L-04-002, Stations - NRC Generic Letter 2004-02, Potential Impact of Debris Blockage on Emergency Recirculation During Design Basis Accidents at Pressurized-Water Reactors Fleet Response to Request for Additional Information
| ML23128A162 | |
| Person / Time | |
|---|---|
| Site: | Millstone, Surry, North Anna  (DPR-065, DPR-032, DPR-037, NPF-049, NPF-004, NPF-007) |
| Issue date: | 05/08/2023 |
| From: | James Holloway Dominion Energy Nuclear Connecticut, Dominion Energy Services, Virginia Electric & Power Co (VEPCO) |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| 23-044, GL-04-002 | |
| Download: ML23128A162 (1) | |
Text
Dominion Energy Services. Inc.
5000 Dominion Boulevard, Glen Allen, VA 23060 Dominion Energy.com U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC 20555-0001 May 8, 2023 DOMINION ENERGY NUCLEAR CONNECTICUT, INC.
VIRGINIA ELECTRIC AND POWER COMPANY MILLSTONE POWER STATION UNITS 2 AND 3 NORTH ANNA POWER STATION UNITS 1 AND 2 SURRY POWER STATION UNITS 1 AND 2 P
Dominion
- t
- $! Energy~
Serial No.:
23-044 NRA/GDM:
RO Docket Nos.:
50-336/423 50-338/339 50-280/281 License Nos.: DPR-65 NPF-49 NPF-4/7 DPR-32/37 NRC GENERIC LETTER 2004-02, "POTENTIAL IMPACT OF DEBRIS BLOCKAGE ON EMERGENCY RECIRCULATION DURING DESIGN BASIS ACCIDENTS AT PRESSURIZED-WATER REACTORS" FLEET RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION By letters dated May 27, 2021 (ADAMS Accession No. ML21147A477), April 15, 2021 (ADAMS Accession No. ML21105A433), and February 25, 2021 (ADAMS Accession Nos.
ML21056A557 and ML21056A541), Dominion Energy Nuclear Connecticut, Inc. and Virginia Electric and Power Company (collectively, Dominion Energy) submitted final supplemental responses for Millstone Power Station (MPS) Units 2 and 3, and North Anna and Surry Power Stations (NAPS and SPS) Units 1 and 2, respectively, to Generic Letter (GL) 2004-02, "Potential Impact of Debris Blockage on Emergency Recirculation during Design Basis Accidents at Pressurized-Water Reactors," dated September 13, 2004. By letter dated November 7, 2022 (ADAMS Accession No. ML22312A442), Dominion Energy responded to a fleet request for additional information (RAI) associated with the final supplemental responses noted above.
By email dated March 9, 2023, the NRG provided a follow-up fleet RAI associated with the station and fleet submittals noted above. A clarification call was also held with the NRG on March 9, 2023, to discuss the NRG RAI as well as Dominion Energy's planned response. Dominion Energy's response to the NRG RAI is provided in Attachment 1 for MPS Units 2 and 3, and in Attachments 2 and 3 for North Anna Units 1 and 2 and SPS Units 1 and 2, respectively.
Serial No.23-044 Docket Nos. 50-336/423/338/339/280/281 GL 2004-02 RAI Response Page 2 of 3 Should you have any questions or require additional information, please contact Mr. Gary D. Miller at (804) 273-2771.
Respectfully, James E. Holloway Vice President - Nuclear Engineering and Fleet Support Commitments contained in this letter: None Attachments:
- 1. Response to Request for Additional Information, Generic Letter 2004-02 Final Supplemental Response - Millstone Power Station Units 2 and 3
- 2. Response to Request for Additional Information, Generic Letter 2004-02 Final Supplemental Response - North Anna Power Station Units 1 and 2
- 3. Response to Request for Additional Information, Generic Letter 2004-02 Final Supplemental Response - Surry Power Station Units 1 and 2 COMMONWEAL TH OF VIRGINIA
)
)
COUNTY OF HENRICO
)
The foregoing document was acknowledged before me, in and for the County and Commonwealth aforesaid, today by James E. Holloway, who is Vice President - Nuclear Engineering and Fleet Support of Dominion Energy Nuclear Connecticut, Inc., and 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 ef"'
day of f-1 A J My Commission Expires: a7Y$+ '5l1ZoZ.3.
GARY DON MILLER Notary Public Commonwealth of Virginia Reg.# 7629412 My Commission Expires August 31, 2013
, 2023.
Serial No.23-044 Docket Nos. 50-336/423/338/339/280/281 GL 2004-02 RAI Response Page 3 of 3 cc:
U.S. Nuclear Regulatory Commission - Region I 2100 Renaissance Blvd, Suite 100 King of Prussia, PA 19406-2713 U.S. Nuclear Regulatory Commission - Region II Marquis One Tower 245 Peachtree Center Avenue, NE Suite 1200 Atlanta, GA 30303-1257 Mr. Richard V. Guzman NRC Senior Project Manager - Millstone Power Station Units 2 and 3 U.S. Nuclear Regulatory Commission One White Flint North, Mail Stop 08 C-2 11555 Rockville Pike Rockville, MD 20852-2738 Mr. G. Edward Miller NRC Senior Project Manager - North Anna Power Station Units 1 and 2 U:S. Nuclear Regulatory Commission One White Flint North, Mail Stop 09 E-3 11555 Rockville Pike Rockville, MD 20852-2738 Mr. L. John Klos NRC Project Manager - Surry Power Station Units 1 and 2 U.S. Nuclear Regulatory Commission One White Flint North, Mail Stop 09 E-3 11555 Rockville Pike Rockville, MD 20852-2738 NRC Senior Resident Inspector Millstone Power Station NRC Senior Resident Inspector North Anna Power Station NRC Senior Resident Inspector Surry Power Station Old Dominion Electric Cooperative Electronically Distributed Serial No.23-044 Docket No. 50-336/423 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION GENERIC LETTER 2004-02 FINAL SUPPLEMENTAL RESPONSE Dominion Energy Nuclear Connecticut, Inc.
(DENC)
Millstone Power Station Units 2 and 3
Serial No.23-044 Docket Nos. 50-336/423 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION GENERIC LETTER 2004-02 FINAL SUPPLEMENTAL RESPONSE MILLSTONE POWER STATION UNITS 2 AND 3 NRCCOMMENT 10 CFR 40.46 requires that plants maintain the ability to provide Jong-term core cooling following any initial LOCA response. To ensure that this cooling is available, licensees demonstrate that in-vessel fiber accumulation will not adversely affect the required function.
Dominion - GL 04-02 Response Follow Up RAJ In its most recent RAJ response for NRG Generic Letter 2004-02, dated November [22],
2022 (ADAMS Accession No. ML22312A442), Dominion Energy provided information regarding the debris amounts used in the fiber penetration calculation for Millstone 2 and 3, North Anna 1 and 2, and Surry 1 and 2. The RAJ response provided details regarding the methodology used to determine the plant-specific amounts of fiber that could penetrate the emergency core cooling system strainers. Upon review of the information, the NRG staff identified some assumptions used in the calculations appeared to be inconsistent with each other and departs from the expected behavior of various size classifications of fibrous debris.
For the plant-specific penetration calculations, some of the assumptions used for the bed thickness corrections were not justified.
Specifically, the NRG staff identified that differences in debris characteristics for the inputs did not appear to be accounted for in the fiber penetration evaluations.
The cumulative bypass fractions are based on the debris bed thicknesses resulting from all fibrous debris that is predicted to transport to the strainers. The assumed bed thicknesses include fine, small, and large pieces of fiber.
However, the penetration testing used to develop the penetration metrics used only fine fiber. The evaluation does not account for differences between the filtering efficiency of fines and pieces larger than fines. The evaluation also assumes that fine fiber, small pieces, and large pieces will transport to the strainers with the same timing which was not justified. In addition, the assumption that small and large pieces of fiber will accumulate uniformly on the strainer appears to have no supporting basis and considering the transport characteristics of the different debris sizes, uniform accumulation is very unlikely to occur. Non-uniform accumulation of debris will affect the filtering efficiency of the fiber bed.
Please provide a justification for the bed thickness justifications used in the penetration analysis.
Page 1 of 5
DOMINION ENERGY RESPONSE Millstone Power Station (MPS) Unit 2 Serial No.23-044 Docket Nos. 50-336/423 In response to the NRC RAI (Reference 2) and as discussed during the associated clarification call, a more conservative theoretical bed thickness was determined based on the assumption that only fines will transport to the strainer. This assumption provides consistency with the Point Beach fiber fines-only bypass tests. Assuming a fines-only debris bed results in a significantly thinner debris bed and a higher cumulative bypass fraction.
The bypass fraction based on a fines-only sump strainer debris bed was calculated to be 0.0862 (8.62%), compared to 0.019 (1.9%) given in Section 3.n.1 of Reference 3. The Reference 3, Section 3.n.1, value was determined assuming fine, small, and large pieces of fiber contribute to the build-up of a debris bed on the sump strainer.
The fibrous debris load reaching the reactor vessel was determined using Equation 6-39 of Reference 1, Volume 1. The Emergency Core Cooling System (ECCS) flow rate, Containment Spray System (CSS) flow rate, and initial sump fiber load from Section 3.n.5 of Reference 3 remain applicable for use with Equation 6-39. The updated bypass fraction, assuming a fines-only strainer debris bed, was used as additional input as discussed above.
MPS Unit 2 is a Combustion Engineering (CE) design with Framatome fuel.
The applicable core inlet fiber load threshold is provided in Table 6-5 of Reference 1, Volume 1, for CE plants. The total reactor vessel fiber load is calculated to be 89.90 grams/Fuel Assembly (g/FA), assuming the formation of a fines-only theoretical debris bed at the sump strainer. While this exceeds the core inlet fiber limit, it is less than the total in-vessel debris limit provided in Section 6.4 of Reference 1, Volume 1. The analyses in Reference 1 conservatively assumed that debris would collect uniformly at the core inlet because that results in the greatest head-loss for a given amount of debris. However, the debris bed is realistically expected to collect non-uniformly. As a result, the amount of debris required to completely block the core inlet would be greater than that assumed in the analyses [Technical Evaluation Report (TER), Section 5.2.1, of Reference 1]. Therefore, the core inlet fiber limit is not a critical parameter, and the current long-term core cooling (L TCC) analyses remain applicable.
In the unlikely scenario where the core inlet did become completely blocked, alternate flow paths (AFP) will allow flow to reach the core. The plant-specific AFP resistance for MPS Unit 2 is documented in RAI Table 4.3-7 of Reference 1, Volume 4, and is discussed in Section 3.n.14 of Reference 3. The MPS Unit 2-specific AFP resistance is significantly less than the AFP resistance assumed in the Reference 1 analyses and would thus be effective to maintain core cooling. Regarding the unbounded minimum and maximum ECCS recirculation flow rates identified in Section 3.n.15 of Reference 3, the justifications for their acceptability at MPS Unit 2 remain applicable with the use of the in-vessel debris limit.
Page 2 of 5
References Serial No.23-044 Docket Nos. 50-336/423
- 1. Westinghouse Report, WCAP-17788, Rev. 1, "Comprehensive Analysis and Test Program for GSl-191 Closure (PA-SEE-1090)," December 2019.
- 2. Email from Ed Miller, NRG Dominion Energy Fleet Senior Project Manager, to Gary D. Miller, Dominion Energy Services, dated Thursday, March 9, 2023, 3:59 PM,
Subject:
RAI for GL 2004-02 Response."
- 3. Letter from M. D. Sartain (Dominion Energy) to USNRC, "Dominion Energy Nuclear Connecticut, Inc., Millstone Power Station Unit 2, NRG Generic Letter 2004-02,
'Potential Impact of Debris Blockage on Emergency Recirculation During Design Basis Accidents at Pressurized-Water Reactors' Final Supplemental Response," dated May 27, 2021. Dominion Serial No.21-040. ADAMS Accession Number ML21147A477.
Millstone Power Station (MPS) Unit 3 In response to the NRG RAI (Reference 2) and as discussed during the associated clarification call, a more conservative theoretical bed thickness was determined based on the assumption that only fines will transport to the strainer. This provides consistency with the Point Beach fiber fines-only bypass tests. Assuming a fines-only debris bed results in a significantly thinner debris bed and a higher cumulative bypass fraction. The bypass fraction based on a fines-only sump strainer debris bed was calculated to be 0.1217 (12.17%), compared to 0.035 (3.5%), given in Section 3.n.1 of Reference 3, which was calculated assuming all fiber sizes (fines, small, and large pieces) transported to the sump strainers.
The fibrous debris load reaching the reactor vessel was determined using Equation 6-39 of Reference 1, Volume 1. The ECCS flow rate, Recirculation Spray System (RSS) flow rate, and sump fiber load from Section 3.n.5 of Reference 3 remain applicable for use with Equation 6-39. The updated bypass fraction assuming a fines-only sump strainer debris bed was used as additional input as discussed above.
MPS Unit 3 is a Westinghouse 4-loop upflow design with Westinghouse fuel.
The applicable core inlet fiber threshold is provided in Table 6-3 of Reference 1, Volume 1.
The total reactor vessel fiber load is calculated to be 56.70 g/FA assuming the formation of a fines-only theoretical debris bed at the sump strainer. While this exceeds the core inlet fiber limit provided in Table 6-3 of Reference 1, it is less than the total in-vessel debris limit in Section 6.4 of Reference 1. The analyses in Reference 1 conservatively assumed that debris would collect uniformly at the core inlet because that results in the greatest head-loss for a given amount of debris. However, the debris bed is realistically expected to collect non-uniformly. As a result, the amount of debris required to completely block the core inlet would be greater than that assumed in the analyses [TER Section 5.2.1 of Page 3 of 5
Serial No.23-044 Docket Nos. 50-336/423 Reference 1]. Therefore, the core inlet fiber limit is not a critical parameter.
In the unlikely scenario where the core inlet did become completely blocked, alternate flow paths (AFP) will allow flow to reach the core. The plant-specific AFP resistance for MPS Unit 3 is documented in Table RAl-4.2-24 of Reference 1, Volume 4, and is discussed in Section 3.n.14 of Reference 3. The MPS Unit 3-specific unadjusted AFP resistance is similar to, but less than, the AFP resistance assumed in the Reference 1 analyses and would thus be effective to maintain core cooling.
Section 3. n.15 of Reference 3 discusses the consistency between the minimum ECCS flow per FA assumed in the Reference 1 AFP analyses and the minimum ECCS flow per FA at MPS Unit 3.
The minimum ECCS recirculation flow at MPS Unit 3 is 6 gpm/FA versus the minimum analyzed flow of 8 gpm/FA in Reference 1.
In Reference 3, it was stated that the minimum ECCS flow is acceptable because the resulting fiber load at this flow rate is non-limiting and the limiting fiber load at the highest ECCS flow rate was well below the core inlet limit. However, the sump strainer bypass fraction has since been recalculated based on the RAI in Reference 2, and the core inlet fiber limit is now exceeded for the limiting configuration. As a result, the unbounded minimum ECCS recirculation flow will be justified herein by accounting for remaining excess conservatisms in the Reference 1 analyses compared to plant-specific parameters at MPS Unit 3.
Justification for Unbounded Minimum EGGS Recirculation Flow Rate The minimum plant-specific ECCS flow rate during recirculation is less than the minimum analyzed ECCS flow rate used to develop Kmax in Reference 1. As noted in the TER of Reference 1 (p. 43), debris bed resistance increases as ECCS flow rate decreases, so an unbounded low flow has the potential to cause the Kmax used in the calculation to be non-conservative.
For Westinghouse Upflow plants in Reference 1, the maximum core inlet resistance that could be tolerated prior to reaching tbrock, known as Kmax, was determined in Section 8 of Volume 4. Kmax was determined by applying instantaneous partial core inlet blockage at the time of sump switchover (SSO) and then complete core inlet blockage at the time of tbrock. The assumption that the maximum resistance that can be tolerated occurs at the time of SSO is substantially conservative. It is not possible for the fiber load associated with the core inlet limit to arrive at the core inlet and form a uniform bed coincident with the initiation of SSO. Realistically, fibrous debris would arrive over a period of time throughout sump recirculation until penetration at the sump strainers becomes negligible.
The fibrous debris would also take time to travel to the strainers at the time of SSO, penetrate the strainers, and travel to the reactor vessel, and finally to the core inlet where a non-uniform debris bed would begin to form.
Section 5.2.1 of the TER included in Reference 1 discusses that some amount of fiber in a debris bed is required before the core inlet begins to experience resistance and thus Page 4 of 5
Serial No.23-044 Docket Nos. 50-336/423 induce a reduction in flow through the core inlet.
The lower than analyzed ECCS recirculation flow rate would delay the initiation of resistance at the core inlet debris bed, thereby allowing decay heat to dissipate and reduce the coolant requirement to offset boiloff. A lower ECCS flow rate would also result in reduced fiber penetration through the sump strainers compared to the limiting fiber case and slower transport of fibrous debris to the reactor vessel relative to the Reference 1 analyses and thus slow the formation of a debris bed at the core inlet.
The combination of delayed debris arrival and delayed initiation of resistance across the core inlet debris bed ensures an unbounded ECCS recirculation flow rate would offset any non-conservatisms in the value of Kmax because of an unbounded low ECCS flow rate, and LTCC is ensured.
References
- 1. Westinghouse Report, WCAP-17788-P, Rev. 1, "Comprehensive Analysis and Test Program for GSl-191 Closure (PA-SEE-1090)," December 2019.
- 2. Email from Ed Miller, NRC Dominion Energy Fleet Senior Project Manager, to Gary D. Miller, Dominion Energy Services, dated Thursday, March 9, 2023, 3:59 PM,
Subject:
RAI for GL 2004-02 Response."
- 3. Letter from Mark D. Sartain (Dominion Energy) to USN RC, "Dominion Energy Nuclear Connecticut, Inc., Millstone Power Station Unit 3, NRC Generic Letter 2004-02,
'Potential Impact of Debris Blockage On Emergency Recirculation During Design Basis Accidents at Pressurized-Water Reactors' Final Supplemental Response,"
Serial No.21-016, April 15, 2021.
Page 5 of 5 Serial No.23-044 Docket Nos. 50-338/339 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION GENERIC LETTER 2004-02 FINAL SUPPLEMENTAL RESPONSE Virginia Electric and Power Company (Dominion Energy Virginia)
North Anna Power Station Units 1 and 2
Serial No.23-044 Docket Nos. 50-338/339 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION GENERIC LETTER 2004-02 FINAL SUPPLEMENTAL RESPONSE NORTH ANNA POWER STATION UNITS 1 AND 2 NRC COMMENT 10 GFR 40.46 requires that plants maintain the ability to provide long-term core cooling following any initial LOGA response. To ensure that this cooling is available, licensees demonstrate that in-vessel fiber accumulation will not adversely affect the required function.
Dominion - GL 04-02 Response Follow Up RAJ In its most recent RAJ response for NRG Generic Letter 2004-02, dated November [22],
2022 (ADAMS Accession No. ML22312A442), Dominion Energy provided information regarding the debris amounts used in the fiber penetration calculation for Millstone 2 and 3, North Anna 1 and 2, and Surry 1 and 2. The RAJ response provided details regarding the methodology used to determine the plant-specific amounts of fiber that could penetrate the emergency core cooling system strainers. Upon review of the information, the NRG staff identified some assumptions used in the calculations appeared to be inconsistent with each other and departs from the expected behavior of various size classifications of fibrous debris.
For the plant-specific penetration calculations, some of the assumptions used for the bed thickness corrections were not justified. Specifically, the NRG staff identified that differences in debris characteristics for the inputs did not appear to be accounted for in the fiber penetration evaluations.
The cumulative bypass fractions are based on the debris bed thicknesses resulting from all fibrous debris that is predicted to transport to the strainers. The assumed bed thicknesses include fine, small, and large pieces of fiber.
However, the penetration testing used to develop the penetration metrics used only fine fiber. The evaluation does not account for differences between the filtering efficiency of fines and pieces larger than fines. The evaluation also assumes that fine fiber, small pieces, and large pieces will transport to the strainers with the same timing which was not justified. In addition, the assumption that small and large pieces of fiber will accumulate uniformly on the strainer appears to have no supporting basis and considering the transport characteristics of the different debris sizes, uniform accumulation is very unlikely to occur. Non-uniform accumulation of debris will affect the filtering efficiency of the fiber bed.
Please provide a justification for the bed thickness justifications used in the penetration analysis.
Page 1 of 3
DOMINION ENERGY RESPONSE Serial No.23-044 Docket Nos. 50-338/339 In response to the NRG RAI (Reference 2) and as discussed during the associated clarification call, a more conservative theoretical bed thickness was determined based on the assumption that only fines will transport to the strainer. This provides consistency with the Point Beach fiber fines-only bypass tests. Assuming a fines-only debris bed results in a significantly thinner debris bed and a higher cumulative bypass fraction. The bypass fraction based on a fines-only sump strainer debris bed was calculated to be 0.3098 (30.98%), compared to 0.074 (7.4%) given in Section 3.n.1 of Reference 3. The Reference 3, Section 3.n.1 value was determined assuming fine, small, and large pieces of fiber contribute to the build-up of a debris bed on the sump strainer.
The fibrous debris load reaching the reactor vessel was determined using Equation 6-39 of Reference 1, Volume 1. The ECCS flow rate, RSS flow rate, and initial sump fiber load from Section 3.n.5 of Reference 3 remain applicable for use with Equation 6-39. The updated bypass fraction assuming a fines-only sump strainer debris bed was used as additional input as discussed above.
North Anna Power Station (NAPS) is a Westinghouse 3-loop upflow design with Westinghouse fuel. The applicable core inlet fiber threshold is provided in Table 6-3 of Reference 1, Volume 1. The total reactor vessel fiber load is calculated to be 98.1 g/FA, assuming the formation of a fines-only theoretical debris bed at the sump strainer. While this exceeds the core inlet fiber limit, it is less than the total in-vessel debris limit provided in Section 6.4 of Reference 1, Volume 1. The analyses in Reference 1, Volume 1, conservatively assumed that debris would collect uniformly at the core inlet because that results in the greatest head-loss for a given amount of debris. However, the debris bed is realistically expected to collect non-uniformly. As a result, the amount of debris required to completely block the core inlet would be greater than that assumed in the analyses.
Therefore, the core inlet fiber limit is not a critical parameter and the current L TCC analyses remain applicable.
In the unlikely scenario where the core inlet did become completely blocked, alternate flow paths (AFP) will allow flow to reach the core. The plant-specific AFP resistance for NAPS is documented in Table RAl-4.2-24 of Reference 1, Volume 4, and is discussed in Section 3.n.14 of Reference 3. The North Anna-specific AFP resistance is significantly less than the AFP resistance assumed in the Reference 1 analyses and would thus be effective to maintain core cooling.
Page 2 of 3
References Serial No.23-044 Docket Nos. 50-338/339
- 1. PWROG Report, WCAP-17788-P, Rev. 1, "Comprehensive Analysis and Test Program for GSl-191 Closure (PA-SEE-1090)," December 2019.
- 2. Email from Ed Miller, NRC Dominion Energy Fleet Senior Project Manager, to Gary D. Miller, Dominion Energy Services, dated Thursday, March 9, 2023 3:59 PM,
Subject:
RAI for GL 2004-02 Response."
- 3. Letter from M. D. Sartain (Dominion Energy) to USN RC, "Virginia Electric and Power Company, North Anna Power Station Units 1 and 2, NRC Generic Letter 2004-02,
'Potential Impact of Debris Blockage on Emergency Recirculation During Design Basis Accidents at Pressurized-Water Reactors' Final Supplemental Response," dated February 25, 2021. Dominion Serial No. 21 014. ADAMS Accession Number ML21056A557.
Page 3 of 3 Serial No.23-044 Docket Nos. 50-280/281 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION GENERIC LETTER 2004-02 FINAL SUPPLEMENTAL RESPONSE Virginia Electric and Power Company (Dominion Energy Virginia)
Surry Power Station Units 1 and 2
Serial No.23-044 Docket Nos. 50-280/281 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION GENERIC LETTER 2004-02 FINAL SUPPLEMENTAL RESPONSE SURRY POWER STATION UNITS 1 AND 2 NRC COMMENT 10 GFR 40.46 requires that plants maintain the ability to provide long-term core cooling following any initial LOGA response. To ensure that this cooling is available, licensees demonstrate that in-vessel fiber accumulation will not adversely affect the required function.
Dominion - GL 04-02 Response Follow Up RAJ In its most recent RA/ response for NRG Generic Letter 2004-02, dated November [22],
2022 (ADAMS Accession No. ML22312A442), Dominion Energy provided information regarding the debris amounts used in the fiber penetration calculation for Millstone 2 and 3, North Anna 1 and 2, and Surry 1 and 2. The RAJ response provided details regarding the methodology used to determine the plant-specific amounts of fiber that could penetrate the emergency core cooling system strainers. Upon review of the information, the NRG staff identified some assumptions used in the calculations appeared to be inconsistent with each other and departs from the expected behavior of various size classifications of fibrous debris.
For the plant-specific penetration calculations, some of the assumptions used for the bed thickness corrections were not justified. Specifically, the NRG staff identified that differences in debris characteristics for the inputs did not appear to be accounted for in the fiber penetration evaluations.
The cumulative bypass fractions are based on the debris bed thicknesses resulting from all fibrous debris that is predicted to transport to the strainers. The assumed bed thicknesses include fine, small, and large pieces of fiber.
However, the penetration testing used to develop the penetration metrics used only fine fiber. The evaluation does not account for differences between the filtering efficiency of fines and pieces larger than fines. The evaluation also assumes that fine fiber, small pieces, and large pieces will transport to the strainers with the same timing which was not justified. In addition, the assumption that small and large pieces of fiber will accumulate uniformly on the strainer appears to have no supporting basis and considering the transport characteristics of the different debris sizes, uniform accumulation is ve,y unlikely to occur. Non-uniform accumulation of debris will affect the filtering efficiency of the fiber bed.
Please provide a justification for the bed thickness justifications used in the penetration analysis.
DOMINION ENERGY RESPONSE In response to the NRC RAI (Reference 2) and as discussed during the associated clarification call, a more conservative theoretical bed thickness was determined based on the assumption that only fines will transport to the strainer. This provides consistency Page 1 of 3
Serial No.23-044 Docket Nos. 50-280/281 with the Point Beach fiber fines-only bypass tests. Assuming a fines-only debris bed results in a significantly thinner debris bed and a higher cumulative bypass fraction. In the case of a fines-only debris bed, the theoretical bed thickness is 0.05179 inches. Since this bed thickness is insignificant, it is therefore appropriate to use a bypass fraction associated with the clean plant method as described in Section 4.6 of Reference 4.
Accordingly, a bypass fraction of 0.45 (45%) is assigned, compared to 0.121 (12.1%)
given in Section 3.n.1 of Reference 3.
The Reference 3, Section 3.n.1, value was determined assuming fine, small, and large pieces of fiber contribute to the build-up of a debris bed on the sump strainer. Reference 4, Section 4.6, notes the clean plant method assumes a 75% transport fraction of all fibrous debris inside containment. The results described herein utilized a 100% transport fraction, which provides a bounding in-vessel fiber load.
The fibrous debris load reaching the reactor vessel was determined using Equation 6-39 of Reference 1, Volume 1. The ECCS flow rate, RSS flow rate, and initial sump fiber load from Section 3.n.5 of Reference 3 remain applicable for use with Equation 6-39. The updated bypass fraction assuming a fines-only sump strainer debris bed was used as additional input as discussed above.
Surry Power Station (SPS) Units 1 and 2 are Westinghouse 3-loop upflow and downflow designs, respectively, and use Westinghouse fuel. SPS Unit 1 was recently converted to an upflow reactor vessel (RV) design as part of subsequent license renewal (SLR) modifications. SPS Unit 2 will be converted to an upflow RV design in the near future.
The applicable core inlet fiber threshold is provided in Table 6-3 of Reference 1, Volume 1. The total reactor vessel fiber load is calculated to be 74.1 g/FA, assuming the formation of a fines-only theoretical debris bed at the sump strainer and use of the clean plant method described in Section 4.6 of Reference 4. While this exceeds the core inlet fiber limit, it is less than the total in-vessel debris limit in Section 6.4 of Reference 1, Volume 1. The analyses in Reference 1, Volume 1, conservatively assumed that debris would collect uniformly at the core inlet because that results in the greatest head-loss for a given amount of debris. However, the debris bed is realistically expected to collect non-uniformly. As a result, the amount of debris required to completely block the core inlet would be greater than that assumed in the analyses. Therefore, the core inlet fiber limit is not a critical parameter and the current L TCC analyses remain applicable.
In the unlikely scenario where the core inlet did become completely blocked, alternate flow paths (AFP) will allow flow to reach the core. The plant-specific AFP resistance for SPS is documented in Table RAl-4.2-24 of Reference 1, Volume 4, and is discussed in Section 3.n.14 of Reference 3. The Surry-specific AFP resistance is significantly less than the AFP resistance assumed in the Reference 1 analyses and would thus be effective to maintain core cooling.
Page 2 of 3
References Serial No.23-044 Docket Nos. 50-280/281
- 1. PWROG Report, WCAP-17788-P, Rev. 1, "Comprehensive Analysis and Test Program for GSl-191 Closure (PA-SEE-1090)," December 2019.
- 2. Email from Ed Miller, NRC Dominion Energy Fleet Senior Project Manager, to Gary D. Miller, Dominion Energy Services, dated Thursday, March 9, 2023, 3:59 PM,
Subject:
RAI for GL 2004-02 Response."
- 3. Letter from M. D. Sartain (Dominion Energy) to USNRC, "Virginia Electric and Power Company, Surry Power Station Units 1 and 2, NRC Generic Letter 2004-02, 'Potential Impact of Debris Blockage on Emergency Recirculation During Design Basis Accidents at Pressurized-Water Reactors' Final Supplemental Response," dated February 25, 2021. Dominion Serial No.21-015. ADAMS Accession Number ML21056A541.
- 4. PWROG Report, PWROG-16073-P, Rev. 0, "TSTF-567 Implementation Guidance, Evaluation of In-Vessel Debris Effects, Submittal Template for Final Response to Generic Letter 2004-02 and FSAR Changes," February 2020.
Page 3 of 3