ML18355A610

Issuance of Amendments to Revise the Licensing Basis for Protection from Tornado Generated Missiles

ML18355A610
Person / Time
Site:	Mcguire, McGuire
Issue date:	01/25/2019
From:	Michael Mahoney Plant Licensing Branch II
To:	Teresa Ray Duke Energy Carolinas
Mahoney M, NRR/DORL/LPL2-1, 415-3867
References
EPID L-2017-LLA-0412
Download: ML18355A610 (27)

Text

UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 January 25, 2019 Mr. Thomas D. Ray Vice President McGuire Nuclear Station Duke Energy Carolinas, LLC 12700 Hagers Ferry Road Huntersville, NC 28078-8985

SUBJECT:

MCGUIRE NUCLEAR STATION, UNITS 1 AND 2- ISSUANCE OF AMENDMENTS TO REVISE THE LICENSING BASIS FOR PROTECTION FROM TORNADO-GENERATED MISSILES (EPID NO. L-2017-LLA-0412)

Dear Mr. Ray:

The U.S. Nuclear Regulatory Commission has issued the enclosed Amendment No. 312 to Renewed Facility Operating License No. NPF-9 and Amendment No. 291 to Renewed Facility Operating License No. NPF-17 for the McGuire Nuclear Station, Units 1 and 2 (McGuire),

respectively. The amendments are in response to your application dated December 8, 2017, as supplemented by letters dated July 3 and November 1, 2018.

The amendments approve modifications to the McGuire, Units 1 and 2, Updated Final Safety Analysis Report (UFSAR) to describe the methodology (TORMIS (Tornado Missile Risk Analysis Methodology) computer code), and results of the analyses performed to evaluate the protection of the plant's structures, systems, and components from tornado-generated missiles.

A copy of the related Safety Evaluation is also enclosed. A Notice of Issuance will be included in the Commission's biweekly Federal Register notice.

Sincerely,

/lM Michael Mahoney, Project Manager Plant Licensing Branch 11-1 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Docket Nos. 50-369 and 50-370

Enclosures:

1. Amendment No. 312 to NPF-9

2. Amendment No. 291 to NPF-17

3. Safety Evaluation cc: Listserv

UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 DUKE ENERGY CAROLINAS, LLC DOCKET NO. 50-369 MCGUIRE NUCLEAR STATION, UNIT 1 AMENDMENT TO RENEWED FACILITY OPERATING LICENSE Amendment No. 312 Renewed License No. NPF-9

1. The Nuclear Regulatory Commission (the Commission) has found that:

A. The application for amendment to the McGuire Nuclear Station, Unit 1 (the facility), Renewed Facility Operating License No. NPF.:.9, filed by Duke Energy Carolinas, LLC (the licensee), dated December 8, 2017, as supplemented by letters dated July 3 and November 1, 2018, complies with the standards and requirements of the Atomic Energy Act of 1954, as amended (the Act), and the Commission's rules and regulations as set forth in 10 CFR Chapter I; B. The facility will operate in conformity with the application, the provisions of the Act, and the rules and regulations of the Commission; C. There is reasonable assurance (i) that the activities authorized by this amendment can be conducted without endangering the health and safety of the public, and (ii) that such activities will be conducted in compliance with the Commission's regulations set forth in 10 CFR Chapter I; D. The issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public; and E. The issuance of this amendment is in accordance with 10 CFR Part 51 of the Commission's regulations and all applicable requirements have been satisfied.

Enclosure 1

2. Accordingly, by Amendment No. 312, Renewed Facility Operating License No. NPF-9, is amended to authorize revision to the Updated Final Safety Analysis Report (UFSAR), as set for in the application dated December 8, 2017, as supplemented by letters dated July 3 and November 1, 2018. The licensee shall update the USFAR to incorporate the changes as described in the licensee's application dated December 8, 2017, as supplemented by letters-dated July 3 and November 1, 2018, and the NRC staff's safety evaluation attached to this amendment.

3. This license amendment is effective as of its date of issuance and shall be implemented within 120 days of issuance. The UFSAR changes shall be implemented in the next periodic update to the UFSAR in accordance with 10 CFR 50.71(e) following the implementation period.

FOR THE NUCLEAR REGULATORY COMMISSION

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Michael T. Markley, Chief Plant Licensing Branch 11-1 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Dateoflssuance:January 25, 2019

UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 DUKE ENERGY CAROLINAS. LLC DOCKET NO. 50-370 MCGUIRE NUCLEAR STATION. UNIT 2 AMENDMENT TO RENEWED FACILITY OPERATING LICENSE Amendment No. 291 Renewed License No. NPF-17

1. The Nuclear Regulatory Commission (the Commission) has found that:

A. The application for amendment to the McGuire Nuclear Station, Unit 2 (the facility), Renewed Facility Operating License No. NPF-17, filed by the Duke Energy Carolinas, LLC (the licensee), dated December 8, 2017, as supplemented by letters dated July 3 and November 1, 2018, complies with the standards and requirements of the Atomic Energy Act of 1954, as amended (the Act), and the Commission's rules and regulations as set forth in 10 CFR Chapter I; B. The facility will operate in conformity with the application, the provisions of the Act, and the rules and regulations of the Commission; C. There is reasonable assurance (i) that the activities authorized by this amendment can be conducted without endangering the health and safety of the public, and (ii) that such activities will be conducted in compliance with the Commission's regulations set forth in 10 CFR Chapter I; D. The issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public; and E. The issuance of this amendment is in accordance with 10 CFR Part 51 of the Commission's regulations and all applicable requirements have been satisfied.

Enclosure 2

2. Accordingly, by Amendment No. 291, Renewed Facility Operating License No. NPF-17, is amended to authorize revision to the Updated Final Safety Analysis Report (UFSAR),

as set for in the application dated December 8, 2017, as supplemented by letters dated July 3 and November 1, 2018. The licensee shall update the USFAR to incorporate the changes as described in the licensee's application dated December 8, 2017, as supplemented by letters dated July 3 and November 1, 2018, and the NRC staff's safety evaluation attached to this amendment.

3. This license amendment is effective as of its date of issuance and shall be implemented within 120 days of issuance. The UFSAR changes shall be implemented in the next periodic update to the UFSAR in accordance with 10 CFR 50. 71 (e) following the implementation period.

FOR THE NUCLEAR REGULATORY COMMISSION Michael T. Markley, Chief Plant Licensing Branch 11-1 Division of Operating Reactor Licensing Office of Nuclear Reactor Regulation Date of Issuance: January 25, 2019

UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO. 312 TO RENEWED FACILITY OPERATING LICENSE NO. NPF-9 AND AMENDMENT NO. 291 TO RENEWED FACILITY OPERATING LICENSE NO. NPF-17 DUKE ENERGY CAROLINAS. LLC MCGUIRE NUCLEAR STATION, UNITS 1 AND 2 DOCKET NOS. 50-369 AND 50-370

1.0 INTRODUCTION

By letter to the U. S. Nuclear Regulatory Commission (NRC, Commission) dated December 8, 2017, as supplemented by letters dated July 3 and November 1, 2018. (Agencywide Documents Access and Management System (ADAMS) Accession Nos. ML17352A404, ML18303A103, and ML18313A133, respectively), Duke Energy Carolinas, LLC (Duke Energy, the licensee) submitted an application requesting approval to change the Updated Final Safety Analysis Report (UFSAR) for the McGuire Nuclear Station (McGuire), Units 1 and 2.

The amendments would modify the McGuire, Units 1 and 2, UFSAR, to describe the methodology (TORMIS (Tornado Missile Risk Analysis Methodology) computer code), and results of the analyses performed to evaluate the protection of the plant's structures, systems, and components (SSCs) from tornado-generated missiles.

The supplemental letters, dated July 3 and November 1, 2018, provided additional information that clarified the application, did not expand the scope of the application as originally noticed, and did not change the NRC staffs original proposed no significant hazards consideration determination as published in the Federal Register on June 5, 2018 (83 FR 26100).

2.0 REGULATORY EVALUATION

2.1 System Descriptions and Requirements The licensee provided the following in Section 2.1, "Current Licensing Basis for Tornado Missile Protection" of their December 8, 2017 letter:

The MNS [McGuire Nuclear Station] CLB [current licensing basis] for tornado missile protection is presented in UFSAR Sections 3.5.1.3 and 3.5.2.8. These sections state in part; All Category 1 structures exposed to the probable tornado Enclosure 3

generated missiles tabulated in UFSAR Table 3-8 are designed to withstand their effect. The integrity of all Category 1 structures is not impaired by these missiles.

This is accomplished by designing the exposed structure of steel reinforced concrete capable of withstanding the impact of tornado generated missiles ....

The licensee provided the following in Section 2.2, "Conditions Proposed Change in Intended to Resolve" of their December 8, 2017 letter:

Each MNS Reactor Building has two Main Steam Doghouses, an interior and exterior. Each Doghouse is a free-standing reinforced concrete structure composed of concrete frames, slabs, and walls. The Doghouses provide environmental shielding and missile protection for the main steam lines and safety valves (MSSV), steam generator power operated relief valves (PORVs),

main steam isolation valves (MSIV), Turbine Driven Auxiliary Feedwater (TD AFW) exhaust piping (TE), and the feedwater lines and isolation valves (CFIV).

Each Doghouse has ventilation openings or windows on three sides under "gull wing walls" near the top ....

. . . Each MNS spent fuel pool is housed in a concrete and steel superstructure.

The concrete superstructure encloses the spent fuel pool except for the North end of the structure, which is enclosed by a steel structure with siding. The concrete structure provides protection from turbine generator, tornado winds and tornado missiles. The North end of the spent fuel building does not provide tornado missile protection ...

. . . The four VC/YC [Control Room Area Ventilation] outside air intakes are located on top of the Auxiliary Building roof, two near the south end of the Unit 1 reactor building, and two near the south end of the Unit 2 reactor building ...

2.2 Licensee's Proposed Changes Current, McGuire, Units 1 and 2, UFSAR, Section 3.5.1.3, "Tornado Generated Missiles,"

states, in part:

All Category 1 structures exposed to these probable missiles are designed to withstand their effect. A tabulation of the tornado generated missiles is given in Table 3-8.

Revised, McGuire, Units 1 and 2, UFSAR, Section 3.5.1.3, "Tornado Generated Missiles," will state, in part:

Category 1 structures exposed to these design basis missiles are designed to withstand their effect with the exception of those Structures Systems and Components included in the TORMIS probabilistic tornado risk analysis listed in Table 3-63 and as discussed in Section 3.5.2.8.1.1. A tabulation of the design basis tornado generated missiles is given in Table 3-8.

Current, McGuire, Units 1 and 2, UFSAR, Section 3.5.2.8, also titled "Tornado Generated Missiles," states, in part:

Table 3-8 provides a summary of the tornado-generated missiles. The integrity of all Category 1 structures is not impaired by these missiles. This is accomplished either deterministically by designing the exposed structure of steel reinforced concrete capable of withstanding the impact of tornado-generated missiles ...

Revised, McGuire, Units 1 and 2, UFSAR, Section 3.5.2.8, "Tornado Generated Missiles," will state, in part:

Table 3-8 provides a summary of the design basis tornado-generated missiles.

The integrity of Category 1 structures is not impaired by these missiles. This is accomplished by designing the exposed structure of steel reinforced concrete capable of withstanding the impact of tornado-generated missiles. Modifications to existing or the design of new Category 1 structures shall conform to the requirements of NRC RIS 2008-14 RIS 2008-14 requires no action on the part of a license, nor does RIS 2008-14 impose any requirements.

Table 3-63 provides a list of Category 1 structures, systems, and components that have been designed to withstand the impact of design basis tornado-generated missiles. These SSCs were probabilistically shown that they will not be impacted or will not be damaged beyond an acceptable criteria if impacted as discussed in Section 3.5.2.8.1.3.

The licensee will add a new Section 3.5.2.8.1, "Probabilistic Tornado Missile Risk Analysis," and it will state:

3.5.2.8.1 Probabilistic Tornado Missile Risk Analysis A probabilistic tornado missile risk analysis (Reference 7) was completed using the TORMIS computer code which is based on the NRC approved methodology detailed in References 8, 9, and 10. The TORM IS analysis was performed in accordance with the guidance described in NRC TORMIS Safety Evaluation Report (Reference 11) and as clarified by Regulatory Issue Summary (RIS) 2008-14 (Reference 12).

3.5.2.8.1.1 Scope The TORMIS analysis (Reference 7) includes plant components identified as necessary to safely shutdown the plant and maintain a shutdown condition that are located in areas not fully protected by missile barriers designed to resist impact from design basis tornado generated missiles. The plant components (also referred to as ["]targets["]) included in the analysis are listed in Table 3-63 and additional details regarding these targets (i.e. specific identification, description, location, and portion) are included in Reference 7, Volume 3.

3.5.2.8.1.2 TORMIS Computer Code The TORMIS (TORnado MISsile Risk Analysis Methodology) computer code uses a Monte Carlo simulation method that simulates tornado strikes on a plant.

For each tornado strike the tornado field is simulated; missiles are injected and flown; and the missile impacts on structures, systems, and components (SSCs) are analyzed. These models are linked to form an integrated time history simulation methodology. By repeating these simulations, the frequencies of missiles impacting and damaging individual plant components (targets) and groups of targets are estimated. Statistical convergence of the results is achieved by performing multiple replications with different random number seeds.

3.5.2.8.1.3 Analysis The TORMIS results show that the arithmetic sum of damage frequencies for all target groups affecting the individual Units are lower than the acceptable threshold frequency of 1.0E-06 per year per Unit as established in Reference 13.

The following limiting inputs and assumptions were used in the analysis (Reference 7):

a. A site specific tornado hazard curve and data set for McGuire was develop using statistical analysis of the NOAA/National Weather Service Storm Prediction Center tornado data for the years 1950 through 2016.

The analysis utilizes the Enhanced Fujita (EF) Scale wind speeds in the TORMIS simulations.

b. The missile characteristics and locations are based on plant walk down surveys and plant drawings. The plant walk downs were conducted during both non-outage and outage periods to capture both conditions. A stochastic (time dependent) model of the missile population is implemented in TORMIS. The stochastic approach to the missile population varies the missile populations in each of the TORM IS replications to account for predictable changes in plant conditions (i.e.

increased missiles during outages) and the randomness inherent in the total number of missiles present at the plant at any given time.

c. Finite element analysis calculations were performed to determine the missile damage threshold velocity for tornado generated missiles that would cause unacceptable damage to selected targets which is then used as an input in the TORMIS model.

d. Boolean combinations of targets were developed, and the logic was applied to targets or target groups to account for redundancies in the system design or for the TORM IS modeling of a component as multiple targets. The failure logic for redundancy of the MainSteam lines when missile damage to the PORVs and MSSVs is beyond acceptable criteria, is that the Unit can sustain damage to one of four MainSteam line and the damage can be in multiple places on the same MainSteam line (PORVs, MSSVs, or associated components). Damage, beyond the acceptable criteria, on more than one line is considered a failure in TORMIS space.

The failure logic for the Control Room Air Ventilation System (CRAVS)

Intakes (VC/YC Air Intakes) and Spent Fuel Pools (SPF) is simultaneous tornado generated missile impacts to all the Unit 1 and Unit 2 VC/YC Air Intakes AND the entry of a tornado generated missile into either the Unit 1 or Unit 2 SFP that would impact any Spent Fuel assemblies above acceptable critical velocities.

e. Any tornado generated missile strikes to the VC/YC Air Intakes were conservatively assumed to crimp the Intakes closed.

f. The Utility Port Barriers in the Doghouse Upper Openings are conservatively taken into account for their resistance to a conservative selection of tornado generated missiles entering the Doghouse Upper Openings.

g. All tornado generated missiles are conservatively assumed to strike with an end-on, collinear impact.

The licensee will add new Table 3-63, "Structures, Systems, and Components Included in TORMIS Analysis Not Designed for Design Basis Tornado Generated Missiles". Table 3-63, will state:

Table 3-63. Structures, Systems and Components Included In TORMIS Analysis Not Designed for Design Basis Tornado Generated Mlsslles3 Category 1 SSC Unit I & 2 Main Steam Safety Valves (MSSVs) Exhaust Piping and associated supports' 2 Unit I & 2 Steam Generator Power Operated Relief Valves (PORVs) and associated piping and supports' 2 Unit I & 2 Turbine Driven Auxiliary Feedwater (TD AFW) Exhaust (TE) Pipe2 Unit I & 2 Control Room Air Ventilation System (CRAVS) Intakes (VCNC Intakes) 2 Unit l & 2 Spent Fuel Building (north facing wall)

Notes:

1. The SSCs located in the Unit 1 Exterior Doghouse are not included as they have positive tornado missile protection.

2. Only the portion of the Structure that is not protected from the Design Basis tornado generated missiles are included. The Design Basis tornado generated missiles have a horizontal only projection.

3. Additional details and target areas can be found in Section 3.5.6 Reference 7.

The licensee will add new references 7 through 13 in Section 3.5.6. The new references are as follows:

7. MCC-1139.01-00-0298, "MNS Tornado Missile TORMIS Analysis".

8. Electric Power Research Institute Report, EPRI NP-768, "Tornado Missile Risk Analysis", May 1978.

9. Electric Power Research Institute Report, EPRI NP-769, "Tornado Missile Risk Analysis -Appendices", May 1978.

10. Electric Power Research Institute Report, EPRI NP-2005, Volumes 1 and 2, "Tornado Missile Risk Evaluation Methodology", August 1981.

11. NRC Safety Evaluation Report, "Electric Power Research Institute (EPRI)

Topical Reports Concerning Tornado Missile Probabilistic Risk Assessment (PRA) Methodology", October 26, 1983 (Adams ML080870291 ).

12. NRC Regulatory Issue Summary 2008-14, "Use of TORMIS Computer Code for Assessment of Tornado Missile Protection", June 16, 2008 (Adams ML080230578).

13. Memorandum from Harold Denton, NRR Director, to Victor Stello, Deputy Executive Director for Regional Operations and Generic Requirements, "Position of use of Probabilistic Risk Assessment in Tornado Licensing Action," dated November 7, 1983 (Adams ML030020331).

The licensee will add the following statement to UFSAR Section 2.3.1 (page 2.3 - 2):

The tornado parameters and tornado frequency values used in the probabilistic tornado risk analysis (TORMIS) described in Section 3.5.2.8.1 are found in Reference 5.

The licensee will add a new reference 5 to UFSAR Section 2.3. 7, and will state as follows:

5. MCC-1139.01-00-0298, "MNS Tornado Missile TORMIS Analysis".

2.3 Applicable Regulations and Guidance The NRC requires that nuclear power plants be designed to withstand the effects of natural phenomena, including tornado and high-wind-generated missiles, so as not to adversely impact the health and safety of the public in accordance with the requirements of Title 1O of the Code of Federal Regulations (10 CFR) Part 50, Appendix A, General Design Criterion (GDC) 2, "Design Bases for Protection against Natural Phenomena," and GDC 4, "Environmental and Dynamic Effects Design Bases."

Standard Review Plan (SRP), Sections 3.5.1.4 and 3.5.2 of NUREG-0800, contain the acceptance criteria for tornado missile protection. These criteria generally specify that SSCs that are important to safety be provided with sufficient, positive tornado missile protection (i.e.,

barriers) to withstand the maximum credible tornado threat. Methods acceptable to the NRC to comply with these regulations are described in Regulatory Guides (RG) 1.117, "Tornado Design Classification," Revision 1, April 1978 and RG 1. 76, "Design-Basis Tornado and Tornado Missiles for Nuclear Power Plants," Revision 1, March 2007. The appendix to RG 1.117 lists the types of SSCs that should be protected from design basis tornadoes. However, SRP Section 3.5.1.4 describes relaxation of the above deterministic criteria if it can be demonstrated that the frequency of damage to unprotected essential safety-related features is sufficiently small.

To use this probabilistic criterion, the Electric Power Research Institute (EPRI) developed the tornado missile probabilistic methodology described in two topical reports, EPRr NP-768 and NP-769, "Tornado Missile Risk Analysis and Appendices," issued May 1978, and EPRI NP-2005, "Tornado Missile Risk Evaluation Methodology," Volumes I and II, issued August 1981. These topical reports document the TORMIS methodology. The TORMIS methodology employs Monte Carlo random sampling techniques to assess the frequency of tornado missile strikes that will cause unacceptable damage to safety-related plant equipment.

The NRC staff issued a safety evaluation report (SER), dated October 26, 1983 (ADAMS Accession No. ML080870291 ), which concludes that the TORM IS methodology may be used when assessing the need for positive tornado missile protection for specific safety-related plant features in accordance with the criteria of SRP Section 3.5.1.4. The NRC staff further concluded that the methodology had limitations for its use and that licensees must consider five plant-specific points and provide appropriate information regarding its use. These five points are discussed and evaluated in Section 3.2 of this safety evaluation (SE).

On June 16, 2008, the NRC issued Regulatory Issue Summary (RIS) 2008-14, "Use of TORM IS Computer Code for Assessment of Tornado Missile Protection," (ADAMS Accession No. ML080230578). This RIS addresses: (1) the NRC staff position on the use of TORMIS for assessing nuclear power plant tornado missile protection, (2) issues identified in recent license amendment requests to use TORMIS computer code, and (3) information needed in license amendment applications using TORMIS computer code. As specified in RIS 2008-14, use of the TORMIS methodology may be approved for situations where: (1) a licensee identifies existing plant SSCs that do not comply with the current licensing basis for positive tornado missile protection of the plant and (2) it would require costly modifications to bring the plant into compliance with the current licensing basis. The TORMIS methodology is not approved for justification of existing missile barrier removal, either temporarily or permanently.

Although the TORM IS methodology utilizes acceptance criteria for the frequency of tornado-induced loss of system function, the NRC approval for implementation of TORM IS is not a risk-informed approach. As such, approval of TORM IS allows an alternate method for meeting regulatory guidance under very specific circumstances with respect to the evaluation of specific plant features where additional costly tornado missile protective barriers or alternate systems are under consideration. A licensee may submit a license amendment application utilizing a risk-informed change process consistent with the guidelines of RG 1.174, "An Approach for Using Probabilistic Risk Assessment in Risk Informed Decision on Plant-Specific Changes to the Licensing Basis." If a risk-informed process was proposed, it would have to meet the five key principles described in RG 1.174.

In its application dated December 8, 2017, the license states, "The MNS [McGuire, Units 1 and 2] analysis utilized a probabilistic approach implemented through the application of the TORMIS software program as described in Regulatory Issue Summary (RIS) 2008-14." Based on the licensee's statement, the NRC does not consider the licensee's request a risk-informed application and, therefore, the NRC did not use the guidance in RG 1.174 for this evaluation.

3.0 TECHNICAL EVALUATION

3.1 Background In its letter dated December 8, 2017, the licensee states the following in Section 2.2:

In September 2009, while preparing for the NRC Component Design Basis Inspection (CDBI), it was identified that the TD AFW TE piping projected slightly outside of the gull wing walls of the interior Doghouses. While investigating the design basis and requirements of the TE piping, Engineering discovered that there were other potential tornado missile targets in the Doghouse window openings (PORVs, MSSVs) and components located behind exterior doors. The components in the top of the Doghouses are somewhat protected from missiles by the heavy roof and gull wing walls.

It was not obvious that the Doghouse window openings offer the desired protection from a horizontal tornado missile. A design basis search, including calculations, correspondence, licenses basis documentation and specifications, did not lead to a firm position for the tornado missile protection acceptability of the current configuration. MNS determined the condition described above to be non-conforming with respect to 10 CFR 50, Appendix A, General Design Criterion (GDC) 2, "Design Basis for Protection Against Natural Phenomena."

The initial resolution plan to resolve the non-conformances was to install steel missile barriers in the Doghouse window openings up to the bottom of the gull wing walls,

• remove the portion of the TE piping that protrudes from the windows, and replace the exterior doors. The missile barrier modification was completed on the Unit 1 Exterior Doghouse window openings and the exterior doors on the Exterior Doghouses were replaced with missile doors (see Enclosure 4). Due to significant cost overruns, design and installation issues, and schedule delays, the missile barrier modifications for the remaining three Doghouse windows were suspended.

In its letter dated December 8, 2017, the licensee states the following in Section 2.3:

As identified above, MNS has identified existing plant SSCs that do not fully comply with the CLB for tornado missile protection and it would require costly modifications to bring the plants into compliance with the CLB. As such, a TORMIS analysis, using methodology approved by the NRC, has been performed to address the identified deficiencies.

The current licensing basis for tornado missile protection is contained in UFSAR Section 3.1, states, in part:

Criterion 2 - Design Bases for Protection against Natural Phenomena - "Structures, systems, and components important to safety shall be designed to withstand the effects of natural phenomena such as earthquakes, tornadoes, hurricanes, floods, tsunamis, and seiches without loss of capability to perform their safety functions ... "

Criterion 4 - Environmental and Missile Design Bases - "Structures, systems and components important to safety shall be designed to accommodate the effects of and to be compatible with the environmental conditions associated with normal operation, maintenance, testing, and postulated accidents, including loss-of-coolant accidents.

These structures, systems and components shall be appropriately protected against

dynamic effects, including the effects of missiles, pipe whipping, and discharging fluids, that may result from equipment failures and from events and conditions outside the nuclear power unit."

The NRC staffs approval of licensee's application using TORMIS is subject to the appropriate resolution of five specific concerns identified in the SER for the EPRI TORM IS methodology (ADAMS Accession No. ML080870291 ). These specific concerns are related to the assumptions used in the input parameters for the analysis (e.g., locations and numbers of potential missiles presented at a specific site, wind speed, wind speed near the ground, etc.).

The NRC staff reviewed the submittal with respect to: (1) the five specific concerns related to the NRC approval of the TORM IS methodology, and (2) the acceptability of the TORM IS analysis for calculating the appropriate mean strike and damage probabilities and of the TORMIS results against the guidance provided in the SER on EPRI TORMIS methodology and RIS 2008-14.

The McGuire TORM IS results provide estimated probabilities of tornado missile hits and damage to modeled targets. There are 5 individual unprotected safety-significant targets modeled in McGuire TORMIS, as shown in the licensee's December 8, 2017 letter in , Table 2-1, "MNS Safety Related Unprotected Targets." The licensee considered systems or portions of systems such as, Main Stream Safety Valves (MSSV) exhaust piping and supports, Steam Generator Power Operated Relief Valves (SG PORVs), Turbine Driven Auxiliary Feedwater (TD AFW) exhaust piping (TE), Portion of Spent Fuel Pool Buildings and Control Room Area Ventilation Air Intakes.

In memorandum dated November 7, 1983, "Position on the Use of Probabilistic Risk Assessment in Tornado Missile Protection Licensing Actions" (ADAMS Accession No. ML080870287), the NRC summarizes the position on use of probabilistic risk assessment in tornado missile protection licensing actions. This memo, which is used in probabilistic tornado missile reviews, states that an expected rate of occurrence of potential exposures in excess of the 10 CFR 100 guidelines of approximately 1.0E-06 per year is acceptable if, when combined with reasonable qualitative arguments, the risk can be expected to be lower.

As noted in the McGuire TORM IS results, the aggregate damage frequency for each unit is within the acceptance criterion of 1.0E-06 per year.

3.2 Implementation of the TORM IS Methodology The NRC TORMIS SER, dated October 26, 1983, approving the TORMIS methodology, identifies that licensees using the TORM IS methodology are to consider and address five points in their applications. The NRC's evaluation of the licensee's responses, as provided in the licensee's December 8, 2017 letter, with respect to these five points is described below:

3.2.1 Evaluation of Point 1 The first point to be is evaluated is, "Data on tornado characteristics should be employed for both broad regions and small areas around the site. The most conservative values should be used in the risk analysis or justification provided for those values selected."

For TORMIS submittal, tornado data from the NOAA Storm Prediction Center (SPC) for both large regions and small areas around the site were considered in the development of the site specific tornado characteristics for McGuire. A broad 15.4° x 15.4° latitude longitude square

was used as the starting region. This large area covered 581,423 square miles of land and included 16,288 tornadoes from the NWS SPC tornado data set for the years 1950 to 2016 (67 years). Within this broad region, the tornado risk was quantified for 1°, 1.4 °, and 2° cells.

These sub-regions contains 73,377 square miles of land and contained 2,029 tornadoes in the 67 year period, producing an average of 30.28 per year. Using TORMIS trending methodology, the calculated occurrence rate to reflect McGuire sub-region reporting trends is 7.99 E-04 tornadoes per square mile per year.

Based on licensee use of most recent data, the NRC staff finds use of recent data reasonable for use in McGuire TORMIS application.

3.2.2 Evaluation of Point 2 The second point to be evaluated is, "The EPRI study [EPRI NP-768, NP-689 and NP-2005]

proposes a modified tornado classification, F'-scale, for which the velocity ranges are lower by as much as 25% than the velocity ranges originally proposed in the Fujita, F-scale. Insufficient documentation was provided in the studies in support of the reduced F'-scale. The F-scale tornado classification should therefore be used in order to obtain conservative results."

In section 3.2 of its letter dated December 8, 2017, the licensee stated that the original Enhanced Fujita (EF) scale wind speeds were utilized in the TORMIS analysis. The hazard curve developed for the McGuire analysis does not utilize either the SER specified Fujita (F) scale or the SER prohibited modified Fujita (F') scale. Instead the analysis utilizes the EF scale wind speeds as per NUREG/CR-4461 from the SER (Reference 11). Although the 1983 NRC SER called for the use of the F-scale of tornado intensity for assigning tornado wind speeds to each intensity category (F1-F5), the NRC subsequently adopted the EF scale in the positions of NRC RG 1.76, Revision 1 that are based on NUREG/CR-4461, Rev 2 (ADAMS Accession No. ML070810400).

While McGuire TORMIS analysis used the EF-Scale, UFSAR Section 3.3.2.1 defines current licensing basis for McGuire as rotational wind-speed is 300 mph (miles per hour) and translational speed of tornado is 60 mph. It should be noted that the use of the EF scale wind speeds shall be limited to evaluation of unprotected equipment analyzed within this specific TORMIS submittal, unless changes to UFSAR and licensing basis are later approved.

Based on limitations of use and consistency with recent design basis tornado defined in revision 1 of RG 1. 76 , the NRC staff concludes that the use of the EF scale is reasonable for analyzing SSC's within this TORMIS application.

3.2.3 Evaluation Point 3 The third point to be evaluated is, "Reductions in tornado wind speed near the ground due to surface friction effects are not sufficiently documented in the EPRI study. Such reductions were not consistently accounted for when estimating tornado wind speeds at 33 feet above grade on the basis of observed damage at lower elevations. Therefore, users should calculate the effects of assuming velocity profiles with ratios Vo (speed at ground level)/ V33 (speed at 33 foot elevation) higher than that in the EPRI study. Discussion of the sensitivity of the results to changes in the modeling of the tornado wind speed profile near the ground should be provided."

The Licensee indicated TORMIS simulations were performed with the TORMIS rotational velocity Profile 3, which has increased near ground wind speeds over Profile 5, which was used in the 1981

EPRI TORMIS reports .To address the reductions in tornado missile speed near the ground due to surface friction effects that are not sufficiently documented in the EPRI study, the McGuire runs were made with higher near ground wind speeds than in the EPRI study. The licensee performed additional sensitivity studies by comparing EPRI profiles from the original EPRI TORMIS. The comparison shows that differences in results were negligible for a missile hit. It was noted that some sensitivity was observed for targets with very low damage frequencies

(<10 E-08), however, differences were negligible when aggregated over the target groups. The licensee concluded the McGuire TORMIS results are not sensitive to using EPRI Profile 3 instead of EPRI Profile 5 (from EPRI NP-2005). The use of Profile 3 with higher near-ground wind speeds is conservative when compared to Profile 5. The NRC staff concludes that this modeling is acceptable.

3.2.4 Evaluation of Point 4 The fourth point to be evaluated is, "The assumptions concerning the locations and numbers of potential missiles presented at a specific site are not well established in the EPRI studies.

However, the EPRI methodology allows site-specific information on missile availability to be incorporated in the risk calculation. Therefore, users should provide sufficient information to justify the assumed missile density based on site-specific missile sources and dominant tornado paths of travel."

The licensee stated that its contractor performed a plant walk-down of McGuire, Units 1 and 2, to characterize the missile sources and obtain plant information and a follow-up walk-down was completed during a refueling outage to quantify the number of outage missiles. The survey walk-down uses a systematic, documented process to provide inputs on the type, quantity, and elevation of missiles in each missile source zone and structure for use in developing inputs for the TORMIS analysis. The mean number of potential missiles simulated for EF5 tornadoes was estimated 298,856, including structural-origin (i.e. building deconstruction) and zone missile sources. Missile sources were catalogued and modeled to a distance of approximately 2,500 feet. A source of conservatism is considering that all components are assumed to fail and become unrestrained potential missiles, whereas, in reality, a large portion of the structural components will generally remain connected to one another or attached to the foundation.

As noted above, the licensee is to provide sufficient information to justify the assumed missile density based on site-specific missile sources. The NRC staff was unable to locate details of missiles depicting the type, quantity or density of missile in each zone. Also, the basis for a large amount of the 214,000 deconstruction missiles was missing. The NRC staff issued request for addition information (RAl)-02, (dated May 18, 2018, ADAMS Accession No. ML18138A466), requesting the licensee to provide details of the estimated missile density and count. In its response, the licensee defined their process for estimating missile zones and quantity. Additional tables were provided showing total number of missiles and missile type for each zone. The total number of missiles produced by structural failure is based on the expected damage of various building types based on near-ground wind speeds. The approach includes:*

(1) defining the inventory of all potential missiles generated by failure of structures, and (2) determining the fraction of the missile inventory that will be produced by each level of tornado intensity (EF Scale).

This approach is reasonable to derive conservative missile density and count.

This is a reasonable missile density in comparison to some other plants that reported less than 250,000 total missiles. Therefore, the NRC staff concludes the postulated missile count is acceptable.

3.2.5 Evaluation of Point 5 The fifth, and last, point to be evaluated is, "Once the EPRI methodology has been chosen, justification should be provided for any deviations from the calculational approach."

The McGuire TORMIS analysis uses the new "Missile Path Through" option in TORMIS computer code. This option allows missile trajectories to be continued through targets that are impacted by missiles with velocities greater than their failure velocities. As clarified by licensee, this approach does not deviate from the EPRI methodology, but improves the functionality of the ricochet routine from EPRI Report NP-769, "Tornado Missile Risk Analysis -Appendices,"

dated May 1978, to account for missiles that can get through non-qualified missile barriers such as the Utility Port Barriers (UPBs) in the upper openings of the McGuire Doghouses.

Based on the above, the NRC staff determined that the licensee considered and appropriately addressed each of the five points described in the NRC TORM IS SER, dated October 26, 1983.

3.2.6 Evaluation of considerations in RIS 2008-14 The NRC issued RIS 2008-14, to inform addressees of the following: (1) the NRC staff position on the use of the TORM IS computer code for assessing nuclear power plant tornado missile protection; (2) issues identified in recent license amendment requests to use the TORMIS computer code; (3) information needed in license amendment applications using the TORMIS computer code. This RIS requires no action or written response on the part of an addressee.

In RIS 2008-14, the NRC described how questions (i.e., requests for additional information) concerning license amendment applications fell into three general areas:

(1) Licensees did not fully satisfy the first four points identified in the SER approving the TORMIS methodology (identified above).

(2) Licensees did not fully address the fifth point identified in the SER and explain how the methodology was implemented when the parameters used differed from those specified in the TORMIS methodology.

(3) Licensees used the TORM IS methodology to address situations for which the methodology was not approved.

Examples of the questions where provided by the RIS. To assure that its application addressed the areas of past concern described in the RIS, the licensee addressed the applicable examples.

As example "a." of question general area "(1)" above, RIS 2008-14 stated that some licensees did not provide adequate justification that the analysis used the most conservative value for tornado frequency. The licensee addressed this example by indicating the TORMIS produced hazard curves are more conservative than the NUREG/CR-4461 hazard curve for McGuire. In Section 3.3 of the application, the licensee indicates McGuire values do not bound the NUREG/CR-4461 values above 180 mph. For the regions above 200 mph, the use of TORM IS exceedance curve may be non-conservative for EF5 criteria, compared to the NUREG/CR-4461 exceedance values. The NRC staff requested additional details of non-conservative and bounding values above 180 mph in RAl-3 (ADAMS Accession No. ML18138A466). In its response dated July 3, 2018, the licensee performed sensitivity analyses to verify the site-specific values used contain sufficient conservatism. As discussed in the licensee's RAl-03

response and shown in Table 4 of the response, the TORMIS frequency related to EFS is lower than the NUREG/CR-4461 value, but all other frequencies are considerably higher for the other EF scales (EF1-EF4). The sensitivity study concluded the effect of increasing EFS frequencies produced a minimal impact on the McGuire missile damage frequencies.

The analysis also concludes missile damage frequencies from much more frequent and lower intensity tornadoes (EF 1-4) dominate the tornado missile risk for McGuire. Based on the additional studies conclusions, it appears the McGuire DH EF (Doghouse Enhanced Fujita) site-specific curve produces missile damage frequencies that are reasonably higher than those that would be produced using the NUREG/CR-4461 EF curve. The NRC staff finds the additional information and sensitivity study results provide sufficient justification to ensure overall results are conservative when compared to NUREG/CR-4661 hazard curve.

As example "d." of question general area "(2)" above, RIS 2008-14 stated that some licensees took credit for nonstructural members. The licensee addressed this example by describing how the doghouse openings contain non-safety-related barriers (UPBs) to protect internal components from missiles. According to LAR, the UPBs in the openings at the top of the Doghouses are credited for their ability to resist or slow down missiles impacting them. The licensee used a method with TORMIS for evaluating missiles passing through openings and calculating impact conditions to screen-out missiles that can't pass through the opening. The NRC staff issued RAl-04 (ADAMS Accession No. ML18138A466) requesting the licensee to provide details of a reduced impact velocity or crediting non-structural members related to the UPBs. In response, the licensee described the process for calculating velocities at which UPB damage could occur for less-damaging missile types for screening when this velocity is not exceeded. These less-damaging missiles were defined as metal siding, wood plank, wood beam, and plywood missiles. TORMIS input assumes that all other missile types will pass through the UPBs unimpeded. Impact velocity of lighter weight missiles was determined based on the minimum amount of kinetic energy that is lost for a given missile type that penetrates a given barrier. The missile velocities for wood planks, metal siding, and plywood missiles was determined using a deterministic Finite Element Analysis (FEA) and used as input to TORM IS.

The missile velocities for the other 19 TORMIS missile types did not take credit for screen, which allows all of these missile types to pass through the UPBs unimpeded with no reduction in velocity The NRC staff finds the approach of crediting barrier resistance reasonable for lightweight TORMIS-generated tornado missiles.

As specified in RIS 2008-14, the TORM IS methodology is approved for situations where ( 1) a licensee identifies existing plant SSCs that do not comply with the current licensing basis for positive tornado missile protection of the plant and (2) it would require costly modifications to bring the plant into compliance with the current licensing basis. The UFSAR markups in its letter dated December 8, 2017, could be considered an improper future use of TORM IS.

Therefore, the NRC staff issued RAl-01 (ADAMS Accession No. ML18138A466), to request clarification in the UFSAR 3.5.2.8 markup regarding future use of TORM IS to modifications of existing and design of new Category 1 SSCs. In their response dated July 3, 2018 (ADAMS Accession ML18303A103), the licensee clarified (including providing revised proposed UFSAR markups) that the licensing basis will contains limitations that modifications to existing or the design of new Category 1 structures shall conform to RIS 2008-14. The NRC staff finds this sufficient to ensure future use of TORMIS methodology will be used as currently approved.

3.3 Results of the McGuire, Units 1 and 2, TORM IS Analysis In Section 3.5.2 of the proposed UFSAR revision, the licensee states that a TORM IS analysis was completed and includes plant components located in areas not fully protected by missile barriers designed to resist impact from design-basis tornado missiles. Structures, Systems and Components (SSCs) included in the TORMIS probabilistic tornado risk analysis are listed in Table 3-63. The licensee states that the results of the TORM IS analysis predict a site mean aggregate tornado missile damage probability of 3.59E-7 per year (Unit 1) and 8.1 OE-7 per year (Unit 2) for the identified scope of safety-related targets. The licensee states that MNS TORM IS tornado missile risk analysis results show that the arithmetic sum of damage frequencies for all target groups affecting the individual Units are lower than the acceptable threshold frequency of 1.0E-6 per year per Unit.

In "Position On the Use of Probabilistic Risk Assessment In Tornado Missile Protection Licensing Actions," (ADAMS Accession No. ML080870287) the NRC staff states that the guidance of SRP Section 2.2.3, "Evaluation of Potential Accidents," is applicable to tornado missiles. SRP Section 2.2.3 supports this probability and identifies an acceptance criterion of 1.0E-6 for the expected rate of occurrence for potential exposures in excess of 10 CFR Part 100 guidelines per year when combined with reasonable qualitative arguments that show the realistic probability is lower.

The McGuire TORM IS results provide estimated probabilities of tornado missile hits and damage to modeled targets. There are 5 individual unprotected safety-significant targets modeled in McGuire TORMIS, as shown in LAR Enclosure 1, Table 3-2, "TORMIS Results by Individual Target." As shown in Table 3-2 of the LAR, the four targets with the highest hit frequencies are the VC/YC (Control Room Area Ventilation) Air Intakes (targets 82-85) located outside of the doghouses next to the Reactor Buildings. Since the VC/YC components appear small and protected from surrounding buildings, the NRC staff requested the licensee to provide additional information which explains this result. In response, the licensee described some factors that could impact results, such as location of outside components, elevation of components and proximity of missile sources.

Based on review of the submittal, the NRC staff finds that because of not requiring unique tornado missile protection for identified targets, which have been analyzed by the TORMIS computer code, the probability of a malfunction of equipment important to safety will increase slightly. However, the frequency of a tornado-generated missile damaging these targets is less than 1.0E-6 per year.

3.4 Component Evaluation Due to complexity with use of TORM IS and post-processing modeling of specific components, additional attention was needed regarding proposed use of TORM IS methodology and Boolean approach for Main Steam components, control room intake and spent fuel north wall configuration to assist in understanding the application results and conclusions.

Table 3-5 in Section 3.1.7 of their December 8, 2017 letter, the licensee provided the arithmetic sum over all target groups mean damage frequencies of 3.59E-07 yr-1 and 8.10E-07 yr-1 for Unit 1 and Unit 2, respectively. Therefore, each unit is within the threshold frequency of 1.0E-06 yr-1 established in the 1983 TORMIS SER and meets the acceptance criteria of 1.0E-06 yr-1 established in NUREG-0800, SRP, Section 2.2.3, "Evaluation of Potential Accidents."

In Enclosure 1, Section 3.1.5 of their December 8, 2017 letter, the licensee stated that Boolean logic is used in the McGuire TORMIS analysis. The licensee stated in Enclosure 1, Section 3.1.5 of their December 8, 2017 letter that the Boolean logic for the Unit 1 and 2 Spent Fuel Pools (SFPs) and the Unit 1 and 2 Control Room Air Ventilation System Intakes (VC/YC Air Intakes) defined failure is both VC/YC Air Intakes failing by wind missile and missile damage to fuel assemblies in either SFP. Additionally, the Boolean logic for the main steam (MS) lines defines the failure as missile damage on more than one line.

The McGuire TORM IS results provide estimated probabilities of tornado missile hits and damage to modeled targets. There are 5 individual unprotected safety-significant targets modeled in McGuire TORMIS, as shown in LAR Enclosure 1, Table 3-2, "TORMIS Results by Individual Target." As shown in Table 3-2 of the LAR, the four targets with the highest hit frequencies are the VC/YC (Control Room Area Ventilation) Air Intakes (targets 82-85) located outside of the doghouses next to the Reactor Buildings. Since the VC/YC components appear to be small and protected from surrounding buildings, the NRC staff requested the licensee to provide additional information which explains this result, RAl-05 (ADAMS Accession ML18138A466). In their response, in the July 3, 2018 letter, the licensee described some factors that could impact results, such as location of outside components, elevation of components and proximity of missile sources. Based on these factors, the NRC staff finds the licensee's comparison of VC/YC to other exposed components sufficient to address the RAl-05 concern. "Targets Excluded from the TORM IS Analysis" of the December 8, 2017 letter, contains a list of SSC's that were not included in TORMIS analysis, including their basis for exclusion. The intent of this TORM IS application review is to address licensee-identified existing plant SSCs that do not comply with the current licensing basis for positive tornado missile protection. The NRC staff reviewed Enclosure 3 and concludes that the SSCs listed and the basis for exclusion is appropropriate.

The components included in the TORMIS analysis are discussed below.

3.4.1 MSSV, PORVs and TE Each McGuire Reactor Building (Units 1 and 2) has two Main Steam Doghouses, an interior and exterior. Each Doghouse is a free-standing reinforced concrete structure composed of concrete frames, slabs, and walls. The Doghouses provide environmental shielding and missile protection for the Main Steam Lines and Safety Valves (MSSV), SG Power Operated Relief Valves (PORVs), Main Steam Isolation Valves (MSIV), TD AFW exhaust piping (TE}, and Feedwater Lines and Isolation Valves. Each Doghouse has ventilation openings or windows on three sides under "gull wing walls" near the top of the structure.

Openings at the top of the Doghouses contain UPBs or mesh-type screens. While these barriers cannot resist the McGuire design basis tornado missiles, they do offer resistance to lightweight tornado missiles such as metal siding, wood planks, and plywood.

In September 2009, McGuire discovered that there were other potential tornado missile targets in the Doghouse window openings (PORVs, MSSVs) and components located behind exterior doors. The components in the top of the Doghouses are somewhat protected from missiles by the heavy roof and gull wing walls. McGuire determined the condition described above to be non-conforming with respect to GDC 2 for Protection against Natural Phenomena. Missile

protection was only installed on Unit 1 Exterior Doghouse window openings and the exterior doors.

The components in the Doghouse window openings that are currently unprotected from design bases horizontal tornado generated missiles are; the PORVs including upstream piping, downstream piping, and piping supports, MSSV downstream piping, and a portion of the TD AFW TE piping.

• SG PORVs (8, four per unit) components were included in TORMIS, including piping upstream and downstream from the valves and associated pipe supports. These components are included where they are exposed to horizontal missiles coming through the upper level Doghouse openings and above the roof.

• MSSV exhausts (40, three exhaust per train x four trains per unit x two units) are included where exposed to horizontal missiles coming through the upper level and above the roof. FEA is used to determine missile impact criteria that failure of the MSSV exhaust supports that could allow the pipes to either fall or swing into the MSSVs themselves, potentially causing an uncontrolled main steam leak.

• AFW TE piping (two, one per unit) are included in the interior Doghouses of each unit where exposed to horizontal missiles passing through the upper level Doghouse openings. These pipes are not exposed above the roof.

The NRC staff requested in RAl-06.a (ADAMS Accession No. ML18138A466), that the licensee describe the basis for all failure criteria used in the McGuire TORM IS analysis where Boolean logic is used. RAl-06.b requested that the licensee justify comparing McGuire results using selected failure criteria against the criteria in SRP 2.2.3, which states that "the expected rate of occurrence of potential exposures in excess of the 10 CFR 50.34 (a)(1) requirements as they relate to the requirements of 10 CFR Part 100 guidelines by an order of magnitude of 1.0E-6 per year is acceptable if, when combined with reasonable qualitative arguments, the realistic probability can be shown to be lower," and describe how the use of the selected failure criteria is consistent with the application of the approved TORM IS methodology.

In the response to RAl-06.a, in its letter dated July 3, 2018, the licensee stated that the failure criteria used for the main steam line (including the MSSVs and the PORVs) Boolean logic in the McGuire TORMIS analysis was defined in a main steam line redundancy analysis performed in support of this LAR. This analysis determined an acceptable level of tornado missile damage to the main steam lines such that the plant response to the resulting damage and loss of function was bounded by the applicable McGuire UFSAR Chapter 15 accident analyses. The failure criteria resulting from this analysis can generally be stated that if a unit does not have the system/component configuration required to mitigate a tornado event, concurrent with a loss of offsite power (LOOP), and complete a controlled unit cooldown, then it fails. The licensee stated that successful mitigation requires that three of the unit's four main steam lines remain intact and at least one of the PORVs associated with any of the three intact main steam lines is undamaged and fully functional. If this is not the case, then the failure criteria is met. The licensee also stated that the failure criteria for the Unit 1 and 2 SFPs and the Unit 1 and 2 VC/YC Air Intakes is that a tornado would need to produce missiles that impact and damage the VC/YC Intakes of both units and have a damaging missile from the same tornado entering at least one of the SFPs. In Table 8 of its July 3, 2018 letter (response to RAl-06.e), the license provides the damage frequency for each possible combination of events that would lead to failure based on the failure criteria and that the overall failure frequency for the system is then

computed as the arithmetic sum of the three event combinations that are labeled as "Fail" in Table 8.

As defined in its letter dated December 8, 2017 and further explained in its letter dated July 3, 2018, portions of the Main Steam System are included in the TORMIS analysis, but are not fully protected which is not in accordance with the current licensing basis. In this first RAI response, the licensee defined failure criteria as requiring three of four main steam lines to shut down the plant which the staff finds acceptable, only if the licensing basis requires protection of three main steam lines against tornado missiles. In its correspondence dated October 11, 2018 (ADAMS Accession No. ML18285A081), the NRC staff requested in RAl-07, that the licensee demonstrate that McGuire's current licensing basis requires protection of three out of four main steam lines against tornado missiles and to alternatively provide TORM IS results assuming any main steam line hit is considered a failure and show that the results meet the acceptance guidelines.

In response to RAl-07, in its dated November 1, 2018, the licensee stated that each of the main steam lines has, in order; one power relief valve (PORV), five safety valves, and one isolation valve, before they join in a common header. PORVs (one per steam line) are upstream of the MSIVs to provide atmospheric steam relief capacity as a means of heat dissipation in the event of a loss of normal heat sink capabilities. Technical Specification (TS) 3.7.4, "Steam Generator Power Operated Relief Valves (SG PORVs)" requires 3 of the 4 steam-line PORVs to be Operable in Modes 1 through 4. If the main condensers are not available during normal unit shutdown, or sudden load rejection on turbine trip, the PORVs and the spring-loaded safety valves (MSSVs, five per steam line and located immediately outside the containment) can discharge full main steam flow to the atmosphere and affect safe reactor shutdown. MSIVs are provided in each steam generator steam line immediately downstream of the MSSVs to isolate each individual steam generator and prevent reverse flow in the event of a steam line rupture.

The MSIVs are located as close to the containment as possible and protected in service by reinforced concrete Doghouses and are restrained to prevent pipe whip damage if pipe break occurs within the Doghouse.

The licensee also stated that neither the main steam piping nor the MS IVs are exposed to tornado-generated missiles, but that portions of the MSSV vent stacks, which are not physically connected to the MSSVs (the MSSVs themselves are not targets), and portions of the steam-line PORVs and associated piping (upstream PORV block valves) are not targets.

Steam leak due to damage of a PORV could likely be isolated by closure of the associated PORV block valve, because these block valves are fully protected from missiles. It is important to note that PORV block valves do not receive class 1E safety-related power, so local manual operation will be required to actuate the valves. These PORV block valves are located at a lower elevation than the PORVs and valve operation is accessible.

Because only portions of the steam-line PORVs are not protected and the PORVs may be manually isolated by the PORV block valves, which are not exposed to tornado missiles, the NRC staff finds that the Boolean login in the McGuire TORM IS analysis related to protection of main steam lines against tornado missiles is acceptable for this application.

3.4.2 SFP and VCNC Intake The SFPs and VCNC System outside air intakes were added to the TORMIS analysis since they are not fully protected from licensing basis horizontal missiles. Each McGuire SFP is

housed in a concrete and steel superstructure. The concrete superstructure encloses the SFP except for the North end of the structure, which is enclosed by a steel structure with siding.

Although added to the TORM IS analysis, the NRC staff previously reviewed and accepted the North end of the SFP as an unprotected structure. Section 9.1.2 of, NUREG-0422, "Safety Evaluation Report - McGuire Nuclear Station, Units 1 and 2," dated March 1, 1978 (ADAMS Accession No. ML16029A007), states, in part, "The design of the spent fuel pit walls will prevent tornado missiles traveling in a horizontal direction from penetrating the pit. However, the north end of the building, which contains the fuel receiving area and the new fuel storage vault, does not provide tornado missile protection for certain trajectories (see Section 3.5.1 ). We have accepted the design of the fuel building and the spent fuel pit in regard to tornado missile protection based on its similarity to those of the previously approved plants."

McGuire design contains four VC/YC outside control room air intakes located on top of the Auxiliary Building roof, two near the South end of the Unit 1 reactor building, and two near the South end of the Unit 2 reactor building. The intakes are used to pressurize the control room (via filtration trains) to prevent possible unfiltered in-leakage to the control room from other paths, which otherwise may cause unacceptable doses to the control room operators.

During normal operation, the air handling unit recirculates and cools the air in the Control Room.

No outside air is provided. Make up air is provided by routine door openings. The outside air intakes are not required during normal VC system operation.

Upon a loss-of-coolant accident {LOCA) and/or LOOP from the engineered safety feature actuation signal system, the Outside Air Pressurization Filter Train (OAPFT) fans start to provide filtered outside air thru the VC system outside air intakes and to pressurize the Control Room. During a radiological release event, the OAPFTs maintain Control Room Operator dose within regulatory limits. The Control Room ventilation and cooling subsystem (CR-AHU) also remains in service.

Therefore, the CR-AHU will continue to function normally if all four outside air intakes were to be damaged by tornado missiles by operation in recirculation mode without fresh air intake.

As shown in Section 15.10.1 of UFSAR, an analysis was performed to determine the dose consequences in the event of a postulated tornado missile accident. A tornado-generated missile is assumed to breach the fuel building and rupture twelve assemblies in region 1 of the spent fuel pool. The VC/YC System is assumed to be in service within 30 minutes of this event to limit dose to the Control Room Operators. Outside air is drawn in through the intakes and filtered. The licensee concluded the resulting doses at the Control Room are below the 10 CFR 50.67 limits.

The licensee used Boolean logic to analyze failure combinations for VC/YC Air Intakes and SFP to determine whether the intakes for both units are impacted when damaging missiles enter the SFP. To further understand these combinations, the NRC staff issued RAl-06 requesting further description of basis for the failure criteria used for VC/YC Air Intakes and SFPs. In the response to RAl-06, in their July 3, 2018 letter, the licensee provided a description of how Boolean logic was applied and included Table 8 showing all combinations used.

The licensee included one Boolean combination in Table 8 resulting in loss of VC/YC function as being an acceptable configuration. The NRC staff issued RAl-07 requesting the licensee to provide the basis for use of Boolean combinations that classify loss of safety function as a

system success and/or acceptable configuration. In their response, the licensee clarified that VC/YC outside air supply is unavailable, but air handling unit recirculates and cools the air in the Control Room and makeup air is available by routine door openings. The licensee indicated the Control Room environment would remain habitable without the VC system outside air intakes and the filtration package and fans. Based on the licensee's response to RAl-07, the failure criteria for loss of bot.h units VC/YC intakes is acceptable to the NRC staff, since air handling unit recirculation remains available without use of VC/YC outside air supply.

The NRC staff reviewed the Boolean TORMIS failure combinations to address radiological concerns defined in Table 3-2 of the licensee's letter dated December 8, 2017, against the current licensing basis and concluded the following:

• Failure of only one unit's SFP fuel cladding (no VC/YC failure) does not result in a safety concern, due to properly functioning VC/YC air intakes. Also, failure consequences of an SFP fuel handling accident has been analyzed in McGuire's UFSAR, Chapter 15, "Handling Accident in Fuel Building."

• Failure of only one unit's VC/YC intake (No SFP failure) does not result in a safety concern, since control room function remains functional from other unit intake or alternate actions can be performed to address failure.

• Failure combination of a single unit's SFP and VC/YC has been previously analyzed in licensing basis (UFSAR Section 15.10) concluding dose consequences are below 50.67 limits.

• Failure combination of both units' VC/YC intakes without SFP failure does not result in a safety concern. While VC/YC outside air supply is unavailable, air handling unit recirculates and cools the air in the Control Room. Make up air is available by routine door openings.

• Failure combinations involving fuel failure in both units' SFP has negligible probability of occurrence and not considered credible. Simultaneous independent events, like two fuel handling accidents in different SPFs, are not considered credible. The accident dose acceptance criteria are based on a single accident and fuel handling accident in one unit should be considered for licensing purposes.

• Failure combination of both units' VC/YC intakes and either units' SFP fuel failure is modeled as an unacceptable result or failure in TORM IS assessment.

In RAl-6.d (ADAMS Accession No. ML18138A466), the NRC staff requested that the licensee clarify the criteria for VC/YC Air Intakes and SFPs and address any discrepancies between the failure criteria in Enclosure 1, Section 3.1.5 and Enclosure 2, Section 3.5.2.8.1.3d of the licensees December 8, 2017 letter. RAI 6.e requested that the licensee provide details on how the damage frequency for VC/YC Air Intakes and SFPs was derived and to explain the apparent discrepancy between Enclosure 1, Table 3.2 and Enclosure 1, Section 3.1.5 of the licensee's letter dated December 8, 2017.

In its letter dated July 3, 2018, the licensee stated that the apparent discrepancy seems to be from wording in Enclosure 1, Section 3.1.5 of its letter dated December 8, 2017, and was corrected to state, "The Boolean logic for the Unit 1 and Unit 2 Spent Fuel Pools and the Unit 1

and 2 VC/YV Air Intakes is that failure is defined as the VC/YC Air Intakes of both Units failing by wind missile and missile damage to fuel assemblies in either of the Spent Fuel Pools." The licensee also stated that the individual target damage frequencies in Table 3-2 of Enclosure 1 of its letter dated December 8, 2017, do not include any effects of the Boolean logic discussed at the end of Section 3.1. 5 on page 13 of Enclosure 1. To reconcile the relatively high damage frequencies for targets 82-85 (VC/YC air intakes) from Table 3-2 with the relatively low Boolean frequency (i.e. 5.41 E-08 yr-1) reported on page 13 of Enclosure 1, the location of the targets with respect to each other needed to be taken into consideration. The differences were quantitatively resolved by considering the 16 possible combinations of the four failure events:

1. Missile impact on either portion of the Unit 1 VC/YC Air Intakes

2. Missile impact on either portion of the Unit 2 VC/YC Air Intakes

3. Damaging missile impact on fuel assemblies in Unit 1 SFP

4. Damaging missile impact on fuel assemblies in Unit 2 SFP In UFSAR, Section 15.10.1, McGuire had previously analyzed tornado impact on unprotected SFP North wall configuration, but has chosen to incorporate the non-compliance into TORMIS input by employing Boolean Logic. Boolean "intersection" or "AND" logic is typically not used, nor approved, for post-TORMIS assessment. However, McGuire's licensing basis contains additional deterministic evaluations to disposition dosage concerns for the various combinations of the Boolean approach to support the multiple component SFP and VC/YC assessment.

While use of shared components between units is acceptable as TORMIS input, combining multi-unit components necessitated the additional justification provided by the licensee.

Based on above, the NRC staff finds the licensee approach to define failure combinations for dose assessment on control room intakes resulting from tornado missile strike seems reasonable and appropriately modeled in TORMIS application for McGuire, Units 1 and 2.

3.5 Technical Evaluation Summary Based on the above, the NRC staff concludes that the licensee has adequately addressed the items identified in the NRC SER approving the TORMIS methodology and also concludes that the EPRI TORMIS methodology is implemented appropriately in accordance with the guidance provided in the 1983 TORMIS SER and RIS 2008-14. The NRC staff concludes that implementation of Boolean logic is acceptable for this application. Furthermore, the NRC staff concludes that the reported results are within the bounds of NRC guidance and are acceptable.

The NRC staff finds that the licensee's use of the TORM IS-methodology will provide reasonable assurance of compliance with the Commission's regulations, and accordingly the health and safety of the public will not be endangered. The NRC therefore approves the request to revise the UFSAR to appropriately reflect McGuire, Units 1 and 2 use of NRG-approved methodology for TORM IS analysis

4.0 STATE CONSULTATION

In accordance with the Commission's regulations, NRC staff notified the North Carolina State official of the proposed issuance of the amendments on December 19, 2018. The State official confirmed on January 16, 2019, that the State of North Carolina had no comments.

5.0 PUBLIC COMMENTS On June 5, 2018, the NRC staff published a "Notice of Consideration of Issuance of Amendments to Facility Operating Licenses and Combined Licenses and Proposed No Significant Hazards Consideration Determination," in the Federal Register associated with the proposed amendment request (83 FR 26100). In accordance with the requirements in 10 CFR 50.91, the notice provided a 30-day period for public comment on the proposed no significant hazards consideration determination. One comment from a member of the public was received, however it was not related to the proposed no significant hazards consideration determination or to the proposed amendment request. The comment can be found at www.regulations.gov, reference NRC-2018-0105-0002.

6.0 ENVIRONMENTAL CONSIDERATION

The amendments change a requirement with respect to the installation or use of facility components located within the restricted area as defined in 10 CFR Part 20. The NRC staff has determined that the amendments involve no significant increase in the amounts and no significant change in the types of any effluents that may be released offsite and that there is no significant increase in individual or cumulative occupational radiation exposure. The Commission has previously issued a proposed finding that the amendments involve no significant hazards consideration, and there has been no public comment on this finding (83 FR 26100: June 5, 2018). Accordingly, the amendments meet the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9). Pursuant to 10 CFR 51.22(b}, no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendments.

7.0 CONCLUSION

The Commission has concluded, based on the considerations discussed above, that: (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) there is reasonable assurance that such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendments will not be inimical to the common defense and security or to the health and safety of the public.

Principal Contributors: G. Curran, NRR K. Tetter, NRR M. Mahoney, NRR Date of issuance: January 25, 2019

ML18355A610 *b memorandum +by e-mail OFFICE DORL/LPL2-1 /PM DORL/LPL2-1 /LA DSS/SCPB/BC* DRNAPLB/BC*

NAME MMahoney KGoldstein SAnderson GCasto DATE 01/04/2019 01/04/2019 12/14/2018 12/14/2018 OFFICE OGC- NLO+ DORL/LPL2-1 /BC DORL/LPL2-1/PM NAME DRoth MMarkley MMahoney DATE 01/17/2019 01/25/2019 01/25/2019