ML16337A298

11/29/2016 Slides from Public Meeting with Southern Nuclear to Discuss Vogtle GSI-191 Resolution Plan and Current Status

ML16337A298
Person / Time
Site:	Vogtle
Issue date:	11/29/2016
From:	Southern Nuclear Operating Co
To:
Marshall M L, NRR/DORL/LPLI-2, 415-2871
References
CAC MF8488, CAC MF8489
Download: ML16337A298 (73)

Text

NRC Public Meeting Vogtle GSI-191 Resolution Plan and Current Status November 29, 2016 2 Agenda 8:00 - Introductions and Opening Remarks 8:15 - Overall Schedule for Implementation 8:30 - Specific Technical Topics Hybrid LOCA frequency methodology sensitivity Credit for containment accident pressure Fiber quantity required for chemical head loss Partially submerged breaks 9:45 - Break 10:00 - SNC Submittal Table of Contents Overview 10:15 - Overview of Risk Quantification and Results PRA model/interface description Risk quantification, sensitivity, and uncertainty quantification results 11:15 - Safety Margin vs. Operating Margin 11:30 - Method for Evaluating Plant Modifications 11:45 - Conclusions and Closing Remarks 3 Purpose of Meeting Obtain staff feedback on methods SNC is using to address specific technical issues (follow-up from earlier discussions) Provide preliminary information on layout and content of Vogtle license amendment request (SNC Submittal ) submittal to help staff know what to expect and identify potential gaps Discuss timing of submittal relative to WCAP-17788 and South Texas Project (STP) risk-informed GSI-191 pilot project approvals 4 Background Information - Vogtle Plant Layout Westinghouse 4-loop PWR (3,626 MWt per unit) Large dry containment Two redundant ECCS and CS trains Each train has an RHR pump, a high head pump, an intermediate head SI pump, and a CS pump SI and high head pumps piggyback off of the RHR pump discharge during recirculation. Maximum design flow rates: RHR 3,700 gpm/pump CS 2,600 gpm/pump Two independent and redundant containment air cooling trains 5 Background Information - Strainer arrangement Two RHR and CS pumps each with their own strainer Each GE strainer is similar with four stacks of disks RHR strainer (current): 18-disk tall, 765 ft2, 4.9 ft tall RHR strainer (modified): 16-disk tall, 677.6 ft2, 4.4 ft tall CS strainer: 14-disk tall, 590 ft2, 4 ft tall Perforated plate with 3/32" diameter holes RHR B CS B RHR A CS A 6 Background Information - Plant Response to LOCAs Plant response includes the following general actions: Accumulators inject (breaks larger than 2 inches) ECCS injection is initiated from the RWST to the cold legs via RHR, SI, and High Head pumps Containment spray is initiated from the RWST via CS pumps Realignment of RHR pumps to cold leg recirculation begins at RWST low-low alarm CS pumps switched to recirculation at RWST empty alarm CS pumps secured no earlier than 1.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> after start of recirculation, and probably before 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> depending on pressure and dose rate RHR pumps switched to hot leg recirculation at 7.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> Agenda 8:00 - Introductions and Opening Remarks 8:15 - Overall Schedule for Implementation 8:30 - Specific Technical Topics Hybrid LOCA frequency methodology sensitivity Credit for containment accident pressure Fiber quantity required for chemical head loss Partially submerged breaks 9:45 - Break 10:00 - SNC Submittal Table of Contents Overview 10:15 - Overview of Risk Quantification and Results PRA model/interface description Risk quantification, sensitivity, and uncertainty quantification results 11:15 - Safety Margin vs. Operating Margin 11:30 - Method for Evaluating Plant Modifications 11:45 - Conclusions and Closing Remarks 8 Schedule Update - Original Milestones Identified in May 2013 Letter Milestone Expected Completion Date Current Status Develop containment CAD model to include pipe welds Complete Complete Conduct meeting with NRC 3rd Quarter 2013 Complete Modify probabilistic risk assessment (PRA) to include Strainer and Core Blockage events 4th Quarter 2013 Complete Perform Chemical Effects testing 1st Quarter 2014 Complete Perform thermal and hydraulic modeling of RCS, Core, and Containment conditions 1st Quarter 2014 Complete Perform Strainer Head Loss and Bypass testing to establish correlation for range of break sizes 2nd Quarter 2014 Complete1 Assemble base inputs for CASA Grande 2nd Quarter 2014 Complete2 Evaluate Boric Acid Precipitation impacts 3rd Quarter 2015 Complete3 Finalize inputs to CASA Grande 3rd Quarter 2015 Complete2 Complete Sensitivity Analyses in/for CASA Grande 4th Quarter 2015 Complete2 Integrate CASA Grande results into PRA 1st Quarter 2016 Complete2 Licensing Submittal for VEGP To be established through discussions with NRC - tentatively September 2016 Projected 1st Quarter 20174 1 Using 2009 Vogtle test results for strainer head loss. 2 Using NARWHAL instead of CASA Grande. 3 Using PWROG WCAP-17788. 4 Prior to SE on STP Pilot Project or SE on WCAP-17788.

9 Schedule Update Milestone Expected Completion Date Draft of Full Submittal 11/11/2016 (actual) SNC Review Process Complete February 2017 Issue Vogtle Submittal to NRC *Includes Containment Sump TS similar to Traveler being developed by PWROG March 2017 NRC Issues STP SE April 20171 NRC Acceptance Review June 2017 NRC Issues WCAP-17788 SE2 Fall 20171 NRC SE with LAR Approval June 2018 Vogtle Unit 1 Outage - RHR Strainer Modifications Fall 2018 Vogtle Unit 2 Outage - RHR Strainer Modifications Spring 2019 1 Estimated. 2 WCAP-17788 is referenced in Vogtle submittal. .

Agenda 8:00 - Introductions and Opening Remarks 8:15 - Overall Schedule for Implementation 8:30 - Specific Technical Topics Hybrid LOCA frequency methodology sensitivity Credit for containment accident pressure Fiber quantity required for chemical head loss Partially submerged breaks 9:45 - Break 10:00 - SNC Submittal Table of Contents Overview 10:15 - Overview of Risk Quantification and Results PRA model/interface description Risk quantification, sensitivity, and uncertainty quantification results 11:15 - Safety Margin vs. Operating Margin 11:30 - Method for Evaluating Plant Modifications 11:45 - Conclusions and Closing Remarks Hybrid LOCA Frequency Methodology Sensitivity 12 Hybrid LOCA Frequency Methodology Sensitivity Using top-down methodology for risk quantification (base case for GSI-191) Using hybrid methodology sensitivity (three cases) to quantify uncertainty associated with top-down methodology Analyzed welds within Class 1 pressure boundary Degradation mechanisms (DMs) for each weld determined based on ISI program Design and construction (D&C) defects (applies to all welds) Thermal fatigue (TF) Vibration fatigue (VF) Intergranular stress corrosion cracking (IGSCC) Primary water stress corrosion cracking (PWSCC) (most welds exposed to PWSCC at Vogtle have been mitigated) 13 Hybrid LOCA Frequency Methodology Sensitivity Each weld ranked as high, medium, or low failure probability based on DM D&C only exposure classified as low rupture probability PWSCC exposure classified as high rupture probability Exposure to any other DM (TF, VF, or IGSCC) classified as medium rupture probability Probability weights assigned with a difference of two orders of magnitude to maximize effects of sensitivity analysis High = probability weight of 10,000 Medium = probability weight of 100 Low = probability weight of 1 14 Hybrid LOCA Frequency Methodology Sensitivity Three sensitivities cases evaluated Sensitivity 1 used weighting described on previous slide Sensitivity 2 assumed high rupture probability welds are no more likely than medium rupture probability welds Sensitivity 3 assumed high and medium rupture probability welds are no more likely than low rupture probability welds (i.e., all welds treated equally, which is equivalent to the top-down approach) Results from the three sensitivity cases are dependent on which weld category has the most GSI-191 failures 15 Hybrid LOCA Frequency Methodology Sensitivity Equipment Configuration Base Case Sensitivity 1 Sensitivity 2 Sensitivity 3 LBLOCA CFP (No pump failures) 0.0117 1.64E-05 0.00145 0.0117 LBLOCA CFP (2 CS pump failures) 0.0177 2.47E-05 0.00219 0.0177 LBLOCA CFP (1 train failure) 0.0673 4.95E-04 0.0141 0.0673 CDF 2.44E-08 4.83E-11 3.22E-09 2.44E-08 Results show: Low rupture probability welds have highest GSI-191 failure probability High rupture probability welds have lowest GSI-191 failure probability Top-down approach is conservative for Vogtle Credit for Containment Accident Pressure 17 Credit for Containment Accident Pressure No accident pressure credited for pump NPSH margin Assuming vapor pressure when sump temperature above 210.96 °F Assuming -0.3 psig (TS minimum) when sump temperature below 210.96 °F Accident pressure is credited for degasification and flashing 2.5 psi credited to limit degasification Approximately 3.5 psi credited to prevent flashing failures 18 Credit for Containment Accident Pressure Pool temperature near or above 210.96 °F for approximately first 120 minutes Containment pressure above 19 psig for first 133 minutes (8,000 seconds) Containment pressure above 5 psig for first 11.5 days (106 seconds) Never drops below 4.5 psig within 30-day mission time Containment pressure with maximum safety injection Fiber Quantity Required for Chemical Head Loss 20 Fiber Quantity Required for Chemical Head Loss Chemical head loss not applied until a filtering fiber bed accumulates over entire strainer Test results show that this filtering fiber bed is greater than 0.45 inches Maximum head loss for each type of precipitate applied with fiber bed >0.45 inches and any chemical precipitate Calcium phosphate starts precipitating immediately for all breaks Sodium aluminum silicate (SAS) precipitates no later than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> for all breaks Head loss extrapolation is based on test results for chemical head loss and is therefore only applied if criteria for applying chemical head loss are met 21 Fiber Quantity Required for Chemical Head Loss Debris Fiber1 Calcium Phosphate SAS Head Loss Contribution Conventional Debris 3 N/A N/A 0.625 ft >3.1 ft3 N/A N/A 5.46 ft Calcium Phosphate Debris 3 lbm N/A 0 ft >2.483 ft3 0 lbm N/A 0 ft >0 lbm N/A 1.11 ft Sodium Aluminum Silicate Debris 3 N/A lbm 0 ft >2.483 ft3 N/A 0 lbm 0 ft N/A >0 lbm 5.24 ft Extrapolation Constant 3 lbm N/A 0 ft >2.483 ft3 lbm N/A 3.89 ft Maximum Total >3.1 ft3 >0 lbm >0 lbm 15.7 ft 1 Fiber debris quantities and headloss at test conditions and scale with 65.57 ft2 test strainer.

Partially Submerged Breaks 23 Partially Submerged Breaks With plant modifications to refueling water storage tank (RWST) switchover procedure and RHR strainer height, all breaks will have a fully submerged strainer when RWST injection is complete Valve to RHR strainers is automatically opened at low-low level alarm (valve to RWST closed at empty level alarm) CS pumps switched over to recirculation at empty level alarm Strainers are fully submerged when flow through RHR strainers begins for all breaks with the exception of large reactor cavity breaks with high pressure that initiate containment sprays Strainers are fully submerged for all breaks by the time the RWST reaches the empty level 24 Partially Submerged Breaks Limited scenarios where RHR strainers partially submerged Large breaks in Rx cavity that initiate CS Strainers fully submerged approximately 6-11 minutes after flow through RHR strainers begins ft when RHR and CS pump suction from RWST is isolated Modified RHR Strainer All heights are measured from floor 4.438 ft Minimum LBLOCA outside Rx Cavity at Low-Low Level Switchover = ~4.5 ft Minimum LBLOCA Long Term = ~5.3 ft Minimum Rx Cavity LBLOCA at Low-Low Level Switchover =

~3.1 ft Agenda 8:00 - Introductions and Opening Remarks 8:15 - Overall Schedule for Implementation 8:30 - Specific Technical Topics Hybrid LOCA frequency methodology sensitivity Credit for containment accident pressure Fiber quantity required for chemical head loss Partially submerged breaks 9:45 - Break 10:00 - SNC Submittal Table of Contents Overview 10:15 - Overview of Risk Quantification and Results PRA model/interface description Risk quantification, sensitivity, and uncertainty quantification results 11:15 - Safety Margin vs. Operating Margin 11:30 - Method for Evaluating Plant Modifications 11:45 - Conclusions and Closing Remarks Agenda 8:00 - Introductions and Opening Remarks 8:15 - Overall Schedule for Implementation 8:30 - Specific Technical Topics Hybrid LOCA frequency methodology sensitivity Credit for containment accident pressure Fiber quantity required for chemical head loss Partially submerged breaks 9:45 - Break 10:00 - SNC Submittal Table of Contents Overview 10:15 - Overview of Risk Quantification and Results PRA model/interface description Risk quantification, sensitivity, and uncertainty quantification results 11:15 - Safety Margin vs. Operating Margin 11:30 - Method for Evaluating Plant Modifications 11:45 - Conclusions and Closing Remarks 27 SNC Submittal Table of Contents Overview Enclosure 1 Provides a high level overview of all enclosures Organized with same layout as draft RG 1.229 Section C Enclosure 2 Provides a detailed description of plant-specific GSI-191 models (including proprietary information and affidavits for withholding proprietary information) Organized in accordance with the content guideline for GL 2004-02 responses Includes responses to previous Vogtle GSI-191 RAIs Primarily intended for NRC reviewers with expertise in GSI-191 Enclosure 3 Description of risk quantification using NARWHAL and the Vogtle GSI-191 PRA model Organized with same layout as draft RG 1.229 Appendix A Describes how all of the individual parts are combined to quantify risk Primarily intended for NRC reviewers with expertise in PRA 28 SNC Submittal Table of Contents Overview Enclosure 4 Provides a summary of defense-in-depth and safety margin Shows that health and safety of the public are not adversely affected by potential debris-related failures Enclosure 5 Provides a request for exemptions to specific requirements in 10 CFR 50.46(a)(1), General Design Criterion (GDC) 35, GDC 38, and GDC 41 to allow Vogtle to use a risk-informed approach Enclosure 6 Provides a license amendment request (LAR) requesting approval to change Vogtle's licensing basis to support risk-informed resolution of GSI-191 Includes FSAR markup Includes TS changes RWST water level surveillance (reduces minimum water level by 2% for 7 days) Containment Sump (allows 90 days to evaluate discovered conditions) Enclosure 7 Duplicate of Enclosure 2 with proprietary information redacted for public release Agenda 8:00 - Introductions and Opening Remarks 8:15 - Overall Schedule for Implementation 8:30 - Specific Technical Topics Hybrid LOCA frequency methodology sensitivity Credit for containment accident pressure Fiber quantity required for chemical head loss Partially submerged breaks 9:45 - Break 10:00 - SNC Submittal Table of Contents Overview 10:15 - Overview of Risk Quantification and Results PRA model/interface description Risk quantification, sensitivity, and uncertainty quantification results 11:15 - Safety Margin vs. Operating Margin 11:30 - Method for Evaluating Plant Modifications 11:45 - Conclusions and Closing Remarks PRA Model/Interface Description 31 PRA Model/Interface Description Current Vogtle model of record for internal events modified for GSI-191 evaluation (GSI-191 PRA model) GSI-191 conditional failure probabilities calculated using NARWHAL software for each equipment configuration NARWHAL results entered in GSI-191 PRA model to calculate CDF and LERF 32 PRA Model/Interface Description 33 PRA Model/Interface Description - PRA Model Changes 34 PRA Model/Interface Description - PRA Model Changes Risk Quantification Results 36 Risk Quantification Results Breaks postulated at all Class 1 ISI welds inside first isolation valve Over 27,000 breaks evaluated including 1/2-inch breaks up to DEGBs on every weld Smallest break size that fails is a 20-inch partial break when all equipment is available and a 12-inch partial break for any equipment configuration (single train failure) 36 welds had at least one break assigned to failure when all equipment was available (48 welds for single train operation) Failure mode was due to an RHR strainer debris limit (fiber and/or chemical precipitate) No core failures were predicted 37 Risk Quantification Results Break sizes with failures (all equipment available) Break sizes with failures (single train failure) 38 Risk Quantification Results 36 welds with failures (all equipment available) 48 welds with failures (single train failure) 39 Risk Quantification Results Conditional failure probabilities (CFPs) for Pipe break large LOCA initiating event scenarios Equipment Configuration Core Strainer A and B Strainer A Only Strainer B Only No Equipment Failure 0 0.0117 0 0 RHR Pump B Failure 0 N/A 0.0615 N/A Charging Pump B Failure 0 0.0117 0 0 SI Pump B Failure 0 0.0117 0 0 Train B (ECCS and CS) Failure 0 N/A 0.0673 N/A CS Pump B Failure 0 0.0139 0 0 Both CS Pumps Failure 0 0.0177 0 0 40 Risk Quantification Results - Secondary Side Breaks Secondary side breaks inside containment (SSBI) were evaluated in a manner similar to primary side breaks with the following exceptions: Breaks evaluated in approximately 5 ft intervals on main steam and feedwater piping All breaks assumed to be DEGBs Smaller ZOI sizes due to reduced pressure and temperature Lower emergency core cooling system (ECCS) flow rate Containment spray assumed to initiate for all breaks PRA model accounts for SSBI initiating event frequency and low probability failure sequences that would lead to ECCS recirculation (e.g., failure to terminate safety injection or a stuck open PORV) 41 Risk Quantification Results - Secondary Side Breaks CFPs for SSBI - feedwater line break initiating events Equipment Configuration Core Strainer A and B Strainer A Only Strainer B Only No Equipment Failure 0 0 0 0 Charging Pump B Failure 0 N/A 0 N/A CS Pump B Failure 0 0 0 0 Both CS Pumps Failure 0 0 0 0 42 Risk Quantification Results - Secondary Side Breaks CFPs for SSBI - main steam line break initiating events Equipment Configuration Core Strainer A and B Strainer A Only Strainer B Only No Equipment Failure 0 0 0 0 Charging Pump B Failure 0 N/A 0 N/A CS Pump B Failure 0 0 0 0 Both CS Pumps Failure 0 0.475 0 0 43 Risk Quantification Results Equipment Configuration (ry-1) (ry-1) Risk increase from GSI-191 failures for high-likelihood LOCA configurations 2.30E-08 3.06E-11 Bounding risk increase from GSI-191 failures for unlikely LOCA configurations 1.40E-09 4.06E-12 Risk increase from GSI-191 failures for SSBIs 1.39E-09 8.25E-11 Total risk increase associated with GSI-191 2.58E-08 1.17E-10 Baseline risk from GSI-191 PRA model CDF = 2.35E-06/reactor-year LERF = 6.83E-09/reactor-year 44 Risk Quantification Results (2.35E-06, 2.58E-08) 45 RISK Quantification Results (6.83E-09, 1.17E-10)

Sensitivity Analysis Results 47 Sensitivity Analysis Results Varied one parameter at a time Consistent methodology used to select minimum and maximum values for each parameter Nominal defined as value used in base case NARWHAL model If nominal value was skewed in conservative direction, minimum [maximum] value was assumed to be 10% lower [higher] than the nominal value For all other cases, the minimum and maximum values were determined by the available information (design limits preferentially used if available, or minimum/maximum values from analytical ranges) If no information was available for the range of a given input, then the minimum

[maximum] value was assumed to be 25% lower [higher] than the nominal value Results were used to rank the sensitivity for each input parameter using a tornado diagram 48 Sensitivity Analysis Results Uncertainty Quantification Results 50 Uncertainty quantification Results Uncertainty quantification includes parametric uncertainty and model uncertainty Parametric uncertainty addressed in a bounding manner by simultaneously varying all parameters in the conservative direction (multiple simulations run to determine worst case conditions for both the strainers and the core) Model uncertainty addressed by postulating alternate models for each model where no consensus exists 51 Uncertainty Quantification results Strainer Failure Cases (2x2 Matrix) Water volume Min (equivalent to base case) Max (~500,000 lbm additional water) Max RHR flow rate (4,500 gpm) Min CS flow rate (1,950 gpm) CS duration Min (120 minutes) Max (30 days) Min Penetration (0%) Max LOCA frequency (95th percentile) Core Failure Cases (2x2 Matrix) Water volume Min (equivalent to base case) Max (~500,000 lbm additional water) RHR flow rate Min (2,775 gpm) Max (4,500 gpm) Min CS flow rate (1,950 gpm) Max hot leg switchover (HLSO) time (563 minutes) Min CS duration (120 minutes) Max LOCA frequency (95th percentile) 52 Uncertainty Quantification results Sensitivity Case Description CDF Strainer Case 1 Min water volume and min CS duration 1.21E-07 Strainer Case 2 Min water volume and max CS duration 1.21E-07 Strainer Case 3 Max water volume and min CS duration 1.16E-07 Strainer Case 4 Max water volume and max CS duration 1.16E-07 Core Case 1 Min water volume and min RHR flow rate 6.95E-08 Core Case 2 Min water volume and max RHR flow rate 1.14E-07 Core Case 3 Max water volume and min RHR flow rate 7.06E-08 Core Case 4 Max water volume and max RHR flow rate 1.09E-07 53 Uncertainty Quantification results The following model comparisons were evaluated to quantify uncertainty Break model (continuum vs. DEGB-only) LOCA frequencies (geometric mean vs. arithmetic mean) LOCA frequency allocation (top-down vs. hybrid) CS actuation (hot leg breaks larger than 15 inches vs. multiple options including no breaks and all breaks larger than 2 inches) Aluminum metal release equation (UNM vs. WCAP-16530) WCAP-17788 msplit equation (logarithmic fit vs. linear fit) Fiber bed thickness required for chemical head loss (0.45 inches vs. 0 inches) 54 Uncertainty Quantification results Risk Results Summary 56 Results Summary CDF and LERF are both well below the RG 1.174 Region III thresholds for defining the effects of debris as very low risk Sensitivity analysis showed that the results are most sensitive to the following top five parameters (in order of importance): Strainer debris limits Reactor vessel hot leg break fiber limit LOCA frequency values RHR pump flow rate ZOI debris quantity Uncertainty quantification showed that there is high confidence that the risk is very low Parametric evaluation showed that even with the worst case values for each input parameter, risk is still in Region III Model uncertainty quantification showed that even with alternative models, risk is still in Region III Agenda 8:00 - Introductions and Opening Remarks 8:15 - Overall Schedule for Implementation 8:30 - Specific Technical Topics Hybrid LOCA frequency methodology sensitivity Credit for containment accident pressure Fiber quantity required for chemical head loss Partially submerged breaks 9:45 - Break 10:00 - SNC Submittal Table of Contents Overview 10:15 - Overview of Risk Quantification and Results PRA model/interface description Risk quantification, sensitivity, and uncertainty quantification results 11:15 - Safety Margin vs. Operating Margin 11:30 - Method for Evaluating Plant Modifications 11:45 - Conclusions and Closing Remarks 58 Safety Margin Vs. Operating Margin Safety margin is defined as the response to the question, "What aspects of the analysis increase confidence that a declared success is a success?" Numerous conservatisms are used throughout the risk-informed GSI-191 evaluation Some conservatisms are used for operating margin to allow for future design changes, etc. Some conservatisms are used for safety margin to provide confidence that the risk is not underestimated Safety margin built in throughout analysis (see backup slides) 59 Operating Margin Examples Item Actual Value Value Used Operating Margin Unqualified epoxy coatings 2,700.6 lbm 2,729 lbm 28.4 lbm Unqualified alkyd coatings 30.6 lbm 59 lbm 28.4 lbm Unqualified IOZ coatings 27.6 lbm 56 lbm 28.4 lbm Latent debris 60 lbm 200 lbm 140 lbm Miscellaneous debris 4 ft2 50 ft2 46 ft2 Unsubmerged aluminum metal 741.3 ft2 926.6 ft2 185.3 ft2 Submerged aluminum metal 278.7 ft2 348.4 ft2 69.7 ft2 Agenda 8:00 - Introductions and Opening Remarks 8:15 - Overall Schedule for Implementation 8:30 - Specific Technical Topics Hybrid LOCA frequency methodology sensitivity Credit for containment accident pressure Fiber quantity required for chemical head loss Partially submerged breaks 9:45 - Break 10:00 - SNC Submittal Table of Contents Overview 10:15 - Overview of Risk Quantification and Results PRA model/interface description Risk quantification, sensitivity, and uncertainty quantification results 11:15 - Safety Margin vs. Operating Margin 11:30 - Method for Evaluating Plant Modifications 11:45 - Conclusions and Closing Remarks 61 Method for Evaluating Plant Modifications Plant modifications that affect GSI-191 inputs will be evaluated as part of the modification process (including 50.59 screening) If modification does not exceed operating margin values (i.e., input values used in NARWHAL base case), modification is acceptable If modification does exceed operating margin values, GSI-191 risk will be re-quantified by running NARWHAL with new input values and calculating CDF and LERF using GSI-191 PRA model If risk result is within NRC-approved limits (e.g., RG 1.174 Region III) and there is no reduction in safety margin or defense-in-depth, modification is acceptable (i.e., the change does not result in a "more than minimal" accident consequence) If risk result exceeds NRC-approved limits and/or the modification affects safety margin or defense-in-depth, modification can only be made with a new license amendment Modifications that are acceptable without a license amendment will be documented and subject to the normal review process by NRC inspectors Agenda 8:00 - Introductions and Opening Remarks 8:15 - Overall Schedule for Implementation 8:30 - Specific Technical Topics Hybrid LOCA frequency methodology sensitivity Credit for containment accident pressure Fiber quantity required for chemical head loss Partially submerged breaks 9:45 - Break 10:00 - SNC Submittal Table of Contents Overview 10:15 - Overview of Risk Quantification and Results PRA model/interface description Risk quantification, sensitivity, and uncertainty quantification results 11:15 - Safety Margin vs. Operating Margin 11:30 - Method for Evaluating Plant Modifications 11:45 - Conclusions and Closing Remarks 63 Schedule Update Milestone Expected Completion Date Draft of Full Submittal 11/11/2016 (actual) SNC Review Process Complete February 2017 Issue Submittal to NRC March 2017 NRC Issues STP SE April 20171 NRC Acceptance Review June 2017 NRC Issues WCAP-17788 SE2 Fall 20171 NRC SE with LAR Approval June 2018 Vogtle Unit 1 Outage - RHR Strainer Modifications Fall 2018 Vogtle Unit 2 Outage - RHR Strainer Modifications Spring 2019 1 Estimated. 2 WCAP-17788 is referenced in Vogtle submittal. .

64 Purpose of Meeting Obtain staff feedback on methods SNC is using to address specific technical issues (follow-up from earlier discussions) Provide preliminary information on layout and content of Vogtle license amendment request (LAR) submittal to help staff know what to expect and identify potential gaps Discuss timing of submittal relative to WCAP-17788 and South Texas Project (STP) risk-informed GSI-191 pilot project approvals

Backup Slides 67 RHR Pump Suction Flow Path Elevations 68 Credit for Containment Accident Pressure NARWHAL calculates flashing and degasification at a specific reference elevation (specified at strainer midpoint for Vogtle) Gives average degasification assuming uniform flow across strainer height Difference in hydrostatic head from top of strainer ~1 psi 2.5 psi accident pressure credited gives total of ~3.5 psi credited for flashing at top of strainer Modified RHR Strainer Top of strainer: 4.438 ft All heights are measured from floor Reference elevation:

2.552 ft 69 Safety Margin All frequency associated with secondary side breaks is allocated to DEGBs Random pump failures are assumed to occur at switchover to recirculation Initial containment pressure is at technical specification (TS) minimum of -0.3 psig No credit taken for containment accident pressure in NPSH calculations and minimal credit taken for degasification and flashing calculations Design basis containment and sump temperature profiles used for all break sizes Large break flow rate used for all break sizes to calculate strainer head loss and NPSH margin With the exception of shadowing by concrete walls, no credit was taken for structures or restraints that would limit the quantity of debris generated within a break ZOI 70 Safety Margin 100% failure of unqualified coatings for all breaks Unqualified epoxy fails as 100% particulate Unqualified coatings fail at the start of the accident Maximum pH for chemical release and minimum pH for solubility No aluminum remains in solution after the solubility limit has been reached or 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> (whichever comes first) All insulation debris is assumed to be in the sump for the chemical release calculation Fine debris has a high condensate washdown fraction (10%) when sprays are not initiated Fine debris has a high spray washdown fraction (100%) when sprays are initiated Fine debris has a high recirculation transport fraction (100%) for all breaks Small and large pieces of fiberglass transport at the incipient tumbling velocity for the respective debris sizes 71 Safety Margin Small and large pieces of fiberglass debris have a high containment pool erosion fraction (10%) A strainer is assumed to fail any time the accumulated debris quantities exceed the tested debris quantities Miscellaneous debris (e.g., tags or labels) all transports to the strainers prior to any other debris and reduces the effective strainer area Debris head loss was conservatively calculated using a rule-based approach (i.e., if the accumulation of a given debris type exceeds a certain threshold, a bounding head loss is automatically applied) Strainer head loss testing was conservatively performed using a strainer module with fewer disks and scaled up to the full height strainers based on the area ratios Calcium phosphate head loss was applied for all breaks that generate and transport a sufficient quantity of fiber debris The chemical head loss was extrapolated to 30 days and the extrapolation constant was applied 450 minutes after the start of the event 72 Safety Margin Strainer failure is assumed in all cases where the head loss meets or exceeds the structural margin of the strainer All gas voids formed by degasification were assumed to transport to the pumps Pump NPSH required was adjusted for gas voids based on very conservative guidance The fiber penetration correlation ignores effects of fiber and particulate interactions and accumulation of pieces of fiberglass Maximum boil-off flow rate with additional 20% margin used to calculate debris accumulation on core inlet for cold leg breaks Fiber limits associated with core blockage and boron precipitation are based on bounding tests and analyses 73 Safety Margin - Example with Detailed Description Topic Conservatism Credited as Safety Margin Realistic Conditions Impact on Evaluation Scenario Frequency All frequency associated with secondary side breaks is allocated to DEGBs Smaller breaks on main steam and feedwater piping are much more likely than DEGBs, and would generate significantly lower debris quantities Overall likelihood of failure is over-predicted for secondary side breaks Scenario Frequency Random pump failures are assumed to occur at switchover to recirculation Random pump failures can occur at the beginning of the event, at the start of recirculation or any time during the event Failures at the start of the event would delay switchover to recirculation, failures later in the event would result in distribution of debris across more strainers Thermal-Hydraulics Initial containment pressure is at technical specification (TS) minimum of -0.3 psig Containment pressure would be above TS minimum NPSH margin is under-predicted and degasification and flashing are over-predicted Thermal-Hydraulics No credit taken for containment accident pressure in NPSH calculations and minimal credit taken for degasification and flashing calculations The post-LOCA containment pressure would be significantly higher than the saturation pressure NPSH margin is under-predicted and degasification and flashing are over-predicted