ML22131A351

License Amendment Request for Application of Risk-Informed Approach for Tornado Classification of the Fuel Handling Trolley Support Structure

ML22131A351
Person / Time
Site:	Surry
Issue date:	05/11/2022
From:	Lawrence D Virginia Electric & Power Co (VEPCO)
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
22-027
Download: ML22131A351 (31)

Text

V IRGINIA E LECTRIC AND P OWER C OMPANY RICHMOND, VIRGINIA 2 3 2 61 May 11, 2022 10 CFR 50.90 United States Nuclear Regulatory Commission Serial No.: 22-027 Attention: Document Control Desk NRA/GDM: RO Washington, D. C. 20555 Docket Nos.: 50-280/281 License Nos.: DPR-32/37 VIRGINIA ELECTRIC AND POWER COMPANY SURRY POWER STATION UNITS 1 AND 2 LICENSE AMENDMENT REQUEST FOR APPLICATION OF RISK-INFORMED APPROACH FOR TORNADO CLASSIFICATION OF TH E FUEL HANDLING TROLLEY SUPPORT STRUCTURE Pursuant to 10 CFR 50.90, Virginia Electric and Power Company (Dominion Energy Virginia) requests an amendment to the Surry Power Station (SPS) Units 1 and 2 Subsequent Renewed Operating Licenses DPR-32 and DPR-37, respectively. NRC approval is requested to apply a risk-informed approach to demonstrate the Fuel Handling Trolley Support Structure (FHTSS), as designed, meets the intent of a tornado resistant structure (i.e., Tornado Criterion T") under the current SPS licensing basis for a 360 miles per hour (mph) maximum tornado wind speed. The proposed approach for the risk-informed analysis utilizes the acceptance criteria in Regulatory Guide (RG) 1.174, Revision 3, "An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis," similarly to how they were applied in NUREG-1738, "Technical Study of Spent Fuel Pool Accident Risk at Decommissioning Nuclear Power Plants." The risk-informed analysis shows the change in risk of damage to spent fuel assemblies from tornado events with wind speeds up to 360 mph is small and within the acceptance limits of RG 1.174, Revision 3.

The risk-informed approach being used to demonstrate the FHTSS meets the intent of a Tornado Criterion 'T' structure in the SPS Updated Final Safety Analysis Report (UFSAR) is considered a change to an element of a method of evaluation that requires prior NRC approval per 10 CFR 50.59(c)(2)(viii). No changes to the SPS Technical Specifications (TS) are required by this license amendment request (LAR). A discussion of the proposed change, including the supporting technical basis and risk analysis, is provided in , and reference drawings of the FHTSS are provided in Attachment 2. An external peer review of the LAR is provided in Attachment 3. Upon NRC approval of the LAR, the SPS UFSAR will be updated to reflect the approved methodology change that demonstrates the FHTSS meets the intent of a Tornado Criterion "T" structure. A mark-up of the SPS UFSAR pages indicating the proposed change is provided in .

Dominion Energy Virginia has evaluated the proposed amendment and has determined it does not involve a significant hazards consideration as defined in 10 CFR 50.92. The

Serial No.22-027 Docket Nos. 50-280/281 License Amendment Request Page 2 of 3 basis for this determination is included in Attachment 1. We have also determined operation with the proposed change will not result in a significant increase in the amount of effluents that may be released offsite or a significant increase in individual or cumulative occupational radiation exposure. Therefore, the proposed amendment is eligible for categorical exclusion from an environmental assessment as set forth in 10 CFR 51.22(c)(9). Pursuant to 10 CFR 51.22(b), no environmental impact statement or environmental assessment is needed in connection with the approval of the proposed change.

The LAR has been reviewed and approved by the SPS Facility Safety Review Committee.

Dominion Energy Virginia requests approval of the proposed amendment by May 31, 2023, with a 90-day implementation period.

If you have any questions or require additional information, please contact Mr. Gary D. Miller at (804) 273-2771.

Douglas C. La ence Vice President - Nuclear Engineering and Fleet Support Commitment made in this letter:

1. Upon NRG approval of the LAR, the SPS UFSAR will be updated to reflect the approved methodology change that demonstrates the FHTSS meets the intent of a Tornado Criterion "T" structure.

COMMONWEAL TH OF VIRGINIA )

)

COUNTY OF HENRICO )

The foregoing document was acknowledged before me, in and for the County and Commonwealth aforesaid, today by Mr. Douglas C. Lawrence, who is Vice President - Nuclear Engineering and Fleet Support, of Virginia Electric and Power Company. He has affirmed before me that he is duly authorized to execute and file the foregoing document in behalf of that company, and that the statements in the document are true to the best of his knowledge and belief.

Acknowledged before me this jJ_+I, day of t1oj , 2022.

My Commission Expires: Ojy,s-t 3,1, ZcZ'::,

GARY DON MILLER Notary Public Commonwealth of Virginia Reg. # 7629412 My Commission Expires August 31, 20.Z,

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Serial No.22-027 Docket Nos. 50-280/281 License Amendment Request Page 3 of 3 Attachments:

1. Discussion of Change

2. Fuel Handling Trolley Support Structure Drawings

3. SGH Letter, "Review of Tornado Wind Fragility Analysis of Surry Fuel Handling Trolley Support Structure," dated April 18, 2022

4. Mark-up of SPS UFSAR Indicating Planned Changes cc: U.S. Nuclear Regulatory Commission - Region II Marquis One Tower 245 Peachtree Center Ave., NE Suite 1200 Atlanta, GA 30303-1257 NRC Senior Resident Inspector Surry Power Station Mr. L. John Klos NRC Project Manager - Surry U.S. Nuclear Regulatory Commission One White Flint North Mail Stop 09 E-3 11555 Rockville Pike Rockville, MD 20852-2738 Mr. G. Edward Miller NRC Senior Project Manager - North Anna U.S. Nuclear Regulatory Commission One White Flint North Mail Stop 09 E-3 11555 Rockville Pike Rockville, MD 20852-2738 State Health Commissioner Virginia Department of Health James Madison Building - 7th floor 109 Governor Street Suite 730 Richmond, VA 23219

Serial No.22-027 Docket Nos. 50-280/281 Attachment 1 DISCUSSION OF CHANGE Virginia Electric and Power Company (Dominion Energy Virginia)

Surry Power Station Units 1 and 2

Serial No.22-027 Docket Nos. 50-280/281 Attachment 1 TABLE OF CONTENTS 1

SUMMARY

DESCRIPTION ......................................................................................2 2 DETAILED DESCRIPTION .......................................................................................2 2.1 Fuel Building Design .... .......... ... ........ ... ..... .................................... ....... .. ........ 2 2.2 Current Licensing Basis ......................................... .... .................................... 3 2.3 Reason for the Proposed Change ........ .................. ...... ............................. .. .. .4 3 TECHNICAL EVALUATION .....................................................................................4 3.1 Tornado Wind Fragility for the FHTSS ................. .. ........................................ 5 3.2 Initiating Event Frequencies ................... ........................................ ... ..... .... .... 6 3.3 Tornado Wind Fragility Estimates ........ .. ........................................................ 7 3.4 Probabilistic Assessment of Impact to Spent Fuel Damage Frequency ..... .... 7 3.5 Comparison of Calculated Risks with Acceptance Criteria ........ .. .. ............. ... .8 3.6 Treatment of Uncertainty .... .................... ........................................................ 8 4 REGULATORY EVALUATION ................................................................................. 9 4.1 Applicable Regulatory Requirements/Criteria ................................................. 9 4.2 No Significant Hazards Consideration Determination Analysis ...... ..... ..... .... 10 5 ENVIRONMENTAL CONSIDERATION ..................................................................13 6 CONCLUSION ........................................................................................................ 14 7 REFERENCES ........................................................................................................14 Page 1 of 15

Serial No.22-027 Docket Nos. 50-280/281 Attachment 1 DISCUSSION OF CHANGE 1

SUMMARY

DESCRIPTION Pursuant to 10 CFR 50.90, Virginia Electric and Power Company (Dominion Energy Virginia) requests an amendment to the Surry Power Station (SPS) Units 1 and 2 Subsequent Renewed Operating Licenses DPR-32 and DPR-37, respectively. NRC approval is requested to apply a risk-informed approach to demonstrate the Fuel Handling Trolley Support Structure (FHTSS), as designed, meets the intent of a tornado resistant structure (i.e., Tornado Criterion 'T') under the current SPS licensing basis for a 360 miles per hour (mph) maximum tornado wind speed.

The proposed approach for the risk-informed analysis utilizes the acceptance criteria in Regulatory Guide (RG) 1.174, Rev. 3, "An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis" [1], similarly to how they were applied in NUREG-1738, Technical Study of Spent Fuel Pool Accident Risk at Decommissioning Nuclear Power Plants" [2]. The risk-informed analysis shows the change in risk of damage to spent fuel assemblies from tornado events with wind speeds up to 360 mph is small and within the acceptance limits of RG 1.174, Rev. 3 [1]. Radiological consequences from the impact of potential falling members of the FHTSS upon spent fuel assemblies during a tornado event are reasonably expected to be bounded by the fuel handling accident in the spent fuel pool (SFP), as described in Section 14.4.1.3 of SPS Updated Final Safety Analysis Report (UFSAR) [3]. Potential falling structural members of the FHTSS, due to a postulated collapse of the FHTSS during a tornado wind event, will not result in perforation of the reinforced concrete walls or the floor mat of the SFP. Any potential minor leakage from the SFP caused by perforation the SFP liner under the impact of a single falling member of the FHTSS is bounded by the analysis of a cask-drop event in the SFP as described in Section 9.12.4.13 of SPS UFSAR [3].

The risk-informed approach being used to demonstrate the FHTSS meets the intent of a Tornado Criterion "T" structure in the SPS UFSAR [3] is considered a change to an element of a method of evaluation that requires prior NRC approval per 10 CFR

50. 59(C)(2)(viii).

2 DETAILED DESCRIPTION 2.1 Fuel Building Design The SPS Fuel Building (FB) is bounded by the Auxiliary Building, Decontamination Building, and the two (2) Reactor Containment Buildings. The lower portion of the FB comprises the SFP with reinforced concrete walls extending from elevation 6' -1 O" to Page 2 of 15

Serial No.22-027 Docket Nos. 50-280/281 Attachment 1 47'-4". The lower portion of the west-side of the FB consists of reinforced concrete walls and slabs that house mechanical and electrical equipment located below grade level (i.e., below elevation 26'-4"). An intermediate 6-foot-thick reinforced concrete wall separates the new fuel storage area on the west side of the SFP from the irradiated spent fuel storage area on the east side. The SFP cooling pumps and purification pumps are located below the new fuel storage pit at elevation 6'-10". The motor control centers for the pumps are located on a stairway landing at elevation 16'-10". As described in Section 9.10.4.14 of the SPS UFSAR [3], no equipment required for safe shutdown is located in the FB.

The upper portion of the FB is comprised of a steel superstructure that provides a weather cover for the spent fuel assemblies in the SFP. The FB steel superstructure includes the low-bay area on the west side of the building (i.e., structural Bents 1A, 1B & 2), with an approximate roof elevation of 75 ft., and the FHTSS (i.e., structural framing for the high-bay portion of the FB steel superstructure) on the east side, with a high roof elevation of 97 ft., as illustrated in Attachment 2, that houses the FB 125-ton fuel cask trolley. The FB 125-ton fuel cask trolley moves only in a north-south direction over an area at one end of the SFP. Spent fuel racks are excluded from this area. Therefore, spent fuel casks and other heavy objects cannot be moved over irradiated spent fuel. The 125-ton fuel cask trolley is required by station procedures to be moved to the north end of the Crane Enclosure, outside of the FB, during abnormal weather conditions, including declaration of a Tornado Warning, to prevent it from potentially falling into the cask loading area of the SFP during a tornado.

2.2 Current Licensing Basis Table 15.2-1, "Structures, Systems, and Components Designed for Seismic and Tornado Criteria," of the SPS UFSAR [3] designates the FB steel superstructure, and the FHTSS specifically, as Tornado Criterion "T" for tornado winds only, and over the spent fuel pit only, where "T" refers to systems and components that will not fail during the design tornado. Based on Table 15.2-1, the spent fuel assemblies are categorized as Tornado Criterion "P" for horizontal missiles only, which means the spent fuel assemblies are only required to be protected against horizontal tornado missiles during a tornado event by the Tornado Criterion 'T' reinforced concrete walls of the SFP.

SPS UFSAR [3], Section 15.2.3, specifies the existing licensing basis tornado characteristics as follows:

• Rotational velocity = 300 mph

• Translational velocity= 60 mph

• Pressure drop of 3 psi in 3 seconds

• Overall diameter = 1200 ft

• Radius of maximum winds = 200 ft Page 3 of 15

Serial No.22-027 Docket Nos. 50-280/281 Attachment 1 Based on the above, structures that are designed to resist tornadoes shall withstand the licensing basis maximum tornado wind speed of 360 mph (i.e., 300 mph rotational velocity

+ 60 mph translational velocity).

2.3 Reason for the Proposed Change Dominion Energy Virginia identified that the existing design basis calculation for the FB does not consider a maximum tornado wind speed of 360 mph as stated in the SPS UFSAR [3]. The design basis calculation of the FB steel superstructure includes separate analyses for two (2) wind load cases: 1) loads from 150 mph wind acting on the entire external building sail area (i.e., siding will remain intact), and 2) loads from the 300 mph tornado wind acting on exposed bare structural steel members (i.e., siding is blown off of the building). The design basis calculation included a two-dimensional (2-D) frame analysis of the low-bay portion of the fuel building steel superstructure under the loads from a 300 mph tornado wind. However, such an analysis was not performed for the high-bay portion (i.e., FHTSS). Specifically, structural Bents 3 and 4 of the FHTSS were not analyzed in the design basis calculation of the FB. The low-bay portion of the FB steel superstructure was later analyzed under loads from a 360 mph tornado wind in the north-south direction. The FHTSS was also not included in the scope of this later calculation. Therefore, further analysis was needed to investigate structural performance of the FHTSS under licensing basis tornado wind loads.

3 TECHNICAL EVALUATION The new analysis for the FHTSS follows a risk-informed approach to calculate the risk of collapse of the FHTSS under the loads from 360 mph maximum tornado winds that can lead to spent fuel failure in the SFP. The risk of spent fuel failure is estimated as the product of the probability of FHTSS collapse and the initiating event (i.e., tornado) frequency summed over a range of tornado wind speeds up to 360 mph. Tornadoes with wind speeds larger than 360 mph contribute less than 0.1 % of the total risk because they have very low frequency of occurrence. Therefore, these tornadoes are not considered in the analysis. The probability of FHTSS collapse at a given wind speed is obtained from a tornado wind fragility curve that follows a log normal distribution and is constructed using median structural capacity of the FHTSS and a generic composite variability. The initiating event frequencies are estimated from the tornado wind exceedance frequencies that follow a logarithmic trend line calibrated with tornado exceedance frequencies in Table 6-1 of NUREG/CR 4461, Rev. 2 [4]. The estimated risk to spent fuel assemblies was compared to the acceptance criteria for base risk and delta risk corresponding to the Core Damage Frequency (GDF) in RG 1.174, Rev. 3 [1]. Results from this comparison show the change in risk of damage to spent fuel assemblies from tornado events with wind speeds up to 360 mph is small, and within the acceptance limits of RG 1.174, Rev.

3 [1]. Since a PRA model was not used, there are no PRA Quality considerations for this analysis.

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Serial No.22-027 Docket Nos. 50-280/281 Attachment 1 3.1 Tornado Wind Fragility for the FHTSS The tornado wind fragility curve for the FHTSS was constructed using the median wind speed capacity of the FHTSS and a generic composite variability assuming a lognormal distribution for the tornado fragility, which is consistent with typical practice for wind fragility analysis. This approach is similar to what is widely used in the Seismic Probabilistic Risk Assessment (SPRA) methodology. The lognormal fragility model is used extensively in the development of fragilities for wind and other hazards and is discussed in more detail in EPRI 3002012994 [5], "Seismic Fragility and Seismic Margin Guidance for Seismic Probabilistic Risk Assessments." This approach is consistent with the ASME/ANS Probabilistic Risk Assessment Standard per the note in Table 7-2.2-2(a) of the NRC endorsed ASME/ANS RA-Sa- 2009 [6], "Standard for Level 1/Large Early Release Frequency Probabilistic Risk Assessment for Nuclear Power Plant Applications,"

which states "Wind fragility is evaluated using the same general methodology as for seismic fragilities."

The median tornado wind speed capacity is defined as the maximum tornado wind speed the structure can withstand based on its median capacity. The median capacity of each structural member of the FHTSS is defined as its unfactored (nominal) strength, as listed in AISC 360-16 [1 O], and using median linear elastic material properties. Based on results from analysis of the FHTSS computer model, the wind speed that causes at least one ( 1) structural member to reach the maximum interaction ratio (i.e., demand-to-capacity ratio equal to 1.0) is considered to be the median wind speed capacity of the structure.

To estimate the median structural capacity of the FHTSS, a three-dimensional (3-0) computer model of the structural framing between column lines 10 3/8 and 11 1/2 was created in SAP2000 structural analysis software. Additionally, to investigate the behavior of the beams spanning to the west of Bent 3 (Attachment 2), a portion of the low-bay of the FB steel superstructure between column lines 10 3/8 and 9 3/4 was also included in the computer model. These beams span over the SFP and their inclusion in the computer model would allow assessment of the effect of high-bay lateral drifts under tornado wind loads on these beams. The base of the structural steel columns in the SAP2000 computer model is located at elevation 43'-2" on top of the reinforced concrete walls of the SFP.

Only the bare structural steel sail area of the FHTSS was considered in the computer model for calculation of the loads due to tornado winds above 150 mph because the exterior metal siding and fastening to the building girts of the FB are specified in the SPS design specification to fail under loads from wind speeds above 150 mph. Structural beams, columns, and bracings were modeled as linear beam elements having six (6) degrees of freedom (DOFs) at each end.

The drag coefficients for wind load evaluations used in the design basis calculations of the FB were based on ASCE 3269 [7]. In the design basis calculations of the FB, the drag coefficients were conservatively selected from Table 3 of ASCE 3269 [7] for infinitely Page 5 of 15

Serial No.22-027 Docket Nos. 50-280/281 Attachment 1 long structural members. Application of these drag coefficients to structural members comprising the 3-D frame structure of the FHTSS is conservative, since this approach ignores the shielding effect of closely spaced structural members. Therefore, for estimation of the wind loads applied to the bare structural members in the 3-D computer model of the FHTSS, the drag coefficients were calculated based on the guidelines in an ASCE publication [8] that is also referenced in Section 29.4 of ASCE 7-16 [9]. Appendix 5A of this ASCE publication [8] provides a detailed method of calculating the wind forces on open frame structures and provides guidelines for determination of force (i.e., drag) coefficients. Based on the guidelines in Appendix 5A of this ASCE publication [8], force coefficients of 3.2 and 2.6 were estimated for lower and upper elevations, respectively.

The force (drag) coefficients were used along with the effective solid area of the windward frame (i.e., the solid area of all elements of the windward frame, in the plane of the frame, projected normal to the wind direction) to calculate the total wind force acting on the FHTSS. The total force was applied as a distributed load to all structural members on the windward frame proportional to their effective solid area.

Based on results from the analysis of the FHTSS computer model, a median wind speed capacity of 210 mph was calculated. A comparative study showed the FHTSS will experience a much smaller base shear and moment due to a 150 mph wind acting on the FHTSS with siding intact, as compared to the base shear and moment due to a 210 mph wind applied on the bare structural steel framing of the FHTSS. Therefore, the calculated fragility with a median wind speed capacity of 210 mph and the corresponding risk will not be affected by the presence of siding during tornado wind speeds of 150 mph or lower.

To construct the tornado fragility curve, the estimated median wind speed capacity was combined with variability parameters. Based on results presented in Reference [11], a generic composite variability of 0.175 was used along with the median wind speed capacity to construct a lognormal wind fragility for the FHTSS. A sensitivity analysis, as described in Section 3.6, shows the risk of spent fuel failure would still be acceptable per the guidelines in RG 1.174, Rev. 3 [1] even if the variability was twice the typical value as stated above. Therefore, the use of a generic variability for the purpose of this study is justified.

An external peer review of the tornado wind fragility analysis of the FHTSS was performed by (Simpson Gumpertz & Heger - SGH) who has extensive experience in developing wind fragilities. The peer review concluded the FHTSS wind fragility evaluation is consistent with the state of practice and the results are reasonable (See Attachment 3).

3.2 Initiating Event Frequencies Initiating event frequencies for tornadoes were developed using the data in Table 6-1 of NUREG/CR 4461, Rev. 2 [4]. Tornado wind speed estimates from the Fujita Scale for events with a frequency of 1.0E-5, 1.0E-6, and 1.0E-7 events per year were used since these data are more conservative than the Enhanced Fujita Scale. This data was used Page 6 of 15

Serial No.22-027 Docket Nos. 50-280/281 Attachment 1 with the process described in NEI 17-02, Rev. 1B [12] to develop a site-specific hazard curve to describe the expected frequency of tornadoes that exceed wind speeds from 70 to 360 mph in 10 mph increments. This range was selected because it encompasses the range of high wind events considered in the SPS UFSAR [3]. The estimated initiating event frequencies range from 9.37E-5/yr for the smallest Fujita prime F'1 tornado intensity to 6.83E-10/yr for the largest F'6 tornado intensity.

3.3 Tornad o Wind Fragility Estimates The lognormal tornado wind fragility curve described above was used to produce fragility estimates for each tornado wind speed interval in the scope of the analysis. These fragility estimates ranged from 2.01 E-9 at 70 mph to 0.445 at 200 mph. The fragility was conservatively assumed to be 1.0 at wind speeds above the calculated median wind speed capacity of 210 mph (i.e., the FHTSS was conservatively assumed to collapse at wind speeds above the calculated median wind speed capacity of 210 mph).

3.4 Probabilistic Assessment of Impact to Spent Fuel Damage Frequency The generated hazard curve was convolved with the fragility curve to estimate the Spent Fuel Damage Frequency (SFDF) associated with the current design. The frequency for a tornado of each assessed wind speed was multiplied by the failure rate from the fragility curve for that wind speed. A summation of these values was taken to calculate the total SFDF of 1.97E-6/yr.

Several conservative assumptions were made in the estimation of the total SFDF. The post-yield capacity of structural members was ignored, conservatively, and structural members were assumed to fail once they experience yielding under the applied loads.

Additionally, the FHTSS was conservatively assumed to collapse at wind speeds above the calculated median wind speed capacity of 210 mph, while the actual failure probability remains less than 100% for wind speeds exceeding 210 mph, and the structure is likely to remain stable even when some members experience local failure. Furthermore, it is conservatively assumed that once failed, structural members would fall and directly impact the spent fuel assemblies and cause spent fuel damage.

The possibility of Large Early Release being caused by tornado wind damage to the FHTSS was considered. A potential radioactive release resulting from spent fuel damage is expected to be bounded by the fuel accidents analyzed under the design basis, including the cask-drop analysis and the fuel handling accident, because it was conservatively assumed in the cask-drop analysis that all 324 fuel assemblies stored in the first three rows of storage racks adjacent to the cask loading area, under the FHTSS, would fail. The estimated releases associated with these accidents are less than the release criteria for Large Early Release Frequency (LERF). Therefore, it is concluded that a Large Early Release cannot be caused by a high wind induced failure of the FHTSS.

Page 7 of 15

Serial No.22-027 Docket Nos. 50-280/281 Attachment 1 Additionally, SFP integrity was evaluated against potential impact loads from falling structural members of the FHTSS. Considering the low likelihood of multiple falling members targeting the SFP with an angle of attack that can result in localized damage to the SFP, only single impacts from the heaviest falling members were considered. It was conservatively assumed the worst-case falling member would impact the SFP with its longitudinal axis perpendicular to the impacted surface such that it results in the most severe localized damage. As additional conservatism, no credit was taken for energy dissipation by the SFP water and the existing crush pads in the cask loading area. The evaluation showed potential falling structural members from the FHTSS, due to a postulated collapse of the FHTSS under tornado winds, will not cause a perforation in the reinforced concrete walls or the floor mat of the SFP. Falling members from the FHTSS may cause local perforation of the SFP liner, especially if these members hit the SFP along a sharp edge. However, no significant leakage, i.e., more than 5 gallon per minute (gpm) as described in Section 9.12.4.13 of SPS UFSAR [3] for the cask-drop in the SFP, is expected due to a potential SFP liner perforation under the worst-case impact of a single falling member of the FHTSS because there will be no perforation (i.e., through cracking) in the reinforced concrete walls or the floor mat of the SFP.

3.5 Comparison of Calculated Risks with Acceptance Criteria The fuel damage frequency from high wind events associated with the current FHTSS design was estimated to be 1.97E-06/yr. The SFDF associated with a FHTSS design that meets the current design basis requirements was conservatively assumed to be zero, so the total SFDF calculated was treated as incremental risk from leaving the as-built design in place. This incremental risk value is considered 'small' by the acceptance guidelines in RG 1.174, Rev. 3 [1], and is considered reasonable in cases where the total CDF is less than 1.0E-4 events per reactor year. In the study on SFP risk documented in NUREG-1738 [2], the total spent fuel damage frequency was assessed to be on the order of 1.0E-6/yr or 1.0E-7/yr. Therefore, it is reasonable to assume the total SFDF for the SPS SFP is less than 1.0E-4/yr.

It is therefore concluded the overall risk associated with the wind speed affecting the FHTSS is acceptably small per the RG 1.174, Rev. 3 [1] acceptance guidelines, and the current design does not present a nuclear safety concern to the public or to the environment.

3.6 Treatment of Uncertainty There is uncertainty present in this risk assessment due to the nature of rare events like tornadoes and the complexity of assessing their impact on structures. Conservatism was used throughout this analysis to ensure the estimates maintained a conservative bias to bound these uncertainties. For the event frequencies, credit was not taken for the reduced frequencies characterized by the Enhanced Fujita Scale data in NUREG/CR 4461, Rev. 2 [4]. For the fragilities, the structural failure rate was assumed to be 1.0, Page 8 of 15

Serial No.22-027 Docket Nos. 50-280/281 Attachment 1 which means the FHTSS is assumed, conservatively, to collapse at wind speeds above the calculated median wind speed capacity of 210 mph. This is a conservative assumption because structures reaching their median capacity do not necessarily fail until they develop enough plastic hinges to initiate instability. With regard to impact to fuel, spent fuel damage was assumed to take place any time the analysis indicated a structural member would experience plastic deformation, and no credit was taken for post-yield capacity of structural members. The conservative bias resulting from the use of these conservative assumptions ensures the results can be considered reasonable despite the presence of uncertainties.

Additionally, a sensitivity study was performed by doubling the composite variability in the fragility curve from 0.175 to 0.35. The SFDF in the sensitivity study was estimated to be 7.56E-06/yr. This demonstrates that the risk assessment is sensitive to the composite variability, but the incremental SFDF would still be assessed as 'small' per the guidelines in RG 1.174, Rev. 3 [1] even if the variability was twice the typical value.

4 REGULA TORY EVALUATION The proposed LAR demonstrates the FHTSS meets the intent of a Tornado Criterion "T" structure in the SPS UFSAR [3] for licensing basis 360 mph tornado winds only using a risk-informed approach. The following regulatory requirements have been reviewed and a No Significant Hazards Consideration Determination has been performed as discussed below.

4.1 Applicable Regulatory Requirements/Criteria 10 CFR Requirements/ General Design Criteria (GDC): General Design Criterion (GDC) 2, "Design bases for protection against natural phenomena," of 10 CFR 50, Appendix A requires that structures, systems, and components important to safety shall be designed to withstand the effects of natural phenomena such as tornadoes without loss of capability to perform their safety functions. GDC 2 also requires that "the design bases for these structures, systems, and components shall reflect: (1) Appropriate consideration of the most severe of the natural phenomena that have been historically reported for the site and surrounding area, with sufficient margin for the limited accuracy, quantity, and period of time in which the historical data have been accumulated, (2) appropriate combinations of the effects of normal and accident conditions with the effects of the natural phenomena and (3) the importance of the safety functions to be performed."

In addition, GDC 4, "Environmental and dynamic effects design bases," of 10 CFR 50, Appendix A states, in part, that "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, Page 9 of 15

Serial No.22-027 Docket Nos. 50-280/281 Attachment 1 and components shall be appropriately protected against dynamic effects, including the effects of missiles... that may result from equipment failures and from events and conditions outside the nuclear power unit."

The GDC included in Appendix A to 10 CFR Part 50 did not become effective until May 21, 1971. The Construction Permit for SPS Units 1 and 2 was issued prior to May 21, 1971; consequently, SPS was not subject to GDC requirements [13]. However, SPS UFSAR [3], Section 1.4.2, "Performance Standards," Section 1.4.40, "Missile Protection," Section 2.2.2.1, "Tornadoes," and Section 15.2.3, 'Tornado Criteria," meet the intent of GDC 2 and GDC 4. The FHTSS meets the intent of GDC 2, in general, because the reinforced concrete structure of the FB, as well as the FB steel superstructure and the FHTSS, are all Tornado Criterion 'T' for tornado winds, and therefore, can withstand loads from licensing basis tornado winds and provide tornado protection to the equipment housed within the FB. The FHTSS also meets the intent of GDC 4, in general, because spent fuel storage racks are protected against the impact of horizontal missiles (i.e., Tornado Criterion "P" for horizontal missiles). Potential damage to the spent fuel assemblies due to the impact of potential falling structural members of the FHTSS during a tornado wind event is not investigated separately; however, their radiological consequences are reasonably expected to be bounded by a release due to a cask-drop and a fuel handling accident in the SFP, as described in Section 14.4.1.3 of SPS UFSAR [3].

RG 1.174, "An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis," Rev. 3 [1], describes the risk-informed approach as an acceptable approach to the NRC for developing licensing basis changes considering engineering issues and risk insights. Risk acceptance guidelines are provided in this RG in terms of base value of risk metrics of CDF and LERF, and the change in these metrics (..6.CDF and .6.LERF). The proposed LAR uses a risk-informed approach that shows the change in risk of damage to spent fuel assemblies from tornado events with wind speeds up to 360 mph is small and within the acceptance limits of RG 1.174, Rev. 3 [1].

4.2 No Significant Hazards Consideration Determination Analysis Virginia Electric and Power Company (Dominion Energy Virginia) requests an amendment to the Surry Power Station (SPS) Units 1 and 2 Subsequent Renewed Operating License Nos. DPR-32 and DPR-37, respectively. The proposed amendment would revise the SPS Updated Final Safety Analysis Report (UFSAR) to demonstrate the Fuel Handling Trolley Support Structure (FHTSS) meets the intent of a Tornado Criterion

'T' structure for licensing basis tornado winds only, using a risk-informed approach as a new method of evaluation.

Dominion Energy Virginia has evaluated the proposed change using the criteria in 10 CFR 50.92 and determined the proposed change does not involve a significant Page 10 of 15

Serial No.22-027 Docket Nos. 50-280/281 Attachment 1 hazards consideration. The following information is provided to support a finding of no significant hazards:

1. Does the proposed change involve a significant increase in the probability or consequences of an accident previously evaluated?

Response: No The proposed license amendment request (LAR) demonstrates the FHTSS meets the intent of a Tornado Criterion "T" structure in the SPS UFSAR, for licensing basis tornado winds only and using a risk-informed approach as a new method of evaluation. The proposed risk-informed approach shows the change in risk of damage to spent fuel assemblies from tornado events with wind speeds up to 360 mph is small and within the acceptance limits of RG 1.174, Rev. 3. Additionally, radiological consequences from the impact of potential falling members of the FHTSS upon spent fuel assemblies during a tornado event are reasonably expected to be bounded by releases due to the fuel handling accident in the spent fuel pool (SFP), as described in Section 14.4.1 .3 of SPS UFSAR. Potential falling structural debris due to a postulated collapse of the FHTSS during a tornado wind event will not result in perforation of the reinforced concrete walls or the floor mat of the SFP.

Any potential minor leakage from the SFP caused by the SFP liner perforation under the impact of a single falling member, is bounded by the analysis of a cask-drop in the SFP, as described in Section 9.12.4.13 of SPS UFSAR. Under the proposed change, the design functions of the SPS FHTSS are not changed. The proposed change does not implement plant physical changes or result in plant operation in a configuration outside the plant safety analyses or design basis.

Therefore, the proposed change does not involve a significant increase in the probability or consequences of any accidents previously evaluated.

2. Does the proposed change create the possibility of a new or different kind of accident from any accident previously evaluated?

Response: No The proposed LAR uses a risk-informed approach as a new method of evaluation to demonstrate the FHTSS meets the intent of a tornado resistant (i.e., Tornado Criterion "T) structure in the SPS UFSAR for licensing basis 360 mph tornado winds only. The risk-informed approach shows the change in risk of damage to spent fuel assemblies from tornado events with wind speeds up to 360 mph is small and within the acceptance limits of RG 1.174, Rev. 3. Radiological consequences, structural impacts, and potential leakages from the SFP under the impact of falling structural debris due to a postulated collapse of the FHTSS during a tornado wind event will not create the possibility of a new or different kind of accident from any accident Page 11 of 15

Serial No.22-027 Docket Nos. 50-280/281 Attachment 1 previously evaluated.

The proposed LAR does not alter the ability of the FHTSS, or the Fuel Building (FB) in general, to perform their intended design functions. As a result of the proposed LAR, no new effects on existing equipment are created nor are any new malfunctions introduced. Furthermore, the proposed change does not implement plant physical changes or result in plant operation in a configuration outside the plant safety analyses or design basis.

Therefore, based on the above evaluation, the proposed change does not create the possibility of a new or different kind of accident from any accident previously evaluated.

3. Does the proposed change involve a significant reduction in a margin of safety?

Response: No The proposed LAR demonstrates the FHTSS meets the intent of a Tornado Criterion "T" structure in the SPS UFSAR, for licensing basis tornado winds only, and using a risk-informed approach. The risk-informed approach shows the change in risk of damage to spent fuel assemblies from tornado events with wind speeds up to 360 mph is small and within the acceptance limits of RG 1.174, Rev. 3. Additionally, radiological consequences from the impact of potential falling members of the FHTSS upon spent fuel assemblies during a tornado event are reasonably expected to be bounded by releases due to the fuel handling accident in the SFP, as described in Section 14.4.1.3 of SPS UFSAR. Impact loads from falling structural members of the FHTSS will not result in perforation of the reinforced concrete walls or the floor mat of the SFP. Therefore, no significant leakage, i.e., more than 5 gpm as described in Section 9.12.4.13 of SPS UFSAR for a cask-drop in the SFP, is expected due to a potential SFP liner perforation under the worst-case impact of a single falling member of the FHTSS, because there will be no perforation (i.e., through cracking) in the reinforced concrete walls or the floor mat of the SFP.

Therefore, the proposed change does not involve a significant reduction in a margin of safety.

Based upon the above, Dominion Energy Virginia concludes the proposed amendment presents no significant hazards consideration under the standards set forth in 10 CFR 50.92(c), and, accordingly, a finding of "no significant hazards consideration" is justified.

Page 12 of 15

Serial No.22-027 Docket Nos. 50-280/281 Attachment 1 5 ENVI RONMENTAL CONSIDERATION The proposed amendment meets the eligibility criterion for categorical exclusion set forth in 10 CFR 51.22(c)(9) as follows:

(i) The proposed change involves no significant hazards consideration.

As described in Section 4.2 above, the proposed change involves no significant hazards consideration.

(ii) There are no significant changes in the types or significant increase in the amounts of any effluents that may be released off-site.

The proposed LAR does not alter the design function or operation of any plant structure, system, or component. The FHTSS will continue to meet its specific structural performance criteria such that it poses no threat to the reinforced concrete walls or the floor mat of the SFP. Radiological consequences from the impact of potential falling members of the FHTSS upon spent fuel assemblies during a tornado event are reasonably expected to be bounded by releases due to the fuel handling accident in the SFP as described in Section 14.4.1.3 of SPS UFSAR. Potential falling structural debris due to a postulated collapse of the FHTSS during a tornado wind event will not result in perforation of the reinforced concrete walls or the floor mat of the SFP. Any potential minor leakage from the SFP caused by perforation of the SFP liner under the impact of a single falling member is bounded by the analysis of a cask-drop in the SFP as described in Section 9.12.4.13 of SPS UFSAR. The proposed change does not involve the installation or modification of any new equipment that may affect the types or amounts of effluents that may be released off-site. Therefore, there are no significant changes in the types or significant increase in the amounts of any effluents that may be released off-site.

(iii) There is no significant increase in individual or cumulative occupational radiation exposure.

The proposed change does not implement plant physical changes or result in plant operation in a configuration outside the plant safety analyses or design basis. The FHTSS will continue to meet its specific structural performance criteria such that it poses no threat to the reinforced concrete walls or the floor mat of the SFP. The risk-informed approach shows the change in risk of damage to spent fuel assemblies from tornado events with wind speeds up to 360 mph is small and within the acceptance limits of RG 1.174, Rev. 3. Furthermore, radiological consequences from the impact of potential falling members of the FHTSS upon spent fuel assemblies during a tornado event are reasonably expected to be bounded by releases due to the fuel handling accident in the SFP as described in Section 14.4.1.3 of SPS UFSAR.

Based on the above, Dominion Energy Virginia concludes that, pursuant to 10 CFR Page 13 of 15

Serial No.22-027 Docket Nos. 50-280/281 Attachment 1 51.22(b), no environmental impact statement or environmental assessment needs to be prepared in connection with the proposed amendment.

6 CONCLUSION The proposed LAR uses a risk-informed approach as a new method of evaluation to demonstrate the FHTSS meets the intent of a tornado resistant (i.e., Tornado Criterion

'T') structure in the SPS UFSAR [3] for licensing basis 360 mph tornado winds only. The proposed approach for the risk-informed analysis utilizes the acceptance criteria in RG 1.174, Rev. 3 [1], similarly to how it is applied in NUREG-1738 [2]. The risk-informed analysis shows the change in risk of damage to spent fuel assemblies from tornado events with wind speeds up to 360 mph is small and within the acceptance limits of RG 1.174, Rev. 3 [1]. Radiological consequences from impact of potential falling members of the FHTSS upon spent fuel assemblies during a tornado event are reasonably expected to be bounded by the fuel handling accident in the SFP as described in Section 14.4.1.3 of SPS UFSAR [3]. Potential falling structural members of the FHTSS due to a postulated collapse of the FHTSS during a tornado wind event will not result in perforation of the reinforced concrete walls or the floor mat of the SFP. Any potential minor leakage from the SFP caused by the SFP liner perforation under the impact of a single falling member of the FHTSS is bounded by analysis of a cask-drop in the SFP as described in Section 9.12.4.13 of SPS UFSAR [3].

Therefore, Dominion Energy Virginia concludes, based on the considerations discussed herein, 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) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.

7 REFERENCES

[1] U.S. NRC, Regulatory Guide 1.174, Revision 3, "An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis," January 2018 (ML17317A256 ).

[2] U.S. NRC, NUREG 1738, "Technical Study of Spent Fuel Pool Accident Risk at Decommissioning Nuclear Power Plants," February 2001 (ML010430066).

[3] Surry Power Station UFSAR, Revision 53.02.

[4] U.S. NRC, NUREG/CR-4461, Revision 2, "Tornado Climatology of the Contiguous United States," February 2007 (ML070810400).

Page 14 of 15

Serial No.22-027 Docket Nos. 50-280/281 Attachment 1

[5] EPRI Report 3002012994, "Seismic Fragility and Seismic Margin Guidance for Seismic Probabilistic Risk Assessments," Electric Power Research Institute, Palo Alto, CA, 2018.

[6] American Society of Mechanical Engineers (ASME)/American Nuclear Society (ANS) Standard ASME/ANS RA-Sa-2009, "Standard for Level 1/Large Early Release Frequency Probabilistic Risk Assessment for Nuclear Power Plant Applications," Addendum A to RA-S-2008, ASME, New York, NY, American Nuclear Society, La Grange Park, Illinois, February 2009.

[7] ASCE Paper 3269, "Wind Forces on Structures," Transactions of the American Society of Civil Engineers, Vol. 126, Part II, 1961.

[8] ASCE publication, "Wind Load Design for Petrochemical and Other Industrial Facilities," American Society of Civil Engineers, Reston, VA, 2nd Ed., 2020.

[9] ASCE/SEI 7-16, "Minimum Design Loads and Associated Criteria for Buildings and Other Structures," American Society of Civil Engineers, 2016.

[10] ANSI/AISC 360-16, "Specifications for Structural Steel Buildings," American Institute of Steel Construction, Chicago, IL, USA, 2016.

[11] Lawrence Twisdale, "Non-parametric method for wind pressure fragility analysis,"

Applied Research Associates, Inc., PSA 2017, Pittsburgh, PA, September 24-28, 2017.

[12] NEI 17-02, Rev. 18, 'Tornado Missile Risk Evaluator (TMRE)," September 2018, as implemented and approved at Shearon Harris Nuclear Power Plant (ML18347A385).

[13] NRC letter, "SECY-92-223 - Resolution of Deviations Identified During Systematic Evaluation Program," September 18, 1992 (ML12256B290).

Page 15 of 15

Serial No.22-027 Docket Nos. 50-280/281 Attachment 2 FUEL HANDLING TROLLEY SUPPORT STRUCTURE (FHTSS) DRAWINGS Virginia Electric and Power Company (Dominion Energy Virginia)

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Serial No.22-027 Docket Nos. 50-280/281 Attachment 3 SGH LETTER, "REVIEW OF TORNADO WIND FRAGILITY ANALYSIS OF SURRY FUEL HANDLING TROLLEY SUPPORT STRUCTURE," DATED APRIL 18, 2022 Virginia Electric and Power Company (Dominion Energy Virginia)

Surry Power Station Units 1 and 2

18 April 2022 Mr. Jeremy Graham Dominion Energy I Nuclear Engineering & Fuel Department 5000 Dominion Boulevard - Innsbrook 2SE Glen Allen, VA 23060 Project 218022 - Review of Tornado Wind Fragility Analysis of Surry Fuel Handling Trolley Support Structure

Dear Mr. Graham:

Simpson Gumpertz & Heger (SGH) is pleased to present herein our final review results for the Dominion Energy (DE) tornado wind fragility analysis for the Fuel Handling Trolley Support Structure (FHTSS) portion of the Fuel Building (FB) at Surry Power Station (SPS) in southeastern Virginia.

Background

Our 3 June 2021 letter to you summarized our review of the 29 April 2021 draft of DE Calculation CE-2085, Tornado Wind Fragility Analysis of Surry Fuel Handling Trolley Support Structure. Our review process included several teleconferences with DE to understand the technical approach and intended use of the fragility results, as well as a review of the draft calculation documentation and associated electronic files (e.g., supporting MS Excel spreadsheets and SAP2000 models). We provided comments, suggestions, and recommendations to DE throughout the review process. Our review included the following:

• Inputs and assumptions

• Structure model development

• Estimation of wind loads

• Determination of the median wind capacity

• Estimation of variability and tornado wind fragility We concluded from our review that the overall wind fragility evaluation approach is consistent with the current state of practice and the results are reasonable, or perhaps somewhat conservative. We concurred that the tornado wind fragility results were appropriate to be used in a risk-based assessment such as that in License Basis Document Change Request (LBDCR)22-027.

SIMPSON GUMPERTZ & HEGER 4695 MacArthur Court, Suite 500, Newport Beach. CA 92660 949.930.2500 sgh.com

Mr. Jeremy Graham 18 April 2022 Project 218022 Follow-up Review Subsequent to our review of the fragility analysis summarized in our 3 June 2021 letter, DE requested that SGH also review LBDCR 22-027, which uses the tornado wind fragility from DE Calculation CE-2085 to support a risk-based assessment of the FHTSS. The intent of the follow-up review is to ensure that the final fragility evaluation is substantively consistent with the 29 April 2021 draft previously reviewed by SGH, and to review how the fragility is used in the LBDCR to ensure the application is consistent with the underlying inputs, assumptions, and methods used in the fragility evaluation. This follow-up review was performed in two steps:

1. DE provided a draft LBDCR 22-027 dated 24 February 2022. SGH reviewed this draft and provided comments.

2. DE addressed SGH's comments and provided a revision dated 5 April 2022. SGH reviewed the revision to confirm our comments have been addressed appropriately and issued this letter to document our review.

Our initial review of the 24 February 2022 draft focused on the following attachments of LBDCR 22-027: - Discussion of Change - SU-CALC-STR-CE- 2085, Revision 0, Tornado Wind Fragility Analysis of Surry Fuel Handling Trolley Support Structure, dated 9 June 2021 Attachment 5 1 - NOTEBK-PRA-SPS-RA.Ll.016, Revision 0, Fuel Building Trolley Support Structure Risk Analysis, draft version, provided on 24 February2022 Our review focused on the development of the tornado wind fragility and its use in the risk quantification. While we also reviewed other aspects of the LBDCR, our work was not an exhaustive review of the input hazard, risk methodology, or conclusions.

We found that Revision O of DE Calculation SU- CALC-STR-C E-2085 has the same technical content and results provided in the 29 April 2021 draft version we originally reviewed. As we concluded before, we found the tornado wind fragility development to be consistent with the state of practice and the results to be reasonable, and perhaps somewhat conservative. We concur with the tornado wind fragility results and agree they are appropriate to be used in the risk-based assessment.

We have found the results and conclusions presented in these documents to be aligned with our previous review of the source information. We provided comments to DE that were primarily either editorial in nature or suggestions to provide additional detail and clarity to 1 This attachment was renumbered as Attachment 6 in a later revision of the LBDCR dated 5 April 2022.

Mr. Jeremy Graham 18 April 2022 Project 218022 support the conclusions drawn. We did not find any technical issues or have any objections to the conclusions in these attachments.

In response to our comments on the 24 February 2022 LBDCR, DE revised the LBDCR and provided a revision dated 5 April 2022. The revision includes a new attachment: 2 - SU-CALC-STR-CE-2085, Revision 0, Addendum A, Comparison of the Applied Tornado Wind Loads on the FHTSS with and without Siding, dated 30 March 2022 We found that our comments on the 24 February 2022 version of the LBDCR were adequately addressed in the 5 April 2022 version, and we have no further substantive comments. We provided some minor editorial comments to DE via email that may be incorporated at DE's discretion.

We have also performed a review of the new Attachment 5. We have completed a high-level review of the calculation but did not perform a detailed number check of the inputs and computations. The intent of the new Attachment 5 is to validate the previous judgment that the analysis results of the bare structural steel framing (which is consistent with the expected damage state at the 210-mph median windspeed capacity) can be reasonably or conservatively scaled to lower windspeeds at which the FHTSS siding remains attached to the structure (i.e.,

< 50 mph). The calculation concludes that the base shears and base moments are greater when the siding is removed than when it is intact. Therefore, the lognormal fragility that is derived from the 210-mph median capacity that assumes siding is detached conservatively characterizes the failure probability at lower windspeeds at which siding will remain intact.

Based on this follow-up review, we conclude that the FHTSS wind fragility evaluation is consistent with the state of practice and the results are reasonable. DE has addressed our comments satisfactorily, and we have no further comments on the evaluation.

Sincerely yours, Associate Principal Senior Project Manager l:\OC\Projects\2021\218022.00-FHTS\WP\002FFGrant-L-218022.00Jm.docx 2

This does not replace the previous Attachment 5 but causes it to be renumbered to Attachment 6.

Serial No.22-027 Docket Nos. 50-280/281 Attachment 4 MARK-UP OF UFSAR PAGES INDICATING PROPOSED CHANGES Virginia Electric and Power Company (Dominion Energy Virginia)

Surry Power Station Units 1 and 2

Revision 53.02-Updated Online 02/24/22 SPS UFSAR 15.6-2 See following page for Insert of the Fuel Building steel superstructure. All concrete work is designed in accordance with the Building Code Requirements for Reiriforced Concrete, serial designation 318-63 of the American Concrete Institute. Access and egress requirements, as well as fire ratings of walls and floor systems, satisfy the requirements of the Basic Building Code of the Building Officials Conference of America, 1966 issue.

Under the design-basis accident loading, the allowable stresses do not exceed 90% of the minimum yield strength of the structural steel. From mill test reports, the yield strength of structural steel is 42,000 psi, with an ultimate strength of 63,000 psi. Using a minimum yield of 36,000 psi for A36 steel, the design-allowable stress is 90% of 36,000 = 32,400 psi.

Design-allowable stress for structural steel is ~;* ~~~ = 51.5% of the ultimate strength.

Tests on special reinforcing steel with a minimum yield of 50,000 psi have resulted in yield strength of 55,500 psi, with an ultimate strength of 90,000 psi; with a design-allowable stress of 90% of minimum yield, the design-allowable stress is 0.9 x 50,000 = 45,000 psi.

Design-allowable stress on reinforcing is :~: ~~~ = 50% of ultimate strength.

Concrete continues to increase in strengt!t beyond the 28-day strength of 3000 psi. The Bureau of Reclamation Concrete Manual indi9ates that Type II cement concrete can be expected to increase in strength approximately 30% in 1 year from the 28-day strength.

Approximate 28-day strength for 3000-psi concrete from test reports= 3800 psi Design allowable 85% of 3000 psi = 2500 psi Ultimate strength in 1 year= 1.3 x 3800 = 4950 psi Design allowable is ~ !~~ = 51 % of ultimate strength in 1 year.

The above figures show that, for structures designed for the design-basis accident loading, structural steel and reinforced concrete are designed at approximately 50% of their ultimate strength. In the design of concrete structural members under design-basis accident conditions, concrete strength is not the c;ontrolling factor.

Allowable soil bearing values for foundations are determined from the soil boring logs and the results of triaxial shear tests of the soil. Applicable factors of safety are applied to the test results.

15.6.2 Reactor Coolant System Supports The reactor coolant system includes the reactor vessel, three steam generators, three reactor coolant pumps, and a pressurizer for each unit. Structures are provided to support these heavy vessels and equipment, and to ensure system integrity during normal operation and design-basis accident conditions.

II NSERTI By license amendment issuance (Reference 12), the NRC approved the application of risk-informed methodology and acceptance criteria, in accordance with US NRC Regulatory Guide (RG) 1.174, Rev. 3 (Reference 13), for tornado classification of the Fuel-Handling Trolley Support Structure (FHTSS), which is the high bay structure of the Fuel Building. The risk-informed analysis calculates the risk of fuel damage introduced to spent fuel assemblies stored in the spent fuel pool (SFP) due to the postulated collapse of the FHTSS from tornado events with wind speeds up to 369 mph. 'The risk of fuel damage is estimated as the sum of the products of the probability of FHTSS collapse, and the initiating event (i.e., tornado) frequency, calculated over a range of tornado wind -peeds'1,1p to 360 mph. The probability of the FHTSS collapse at a given wind speed is obtained from a orna o fragility curve, which is assumed to follow a lognormal distribution, using the median structural capacity of the FHTSS, and an industry-approved generic composite variability. The initiating eve*n frequencies are estimated from tornado wind exceedance frequencies that follow a logarithm.ic trend line, calibrated with tornado exceedance frequencies in Table 6-1 of US NRC NUREG/CR 4461, Rev. 2 (Reference 14).,E stimated risks were compared to the acceptance criteria for bas~ risk a:nd delta risk correspondin to the Core Damage Frequency (CDF) in RG 1.174, Rev. 3.

The risk-informed analysis shows the change in risk of dain,~e to spent fuel assemblies, from tornado events with wind speeds up to 360 mph, is .small, and within t e acceP.tance limits ofRG 1.174, Rev. 3. Radiological consequences from impact of potential falling members ofFHTSS upon spent fuel assemblies during a tornado event are reasonably 'expected to* be bounded;by the Fuel Handling Accident in the SFP, as described in Section 14.4.1.3 ofSPS Updated Final Safety Analysis Report (UFSAR). The potential minor leakage due to the impact of a single f<Jl}ing member is bounded by that due to a cask-drop, as described in Section 9.12.4.13 of SPS SAR. It is also demonstrated that potential falling structural debris, due to the po_stuh1tedcollapse of the FHTSS during a tornado wind event, will not result in perforation of reinforced concrete walls and floor mat of the SFP.

It is demonstrateo that the nuclear safety risk of a postulated FHTSS collapse due to tornado events with wind speeds up to 360 mph *s acceptably low, and the FHTSS meets applicable criteria in Table 15.2-1 by risk-fafo rmed methodology.

Revision 53.02-Updated Online 02/24/22 SPS UFSAR 15.6-13

15.6 REFERENCES

1. Westinghouse Topical Report WCAP 9558, Revision 2, Mechanistic Fracture Evaluation of Reactor Coolant Pipe Containing a Postulated Circumferential Throughwall Crack, May 1981.

2. Westinghouse Topical Report WCAP 9787, Tensile and Toughness Properties of Primary Piping Weld Metal for Use in Mechanistic Fracture Evaluation, May 1981.

3. Letter from Vepco to NRC,

Subject:

Request for Partial Exemption from General Design Criterion 4, dated November 5, 1985 (Serial No.85-136).

4. Letter from Vepco to NRC,

Subject:

Request for Partial Exemption from General Design Criterion 4 - Supplement, dated December 3, 1985 (Serial No. 85-136A).

5. Letter from Vepco to NRC,

Subject:

Partial Exemption from General Design Criterion 4 -

Request for Additional Information, dated December 27, 1985 (Serial No. 85-136B).

6. Letter from Vepco to NRC,

Subject:

Partial Exemption from General Design Criterion 4 -

Request for Additional Information, dated January 14, 1986 (Serial No. 85-136C).

7. Amendment to General Design Criterion 4 (GDC-4), 10 CFR Part 50, Appendix A, published in Federal Register 51 FR 12502, effective May 12, 1986.

8. Letter from Vepco to NRC,

Subject:

Proposed License Amendment - GDC 4, dated April 30, 1986 (Serial No.86-245).

9. Letter from NRC to Vepco transmitting Surry Unit 1 and 2 License Amendments No. 108 and related safety evaluations, dated June 16, 986.

10. Letter from Vepco to NRC,

Subject:

Generic Letter 87-11, dated September 12, 1988 (Serial No.88-371).

11 . Manual of Steel Construction, 7th Edition, American Institute of Steel Construction.

l

12. Lett_er from -- (NRC) to Sun y Power Station, Unit Nos. 1 and 2, Issuance of License Amendment Regarding Application of Risk-Informed Methodology for Tornado Classification of the Fuel-Handling Trolley Support Structure, Serial No.

XX-XXX, Date TBD

13. U.S. Nuclear Regulatory Commission, An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to The Licensing Basis, Regulatory Guide 1.174, Rev. 3, January 2018

14. U.S. Nuclear Regulatory Commission NUREG/CR-4461, Rev. 2, Tornado Climatology of the Contiguous United States, Pacific Northwest National Laboratory, February 2007