ML11301A026
| ML11301A026 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 10/25/2011 |
| From: | Heacock D Virginia Electric & Power Co (VEPCO) |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| Shared Package | |
| List: | |
| References | |
| 11-544C | |
| Download: ML11301A026 (6) | |
Text
SECURITY-RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390 VIRGINIA ELECTRIC AND POWER COMPANY RICHMOND, VIRGINIA 23261 October 25, 2011 10 CFR 100, Appendix A U.S. Nuclear Regulatory Commission Serial No.:
11-544C Attention: Document Control Desk NL&OS/ETS R1 Washington, DC 20555 Docket Nos.:
50-338/339 License Nos.:
NPF-4/7 VIRGINIA ELECTRIC AND POWER COMPANY (DOMINION)
NORTH ANNA POWER STATION UNITS I AND 2 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION (RAI) REGARDING THE REACTOR FUEL TECHNICAL AUDIT During September 19 through 22, 2011, the NRC staff conducted an audit of the post-earthquake fuel inspections at the North Anna Power Station. As a follow-up to the audit, the NRC reviewed additional vendor calculations on October 18, 2011.
As a result of the follow-up audit and a telephone call on October 20, 2011, the NRC staff requested additional information regarding the engineering calculations/evaluations performed to support the functionality of the reactor fuel. In the attachments to this letter, Dominion is providing responses to the additional information requested.
Dominion requests that Attachment 2 to this letter, which contains Security-Related Information, be withheld from public disclosure in accordance with 10 CFR 2.390.
If you have any questions or require additional information, please contact Mr. Thomas Shaub at (804) 273-2763.
Sincerely, D.A. Heacock President and Chief Nuclear Officer Attachments
- 1. Response to Request for Additional Information - Reactor Fuel Audit
- 2. Unit 1 and Unit 2 Reactor Coolant System/Containment Drawings There are no commitments made in this letter.
COMMONWEALTH OF VIRGINIA COUNTY OF HENRICO
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The foregoing document was acknowledged before me, in and for the County and Commonwealth aforesaid, today by David A.
Heacock who is President and Chief Nuclear Officer of Virginia Electric and Power Company. He has affirmed before me that he is duly authorized to execute and file the foregoing document on behalf of that Company, and that the statements in the document are true to the best of his knowledge and belief.
Ginger Lynn Rutherford Acknowledged before me this J.._-I7ay of C 2011.
NOTARY PUBLIC Commonwealth of Virginia My Commission Expires:.-
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My Commission Expires 4/30/2015 y
Notary Public 4 to this letter contains Security-Related Information which is to be withheld from public disclosure under 10 CFR 2.390. Upon removal of Attachment 2, this letter is decontrolled.
SECURITY-RELATED INFORMATION - WITHHOLD UNDER 10 CFR 2.390
Serial Number 11-544C Docket Nos. 50-338/339 Page 2 of 2 cc:
Regional Administrator U.S. Nuclear Regulatory Commission - Region II Marquis One Tower 245 Peachtree Center Ave., NE Suite 1200 Atlanta, Georgia 30303-1257 NRC Senior Resident Inspector North Anna Power Station M. Khanna NRC Branch Chief-Mechanical and Civil Engineering U. S. Nuclear Regulatory Commission One White Flint North Mail Stop 08 G-9E3 11555 Rockville Pike Rockville, MD 20852-2738 R. E. Martin NRC Project Manager U. S. Nuclear Regulatory Commission One White Flint North Mail Stop 08 G-9A 11555 Rockville Pike Rockville, MD 20852-2738 P. G. Boyle NRC Project Manager U. S. Nuclear Regulatory Commission One White Flint North Mail Stop 08 G-9A 11555 Rockville Pike Rockville, MD 20852-2738 J. E. Reasor, Jr.
Old Dominion Electric Cooperative Innsbrook Corporate Center 4201 Dominion Blvd.
Suite 300 Glen Allen, Virginia 23060
Serial Number 11-544C Docket Nos. 50-338/339 Response to Request for Additional Information Reactor Fuel Evaluation Virginia Electric and Power Company (Dominion)
North Anna Power Station Units 1 and 2
Serial No. 11-544C Docket Nos. 50-338/339 Page 1 of 3 BACKGROUND During September 19 through 22, 2011 the NRC staff conducted an audit of the post-earthquake fuel inspections at the North Anna Power Station. As a follow-up to the audit, the NRC reviewed additional vendor calculations on October 18, 2011. As a result of the follow-up audit and a subsequent telephone call on October 20, 2011, the NRC staff requested additional information regarding the engineering calculations/evaluations performed to support the functionality of the reactor fuel. Dominion is providing responses to the additional information requested below.
Reactor Fuels Clarifications associated with the NRC's technical audit of reactor fuel Discuss how the vertical component of the seismic event is treated in the AREVA analysis for Advanced Mark-BW fuel.
Dominion Response The existing design analysis for the AREVA Advanced Mark-BW fuel at North Anna is documented in report BAW-2414, Rev. 7. Section 7.3 of BAW-2414 discusses the faulted conditions analyses performed for seismic and LOCA events.
The faulted analysis addresses horizontal effects (for LOCA and seismic events) and vertical effects (LOCA only).
This analysis methodology is consistent with that documented in BAW-10239 (Advanced Mark-BW Fuel Assembly Mechanical Design Report).
As stated in Section 5.3.4 of BAW-10239, the faulted analysis addresses both the horizontal (LOCA and seismic) and vertical (LOCA) effects. The AREVA analyses in response to the August 23, 2011 earthquake were performed in the manner stated in Section 5.3.4 of BAW-1 0239.
As discussed during the audit and documented in a previous RAI response (Dominion letter dated October 3, 2011, Serial No. 11-544A), the control rods inserted during the earthquake and RCCA drag tests were subsequently performed. The RCCA drag testing confirmed that the fuel assembly guide tubes were not impacted by the August 23, 2011 earthquake. Separately, the vessel flange carries the load of the core plate and internals, so the fuel is not subjected to excessive mechanical loadings from these components during a seismic event.
Further, the top nozzle springs are designed to prevent fuel assembly liftoff during normal operation by being slightly compressed. During a seismic event, these springs would also help absorb some, if not all, of the load applied in the vertical direction due to the seismic event.
Serial No. 11-544C Docket Nos. 50-338/339 Page 2 of 3 Provide a discussion of how the AREVA Methodology addresses loads for SSE and LOCA Analysis.
Dominion Response In the North Anna seismic safe shutdown earthquake (SSE) and loss of coolant accident (LOCA) analysis of record (BAW-2414), the LOCA loads produce permanent deformations and the SSE loads do not.
In this case, combining the SSE and LOCA loads by the square root sum of the squares (SRSS) method does not apply, since the LOCA-only loads already exceed the elastic load limit (i.e. cannot combine linear and non-linear responses by SRSS). The method used to combine LOCA and SSE events is as follows:
- 1. Acceleration response spectra of the upper core plate motions were computed for both the worst-case LOCA and the SSE loads.
- 2. The SRSS combination of the acceleration response spectra were then computed around the peak of the LOCA response spectra near the third mode frequency of 10-12 Hz. The third mode frequency was chosen because this corresponds to the peak LOCA-only response, which is governing.
- 3. The maximum permanent grid deformation for the LOCA-only loads was computed with the core model (finite element model) used in the core seismic-LOCA analysis.
- 4. The total deformation corresponding to the SSE and LOCA combination load was computed by taking the total permanent deformation from the LOCA-only deformation (from Step 3) times the ratio of the combined response spectra value (from Step 2) to the LOCA-only response spectra (Step 1).
The peak acceleration values from the core plate motion spectra for LOCA and SSE at 12 Hz are 11g and 1g, respectively. The SRSS combination of these peak accelerations yields 11.05g. The ratio of the SRSS combined accelerations to LOCA accelerations is 11.05/11 = 1.01. Therefore, the permanent grid deformation obtained from the LOCA-only loads was multiplied by 1.01 to approximate the combined (or total) deformation.
The approach employed in the AREVA August 23, 2011 earthquake calculation (32-9170114-000) used the SRSS method to combine the LOCA and seismic loads in the X-direction. This was appropriate because both the LOCA and the seismic loads were below the load limit of the grid and therefore exhibit linear behavior. However, the LOCA loads in the Z-direction exceed the load limit of the grid and therefore exhibit non-linear behavior after grid deformation occurs. Thus, a more technically rigorous approach was used to assess the combined impact on the grids. As discussed in the AREVA calculation, the LOCA and seismic loads were combined assuming two scenarios: 1) the LOCA occurs at the beginning of the seismic event and 2) the LOCA occurs after the seismic event. The AREVA calculation concludes that the results from this approach are bounded by the worst-case SSE+LOCA analysis of record. Both approaches show that the SSE is a very small contributor to the combined SSE+LOCA event.
Serial No. 11-544C Docket Nos. 50-338/339 Page 3 of 3 Provide an assessment of the August 23, 2011 earthquake for the mixed cores duirng fuel transition.
Dominion Response While the RFA-2 fuel was not physically at North Anna during the August 23, 2011 earthquake, the NRC's concern appears to be a hypothetical scenario in which an earthquake similar to the August 23, 2011 earthquake occurs while the North Anna cores contain both Westinghouse RFA-2 fuel and AREVA Advanced Mark-BW fuel. In support of the RFA-2 fuel transition, Westinghouse and AREVA performed grid crush analyses for both the mixed core configurations and their respective full core configuration.
These analyses are performed with the combined SSE + LOCA core plate motions.
The Westinghouse document, VRA-11-66, and AREVA calculation, 32-9170114-000, (both available during the audit) concluded that the SSE + LOCA core plate motions used in the fuel transition analyses bound the August 23, 2011 seismic event + LOCA core plate motions. Thus, the fuel transition grid crush analyses performed by both vendors bounds a grid crush analysis performed for the August 23, 2011 earthquake. While the August 23, 2011 earthquake was not specifically modeled for a mixed core configuration, a grid crush analysis is not necessary as it would be bounded by the current fuel transition analyses, which did consider mixed core configurations.
Provide a discussion or containment drawings to show core symmetry.
Dominion Response As documented in a previous RAI response (Dominion letter 11-544B, dated October 17, 2011), directionally dependent forces would impact the Unit 1 and Unit 2 cores in the same manner due to the symmetric characteristics of the core and the fuel assemblies. The attached drawings of the Unit 1 and Unit 2 reactor coolant piping show that the cores are 1800 rotated from each other.
Clarification of the Root Cause Evaluation (RCE) discussion of East/West plane motion Dominion Response On page 14 (Figure 6) of the RCE, a statement is made about the East/West plane being the initial/predominant seismic motion. However, other documentation stated the strong motion of the earthquake was primarily in the North/South direction.
Dominion's subsequent review of the data shows that, very briefly, the initial and predominant motion leading to the reactor trip was in the east-west direction as stated in the RCE. The strong motion in the north-south direction occurred subsequent to the initial east-west motion.