PNP 2013-024, Response to NRC Letter, Palisades Nuclear Plant - Summary of Conference Call Regarding the Spring 2012 Steam Generator Inspections (TAC ME8129).

Response to NRC Letter, Palisades Nuclear Plant - Summary of Conference Call Regarding the Spring 2012 Steam Generator Inspections (TAC ME8129).

ML13171A010
Person / Time
Site:	Palisades
Issue date:	06/19/2013
From:	Gustafson O Entergy Nuclear Operations
To:	Office of Nuclear Reactor Regulation, Document Control Desk
References
PNP 2013-024, TAC ME8129
Download: ML13171A010 (14)

Text

• Entergy

-=:::.

Entergy Nuclear Operations, Inc.

Palisades Nuclear Plant 27780 Blue Star Memorial Highway Covert, MI 49043-9530 Tel 269 764 2000 Otto W Gustafson Licensing Manager PNP 2013-024 June 19, 2013 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001

SUBJECT:

Response to NRC letter, "Palisades Nuclear Plant - Summary of Conference Call Regarding the Spring 2012 Steam Generator Inspections (TAC No. ME8129)"

Palisades Nuclear Plant Docket 50-255 License No. DPR-20

References:

1. ENO letter to NRC, "Palisades, Notice of Deviation from EPRI Steam Generator Management Program: PWR Steam Generator Examination Guidelines: Revision 7," dated April 5, 2012 (ADAMS Accession Number ML121000292)

2. NRC letter to ENO, "Palisades Nuclear Plant - Summary of Conference Call Regarding the Spring 2012 Steam Generator Inspections (Tac No.

ME8129)" dated July 17,2012 (ADAMS Accession Number ML12188A578)

Dear Sir or Madam:

Entergy Nuclear Operations, Inc. (ENO) submitted Reference 1 to the Nuclear Regulatory Commission (NRC) providing a notice of deviation from Electrical Power Research Institute (EPRI) Steam Generator Management Program: Pressurized Water Reactor (PWR) Steam Generator Examination. ENO received a letter, Reference 2, containing comments based on conference calls held between the NRC and ENO on April 12, 2012 and April 20, 2012. Attached is ENO's response to the comments listed in Reference 2.

This letter contains no new or revised commitments.

PNP 2013-024 Page 2 Sincerely, owg/jpm : Response to NRC comments on "Notification of Deviation from EPRI Steam Generator Management Program: PWR Steam Generator Examination Guidelines: Revision 7" cc: Administrator, Region III, USNRC Project Manager, Palisades, USNRC Resident Inspector, Palisades, USNRC

Attachment 1 Response to NRC comments on "Notice of Deviation from EPRI Steam Generator Management Program:

PWR Steam Generator Examination Guidelines: Revision 7" Comments received by NRC letter July 17. 2012 Nuclear Regulatory Commission (NRC) Comment

1. The April 5, 2012, letter does not appear to provide adequate justification for the deviation.

Entergy Nuclear Operations, Inc (ENO) Response

1. ENO's position, with agreement from industry technical experts, is that the deviation is technically justified with the following clarifications provided to demonstrate that the Combustion Engineering (CE) plant data (CE axial outside diameter stress corrosion cracking (ODSCC>> and the Electrical Power Research Institute (EPRI) approved examination technique specification sheet (ETSS) 128432 data are from different statistical populations.

NRC Comment

2. All data should be included in the sizing correlation unless it is excluded through application of objective data exclusion criteria (e.g., all Westinghouse data will be excluded). As a result, it may be acceptable to segregate the Combustion Engineering (CE) data from the total data set.

ENO Response

2. It can be demonstrated through statistical T-tests that the CE data subset (Le.,

the CE portion of the data used for the Appendix 128432 qualification) is statistically removed from the remaining Appendix 128432 dataset. Even though the CE portion of the Appendix I data will be removed, the proposed sizing procedure for the Appendix I deviation remains identical to the EPRI approved Appendix I ETSS 128432 sizing procedure with exception of a change to the Y-intercept value. Specifically, the deviation will continue to maintain the ETSS 128432 slope and standard error of maximum depth measurement. The only change is a decrease in the V-intercept value, through use of the CE data subset, to more truly reflect the CE plant population. The CE data subset is one of four components making up the combined fleet dataset. The combined fleet dataset includes CE, Westinghouse, Once-Through Steam Generator (OTSG),

and Lab specimen data and is the dataset used to qualify the EPRI Appendix I, ETSS 128432 technique. This dataset will be referred to as the combined fleet dataset (128432, All Data). The CE data subset (128432, CE Data) is the CE Page 1 of 12

portion of the combined fleet dataset. A third dataset, used in the deviation to Appendix 128432, is the CE data identified in EPRI Report 1014983, "Steam Generator In Situ Pressure Test Guidelines, Revision 3." This dataset will be referred to as the CE in situ dataset (CE Data, EPRI1014983). These datasets are summarized in the following table.

Dataset Key Symbol, Source # Data Points 128432, All Data * , 6. , ..

Figures 1,2, and 3

, 0 CE, W, OTSG, Lab 128432, CE Data

• CE *14(in-situ) + 2( other)

= 16 CE Data, EPRI 0 CE Plants 84 (in-situ) 1014983

• 14 (in-situ) points are pulled from the 84 (in-situ) points population (CE Data, EPRI 1014983)

While it can be demonstrated that the CE plant population (

• and 0 , 86 data points) is statistically further removed from the combined fleet dataset (128432, All Data), a compromise position was taken to use only the CE data subset ( * ,

128432, CE Data) in the deviation to Appendix 128432. The compromise was based on a request by the EPRI peer review committee to include only the CE data used in the original Appendix I, ETSS 128432 qualification and not to amend or detract from this dataset. Applying this request, use of the CE data subset rather than the CE population, conservatively adds more than 5 % percent through wall (%TW) to each reported non-destructive examination (NDE) depth measurement. Therefore, the deviation to Appendix I uses the original CE data along with the original Appendix 128432 values, for slope and standard error of maximum depth estimate, with the only change being a decrease in the y-intercept value. This minimal change will continue to ensure continuity with the EPRI pedigree already established through the well vetted Appendix I protocol.

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NRC Comment

3. Statistical tests should be performed to justify that the CE data is statistically different than the total data set.

4. Even if the CE data set (i.e., the remaining data set after application of the data exclusion criteria) does not meet all the requirements of Appendix I of the PWR Steam Generator Examination Guidelines for sizing, it may be useful to develop a correlation to understand the uncertainties associated with the sizing method.

ENO Response 3&4. Statistical T -tests will demonstrate that the CE data subset is statistically different from the combined fleet dataset by showing that both datasets exhibit statistically similar slopes yet statistically different V-intercept values. Figure 1 shows a plot of maximum axial ODSCC crack depth versus +Point probe voltage using the combined fleet dataset (128432, All Data). Note that all of the data below 30

%TW is CE plant pulled tube data ( * ). Figure 2 is a re-plot of Figure 1 but with the addition of the CE in situ dataset ( 0 , CE Data, EPRI 1014983), where the plot now illustrates the entire CE plant population (84 in situ (includes 14 common points) plus 2 other). Notice that most of the CE plant data lies well to the right of the combined fleet data best fit line (128432 Fit, All Data). In fact, only 6 of the CE data points lie to the left of the best fit line. This is a clear indication that there is a significant difference between the CE plant data and the other data sets comprising ETSS 128432 (Le., Westinghouse, OTSG, and Lab Specimens).

An evaluation of this difference will demonstrate that while the slopes are statistically similar the V-intercept values are from different statistical populations.

The slope of the combined fleet dataset plot of maximum depth %TW versus +Pt Probe 300 kHz Voltage, volts, peak to peak (In(Vpp)) is 20.91 with a standard error of measurement of 1.68. The slope of the CE data subset is 20.69 with a standard error of measurement of 1.84. The resulting combined standard error of both data sets is 2.49 (1.68 2 + 1.842 )112 . Application of the T-test, after taking the difference between the slopes and dividing by the combined standard error, shows that any difference in the slope values can be stated with only 7%

confidence. Such a low confidence value indicates that there is not a statistically significant difference in slopes between the combined fleet dataset and the CE data subset. That is, the slopes are nearly equal. The concept of nearly equal slopes is visually demonstrated in Figure 3. It is not surprising that the two slopes are statistically similar, in light of the fact that the industry slope for maximum depth %TW versus In(Vpp) for crack like outside diameter (00) degradation is well established. In fact, the slope is the same for both axial ODSCC and circumferential ODSCC. This concept is readily apparent as illustrated in Figure 8-7 of Revision 4 of the EPRlln Situ Pressure Testing Guidelines (EPRI Report Page 3 of 12

1025132). This point is further emphasized by viewing Figure 4 of this report where it is instructive to consider that the slope of the correlation line for partial depth electrical discharge machining (EDM) notches, whether axial or circumferential, is essentially the same as the slope of the combined fleet data correlation line.

The second point, concerning the difference in V-intercept values between the CE data subset and the combined fleet dataset, is illustrated in Figure 3. Here a clear difference exists between the V-intercept value for the combined fleet data best fit line (128432 Fit, All Data) and the best fit line to the CE data subset (128432 Fit, CE Data). Note that the V-intercept equals the maximum depth for In(Vpp) at 1.0 volts. The V-intercept corresponding to the combined fleet dataset is 78.98 %TW while the V-intercept corresponding to the CE data subset is 67.75

%TW. This difference of 11.23 %TW is 2.63 times larperthan the combined standard error of the intercept values (2.392 + 3.542 )11

• Through application of the T-distribution, a difference in V-intercept values is shown between the two datasets, at 98% confidence. The actual difference in V-intercepts between the combined fleet dataset ( * ,b., & , 0 ) and the CE plant population ( 0 ) is understated since Figure 2 shows that the majority of the CE data subset is on the low voltage side (approximately 0.05 to 0.24 Volts) of the CE plant population

( 0 ). The majority of the CE plant population voltages range from approximately 0.08 to 0.49 Volts.

Degradation morphology can change the Y-intercept value of the sizing correlation line. As the width of EDM notch is decreased the +Point probe voltage also decreases. Similarly, the voltage is decreased by the presence of ligaments across the crack faces as they provide current paths. Hence, crack morphology will have an effect on the V-intercept value of the sizing correlation line. The case for considering Palisades Nuclear Plant (PNP) as best represented by the CE plant data was made, in the deviation request, using multiple cycle Monte Carlo probabilistic projections for the number and severity of ODSCC degradation. The best representation of the history of degradation progression, at PNP, required use of a probability of detection (POD) curve that is more adverse than the ETSS 128413 POD curve and a sizing correlation that is not as severe as 128432. That is, a POD curve equal to or better (less conservative) than that of Appendix 128432 was clearly inadequate in matching past inspection results. It was only through the use of more aggressive POD curves (San Onofre Nuclear Generating Station (SONGS) and OTSG with the OTSG being more aggressive) that previous inspection results were better able to be duplicated or matched via computer simulation. Going forward, the more aggressive OTSG POD is planned to be made part of the operational assessment to successfully predict degradation severity at PNP.

Since the majority of the CE plant population data, Figure 2, is overwhelmingly skewed to the lower depths, with only two of the CE data points being above 49 Page 4 of 12

%TW, a reliable slope for the CE plant population cannot be obtained. However, by using the argument where the industry slope for maximum depth versus In(Vpp) for crack like 00 degradation is well established, and through the use regression analysis where the slope is set equal to the combined fleet dataset slope of 20.91 , the V-intercept value for the CE plant population can be determined. The resulting V-intercept value was calculated to be 62.21 %TW with a standard deviation of 12.57 %TW. The 62.21 %TW V-intercept is 16.77

%TW less than the combined fleet data (128432, All Data) V-intercept value of 78.98 %TW and approximately 7.0 times the standard error of estimate (2.39) of the combined fleet data intercept value. Hence statistically speaking, the CE plant data and the ETSS 128432 data are certainly from different statistical populations.

There is a very reasonable case to use the CE plant population data, with a specified slope of 20.91, an intercept of 62.21 %TW, and a standard error of 1.37 to obtain a valid sizing correlation. Figure 5 demonstrates the difference in upper 95 th percentile depth estimates between the proposed deviation using the CE data subset ( * ,128432, CE Data) and that using the CE plant population data

(

• and 0 , 86 data points). The proposed deviation (using only the CE data subset) leads to upper 95th percentile depth estimates that are larger than would be obtained using a correlation based on the CE plant population data.

Since EPRI peer review approval requires use of Appendix I data (requiring the CE data subset be used rather than the CE population data), a compromise position was taken such that the more conservative V-intercept (67.75 %TW rather than 62.21 %TW) be used. Thus, the proposed sizing procedure, for the Appendix I deviation, is based on using the well vetted CE data subset (128432, CE Data) where the combined fleet dataset (128432, All Data) slope (20.91) and standard error of maximum depth estimate (1.68) are maintained while only changing the V-intercept value from 78.98 %TW to 67.75 %TW. Based on the forgoing, there is a strong technical basis to accept the proposed sizing change, which is supported by the EPRI peer review process.

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Figure 1 Maximum Axial ODSCC Depth versus +Pt Probe Voltage, ETSS Appendix 128432 100 D

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NRC Requesf

5. If the Westinghouse and CE data are from different statistical populations, assessment of the probability of detection for the data sets may be needed.

ENO Response

5. ENO, with agreement from industry technical experts and by benchmarking previous PNP inspection results to computer simulations, has determined that use of the Appendix 128432 POD curve is inadequate. It was only through the use of more aggressive POD curves (SONGS and OTSG) that previous inspection results were better able to be duplicated or matched. The SONGS POD curve is the outcome of a pulled tube POD program sponsored by SONGS Unit-2 in the late 1990's. The OTSG POD curve is a result of an OTSG program to improve the probability of detection for freespan ODSCC. Of the two POD curves, the more aggressive (OTSG) curve was used since it better benchmarked historical PNP data. The inadequacy of the 128432 POD curve was originally identified in the deviation request. Specifically, the deviation request states: "any proposed use of a probability of detection (POD) curve equal or better than that of EPRI Appendix 128413 is inadequate in matching inspection data for axial ODSCC at eggcrate intersections." Two alternative POD curves were proposed. Both are considerably more adverse than Appendix I POD curves.

NRC Requesf

6. If there are not enough data points in the CE data set, a strategy for obtaining additional data could be developed which satisfies the industry guidelines for qualifying techniques. Additional confidence in the sizing methods could be obtained from the removal of tubes for destructive examination.

ENO Response

6. There are enough data points in the CE plant subset of ETSS Appendix I to obtain a valid sizing correlation based on a standard deviation of approximately 12.6 for %TW not being accounted for in the regression of In(Vpp). The standard deviation of 12.6 is taken from the Appendix 128432 regression parameters.

Most of the CE plant data is not vetted according to the Appendix I protocols.

Even if the data was vetted, the database would have to be expanded by including larger crack depths (Le., the majority of the CE plant database, on average, is less than 32 %TW). The number of these larger depth indications, 60 Page 11 of 12

%TW and greater, would have to be substantial to balance the database. Since the degradation at PNP is being managed in an effective manner through tube plugging, in line with growth rates such that large %TW indications (60 %TW and greater) are the exception and not the rule, tube pulls is not considered a viable path (Le., since it is highly unlikely that a number of large crack depths will be encountered).

NRC Requesf

7. Successful performance of in-situ pressure tests on indications larger than the in-situ pressure test screening does not necessarily imply that the sizing methods are conservative since the screening criteria are intended to be conservative.

ENO Response

7. ENO agrees that successful in-situ pressure tests do not, in and of themselves, validate that one sizing method is more conservative than another. However, it is a validation that sizing methods combined with screening criteria are conservative with margin. The deviation request presented this data as a quantitative analysis of the probabilities of past degradation passing in-situ tests including in-situ tests at elevated test pressures. It concluded: "Therefore the fact that all three indications passed in situ testing offers no strong evidence that sizing per Appendix 128432 for axial ODSCC indications is overly conservative."

References:

[1] EPRI "Steam Generator Management Program: Pressurized Water Reactor Steam Generator Examination Guidelines: Revision 7," report number 1013706, dated October 2007 (Adams Accession No. ML080450582).

[2] EPRI "Steam Generator In Situ Pressure Test Guidelines Revision 3," report number 1014983, dated August 2007 (Adams Accession No. ML072970246).

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