, volts Figure 2.6 Voltage Growth Rate Versus BOC Voltage, SG 11 14 FA-PI1-016 ForeLine Associates, LLC LLC 14 FA - PIH - 016

SG 12 0.6 S0.4 S0.2

. 0 0

L_

-0.2

-0.4

-0.6 0 0.5 1 1.5 2 BOC Voltage, volts Figure 2.7 Voltage Growth Rate Versus BOO Voltage, SG 12 ForeLine Associates, LLC 15 FA - P11 - 016

Section 3 EOC 22 Voltage Projections The standard methodology to develop EOC projections of bobbin voltage indications for application of a voltage based ARC consists of using a constant probability of detection (POD) value of 0.6, consideration of probe wear and analyst variation effects on voltage measurements uncertainties and finally voltage growth over the cycle of interest. The number of projected indications is given by:

N= ND NP POD where N is the projected number of indications, ND, is the detected number of indications in the prior cycle, Np is the number of these indications removed from service and POD is the probability of detection, taken as a constant of 0.6. This almost always provides a conservative projection as is historically true at Prairie Island Unit 1.

If the number of tubes removed from service is small, the above equation predicts a 66.6% increase in indications from one cycle to the next. At EOC 21, the increase in number of indications was 6.4% in SG 11 and 25.1% in SG 12. The projected number of indications for Cycle 22 is 605 in SG 11 and 326 in SG 12.

The projected magnitude of these indications in terms of bobbin voltage is presented in Figure 3.1 as a bar chart of number of indications in voltage bins with 0.1 volt steps. In developing these projections via a Monte Carlo approach, voltage measurement uncertainties were included as a percentage of the reported BOC voltage and added to the reported BOC voltage. The mean uncertainty for probe wear and analyst variation effects was taken as 0 %. For probe wear, the measurement uncertainty was taken from a normal distribution with a standard deviation of 7.0 %. These uncertainty percentage values were truncated at +/- 15%. No truncation was applied to the analyst uncertainty in bobbin voltage measurement. Again, this uncertainty is expressed as a ForeLine Associates, LLC 16 FA - PI1 - 016

percentage of the reported bobbin voltage. The analyst uncertainties are sampled from a normal distribution with a mean of 0 % and a standard deviation of 10.3%.

After probe wear and analyst uncertainties are applied to a BOC bobbin indication, voltage growth is selected using a growth rate from one of the growth rate distributions of Figure 2.4. The projected operating time for Cycle 22 at Prairie Island Unit I is 1.695 EFPY. Voltage projections are determined for each projected indication in the SG of interest producing one simulation of a steam generator operating cycle. Burst pressures and SLB leak rates are determined for each projected EOC indication in a steam generator. Typically 100,000 simulations of a steam generator operating cycle are performed creating the statistics to calculate the probability of burst at SLB differential pressure with a 95% confidence and a projected 95th percentile SLB leak rate at 95% confidence.

Figure 3.1 shows that the most likely voltage of an EOC indication for the upcoming Cycle 22 is about 0.5 volts. The maximum voltage observed in any simulation keeps increasing as the number of simulations increases since the analyst uncertainty is unbounded. Integrating the tail of projected fractional indications to about 0.5 leads to a reasonable projected maximum EOC voltage. These values for EOC 22 are 2.2 volts for SG 11 and 2.0 volts for SG 12. Previous projections for the past two cycles are about at these levels (> 2.0 volts) but no indication in the past two cycles at Prairie Island Unit 1 has exceeded 1.8 volts. Table.3.1 provides a tabulation of projected EOC 22 indications.

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Table 3.1 Tabulation of Projected EOC 22 Voltage Distributions TT SGI 1 SG12 Top of Bin Projected Number Top of Bin Projected Number Volts at EOC 22 Volts at EOC 22 0.1 1.69 0.1 1.21 0.2 19.36 0.2 9.97 0.3 43.85 0.3 28.71 0.4 76.55 0.4 45.84 0.5 97.11 0.5 51.28 0.6 99.77 0.6 49.90 07 86.26 0.7 41.17 0.8 65.27 0.8 31.05 0.9 44.59 0.9 21.72 1 28.31 1 14.84 1.1 16.87 1.1 10.12 1.2 9.76 1.2 6.92 1.3 5 69 1.3 4.71 1.4 3.42 1.4 3.16 1.5 2.17 1.5 2.08 1.6 1.45 1.6 1.34 1.7 1.01 1.7 0.84 1.8 0.72 1.8 0.50 1.9 0.52 1.9 0.29 2 0.36 2 0.16 2.1 0.24 2.1 0.09 2.2 0.15 2.2 0.05 2.3 0 09 2.3 0 03 24 0.05 2.4 001 2.5 0.03 2.5 0.01 2.6 0.01 2.6 0.00 2.7 0.01 2.7 0.00 2.8 0.00 2.8 0.00 29 0.00 2.9 0.00 3 0.00 3 0.00 3.1 0.00 3.1 0.00 3.2 0.00 3.3 0.00 3.4 0.00 3.5 0.00 3.6 0.00 3.7 0.00 Total 605 Total 326 I.L.

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U 0

z 0

ILl zU 0

ix 0

a.

010203040506070809 1 1.112 13 14 1516 1.7 18 19 2 21222324 2526272829 3 PROJECTED EOC 22 BOBBIN VOLTS Figure 3.1 Projected EOC 22 Voltage Distributions, SG 11 and SG 12 19 FA-PI1-016 Associates, LLC ForeLine Associates, 19 FA - PI1 - 016

Section 4 LEAK AND BURST ANALYSIS METHODOLOGIES Pulled tube leak rate test results have led to an updating of correlation coefficients used in the application of the bobbin voltage based repair criteria 6 . Updated correlations of burst pressure with bobbin voltage have relatively little impact on structural integrity analyses. The probability of leakage at SLB conditions versus bobbin voltage has also changed based on tube pull results over the past year. However, these changes are not dramatic and do not require any changes to the standard calculation methodology 5.

New leak rate measurements at 2560 psi SLB differential pressure conditions have resulted in new correlation coefficients between SLB leak rate and bobbin voltage.

While the absolute changes are not substantial, the degree of correlation between SLB leak rate and bobbin voltage, expressed as the "p" value, has exceeded the accepted value of 0.05 needed to apply a linear correlation between the log of the SLB leak rate and the log of the bobbin voltage. The revised value of p is 0.076. The p value for the correlation of log leak rate and log voltage for an SLB differential pressure of 2405 psi remains below 0.05. Hence, by previous standardized procedures, a leak rate correlation could be applied at a differential pressure of 2405 psi but not at 2560 psi.

This runs counter to physical behavior and the use of a leak rate correlation for 0.750 inch diameter tubing at either pressure differential. Sampling of an SLB leak rate distribution which is uncorrelated with bobbin voltage leads to grossly overestimating SLB leak rates. In this approach a very high leak rate would be just as probable for extremely low-level bobbin voltages as for voltages that exceeded the 2.0 volt limit by an order of magnitude.

At an NEI/NRC meeting in February, 20029 a proposal was advanced to sample the slope of the log leak rate/ log voltage correlation, as is done in the standard calculation methodology, but when the slope is zero or less, no correlation is assumed. When the slope is greater than zero, the correlation is applied. The approach can be considered as sampling of the p value. The net effect is using uncorrelated leak rates a fraction of the time. This fraction is about p. With the revised database, the value of p is 0.076.

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Thus, an uncorrelated leak rate effect is included and the basic requirement of not using a leak rate correlation with a p value less than 0.05 is essentially met.

The above procedure was followed in the present analysis for calculations at a differential SLB pressure of 2560 psi. Prairie Island has established the basis to implement an SLB pressure differential of 2405 psi but has not made a submittal to the NRC. SLB leak rate calculations are presented in this report at 2405 psi for information purposes. Use of an SLB leak rate correlation for all calculations at 2405 psi is justified by the revised leak rate correlations with a p value of 0.023.

When Monte Carlo sampling leads to conditions requiring use of an uncorrelated leak rates at 2560 psi, the distribution of leak rates in Figure 4.1 is sampled, regardless of the bobbin voltage. The ordinate is the cumulative distribution function and the abscissa is the log of the SLB leak rate expressed in terms of liters per hours at room temperature density. The logarithm values are to the base 10. The irregular line is the revised 2560 psi SLB leak rate database. The dotted smooth curve is the inferred parent population of SLB leak rates given by a log normal distribution using best estimate mean and standard deviation values from the test database. The smooth solid curve is generated from 100,000 Monte Carlo simulations of SLB leak rates applying Student "t" statistics to the mean leak rate estimates and Chi Squared statistics to the standard deviation. It is seen that the variation added to the distribution of leak rates from a relatively complete statistical treatment is rather small.

Figures 4.2 contains the latest revised probability of leak and leak rate correlations parameters used for calculations at 2560 psi. The Monte Carlo simulation methodology, other than for occasional use of uncorrelated leak rate selections, followed standard practice and was benchmarked versus Bobbin Voltage ARC Round Robin results.

The burst pressure analysis methodology followed standard practice per WCAP 14277.

The latest burst pressure database correlation coefficients were applied. These 21 FA P11 016 Associates, LLC ForeLine Associates, LLC 21 FA - PI1 - 016

parameters are presented in Figure 4.2. Section 5 presents the results of SLB leak rate and burst pressure calculations.

I 0.9 08 07 06 Rankj MQko 05 Rankj 04 03 0.2 0.1 0

-4 -3 -2 -1 0 1 2 3 4 5 6

-4 SORTLQ3) , MQ k, 1 , SORTLQ21 6 Figure 4.1 Distribution of SLB Leak Rates at 2560 psi for Use in Calculations Without a Leak Rate Correlation Equation, Abcissa and Ordinate Described in Text 22 FA-PIl -016 ForeLine Associates, LLC Associates, LLC 22 FA - P11 - 016

SGDB Variable I! ::il Burst Intercept 7.4934 Burst Slope -2.3775 Burst Sig 0.8861 Burst V(OJ 0.009427291 Burst V(1J -0.004564115 Burst V(2) 0.015630532 Burst DOF 95 Leak Intercept -0.0691 Leak Slope 0.717 Leak Sig 0.8108 Leak V[OJ 0.310201604

~~I "".4> 4SW Leak V(I) -0.260564345 Leak V(2) 0.236125369 Leak DOF 27 POL Intercept -5.1017 POL Slope 7.3483 POL VIOJ 1.3742 POL Vil -1.7365 POL V(2) 2.6428 POL DOF 113 4

41-Figure 4.2 Latest Revised Leak rate and Burst Pressure Correlation Parameters 23 FA-PJ1-016 ForeLine Associates, LLC LLC 23 FA - P11 - 016

Section 5 LEAK AND BURST ANALYSIS RESULTS The as found distributions of bobbin voltage DSI indications at TSP intersections at EOC 21 were used to perform condition monitoring evaluations. See Table 2.1 and Figure 2.1. Voltage measurement uncertainties were applied to each indication and then leak rate and burst pressures calculations were performed. SLB leak rates on a per generator basis were developed and the 95th percentile SLB leak rate was determined at 95% confidence. The probability of burst at SLB pressures was determined at 95% confidence. SLB leak rates are reported in terms of room temperature density with units of gpm. The SLB leak rate calculation of record is based on the "p value sample" method and is highlighted and listed in Table 5.1 as the 92.4% correlation. For information purposes, 95/95 SLB leak rates are reported for a differential pressure of 2405 psi and 100% use of the leak rate correlation at 2560 psi.

Operational assessments considered the DSI indications returned to service and the undetected population of indications developed from a POD of 0.6. Voltage measurement uncertainties were applied and then voltage growth was added via a Monte Carlo process. The projection numbers and voltages of indications at EOC 22 were then used in leak rate and burst pressure calculations.

Condition monitoring calculations are presented in Table 5.1. The probability of burst is well below the allowable value of 0.01 at EOC 21. The 95/95 SLB leak rates are well below the Prairie Island Unit 1 limit of 1.0 gpm.

Operational assessment calculation results for a cycle length of 1.695 EFPY are listed in Table 5.2. The probability of burst at EOC 22 is small compared to the allowable limit of 0.01. The projected 95/95 SLB leak rate at 2560 psi is 0.833 gpm 24 FA P11 016 Associates, LLC ForeLine Associates, 24 FA - P11 - 016

Table 5.1 Summary of Condition Monitoring Calculations Parameter SG 11 SG 12 BOC 21 Number of Indications 365 204 Probability of Burst at 2560 psi 0.00006 0.00006 Probability of Burst at 2405 psi 0.00095 0.00095 95195 SLB Leak Rate at 2560 psi 92.4% Correlation 0.261 0.113 p= 0.076 gpm at RT density 95/95 SLB Leak Rate at 2405 psi 100% Correlation 0.0499 0.0272 gpm at RT density 95195 SLB Leak Rate at 2560 psi 100% Correlation 0.183 0.0862 gpm at RT density 25 FA-PIl -016 ForeLine Associates, LLC LLC 25 FA - P11 - 016

Table 5.2 Summary of Operational Assessment Calculations Parameter SG 11 SG 12 EOC 22 Number of Indications 605 326 Probability of Burst at 2560 psi 0.000126 0.000095 Probability of Burst at 2405 psi 0.000095 0.00006 95195 SLB Leak Rate at 2560 psi 92.4% Correlation 0.833 0.429 p= 0.076 gpm at RT density 95/95 SLB Leak Rate at 2405 psi 100% Correlation 0.163 0.091 gpm at RT density 95/95 SLB Leak Rate at 2560 psi 100% Correlation 0.585 0.306 gpm at RT density at EOC 22 for SG 11. Projected SLB leakage from other sources is 0.057 gpm. A total of 0.890 gpm meets the 1.0 gpm limit for SG 11. At 2405 psi the projected SLB leak rate drops to 0.163 gpm. The effect of uncorrelated leak rates is seen to increase the ForeLine Associates, LLC 26 FA - P11 - 016

projected SLB leak at 2560 psi by a factor of 1.42 compared to use of fully correlated leak rates. The projected 95/95 SLB leak rate in SG 12 is less than half of the allowable limit.

As a final measure of degradation severity the upper voltage repair limit, based on a 1.695 EFPY cycle length and a default voltage growth rate of 30% and measurement uncertainty of 20% is 4.49 volts.

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Section 6

SUMMARY

AND CONCLUSIONS This report describes the Bobbin Voltage Alternate Repair Criteria for steam generator tubing applied at Prairie Island Unit 1, EOC 21. Bobbin voltage data at tube support plates is summarized, with analyses of leak rate and probability of tube burst under postulated steam line break conditions both in terms of condition monitoring at EOC 21 and operational assessment for cycle 22. Projected bobbin voltage distributions for the conclusion of the next operating cycle are included along with calculated probabilities of burst and projected leak rates at postulated SLB conditions. The latest available data for burst pressure and leak rate correlations has been applied. Present results support continued application of the 2.0-volt repair criteria for TSP indications per NRC Generic Letter 95-05.

The maximum voltage DSI indications were 1.73 volts in SG 11 and 1.43 volts in SG 12.

The composite population voltage growth rate was -0.13 volts per EFPY for Cycle 21.

The Upper Voltage Repair Limit for a projected cycle length of 1.695 EFPY using default voltage growth and measurement uncertainties is 4.49 volts. Condition monitoring burst probabilities were 6E-5 for both SG 11 and SG 12. Projected SLB burst probabilities for EOC 22 are 1.26E-4 and 9.5E-5 for SG's 11 and 12 respectively. Condition monitoring 95/95 SLB leak rates at 2560 psi is 0.261 gpm for SG 11 and 0.113 gpm for SG 12.

Inclusion of a lack of leak rate correlation effect led to a projected 0.833 gpm 95/95-leak rate for SG 11 at EOC 22 at 2560 psi. This value drops to 0.163 gpm at 2405 psi.

Consideration of other leakage sources leads to a total projected leak rate of 0.890 gpm in SG 11 at EOC 22 for a pressure differential of 2560 psi. This meets the 1.0 gpm limit.

The projected 95/95 SLB leak rate for SG 12 at EOC 22 is 0.429 gpm.

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Section 7 REFERENCES

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.

2. 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.

3. SG-98-03-002, "Prairie Island Unit-1 Cycle 19 Voltage Based Repair Criteria 90 Day Report, "Westinghouse Nuclear Services Division, March, 1998.

4. SG-99-07-006, "Prairie Island Unit-1 Cycle 20 Voltage-Based Repair Criteria 90 Day Report, "Westinghouse Nuclear Services Division, July, 1999.

5. WCAP-14277, Revision 1, "SLB Leak Rate and Tube Burst Probability Analysis Methods for ODSCC at TSP Intersections," Westinghouse Nuclear Services Division, March, 1998.

6. Addendum-5 to EPRI Report NP-7480-L, "Steam Generator Outside Diameter Stress Corrosion Cracking at Tube Support Plates - Database for Alternate Repair Criteria, "to bee published.

7. "Steam Generator Integrity Assessment Guidelines: Revision 1", EPRI Report TR 107621-R1, March, 2000.

8. NMC Procedure, "Eddy Current Examination Requirements for the Steam Generator Voltage Based Repair Criteria", Prairie Island Nuclear Generating Plant Maintenance Procedure, D27.25, November 29, 2001.

9. Keating, R. F., "ODSCC ARC Leak Rate Correlation and Simulation by Monte Carlo Methods", NEI/NRC Meeting, February 21, 2002.

10. "Appendix B to Steam Generator In Situ Pressure Test Guidelines", EPRI Report TR-1 07620-R1, to be published.

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