RS-20-057, Response to Request for Additional Information Regarding Emergency License Amendment Request for a One-Time Extension of the Steam Generator Tube Inspections

Response to Request for Additional Information Regarding Emergency License Amendment Request for a One-Time Extension of the Steam Generator Tube Inspections

ML20104C139
Person / Time
Site:	Braidwood
Issue date:	04/13/2020
From:	Demetrius Murray Exelon Generation Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
RS-20-057
Download: ML20104C139 (13)

Text

4300 Winfield Road Warrenville, IL 60555 630 657 2000 Office RS-20-057 10 CFR 50.90 April 13, 2020 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Braidwood Station, Unit 2 Renewed Facility Operating License No. NPF-77 NRC Docket No. STN 50-457

Subject:

Response to Request for Additional Information Regarding Emergency License Amendment Request for a One-Time extension of the Steam Generator Tube Inspections

References:

1) Letter from D. Murray (Exelon Generation Company, LLC) to U.S. Nuclear Regulatory Commission, "Emergency License Amendment for a One-Time extension of the Steam Generator Tube Inspections," dated April 6, 2020 (ADAMS Accession No. ML20097J188)

2) Email from J. Wiebe (U.S. Nuclear Regulatory Commission) to L. Zurawski (Exelon Generation Company, LLC), "Preliminary RAIs for Exelon's April 6, 2020, Application to Defer Braidwood, Unit 2, Steam Generator Inspections,"

dated April 10, 2020 In Exelon Generation Company, LLC (EGC) letter dated April 6, 2020 (Reference 1), EGC requested an amendment to the Technical Specifications (TS) for Renewed Facility Operating License No. NPF-77 for Braidwood Station, Unit 2 (Braidwood). The proposed amendment request revises TS 5.5.9, "Steam Generator (SG) Program," for a one-time revision to the frequency for Steam Generator Tube Inspections.

In NRC email dated April 10, 2020 (Reference 2), the NRC determined that additional information is needed to complete its review. During the clarification call held with the NRC on April 10, 2020, the NRC asked for additional information. Responses to requested information provided in Reference 2 and the additional questions discussed on April 10, 2020 are included in Attachment 1.

EGC has reviewed the information supporting the no significant hazards consideration and the environmental consideration that were previously provided to the NRC in Attachment 1 of the Reference 1 letter. The additional information provided in this submittal does not affect the conclusion that the proposed license amendment does not involve a significant hazards consideration. This additional information also does not affect the conclusion that there is no need for an environmental assessment to be prepared in support of the proposed amendment.

U.S. Nuclear Regulatory Commission April 13, 2020 Page 2 There are no regulatory commitments contained within this submittal. Should you have any questions concerning this submittal, please contact Ms. Lisa Zurawski at (630) 657-2816.

I declare under penalty of perjury that the foregoing is true and correct. Executed on the 13th day of April 2020.

Respectfully, Dwi Murray Sr. Manager - Licensing Exelon Generation Company, LLC

Attachment:

1) Response to Request for Additional Information cc:

NRC Regional Administrator - Region III NRC Senior Resident Inspector - Braidwood Station NRC Project Manager, NRR - Braidwood Station i M

ATTACHMENT 1 Response to Request for Additional Information

ATTACHMENT 1 Response to Request for Additional Information Page 1 of 10 By letter dated April 6, 2020 (Reference 1) Exelon Generation Company, LLC (EGC) requested an amendment to the Technical Specifications (TS) for Renewed Facility Operating License No.

NPF-77 for Braidwood Station, Unit 2. The proposed amendment request revises TS 5.5.9, "Steam Generator (SG) Program," for a one-time revision to the frequency for Steam Generator Tube Inspections.

By email dated April 10, 2020 (Reference 2), the NRC determined that additional information is needed to complete its review. As discussed during the clarification call held with the NRC on April 10, 2020, the NRC verbally asked two additional questions. EGC's interpretation of these questions are as follows:

x How does operating time and growth affect the [cumulative probability] distribution shown on the referenced Figure 1 of Attachment 2 of Reference 1? The indication counts do not seem to match the table of OA output results.

x Are there any other TS affected by the proposed license amendment request (Reference 1), specifically TS 3.4.13, "RCS Operational LEAKAGE," and TS 3.4.19, "Steam Generator (SG) Tube Integrity"?

This attachment provides the responses to the requested information provided in Reference 2 and verbal questions discussed on April 10, 2020 as described above.

Request for Additional Information (RAI) Basis The TS for all PWR plants require that an SG program be established and implemented to ensure that SG tube integrity is maintained. SG tube integrity is maintained by meeting the performance criteria specified in Section 5.5.9 of the Braidwood U2 TS for structural and leakage integrity, consistent with the plant design and licensing basis.

1. Wear at support structures Following the Spring 2017 SG inspections (A2R19), a 39% through-wall (TW) flaw from Anti-Vibration Bar (AVB) wear was returned to service. The Operational Assessment (OA) performed at that time supported operation for two cycles to Spring 2020 (A2R21). Table 6 in Attachment 1 of the LAR projects a measured depth of 52% TW at the Fall 2021 inspection (A2R22). Please provide the following information about the evaluation:

a. Notes 1 and 2 of Table 6 state that the basis for selection was "paired wear indications" at A2R17 and A2R19. Please explain the "paired wear indications" methodology and clarify how the 95th percentile paired wear indication growth rates were determined, including how the methodology and rates compare to historical wear growth rates determined for Braidwood Unit 2.

b. Table 6 does not appear to account for NDE measurement uncertainty, beyond the statement that the arithmetic method of uncertainty was used in the evaluations. Please clarify how NDE measurement uncertainty was used in the OA projections in Table 6.

ATTACHMENT 1 Response to Request for Additional Information Page 2 of 10

2. Axial ODSCC in high residual stress tubing For Figure 1 on page 10 of Attachment 2, please clarify the following:

a. Are the blue dots in Figure 1 made using the EPRI upper bound structural average growth rate?

b. Is the yellow curve made using only the four indications observed in the A2R10 outage?

c.

It appears the indications from A2R10 are also plotted on the graph as yellow dots.

However, it appears the yellow dot at approximately 42% TW may not be graphed correctly. Our understanding of the A2R10 indications is that the smallest indication was only 26% TW.

d. In the last paragraph on page 10, please clarify the following:

i.

"When the model is rerun using the number of indications observed in SG C, the predicted depth distribution significantly bounds the observed depth distribution, suggesting that the applied growth rate is conservative." The underlined portion of this sentence appears to be implying that the yellow curve significantly bounds the yellow dots in Figure 1, which does not seem validated by the good fit of the yellow curve to the yellow dots. Please clarify if the sentence is referencing a different "observed depth distribution."

ii.

Please elaborate on this sentence, "The simulation software was used to optimize the growth rate for the observed A2R10 indications."

iii.

"This effort shows that an alternate non-linear function form benchmarks well to the observed A2R10 depths and this growth rate distribution is also bounded by the upper bound default of the EPRI IAGL." The first half of this sentence (underlined) appears to indicate that the yellow curve and the yellow dots in Figure 1 are in good agreement; however, the remainder of the sentence appears to indicate that if the upper bound default growth rates of the EPRI IAGL were simulated and plotted on Figure 1, it would bound the yellow curve. Please confirm our understanding.

3.

Please discuss if the bobbin coil mix residual voltage screening criteria used in 2017 at the non-high stress tubing TSP intersections, to determine if a supplemental examination is needed, would have detected the 2011 indication in SG C, R44 C47 that was subsequently in-situ pressured tested in 2012.

ATTACHMENT 1 Response to Request for Additional Information Page 3 of 10 Request for Additional Information:

RAI 1.a.

Wear at support structures Following the Spring 2017 SG inspections (A2R19), a 39% through-wall (TW) flaw from Anti-Vibration Bar (AVB) wear was returned to service. The Operational Assessment (OA) performed at that time supported operation for two cycles to Spring 2020 (A2R21). Table 6 in of the LAR projects a measured depth of 52% TW at the Fall 2021 inspection (A2R22). Please provide the following information about the evaluation:

a. Notes 1 and 2 of Table 6 state that the basis for selection was "paired wear indications" at A2R17 and A2R19. Please explain the "paired wear indications" methodology and clarify how the 95th percentile paired wear indication growth rates were determined, including how the methodology and rates compare to historical wear growth rates determined for Braidwood Unit 2.

Response to RAI 1.a.

Paired wear indications are simply repeat indications which exhibit wear in two successive inspections such that a growth rate for the operating period between inspections can be established.

The observed wear indication growth rates at A2R17 (for the A2R16 to A2R17 interval) and A2R19 (for the A2R17 to A2R19 interval) were computed by dividing the change in indication depth with the length of the inspection interval. Each change in indication depth was determined by comparing "paired" inspection data for indication depth at the current inspection (A2R17 and A2R19) with inspection data for the same indication from the preceding inspection (A2R16 and A2R17, respectively).

The population of growth rates at A2R17 and A2R19 were individually fitted to normal distributions and the 95th percentile growth rates for A2R17 and A2R19 were determined from the fitted distributions. The 95th percentile growth rate for A2R17 (2.9% Through Wall / Effective Full Power Year (TW/EFPY)) was larger than the 95th percentile growth rate for A2R19 (2.5%TW/EFPY). The 95th percentile growth rate for A2R16 to A2R17 was conservatively used to project wear indication growth rate for the three-cycle period covering A2R20, A2R21, and A2R22 which yielded the 52%TW prediction at A2R22.

It should also be noted that even if the singular largest Anti-Vibration Bar (AVB) wear growth rate observed over all SGs in terms of %TW/EFPY for the A2R17 to A2R19 period is used, 4.57%TW/EFPY, the A2R22 depth remains bounded by the 69%TW condition monitoring (CM) limit.

RAI 1.b.

b. Table 6 does not appear to account for Non-Destructive Examination (NDE) measurement uncertainty, beyond the statement that the arithmetic method of uncertainty was used in the evaluations. Please clarify how NDE measurement uncertainty was used in the OA projections in Table 6.

ATTACHMENT 1 Response to Request for Additional Information Page 4 of 10 Response to RAI 1.b.

The NDE measurement uncertainties for the eddy current technique used can either be applied to the initial flaw depth where the projected end-of-cycle depth after adding a growth allowance is compared against the end-of-cycle (EOC) structural limit (SL), or, can be applied to the SL to define the CM limit. For this OA, the NDE uncertainties were applied in the CM limit which then permits projection of as-reported A2R19 depths forward to A2R22 and then are compared against the CM limit.

RAI 2.a.

Axial ODSCC in high residual stress tubing For Figure 1 on page 10 of Attachment 2, please clarify the following:

a. Are the blue dots in Figure 1 made using the EPRI upper bound structural average growth rate?

Response to RAI 2.a.

Yes. The blue dot data are the cumulative probability distribution of maximum depth for all flaws generated within the model, both simulated detections and simulated non-detections.

RAI 2.b.

b. Is the yellow curve made using only the four indications observed in the A2R10 outage?

Response to RAI 2.b.

No. The yellow curve is not a curve fit to the observed indication data. The yellow line is the cumulative probability distribution of simulated detected indications using the Electric Power Research Institute (EPRI) Steam Generator Integrity Assessment Guidelines (IAGL) default upper bound growth rate. This distribution is based on an assumed susceptible population of seven flaws. This figure is only included to show what set of conditions would produce benchmarking of the SG 'C', A2R10 indication depths.

RAI 2.c.

c.

It appears the indications from A2R10 are also plotted on the graph as yellow dots.

However, it appears the yellow dot at approximately 42% TW may not be graphed correctly.

Our understanding of the A2R10 indications is that the smallest indication was only 26%

TW.

ATTACHMENT 1 Response to Request for Additional Information Page 5 of 10 Response to RAI 2.c.

During the teleconference with NRC on April 10, 2020, it was suggested that SCC depth sizing in 2003 may have been based on +Point'1 phase angle analysis whereas the current standard is amplitude-based depth sizing using the +Point'coil signal amplitude, thus explaining differences in reported sizing results.

EGC has reviewed the engineering analyses performed in 2003 and later analyses performed after reporting of the 2012 indications. These documents indicate that the 2003 analysis estimated flaw depths at 65%TW, 54%TW, and 42%TW. These depths were based on an amplitude-based depth sizing regression developed using only pulled tube data from Westinghouse supplied tubing. The Braidwood Unit 2 tube bundles were supplied by Westinghouse-Blairsville. Another document from the 2012 time period indicates that the 2003 indications were later resized using the EPRI Examination Technique Specification Sheet (ETSS) I28432 sizing regression, which includes Westinghouse tubing samples, Combustion Engineering (C-E) tubing samples, Babcock & Wilcox (B&W) tubing samples, and lab cracks.

Using the ETSS I28432 depth sizing regression, the estimated depths of the 2003, SG 'C' indications are 66%TW, 53%TW, and 26%TW. Thus, it appears that the difference in estimated depths of the shallowest indication is due to the use of different +Point' amplitude-based sizing regressions.

RAI 2.d.i.

d. In the last paragraph on page 10, please clarify the following:

i.

"When the model is rerun using the number of indications observed in SG C, the predicted depth distribution significantly bounds the observed depth distribution, suggesting that the applied growth rate is conservative." The underlined portion of this sentence appears to be implying that the yellow curve significantly bounds the yellow dots in Figure 1, which does not seem validated by the good fit of the yellow curve to the yellow dots. Please clarify if the sentence is referencing a different "observed depth distribution."

Response to RAI 2.d.i.

The intent of Figure 1 was only to show that the software provides diagnostic output which can be used to ensure the model predictions are consistent with in plant observed results.

Benchmarking attempts to recreate the observed SG 'C' A2R10 indication depths using the number of observed A2R10 indications and the upper bound growth rate produced a simulated detected depth distribution which overestimated the as-found A2R10 distribution. That is, the simulated detected depth distribution resides to the right of the "Observed" data. This implies the applied growth distribution is an overestimate of the actual performance for the combination of the number of flaws reported at A2R10, initiation function, and growth rate. Note: This curve (not shown) is not to be confused with the yellow curve presented in Figure 1. The IAGL upper bound default growth curve is shown to be conservative for this mechanism and was applied in the OA.

1 +Point and X-Probe are trademarks or registered trademarks of Zetec, Inc., its subsidiaries and/or affiliates in the United States of America and may be registered in other countries through the world. All rights reserved. Unauthorized use is strictly prohibited. Other names may be trademarks of their respective owners.

ATTACHMENT 1 Response to Request for Additional Information Page 6 of 10 RAI 2.d.ii.

ii.

Please elaborate on this sentence, "The simulation software was used to optimize the growth rate for the observed A2R10 indications."

Response to RAI 2.d.ii.

The software used for the tube integrity analysis can be used to determine a best estimate of the actual growth rate over the operating period through an iterative process. When the Cumulative Distribution Function (CDF) of the simulated detected depths converges with the CDF of observed depths the model is optimized. An example of this convergence is shown on Figure 1. For the number of indications observed in SG 'C' at A2R10 used as the susceptible population and an acute initiation function (i.e., all indications initiate within a short operating period window) the growth rate distribution which best benchmarks the A2R10 data is found to be a Beta function. Note that the IAGL upper bound default growth was applied in the OA analysis.

RAI 2.d.iii.

iii.

"This effort shows that an alternate non-linear function form benchmarks well to the observed A2R10 depths and this growth rate distribution is also bounded by the upper bound default of the EPRI IAGL." The first half of this sentence (underlined) appears to indicate that the yellow curve and the yellow dots in Figure 1 are in good agreement; however, the remainder of the sentence appears to indicate that if the upper bound default growth rates of the EPRI IAGL were simulated and plotted on Figure 1, it would bound the yellow curve. Please confirm our understanding.

ATTACHMENT 1 Response to Request for Additional Information Response to RAI 2.d.iii.

As stated in the response to 2.d.ii, the best estimate growth function which produces acceptable benchmarking of the A2R10 indications is a Beta function. This function produces larger growth rates at lower probabilities however the lognormal IAGL upper bound default growth function produces larger growth rates at the higher probabilities. As the higher probability growth rates affect the burst and leakage analyses results, use of the lognormal upper bound default growth will produce a conservative analysis. The lognormal IAGL default upper bound growth function was applied in the Braidwood OA analyses. The plot below compares the best estimate Beta growth function with the IAGL upper bound default growth function and shows the upper bound default growth function is conservative compared to the best estimate Beta growth at higher probabilities.

0.9 0.8

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0.7

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0.5

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0.4 E

0.3

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0.2 0.1

RAI 3

Comparison of Lognormal Upper Bound Growt h and Opt imized Beta Growt h Funct ion for Axial ODSCC at TSPs on High Residual Stress Tubes

+

+

Upper Bound Defa.J t OptimE!ed Beta t-

+

+

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10 15 20 Growth Rat e (%TW/ EFPY) 25 Please discuss if the bobbin coil mix residual voltage screening criteria used in 2017 at the non-high stress tubing TSP intersections, to determine if a supplemental examination is needed, would have detected the 2011 indication in SG C, R44 C47 that was subsequently in-situ pressured tested in 2012.

Response to RAI 3 The bobbin coil mixed residual screening criteria is used to proactively screen potential high noise condition intersections to provide a defense-in-depth approach for detection of axial Outer Diameter Stress Corrosion Cracking (ODSCC) at Tube Support Plate (TSP) intersections on non-high residual stress tubes. This methodology was developed in response to observed conditions in original C-E SGs and effectively identified SCC indications at locations where the bobbin coil analysis did not produce a flaw report. The mixed residual screening criteria is implemented via computer-based signal screening processes.

Page 7 of 10

ATTACHMENT 1 Response to Request for Additional Information Page 8 of 10 The mixed residual screening criteria would not have identified the 2011 signal on SG 'C' R44 C47 at 03H; however, the criteria's intent is focused on non-high residual stress tubes since this population represents a much larger tube population than high residual stress tubes. The mixed residual screening criteria is developed using detection theory with application of the IAGL typical default growth rate. High residual stress tubes are inspected at Braidwood Unit 2 using a combined 100% full length bobbin inspection followed by 100% X-Probe'inspection at all hot leg and cold leg TSP intersections. This process applies Design of Experiments Theory to improve the overall inspection adequacy on high residual stress tubes. Had this inspection practice been applied at the A2R10 outage it is likely that the combined inspection program consisting of bobbin inspection with subsequent X-Probe'inspection would have identified this signal in 2011 (A2R15).

The OA analyses for high residual stress tubes includes two OA models; an acute initiation model and a low Weibull slope model. The low Weibull slope model includes a very conservative susceptible tube population size allocation. The susceptible population size applied in the low Weibull slope model exceeds the total number of Alloy 600 Thermally Treated (A600TT) axial ODSCC indications at TSP intersections on high residual stress tubes for all affected plants and all affected SGs combined. This population size is assumed to exist within one SG at Braidwood. Thus, the low Weibull slope model accounts for initiation of additional high residual stress tube indications that may not be detected by the applied inspection process.

A recent evaluation of the growth rate of this indication (on SG 'C' R44 C47 at 03H) concludes that the actual growth rate is bounded by the IAGL typical default growth rate, thus the low Weibull slope model, which applies the IAGL upper bound default growth rate, is a conservative assessment for this condition.

Question 4:

During the clarification call held with the NRC on April 10, 2020, the NRC asked a verbal question. EGC's interpretation of the question is as follows:

How does operating time and growth affect the [cumulative probability] distribution shown on the referenced Figure 1 of Attachment 2 of Reference 1? The indication counts do not seem to match the table of OA output results.

Response to Question 4:

The referenced figure (Figure 1 of Reference 1) represents the simulated depth distribution at the A2R10 inspection for a specific initiation function, growth rate, and susceptible population size. The tube integrity software uses a Weibull initiation function, described by a Weibull slope, characteristic life, and susceptible population size to initiate flaws as a function of time. The actual operating period between the operating cycles for Braidwood Unit 2 is modelled so the growth of indications at each inspection can be evaluated and compared with the NDE data.

The purpose of the figure (Figure 1 of Reference 1) is to show that ample diagnostic data are generated by the software to adequately benchmark actual plant experience. As flaws are initiated within the model, they are subsequently "grown" following their initiation point by application of the growth rate distribution. Once initiated the indications grow over each cycle by the applied growth rate. The predicted depths for subsequent outages are tracked in the

ATTACHMENT 1 Response to Request for Additional Information Page 9 of 10 model except for those indications which were detected during normal eddy current testing and removed from service by plugging.

Thus, the model can provide an estimate of the indication depth distribution at any selected point in time and this estimate can be specifically configured based on the actual plant inspection programs.

The OA results table (Table 5 of Reference 1) provided in the license amendment request presents the number of assumed missed indications at the most recent inspection and the predicted number of indications at A2R22, along with the calculated probability of burst for comparison with the structural integrity performance criteria (SIPC) and probability of leakage exceeding the AILPC value for Braidwood Unit 2 for each evaluated mechanism. As seen in this table, the predicted number of indications at A2R22 is greater than the number of assumed missed indications at the most recent inspection. The final OA includes a table of results for each mechanism that shows the progression of indication initiation over the time period used in the analysis.

Question 5:

During the clarification call held with the NRC on April 10, 2020, the NRC asked a verbal question. EGC's interpretation of the question is as follows:

Are there any other TS affected by the proposed license amendment request (Reference 1),

specifically TS 3.4.13, "RCS Operational LEAKAGE," and TS 3.4.19, "Steam Generator (SG)

Tube Integrity"?

Response to Question 5:

As required by 10 CFR 50.36(c)(2)(i), the TS will include Limiting Conditions for Operation (LCO), which are the lowest functional capability or performance levels of equipment required for safe operation of the facility. 10 CFR 50.36(c)(3) requires TS to include items in the category of SRs, which are requirements relating to test, calibration, or inspection to assure that the necessary quality of systems and components is maintained, that facility operation will be within safety limits, and that the LCO will be met.

TS LCO 3.4.13.d, "RCS Operational LEAKAGE" requires RCS operational LEAKAGE shall be limited to 150 gallons per day primary to secondary LEAKAGE through any one steam generator (SG). TS LCO 3.4.19, "Steam generator (SG) Tube Integrity," requires that SG tube integrity shall be maintained and all SG tubes satisfying the tube plugging criteria shall be plugged in accordance with the Steam Generator Program. During the period of Braidwood Unit 2 operation until A2R22 (October 2021), all three SG performance criteria of structural integrity, accident induced leakage, and operational LEAKAGE (i.e., primary to secondary LEAKAGE) will continue to be met. Each inspected SG tube that satisfied the tube plugging criteria in A2R19 was plugged in accordance with TS 5.5.9, "Steam Generator (SG) Program."

The completed Operational Assessment evaluated that the SG performance criteria will continue to be met until the next SG tube inspection during A2R22 (October 2021). The tube integrity determination was based on the condition of all the tubes remaining in service at A2R19 and the estimated growth of the degradation prior to the next SG tube inspection. The SG plugging criteria applied in A2R19 allows for flaw growth between inspections while still

ATTACHMENT 1 Response to Request for Additional Information Page 10 of 10 providing assurance that all the SG performance criteria will continue to be satisfied until A2R22.

Therefore, EGC has determined that the proposed one-time revision to defer the Steam Generator inspection to be performed after three operating cycles following refueling outage A2R19 does not alter Braidwood Unit 2 compliance with TS LCOs 3.4.13.d and 3.4.19 or the requirements of 10 CFR 50.36(c)(2)(i) and 10 CFR 50.36(c)(3).