ML19259A036

Summary Report for the Audit of the Response to 10 CFR 50.54(F) Information Request -Flood-Causing Mechanism Reevaluation

ML19259A036
Person / Time
Site:	Millstone
Issue date:	10/07/2019
From:	Joseph Sebrosky Beyond-Design-Basis Management Branch
To:	Stoddard D Dominion Nuclear Connecticut
Sebrosky J, NRR/DLP, 415-1132
References
EPID L-2015-JLD-0011, EPID L-2015-JLD-0012
Download: ML19259A036 (18)

Text

UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 October 7, 2019 Mr. Daniel G. Stoddard Senior Vice President and Chief Nuclear Officer Dominion Nuclear Connecticut, Inc.

Millstone Power Station Innsbrook Technical Center 5000 Dominion Boulevard Glen Allen, VA 29060

SUBJECT:

MILLSTONE POWER STATION, UNITS 2 AND 3 -

SUMMARY

REPORT FOR THE AUDIT OF THE RESPONSE TO 10 CFR 50.54(!) INFORMATION REQUEST - FLOOD-CAUSING MECHANISM REEVALUATION (EPID NOS. L-2015-JLD-0011 AND L-2015-JLD-0012)

Dear Mr. Stoddard:

The purpose of this letter is to provide the U.S. Nuclear Regulatory Commission's (NRC) audit summary report associated with the Millstone Power Station, Units 2 and 3 (Millstone) post-Fukushima reevaluated flood hazard. The regulatory audit provided insights to the staff's development of the Millstone flood hazard reevaluation report (FHRR) staff assessments that were issued on October 3, 2018, and October 7, 2019 (Agencywide Documents Access and Management System (ADAMS) Accession Nos. ML18256A200 and ML19246A116, respectively). These staff assessments document the resolution of the issues associated with the staff's review of the FHRR including issues that were identified during the audit process.

By letter dated March 12, 2012 (ADAMS Accession No. ML12053A340), the NRG issued a request for information pursuant to Title 10 of the Code of Federal Regulations, Section 50.54(f)

(hereafter referred to as the 50.54(f) letter). The request was issued as part of implementing lessons learned from the accident at the Fukushima Dai-ichi nuclear power plant. Enclosure 2 to the 50.54(f) letter requested that licensees reevaluate flood-causing mechanisms using present-day methodologies and guidance. By letter dated March 12, 2015 (ADAMS Accession No. ML15078A204), as supplemented by letter dated January 4, 2019 (ADAMS Accession No. ML19011A110), Dominion Nuclear Connecticut, Inc. (Dominion, the licensee) responded to this request for Millstone by providing the FHRR.

By letter dated June 15, 2015 (ADAMS Accession No. ML15153A077), the NRG notified Dominion of the staff's plan to perform a regulatory audit of Millstone's supporting calculations of the FHRR. The technical audit was performed consistent with NRC Office of Nuclear Reactor Regulation, Office Instruction LIC-111, "Regulatory Audits," dated December 29, 2008 (ADAMS Accession No. ML082900195). This audit summary report closes out the audit process associated with the Millstone FHRR review. Based on the issuance of the October 3, 2018, and October 7, 2019, FHRR staff assessments and the issuance of this audit summary report the NRC will close EPID Nos. L-2015-JLD-0011 and L-2015-JLD-0012.

D. Stoddard If you have any questions, please contact me at (301) 415-1132 or by e-mail at Joseph.Sebrosky@nrc.gov.

Sincere!

./

Docket Nos. 50-336 and 50-423

Enclosure:

Audit Summary Report cc w/encl: Distribution via Ustserv

NUCLEAR REGULATORY COMMISSION AUDIT REPORT FOR THE AUDIT OF DOMINION NUCLEAR CONNECTICUT INC.'S FLOOD HAZARD REEVALUATION REPORT SUBMITTALS RELATING TO THE NEAR-TERM TASK FORCE RECOMMENDATION 2.1-FLOODING FOR MILLSTONE POWER STATION, UNITS 2 AND 3 (EPID NOS. L 2015 JLD-0011 AND L-2015-JLD-0012)

BACKGROUND AND AUDIT BASIS By letter dated March 12, 2012 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML12053A340), the U.S. Nuclear Regulatory Commission (NRG) issued a request for information to all power reactor licensees and holders of construction permits in active or deferred status, pursuant to Title 10 of the Code of Federal Regulations (10 CFR), Section 50.54(1) "Conditions of license" (hereafter referred to as the "50.54(1) letter"). The request was issued in connection with implementing lessons-learned from the 2011 accident at the Fukushima Dai-ichi nuclear power plant, as documented in The Near-Term Task Force Review of Insights from the Fukushima Dai-ichi Accident. Recommendation 2.1 in that document recommended that the NRC staff issue orders to all licensees to reevaluate seismic and flooding hazards for their sites against current NRC requirements and guidance.

Subsequent staff requirements memoranda associated with Commission Papers SECY 11-0124 and SECY-11-0137, instructed the NRC staff to issue requests for information to licensees pursuant to 10 CFR 50.54(1).

By letter dated March 12, 2015 (ADAMS Accession No. ML15078A204), as supplemented by letter dated January 4, 2019 (ADAMS Accession No. ML19011A110), Dominion Nuclear Connecticut, Inc. (Dominion, the licensee) responded to this request for Millstone Power Station, Units 2 and 3 (Millstone) by providing the flood hazard reevaluation report (FHRR). By letter dated June 15, 2015 (ADAMS Accession No. ML15153A077), the NRC notified Dominion of the staff's plan to perform a regulatory audit of Millstone's supporting calculations of the FHRR. The regulatory audit provided insights to the staff's development of the Millstone FHRR staff assessments that were issued on October 3, 2018, and October 7, 2019 (ADAMS Accession Nos. ML18256A200 and ML19246A116, respectively).

This audit summary was completed in accordance with the guidance set forth in NRC Office of Nuclear Reactor Regulation. Office Instruction LIC-111, "Regulatory Audits," dated December 29, 2008 (ADAMS Accession No. ML082900195).

AUDIT LOCATION AND DATES The staff notes that on December 21, 2016 (ADAMS Accession No. ML16308A226), the NRG staff sent the licensee a summary of the staff's review of Millstone reevaluated flood-causing mechanisms. This letter did not include the staffs conclusions associated with the storm surge flood-causing mechanism and stated that the staff's evaluation of the storm surge analysis was ongoing. The October 3, 2018, letter provided the staff's assessment of flood mechanisms, Enclosure

other than storm surge, that supported the staff's conclusions summarized in the December 21, 2016, letter.

As noted in the October 3, 2018, FHRR staff assessment for flood mechanisms other than storm surge, the staff used the audit process for other flood hazard mechanisms including the staff's review of the licensee's site-specific probable maximum precipitation flood hazard analysis. The audit process for hazards other than storm surge included the licensee making calculation packages available to the NRC staff via an electronic reading room. The calculation packages were used to expand upon and clarity the information provided on the docket.

Because the October 3, 2018, FHRR staff assessment documents the staff's review of flood mechanisms other than storm surge and describes the audit process used for these mechanisms an audit report for these mechanisms is not necessary. The audit dates and issues discussed in the audit summary are associated with the staffs review of the storm surge flood mechanism. The audit was completed by document review in conjunction with the use of the licensee's established electronic reading room and interactions with the licensee on March 2 and 3, 2017, July 20, 2017, October 26, 2017, January 25, 2018, April 4, 2018, and June 26, 2018. Table 1 provides a list of the participants for the various audit interactions.

AUDIT ACTIVITIES In general, the audit activities consisted mainly of the following actions:

• Review background information on site topography and geographical characteristics of the watershed.

• Review site physical features and plant layout.

• Understand the selection of important assumptions and parameters that would be the basis for evaluating the individual flood causing mechanisms described in the 50.54(f) letter.

• Review model input/output files to computer analyses Table 2 summarizes specific technical topics (and resolution) of important items that were discussed and clarified during the audit.

EXIT MEETING/BRIEFING The Millstone FHRR staff assessments were issued on October 3, 2018, and October 7, 2019 (ADAMS Accession Nos. ML18256A200 and ML19246A116, respectively). These staff assessments document the resolution of the issues associated with the staffs review of the FHRR including issues that were identified during the audit process.

There are no outstanding information needs remaining as a result of this audit.

Table 1 - Audit Attendees Audit Date Name Organization March 2 and 3, 2017 Michelle Bensi NRC/NRO/DLSE Chris Cook NRC/NRO/DLSE Steve Breithaupt NRC/NRO/DLSE Brad Harvey NRC/NRO/DLSE Laura Quinn-Willingham NRC/NRO/DLSE Mike Lee NRC/NRO/DLSE Lyle Hibler NRC/NRO/DLSE Kevin Quinlan NRC/NRO/DLSE Thomas Weaver NRC/RES WorldWinds, Inc. (NRC Patrick Fitzpatrick Contractor\

Taylor Engineering (NRC Chris Bender Contracto;,

Lauren Gibson NRC/NRR/JLD James Kasper Dominion Energy

-~

Paul Young Dominion Energy Daniel Cabonor Dominion Energy Paul Phelps Dominion Energy

~-

Wanda Craft Dominion Energy i Daniel Stapelton GZA '

Michael Mobile GZA Bin Wang GZA '

Lee Branscome CCC Douglas Stewart CCC Michael Kerst Zachry Stephen Superson Zachry Michelle Bensi NRC/NRO/DSEA/RHM1 July 20, 2017 Christopher Cook NRC/NRO/DSEA/RHM1 Frankie Vega NRC/NRR/JLD Nathan Sanfilippo NRC/NRR/JLD Thomas Weaver NRC/RES/DE/SGSEB Wanda Craft Dominion Energy Paul Young Dominion Energy

Audit Date Name Organization Michael Moore GZA Bin Wang GZA David Leone GZA ___ ,,

Mike Kerst Zachry

--~--

Stephen Superson Zachry Stephanie Devlin-Gill NRCINROIDSEA/RHM October 26, 2017 Nilesh Chokshi NRCINROIDSEA Christopher Cook NRCINROIDSEA Cliff Munson NRCINRO/DSEA Hosung Ahn NRCINROIDSEA Dan Barnhurst NRC/NRO/DSEA Joseph Kanney NRCIRESIDRA Thomas Weaver NRC/RESIDRA f--

Meredith Carr NRCIRESIDRA Wanda Craft, et. al, Dominion Energy Dan Barnhurst NRC/NROIDSEA/RHM January 25, 2018 Stephanie Devlin-Gill NRCINROIDSEA Hosung Ahn NRCINROIDSEA Cliff Munson I NRC/NROIDSEA Joseph Kanney NRG/RES Frankie Vega NRR/DLPIPBMB Mo Shams NRRIDLPIPBMB Brandford Stanley Dominion Energy Cathie Tiernan Dominion Energy Mayo Oppenhimer Dominion Energy Wanda Craft Dominion Energy i James Kasper Dominion Energy William Webster Dominion Energy Michael Mable GZA Dan Stapleton GZA Mike Kerst Zachry Stephen Superson Zachry

Audit Date Name Organization Stephanie Devlin-Gill NRC/NRO/DSEA/RHM April 4, 2018 Dan Barnhurst NRC/NRO/DSEA/RHM Cliff Munson NRC/NRO/DSEA Hosung Ahn NRC/NRO/DSEA/RHM Thomas Weaver NRC/RES/SGSEB Joseph Kanney NRC/RES/DRA/FXHAB Frankie Vega NRC/NRR/DLP/PBMB Wanda Craft Dominion Energy Craig Sly Dominion Energy Cathie Tiernan Dominion Energy Mike Kerst Zachary Stephen Superson Zachary David Leone GZA Bin Wang GZA Dan Stapleton GZA Dan Barnhurst NRC/NRO/DSEA/RHM June 26, 2018 Stephanie Devlin-Gill NRC/NRO/DLSE Hosung Ahn NRC/NRO/DLSE Christopher Cook NRC/NRO/DLSE Joseph Kanney NRC/RES/DRA/FXHAB Meredith Carr NRC/RES/DRA/FXHAB Thomas Weaver NRC/RES/DE/SGSEB Brett Titus NRC/NRR/DLP/PBMB Wanda Craft Dominion Energy Craig Sly ' Dominion Energy Cathie Tiernan Dominion Energy

, Bin Wang Dominion Energy Stephen Superson Zachry Mark Pellin Zachry Dan Stapleton GZA David Leone GZA

6 Table 2: Millstone Power Station, Units 1 and 2 Information Needs and Responses Information NRC Response Information Need Description Licensee Response Need No.

Overview of Evaluation During the 02 March 2017 audit. the licensee provided a description of the This information need was

, JPM integral used for their PSSA analysis. Staff also notes that the closed based on the following:

licensee's formulation of the JPM is similar to Equation (2) m Tom, et al, 1 The licensee is requested to prepare an overview (2010). Staff noted that the equation provided in the licensee's presentation 1) 1nformalion provided 1n presentation describing the probabilistic evaluation of the licensee's during the first day (02 March 2017) was missing terms related to conditional storm surge, including components that were leveraged probabilities and the distribution of errors, though staff also notes the error supplemental FHRR from the statistical analysis initially used lo support the term is included in the second day's presentation (02 March 2017). The dated January 4, 2019 deterministic assessment. presentation also notes the probabilities are condiUonal. though formation (ADAMS Accession No.

does not expliCltly include them. The licenses continuous and discrete forms ML19011A110),

As part of the presentation. the licensee is requested to: of the JPM are as follows:

2) information provided to

1. Provide mathematical expressions associated with key components of the evaluation, including the P[rrmax11 yr) > rr] = .l. f . . i f,(:!)P[11m(!) + f > I/] de!"'

staff via an Electronic Reading Room (ERR).

JPM integral showing the explicit components utilized in the site-specific assessment (e.g .. "'L,_," A,P[rrm(&) + E > I/]

3) the NRG staffs independent analysis as provide an expanded expression showing product documented in the of conditional probabilities used to represent the with Attachment to the NRG joint distribution of storm parameters and the Supplemental Staff mathematical expression showing the inclusion of  :!, = (Vmax,, Rmax.1, Vr.,, 8;, land{ all,) Assessment dated the error term). October 7, 2019 (ADAMS A =annual omni-directional storm frequency of the area of interest rr =surge Accession No

2. Clarify terminology utilized in Zachry Calculation elevation of interest: 1/m = modeled surge elevation: P[l/m (:!. )] = cumulative ML19246A116).

No.14-161 (e.g., Section 6.2.3 appears to use AEP flood probability presented by flood -frequency curve: " = error and

{"annual exceedance probabilities" for quantities =

uncertainty related to additional Hood elevation: i JPM storm index: n =

that appear to be probability masses or frequencies total number of JPM storms: Vmax., = maximum wind speed of i-th JPM of occurrence of specific parameter combinations). =

storm: Rmax.l maximum radius of the the Hh JPM storm: V1 , = forward speed of the i-th JPM storm: e, = compass heading of the i-th JPM storm:

The information needs that follow (i.e .. information landfa/1, = landfall location of the i-th JPM storm.

needs 2 through 10) are intended to support the The licensee indicated in the audit information that there were two versions licensee in developing the requested overview of the function of '7max(x): one that used central pressure deficit as the storm presentation by focusing on areas initially identified by intensity parameter and another that used maximum wind speed as the staff as requiring clarification or for which additional storm intensity parameter.

details or discussions are judged to be beneficial to The licensee's method for estimating the surge response was an optimal staffs understanding of the licensee's evaluation.

sampling scheme (JPM-OS), which is used to increase the computational efficiency of the estimation of surge height for the numerous storm parameter combinations. The licensee's JPM-OS is based Equations (3),

(4), and (5) in Toro, et al (2010). which calls the method the response surface JPM-OS scheme (JPM-OS-RS). The RS scheme estimates a change in surge response as the linear combination of incremental variations of surge response to each storm parameter (i.e .. the slope of the surge elevation to parameter line) times the change/difference in the parameter with respect to a parameter reference value. The surge-response changes for each narameter are added to the sume of the reference set to orovide an

7 Information NRC Response Information Need Description Licensee Response Need No.

estimate of the surge for the set of storm parameters. The licensee provides the following equation of the ctiange in response to change in forward speed:

The licensee provides an example equation of the variation in surge with respect to forward speed at a particular location 4'kmn(V,,rnx,Rm 0 x,O,Vr,LF)- 4'em 0 n(Vmax,Rmax*O,Vro,LF) + 1f1atm with

_ a,p.,,,m(Vmax, Rmax, o,v,, LF) ( )

'f1rnrn - av v, - v,o

(/)kmll = surge response function with k. m. m indicating the specific landfall location, the forward speed. and track angle and v,,, = reference value for forward speed.

I Storm Rate This information need was According to the information provided by the licensee, the storm rate of the closed based on the following:

Offshore Inner Region (OIR) was 0.20371 storms per year over the 400-km 2

Background:

Calculation No.14-161 identifies the diameter capture zone. This value was developed for the P-PMSS 1) information provided in annual storm frequency for the study area as 0.20371 calculation. The calculation consists of computing the arithmetic mean of an the licensee's stonns per year, which is then used to oompute an adjusted annual frequency computed in a program named "filter-WRT.f."' supplemental FHRR The adjustment to the number of storms is made by use of a OIR spatial filter dated January 4, 2019 omnt-direct,onal rate by dividing by the approximate that scales the annual frequency value by the count of OIR to IR stonns. {ADAMS Accession No.

length of coasUine over which the annual frequency was Ml19011A110),

evaluated (216nm}. The capture zone {OIR) is centered on the Atlantic Coast of eastern Long Island. Only storms with wind speeds exceeding 40 knots are included in the 2) information provided to Calculation No.14-034, Attachment F provides the estimation of storm rate. Within the capture zone. 7,957 storms from the staff via an ERR.

following computed values: WRT data are included, while for HURDAT2 data there are 50 storms within the capture zone. 3) the NRC staff's Annual frequency for the inner region independent analysis as Comparison of the storm rates between WRT and HURDAT2 data are made documented in the (IR}=0.257775 Annual frequency for the offshore IR {OIR) for maximum velocities greater than 40 knots (with one exception): Attachment to the NRG Supplemental Staff

=0.20371

• ForWRT data Assessment dated 0 Within the IR= 0.2578 storms/year(= 0.00064 October 7. 2019 (ADAMS Calculation No.14-034. Section 6.3. 1.1 identifies that an storms/yr/km in 400-km wide capture zone) Accession No.

"annual frequency factor" of 0.549 was "calculated by Ml19246A116).

0 Within the OIR = 0.2037 storms/year(= 0.00051 dividing the number of HURDAT2 storm tracks storms/yr/km) (used of P-PMSS analyses) intersecting the IR {i.e., 89 storms) by the period of

• For HURDAT2 data I record (i.e., 162 years) associated with the dataset.* C Within the IR = 0.5494 storms/year(= 89 storms in 162 years) with maximum velocities greater than O knots{=

I 0.00137 storms/vrlkml

8 Information NRC Response Information Need Description Licensee Response Need No.

Request: The licensee is requested to: 0 Within the OIR ~ 0.308 stomislyear (=50 stom,s in 162 years)(= 0.00077 storms/yr/km}

1 Describe the process used to compute the annual storm frequency for the IR and the OIR using the The licensee conveyed information from Dr. Keriy Emanuel who supplied the WRT (synthetic) data set. WRT data.

2. Describe the consistency between the annual

• Seeding rates empirically determined to produce a specific number storm frequency computed using the WRT storms per year within a given basin (synthetic} data set and the HURDAT2 dataset.

• Seeding fixed for all time in a given reanalysis

• Simulation forward in downscaled environment

3. Discuss the variation in the computed annual

• Ratio of stonn intersecting the IR to total generated stonns results frequency computed using the WRT with time and in variable annual frequency length of synthetic record {see Calculation No.14-034. Figure 38).

4. Discuss the effect of the underlyrng data generating process for the WRT data set on the computed annual stonn frequency.

Storm Parameter Discretization This infonnation need was The licensee setup an ERR which provided the staff with additional closed based on the following:

information.

3

Background:

Flood hazard reevaluation report (FHRR) 1) infonnation provided in Section 2.4.1.3 and Calculation No.14-161. Section the licensee's 6.2.1 describe the following hurricane parameter supplemental FHRR discretization: dated January 4. 2019 (ADAMS Accession No. ML19011A110).

• fdrr: 11 bearing values spanning -50' to 50' in 10° intervals 2) information provided to

• lspd: 8 values spanning 15 to 50 kl discretized in 5 staff via an ERR, kt intervals

• RMW: 9 values spanning 15 to 55 nm discretized in 3) the NRC staff's 5 nm intervals independent analysis as

• Vm: 8 values spanning range of 70 to 140 kt documented in the discretized in 10 kt intervals Attachment to the NRC

• Landfall locations* 5 locations spanning the Supplemental Staff distance between NWS 23 Mile Posts 2550 and Assessment dated 2650 in 25nm intervals October 7, 2019{ADAMS Accession No.

Request: The licensee is requested to: ML19246A116).

1 Describe the basis for the ctiosen parameter (1.e.,

fdir. fspd, RMW, Vm) discretization and discuss any associated sensitivity studies.

2. Describe {a) the basis for the selection of the discrete landfall locations. (b) the variation of su_rqe

9 Information NRC Response Information Need Description Licensee Response Need No.

~-* with landfall location, and (c) any sensitivity studies performed to show whether further refinement of track spacing will substantially affect results.

3. Describe the basis for parameter ranges selected.

4. Clarify whether the definition of synthetic RMW observations in the synthetic (WRT) database is consistent with the definition of Rmax conventionally used in the JPM integration and in numerical models and discuss wheltler observed historical Rmax values are consistent with the RMW parameter range considered 5 Clarify how the values of forward speed less than I 15kt were utilized in the analysis and how they were used to support the interpolation and extranolation to lower forward sneeds. -

~----

The licensee setup an ERR which provided the staff with addiUonal Statistical Anal~is of HUR DAT and WRT Datasets This infonnation need was infonnation.

closed based on the lolloW1ng:

4

Background:

Calculation No.14-034 descnbes the 1) infonnation provided in stat1st1cal analysis of historical data (Section 6.3.1) and the licensee's statistical analysis and verification of synthetic data supplemental FHRR

{Section 6.3.2). The descriptions include analysis of dated January 4, 2019 (ADAMS Accession No.

geographic filter regions, development of univariate ML19011A110), '

distributions using a non-parametric kernel-based method, extreme value analysis. analysis of hurricane 2) infonnation provided to parameter co-variability, analysis of autocorrelation. and staff via an ERR.

comparison of distributions developed based on the historical and synthetic dataset. Galculation No.14-034. 3) the NRC staff's Section 6.3.3.4 also describes used of a tree-structured independent analysis as documented in the approach and development ofthe "3M dataset."'

Attachment to the NRC Supplemental Staff Request: The licensee is requested to clarify: Assessment dated October 7, 2019 (ADAMS

1. The relationship between (a) the analyses Accession No.

described in Calculation No.14-034, Sections 6.3.1 ML19246A116).

and 6.3.2 (including development of univariate distributions using a non-parametric kernel based method, extreme value analysis. analysis of hurricane parameter co-variability, and analysis of autocorrelation} and (b} the "tree-structured" calculation procedure and development of the 3M dataset.

10 Information NRC Response Information Need Description Licensee Response Need No.

2. How the items discussed in item (1) relate to the joint probabilities developed to support the probabilistic assessment of the storm surge hazard.

3. Discuss the consistency between the historical data and the synthetic data, particularly with respect to parameter Vm (see Calculation No.14-034, Figure 36).

Note: lnfonnation requests related specifically to the "tree-structured approach" are provided in Error!

Reference source not found. and information requests related to the "3M dataset" are provided in Error!

Reference source not found.

The licensee developed a tree of the stonn parameter combinations from the S]lnthetic Hurricane Parameter Co-variabilit:it This information need was WRT data. The tree provides a means of esilmating the parameter co-closed based on the following:

variability. For the four parameters considered (maximum wind speed : 1.

5

Background:

Calculation No.14-034, Sections 6.3.1.3 forward speed: 2, forward direction: 3, and radius of maximum winds: 4). 1) rnformation provided in and 6.3.3.4 describe a co-variability analysis for the 24 different pennutations of the parameter ordering are possible (1234, the licensee's 1243, 1324, ... , 4312. 4321). The method first reads the WRT data for the supplemental FHRR HURDAT2 and WRT datasets. respectively. In addition.

four parameters into separate arrays. Then the method loops through the 24 dated January 4. 2019 Section 6.3.3.4 describes a "tree-structured conditional different pennutations. (ADAMS Accession No.

probability calculation procedure" that was used to ML19011A110},

develop conditional distributions of hurricane For the parameter in the first position/generation. a PDF and CDF are parameters. generated from the all the WRT data for the first generation parameter and

2) infonnation provided to five partitions are made (the tercile bounds) with all the WRT data for the staff via an ERR, first-generation parameter.

Request: The licensee is requested to:

For the parameter 1n the second position/generation and using only the WRT 3) the NRC staff's

data from one of the partitions of the first-generation parameter, a PDF and independent analysis as 1 Provide an overview of the tree-structured CDF are generated for the second-generation parameter, and five further documented in the approach with emphasis on mathematical representations (e.g .. mathematical expressions for partitions are made. This process continues recursively for the third- and Attachment to the NRC '

fourth-generation parameters. Supplemental Staff derived conditional distributions).

Assessment dated The output of this recursive method are PDFs and CDFs conditioned on the October 7, 2019 (ADAMS

2. Discuss the data filtering or sub-dividing procedure previous generation's parameters. This produces 3,744 distributions Accession No.

at each "layer'" of the tree-structured approach and ((1 +5+25+125)*24 : 3,744). The sizes (number ofWRT parameter ML19246A116}.

provide samples sizes associated with each sub- combinations) of the distributions decreases with the level of conditioning:

divided data set.

• pl {first generation, unconditional}, n: 23,993

3. Discuss the basis for the discretization of the parameter space {e.g., bin sizes, bin numbers} and any sensitivity studies performed.

• p2 (second generation, conditioned on p1 quintile). n = 4.798 p3 (third generation, conditioned on p1 and p2 quintiles), n: 959

• p4 (fourth generation. conditioned on p1. p2. and p3 quintiles), n:

191

4. Discuss the approach used to estimate and validate distributions developed using the tree- To generate the 3M data set, the outer loop of the method is run 3,000.000 structured approach. times. One of the 24 permutations of parameters is sampled, and the distributions for each parameter are sampled. The first-generation, unconditioned CDF is samoled and the nuintile identified. For that nuintile,

11 Information NRC Response Information Need Description Licensee Response Need No.

5. Describe how the procedure was used to develop the second-generation. conditional CDF is sampled, and the quintile the "3M dataset: identified. This is repeated for the third- and fourth-generation parameters of the permutation. The output for each run through the loop is the sampled Note: Information requests specifically related to the 3M parameter values for that permutation set dataset are provided in Error! Reference source not found ..

Develogment of "3M Data Set" See IN 5. This information need was closed based on the following:

6

Background:

Calculation No.14-034 identifies an 1) information provided in "extension to the WRT storm set" that is called the the licensee's "3,000,000 (3M) data set" and that was used to supplemental FHRR "characterize non-linear co-variability." Calculation dated January 4, 2019 (ADAMS Accession No.

No.14-034, Section 3.3.3.3 identifies a "tree-structured" ML19011A110),

method of developing conditional probability density functions (PDFs) for the following four parameters from 2) infonnation provided to the WRT stonn set: staff via an ERR,

3) the NRC staffs

• stonn bearing (i.e., translational direction, fdir}

independent analysis as

• forward speed (fspd}

documented in the

• radius of maximum winds (RMW)

Attachment to the NRC

• maximum (i.e., 1-minute average at an altitude of Supplemental Staff 10 meters} wind speed (Vm)

Assessment dated October 7. 2019 (ADAMS Calculation No.14-034, Section 3.3.3.3 further states Accession No.

that a "randomized sampling process was applied to ML19246A116}.

generate synthetic vectors based on the conditional distributions."

Calculation No.14-161 states the following with respect to development of joint probabilities used in the probabilistic stonn surge assessment "Joint probab1/il1es were calculated for each synthetic storm in a manner thaf recovered paramefer co-vanability, as reflected withrn fhe 3M data set, which was used as input to this calculat1on. The joint probabllity for STORMID = i, or CPr(STORMIO=i), was calculated by querying the 3M dafa set. The number of parameter combinatmns within the 3M data set matching (i.e .. as defined by upper and lower bounds defined by the tabulated PDF results) the parameter specifications (i.e ..

12 Information NRC Response Information Need Description Licensee Response Need No.

Fdir, Fspd, RMW and Vm) associated with a given JPM Storm Set combination was counted. The resulting count was then divided by the total number of parameter combinations within the 3M data set (i.e., 3,000,000) to calculate the joint probability. Based on this calculation process, parameter combinations for which no matches existed within the 3M dataset were assigned CPr values of O (i.e .. less than 1 in 3,000,000or approximately 3.33e-7)"

Request: The licensee is requested to clarify:

1 The process used to develop the 3M dataset.

2. How the 3M dataset was used to compute the joint '

probability of storm parameter combinations.

The licensee is further requested to identify relevant supporting calculation tools (e.g., Excel Spreadsheets or FORTRAN scripts) and provide a walk-through" of those tools.

Since the lime when the information presented in the 02 March 2017 audit Use of JPM-OS meeting, the licensee has modified the approach used in the stillwater storm This information need was surge analysis. Former1y, the approach relied on the synthetic data provided closed based on the following:

7

Background:

Calculation No.14-161, Section 6.2.6 by WRT from which a 3M data set was developed for use to compute the 1) information provided in describes the identification of the JPM-OS storm set. probabilities of storm parameter combinations. Currently. the licensee has the licensee's relegated that analysis to a branch (or a few branches) of the logic tree. supplemental FHRR Request: The licensee is requested to discuss: Most other branches rely on data from NACCS (USAGE, 2015) from which dated January 4. 2019 the licensee draws the JPM-OS set. Consequen~y. it is only for the few (ADAMS Accession No.

The process used to identify the JPM-OS set (Step WRT-based branches that the licensee identified the JPM-OS set. ML19011A110).

1 6). including the basis for parameter combinations For Item 1, the licensee used the SLOSH model to simulate a many storms selected for the reference set and sensitivity tests. with a range of parameters since it is computationally efficient. but the results 2) information provided to are not as accurate as those from ADCIRC, which is not computationally staff via an Electronic

2. The development and application of the parameter- efficient. From the SLOSH simulation, evaluated the surge sensitivity to Reading Room (ERR).

specific slope terms. heading (-50 deg to +50 deg) and five land fall locations. Selected the parameter combination ofVmax = 100 kt: Vf = 25 kt: and Rmax = 35 nm to 3) the NRC staff's

3. The validation of the OS-set. including validation of independent analysis as provide low-probability surge elevations. Using 11 headings and five landfall extrapolated values. documented in the locations and the low-probability storm parameter combination, 55 simulations are available for the JPM-OS. An additional 16 test storms were Attachment to the NRG The licensee is also requested to provide the above selected from which to compute slopes (Item 2). In addition to the SLOSH Supplemental Staff information in conjunction with a 'walkthrough" of model runs, ADCIRC was run for these 71 simulations. Assessment dated Attachment G of Calculation No.14-161 (spreadsheet October?. 2019(ADAMS JPM_JPM-OS _Calculation_ 10-8-14.xlsx). For Item 2. the equations for the JPM-OS-RS are discussed in Information Accession No.

Need No. 3 of this audit report including development of the slope terms. ML 1g246A 116).

13 Information NRC Response Information Need Description Licensee Response Need No.

For Item 3, the licensee compared the complete JPM surge-frequency curve generated from 26,970 SLOSH simulations and compared with surge estimates (OS) using the slope terms calculated from the SLOSH results.

The results were found to be comparable: e.g., at 1E-04 event probability the complete-set elevation is approximately 13 ft NAVD88, while the OS-set elevation is only slightly higher {<1 ft difference).

The licensee setup an ERR which provided the staff with additional Extrapolation in OS set This information need was information.

closed based on the following:

8

Background:

Calculation package 14-161, Attachment 1) information provided in G, sheet *os_Model_Results' provides surge elevations the licensee's computed using ADCIRC for a selected number of supplemental FHRR parameter combinations referred to as the reference set dated January 4, 2019 (ADAMS Accession No.

and the sensitivity set. The computed surge elevations ML19011A110).

(as computed by ADCIRC) in the reference set range from a minimum of 3. 7 ft to a maximum of 12.3ft 2) information provided to NAVD8. The sensitivity set spans a range of 3.8ft to staff via an Electronic 15.9ft. Calculation No.14-161, Attachment G, sheet Reading Room (ERR),

'ADCIRC_OS_Complete_Set' shows interpolated and extrapolated values of surge elevations based on the 3) the NRG staffs independent analysis as reference set. The reported Stillwater elevation documented in the associated with an annual exceedance probability of 1E- Atlochment to the NRC 6 is 19.7 ft NAVD88 {this value includes the contribution Supplemental Staff of an uncertainty term and sea-level rise}. Assessment dated October 7, 2019 (ADAMS Request: The licensee is requested to: Accession No. ML19246A116).

1 Discuss the validation of the OS approach for estimation of surge elevations for parameter combinations associated with surge elevations in excess of 12.3ft NAVD88 (i.e., the maximum surge elevation included in the reference set).

! 2. Given the magnitude of surge elevations of i relevance to estima~on of the stiltwater elevation associated with an annual exceedance probability of 1 E-6, discuss the effects of extrapolations on the final estimate of the 1E-6 surge elevation.

3. Provide a comparison of the surge heights computed using ADCIRC in conjunction with the deterministic evaluation (see Calculation No.14-162, Section 7.1) with the surge heights computed for similar parameter combinations usinq the

14 Information NRC Response Information Need Description Licensee Response Need No.

extrapolation procedure associated with development of the ADCIRC OS (see Attachment G of Calculation No.14-161, sheet

'ADCIRC_OS_Complete_Set'}

Treatment of Uncertainty The licensee estimates uncertainty as the standard deviation computed from This information need was the uncertainty of four different components of the storm surge analysis. closed based on the following:

9

Background:

Calculation No.14-161. Section 6.41 This takes the form 1} information provided 1n describes the adjustments made for uncertainty, error, the licensee's a=Ja'+(brr)'

and sea level rise. The standard deviation of the error supplemental FHRR term includes contributions from tidal variability, in which dated January 4, 2019 numerical surge and wind field models. sampling (ADAMS Accession No. ML19011A110},

uncertainty. and error within the "3M dataset" a=~

b = F(E1,E4) 2) information provided to Request: The licensee is requested to: staff via an Electronic E,= uncertainty due to coincidence of tide with maximum storm surge Reading Room (ERR),

1. Discuss the basis for the values assigned to each error component.

=

r-, uncertainty due to the numerical surge model (i.e., ADCIRC model 3) the NRC staff's results) independent analysis as 2 Describe whether the magnitude of the numerical Ea = uncertainty due to sample (aleatory variability associated with maximum documented in the Attachment to the NRC model error tern, is similar to previously performed wind speed}

studies regarding the accuracy of ADCIRC (e.g., Supplemental Staff existing FEMA studies and available literature). =

E1 error within the 3M data set with respect to the WRT data associated Assessment dated with maximum wind speed> 96 knots. October 7, 2019 (ADAMS

3. Provide mathematical details regarding the Accession No.

The *a' term represents a constant value while the 'b' term represents the ML19246A116).

approach used to incorporate error into the JPM surge-varying component of uncertainty.

integration The licensee estimates the uncertainty due to tides (E,) to be 1.16 ft. denved from the tidal difference of 1.57 ft between MHW and MSL at the Newport. RI tide gage (NOAA Station 8452660) multiplied by a factor of 0.74 to account for attenuation from the Newport gage to the Watch Hill Point. RI tide station (NOAA Station 8458694) near the Millstone site.

The licensee estimates the uncertainty due to model error (t,) to be 2 times the ADCIRC model error of 0.39 ft for Storm Sandy (2012), i.e .. 0.78 ft.

The licensee's estimates for E3 and Ei are based on resampling of the 3M data. For E1 , the licensee developed a relation of the difference of the maximum wind speed at 95% CL as a function of mean maximum wind speed. For r 4

• the licensee examined difference in intensity (maximum wind speed) between the WRT and 3M data. Together, E, and c. were considered the aleatory variability and error (maximum wind speed) of the surge response as a function of surge elevation. The relation was fitted to a

L linear function with a mean of zero that had a slope of 0.28.

15 Information NRC Response Information Need Description Licensee Response Need No.

From these analyses. the licensee estimated the *a' term to be 1 40 ft and the 'b' tern, to be 0.28 ft/ft.

Treatment of Baroclinic Energl!: Sources in The licensee setup an ERR which provided the staff with additional This information need was Development of WRT Data Set information. dosed based on ttle following:

10 1) information provided in

Background:

Calculation No.14-034, Sec~on 6.3.2.1 the licensee's provides a summary description of the synthetic supplemental FHRR hurricane data. dated January 4, 2019 (ADAMS Accession No ML19011A110),

Information requested: The licensee is requested to summarize (a} key assumptions associated with the 2) information provided to data generating process used to develop the WRT data staff via an Electronic and {b) any limitations of the data generating process Reading Room (ERR),

(e.g .. how the process treats coastal effects on storms and the contribution of baroclinic energy sources). 3) the NRG staffs independent analysis as documented in the Attachment to the NRG Supplemental Staff Assessment dated October 7, 2019 (ADAMS Accession No ML19246A116).

Table 2 References Toro, GR, Resio, OT, Divoky, D, Niedoroda, AW, and Reed, C., 2010. "Efficient joint probability methods for hurricane surge frequency analysis,"

Ocean Engineering: 37 (125-134).

USAGE, 2015. "North Atlantic Coast Comprehensive Study (NACCSO Coastal Storm Model Simulations: Waves and Water Levels," Cialone, M. A.,

Massey, T. C., Anderson, M. E., Grzegorzewski. A. S., Jensen, R. E., Cialone, A., Mark, D. J., Pevey, K. C., Gunkel, B. L., McAlpin, T. 0., 2015, Vicksburg, Mississippi: U.S. Army Engineer Research and Development Center, Technical Report ERDC/CHL TR-15-14 Zachry Calculation No.14-034, Revision 0, Probable Maximum Hurricane for Millstone Power Station, GZA GeoEnvironmental, Inc., 2014.

Zachry Calculation No.14-161, Revision 0, Probabilistic Storm Surge for Millstone Power Station, GZA GeoEnvironmental, Inc., 2015.

,,* ML19259A036 *via email OFFICE NRR/DLP/PBMB/PM NRR/DLP/PBMB/LA' 'NRO/DLSE/RHM/BC (A)'

JSebrosky SLent DBarnhurst

-NAME -

9/17/2019 DATE 09/17/2019 9/17/2019 OFFICE NRR/DLP/PBMB/BC (A) NRR/DLP/PBMB/PM NAME BTitus JSebrosky --

DATE 09/17/2019 10/7/2019